Materials and methods This study was based on five treatments: CK (unfertilized control), NPK (chemical fertilizer alone), NPKS1 (NPK plus crop residues), NPKS2 (NPK plus double amounts of crop residues), and NPKM (NPK plus pig manure). Crop yield was determined by harvesting a defined area. Organic C was quantified by dry combustion. A two-step acid hydrolysis technique was used to quantify hydrolysable and non-hydrolysable C fractions.

Results and discussion All organic-treated plots produced significantly higher crop yields than the NPK plots, but only the manure treatment resulted in a significant increase in SOC compared with the NPK treatment after 21 years of experiment. It seems that the effects of organic amendments on SOC depend primarily on the type of organic materials when the application rates were relatively low. This indicated that organic amendments offer relatively short-term soil benefits for plant growth. The pig manure builds SOC over the long term, which provides secondary benefits while also sequestering C.

Conclusions Overall, manure integrated with mineral fertilizer should be recommended to maintain the SOC content and increase crop yield in the Mollisols.