The experiment began in the 1994 kharif season at Pusa, Bihar, and was evaluated during 2023–2024. Its five randomized treatments included uncultivated soil, rice–wheat cultivation with no residue returned, and residue return at 25%, 50% and 100% of the available material. Each treatment had four field replications, and composite samples were taken from four depths: 0–5, 5–15, 15–30 and 30–60 centimeters. The site’s calcareous sandy loam initially had a pH of 8.6, an electrical conductivity of 0.36 decisiemens per meter, 0.62% soil organic carbon and 35.6% calcium carbonate. The subtropical site receives about 1,330 millimeters of rain annually and has distinct rainy, winter and summer seasons. Rice was grown under conventional puddling and wheat under no-till. Fertilizer rates, irrigation and other crop-management operations were otherwise the same across cultivated treatments, while the amount of residue returned was the defining difference among them.

Residue handling changed partway through the trial. Before 2017, crop residues were incorporated into the soil. From the 2017 rice crop onward, rice residue was used as mulch in the following wheat crop, while wheat straw continued to be incorporated. The uncultivated plot served as a long-term reference condition and was not a direct measurement of the soil’s carbon stock when the trial began. Soil samples were cleared of debris and stones, passed through an 8-millimeter sieve and separated by wet sieving into large macroaggregates larger than 2 millimeters, small macroaggregates from 0.25 to 2 millimeters, microaggregates from 0.053 to 0.25 millimeters and a silt-plus-clay fraction below 0.053 millimeters. Bulk density was measured for each layer, and carbon and nitrogen contents were determined within the separated fractions.

Returning residue consistently improved the size and stability of water-stable aggregates. Mean weight diameter, geometric mean diameter, the proportion of macroaggregates and the aggregate ratio all increased with the amount of residue returned, with the highest values generally recorded under complete residue return. The clearest responses occurred near the surface and became weaker with depth. Relative to the uncultivated reference, complete residue return increased mean weight diameter by 71% in the uppermost layer, 55% at 5–15 centimeters, 47% at 15–30 centimeters and 40% at 30–60 centimeters. Complete retention also produced the largest macroaggregate share at every depth and the smallest microaggregate and silt-plus-clay fractions. Bulk density was slightly lower with residue addition, although treatment differences were not statistically significant across depths. The pattern nevertheless indicated a shift from fragmented, less stable soil toward a structure dominated by larger aggregates capable of physically enclosing and protecting organic matter.

Carbon and nitrogen followed the aggregate-size gradient. Macroaggregates contained the highest concentrations of both soil organic carbon and total nitrogen, microaggregates were intermediate, and the silt-plus-clay fraction had the lowest concentrations in every treatment and depth. Macroaggregates contributed more than half of the soil carbon and nitrogen in the 0–5-centimeter layer, although their relative contribution declined as the profile deepened. Complete residue return increased aggregate-associated organic carbon across the profile: relative to the uncultivated reference, concentrations rose by 55–140% in macroaggregates, 52–67% in microaggregates and 107–199% in the silt-plus-clay fraction. Total nitrogen increased in parallel, and the bulk-soil ordering in the upper 30 centimeters was complete return, 50% return, 25% return, no residue and uncultivated soil. These results show that repeated residue inputs supplied organic material beyond the coarse surface layer and promoted its incorporation into multiple soil pools. Because the researchers did not directly trace carbon with isotopes or separately quantify undecomposed, particulate, microbial-derived and mineral-associated fractions, they could not determine the precise contribution of each pool.

The profile-scale stock calculations supplied the clearest numerical contrast. Total organic carbon in the upper 60 centimeters ranged from about 33 to 62 megagrams per hectare and generally rose as the residue-return rate increased. The uncultivated reference contained 32.78 megagrams per hectare, the no-residue treatment 34.25, the 25% treatment 41.01, the 50% treatment 44.68 and the complete-return treatment 62.15. The estimated sequestration rate over the 30-year period was 0.057 megagrams per hectare per year for uncultivated soil, 0.107 for the no-residue treatment, 0.33 for 25% return, 0.45 for 50% return and 1.04 for complete return. These are study-specific estimates based on measured stocks and assumed baseline information, not direct observations of atmospheric carbon removal. Estimated annual straw-carbon inputs ranged from 833 kilograms per hectare in the no-residue treatment to 6,731 kilograms per hectare under complete return. Yet the proportion of straw carbon converted to soil organic carbon remained similar across treatments, between 12.7% and 15.5%, indicating that the principal distinction was the scale of the annual carbon input rather than a dramatic change in conversion efficiency.

Total nitrogen storage increased with residue return and paralleled the carbon trend. Relative to uncultivated soil, the estimated increase in the 0–60-centimeter nitrogen stock ranged from 4.2% without residue to 54.9% with complete residue return. Residue retention also increased vertical stratification of carbon, nitrogen and their carbon-to-nitrogen ratios. Stratification ratios compare concentrations in the uppermost layer with those in progressively deeper layers, so larger values indicate stronger concentration near the surface. The 50% treatment had the highest surface-to-5–15-centimeter soil-organic-carbon ratio, while complete return had the highest values for the broader comparisons with 15–30- and 30–60-centimeter layers. The three residue-added treatments had statistically similar total-nitrogen stratification ratios, whereas the uncultivated treatment had the lowest values. These patterns show where organic matter accumulated, but they do not by themselves demonstrate a change in nutrient-use efficiency or crop nutrient uptake.

Residue management was accompanied by signs of greater biological activity in the surface soil. The geometric mean of five measured enzyme activities—dehydrogenase, β-glucosidase, alkaline phosphatase, acid phosphatase and phenol oxidase—was highest under complete residue return, followed by 50%, 25%, no residue and uncultivated soil. The reported geometric mean was 37.55 for complete return in the 0–5-centimeter layer, compared with 32.32 for 50% return, 28.14 for 25% return, 28.48 for no residue and 22.03 for uncultivated soil. Values in the 0–5-centimeter layer exceeded those at 5–15 centimeters. The enzyme results are consistent with a larger supply of decomposable substrate and more active cycling of carbon and nitrogen, although enzyme activity is a proxy for biological functioning rather than a direct measurement of microbial diversity, specific pathways or net mineralization. The experiment did not identify microbial taxa, microbial community shifts or the relative roles of roots, residue decomposition and mineral binding in producing the observed changes.

Multivariate analysis supported an aggregate-mediated explanation for the field pattern. A global redundancy analysis found that soil depth accounted for 73.6% of the variation in the measured physical, chemical and aggregate-related variables, while residue management accounted for 23.2%; together, the model explained 92.9% of the variation. Treatments receiving more residue clustered near variables representing larger, more stable aggregates and greater carbon and nitrogen storage, while the uncultivated and no-residue treatments occupied the opposite side of the ordination. A partial least squares structural equation model then linked greater straw-carbon input to stronger aggregation, aggregate-associated carbon and nitrogen, and ultimately higher profile-scale carbon and nitrogen stocks. The model reported coefficients of determination of 0.96 for aggregation, 0.97 for aggregate-associated soil organic carbon, 0.97 for aggregate-associated total nitrogen and 0.99 for both profile-scale carbon and nitrogen stocks. These figures describe performance within the specified dataset, not independent predictive accuracy: aggregate-associated carbon and total carbon stocks are mathematically related variables, so their shared variance inflates the apparent explanatory power.

Taken together, the observations show that three decades of residue retention altered both the quantity and the apparent physical protection of soil organic matter in this calcareous rice–wheat system. The simplest treatment—returning all available residue—produced the largest macroaggregate fraction, the strongest aggregate-associated enrichment of carbon and nitrogen, the highest measured profile carbon stock and the greatest enzyme activity. The evidence is consistent with a continuing stabilization pathway in which fresh plant carbon feeds microbial transformation and aggregate formation, aggregate structure protects organic material from decomposition, and finer particles provide additional opportunities for organo-mineral association. That pathway remains a mechanistic interpretation rather than definitive proof of causation: the structural model establishes statistical relationships among measured variables, while the field comparison does not trace individual carbon molecules or measure greenhouse-gas emissions. The results therefore show a strong field association between residue retention, improved soil structure and greater carbon and nitrogen retention under the specific climatic and agronomic conditions of the eastern Indo-Gangetic Plains.

Subject of Research: Long-term crop residue effects on soil aggregation and carbon–nitrogen retention in a calcareous rice–wheat system

Article Title: Long-term crop residue management drives soil aggregation and carbon–nitrogen retention in a calcareous rice–wheat system of the eastern Indo-Gangetic Plains

Article References: Jose, A. T., Das, S., Mandal, D., Bhattacharyya, R., Singh, S. K., Dey, A., Shrivastava, M., Singh, R. K., Saini, P., & Aditya, K. (2026). Long-term crop residue management drives soil aggregation and carbon–nitrogen retention in a calcareous rice–wheat system of the eastern Indo-Gangetic Plains. Frontiers in Agronomy, 8, Article 1924145. https://doi.org/10.3389/fagro.2026.1924145

Image Credits: AI Generated

DOI: 10.3389/fagro.2026.1924145

Keywords: crop residue, soil aggregation, soil carbon, soil nitrogen, rice–wheat system, calcareous soil, macroaggregates, carbon sequestration, long-term field experiment, Indo-Gangetic Plains, Long-term, crop