Continuous Biogas Slurry Substitution of Chemical Fertilizer with Wheat/Maize Straw Return Alters Surface and Subsoil Physicochemical and Aggregate Properties Differently

Biogas slurry is increasingly adopted as a substitute for chemical fertilizer to improve soil fertility and control soil degradation. But little is known about its long-term effects on different soil layers. This study aims to test if biogas slurry substitution alters soil nutrients and aggregates differently in different layers and how the effects are influenced by straw return. A five-year field trial of a winter wheat–summer maize rotation was established to evaluate the effects of biogas slurry substitution [treatments: control without fertilizer (CK); chemical fertilizer (CF); 50% biogas slurry substitution (BSCF); 100% biogas slurry substitution (BS)] on two soil layers (0–20 and 20–40 cm) under equal N, P, and K fertilization in a Fluvisol. The result showed that compared with CK, fertilization was necessary to maintain soil available nitrogen (AN) and available potassium (AK) in the 0–20 cm layer. Also, fertilization had positive effects on soil EC, TN, and AP, especially in the topsoil. In terms of increasing the degree of water-stable macroaggregation, improving aggregate stability and reducing their crushing rates, CF was superior to biogas slurry substitution because of the greater accumulations of soil N, the most important physicochemical factor regulating soil macroaggregation and stability. The effects of biogas slurry substitution changed with soil layers. In surface soil, BD, water-stable macroaggregates, and aggregate stability significantly increased due to the positive regulatory effects of straw, AP, and EC; whereas in subsoil, they decreased. Inorganic fertilization with straw return functions well in soil structural degradation control and N fertility improvement, especially in the topsoil. Whereas in fields without straw return, biogas slurry substitution may be a feasible and economical choice to improve topsoil structure and also an excellent supplement to provide P and K.