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 N₂O emissions (not CO₂ and CH₄) were affected when chars were added at a uniform rate with fertilizer N. Biochar increased N₂O emissions significantly compared to control whereas hydrochar restricted N₂O relative to control and lowered significantly compared to biochar treatments. Biochar with N amendment significantly increased gross NO₃⁻ production (gross nitrification) and N₂O emissions, indicating a linkage between increased nitrifier activity and N₂O emissions. Hydrochar with N amendment showed lower gross nitrification rates and N₂O emissions, indicating a reduced nitrifier activity and N₂O emissions compared to biochar. Interestingly, hydrochar without N amendment showed lowest N₂O emissions with few N₂O uptake events and similar gross NO₃⁻ consumption and production rates, hinting an enhanced soil N₂O 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.

Keywords

Biochar and hydrochar / Boreal legume-forage / Nitrogen fertilization / POC and MAOC / Greenhouse gases / Gross nitrogen transformation

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Funding

Maa- ja MetsätalousministeriÖ(VN/17097/2022)

Walter Ahlströmin Säätiö(VN/17097/2022)

Biotieteiden ja Ympäristön Tutkimuksen Toimikunta(334422)

RIGHTS & PERMISSIONS

The Author(s)