Widespread Soil Organic Carbon Accumulation on Fertilized Cropland: Evidence from Long-Term Experiments Across China.

Soil organic carbon (SOC) plays an important role in sustainable agriculture and climate mitigation. We studied SOC dynamics on China’s cropland based on 25 long-term experiments (LTEs) that were established 20-30 years ago. We analyzed SOC change rates in the top 20cm in response to three common fertilization treatments: mineral nitrogen, phosphorus, and potassium fertilizers (NPK), NPK fertilizers plus farmyard manure (NPKM), and NPK fertilizers combined with straw incorporation (NPKS). On average, SOC stock increased by 0.13, 0.34 and 0.57 Mg C/ha/yr for NPK, NPKS and NPKM, respectively. The greatest increase for each treatment was found in north China for NPK (0.33 Mg C/ha/yr), Yangtze River basin for NPKS (0.49 Mg C /ha/yr), and north and northwest China for NPKM (0.69 Mg C/ha/yr). The smallest SOC increase was in northeast China for all the treatments (0.00, 0.19, 0.44 Mg C/ha/yr for NPK, NPKS and NPKM, respectively). Assuming that 50% of China’s cropland, i.e., ~65 M ha, has been accumulating SOC at a rate similar to that under the NPK fertilization, total carbon sequestration as SOC on China’s cropland would be 8.6 Tg C/yr over the past 2-3 decades. On the other hand, if NPKS fertilization is applied to 100% of China’s cropland, total carbon sequestration may reach 44.5 Tg C/yr, which is about 1.7% of the global land carbon sink.