Greenhouse gas emission responses to different soil amendments on the Loess Plateau, China

Soil amendments are commonly used to reduce greenhouse gas (GHG) emissions by altering the physical, biological, and chemical qualities of soil, but the mechanism by which this is achieved still vague. This two-year study surveyed the influences of biochar (BC), Bacillus mucilaginosus (BM), and Bacillus subtilis (BS) on the physical, enzyme activities, soil microbial and GHG emissions. Results indicated that soil amendment addition decreased the activities of β-glucosidase (BG), cellobiohydrolase (CBH), β-xylosidase (BX), sucrase, cellulase by 8.81%, 8.99%, 22.08%, 59.46%, 74.61%, respectively, and increased N-acetylglucosaminidase (NAG) activity by 155.00%. Soil amendments had little effect on soil microbes, apart from those related to the C and N cycles, such as Proteobacteria, Mortierellomycota and Acidobacteriota. In addition, BC, BM, and BS amendments decreased mean cumulative CO2 emissions by 38.43%, 11.83%, and 38.54%, respectively. Cumulative N2O emissions from the BC treatment increased by 28.17% in 2019 and decreased by 37.22% in 2020. For the BM and BS treatments, cumulative N2O emissions did not significantly change in 2019 but decreased by 4.93% and 57.85% in 2020, respectively. Generally, soil amendments decreased the greenhouse emission intensity (GHGI) and global warming potential (GWP). Moreover, a structural equation model (SEM) revealed factors that directly or indirectly controlled GHG emissions, including NAG, BX, CBH, Proteobacteria, Mortierellomycota and Acidobacteriota. These factors were limited mainly by soil C and N substrates, such as ammonium-nitrogen (NH4+-N), available phosphorus (AP), and soil organic matter (SOM). Overall, soil amendments can reduce GHG emissions in the long term, but they are affected by many factors.However, soil amendments alone have limited effects on reducing GHG emissions on the Loess Plateau. Suggesting combining and applying biochar and plant growth-promoting rhizobacteria (PGPR) may enhance soil quality and alleviate GHG emissions.