Variation trend of soil organic carbon, total nitrogen and the stability of maize yield in black soil under long-term organic fertilization

Objectives This study investigated the different influences of long-term organic, chemical fertilizers and their combined fertilization on soil organic carbon storage and yield stability in northeast black soil region, which would provide valuable reference for obtaining high and stable yield and culturing black soil fertility in Northeast China.

Methods The surveyed long-term fertilization experiment was located Gongzhuling City of Jilin Province, started in 1980. The chosen treatments included no fertilizer control (CK), balanced chemical fertilization (N–P₂O₅–K₂O 150–75–75 kg/hm², NPK), applying manure 30 t/hm², i.e. N 150 kg/hm², P₂O₅ 135 kg/hm² and K₂O 45 kg/hm²(M₂), applying double amount of manure (M₄), chemical and manure combination M₂NPK and M₄ NPK. The maize yield, soil organic carbon and total nitrogen contents were recorded annully.

Results The highest maize grain yield was in M₄NPK treatment, with an average yield of 9637 kg/hm², followed by M₂NPK, with an average yield of 9422 kg/hm², and the lowest in CK treatment, with an average yield of 3551 kg/hm², significantly lower than other treatments. For the first 10 years' consecutive organic fertilization, the soil basic fertility had been significantly improved, and the dependence of maize yield on fertilizer was reduced, the yields in M₂ and M₄ treatments were similar to that in NPK treatment. From then to 2017, the maize yield was annually increased by 3.8% on average in M₂ and M₄ treatments, compared with that in NPK. The fitting equation showed that the contribution rate of fertilizer would be decreased by 9.2% to 12.2% for every 1000 kg/hm² increase in base yield. By the practical monitoring, the contribution of fertilizer to yields stated to decrease after 30 years' organic fertilization. In the organic and inorganic combination treatments, the variation coefficient of maize yield was 19.3% in averaged, and the yield sustainability index SYI was as high as 0.58, which was ranking in high stability level. The difference of soil organic C content among the treatments gradually increased with fertilization years, and became significant since the 11th year. Soil organic carbon content increased significantly in treatments of M₄NPK and M₄. The soil total nitrogen was significantly and positively correlated with soil organic carbon (r = 0.826**). For every 1 g/kg of increase in soil organic carbon would bring an increase of 0.086 g/kg of total nitrogen. Grain yield was significantly and positively correlated with organic carbon and total N.

Conclusions Under the experimental condition, pure organic fertilization mainly contributes to the improvement of soil organic carbon pool at the early stage, and would not increase the yield until the soil organic carbon arriving certain level. The combination of organic and chemical fertilizer could increase soil carbon and nitrogen pools rapidly, and achieve high and stable yield increase. With organic and chemical combination, the annual soil organic carbon increase is 0.35–0.47 g/kg, total nitrogen by 46.3%–84.2%, and maize yield stable index reachs high level (SYI = 0.58). Improving soil fertility can reduce the dependence of maize yield on exogenous fertilizers. The combination rate of fertilizer would be decreased by 9.2%–12.2% for every 1000 kg/hm² of increase in indigenous soil yield. Therefore, the combination of organic and chemical fertilizers should be maintained for keeping increase of soil fertility, which is the base of high and stable yield and sustainable productivity of black soil in Northeast China.