Water availability and abundance of microbial groups are key determinants of greenhouse gas fluxes in a dryland forest ecosystem

Abstract Forests are considered key biomes that could contribute to minimising global warming as they sequester carbon (C) and contribute to mitigate emissions of the potent greenhouse gases (GHG) including nitrous oxide (N 2 O), methane (CH 4 ) and carbon dioxide (CO 2 ). Management practices are prevalent in forestry, particularly in dryland ecosystems, known to be water and nitrogen (N) limited. Irrigation and fertilisation are thus routinely applied to increase the yield of forest products. However, the contribution of forest management practices to current GHG budgets and consequently to soil net global warming potential (GWP) is still largely unaccounted for, particularly in dryland ecosystems. We quantified the long-term effect (six years) of irrigation and fertilisation and the impact of land-use change, from grassland to a Eucalyptus plantation on N 2 O, CH 4 and CO 2 emissions and soil net GWP, within a dryland ecosystem. To identify biotic and abiotic drivers of GHG emissions, we explored the relationship of N 2 O, CH 4 and CO 2 fluxes with soil abiotic characteristics and abundance of ammonia-oxidizers, N 2 O-reducing bacteria, methanotrophs and total soil bacteria. Our results show that GHG emissions, particularly N 2 O and CO 2 are constrained by water availability and both N 2 O and CH 4 are constrained by N availability in the soil. We also provide evidence of functional microbial groups being key players in driving GHG emissions. Our findings illustrate that GHG emission budgets can be affected by forest management practices and provide a better mechanistic understanding for future mitigation options.