Effect on soil C and N ratio (C/N) C/N ratio is an indicator of the characterization of humification degree of organic matter, the higher C/N ratio, the lower the degree of decomposition of organic matter (Baldoek et al. 1992; Li et al. 2004). Soil C/N ratio can reflect the effectiveness of the related elements, as well as the activity of the organic matter in soil aggregates, and it is an important indicator for assessing soil quality such as providing some information concerning availability of elements, as well as on the relative lability of the organic material contained in aggregates (Zibilske and Bradford 2007). After 28 yr of continuous rice cultivation, there were distinct changes in the C/N ratio among the different plots (Fig. 1). Among the three winter green manure plots, the soil C/N ratio was lower in the R-R-MV than in the R-R-WF plot; higher in R-R-RG and R-R-RP than in R-R-WF; lower in the R-R-MV than in R-R-RG and R-R-RP plots. This may be because the C/N ratio of Chinese milk vetch (12.0) is lower than that of rape (26.4) and ryegrass (20.2) (Shen et al. 1989).
The C/N ratio was higher in larger aggregates than in smaller ones, and that of 2-5 mm was the highest (Fig. 3).The C/N ratio had a general downward trend with decreased
soil aggregate size, indicating a higher degree of organic decomposition with smaller soil aggregates.
The C/N ratio of 1-2-mm aggregates was significantly higher in the R-R-RP than in other plots. The C/N ratio of 2-5-mm aggregates was significantly higher in the R-R-RP, R-R-MV and R-R-RG plots than in R-R-WF.The winter green manure plots mainly affected the C/N ratio of 2-5-mm aggregates but had little effect on other
0.0 sized aggregates