Abstract
C:N ratio of crop residues is the key factor dictating the decomposition of crop residues and soil respiration. Crop residue incorporation is one of the best residue management options, which not only enhances soil health, but also reduces environmental pollution. To investigate the effects of various crop residues viz., paddy, sunflower, cotton, and red gram on soil respiration, an incubation study was conducted for 120 days at soil science laboratory, School of Agriculture, SR University, Warangal. Soil respiration was measured by alkali trap method, at different days after incubation. The results indicated that incorporating crop residues significantly increased soil respiration rates, with the highest CO₂ emissions observed in treatments with both residues and nitrogen. Among the treatments, soil with paddy residue and nitrogen showed the highest respiration rate, demonstrating the synergistic effect of residue incorporation and nitrogen addition in enhancing organic matter decomposition. The lowest soil respiration was recorded in soil alone (control) treatment throughout the incubation period. The study concludes that the incorporation of crop residues, especially when combined with nitrogen, significantly enhances soil respiration. The research provides critical insights for developing strategies that promote sustainable agriculture, emphasizing the need for residue retention and appropriate nutrient management to maximize soil productivity while minimizing environmental impacts.
Introduction
Crop residue incorporation in soils is a key practice in agricultural management. Residues provide organic C and N inputs to soils for maintaining or improving the soil stocks of these elements and, ultimately soil health and crop productivity (Janz et al., 2022). Crop residue decomposition may result in the formation of the hotspots of microbial N turnover processes (Janz et al., 2022). Rapid decomposition of soil organic matter, and consequent N loss, is considered as a major limitation for maintaining soil fertility and a threat to the environment. Cox et al. (2004) reported that 80% of the crop residues burning took place during the post-harvest period. There as on behind this is attributed to the crop patterns used to ensure a higher economic return which have limited time between two consecutive crop cultivations. Some farmers even resort to a cycle of three crops a year with a short gap between harvesting and sowing of the next crop. Burning of residues emits a significant amount of Green House Gases (GHGs). Heat from burning of residues elevates soil temperature causing death of beneficial soil organisms also reduces level of C and potentially mineralizable N in the upper (0-15 cm) soil layer. However frequent residue burning leads to complete loss of soil microbial population, though the effect is temporary, as the microbes regenerate. Moreover, residue decomposition in soils potentially releases ammonia gas which is a precursor of secondary aerosol, a harmful pollutant for the environment and human health (Ruijter and Huijsmans, 2012). The major environmental problem today is the global warming due to accumulation of gases like CO2, CH4, N2O and Chlorofluoro Carbons along with water vapour in the atmosphere causing greenhouse effect through trapping outgoing thermal radiation and depletion of ozone layer in stratosphere, affecting several aspects of humanity on planet earth, due to increased temperature (0.3-0.6 °C) at the earth’s surface.
Crop residues with a low C/N ratio (≤15) lead to significant N2O emissions meanwhile, the incorporation of residues with a high C/N ratio (>40) produced insignificant changes or even reduced soil N2O emissions (Akiyama et al., 2020). Specifically, the composition of organic N and C compounds in the soluble, cellulose, and lignin-like fractions determine the N mineralization potential of residues, and this affects the fate of the incorporated residue N, as it may be immobilized in soil fractions or lost to the environment through the gaseous and hydrological pathways (Lashermes et al., 2010). Synchronizing soil N availability with plant requirements can improve the soil–plant system N use efficiency and reduce N losses through leaching below the crop rooting depth and/or from gaseous emissions (Vanlauwe et al., 2001). Previous studies confirm that C and N dynamics are mainly controlled by the C/N ratio (Nicolardot et al., 2001), chemical components (e.g., lignin, total phenol, soluble sugar and nutrients) (Bonanomi et al., 2010) and heterogeneity of plant residues applied to soil. Paul et al. (2014) reported that plant residues with different physical or chemical properties cause differences in soil microorganism activity levels, metabolic pathways and even community structures, thus contributing to different soil C and N dynamics. Addition of organic materials to agricultural soil (with or without chemical fertilizers) is important for replenishing the annual C losses and for improving both the biological and chemical properties of the soils (Goyal et al. 1999). Soil microbial communities are the primary regulators of soil carbon and nutrient cycling processes. Differences in microbial community composition have the potential to affect the fate of carbon and nutrients during decomposition and may therefore influence the retention of C and provisioning of crop nutrients in agro ecosystems.
When these residues are incorporated into soil, they provide a source of carbon and nutrients for soil microbes. This can stimulate microbial activity, leading to increased decomposition of organic matter and nutrient cycling in the soil. Different microbes specialize in breaking down different types of organic residues, so the composition of microbial communities can shift based on the type of organic matter added. Many studies have shown that residues with low C/N ratio are decomposed rapidly and lead to net N mineralization as they satisfy the N demands of microbes (Hadas et al., 2004).
Soil respiration is considered a good estimator of overall biological activity and has been proposed as a descriptor of soil quality (Doran and Parkin, 1994). The soil microorganisms break down complex molecules such as cellulose, hemi-cellulose, proteins and lignin into low-molecular-weight substances, which are then oxidized to CO2 to produce energy or used to provide C for cell growth. The rate of decomposition is determined by the quantity and quality of organic substrates, the efficiency and population dynamics of various decomposer groups, and the soil’s physico-chemical environment including moisture, temperature, oxygen, acidity, and redox potential (Kilham, 1994; Coleman and Crossley, 1996). Soil respiration measurements are increasingly used in studies of soil C cycling to detect early changes in decomposition rate of soil organic matter in response to various soil or crop management practices (Jensen et al., 1996; Rochette and Angers, 1999). The composition of easily and slowly mineralizable organic matter significantly differs among various residues and fresh green materials are generally decomposed faster than straw (Schmatz et al., 2017). Due to a decline in soil fertility and adverse physical conditions, application of organic material and N fertilizer is important for sustainable soil fertility and environment.
Material and Methods
The investigation was carried out in Soil Science Laboratory, School of Agriculture, SRU, Warangal, Telangana, which is located in Warangal district of Telangana state at 79°55' °E longitude and 18°029' N latitude. According to Troll’s climatic classification, it falls under Semi Arid Tropical region (SAT). The experimental site is in Southern Telangana Agro-Climatic Zone. The soil sample for the study was collected from D block (field number 21D), at the SR University college farm. The surface soil (0–15 cm) is collected, air–dried and sieved for chemical analysis. The soil is sandy clay loam, with a pH of 7.48, EC of 0.4 dSm-1 and the organic carbon content was 0.78%.
Four predominant crop residues (Rice, Cotton, Sunflower and Red gram) available on-farm were selected (Table 1). Samples of crop residues were oven dried at 65°C in a hot air oven, and crushed with a willey mill before sieving through a 2 mm sieve. The treatment details were provided in the Table 2.
Experiment details
Table 1. Amount of residue added per 100 g soil
Table 2. Treatment details
T1 Control (Soil with no N and no residue)
T2 Soil + N
T3 Soil + Paddy residue
T4 Soil + Sunflower residue
T5 Soil + Cotton residue
T6 Soil + Red gram residue
T7 Soil + Paddy residue + N
T8 Soil + Sunflower residue + N
T9 Soil + Cotton residue + N
T10 Soil + Red gram residue + N
*Nitrogen (N) @ 80 kg ha-1 will be applied as urea, to T2, T7, T8, T9 and T10; 80 kg ha-1 is chosen because it is the amount employed for rabi maize at the time of sowing as 1/3rd of RDN)
*Recommended dose of Phosphorus (P) is applied uniformly to all treatments.
Pre-incubation
In 500 ml beaker, 100 g weighed soil was taken and the soil was kept at field capacity by adding 13 ml of distilled water and kept in dark for 10 days for pre-incubation. Pre-incubation was done prior to the start of incubation experiment to initiate microbial activity in the soil.
Laboratory incubation
Fresh soil, equivalent to 100 grams of dry soil samples, was pre-incubated, weighed, and transferred into polythene bags. After pre-incubation, residues were thoroughly mixed with the soil as per the treatment requirements and incubated for 120 days. Distilled water was frequently added to maintain a 60% water-filled pore space. The entire experiment followed a completely randomized design and was conducted under controlled room temperature conditions. Soil moisture was monitored and adjusted every five days by weighing the Zip lock bags and adding the necessary amount of distilled water.
The method measures the respiration activity of soil microorganisms as CO2 production per time unit. When soil samples are incubated in a gas tight closed vessel at 30 ºC for 24 hours, the CO2 produced is absorbed in sodium hydroxide (NaOH). Thus after adding barium chloride the sodium carbonate is precipitated as the hardly solute barium carbonate and the unused sodium hydroxide is titrated by hydrochloride acid (Ferreira et al., 2018).
C or CO2 = (B-V) ×N×E
Fig. 1: Set-up for measurement of soil respiration by Alkali Trap Method
Results and Discussion
The result of the soil respiration rate of day two is shown in Fig. 1a. Soil respiration on day 2 was influenced by different residues with or without nitrogen addition. There was significant difference (P < 0.05) among the treatments. The Soil + Red gram residue (T6) without nitrogen have shown the highest respiration rate 12.27 mg g-1, which was on par with soil + cotton residue + N (T9) 11.83 mg g-1, followed by soil + paddy residue + N (T7), soil + sunflower residue + N (T8) and soil + N (T2), which were statistically similar. The result indicated that, among all the treatments, those with addition of nitrogen have recorded higher respiration rate, compared to rest of the treatments. This might be due to the addition of N fertilizer accelerated C mineralization during early stages reported by Wang et al. (2004). The higher soil respiration in soil + red gram residue might be due to red gram residues have a relatively low C: N ratio and microbial community might efficiently utilize the nitrogen available from the red gram residues. Also according to Hadas et al. (2004), the C mineralization of residues in the early stage of decomposition is influenced by the amount of soluble C present in the added plant materials. Use of inorganic nitrogen application when organic residue of high C/N ratio was used might have either minimized the N immobilization or speed up the microbial decay (Jenkinson et al., 1985). In contrast, treatments without nitrogen soil alone
Fig. 1: Impact of different residues decomposition with or without in organic N on Soil Respiration at various days of incubation (2, 4, 6, 8 DAI).
(T1), soil + paddy residue (T3) and Soil + cotton (T5) showed statistically similar but lower respiration rates, likely due to limited nutrient availability. The 34.77% overall increase observed with nitrogen, compared to treatments without nitrogen on day two, which highlights significant role N in enhancing nutrient availability. Additionally, nitrogen contributes to a decrease in the carbon-to-nitrogen (C) ratio of the residue. The result is in line with findings of Jenkinson et al. (1985) reported that Use of inorganic nitrogen application when organic residue of high C/N ratio was used might have either minimized the N immobilization or speed up the microbial decay (Jenkinson et al., 1985).
The soil respiration rates on day 4, is shown in Figure 1b, there was significant differences among treatments, particularly highlighting the influence of residues and nitrogen addition. Soil + Sunflower residue + N (T8) shown significantly higher respiration rate compared to remaining treatments, followed by Soil + Red gram residue + N (T10) 25.63 mg g-1, Soil + Cotton residue + N (T9) 25.50 mg g-1 and soil + N (T2) 24.20 mg g-1, which were statistically similar. The higher soil respiration rate in the Soil + Sunflower residue + N (T8) treatment might be primarily due to sunflower residues having a lower C ratio, which leads to faster decomposition and increased microbial activity when combined with added nitrogen. This combination maximizes microbial growth and CO2 release, whereas other residues with higher C ratios decompose more slowly, resulting in lower respiration rates. Kriauciuniene et al. (2012) and Munthali et al. (2015) reported that initial C/N ratios were the most critical variables in influencing decomposition of plant residues. Use of inorganic nitrogen application when organic residue of high C/N ratio was used might have either minimized the N immobilization or speed up the microbial decay (Jenkinson et al., 1985). The lowest respiration rate on day 4 was observed in soil alone (T1), which was due to the absence of added organic residues and nitrogen. Without these inputs, there are fewer nutrients available for microbial activity, resulting in lower decomposition rates and reduced soil respiration. The 45.51% increase with nitrogen is primarily due to its impact on soil respiration. Nitrogen enhances microbial activity in the soil, which increases respiration rates and overall soil metabolic activity, leading to improved nutrient breakdown and availability.
The soil respiration rates on day 6, is presented in Fig. 1c. It is observed that, there was significant differences among treatments, particularly the influence of differences crop residues and nitrogen addition. Soil + N (T2) recorded the highest respiration rate of 40.83 mg g-1, which was significantly higher than all other treatments, followed by Soil +Sunflower +N treatment (T8), which was 33.23 mg g-1. This might be due to acceleration of mineralization. Also Singh (1991) reported that the nitrogen addition, is known to accelerate the mineralization of easily degradable organic carbon. The overall percentage increase in soil respiration rates due to the addition of nitrogen across all residue amended treatments is approximately 47.69%. This indicates a substantial enhancement in microbial activity and decomposition processes when nitrogen is added to the residues. Soil alone (T1) had the lowest respiration rate of 5.27 (mg g-1), which was significantly lower than all other treatments. This rate reflects minimal microbial activity in the absence of added nutrients or organic matter, serving as a baseline for comparison with the residue and nitrogen-amended treatments.
The soil respiration rates on day 8 is shown in Fig. 1d, reveal significant differences among treatments. Soil + sunflower residue + N (T8) recorded the higher respiration rate of 40.63 mg g-1, significantly higher than all other treatments, followed by Soil + red gram residue + N (T10) showed a respiration rate of 35.07 mg g-1, which was significantly higher than rest of the treatments. On day 8, the high soil respiration rates with sunflower residue + N (T8) and redgram residue + N (T10) are due to enhanced microbial activity. Nitrogen boosts microbial growth by alleviating nutrient limitations, leading to more efficient decomposition of organic matter. The increased decomposition results in higher carbon dioxide release, which lead to higher respiration rates. The percentage increase in soil respiration rates due to the addition of nitrogen across all residue treatments on day 8 is approximately 51.67% which reveals its critical role in enhancing microbial activity. Nitrogen boosts microbial growth and decomposition efficiency by providing essential nutrients, which accelerates organic matter breakdown and CO₂ release. In contrast, the soil alone (T1) had the lowest respiration rate due to the absence of added nutrients and organic matter, limiting microbial activity and decomposition. Previous studies support these findings, including research by Zeng et al. (2010), which highlighted how nitrogen accelerates the breakdown of crop residues, particularly those rich in cellulose and hemicellulose. Similarly, Craine et al. (2007) found that nitrogen availability enhances microbial respiration by reducing the C ratio, allowing microbes to process organic carbon more efficiently. Fierer and Jackson (2006) further demonstrated that nitrogen amendments boost microbial biomass and enzymatic activity, leading to faster organic matter turnover and higher CO₂ emissions. Moreover, Cotrufo et al. (2013) reported that nitrogen additions increase the decomposition rate of recalcitrant plant residues, further contributing to higher soil respiration.
The soil respiration rates on day 10, were presented in Fig. 2a, which indicates significant differences among the treatments. Soil + red gram residue + N (T10) exhibited the highest respiration rate of 46.17 mg g-1, significantly higher than all other treatments. This may be because on day 10 there is active microbial population to process the red gram residues and also red gram residues (legume) have created more favorable environment compared to other residues for the microbes to break down the complex substances, which accelerated the decomposition rate and finally soil respiration. Muhammad et al. (2011) studied the total C mineralized from residues and suggested that decomposition of organic material in soil initially proceeded at faster rate. The decomposition attained a slower rate after about 15 days. It was mineralized within 14 days and remaining plant materials decomposed at a slower rate. The overall percentage increase in soil respiration rates due to the addition of nitrogen across all residue treatments on day 10 is approximately 53.65%. This indicates a significant improvement in soil respiration with the addition of nitrogen, underscoring its substantial impact on enhancing microbial activity and decomposition processes in the soil. Soil alone (T1) recorded the lowest respiration rate of 6.60 mg g-1, significantly lower than all other treatments. This low rate reflects minimal microbial activity in the absence of both added nutrients and organic matter, serving as a baseline for comparison with residue and nitrogen-amended treatments.
The soil respiration rates on day 15, as presented in Fig. 2b. There was significant differences among treatments, particularly the influence of crop residues and nitrogen addition. Soil + paddy residue + N (T7) exhibited significantly higher respiration rate at 48.57 mg g-1, this might be due to combine effect of paddy residue and nitrogen which result to significant increase in mineralization which result in lowering the C: N ratio of paddy residue. The result indicated that paddy residue start mineralization were others residues are almost in the mid of mineralization processes. The overall percentage increase in soil respiration rates due to the addition of nitrogen across all residue treatments on Day 15 is approximately 51.09%. This indicates that nitrogen addition leads to a significant enhancement in soil respiration, emphasizing its critical role in boosting microbial activity and decomposition processes in the soil. The decomposition attained a slower rate after about 15 days. Soil + N (T2) recorded significantly lower respiration rate of 8.57 mg g-1, compared to other treatments. This might be because the microorganisms have utilized the both added nitrogen and mineralized carbon present in the
Fig. 2: Impact of different residues decomposition with or without in organic N on Soil Respiration at various days of incubation (10, 15, 20, 30 DAI).
soil. This suggests that while soil microbial activity is ongoing, the addition of organic residues plays a crucial role in further enhancing respiration rates.
The soil respiration rates on day 20, is presented in Fig. 2c. There was significant difference among treatments. The soil respiration rates observed on day 20 reflect the influence of various crop residues and nitrogen addition on microbial activity. Soil + paddy residue + N (T7) recorded the highest respiration rate of 58.33 mg g-1, significantly higher all other treatments. This high rate can be attributed to the combined effect of paddy residue and nitrogen. Despite paddy residue having a high C: N ratio, the addition of nitrogen helps overcome nitrogen immobilization by providing an immediate nitrogen source for microbes, thus accelerating the decomposition of organic matter and increasing microbial activity. This aligns with findings by Jenkinson et al. (1985), which state that nitrogen addition can enhance decomposition rates, even with residues of high C ratios. Soil alone (T1) had a respiration rate of 9.80 mg g-1, which was significantly lower among all treatments. This is expected, as the absence of added organic material or nitrogen limits microbial activity. The overall percentage increase in soil respiration rates due to the addition of nitrogen across all residue treatments on day 20 is approximately 49.90%. Balasubramanium et al. (1974) observed that the release of CO2 by the soil amended with organic material was significantly more as compared control.
The soil respiration rates on day 30, is presented in Fig. 2d. There was significant difference among treatments. Significantly higher respiration rate was observed in the treatment with soil + red gram residue + nitrogen (T10) 33.37 mg g-1, followed by soil + paddy residue + nitrogen (T7) 25.40 mg g-1 and soil + cotton residues (T5) 22.07 mg g-1, which were on par. These values indicate a strong enhancement of microbial activity due to the combined effects of nitrogen and crop residues. Red gram residues generally have a relatively low C ratio, which supports faster microbial decomposition and higher CO2 emissions. When nitrogen is added, it further stimulates microbial growth and enzyme activity, enhancing the decomposition process and leading to increased soil respiration. Kriauciuniene et al. (2012) and Munthali et al. (2015) reported that initial C/N ratios were the most critical variables in influencing decomposition of plant residues. The overall percentage increase in soil respiration rates due to the addition of nitrogen across all residue treatments on day 30 is approximately 27.57%. This indicates that while nitrogen addition continues to enhance soil respiration, the degree of increase is less pronounced compared to earlier days. This may reflect the varying impacts of nitrogen over time and the dynamic nature of microbial activity and decomposition processes. The lowest respiration rates were recorded in treatments such as soil alone (T1) 10.10 mg g-1, indicating that the absence of crop residues or the sole addition of nitrogen results in significantly lower microbial activity.
The soil respiration rates on day 45, is presented in Fig. 3a, there was significant differences among the treatments involving various crop residues, nitrogen and their combinations. Soil with paddy residue (T3) showed a significantly higher respiration rate of 26.47 mg g-1. The elevated respiration rate in the paddy residue treatment indicates that microbial activity has been strongly stimulated by the paddy residue, which provides a continual source of organic matter for decomposition. Over the 45-day period, the paddy residue has likely undergone substantial decomposition, enhancing microbial activity as nutrients are gradually released and utilized. Saidi et al. (2008) reported that a stable C: N ratio could be achieved after 95 days of decomposition. The soil respiration rate for soil alone (T1) was 10.37 mg g-1 establishing the baseline, which was
significantly lower among all treatments. On Day 45, the overall percentage increase in soil respiration rates due to nitrogen addition across all residue treatments is approximately 19.13 %. This indicates a modest enhancement in soil respiration with nitrogen, suggesting that while nitrogen still influences soil microbial activity and decomposition, its effect has diminished compared to earlier observations. This decrease in the rate of increase might be due to various factors such as changes in microbial activity, the decomposition stage of residues, or nutrient dynamics over time.
The soil respiration rates on day 60, is presented in Fig. 3b, which showed significant differences among the treatments involving various crop residues, nitrogen and their combinations. The higher respiration rate was observed in soil with red gram residue + nitrogen (T10) at 23.57 mg g-1,
Fig. 3: Impact of different residues decomposition with or without in organic N on Soil Respiration at various days of incubation (45, 60, 75, 90 DAI).
followed by soil with paddy residue alone (T3) with a respiration rate of 22.53 mg g-1. On day 60, the higher soil respiration rate in the (T10) treatment is due to advanced decomposition and optimal nutrient conditions. By this time, the red gram residues have fully decomposed, and the added nitrogen has maximized microbial activity and organic matter breakdown, leading to elevated CO₂ emissions. On day 60, the 16.50% increase in soil respiration rates due to nitrogen addition reflects advanced decomposition and stable microbial activity. As decomposition progresses, the impact of nitrogen stabilizes, leading to sustained but moderated increases in microbial respiration. This suggests that while nitrogen continues to enhance microbial activity, the effects have become more consistent over time. The soil alone treatment T1 (10.067 mg g-1) recorded lower respiration rate which was on par with T2 (10.83 mg g-1), T6 (10.98 mg g-1) and T8 (10.93 mg g-1). Reddy et al. (2018) reported that the addition of crop residues significantly enhances soil microbial activity and respiration by providing readily available carbon and nutrients. Similarly, Khan et al. (2012) demonstrated that nitrogen application leads to increased microbial biomass and respiration rates, particularly in soils enriched with organic residues.
The result of the soil respiration rate of day 75 is presented in Fig. 3c. Soil respiration on day 75 was influenced by different residues with or without nitrogen. On Day 75, the soil respiration rates was higher in soil with paddy residue alone (T3) with a respiration rate of 18.8 mg g-1 and it was on par with soil + red gram +N (T10) demonstrating a notable stimulation of microbial activity by paddy residue, though slightly lower than observed on earlier days. On Day 75, the higher soil respiration rate in (T3) is due to the delayed mineralization associated with the high carbon-to-nitrogen (C) ratio of paddy residues. Over time, as nitrogen is gradually released from the residues, microbial activity increases, leading to sustained high respiration rates. This effect is comparable to Soil + red gram residue + Nitrogen (T10), reflecting effective microbial stimulation from both types of residues. Soil with cotton residue (T9) had the lowest respiration rate of 8.67 mg g-1, which was on par with soil alone 9.67 mg g-1. This might due to low C: N ratio of cotton. Use of inorganic nitrogen application when organic residue of high C/N ratio was used might have either minimized the N immobilization or speed up the microbial decay (Jenkinson et al., 1985). The reduction in the percentage increase of soil respiration rates due to nitrogen addition from day 75 might be because nitrogen becomes depleted, its stimulating effect on microbial activity wanes, and advanced decomposition stages lead to a stabilization of microbial processes. Additionally, residues without nitrogen start to mineralize and decompose more steadily, contributing to a natural increase in soil respiration rates independent of added nitrogen.
The result of the soil respiration rate of day 90 is presented in Fig. 3d. Soil respiration on day 90 was influenced by different residues with or without nitrogen. Soil with paddy residue recorded a respiration rate of 16.47 mg g-1, demonstrating a strong stimulation of microbial activity by paddy residue, though this rate is lower than observed on earlier days and it was on par with soil + redgram + N (T10). Soil alone (T1) had a respiration rate of 7.87 mg g-1, the lowest among all treatments, which was on par with T2 (8.55 mg g-1) and T5 (8.40 mg g-1). On Day 90, Soil + paddy residue (T3) has a respiration rate of 16.47 (mg g-1), showing continued microbial activity despite a decrease from earlier peaks due to advanced decomposition and reduced nutrient availability. Soil + redgram residue + nitrogen (T10) maintains a similar respiration rate, supported by the ongoing nutrient benefits of red gram residues and added nitrogen. Saidi et al. (2008) reported that a stable C: N ratio could be achieved after 95 days of decomposition. This indicates that both red gram residue + nitrogen and paddy residue alone have strong impacts on microbial activity, with red gram residue + nitrogen showing a comparable effect to paddy residue alone. On Day 90, the 3.45% increase in soil respiration rates due to nitrogen addition indicates a reduced effect over time. This is likely because of advanced decomposition stages, stabilization of microbial communities, and potential nutrient immobilization, which diminish the impact of additional nitrogen.
The result of the soil respiration rate of day 105 is presented in Fig. 4a. The results of soil respiration measured on day 105 reveal significant differences in microbial activity influenced by various residue treatments with or without nitrogen. The highest respiration rate was observed in the treatment with soil + paddy residue (T3) 14.37 mg g-1 had the highest rates, followed by soil + red gram residue + N 14.40 mg g-1. These two treatments showed statistically difference high respiration levels. In contrast, soil alone exhibited the lowest respiration rate of 4.40 mg g-1, reflecting minimal microbial activity. The addition of nitrogen to soil alone increased this rate to 5.50 mg g-1, a statistically significant improvement, and emphasizing nitrogen’s role in enhancing microbial activity by improving nutrient availability. Saidi et al. (2008) reported that a stable C: N ratio could be achieved after 95 days of decomposition. The observed 5.60 % increase in respiration rates without nitrogen addition across all treatments indicates that microbial communities continue to actively decompose residues even in the absence of added nitrogen. N initially boosts microbial activity, its effect diminishes over time as it is used up, whereas the residual microbial activity continues to drive decomposition and soil respiration.
The results presented in Fig. 4b illustrate significant differences in microbial activity influenced by various residue treatments, particularly with or without nitrogen, on day 120. The highest respiration rate was recorded in the soil + paddy residue treatment (T3) at 12.10 mg g-1, highlighting the effectiveness of paddy residue in promoting microbial activity and stable mineralization. This elevated respiration rate indicates a continuous nutrient release from the paddy residue, providing a rich carbon source for microbes, which sustains activity even as carbon sources in other treatments become depleted. The soil + red gram residue + N treatment (T8) achieved the second-highest respiration rate at 7.80 mg g-1, attributed to the synergistic effect of red gram residue and added nitrogen, enhancing microbial activity. However, the benefits of nitrogen, while significant, do not match the long-term advantages provided by paddy residue. In contrast, the lowest respiration rates were seen in the soil alone (T1) and soil + N (T2), both at 3.67 mg g-1. These low rates reflect limited microbial activity due to the lack of organic residues and the exhaustion of carbon sources in the nitrogen-only treatments. Overall, the increase in respiration linked to organic residues alone is approximately 14.57%, underscoring the crucial role of organic matter in stimulating microbial activity. This percentage reinforces that while nitrogen can enhance microbial processes, the sustained high respiration rates and long-term stability are more effectively supported by high-quality organic residues like paddy. The persistent high respiration associated with paddy residue underscores its superior ability to maintain microbial activity and nutrient cycling over time. The respiration rate of the residue added treatments was high compared to control, throughout the experiment (Fig. 5). Over time, soil respiration steadily decreases after rising gradually and peaking on day 15. This pattern most likely represents a rise in microbial activity at first (which was also evident from our study but data not mentioned) brought on by the breakdown of readily available organic matter, this pattern probably indicates that microbial activity increases first due to the decomposition of easily accessible organic matter and subsequently decreases when these substrates become scarcer. Our finding is similar to Vahdat et al. (2011), who had reported that, CO2-C release, in untreated soils (controls) has shown very slow patterns meanwhile treated soils showed an initial rapid increase, followed by a slower, linear release. Incorporation of paddy residue had demonstrated a progressive rise over time, reaching a peak at 25.27 mg g-1 on day 45. This indicates sustained microbial activity, likely due to the slow decomposition, which might be due to high C: N ratio. This suggests that microbial activity has persisted, most likely as a result of the paddy residue's gradual breakdown. Our result was consistent with the findings of Kobke et al. (2018), who had reported that, legumes typically emit the highest cumulative CO2 emissions, which decrease over time, while cereals exhibit lower emissions but higher stability. Incorporation of sunflower residue led to increase in soil respiration upto day 30, later there was a sharp decrease after. It falls to 1.15 by day 120, indicating that sunflower residue may break down rapidly and cause an early microbial substrate depletion. Cotton residue exhibits a similar pattern to paddy residue i.e., it had shown a gradual increase in respiration that peaks on day 30, then a notable decline by day 120, signifying a slower rate of decomposition.
Fig. 4: Impact of different residues decomposition with or without in organic N on Soil Respiration at various days of incubation (105, 120 DAI).
Fig. 5: Temporal dynamics of impact of crop residue decomposition on Soil Respiration
Rezgui et al. (2021) reported that the mineralization rate decreased due to the increasing recalcitrance of the remaining residues. Lignin content was negatively correlated with C mineralization due to its resistance to microbial breakdown. Incorporation of red gram residue showed a moderate rate of decomposition which peaked on day 15, followed by a steady fall in microbial activity which reflected in decreased respiration. Cogle et al. (1989) estimated that incorporation of straw hastened its decomposition rate only within the first 15 days, thereafter the decomposition rate was similar. Decomposition rate generally peaked between 4 and 15 days. The carbon source however was quickly exhausted. When residues are added once, respiration rates are initially high due to the decomposition of easily available compounds. Thereafter, respiration rates decrease as easily available compounds are depleted. Respiration rates are low in the later stages of decomposition when only more recalcitrant compounds such as lignin-encrusted cellulose and other macromolecules are left (Wang et al. 2004). It is well-known that incorporation of plant residues into the soil results in a rapid increase in microbial activity and biomass followed by gradual decrease (Wang et al. 2004). Sarma et al. (2013) reported that C:N ratio of the different residues were responsible for maximum mineralization of native soil organic matter and added crop residues by increasing microbial activity in soil environment at the particular day of incorporation. Soil + nitrogen treatment showed significant increase in respiration, which peaked on day 6 at 40.83. However, by day 15, respiration drastically drops, suggesting that the initial nitrogen-induced microbial boost is just temporary. On the other hand, respiration dramatically decreases by day 15, indicating that the initial nitrogen-induced microbial boost is transient. After day 20, respiration rates level off and resemble those found in the soil by themselves.
Paddy residue + N treatment, the respiration rate increases dramatically when nitrogen is introduced to paddy residue, peaking at 48.57 on day 15, as compared to paddy residue alone. This suggests that nitrogen significantly accelerates the breakdown of rice residue. Sunflower residue + N treatment showed similar to the impact of nitrogen alone, i.e., respiration peaked early (on day 6), at 40.63. It does, however, decline significantly thereafter, indicating a quick initial breakdown followed by the exhaustion of accessible substrates. Cogle et al. (1989) estimated that incorporation of straw hastened its decomposition rate only within the first 15 days, thereafter the decomposition rate was similar. Sakala et al. (2000) reported rapid CO2 evolution within the first 10 days. Alexander and Scow (1989) reported that, as the decomposition proceeds, the organic matter is not attacked as a whole. Some of the constituents are decomposed readily (sugar and starches), followed by proteins, cellulose, hemicellulose and finally lignin, waxes and tannins. Zeng et al. (2010) reported maximum degradation of hemicellulose as compared to lignin and cellulose during composting. In all the five trials, it was found that the amount of hemicellulose present in the compost at the end of 20 days was less than 10% indicating very rapid degradation of hemicellulose under microbial activity. This implies that nitrogen is essential for promoting cotton residue breakdown. Red gram residue + N treatment showed a peak on day 10, (respiration reaches 46.17), the greatest peak of all the combinations. The C: N ratio of crop residue is a key factor for its degradation. During the initial decomposition phase, low C: N ratio causes manifold increase in the decomposition rate (Golueke, 1992). This implies that red gram residue, when mixed with nitrogen, the result is in line with Wang et al. (2004) also found that addition of N fertilizer accelerated C mineralization during early stages. The C mineralization of residues in the early stage of decomposition is influenced by the amount of soluble C present in the added plant materials (Hadas et al., 2004). Muhammad et al. (2010) reported that, addition of N fertilizer to sugarcane, maize and sorghum residues to soil promoted CO2 emissions significantly, compared to the unfertilized N treatment, during the first 10 days of incubation. Ravali et al. (2024) also reported that, C:N ratio of crop residue is a key factor for its degradation. During the initial decomposition phase, low C: N ratio causes manifold increase in the decomposition rate. Guhe and Deshmukh (1973) found that crop residues like wheat straw incorporated with fertilizer N in soil favorably enhanced the soil microbial ecology, microbial biomass and yield of legume crop. Gallardo and Merino (1993) studied the rate of organic material breakdown depends on the relative proportion of soluble sugars, cellulose, hemicellulose and lignin content.
Conclusion
Incorporating crop residues, particularly when combined with nitrogen, significantly enhances soil respiration. Soil with crop residues showed higher CO2 emissions compared to soil alone. When crop residues were combined with nitrogen, it had further enhanced respiration rates. Paddy residue + nitrogen (T7) exhibited the highest respiration rates, highlighting the role of N in accelerating organic matter decomposition. Crop residues with lower C:N ratio enhanced decomposition and increased nutrient availability. After incorporation, there was rise in CO2 emissions upto 20 DAI, followed by decrease throughout the incubation period. The CO2 emissions reflected carbon mineralization.
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