Biological carbon stabilization and destabilization: peering into microbial hot spots to understand soil carbon turnover and terrestrial energy flows

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Burgeoning population, agricultural intensification and changing climate are threatening global food security. Understanding the amount, composition and stability of soil carbon (C) is needed to enhance soil fertility and agroecosystem sustainability and to establish a fair and transparent approach to evaluating sector contributions to climate change mitigation. Recently, great focus has been on storing or sequestering soil C. While this goal is critical to climate change mitigation, it is important to recognize that soil C not only needs to be stored, but also used to maintain ecosystem function. Plant photosynthate is the primary source of new C, and thus new energy, in agroecosystem soils. Formation of soil organic matter is mediated by microorganisms that utilize plant C during decomposition, releasing energy and nutrients. Understanding these microbial transformations is critical. Soil organic matter is at the heart of soil fertility and productivity. It holds C and nutrients that support plant growth, fuels biological processes, provides habitat for the biota responsible and buffers against short-term stresses (e.g. nutrient imbalance or drought). *** Hot-spots of microbial activity such as plant residue surfaces and microsites within soil aggregates have active populations up to twenty times higher than the bulk soil. Hot spots arise from abundant resource availability (e.g. C). They are temporally dynamic and difficult to study at relevant scales of time and space. Improved understanding of the roles of microbial taxa or community traits that perform decomposition is needed to optimize C cycling processes and for sustainable soil management. To better understand microbial dynamics of C-turnover in C-rich hot spots, the proposed research program will specifically examine how soil microorganisms act as agents of C flows through three projects which address separate, but related aspects of microbial ecology of soil C stabilization and destabilization: 1) a survey of relationships between thermodynamic efficiency of C use and microbial community composition in different agricultural soils, 2) use C-rich buried soils as a model system to identify indicator microorganisms and SOM biomarkers for C-stabilization in C-rich hot-spots, 3) profile microbial communities and C turnover in physical size fractions and aggregates of these C-rich buried soils to better understand the role of microhabitats as potential hot-spots of C stabilization. *** The interrelatedness of macronutrients nitrogen (N), phosphorus (P) and potassium (K) is acknowledged in soil fertilizer management. Explicit integration of C into resource management strategies has the potential to greatly enhance fertilizer use efficiency and to bolster critical functions of air and water filtration, climate change mitigation and food production. In other words, a shift toward a CNPK management frameworks will better support sustainable soil management and ultimately, food security. **