Changes in soil organic carbon and total nitrogen and its relationship under three typical cropping systems in China

Based on data of 20-year continuous observation from 34 sites around the country,we analyzed the characteristics of changes in SOC,TN and the relationship between SOC and TN in dry cropland,wheat-rice system and paddy fields under conventional fertilization.The results showed that SOC and TN contents in topsoil increased significantly in dry cropland,wheat-rice system and paddy fields,while the changes in the soil C/N ratio varied among the three typical agro-ecosystems during the last 20 years of conventional fertilizations.The average of soil C/N ratio in these three types of agro-ecosystems changed from 9.87 in 1985~1990 to 10.32 in 2001~2006.The frequency of soil C/N ratio at the range of 9~11 were about 45% to 48% during the last 20 years.However,the frequency of soil C/N ratio at the range of 7~9 decreased from 27.5% to14.5%,whereas the frequency of soil C/N ratio at the range of 11~13 increased from 15.6% to 27% from 1985 to 2006.Moreover,soil C/N ratio at more than 30% of contentious long-term observation sites in dry cropland increased in the last 20 years.Further analysis indicated that there were significantly positive correlations between soil C/N ratio and the content of available P,available K,and the input amount of phosphorus fertilizer and organic nitrogen in manure,while there was a significantly negative correlation between soil C/N ratio and soil pH.Overall,conventional fertilization accompanied with phosphorus fertilizer and organic manure promoted the accumulations of SOC and TN,and the effect on SOC accumulation exceeded the effect on TN.All these results indicated that application of phosphorus fertilizer and organic manure was the most important factor that affected the relationship between soil organic carbon and nitrogen.