Responses of crop productivity and physical protection of organic carbon by macroaggregates to long-term fertilization of an Anthrosol: Crop productivity and SOC sequestration

Soil organic carbon (SOC) sequestration in the soil of arable land has been recommended as a 'win-win strategy' for improving soil quality and crop productivity, and for mitigating the effects of climate change. With data from a 35-year field experiment, we investigated the effects of organic manure and synthetic fertilizers on the mechanisms of organic carbon (OC) protection in macroaggregates and the relations between crop productivity and OC under an intensive wheat (Triticum aestivum L.) and maize (Zea mays L.) cropping system on an Anthrosol in northern China. The study involved nine treatments consisting of different combinations of synthetic fertilizers and organic manure. Soil samples taken from the 0-10-cm and 10-20-cm layers after 35years of experimentation were separated into four physical fractions within the macroaggregates and analysed for changes in OC. In comparison with the control without any addition of fertilizer, long-term application of nitrogen and phosphorus had no effects on SOC and OC contents of the macroaggregate fractions. In contrast, the application of organic manure significantly increased SOC and OC contents of macroaggregate fractions, apart from mineral-associated organic carbon (MOC). Soil organic carbon was strongly and positively correlated with OC content in the intra-microaggregate particulate organic carbon fraction (iPOC), indicating that SOC sequestration occurred primarily in the form of the physically protected particulate organic carbon fraction in the Anthrosol investigated here. A quadratic-plateau model described the relation between crop yields and SOC well, and the critical SOC content, above which there was no yield response to increasing SOC, was about 9 g kg(-1), or 25 Mg ha(-1). Thus, the input of available C should be adjusted once this critical SOC content has been reached to guarantee the desired level of soil productivity and efficiency of SOC sequestration.