Soil organic carbon dynamics under long-term fertilizations in arable land of northern China.

Soil organic carbon (SOC) data were collected from six long-term experiment sites in the upland of northern China. Various fertilization (e.g. inorganic fertilizations and combined inorganic-manure applications) and cropping (e.g. mono- and double-cropping) practices have been applied at these sites. Our analyses indicate that long-term applications of inorganic nitrogen-phosphorus (NP) and nitrogen-phosphorus-potassium (NPK) result in a significant increase in SOC at the sites with the double-cropping systems. The applications of inorganic NP and/or NPK combined with manure lead to a significantly increasing trend in SOC content at all the sites. However, the application of NPK with crop residue incorporation can only increase SOC content in the warm-temperate areas with the double-cropping systems. Regression analyses suggest that soil carbon sequestration responds linearly to carbon input at all the sites. Conversion rates of carbon input to SOC decrease significantly with an increase of annual accumulative temperature or precipitation, showing lower rates (6.8%-7.7%) in the warm-temperate areas than in the mid-temperate areas (15.8%-31.0%). [ABSTRACT FROM AUTHOR]