Straw combined with siderite enhanced nitrogen retention in biogas slurry returning soil

The biogas slurry application and raw straw returning to soil have become common agricultural practices. However, there is limited knowledge regarding the impact of adding raw straw on nitrogen retention in biogas slurry returning soil. This study compared the effects of different straw pretreatments, including raw straw (RS), biochar (BC), straw plus biochar (SBC), and straw plus siderite (SS), on nitrogen retention in biogas slurry (BS) returning soil. Among these straw pretreatment methods, the SS addition demonstrated the best nitrogen retention, and the cumulative leaching amount of total nitrogen (TN) in soil leaching solution significantly decreased by about 36.32 % (P < 0.05) compared with the control. The RS addition could reduce cumulative TN leaching by 23.23 % (P < 0.05). Differently, the addition of BC and SBC led to an increase of cumulative TN leaching by 21.39 % and 8.59 %, respectively. It was noted that the coexistence of dissimilatory nitrate reduction to ammonium and denitrification was found in RS addition. Moreover, microbial evidence confirmed the addition of straw plus siderite could induce more participation of N-cycle-related bacteria, particularly for Bacteroidota and Anaeromyxobacter, as well as functional genes such as NirK, NorBC, and NosZ. Taken together, straw plus siderite was recognized as one promising method for enhancing nitrogen retention in biogas slurry returning soil, considering its significant environmental and long-term economic benefits (net benefits about 56.4 USD/ha).