Dynamics of soil carbon to nitrogen ratio changes under long-term fertilizer addition in wheat-corn double cropping systems of China

Soil carbon to nitrogen (C:N) ratio is one of the important properties of terrestrial ecosystems. Here, we report a study of soil C:N ratio dynamics in wheat-corn double cropping systems based on four long-term experimental sites in China: three in the temperate zone and one in the sub-tropical zone. We evaluate effects of long-term fertilizer input on soil organic carbon (SOC) and total nitrogen (TN) by comparing three treatments: no added fertilizer (the control), added nitrogen-phosphorus-potassium chemical fertilizers (NPK), and chemical fertilizers combined with manure (NPKM). Our study shows that SOC and TN had different responses to the treatments. There was an increasing trend in SOC, even without fertilizer. However, applying inorganic fertilizers only (NPK) did not maintain TN contents at some sites. The NPKM treatment resulted in a large increase in both SOC (35147%) and TN (33 to 10%) contents, relative to the initial values. The soil C:N ratio showed a significant increase over time at the sub-tropical site but little change at the three temperate sites. Our analysis showed similar C:N ratios (3738) in gross input of organic materials under the NPK treatments. However, the estimated C:N ratio during decomposition was much smaller at the sub-tropical site (23.7) than at the three temperate sites (44.048.2) under the NPK treatments, which may explain the increased soil C:N ratio at the sub-tropical site. Thus, we conclude that variations in soil C:N ratio are not caused by organic matter inputs but by decomposition in the wheat-corn double cropping systems.