Effects of Long-Term Fertilization on the Distribution of Carbon, Nitrogen and Phosphorus in Water-Stable Aggregates in Paddy Soil

We investigated the size distribution of water-stable aggregates and the soil carbon, nitrogen and phosphorus concentration over aggregate size fractions based on a long-term (1990-2006) fertilization experiment in a reddish paddy soil. The results showed that the largest water-stable aggregate (WSA) (>5 mm) and the smallest WSA (<0.25 mm) took up the first largest proportion (38.3%) and the second largest proportion (23.3%), respectively. Application of organic materials increased the proportion of the large WSA (>2 mm) and decreased the proportion of the small WSA (<1 mm), resulting in an increase in the mean weight diameter of WSA, whereas application of chemical fertilizer had little effect. Application of organic materials, especially combined with chemical fertilizers, increased total carbon, nitrogen and phosphorus concentrations in all sizes of WSA, and total carbon, nitrogen and phosphorus were prone to concentrate in the large WSA. Further more, application of organic materials improved the supply effectiveness of available phosphorus, whereas had little influence on the labile carbon in WSA. Application of chemical fertilizers improved concentrations of total and available phosphorus in all sizes of WSA, whereas had little influence on total carbon and nitrogen contents. Economical fertilization model maintained the soil fertility when compared with full dose of chemical fertilizers, indicating that using organic materials could reduce chemical fertilizers by about one third.