Effects of long-term fertilization and cropping regimes on total nitrogen and organic nitrogen forms in a Mollisol of Northeast China

Abstract

A 22-year (1990–2012) field experiment was conducted to assess the effects of different fertilization and cropping regimes on the quantitative changes of total N and organic N forms in the topsoil (0–20 cm) and subsoil (20–40 cm) of a Mollisol in Northeast China. This study included nine fertilizer treatments on maize monoculture [no fertilizer (CK), N, NP, NK, PK, NPK, NPK combined with maize straw (NPKS), NPK combined with pig manure (NPKM), and 1.5’ the rate of NPKM (1.5NPKM)] and one fertilizer treatment with maize-maize-soybean rotation (NPKMR). Compared with the CK treatment, the application of mineral fertilizers alone or in combination with straw generally had no significant effect on the contents of total N and organic N forms, whereas the combined application of mineral fertilizers with manure significantly increased their contents. Manure levels and cropping regimes had no significant effect on the total N content in the topsoil. By contrast, the hydrolysable unknown N content significantly increased with the increasing manure levels, and the amino sugar N content was significantly lower in rotation than in monoculture treatment. Our results imply that manure application integrated with continuous maize cropping can be considered as an optimized strategy for improving soil fertility.