Cultivation effects on the carbon and nitrogen dynamics at depth in oasis farmlands of the Southern Tarim Basin, China

Abstract

The profile distribution of soil organic carbon (SOC) and total nitrogen (TN) are important indicators for predicting and simulating the effects of human activities on soil fertility and quality. However, most studies have focused on these changes in surface soil or the top 100 cm of short-term cultivated farmlands in arid regions. In the present study, farmlands cultivated for 0, 10, 15, 30, 80 and 100 years in the Cele oasis, at the southern margin of the Tarim Basin, were selected to examine cultivation effects on the vertical distribution of SOC and TN to 200 cm, and to try to find a rational approach to increase the SOC and TN stocks in cultivated farmland. The results showed that although SOC concentration and stock increased significantly at 0–20 cm along the chronosequence, the total SOC stock of the 0–200 cm layer was 13.8% lower in 30-year farmlands and 88.3% higher in 100-year farmlands than that in natural soil due to the corresponding decrease in 30-year farmlands and significant increase in 100-year farmlands at 40–200 cm depth. Cultivation also significantly increased TN concentration and stock at 0–20 cm, but the obvious increase of TN stock at 0–200 cm was only found in 100-year farmlands due to the slight change of TN stock at 40–200 cm except for 100-year farmlands. C and N stocks in the 40–200 cm layer of the cultivated farmlands contributed respectively 67.6 and 61.9% of the total SOC and TN stocks of 0–200 cm, further confirming the importance of C and N cycling in the deep layer. The continuous applications of N fertilizers combined with organic manures and irrigation with silt-laden river water contributed greatly to the increase of SOC and TN stocks. These results suggest that the oasis farmland has great potential capacity to accumulate SOC and TN only if long-term cultivation (> 30 years) and the enhancement of organic materials are implemented.