Impact of applicaiton of different types of livestock wastewater on soil fertility, nitrogen balance, and economic benefits of silage corn

The treatment and utilization of livestock wastewater are major challenges in promoting green and sustainable development of the livestock industry in China. The degree of treatment of livestock wastewater, as it transitions from standard discharge to resource utilization, has a substantial impact on its nutrient characteristics. However, the effects of different types of livestock wastewater on crop yield and the economic benefits of their application are not yet fully understood. In this study, various types of livestock wastewater produced at different stages of the treatment process during intensive dairy farming operations were selected. We investigated the effects of mixture of manure and urine, digestated pulp, AO (anoxic-oxic) process effluent, and liquid digestate from oxidation pond, as well as the effects of different application rates (50% nitrogen substitution, 100% nitrogen substitution, and irrigation with no nitrogen substitution) on crop yield, soil fertility, and economic returns. The results indicated that T2 and T3 treatments applying livestock wastewater at a 50% nitrogen substitution rate could increase crop yield by an average of 12.24%, whereas the 100% nitrogen substitution treatments posed a risk of yield reduction. Among these, the highest crop yield was observed with a 50% nitrogen equivalent substitution with AO process effluent, suggesting that the partial substitution of chemical nitrogen fertilizer with livestock wastewater can enhance crop productivity under certain conditions. The application of a mixture of manure and urine had a significant positive effect on soil fertility, particularly in terms of increasing the contents of soil total nitrogen and organic matter. Under the 100% nitrogen substitution treatments, soil total nitrogen and organic matter levels were generally higher than those in the 50% substitution treatments. This suggests that 100% nitrogen substitution using livestock wastewater contributes substantially to soil enrichment and can support sustainable soil health over time. In terms of economic costs, applying livestock wastewater at a 50% nitrogen substitution rate incurred a cost of approximately 12.5 to 13.5 ¥·t⁻¹, whereas the 100% nitrogen substitution treatments were more economical, with costs ranging from 5.0 to 7.0 ¥·t⁻¹. However, when considering both crop yield and the market price of agricultural products, the 50% nitrogen equivalent substitution treatment with digestated pulp produced the best economic returns, reaching up to 38.3 ¥·t⁻¹. Thus, from an economic perspective, a 50% nitrogen substitution treatment with digestated pulp is the optimal strategy for field applications. This approach balances the benefits of improved crop yield, enhanced soil fertility, and maximized economic gain, making it a promising method for sustainable agricultural development.