Sensitive control of N2O emissions and microbial community dynamics by organic fertilizer and soil interactions

Summary

Manure is a key source of N for crops, especially in organic farming systems, but also a driver of N2O emissions from soil. Treatment technologies removing manure organic matter affect soil N2O emissions, but the direction and magnitude of these effects remain uncertain. We explored the effects of four fertilizer materials derived from cattle manure on soil N2O emissions. Treatments included: untreated cattle manure (CA), cattle manure co-digested with grass-clover silage (DD); a liquid fraction (LF) produced by mechanical separation of digestate; and a concentrated fertilizer with NH4+-N and sulfate (NS) produced from stripped H2S and NH3. These fertilizers were surface-applied to a sandy loam (Foulum) and a clay loam soil (Askov) at 55% water-filled pore space (WFPS) in 28-day laboratory experiments with monitoring of CO2 and N2O. Samples were sectioned during or after incubation to describe mineral N and microbial dynamics. Although the WFPS in both soils was 58–61%, N2O emissions varied greatly, and this was explained by differences in water potential, and in the relative gas diffusivity which was approx. 0.011 and 0.030 in Foulum and Askov soil, respectively. Unexpectedly, treatment LF with the lowest manure organic matter input had the highest N2O emissions. Denitrification was the main pathway producing N2O as determined by 15N enrichment of soil NO3−. The vertical distribution of mineral N and microbial activities, and PLFA, indicated that N2O emissions from the organic fertilizers depended on their interaction with the soil, as modified by soil water potential and gas diffusivity at the time of application.