Wetter farming: raising water table and biochar for reduced GHG emissions while maintaining crop productivity in agricultural peatlands

Despite their high agricultural productivity, drained and cultivated peats are highly susceptible to degradation and significant sources of greenhouse gas (GHG) emissions. This study investigates the potential of water table manipulation and biochar application to mitigate GHG losses from agricultural peats. However, balancing the need for agricultural production with securing the ecosystem function of the peat under high water table (WT) conditions poses a significant challenge. Therefore, we grew lettuce in a controlled mesocosm experiment with either a high (HW) or low (LW) water table and monitored emissions of CO2, CH4 and N2O over 4 months using a mesocosm method. Concurrent measurements of soil solution, plant measurements and microbial sequencing allowed identification of the key controls on GHG emissions. Raising the WT significantly reduced CO2 emissions (18%), and N2O emission (40%), but eventually increased CH4 emission (2.5-fold) compared to the Control + LW. Biochar amendment with raised WT provided the strongest reduction in CO2 equivalent GHG emission (4.64 t CO2eq ha−1 yr−1), compared to Control + LW. We found that biochar amendment modified the microbial community composition and diversity (Shannon index 8.9–9.3), lowering the relative abundance of peat decomposers (such as Ascomycota). Moreover, biochar amendments produced 38–56% greater lettuce biomass compared to the unamended controls, irrespective of water table level, suggesting that biochar application could generate economic benefits in addition to reduced GHG emissions. Mechanisms responsible for these effects appeared to be both abiotic (e.g. via effects of the biochar physicochemical composition) and biotic via changing the soil microbiome. Overall, the combination of high-water table and biochar amendment enhanced total soil C, reduced peat decomposition, suppressed CH4 and N2O emissions, and enhanced crop yields.