Effects of the combined application of biochar-based fertilizer and urea on N 2 O emissions, nitrifier, and denitrifier communities in the acidic soil of pomelo orchards

Purpose Nitrous oxide (N 2 O) is a greenhouse gas generated from nitrification and denitrification in the soil, and fertilizers may affect its emission. In this study, the effects of the combined application of a biochar-based fertilizer and urea on N 2 O emissions and nitrifier and denitrifier communities in the acidic soil of a pomelo orchard were investigated. Material and methods Six different treatments were used: no fertilizer (CK), urea only (N), and four concentrations of rice straw biochar-based fertilizer (2.5 g kg −1 , 5 g kg −1 , 10 g kg −1 , and 20 g kg −1 ) plus the same amount of urea (2.5BR, 5BR, 10BR, and 20BR). Soil samples were collected after 60 days for determination of soil chemical properties, N 2 O emissions, and the abundance of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nirS , nirK , and nosZ . Community diversity was investigated using Illumina MiSeq sequencing. Results and discussion 2.5BR significantly promoted N 2 O emissions, and 5BR, 10BR, and 20BR significantly reduced N 2 O emissions. N significantly reduced the abundance of AOA-amoA by 99.99%, and the Shannon index of AOA-amoA significantly decreased compared with that in CK. BR significantly restored the diversity of AOA-amoA . Compared with N, BR significantly reduced the soil NH 4 + -N concentration by 72.67 to 91.97% ( P < 0.05). The NO 3 − -N concentration increased significantly by 1.83 to 2.38 times ( P < 0.05), and the soil pH increased by 0.24 to 3.43 units. Correlation analysis showed that N 2 O emissions were negatively correlated with soil NH 4 + -N and positively correlated with soil pH and C/N. Redundancy analysis (RDA) showed that soil NH 4 + -N and pH were the main environmental factors affecting the nitrifier and denitrifier communities. Heatmap analysis suggested that Proteobacteria ( nirK ) and Rhodanobacter ( nosZ ) played important roles in N 2 O emissions. Conclusions 2.5BR significantly promoted N 2 O emissions, and 5BR, 10BR, and 20BR significantly inhibited N 2 O emissions. The promoting effect of 2.5BR was closely related to soil pH, Rhodanobacter ( nosZ ) and Proteobacteria ( nirK ). The sensitivity of AOA-amoA to BR and urea was higher than that of AOB-amoA . The application of urea alone was not conducive to the survival of AOA-amoA , but the combined application of BR significantly restored the diversity of the AOA-amoA community to its initial level.