Soil fertility and crop yield after manure addition to acidic soils in South China

In agro-ecosystems, the relationship between soil fertility and crop yield is mediated by manure application. In this study, an 8-year field experiment was performed with four fertilizer treatments (NPK, NPKM1, NPKM2, and NPKM3), where NPK refers to chemical fertilizer and M-1, M-2, and M-3 refer to manure application rates of 15, 30, and 45 Mg ha(-1) year(-1), respectively. The results showed that the NPKM (NPKM1, NPKM2, and NPKM3) treatments produced greater and more stable yields (4.95-5.45 Mg ha(-1) and 0.59-0.75) than the NPK treatment (4.01 Mg ha(-1) and 0.50). Crop yields under the NPKM treatments showed two trends, with a rate of decrease of 0.48-0.83 Mg ha(-1) year(-1) during the first 4 years and a rate of increase of 0.10-0.25 Mg ha(-1) year(-1) during the last 4 years. The soil organic carbon (SOC) significantly increased under all treatments. The estimated annual SOC decomposition rate was 0.35 Mg ha(-1) year(-1) and the equilibrium SOC level was 6.22 Mg ha(-1). Soil total nitrogen (N), available N, total phosphorus (P) and available P under the NPKM treatments increased by 0.15-0.26, 15-33, 0.17-0.66 and 45-159 g kg(-1), respectively, compared with the NPK treatment. Manure application mainly influenced crop yield by affecting the soil TN, available N, and available P, which accounted for up to 64% of the crop yield variation. Taken together, applying manure can determine or at least improve the effects of soil fertility on crop yield in acidic soils in South China.