2.2.4 Straw return
The straw-returned crops included wheat, maize, soybean and rapeseed. The annual straw returned to the field was calculated based on the annual production of different crops, the ratio of straw to grain production and the rate of straw returned to the field, and then multiplied by its N, P and K content to calculate the inputs of N, P and K from straw returned to the field in the crop production system during the survey period (1992-2017). For additional details and parameters used in the calculations, see Table S2. The straw return input is calculated as follows:
Inputsr = Cy × Sg × Sn × R (3)
Where
Inputsr nutrient input of straw return for the county (kg);
Cy crop yields (kg);
Sg straw-to-grain yield ratios (%);
Sn nutrient content in straw (g kg-1);
R ratio of straw to field (%).