Methane sink function of grassland soil microbiomes - negative effects of intensive management persist three years after land-use extensification

Abstract

Grassland soils are important methane (CH₄) sinks through CH₄ oxidation by methanotrophs, but intensive management with high nitrogen inputs and grazing densities reduces this potential. While long-term recovery of the CH₄ sink after land-use change is generally established, little is known about the short-term effects of reducing land-use intensity index (LUI) through extensive management in grassland. We did not find an effect on potential CH₄ oxidation rates (PMORs) and the abundances atmospheric CH₄-consuming methanotrophs after three years of LUI reduction (no fertilization, no grazing, and one mowing per year) in 45 intensively managed grassland sites located in three different pedoclimatic regions of Germany. However, we observed a decline in the abundance of CH₄ producing methanogens. Moreover, we found greater PMORs and higher abundance of Upland Soil Cluster γ (USCγ) methanotrophs on additional, historically low LUI sites. Soil bulk density (BD) decreased already after three years of LUI reduction and was even lower in historically low LUI sites. Strong correlations between the abundance of canonical methanotrophs and methanogens highlight a CH₄ filter function that was independent from LUI reduction across regions. Our study consistently shows that three years of LUI reduction are not sufficient to restore the CH₄ sink function of temperate grasslands. However, the lower BD and the decreased abundance of methanogens indicate that LUI reduction may affect the niche-determining boundary conditions for CH₄-cycling microorganisms in the long term.