Long-Term Effect of Fertilizations on Yield Sustainability, Soil Organic Carbon Sequestration and Apparent Phosphorus Balance in Acidic Paddy Soil

Fertilizer management in an intensively managed cropland is vital to achieve sustainable crop yield. Yet, data on the mechanism of yield sustainability response to manure substitution is limited. We quantitatively studied yield sustainability through soil nutrients’ stoichiometry, carbon (C) sequestration, and apparent phosphorus (P) balance in a 35-year long-term trial under double rice cropping pattern in Southern China. The study consisted of five treatments: CK (control), NPK (chemical nitrogen, phosphorus, and potassium fertilizers), NPKM1 (30% manure; 70% chemical NPK fertilizers), NPKM2 (50% manure; 50% chemical NPK fertilizers), and NPKM3 (70% manure; 30% chemical NPK fertilizers). The combined treatments increased yield trend, sustainability index (SYI), C sequestration, P uptake, and sensitivity index, but had little effect on the coefficient of variation of the crop yield compared with the NPK treatment. The apparent P balance in fertilizer treatments has exceeded the environmental risk threshold of 35 kg ha −1 year −1 . The path analysis showed that soil organic carbon (SOC) directly affected total P (TP), while SOC and TP indirectly affected yield by directly changing soil C:N:P stoichiometry. The critical values of C:N, C:P, and N:P ratios above which SYI declined were 10.8, 23.0, and 1.7, respectively. We concluded that organic amendment substitution had significant effect on yield sustainability and C sequestration, functioned as a C-sink, and revealed potential for P buildup. Therefore, substitution of chemical fertilizers by manure should be done with caution to avert excess P accumulation and loss to the environment. Also, the critical values of soil C:N:P stoichiometric ratios should be taken into cognizance during paddy soil management.