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

Abstract

<p>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. </p><p>The carbon dioxide (CO<sub>2</sub>) 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 CO<sub>2</sub> dynamics. The CO<sub>2</sub> emissions correlated linearly with soil moisture and microbial biomass nitrogen. Nitrous oxide (N<sub>2</sub>O) emissions were systematically lower in the amended soils compared to the control soil, and independent of soil nitrate concentrations. Without organic amendments, N<sub>2</sub>O emissions increased exponentially with soil moisture content. Methane (CH<sub>4</sub>) emissions fluctuated throughout the incubation, exhibiting mostly negative values. Consequently, CH<sub>4</sub> emissions played only a minor role in the GHG budget.</p><p>CO<sub>2</sub>, N<sub>2</sub>O and CH<sub>4</sub> emissions, calculated as CO<sub>2</sub> equivalent, exhibited a linear correlation with the moisture gradient. CO<sub>2</sub> dominated the GHG budget up to a moisture of 80% WHC, but was superseded by N<sub>2</sub>O 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 N<sub>2</sub>O dynamics.</p>