Carbon sequestration potential and greenhouse gas emissions of forage sorghum cultivars in Southern Texas

1 Introduction

Climate change is one of the most significant global threats of the 21^st century. It is being accelerated by anthropogenic activities, particularly the release of CO₂ into the atmosphere. However, the terrestrial system acts as a significant carbon sink by sequestering carbon in plants and soils (1). Agricultural practices and photosynthesis mechanisms are key pathways through which atmospheric carbon can enter the terrestrial ecosystem (2). Carbon eventually enters the soil and is sequestered as Soil Organic Carbon (SOC) or soil inorganic carbon through its subsequent cycling (3). The form of carbon storage influences the duration of carbon remaining in the soil before it is released into the atmosphere. Sequestering atmospheric CO₂ as organic carbon in agricultural soils is considered an effective terrestrial strategy, with croplands estimated to store over 140 gigatons of organic carbon in the top 30 cm of soil, accounting for approximately 10% of the total global SOC reservoir (4). Carbon sequestration refers to both natural and anthropogenic activities that remove CO₂ from the atmosphere or prevent emissions from sources and store it in terrestrial ecosystems and aquatic reservoirs (5). In the terrestrial ecosystem, the most effective method of sequestering carbon is as SOC. In addition to mitigating CO₂ emissions, carbon sequestration in the soil provides numerous benefits, including increased soil fertility, improved soil structure and aggregate stability, enhanced water retention capacity, and the ability to reduce toxic elements (6).

The agricultural sector contributes to climate change by emitting greenhouse gases (GHGs) into the environment (2). Therefore, reducing agricultural GHG emissions and increasing soil carbon sequestration could play a critical role in climate change mitigation (7, 8). GHG emissions, including CO₂ and N₂O, are rapidly increasing, and Earth’s climate is continuously warming. Croplands play a significant role in regulating carbon dynamics within the global carbon cycle; however, knowledge of the comprehensive role of the terrestrial biosphere on a regional to global scale under a changing climate remains limited (3). Consequently, the management of agricultural systems to store atmospheric CO₂ as SOC and to decrease GHG emissions has been advocated as a partial solution to the climate change problem (3, 9). These can be biological, chemical, or physical processes that collect and store CO₂ in a stable conditions. Plants utilize carbon fixation through photosynthesis and chemical reactions in the soil, which serve as inorganic CO₂ sequestration mechanisms. Recommended management strategies to raise SOC in croplands include increasing cropping frequency and cultivating high-residue crops. Furthermore, reducing tillage, enhancing plant water-use efficiency, surface mulching, and shading are pivotal to minimizing the soil carbon loss (10). Information on techniques for enriching SOC stocks through the absorption of atmospheric carbon in agricultural soils is important for its implications for climate change mitigation and for its advantages for crop production and sustainability (3).

Plants have a unique ability to uptake atmospheric CO₂ through their stomata and use it for photosynthesis. However, when plants die, they return the sequestered carbon in their roots and emit it from the soil to the atmosphere (11). Additionally, the biological processes that influence carbon uptake and emissions through vegetative leaves and roots rely on daily and seasonal variations in weather parameters and hydrological and climatic variables (12). Hence, CO₂ fluxes over agricultural lands are expected to vary on daily and seasonal time scales. Thus, sustainable management of agroecosystems is critical to maximizing SOC stocks and atmospheric CO₂ sequestration. Recent studies have increasingly emphasized that soil C sequestration cannot be evaluated solely from total biomass or bulk SOC because rhizosphere processes, root-derived C inputs, microbial transformations, soil properties and management practices collectively determine the stabilization and persistence of C in soil. Studies in perennial grass agroecosystems have demonstrated that cultivation can alter labile, active and physically protected soil C pools, highlighting the importance of below-ground C dynamics in evaluating sequestration potential (13). Additionally, unique crop species and a variety of specific traits also play a crucial role in enhancing soil carbon sequestration. Several studies have identified that sorghum (Sorghum bicolor [L.] Moench) is a potential plant for soil carbon sequestration (14–16).

Sorghum (Sorghum bicolor [L.] Moench) is a valuable crop that can be grown in diverse agroclimatic regions worldwide, including semi-arid, subtropical, tropical, and temperate climates (17–20). Based on their characteristics, sorghum can be classified into four main groups: grain sorghum, forage sorghum, energy sorghum, and sweet sorghum. Sorghum is classified as a C4 crop and is renowned for its very high photosynthetic efficiency (21). It will extend root length, density, and water-use efficiency, enabling it to generate substantial biomass and exhibit excellent photosynthetic efficiency. Sorghum cultivars exhibit considerable variation in their carbon sequestration potential, reflecting differences in biomass accumulation, residue quality, and rooting traits (22, 23). Photoperiod-sensitive sorghum cultivars produce exceptionally high above-ground biomass (10–48 Mg ha⁻¹), and when residues are partially retained in the field, they contribute significantly to SOC stock (24, 25).

Forage sorghums are coarse, rapid-growing, warm-season grasses that supply animal feed in mid-summer (18). Forage sorghum is a crucial source of fresh feed and silage globally, with its quality and quantity being closely correlated to the timing of harvest (26–28). In contrast to maize, forage sorghum is better suited to production systems under limited inputs and suboptimal agronomic conditions. Sorghum used around 30% less water than maize forage production and required less nitrogen fertilizer than corn. Forage sorghums resembled grain varieties but exhibited greater height and superior forage quality (29). In 2007, the United States produced approximately 505 million bushels of sorghum grain, cultivated across roughly 7.7 million acres (30). Annually, over 6 million acres are dedicated to forage sorghum, yielding about 58 million tons of sorghum biomass, primarily consisting of cellulose, hemicellulose, and lignin. Forage sorghum cultivars are now developed to achieve high biomass yields. Forage sorghum has the capacity to attain considerable heights (6 to 15 ft) and may yield substantial vegetative biomass (31). There are different rhizomatous forage sorghum varieties that enhance carbon storage in the soil. According to Ferchaud et al. (15), rhizomes, which are an important source of carbon in plants, have the ability to greatly increase the carbon sequestration capacity of grain crops that are extensively cultivated. Several studies have shown that unique traits such as subsurface rhizomes in sorghum are ideally suited to increase atmospheric CO₂ sequestration and enhance SOC stocks. However, few studies have quantified the contribution of rhizomes, roots, and above-ground foliar biomass to atmospheric CO₂ sequestration and SOC accumulation (14, 24).

Agriculture is a significant contributor to climate change by emitting GHG into the atmosphere; However, it also has the potential to promote carbon sequestration through soil and biomass accumulation (7, 22, 32). Forage sorghum is extensively grown in Southern Texas due to its drought resistance. It yields high biomass, which could be a potential solution for higher-carbon-sequestration crops. However, there is very limited information on how different sorghum cultivars influence soil carbon stocks, above-ground and below-ground biomass, and GHG emissions in this study area.

This research study has three objectives: 1) to evaluate the effects of the three sorghum cultivars on selected soil health properties, 2) to evaluate the carbon sequestration potential and dynamics of three forage sorghum cultivars based on SOC content and above-ground and below-ground biomass production and 3) to quantify the GHG emissions (CO₂, and CH₄), from each sorghum cultivar under field conditions.

2 Methodology

The experimental site was on the Prairie View A&M University Research Farm of the College of Agriculture, Food, and Natural Resources, located in Prairie View, Texas, United States of America (Figure 1). The area experiences a humid subtropical climate with hot and humid summers and mild winters (33). The mean annual precipitation is approximately 1,118 mm, with more than 60% of the total occurring from June to October. The site experiences hot summers and cold winters, with January the coldest month with an average of 3°C, while July is the hottest month with an average of 35°C. According to the US Soil Taxonomy, the soil in the experimental field is classified as fine-loamy, siliceous, semiactive, and hyperthermic. Plinthaquic Paleudalfs and Wockley fine sandy loam is the type of soil at the experimental site.

The experiment was conducted over two growing seasons: June 2024 to November 2024 and April 2025 to September 2025. According to the eddy covariance (EC) flux tower data installed in the field, during the 1^st (2024) and 2^nd (2025) growing seasons, the experimental site received 588.3 mm and 510.5 mm of rainfall, respectively. The highest average temperature of 34.5 ^°C was recorded in August in both growing seasons while November (13.4 ^°C) and April (16.6 ^°C) had the lowest temperature in growing seasons 1 and 2, respectively.

2.1 Experimental design and treatments

One annual and two perennial forage sorghum cultivars were evaluated for their potential to sequester carbon. These sorghum cultivars are Sugar Graze Ultra (annual), PSH20TX01 (Sorghum bicolor × S. propinquum) (seeded perennial), and PSH09TX15 (Sorghum bicolor × S. halepense) (rhizomatous perennial). Sugar Graze Ultra is a commercial variety that exceeds 3m in height. PSH20TX01 has larger, non-elongating basal rhizomes, while PSH09TX15 has extensive, elongating rhizomes. The C% values for their forage sorghum varieties are listed in Table 1.

Table 1. Above-ground and below-ground C% of three forage sorghum varieties.

Where, Annual, annual variety (Sugar Graze Ultra); PS, perennial seeded sorghum variety (PSH20TX01); and PR, perennial rhizomatous sorghum variety (PSH09TX15).

The study consisted of 27 plots, with each variety having 9 plots, and each fertilizer application rate had three replicates in each variety (Figure 2). Each experimental plot measured 60 feet × 7 feet, with an inter-row spacing of 3 feet (Figure 3). There were three fertilizer treatments: the recommended rate, the half-recommended rate, and the control (no treatment). Recommended fertilizer rates were: 43 kg/ha of N; 60 kg/ha of P₂O₅; and 115 kg/ha of K₂O. Fertilizer was applied as a single dose two weeks after planting in seasons 1 and 2 for all 27 plots. Fertilizer rates were determined based on laboratory soil analysis conducted prior to planting. Drip irrigation was applied uniformly to all 27 plots throughout the study period for two hours, three days a week.

The experiment was conducted on level terrain with uniform soil compaction. The land had been left fallow for 6 months before starting the experiment following the harvest of cotton (Gossypium hirsutum L.). This fallow period allowed partial regaining of essential nutrients by organic matter decomposition and mineralization. It provided a uniform baseline soil dataset for experimental treatments.

2.2 Soil CO_2, and CH₄ flux measurement

To measure soil carbon dioxide and methane (CO_2, and CH₄) fluxes, soil collars (Supplementary Figure 1b) were installed in each of the 27 plots. The emissions were monitored manually using the LI-7810 Trace Gas Analyzer (LI-COR Bioscience, Lincoln, NE, USA) with a survey chamber (Supplementary Figure 1a). Soil-Flux-Pro software (LI-COR Bioscience) was used to extract soil flux data. The emissions were measured weekly over two growing seasons, between 9:00 am and 11:00 am, as this time is optimal for capturing soil emissions (3). The consistency of the time interval ensures the quality and reliability of the data collected during the research period.

2.3 Biomass yield measurements

At the end of each growing season, above-ground and below-ground biomass (Supplementary Figures 2a, b) within the 2 ft × 2 ft quadrat was clipped at ground level and carefully excavated from each plot. The below-ground biomass was washed to remove all the soil particles. Samples of the aboveground and belowground biomass were oven-dried at 60 °C until a constant weight was reached, and then the dried weight was recorded. Carbon % (C%) of shoots and roots/rhizomes was determined using a Carbon, Hydrogen, Nitrogen, Sulfur (CHNS) analyzer. Thirty milligrams of plant samples were placed in the analyzer, wrapped in aluminum foil, to obtain the reading. Above-ground plant and below-ground plant C stock were calculated (Equations 1, 2). The total plant C stock (Equation 3), total below-ground C stock (Equation 4), and total ecosystem C stock (Equation 5) were calculated (34).

(1) Above-ground plant C stock=Above-ground dry weight×shoot C%

(2) Below-ground plant C stock=Below-ground dry weight×roots and rhizomes C%

(3) Total plant C stock=Above-ground C stock+Below-ground C stock

(4) Total below-ground C stock=soil C stock+Below-ground plant C stock

(5) Total ecosystem C stock=Total below-ground C stock+Above-ground plant C stock

2.4 Soil sampling and laboratory analyses

Soil samples were collected before planting season 1 and after the harvesting seasons of 1 and 2 using two methods. Soil intact core samples were collected from each plot to determine the bulk density of the soil (Supplementary Figures 3a, b). Also, the soil disturbed samples were collected (Supplementary Figures 3c, d) for pH, EC, and total C% analysis. The soil was over-dried at 105 °C until a constant weight was achieved to determine the bulk density (35). Bulk density was calculated from oven-dried soil mass and core volume (Equation 6).

(6) Bulk density=Oven-dried soil weight/soil core volume

Disturbed samples were air-dried, crushed, and sieved using a 2 mm sieve. Processed soil samples were used to measure pH, and EC (Supplementary Figures 4a, b), and to determine total C%. pH and EC were measured using a 1:5 soil water suspension (36).

Soil total carbon percentage (C%) was determined using a CHNS (Carbon, Hydrogen, Nitrogen, and Sulfur) elemental analyzer (Supplementary Figure 4). About 100 milligrams of soil samples were measured, wrapped in aluminum foil, and inserted into the CHNS analyzer, and the respective values were recorded.

2.5 CHNS analysis (elemental analyzer)

A CHNS analyzer (Supplementary Figure 5) was used to determine the total carbon percentage in soil and plants. The samples were subjected to high-temperature (around 950-1000 ^°C) combustion in an oxygen-rich environment. During the combustion, carbon converts into CO₂, hydrogen into H₂O, nitrogen into N₂O_x, and sulfur into SO₂. The gases are separated using gas chromatography and quantified by the thermal conductivity detector.

2.6 Germination test

At the beginning of Season 1 and 2, germination tests were conducted for Sugar Graze Ultra (annual) and PSH20TX01 (seeded perennial) varieties. Two layers of filter paper were placed in a sterilized petri dish, and 5 ml of distilled water was added to moisten the filter paper. 12 seeds were placed inside the petri dish and sealed. Petri dishes were placed in the incubator at 28 ^°C. Sugar Graze Ultra germinated within 4 days, whereas PSH20TX01 took 13 days to germinate. Sugar Graze Ultra showed 100% germination while PSH20TX01 had only 60% germination rate (Supplementary Figures 6a, b).

2.7 Management practices

The field was tilled using a rotary tiller at the beginning of season 1. Sugar Graze Ultra and PSH20TX01 seeds were sown using a roller-type seeder in 1 inch of depth on July 3^rd of 2024 (Supplementary Figure 7a). For PSH09TX15, a shallow furrow was opened using a hoe, and 2–3 inches of rhizome pieces were planted at a depth of ½ to 1 inch. Manual weeding was practiced during the first 3 months of the growing period, and periodic irrigation was supplied using the drip irrigation system (Supplementary Figure 7b). Fertilizer was applied 2 weeks after planting. At maturity, samples of above and below-ground biomass were harvested on November 20^th, 2024. Then, all the annual plants were mowed as low as 2–3 inches, and perennial plants were mowed at 7–8 inches low to facilitate insulation for winter, as well as partition a little more sugar reserves into their rhizomes. Above-ground plants died during winter due to the lower temperature (Supplementary Figure 8a).

The residues in the perennial plots were manually removed to facilitate sprouting in mid-February 2025. 2-4-D herbicides were applied to the annual plots to remove weeds, and prowl was applied to both perennial plots to suppress weed growth at the end of February. During 1^st week of March 2025, the perennial started to regrow but died due to the following -6 ^°C at the end of the week. At the end of March, again (Supplementary Figure 8b), perennials sprouted and continued growing due to the increased temperature.

In the first week of April 2025, the annual plots were tilled using a hand tiller, and seeds were sown using a roller-type seeder. Fertilizer and irrigation application practices continued throughout Season 1 and the crop was harvested after 6-month periods, as Season 01, in September 2025.

2.8 Data analysis

Primarily, three types of samples were collected in this research: soil samples, plant samples, and greenhouse gas flux measurements, all of which aimed to achieve the research objectives. A flow chart is included to outline the steps followed (Figure 4). All data were analyzed using JMP Pro software. To determine differences between varieties, soil and plant data were analyzed using repeated measures analysis, with season treated as the within-subject factor and variety and fertilizer treatments as between-subject factors. The assumption of sphericity was assessed using Mauchly's test, and the Greenhouse–Geisser correction was applied when the sphericity assumption was violated. If the interaction effects were significant, Bonferroni correction was used for the mean separation. Greenhouse gas data were analyzed using repeated-measures ANOVA, as they involved weekly continuous measurements. Tukey HSD test was used for the mean separation.

3 Results

3.1 Basic soil properties

3.1.1 Soil bulk density analysis

The findings indicated that the soil bulk density increased gradually with depth in the three forage sorghum varieties. Also, it varies with soil depth, season, variety, and fertilizer treatment (Table 2). In the surface layer (0–15 cm), the bulk density ranges from 1.29 g/cm³ to 1.77 g/cm³, while the sub-surface soil layer (15–45 cm) had 1.49 g/cm³ to 1.83 g/cm³ (Table 2). Before planting in season 1, bulk density in the 0–15 cm layer ranged from 1.29 to 1.77 g cm⁻³ across the variety × fertilizer combinations. After season 1, values became relatively similar among treatments, ranging from 1.47 to 1.54 g cm⁻³. By the end of season 2, however, bulk density tended to be greater under the annual variety (1.48–1.73 g cm⁻³) than under PS (1.40–1.42 g cm⁻³) and PR (1.38–1.45 g cm⁻³) (Table 2).

Table 2. Soil properties in different forage sorghum varieties over seasons in 0–15 cm depth.

Where, A, annual variety (Sugar Graze Ultra); PS, perennial seeded sorghum variety (PSH20TX01); PR, perennial rhizomatous sorghum variety (PSH09TX15); BP, before planting; AH, after harvesting; R, recommended fertilizer rate; HR, half recommended fertilizer rate; and C, control (no fertilizer).

At 0–15 cm, bulk density was significantly affected by season (p = 0.044) and fertilizer (p = 0.025). Season × variety (p = 0.002), season × fertilizer (p = 0.010), variety × fertilizer (p = 0.042), and season × variety × fertilizer (p = 0.006) interactions were also significant, whereas the overall effect of variety was not significant (p = 0.195) (Table 3). Thus, changes in surface-soil bulk density over time depended on variety and fertilizer treatment.

Table 3. Effects of season, variety, fertilizer, and their interactions on soil properties in two different depths.

Soils in the perennial varieties tend to decrease the bulk density, while the soils in the annual variety have increased the bulk density in 15–45 cm layer over time. There was a significant difference in the soil bulk density after season 2 in PR variety in 15–45 cm (Table 4). Before planting, PR had high bulk density (1.74–1.82 g cm⁻³), but values decreased to 1.49–1.68 g cm⁻³ after season 1 and remained between 1.54 and 1.55 g cm⁻³ after season 2. However, bulk density in annual variety was higher after season 2 (Table 4). The significant season × variety interaction at both 0–15 cm (p = 0.002) and 15–45 cm (p < 0.001) demonstrates that changes in bulk density over time differed among sorghum varieties. Although the annual variety had a lower bulk density than the perennial sorghum varieties before planting in season 1, there was no significant difference. However, after 6 months of planting perennial varieties in season 1, soil bulk density is reduced. Surprisingly, the annual variety showed the lowest bulk density after season 1, and the bulk density gradually increased in PS variety and PR variety in the same season. The PR variety showed a gradual decrease in soil bulk density in the 15–45 cm soil layer. At the end of season 2, PR variety had a significant decrease in bulk density compared to the annual variety and PS variety. The study suggests that perennial sorghum varieties can improve soil physical conditions and overall soil health by reducing compaction compared to annual systems.

Table 4. Soil properties in different forage sorghum varieties over seasons in 15–45 cm depth.

Where, A, annual variety (Sugar Graze Ultra); PS, perennial seeded sorghum variety (PSH20TX01); PR, perennial rhizomatous sorghum variety (PSH09TX15); BP, before planting; AH, after harvesting; R, recommended fertilizer rate; HR, half recommended fertilizer rate; and C, control (no fertilizer).

3.1.2 Total carbon

The surface soil layer showed a higher total C percentage than the subsurface soil layer. In surface soil, total C ranged from 0.19 to 0.49% before planting, 0.27 to 0.42% after season 1, and 0.32 to 0.49% after season 2. The overall effect of season was not significant (p = 0.398), whereas total C differed significantly among varieties (p = 0.033). A significant season × variety interaction was also observed (p = 0.006), indicating that differences among varieties changed over time. Fertilizer and all other interactions were not significant (p > 0.05) (Table 3). The perennial response was particularly evident at 15–45 cm. Total C under PS reached 0.26–0.28% by the end of season 2, while PR reached 0.21–0.26%, compared with 0.18–0.20% under the annual variety. Thus, both perennial varieties generally maintained greater subsurface total C than the annual variety at the end of season 2. Total C was significantly affected by season (p = 0.003) and variety (p < 0.001), with significant season × variety (p = 0.040) and variety × fertilizer (p = 0.011) interactions.

The perennial-seeded sorghum variety showed the highest C content among the other varieties before the planting season, and this value was significantly higher than that of the PR sorghum variety. The annual sorghum variety did not show any significant difference between the perennial-seeded variety and the PR sorghum variety. At the end of season 1, the soil total C of the PS variety was significantly higher than that of the other two varieties at the end of season 2, the soils in the PR sorghum variety had increased the total C, and the soils in the annual variety had a lower total C than in season 1. Both perennial sorghum varieties showed a significantly higher total C than the annual variety in the 15–45 cm soil layer at the end of season 2.

3.1.3 Total soil carbon stock

The results indicated that a 15–45 cm soil layer has a higher soil carbon stock than the surface layer. At 0–15 cm, PR initially had relatively low C stocks, ranging from 0.44 to 0.68 Mg C/ha before planting (Table 4). After season 1, values increased to 0.61–0.71 Mg C/ha, followed by a further increase to 0.83–1.10 Mg C/ha by the end of season 2. Thus, PR showed a clear temporal increase in surface-soil C stock. Variety significantly affected C stock (p = 0.004), and significant season × variety (p = 0.004) and season × fertilizer (p = 0.008) interactions were observed (Table 3).

The PR sorghum variety showed a significantly lower C stock at the end of season 1 than the annual and the PS variety. During the growing period of season 2, the PR sorghum variety increased soil C stock. In contrast, the PS variety decreased the soil C stock in the 0–15 cm soil layer. However, any significant difference in C stock at the end of season 2 among sorghum varieties was not observed.

At depths of 15–45 cm, the PS exhibited the highest C stock in all time periods, reaching 1.32-1.44 Mg C/ha by the end of season 2. C stock increased gradually in the PR sorghum variety over time recording 1.22 Mg C/ha. The PS showed a significantly higher C stock than the PR variety before planting season 1. The annual variety did not show any significant difference from other varieties. At the end of season 1, PS variety showed a significantly higher C stock than the Annual variety. Still, not with the PR variety. Nevertheless, at the end of season 2, PS variety showed a significantly higher C stock than the Annual and PR varieties.

Variety had a highly significant effect on subsurface C stock (p < 0.001), and the variety × fertilizer interaction was significant (p = 0.020). Collectively, the C-stock results indicate distinct responses of the two perennial systems as PS was characterized by relatively high subsurface C stocks, whereas PR showed a notable increase in surface-soil C stock over time (Table 5).

Table 5. Plant carbon storage, total below-ground carbon storage, and total ecosystem carbon storage of three forage sorghum varieties.

Where, A, annual variety (Sugar Graze Ultra); PS, perennial seeded sorghum variety (PSH20TX01); and PR, perennial rhizomatous sorghum variety (PSH09TX15).

3.1.4 Soil pH

Soil pH varied considerably over time, with different temporal responses among the sorghum varieties. At 0–15 cm, PR had pH values ranging from 6.28 to 7.13 before planting and 5.90 to 6.40 after season 1. By the end of season 2, however, soil pH under PR increased to 7.10–7.45. PS also showed an increase from 6.52 to 6.63 at the end of season 2. Season (p < 0.001) and variety (p = 0.005) significantly affected surface-soil pH, and significant season × variety (p < 0.001), variety × fertilizer (p = 0.006), and season × variety × fertilizer (p = 0.018) interactions were detected (Table 5).

At the soil depth of 15–45 cm, both the annual and PR varieties exhibited the same pattern in pH fluctuations. Before the start of season 1, higher pH values were observed, and after the end of season 1, the pH value decreased (Table 4). Before the planting season, the soils in the annual variety had significantly higher pH values than those in the PS and PR perennial sorghum varieties. However, none of the three varieties showed any significant differences among themselves after season 1. At the end of season 2, both perennial varieties showed the same 6.5 pH values in the soil, and soils in the annual variety had slightly higher pH values; however, they did not show any significant difference after 2 years of planting sorghum varieties.

3.1.5 Soil electrical conductivity

Soil EC showed substantial variation among seasons and treatment combinations. At the beginning of season 1, both surface and subsurface values were significantly lower than in the cultivated soil. Also, on both soil surfaces, soil EC increased by the end of season 2 compared to before planting season 1 across all sorghum varieties. In the 0–15 cm layer, EC ranged from approximately 70.28 to 156.23 mS/cm before planting, 46.83 to 125.90 mS/cm after season 1, and 90.68 to 171.78 mS/cm after season 2 (Table 4). Season significantly affected surface-soil EC (p < 0.001), and EC also differed significantly among varieties (p = 0.024). A significant season × variety interaction was detected (p = 0.012), indicating that temporal changes in EC differed among varieties. Fertilizer (p = 0.254), season × fertilizer (p = 0.506), variety × fertilizer (p = 0.195), and the three-way interaction (p = 0.526) were not significant (Table 3).

After season 1, the annual variety showed a significantly lower EC value than the PS and PR varieties in the 0–15 cm soil layer. After season 2, the PS variety showed a considerably lower EC value, but it was not significantly different from the other Annual or PR varieties in the surface layer. In the 15–45 cm layer, EC ranged from 25.83 to 94.86 mS/cm before planting, increased substantially in several treatments after season 1, and ranged from 67.23 to 136.95 after season 2. Season and variety both had significant effects on EC (p < 0.001). The season × variety interaction (p = 0.015) and variety × fertilizer interaction (p = 0.012) was also significant. Importantly, a significant season × variety × fertilizer interaction was detected (p = 0.024), indicating that differences in EC among varieties over time depended on fertilizer treatment. The main fertilizer effect (p = 0.815) and season × fertilizer interaction (p = 0.098) were not significant.

3.2 Above-ground and below-ground biomass variation over sorghum varieties

Figure 5a illustrates the dried above-ground and below-ground biomass of three forage sorghum varieties in season 1 and season 2, while Figure 5b shows the average biomass yield considering total growing periods. Biomass yield varied among the three forage sorghum varieties. Annual variety has higher above-ground biomass than PS and PR varieties. The annual variety yielded significantly higher than the PR variety in both seasons, whereas it yielded significantly more with the PS variety only in season 2 (Figure 5a). There were no significant differences in above-ground biomass yield between PS and PR in season 1, but it was positive during season 2. The lowest above-ground biomass was recorded in the PR variety in both seasons (Figure 5a); however, it showed the opposite pattern for below-ground biomass, with the highest root and rhizome yields in seasons 1 and 2 compared to other varieties (Figure 5a). Both annual and PS varieties did not show any significant difference in below-ground biomass (Figure 5a). Overall, the annual variety has 26.4 Mg/ha of above-ground yield, while PR variety has 24.9 Mg/ha of below-ground biomass (Figure 5b). PS variety showed the middle values for both above and below-ground biomass among annual and PR varieties (Figure 5).

3.3 Total carbon storage and total ecosystem carbon storage

Carbon stock values differed significantly among the three forage sorghum varieties. The highest and the lowest above-ground shoot C storage was observed in the annual and PR varieties, respectively (Table 5). PR variety showed the highest below-ground C stock stored in the roots and rhizomes, and both PS and annual varieties had similar values. An increasing pattern of total plant C stock was observed in PR, PS, and annual forage sorghum varieties, respectively. PR planting system accumulated the highest below-ground C stock, reflecting its extensive rhizomes and root system. Annual and PS varieties showed similar values for total below-ground C storage. Total soil C stock was greater in the PR variety, followed by annual and PS forage sorghum varieties (Table 5). However, the annual variety showed higher above-ground C storage, while the PR variety showed the highest below-ground C storage due to its extensive rhizome stems.

3.3.1 CO₂ emission/sequestration rate on sorghum cultivars

Figure 6 illustrates the CO₂ emissions rate for both seasons 1 and 2, considering the variety and fertilizer treatments. The results revealed no significant differences among varieties, fertilizers, or the combination of variety and fertilizer treatments in season 1. However, the PR variety had the lowest CO₂ emissions among the other treatments, although this difference was not significant. Plots with recommended fertilizer in the annual variety had two outliers (Figure 6a), with the maximum value of 69.22 g CO₂/m²/day and the minimum value as 0.69 g CO₂/m²/day (Table 6). The half-recommended fertilizer rate in the annual variety showed a higher mean value throughout season 1 than the CO₂ emission in the recommended fertilizer rate. However, PS and PR varieties showed decreases in CO₂ emissions across the recommended, half-recommended, and control treatments during season 1, respectively. All the CO₂ emissions in control plots showed the lowest value compared to fertilizer treatments in all forage sorghum varieties in season 1 (Figure 6a; Table 6).

Table 6. Descriptive analysis of the soil CO2 emissions during season 1 and season 2.

Where, A, annual variety (Sugar Graze Ultra); PS, perennial seeded sorghum variety (PSH20TX01); and PR, perennial rhizomatous sorghum variety (PSH09TX15).

The annual sorghum variety showed higher CO₂ emission during season 2 compared to PS and PR varieties in season 2 (Table 6). The recommended fertilizer rates resulted in higher CO₂ emissions across all sorghum varieties (Figure 6b), whereas the control plots tended to release lower CO₂ emissions from the soil. However, there was no significant difference among varieties, fertilizer treatments, or the interaction effect between variety and fertilizer treatments during season 2. Both PS and PR varieties exhibited lower CO₂ emissions during season 2 compared to season 1 (Table 6).

3.3.2 CH₄ emission

Figure 7 illustrates the CH₄ emission during season 1 and season 2 in the three forage sorghum varieties. All the mean CH₄ emissions in all three sorghum varieties were negative over fertilizer treatments in both season 1 and season 2 (Table 7). However, there were no significant differences among varieties, fertilizers, or the interaction between variety and fertilizer in both seasons. The control treatment of the annual sorghum variety had a higher negative CH₄ emission than other fertilizer treatments in the same variety in season 1 (Table 7). Perennial recommended and half-recommended fertilizer rates showed a higher number of CH₄ emission outliers than annual variety or control treatments of the perennial varieties during season 1 (Figure 7a). More outliers were observed during season 1 than season 2 (Figure 7). Both PS and PR varieties exhibited higher CH₄ sinks than the recommended and control treatments in Season 1 (Table 7). The highest CH₄ emission was shown in the PR control plots, while a higher CH₄ sink was observed in the PS half-recommended treated plots (Table 7). The lowest average CH₄ sink was observed in the recommended fertilizer treatment in the A variety, and the highest CH₄ sink was observed in the PS, half-recommended treated plots.

Table 7. Descriptive analysis for the soil CH4 emissions of seasons 1 and 2.

Where, A, annual variety (Sugar Graze Ultra); PS, perennial seeded sorghum variety (PSH20TX01); and PR, perennial rhizomatous sorghum variety (PSH09TX15).

4 Discussion

4.1 Basic soil properties

Perennial rhizomatous forage sorghum varieties have significantly decreased the soil bulk density at the end of season 2 in both surface and sub-surface soil layers. This is explained by the fact that the developed extensive rhizomes and root systems have improved the soil structure and soil aggregation (37). Additionally, over time, the rhizomes contribute root residues to the soil, thereby enhancing its organic matter content (38, 39). It enhances soil moisture retention and provides favorable conditions for microbial activity. Proper aeration and nutrient availability may induce microbial decomposition and reduce soil bulk density, even in deeper soil layers. Plants with an extensive rhizome system have the potential to reduce bulk density more than other rhizomatous varieties, even in the deeper soil layers.

Differences in soil management between annual and perennial systems may have contributed to these responses as well. At the beginning of every season, plots in the annual sorghum variety were tilled and prepared for seeding. Since the PS and PR perennial sorghum varieties were sprouted after the winter season, the respective plots were not plowed. Therefore, the soil disturbance was minimal in the plots of perennial sorghum varieties compared to the annual sorghum variety. The bulk density has gradually increased in the annual sorghum variety plots in both the surface and subsurface soil layers over time, indicating that continuous tillage practices have a negative impact on the soil bulk density.

The fertilizer-dependent response in surface bulk density further indicates that changes in soil physical properties were not controlled solely by crop type. Fertilizer can indirectly affect soil structure by altering plant productivity, root growth, residue production, and microbial activity. Greater plant growth may increase below-ground C inputs and root exploration; however, the magnitude of this response depends on the growth strategy of the cultivar.

Soil total carbon content is a major indicator of soil health and fertility (40). Soil carbon, the largest carbon reservoir, is stored in soil organic matter, which supplies energy and nutrients to soil microorganisms and plays a crucial role in ecosystem functions, including food production and climate regulation (22). Since the total C% increases over time, rhizomatous varieties may release more residues to the soil than the fibrous root system of the annual sorghum variety. Although there was no significant difference among the three varieties in total C%, the rhizomatous varieties in the surface soil layer at the end of season 2, PS and PR varieties, could increase total C% in the soil. It indicates that the gradual improvement of soil fertility is achieved through higher carbon inputs from extensive root and rhizome systems below ground. However, C% in the A sorghum variety slightly decreases over time, indicating that the root system of annual sorghum did not contribute sufficient below-ground carbon input to maintain or increase the C level in the soil.

The higher C% in the surface soil layer than the sub-surface soil layer indicates that carbon input from sorghum leaf litter, residues, and fibrous roots is concentrated near the surface (41, 42). Microbial activity is higher in the surface soil layer due to optimal oxygen, soil moisture, and soil temperature, as well as abundant organic carbon, resulting in rapid microbial decomposition (43, 44).

The interaction between variety and fertilizer in subsurface soil further suggests that nutrient availability can modify these below-ground C inputs. Fertilization can increase plant productivity and C assimilation, but additional above-ground productivity does not necessarily translate directly into greater soil C storage. The proportion of assimilated C allocated to roots and rhizomes differs among plant growth strategies. A perennial rhizomatous variety may allocate a substantial proportion of assimilated C below ground to maintain persistent structures, whereas an annual variety may allocate proportionally more resources toward rapid shoot development and reproduction. Therefore, fertilizer effects on soil C should be considered together with cultivar-specific C allocation rather than as an independent driver.

Soil C stock is proportional to the soil’s total C percentage, as it determines the amount of carbon contained in the soil, considering its bulk density and soil depth. At the beginning of season 1, soil samples did not show homogeneous C stock in the experimental plots in both layers, which could be due to differences in bulk density and C% across plots. Surface soil in the PR variety increased the C stock over time, indicating that the extensive rhizomes have a greater potential to enrich soil carbon content. In the winter (December 2024-March 2025) season (after the first season), the rhizomes remain in the soil, and by the end of April they begin to sprout and emerge as new plants. During season 2 (April 2025-September 2025), the rhizomes continued to develop and expand beneath the soil, forming larger, denser root and rhizome biomass. Therefore, below-ground biomass provided more root residues and contributed to increased soil C input than in season 1. Even though sorghum A variety had lower C% over time, it showed considerably higher C stock in the surface layer, mainly due to its higher bulk density. However, despite the lower bulk density in the PR soil, it revealed a larger C stock due to a significantly higher C percentage over time.

The influence of fertilizer on C stock also appears to depend on variety and soil depth. Nutrient supply can stimulate biomass production and therefore potentially increase C inputs, but whether this translates into greater soil C storage depends on where the additional biomass is allocated and how rapidly residues are decomposed (45).

The subsurface soil layer had a higher carbon stock than the surface layer, primarily because the research considered a deeper soil depth. The surface is only 15 cm thick, and the subsurface is 30 cm thick. Significantly higher C stock was observed in the PS sorghum variety, in the subsurface, mainly due to the increased C% over time. At the beginning of Season 1, there was no significant difference in pH values for all three forage varieties, indicating uniform soil conditions. However, after season 1, the PR variety showed a significantly lower pH, which could be due to the release of organic acids from young growing roots, thereby lowering pH in the rhizosphere (46). High microbial activity around the young roots and rhizomes can produce CO₂ and form carbonic acid in the soil, resulting in a slightly lower pH. However, at the end of season 2, rhizome maturity may reduce acidification and increase the soil pH in PR-grown soil. Improved nutrient cycling and enhanced soil buffering capacity can also be attributed to the higher pH values at the end of season 2 in the surface soil layer.

Although there was no significant difference in EC among varieties at the end of Season 2, EC had increased compared to the pre-planting level in Season 1. It can be attributed to the application of fertilizer, microbial decomposition of organic matter, and nutrient cycling, among other factors (47, 48). The accumulation of soluble ions from these activities could increase the overall EC in the soil across all experimental plots.

4.2 Above and below-ground biomass and C storage of sorghum varieties

Above-ground and below-ground biomass trends reveal distinct patterns in the annual and perennial forage sorghum varieties across both seasons. The commercial annual variety produced more above-ground forage in both seasons, indicating a higher growth rate, which exceeded 3 m in height. Forage sorghum is frequently used as a feed source for livestock and as a bioenergy crop in Texas. The annual sorghum variety produced 26.4 Mg/ha of above-ground biomass, making it more suitable for producers seeking maximum forage biomass harvest in a single season.

In contrast, PS and PR varieties had lower above-ground biomass in both seasons, reaching a maximum height of approximately 1.3 m. Zheng et al. (14) observed that 18.58 Mg/ha for the aboveground biomass for the PS variety, which aligned closely with our findings (17.03 Mg/ha). However, the PR variety showed significantly higher below-ground biomass than the annual and PS varieties due to their extensive rhizome growth. Rhizomes have the capacity to sequestrate CO₂ directly from the atmosphere (14), and the total C% of PS and PR varieties is 42% and 41%, respectively. At the end of season 2, PR showed twice the below-ground biomass as above-ground biomass, while annual and PS varieties showed ratios of 0.6 and 0.9, respectively. This clear difference indicates the stronger carbon storage mechanism in the PR variety than in others. The average below-ground biomass production in the PR variety is recorded as 24.9 Mg/ha, which is similar to the above-ground production of the forage annual variety. Below-ground biomass is more important for carbon sequestration than above-ground biomass, since roots and rhizomes remain in the soil longer and provide organic matter.

The C% of above-ground and below-ground biomass of three forage sorghum varieties did not show larger variations among each other; above-ground C% was approximately 42% and 37-41% in below-ground biomass. Therefore, the carbon stock in the three forage sorghum varieties closely reflected the variation observed in the dry biomass weights. This showed that the carbon storage pattern is directly proportional to the biomass allocation. 19.08 Mg C/ha of the highest ecosystem C stock was observed in the annual sorghum variety, and 60% of it was contributed from the above-ground biomass, and only 35.9% came from the below-ground biomass. Although PR variety has 16.55 Mg C/ha of ecosystem C stock, below-ground C storage contributes 67%, which is more effective for long-term C storage in the environment. Therefore, despite higher total biomass yields, PR varieties provide more stable and long-lasting carbon to the soil.

Global concerns are increasing over the greenhouse effect, which is induced by greenhouse gases. According to the World Meteorological Organization (49), the atmospheric concentration of carbon dioxide, a principal greenhouse gas, attained 420.0 ± 0.1 parts per million (ppm) in 2023. This is a 6% increase from 2013 and is progressing at an annual rate of 2.16 ppm. The approach of carbon sequestration is one of the best solutions to alleviate this situation (14). It involves CO₂ absorption by plants and its long-term storage in soil and below-ground biomass. Therefore, one of the most important sources of organic carbon in soil is root carbon, which accounts for up to 90% of all carbon inputs to arable soil (50). Root-derived C remains longer in the soil than C derived from above-ground crop residue due to its stable chemical composition (51).

4.3 Soil GHG emissions

Although there was no significant difference among varieties or fertilizer treatments in Season 1, distinct trends were identified. The highest CO₂ emissions from the annual sorghum variety in both seasons suggest that an active root system and rapid above-ground biomass turnover have stimulated microbial decomposition. In contrast, both PS and PR varieties exhibit lower CO₂ emissions in season 2 than in season 1, likely due to the formation of stable, mature perennial rhizome structures over time. Among them, PR varieties allocate more C to store in the roots and rhizomes rather than above-ground growth. This observation demonstrated that perennial varieties invest more in long-term below-ground carbon storage and emit less CO₂ through microbial decomposition and root respiration.

Across both seasons, recommended fertilizer-treated plots had slightly higher CO₂ emissions than other fertilized or control plots, likely because fertilizers stimulate microbial activity. Since there were more soluble nutrients near the rhizosphere, root activity may have been increased, releasing CO₂ through respiration, which accounts for the higher CO₂ concentration in the recommended fertilized plots. The control plots consistently showed the lowest CO₂ emissions in both seasons, indicating that fertilizer-driven microbial processes are a key source of soil CO₂ fluxes.

All mean CH₄ fluxes were negative over the sorghum variety and fertilizer in both seasons, indicating that the soil acts as a sink rather than a source for CH₄ emission. Methanotrophic bacteria may account for a negative CH₄ value, as they oxidize CH₄ under aerobic conditions. The soil type at the experimental site is fine sandy loam, a well-drained and aerated soil. These conditions favor methanotrophic activity and result in consistent CH₄ uptake throughout the season. Fertilization has reduced CH₄ uptake, suggesting that microbial activity has competed with methanotrophs for ammonium and altered soil pH. This explained the slightly lower CH₄ in fertilized plots in the annual sorghum variety and the higher negative values in the control plots during season 1.

PS and PR varieties showed more outliers during Season 1, representing short-term changes in soil moisture and microbial activity. Hurricane Beryl affected our experimental plots in June and July 2024, and the soil remained waterlogged for approximately a week after the hurricane. This could be the positive value for CH₄ emissions during season 1. Season 2 exhibited fewer outliers and a more stable CH₄ sink across all varieties, indicating more mature root and rhizome systems, as well as an improved soil structure, which maintained steady CH₄ uptake.

5 Conclusions

The experiment on the carbon sequestration potential of their forage sorghum varieties in Southern Texas examines soil properties, plant biomass production, and the dynamics of soil GHG emissions. The PR variety significantly improved soil properties over time, especially in the 0–15 cm soil layer. Its extensive root-rhizome structure has contributed to higher below-ground carbon allocation, suggesting the potential for higher below-ground carbon inputs. Among perennial sorghum varieties, the PR variety exhibited the highest below-ground biomass over two seasons, indicating greater below-ground C allocation. On the other hand, the annual sorghum variety indicated its importance as a productive forage and as a bio-energy crop by producing the highest above-ground biomass in both seasons. However, it reduced the carbon input to the soil more than perennial forage sorghum varieties did. Even though there was no significant difference in GHG emissions over sorghum varieties, soils in the annual variety showed a higher CO₂ emission, probably due to the rapid above-ground biomass turnover and active microbial decomposition in the rhizosphere. PR variety had lower CO₂ emissions across both cultivated seasons, indicating its tendency to conserve carbon in the below-ground environment and maintain a stable rhizome-root structure. Soils in all experiments act as CH₄ sinks probably due to the activity of methanotrophic bacteria. The PR variety has the potential to enhance soil health, improve below-ground carbon storage, and promote long-term carbon sequestration. A mixed cropping system of annual, PS, and PR varieties can be recommended to achieve both high above-ground biomass production and long-term below-ground carbon storage.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fsoil.2026.1858927/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Martin A.N. Anikwe, Enugu State University of Science and Technology, Nigeria

Reviewed by: Senthilvalavan P., Annamalai University, India

Abdurrahman Şahin, Artvin Coruh University, Türkiye