Variation of organic carbon and nitrogen in aggregate size fractions of a paddy soil under fertilisation practices from Tai Lake Region, China

Long-term changes in agricultural management can affect soil organic carbon (SOC) and nutrient dynamics, which can be monitored by determining the distribution of microbial activity and nutrient pools in soil aggregates of different size fractions. The objective of the present study was to investigate the changes in SOC and total nitrogen (N(t)) distribution in aggregates of different size fractions from a paddy soil (Ferric-accumulic Stagnic Anthrosols) under a long-term fertilisation trial. Undisturbed samples of topsoil (0-5 and 5-15 cm depths) were collected from a field experiment farm located in Tai Lake Region, China, with the plots receiving no fertiliser (NF), chemical fertilisers (CF), chemical fertilisers with straw return (CFS) or chemical fertilisers plus pig manure (CFM). In the surface layer, SOC and N(t) concentrations appeared as a bimodal peak in the 2000-250 and < 2 μm fractions. SOC concentration increased by 38.6, 40.8 and 17.2% and N(t) concentration by 30.0, 16.8 and 38.4% in the 2000-250 μm fractions under CFM, CFS and CF respectively as compared with NF treatment. There were slight changes in SOC and N(t) in the < 2 μm fractions from different fertilisation plots. Continuous addition of manure or straw increased storage of SOC and N(t) mainly in the coarser aggregate fractions. SOC increases due to straw or pig manure application predominated in the 2000-250 μm fractions, with SOC seeming to be physically protected within macro-aggregates. Thus straw and manure are likely to play an important role in carbon and nitrogen storage in paddy soil under long-term combined chemical and organic fertilisation.