Effects of biochar-biogas slurry application on soil microorganisms and functional genes

Soil microorganisms are key drivers of soil nutrient cycling. Understanding the response characteristics of the soil microenvironment under the combined application of biochar and biogas slurry is crucial for improving soil health, enhancing soil quality, and increasing the sustainability of soil for agricultural use. This study used biochar made from animal-derived organic materials and biogas slurry, with a focus on a dryland red-yellow soil cultivated for five consecutive years. The experimental design included conventional fertilization (C0B0), biogas slurry substituting for nitrogen fertilizer (C0B1), 2% biochar (C1B0), and a combined application of 100% biogas slurry substituting for nitrogen fertilizer plus 2% biochar (C1B1), alongside a control with no fertilization (CK). The aim was to explore the effects of biochar and biogas slurry, as substitutes for chemical fertilizers, on soil fertility, carbon sequestration, and the microbial communities and functional genes involved in nitrogen metabolism in dryland soils. The results indicated that compared to the CK and C0B0 treatments, both the separate and combined applications of biochar and biogas slurry significantly increased soil pH, with the C1B1 treatment showing the most substantial increase, up to 35.9%. The C1B1 treatment also demonstrated the most notable increase in total carbon and total nitrogen contents, being 3.4 times and 3.5 times higher than those of CK, respectively. Additionally, the organic carbon content in C1B1 was significantly higher by 39% compared to C1B0. In the C0B1 treatment, nitrate nitrogen content increased significantly by 77.5% compared to C0B0, while ammonium nitrogen content in the C1B0 treatment increased significantly by 21.1% compared to C0B1. The highest abundance of dominant microbial genera associated with carbon fixation and nitrogen metabolism was observed under C1B1 conditions, with increases of 64.0% and 65.7%, respectively, compared to CK. Relative to C0B0, C1B1 significantly increased the abundance of genes associated with carbon fixation and nitrogen metabolism, with acsE and nxrA relative abundances increasing by 97.4% and 54.0%, respectively. The abundance of carbon-fixing genera was significantly positively correlated with the functional genes pccA and frdA, while nitrogen-metabolizing genera showed a significant positive correlation with nxrA, narG, napA, nirK3, and nirS1, and a significant negative correlation with amoA2 and nosZ2. In summary, the combined application of biochar and biogas slurry as a substitute for chemical fertilizers can significantly enhance soil nutrient content, promote microbial biomass, and increase the abundance of genes related to carbon fixation and nitrogen metabolism, thereby contributing to the regulation of soil microecosystem health.