Reduction potential and environmental carrying capacity of straw and livestock manure returning to fields in Hubei Province of China

The large-scale breeding level of livestock and poultry industry in Hubei Province has been significantly improved, and the amount of straw resources has been increased year by year. Returning agricultural wastes to the field can not only reduce agricultural non-point source pollution, but also reduce the application of chemical fertilizers. Based on relevant statistical data and literature, this study collected the amount of livestock and poultry (i.e., pigs, beef cattle, dairy cows, sheep, broilers, layers) in stock, market and growth cycle, and the planting area and economic yield of various crops (i.e., the low hilly area in southeastern Hubei, the low hilly area in northern Hubei, the mountainous area in northwestern Hubei, the plain area in central Hubei, and the mountainous area in southwestern Hubei). Based on the statistical data from 2019 to 2020, We calculated the nutrient supply of livestock and poultry waste, and further evaluated whether the current situation of livestock and poultry breeding in Hubei Province exceeded the maximum allowable amount of soil carrying livestock and poultry waste. According to the ratio of straw to grain of different crops and nutrient content of straw, we analyzed the nutrient resources and the theoretical nutrient returning amount of crop straw. The land carrying capacity index of livestock was the highest in Northern Hubei, reaching to 0.35-0.78. The land carrying capacity index of Central Hubei was only 0.17-0.54, indicating that there would be a large space to develop livestock and poultry breeding. In 2019, the nutrient resources of livestock and poultry waste in Hubei Province were 368 900 t N, 140 300 t P2O5 and 520 600 t K2O, respectively. Based on 65% of livestock manure and fertilizer returning to the field, the total nutrient returning to the field of livestock and poultry manure would be 239 800 t N, 91 200 t P2O5 and 337 700 t K2O, respectively, with the ratio of fertilizer replacement of 17.3%, 11.9% and 56.2%, respectively. The total amount of straw resources of main crops in Hubei Province was the highest in Central Hubei and the lowest in Northwest Hubei. At present, the nutrient resources of straw in Hubei province were 310 700 t N, 99 800 t P2O5 and 683 000 t K2O, and the proportion of theoretical fertilizer replacement were 22.53%, 13.11% and 113.97%, respectively. The livestock and poultry production in different regions of Hubei province did not exceed the local maximum carrying capacity. The livestock and poultry breeding volume in different regions of Hubei Province did not exceed the local maximum carrying capacity. The theory of returning major agricultural wastes in Hubei Province can reduce nitrogen fertilizer consumption by 39.8% and phosphate fertilizer consumption by 25.0%. The total K amount from agricultural wastes return to the field could theoretically meet the demand for potassium of main crops. By calculating the nutrient resources of the main agricultural wastes (i.e., livestock and poultry waste, and straw) in Hubei Province, we evaluated the potential of fertilizer reduction of agricultural waste returning to the field and the carrying capacity of livestock and poultry waste soil, providing theoretical basis and data support for agricultural green development in Hubei Province.