Effects of 14 years of repeated pig manure application on gross nitrogen transformation in an upland red soil in China.

Aims: Long-term application of pig manure can improve soil fertility and alleviate soil acidification, but also increase nitrogen (N) losses in subtropical upland red soils. However, mechanisms driving N losses via nitrate leaching or NO emissions remain unknown. Herein we investigated long-term pig manure applications in upland red soils by assessing soil N transformation dynamics. Methods: Pig manure was applied with or without lime over a 14-year period in four treatments: No manure (CK); Low-rate manure (LM, 150 kg N ha y); High-rate manure (HM, 600 kg N ha y); High-rate manure and lime (HML, 600 kg N ha y and 3000 kg Ca(OH) ha (3y)). N tracing was used to quantify gross N transformation and N dynamics. Results: Prolonged manure application increased soil gross N mineralization and NH immobilization, although the increase was only significant for HM. Both rates were further enhanced by lime addition. Gross autotrophic nitrification also increased with increasing manure application, and further increased with lime addition. In contrast, dissimilatory NO reduction to NH (DNRA) and NO immobilization were negligible irrespective of manure application. Thus, NO produced via autotrophic nitrification was not converted to NH and microbial biomass N, and accumulated in soil. Gross autotrophic nitrification was positively correlated with NO emissions and NO leaching, suggesting it largely determined N losses. Conclusions: Autotrophic nitrification governs N losses in upland red soils receiving repeated manure applications, and attempts to reduce N emissions or N leaching should therefore control this. [ABSTRACT FROM AUTHOR]