Responses of soil organic carbon and crop yields to 33-year mineral fertilizer and straw additions under different tillage systems

Decline in soil organic carbon (SOC) is one of the most important causes of agricultural ecosystem degradation and food insecurity. Multiple agricultural management practices, such as fertilization and straw additions, affect cropland SOC; however, long-term effects of these practices remain ambiguous. In this study, we evaluated how mineral fertilizers, straw additions, and different tillage methods affected SOC and crop yields in a long-term field experiment (1985-2017) in the North China Plain (NCP). Nine treatments consisting of different tillage methods (CT:conventional tillage; NT: no-tillage) combined with applications of inorganic N (N-0, N-1 and N-2: 0 kg, 225 kg and 375 kg urea-N ha(-1) yr(-1)), inorganic P (P-0, P-1 and P-2: 0 kg, 75 kg and 150 kg P2O5 ha(-1) yr(-1)) and straw (S-0, S-1 and S-2: 0 kg, 2250 kg, and 4500 kg ha(-1) yr(-1)) were established. CT-N-0-P-0-S-0 was the control treatment. In all treatments, the SOC content increased rapidly over the first 15 years, which was mainly due to the fact that these plots were wastelands before. The SOC content of treatments with straw additions generally increased faster than treatments without straw additions. Nutrient additions and conventional tillage increased crop yields whereas straw additions had variable effects, with highest crop yields in the CT-N-2-P-2-S-2 treatment. Long-term applications of mineral fertilizer and straw significantly affected SOC and yields (P < 0.0001). Straw additions contributed more than nutrient additions to the explained variance of SOC (19.7 %). P and N contributed most to the total explained variance of wheat yields (43.2 % and 40.9 %, respectively) and maize yields (30.5 % and 38.7 %, respectively). The climate accounted for 23.8 % of the total explained variance of maize yields, however, it contributed only 1.1 % for wheat yields (P = 0.0064). In addition, crop yields were positively correlated with SOC levels. This means that long-term applications of straw can feasibly improve SOC and crop yields, which can satisfy the economic needs of farmers and can also benefit the environment. CT-N-1-P-1-S-2 was eventually identified as the optimal treatment to preserve soil quality and reduce fertilizer use, thereby maintaining the sustainable development of the wheat-maize agricultural system in NCP.