N transformation mechanisms and N dynamics of organic fertilisers as partial substitutes for chemical fertilisers in paddy soils

Purpose Knowledge of nitrogen (N) dynamics of organic fertilisers as partial substitutes for chemical fertilisers could improve the retention of mineral N and optimise fertilisation practices in paddy soils. Materials and methods 15 N tracing was performed to quantify the effect of 36 years of partial substitution of chemical N by organic N on gross N transformation rates in soils, and its relationship to organic N fractions. Treatments were chemical fertilisation (NPK), substitution of chemical N with 30%, 50% and 70% organic N (70F + 30 M, 50F + 50 M and 30F + 70 M), and no fertilisation (control). Results and discussion Partial substitution elevated N mineralisation, ammonium immobilisation, autotrophic nitrification and nitrate consumption rates by 58.4–609.3%, 36.0–304.2%, 0.5–320.0% and 51.4–112.6%, respectively, compared to NPK treatment. However, gross N transformation rates differed with varying organic N substitution proportions. Fifty percent substitution increased mineralisation and immobilisation of the labile organic N pool by increasing the level and distribution of amino acid N fractions in acid-hydrolysable N, resulting in the highest N supply and retention capacity and the lowest N loss potential. Thirty percent substitution also increased the supply and retention capacity of nitrogen, but its integrated effect was inferior to the 50% substitution treatment. However, 70% substitution increased the potential loss of nitrate by increasing the ratio of gross nitrification rate to gross ammonia immobilisation rate (N/I). Partial substitution also increased the yield by 4.26–6.91% ( p < 0.05), but there were no significant differences between organic N substitution proportions. Conclusion Thus, accumulating active fractions of the soil organic N pool enhanced soil N supply and mineral N retention in acidic rice fields. Given optimal sustainable agricultural practices, and considering yield, N retention and potential N losses, a 50% substitution of chemical N with organic N can be recommended for the study region.