RESUMO
The feedbacks between crop yield and soil organic carbon and total nitrogen contents were examined in a long-term experiment, which was established on black loessial soil on the Loess Pla-teau in China. There were six treatments, including CK (no fertilizer), N (single nitrogen fertilizer), NP (chemical fertilizers NP), SNP (straw and chemical fertilizers NP), M (organic manure) and MNP (organic manure and chemical fertilizers NP). Results showed that balanced application of chemical fertilizers, single application of organic manure, the combined application of chemical fertilizers with organic manure and chemical fertilizers coupled with straw returning to the field all significantly increased crop yield and its stability compared with control (CK). The yields of maize and wheat in NP, SNP, M and MNP treatments increased by 92%, 97%, 93%, 141% and 147%, 164%, 139%, 214%, respectively, compared with the control. The annual mean yields of maize and wheat in NP treatment were equal to or higher than those of the local conventional fertilization practices and quite stable among different years, which indicated that the fertilization rates with N 90 kg·hm-2 and P2O5 75 kg·hm-2 were enough for crop growth in wheat-maize rotation system. Application of chemical fertilizer P every other year combined with straw returning to the field (SNP) had similar crop yield values with NP treatment, with the P application amount could be reduced by 50%. The balanced application of chemical fertilizers, organic manure application, the combined application of chemical fertilizers with organic manure, and chemical fertilizers coupled with straw returning to the field could significantly increase soil organic carbon content, whereas chemical fertilizer application had no significant influence on soil total nitrogen content. Across all treatments, the contents of soil organic carbon and total nitrogen were significantly and positively correlated. Under different fertilization treatments, organic carbon sequestration rate was between 15% and 41%. In SNP treatment, the soil organic carbon content enhanced 0.06 g·kg-1 when the amount of organic carbon input every increased 1 t·hm-2, while in CK, N, NP, M and MNP treatments, the increments was between 0.12 and 0.15 g·kg-1. The yields of both maize and wheat were positively correlated with soil total nitrogen content. Maize yield constantly increased with the increases of soil organic carbon content, but wheat yield increased at first and then kept stable with the increases of soil organic carbon content, with a threshold of 6.8 g·kg-1. In conclusion, long-term balanced application of chemical fertilizers, organic manure application, chemical fertilizers combined with manure and chemical fertilizers coupled with straw returning to the field could significantly increase soil organic carbon and total nitrogen contents, consequently resulted in higher crop yield and stubble amount returned to soil, the increase of stubble returned to soil further led to the increase of soil organic carbon and total nitrogen contents, which formed the mutual promotion feedback relationship each other in the black loessial soil region of Loess Plateau in China.
