Controls of Organic Nitrogen Cycling by Soil Microbes

Nitrogen (N) availability in soils exerts a strong control on the carbon (C) cycle, through effects on plant production and on microbial processes, such as organic matter decomposition and soil microbial respiration. The predicted global changes in temperature and precipitation, rising levels of atmospheric carbon dioxide and increasing N deposition call for a better understanding of the soil N cycle and its underlying processes in order to accurately predict future responses of the Earths C and N cycles to environmental changes. Although the importance of N availability for soil C sequestration is well established, we know surprisingly little about the actual bottleneck in the soil N cycle, i.e. the decomposition of soil organic N, and about microbial nitrogen-use efficiency (NUE). Microbial NUE represents the fraction of organic N taken up that is invested into microbial biomass growth, while the excess of organic N is mineralized and recycled to the environment in the form of ammonium. During this mineralization step organic C is released that can be used for energy production (respiration) or biomass production. Microbial NUE therefore is a key biogeochemical parameter determining the fraction of organic matter breakdown products incorporated into microbial biomass and therefore sequestered in soil, as microbial residues comprise a large fraction of stable soil organic matter, and as it causes an intermittent decoupling of the soil N and C cycles. The overarching goal of the micrON-project is therefore to advance our mechanistic understanding of organic N metabolism of soil and litter microbial communities that drive soil N sequestration and soil organic N dynamics. Towards this goal, we will investigate edaphic, climatic and land management controls of soil organic N cycling processes and of microbial NUE in different types of soil across a continental gradient in Europe in natural and managed ecosystems. This will be accompanied by monitoring seasonal changes in the same processes in forest, grassland and arable land at two locations, and by a microcosm experiment using the same soils, amended with different types of labile or complex forms of organic C and N or where soil moisture and temperature are manipulated. The microcosm experiment will help understanding the short-term responses of microbial NUE to environmental changes or resource manipulations, and allow the calibration of a biogeochemical model by model-data fusion approaches. By analyzing microbial N processes and NUE over a wide range of soils differing in latitude, across seasons and after C N amendment or environmental manipulation, and by applying model-data fusion we will contribute substantially to the understanding and the predictability of these key microbial processes.

The MicrON project has clearly shown that soil element cycling processes, including microbial carbon and nitrogen processes, are not driven by the size of enzyme pools or the presence of microbial functional genes but are under strict substrate (resource) control. This represents a clear paradigm shift in soil and microbial ecology that is enzyme and functional gene centered. Decomposition processes are substrate limited and not enzyme limited. The new research task therefore is to tackle what controls substrate availability and accessibility, i.e. linking processes to substrate sorption processes, as well biotic and abiotic source processes. Moreover, the MicrON project has demonstrated that research should not focus on controls of single soil microbial processes but that multiple functions need to be investigated at the same time when one wants to understand soil functioning and ecosystem services provided by soils. This also allows a better linkage between microbial community structure, soil functionality and environmental drivers. Understanding soil carbon and nitrogen cycles at regional and continental scales and how they respond to agricultural management and global change is highly important and relevant to economy and society, and has been the target of the MicrON project team.