Drought Effects on Soil Carbon and Nitrogen Cycling: Role of Fungi and Bacteria

Abstract

Drought has serious impact on the stability and fertility of soil systems, but the consequences for soil microbes and functioning remain unclear. Major soil carbon (C) and nitrogen (N) cycling processes such as microbial biomass C (MBC) and N (MBN), respiration, carbon use efficiency (CUE), extracellular enzyme (EE) activity, gross N mineralization (GNM), and N retention were explored in response to drought, biocides and fertilizer treatments in a seminatural Australian grassland soil. Results showed that the abundances and activities of fungi and bacteria across seasons were responsible for larger variations of CUE compared to drought, and more fungi in dry summer with greater investment in C for EEs production reduced CUE. To investigate their effects on these processes, an initial meta-analysis showed that most biocides had significant negative effects on MBC and MBN, and mixed effects on soil respiration and N cycles. As biocide addition can temporarily increase C and N to surviving microbes, a laboratory experiment was conducted using common fungicides and bactericides. Biocides addition to soil can result in unwanted temporary increases in C and N supply to surviving microbes due to a pulse in microbial necromass, and where biocides can directly be used as sources of C and N. To overcome this issue, the effect of labeled substrate with and without biocides additions in a controlled drought treatment were further investigated. Under drought, fungi produced oxidative enzymes and appeared to be more important in decomposing recalcitrant C substrate than bacteria. Further, drought effects on GNM and plant N uptake were not in synchrony, and compost addition can reduce N loss with drought compared to mineral fertilizers. Finally, this research provides and improved understanding of the links between microbes and soil dynamics with continuing drought stress to overcome the potential rates of soil degradation in grasslands of Australia.