15N tracing study to understand the N supply associated with organic amendments in a vineyard soil

Abstract

Organic materials with low C/N ratio, such as animal manure and compost, have been largely applied to orchard soil to maintain soil organic matter and improve soil fertility. However, little is known about the decomposition characteristics and nitrogen (N) mineralization of added organic materials. Thus, a laboratory incubation study using 15N tracing technique was carried out to investigate the effects of organic materials with low C/N ratio (rapeseed meal and chicken manure) on gross N transformations in a vineyard soil. Our result showed that carbon (C) mineralization of organic material depended on C/N ratio and lignin/N ratio of organic material, while N mineralization was associated with substrate N concentration. The application of organic material with low C/N ratio increased gross N mineralization, NH4 + immobilization, autotrophic nitrification rates, and CO2, N2O, and NO emissions. Heterotrophic nitrification and NO3 − immobilization did not occur, irrespective of organic material amendments. Organic material amendments increased more total inorganic N production (mineralization + heterotrophic nitrification) than total inorganic N consumption (immobilization of NH4 + and NO3 −), leading to increasing net N mineralization rates. In addition, NO3 − consumption (NO3 − immobilization + dissimilatory NO3 − reduction to NH4 +) increased to a lesser extent than NO3 − production (heterotrophic + autotrophic nitrification) following organic material amendments, leading to more rapid accumulation of NO3 − in soils. Our results suggest that organic material with a low C/N ratio can provide readily available N as N fertilizers but accompanied by enhanced risk of N losses through gaseous N emissions and possibly NO3 − leaching and runoff.