Experimental design
This study was conducted from 1982 to 2009. In this area, the common cropping pattern before the study was early rice-late rice-fallow (winter). The soil is a silty light clayey paddy soil from Quaternary red earth, which is classified as Fe-accumuli-Stagnic Anthrosols according to the Chinese
Soil Classification System (Shi et al. 2004). Initial chemical properties of the plowed layer soil (0-15 cm depth) are as follows: pH 6.6, organic C 13.09 g kg-1, total N 1.58 g kg-1, total P 0.82 g kg-1, total K 11.5 g kg-1, available N 156 mg kg-1, available P 7.2 mg kg-1, available K 75 mg kg-1 and slowly available K 123 mg kg-1. The treatments were rice-ricefallow (R-R-WF), rice-rice-rape (R-R-RP), rice-rice-Chinese milk vetch (R-R-MV), rice-rice-ryegrass (R-R-RG). The R-RWF treatment was the control. The experiment was conducted in a randomized complete block design with three replicates. Each plot was 2.5 m×15.0 m. The plots were separated by a 0.6 m wide concrete ridge. From 1982 to 2009, the amounts of N, P2O5 and K2O applied to both early and late rice (basal dressing and top dressing) were 153.0, 84.0, and 129.0 kg ha-