Unique spatial niches created by biochar form a dynamic biogeochemical soil zone, termed the “charosphere”. Charosphere soil has different properties from either soil or biochar, often acting as a hotspot for microbial activity. However, the direction and magnitude of charosphere effects on soil processes and microbial communities remain inconclusive. Herein, we designed a multi-sectional box to separate charosphere soils from biochar under waterlogged conditions, investigating millimeter-scale changes in bacterial communities and distributions of specific genera across the charosphere created by pristine biochar (produced from corn stover at 300 °C), acid-modified biochar and washed biochar. The pristine biochar did not increase bacterial α-diversity but altered bacterial composition by promoting the growth of specific bacterial taxa and suppressing other species. In comparison, washed and acid-modified biochar increased the bacterial α-diversity. The pristine biochar significantly decreased N2O emission and NO3− concentration owing to a lower soil nitrification potential. Abundances of N-cycling functional taxa were higher in the near-charosphere than outer-charosphere zone, especially for Bacillus (N2-fixation), MND1 (ammonia oxidation) and Rhodopseudomonas (denitrification). The spatial distribution of functional genes and genera documents the importance of heterogeneity within the charosphere gradient, providing a novel perspective into functional geometry to understand how biochar reduces N losses from agroecosystems.

Net nitrogen mineralization (N_min) and nitrification regulate soil N availability and loss after severe wildfires in boreal forests experiencing slow vegetation recovery. Yet, how microorganisms respond to postfire phosphorus (P) enrichment to alter soil N transformations remains unclear in N-limited boreal forests. Here, we investigated postfire N-P interactions using an intensive regional-scale sampling of 17 boreal forests in the Greater Khingan Mountains (Inner Mongolia-China), a laboratory P-addition incubation, and a continental-scale meta-analysis. We found that postfire soils had an increased risk of N loss by accelerated N_min and nitrification along with low plant N demand, especially during the early vegetation recovery period. The postfire N/P imbalance created by P enrichment acts as a "N retention" strategy by inhibiting N_min but not nitrification in boreal forests. This strategy is attributed to enhanced microbial N-use efficiency and N immobilization. Importantly, our meta-analysis found that there was a greater risk of N loss in boreal forest soils after fires than in other climatic zones, which was consistent with our results from the 17 soils in the Greater Khingan Mountains. These findings demonstrate that postfire N-P interactions play an essential role in mitigating N limitation and maintaining nutrient balance in boreal forests.

Net nitrogen mineralization (N_min) and nitrification regulate soil N availability and loss after severe wildfires in boreal forests experiencing slow vegetation recovery. Yet, how microorganisms respond to postfire phosphorus (P) enrichment to alter soil N transformations remains unclear in N-limited boreal forests. Here, we investigated postfire N-P interactions using an intensive regional-scale sampling of 17 boreal forests in the Greater Khingan Mountains (Inner Mongolia-China), a laboratory P-addition incubation, and a continental-scale meta-analysis. We found that postfire soils had an increased risk of N loss by accelerated N_min and nitrification along with low plant N demand, especially during the early vegetation recovery period. The postfire N/P imbalance created by P enrichment acts as a "N retention" strategy by inhibiting N_min but not nitrification in boreal forests. This strategy is attributed to enhanced microbial N-use efficiency and N immobilization. Importantly, our meta-analysis found that there was a greater risk of N loss in boreal forest soils after fires than in other climatic zones, which was consistent with our results from the 17 soils in the Greater Khingan Mountains. These findings demonstrate that postfire N-P interactions play an essential role in mitigating N limitation and maintaining nutrient balance in boreal forests.