Terrestrial ecosystems depend on soil organic matter (SOM) for sustaining agricultural productivity and other vital ecosystem services, yet global SOM stocks are threatened by environmental change. Currently, the ability to mitigate SOM losses is limited by our understanding of the processes regulating SOM formation and long-term persistence. In recent years, new concepts of SOM formation and persistence have emerged suggesting that SOM is largely derived from soil microbes. Contrasting sharply with older concepts that emphasize the importance of complex plant-derived compounds in SOM formation, the new emphasis on microbial processes might dramatically improve SOM management if only the mechanisms were better understood. Specifically, there is relatively little known about the microbial physiological traits that regulate microbial production and the fate of microbial products in different soil types. To address this knowledge gap, the broad objective of this research is to link microbial physiological traits to SOM formation and persistence. This objective will be met using a long-term laboratory study to track newly formed SOM derived from microbial processes in different soil types. The soils vary in their clay mineralogy and carbon inputs from plants and, as a result, will select for different microbial communities and physiological traits, which together will presumably lead to differences in the long-term stability of the newly formed SOM. Throughout the experiment, measurements will be made of microbial physiological traits that can be linked to biomass synthesis, rates of SOM formation, SOM chemistry, microbial biodiversity, and SOM persistence.
Results from this work will lead to improvements in current models of SOM formation and stabilization and will characterize the important relationships among microbial physiology, carbon input quality, and the soil environment. This study will make fundamental contributions to microbial ecology and soil and ecosystem science and could change how we mitigate the adverse effects of land-use conversion and climate change on SOM losses. Specifically, this research could encourage new approaches for land-use conservation, restoration, and agricultural management that emphasize the indirect building of SOM by managing microbial biomass production.
Soil organic matter (SOM) is integral to the functioning of healthy ecosystems; it governs nutrient availability for plants, regulates soil greenhouse gas emissions, and contains the largest pool of terrestrial carbon. Yet land-use intensification and global climate change have the potential to deplete SOM stocks. To mitigate potential SOM losses, we must have an accurate understanding of the processes regulating SOM formation and its long-term persistence. Recently, the canonical view that SOM formation is largely composed of decaying plant material has undergone a discipline-wide reassessment. New concepts are emerging that emphasize the turnover and accumulation of soil microorganisms as a primary constituent of SOM rather than plant derivatives. Despite increasing recognition of microbial contributions to SOM, little experimental evidence directly links microbial processes to SOM formation, in part because it is difficult to separate plant-derived SOM from microbial-derived SOM. Thus, it remains unclear how much microbes actually contribute to SOM relative to plants and how variability in microbial traits influencing microbial biomass inputs rates subsequently affect microbial-SOM formation. For example, microbial growth efficiency (MGE; the proportion of microbial biomass produced from a given resource consumed by a microbe) has a direct influence on net microbial biomass production and consequently might affect SOM accumulation rates. To address these uncertainties we used model soils, initially SOM- and microbe- free to 1) test the fundamental theory that significant amounts of SOM can develop solely from soil microorganisms, 2) determine if microbial residues contribute to the characteristically chemically diverse nature of SOM, and 3) understand the influence of microbial communities on the overall abundance and longevity of microbial-derived SOM. The use of initially SOM-free soils allowed us to track newly formed microbial SOM and avoid any background signature of plant carbon, otherwise present in natural soils. Soils were constructed from sterile sand and clay and then inoculated with a natural soil microbial community. Following inoculation, soils were treated weekly with either a carbon energy source intended to either encourage microbial activity and high MGE (glucose) or inhibit microbial activity and MGE (syringol). From these carbon additions, a resource gradient was created of simple organic substrates typical of root exudates that differ in C bioavailability and therefore select for distinct microbial communities and physiological traits. Over the course of a year, we measured differences in microbial SOM accumulation, MGE, microbial community composition, the long-term stability of the newly formed SOM, and the chemistry of the SOM to verify its microbial origin. Microbial SOM accumulation and stability: After one year, we show that soils accumulated between 1 and 1.5 % soil C, within the range of natural soils found in managed ecosystems. More than half of this carbon was resistant to CO2 mineralization, indicating the potential for long-term persistence in the soil. This suggests that a significant amount of stable soil C from microbial residues can be generated within a fairly short time period. Microbial SOM chemistry: Based on chemical analyses of the soils, the majority of the newly formed C was from microbial products such as proteins and lipids, compounds not present in either the initial soils or the weekly carbon additions (Fig. 1). The newly formed SOM also exhibited diverse chemical compositions similar to a natural soil. For example, additions of the simple organic compound, glucose, to initially SOM-free soils, resulted in a soil with 120 unique organic compounds after the glucose had been processed by the microbial community. Most of these compounds were novel and not detected in pure glucose. Thus, we see that as microbes utilize simple carbon resources for growth and biomass production, it is replaced by chemically diverse microbial by-products as the microbial community grows and turns over during the course of a year. Microbial community and physiology effects on SOM: Significant relationships between the types of microbes present in the soil, MGE and the accumulated C concentrations were observed (Fig. 2). Soils exhibiting the highest MGE as well as the highest abundance of fungi relative to bacteria were also those soils that accumulated the most microbial SOM. This suggests that, not only do fungal dominated communities have greater community MGE, but community composition can directly influence how much soil C accumulates due to differences in biomass production. Our experimental results provide direct evidence that significant amounts of chemically diverse SOM accumulates from the turnover of soil microorganism but that the type of microorganisms present and their associated physiologies are important factors in determining the amount of microbial-derived SOM. This work improves current soil carbon models beyond merely a broad conceptual understanding that microbes contribute to SOM, towards ones that can characterize how microbial communities moderate SOM dynamics. Further, the relationships between MGE, microbial communities, and SOM can be used to inform and improve quantitative models that include these traits as variable parameters for predicting changes in soil carbon.
