The impact of applying 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 the green and sustainable development of the livestock industry in China. The degree of treatment livestock wastewater undergoes 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 yields and the economic benefits of their application are not yet fully understood. In this study, we selected various types of livestock wastewater produced at different stages of the treatment process in an intensive dairy farming operation as our experimental subjects. Specifically, we investigated the effects of mixture of manure and urine, digestated pulp, AO process effluent, and liquid digestate from oxidation pond, as well as the impacts 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 applying livestock wastewater at a 50% nitrogen substitution rate could increase crop yields by an average of 12.25%, whereas the 100% nitrogen substitution treatments posed a risk of yield reduction. Among these, the highest crop yields were observed with the 50% nitrogen equivalent substitution with AO process effluent. This suggests that partial substitution of synthetic nitrogen fertilizers with livestock wastewater can enhance crop productivity under certain conditions. For soil fertility, the application of a manure and urine mixture had a significant positive effect, particularly in terms of increasing soil total nitrogen and organic matter. In the 100% nitrogen substitution treatments, soil total nitrogen and organic matter levels were consistently higher than in the 50% substitution treatments. This suggests that full nitrogen substitution using livestock wastewater contributes substantially to soil enrichment, which could 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, while 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 digested pulp produced the best economic returns, reaching up to 38.34 ¥·t⁻¹. Thus, from an economic perspective, the 50% nitrogen substitution treatment with digested pulp is the optimal strategy for field application. 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.