Objectives Organic nitrogen mineralization process plays a crucial role in derterming soil nitrogen supply capacity and crop nitrogen use efficiency. We investigated the effect of commonly used organic fertilizer sources on nitrogen mineralization in soil aggregates of yellow mud fields, aiming to reveal their regulatory mechanisms on soil nitrogen supply.

Methods Soil samples were collected from a 36 years long-term experiment in a yellow mud field. The treatments included: no fertilization control (CK), application of chemical fertilizer alone (NPK), chemical fertilizer plus cow manure (NPKM), and chemical fertilizer plus full rice straw return (NPKS). The wet sieving method was used to separate soil aggregate in sizes of >2 mm and 0.25−2 mm. Then the aggregates were incubated in darkness for 161 days. During incubation, total N, available N, microbial biomass nitrogen content were analyzed periodically.

Results Compared with CK, the NPK, NPKM and NPKS treatments increased cumulative nitrogen mineralization by 10.7%–58.3% in bulk soil, 7.1%–49.4% in >2 mm aggregates, and 36.6%–50.1% in 0.25–2 mm aggregates. The cumulative nitrogen mineralization in bulk soils and >2 mm aggregates under NPKM and NPKS treatments were significantly higher than that under NPK treatment, and the nitrogen mineralization potential were significantly higher under NPKM and NPKS treatments than that under CK and NPK treatment. The nitrogen mineralization potentials in >2 mm aggregates under each fertilization treatment were significantly higher than that in 0.25−2 mm aggregates, but the mineralization rate constant showed the opposite trend, except under NPKS treatment. Nitrogen mineralization accumulation and mineralization potential of all aggregates were positively correlated with total N, alkali-hydrolyzed N, organic carbon and microbial biomass nitrogen (P<0.05), while the nitrogen mineralization accumulation and mineralization potential of undisturbed soil and >2 mm aggregates were negatively correlated with bulk density (P<0.05).

Conclusions Organic nitrogen in >2 mm aggregates is pivotal to the soil nitrogen pool, and that in 0.25−2 mm aggregates influences soil nitrogen supply capacity. Long-term combined application of chemical fertilizers with cow manure and straw could significantly enhance the nitrogen mineralization potential of both >2 mm and 0.25−2 mm aggregates in yellow-mud paddy soil by increasing nitrogen pools and microbial activity, with cow manure showing stronger effects than straw. Cow manure also shows effects on steep up the mineralization rate of 0.25−2 mm aggregates, showing higher nitrogen supply capacity than straw. So cow manure and chemical fertilizer is more effective in regulating the accumulation and supply potential of soil nitrogen in the low-fertility yellow-mud field.