Effects of organic soil amendments on soil greenhouse gas exchange under controlled soil moisture conditions

Organic soil amendments are proposed to mitigate climate change and support soil fertility by introducing recalcitrant carbon into soil. However, the full impact of recycled biosolids on soil greenhouse gas (GHG) dynamics are still unknown. We conducted a laboratory incubation to assess the climatic effects of two biochars (willow and spruce) and two ligneous biosolids on GHG emissions in controlled moisture conditions. The soil used in the incubation was collected from a soil-amendment field experiment on a clay cropland in South-West of Finland. The soil was sieved, air-dried and then individual samples were adjusted to 25%, 50%, 80% and 120% of water filled pore space (WFPS) before being incubated for 32 days in laboratory conditions. Soil GHG fluxes were measured after 1, 5, 12, 20 and 33 days.  

The application of 20–40 Mg ha^-1 of ligneous amendments, two years prior to our experiment, had increased soil pH, soil organic carbon content and plant available water content. The carbon dioxide (CO₂) fluxes were unaffected by the amendment treatments and correlated mainly with soil moisture and microbial biomass. Nitrous oxide (N₂O) emissions were reduced by all amendments compared to the un-amended control. Methane (CH₄) exchange consisted mostly of slight uptake by the soil but played only a minor role in the total GHG budget overall. 

The sum of CO₂, N₂O and CH₄ emissions, calculated as CO₂-equivalents, exhibited a strong linear relationship with soil moisture. Where the GHG budget was dominated by CO₂, it was accompanied by significant N₂O emissions at 120% WFPS. The results indicate that soil moisture critically affects the GHG emissions and that while organic soil amendments may have persisting effects on GHG exchange, they primarily occur in water-saturated conditions through N₂O dynamics.