Effects of biochar, hydrochar and nitrogen fertilization on greenhouse gas fluxes, soil organic carbon pools, and biomass yield of a boreal legume grassland

Abstract Char amendment is an option to lower climatic impact of agricultural soils. However, their effect can vary depending on char and soil properties, vegetation type and their interactions. Nutrient poor and acidic soils of boreal region could benefit from char amendment. We conducted a three-month long mesocosm study representing a typical boreal forage-legume grassland to understand the effects of char application on greenhouse gas (GHG) emissions, soil organic carbon (SOC) pools and biomass yield. We examined biochar and hydrochar for changes in soil properties, gross nitrogen transformation rates, SOC and its fractions, biomass yield and all three major GHG fluxes. We assessed our results from two different perspectives; one, when chars were added at a uniform rate with fertilizer nitrogen (N) following the farmer’s practice and two, when chars were added based on the char C amount without fertilizer N. We show that only N 2 O emissions (not CO 2 and CH 4 ) were affected when chars were added at a uniform rate with fertilizer N. Biochar increased N 2 O emissions significantly compared to control whereas hydrochar restricted N 2 O relative to control and lowered significantly compared to biochar treatments. Biochar with N amendment significantly increased gross NO 3 − production (gross nitrification) and N 2 O emissions, indicating a linkage between increased nitrifier activity and N 2 O emissions. Hydrochar with N amendment showed lower gross nitrification rates and N 2 O emissions, indicating a reduced nitrifier activity and N 2 O emissions compared to biochar. Interestingly, hydrochar without N amendment showed lowest N 2 O emissions with few N 2 O uptake events and similar gross NO 3 − consumption and production rates, hinting an enhanced soil N 2 O reduction/sink mechanism, especially with actively photosynthesizing vegetation. Both chars increased soil particulate organic C (POC) significantly mainly owing to both chars themselves being carbon. The mineral associated organic C (MAOC) remained unaltered. Interestingly, we found significantly lower soil MAOC per unit of char C with biochar than with hydochar amendment, especially when endpoint soil MAOC was compared with initial soil MAOC. Our results suggest that destabilization of MAOC increased more with biochar than with hydrochar, especially with N fertilization and in the presence of actively photosynthesizing vegetation. This was further supported by a significantly greater rise in microbial biomass carbon with hydrochar than with biochar amendment. The total biomass yield remained unaffected. However, biochar with the applied N reduced the timothy grass yield compared to control, implying a reduced uptake of applied N by timothy. Our results shed light on the complex interactions among chars, soil, vegetation and N management. Therefore, future studies should focus on assessing the char amendment impacts including both plant and soil and at the whole agricultural field scale. Chars manufactured from diverse feedstocks need to be investigated for their impacts in diverse agricultural ecosystems, paving the way for their large-scale use. Graphical Abstract