Persisting effects of ligneous organic soil amendments on CO2, N2O and CH4 emissions in relation to moisture content in northern agricultural clay soil

In agriculture, organic soil amendments are envisioned to mitigate climate change through carbon sequestration. However, the full impact of the organic amendments on soil physico-chemical dynamics is still poorly understood. We conducted a laboratory incubation to assess the net climate effect of four ligneous organic amendments: two biochars (willow and spruce) and two fiber byproducts of paper and pulp industry. Soil samples were collected from a soil-amendment field experiment at Qvidja farm, South-West of Finland. Soil samples were sieved, air-dried and adjusted to 30%, 50%, 80% and 110% of water holding capacity (WHC), and incubated for 32 days in standard laboratory conditions. Greenhouse gas (GHG) emissions were measured after 1, 5, 12, 20 and 33 days. 

The carbon dioxide (CO₂) emissions were highest at 80% WHC, and lowest at severely water stressed conditions at 30% WHC. The organic amendments did not have an observable effect on CO₂ dynamics. The CO₂ emissions correlated linearly with soil moisture and microbial biomass nitrogen. Nitrous oxide (N₂O) emissions were systematically lower in the amended soils compared to the control soil, and independent of soil nitrate concentrations. Without organic amendments, N₂O emissions increased exponentially with soil moisture content. Methane (CH₄) emissions fluctuated throughout the incubation, exhibiting mostly negative values. Consequently, CH₄ emissions played only a minor role in the GHG budget.

CO₂, N₂O and CH₄ emissions, calculated as CO₂ equivalent, exhibited a linear correlation with the moisture gradient. CO₂ dominated the GHG budget up to a moisture of 80% WHC, but was superseded by N₂O emissions at 110% WHC. The results indicate that soil moisture content is critically affecting the GHG emissions and that while organic soil amendments may have persisting effects on GHG exchange, they primarily occur through N₂O dynamics.