Soil aggregation and distribution of carbon and nitrogen in different fractions after 41 years long-term fertilizer experiment in tropical rice–rice system

Abstract Inorganic fertilizers and manure application can alter the aggregate distribution as well as aggregate associated organic carbon and nitrogen. It is not fully understood how long term addition of carbon (C) and nitrogen (N) through biomass, farm yard manure (FYM) and inorganic fertilizers are distributed in soil aggregates in a tropical rice–rice system. Therefore, the objectives of this study were to investigate the long-term effects of FYM and inorganic fertilizer in a rice–rice system on (i) distribution of water stable aggregates in the soil, and (ii) amount of total soil organic carbon (TOC), total N and C/N ratio in different aggregate fractions. The total water stable aggregates (WSA) ranged from 71.6% under control to 91.1% under NPK + FYM in the surface soil (0–15 cm). The incorporation of FYM increased the macroaggregates (5–2 mm) by 165.3% and mesoaggregates by 130.7% (2–1 mm) and 282.8% (1–0.5 mm) over control in 0–15 cm soil layer. Mean weight diameter (MWD), varied from 0.43 to 0.78 in 0–15 cm and from 0.40 to 0.72 in 15–30 cm soil layer. MWD was higher under FYM treated plots than inorganic fertilizer alone and unfertilized control plots. Irrespective of treatment, macroaggregates (5–2 mm) had the highest (4.13–14.03 g kg− 1) and microaggregate fraction (0.25–0.1 mm) had the least (3.70–8.89 g kg− 1) TOC content in the 0–15 cm soil layer. The application of FYM either alone or in combination with inorganic fertilizers significantly increased the accumulation of TOC in all aggregate fractions over control, however, the effect was more pronounced in macroaggregates fraction 5–2 mm and microaggregate fraction 0.1–0.053 mm. The combined application of inorganic fertilizers and FYM resulted in an increase in total N content which ranged from 0.43 to 1.0 g kg− 1 in 5–2 mm and from 0.09 to 0.44 g kg− 1 in 0.25–0.1 mm aggregate fractions. The highest C:N ratio was recorded in aggregate fraction of 0.1–0.053 mm irrespective of treatment, whereas aggregate fraction of 0.5–0.25 mm had the least C:N ratio. A higher C:N ratio was observed in 15–30 cm than 0–15 cm soil layer. Hence it was concluded that the long-term application of FYM resulted in C and N accumulation in bulk soil and aggregates, but the accumulation pattern was dependent on aggregate size.