Pathways and mechanisms by which biochar application reduces nitrogen and phosphorus runoff losses from a rice...

Biochar application has the potential to reduce nitrogen (N) and phosphorus (P) losses in agricultural runoff, but little is known about how and to what extent biochar is effective in rice agroecosystems. In this study, in a typical double-rice cropping system, N and P runoff losses and soil carbon (C), N, and P contents (soil CNP contents) were observed under three different biochar application rates (0, 24, and 48 t ha^-1, which were defined as CK, LB, and HB, respectively) from 2017 to 2019. The results showed that the two-year averages of soil total organic C (TOC), total N (TSN), total P (TSP), available P (Olsen P), microbial biomass N (MBN), and microbial biomass P (MBP) contents were generally higher in the biochar treatments than in CK (P < 0.05). Specifically, the TSP, TOC, and MBN contents increased with the increasing biochar application rate, thus demonstrating the significant effects of biochar application on the paddy soil CNP contents and composition. The HB and LB treatments reduced the seasonal mean runoff flow-weighted total N (TN_wc) and total P (TP_wc) concentrations by 32.4% and 42.1%, respectively, compared to CK. Structural equation modeling (SEM) further revealed that the paths and mechanisms by which biochar reduced the TN_wc and TP_wc were different, depending on the different application rates. HB reduced the TN_wc mainly through the direct absorption of N, followed by the indirect inhibition of N mineralization, whereas LB decreased the TP_wc mainly through the strong P sorption capacity of the biochar. The direct effect of HB on the TN_wc was 1.58 times as strong as the indirect effect (path coefficients: -0.68 vs. 0.43, respectively), and the direct effect of LB on the TP_wc was 1.78 times as strong as the indirect effect (path coefficients: -0.89 vs. 0.50, respectively). Given the distinct pathways and mechanisms by which biochar reduced NP runoff losses, in practice, the biochar application rate should be optimized according to a targeted priority of reducing either N or P runoff losses in rice agroecosystems.