How soil microbes assimilate carbon-C, nitrogen-N, phosphorus-P, and sulfur-S is fundamental for understanding nutrient cycling in terrestrial ecosystems. We compiled a global database of C, N, P, and S concentrations in soils and microbes and developed relationships between them by using a power function model. The C:N:P:S was estimated to be 287:17:1:0.8 for soils, and 42:6:1:0.4 for microbes. We found a convergence of the relationships between elements in soils and in soil microbial biomass across C, N, P, and S. The element concentrations in soil microbial biomass follow a homeostatic regulation curve with soil element concentrations across C, N, P and S, implying a unifying mechanism of microbial assimilating soil elements. This correlation explains the well-constrained C:N:P:S stoichiometry with a slightly larger variation in soils than in microbial biomass. Meanwhile, it is estimated that the minimum requirements of soil elements for soil microbes are 0.8 mmol C Kg-1 dry soil, 0.1 mmol N Kg-1 dry soil, 0.1 mmol P Kg-1 dry soil, and 0.1 mmol S Kg-1 dry soil, respectively. Lastly, these findings provide a mathematical explanation of element imbalance in soils and soil microbial biomass, and offer insights for incorporating microbial contribution to nutrient cycling into Earth system models.

Soil dwelling microorganisms are essential components of numerous ecosystem processes and biogeochemical cycles. In particular, they are important actors in terrestrial carbon cycling, producing and turning over soil organic matter. Microbially mediated soil carbon cycling can be influenced by environmental conditions, with soil organic matter dynamics and carbon fate varying across biomes. Drastic alterations to soil habitat conditions brought about through anthropogenic changes to land-use (e.g. agriculture) can greatly influence these processes. However, we are limited in our understanding of how land-use regimes and other environmental conditions control microbially mediated soil carbon cycling. I took three approaches to explore this relationship. First, I examined how bacterial community assembly and composition differed across cropland, old-field, and forest soils. I found that homogeneous selection, whereby selection pressure causes bacterial communities to be more phylogenetically similar to each other than expected by random assembly from a metacommunity, was the dominant bacterial community assembly process across all three land-use types. However, I also found that land-use interacted with soil pH to drive the balance between stochastic and deterministic assembly processes. This result indicates a mechanism by which microbial communities may develop differently across land-use regimes. Second, I examined the overall organic matter turnover across land-use regimes and the identity of the bacterial taxa actively involved in this carbon processing. I found that the dynamics of organic matter turnover and the active bacterial populations involved were distinct across land-use regimes. From these patterns I developed a conceptual model explaining how initial microbial biomass, which is impacted by land-use, may control bacterial activities in organic matter turnover. Finally, I examined the genomic basis of bacterial life history strategies, specifically the copiotroph-oligotroph continuum. Life history strategy can explain both bacterial activity in soil carbon cycling and bacterial response to environmental change. I found that the abundance of transcription factor genes and genes encoding a secretion signal peptide were both genomic signatures of the copiotroph-oligotroph continuum. These signatures can be used to classify diverse microbes based on their life history strategy and may further explain the biological drivers of these strategies. I also developed a toolkit, MetaSIPSim, that simulates metagenomic DNA-stable isotope probing datasets. Such datasets can be used to improve metagenomic DNA-stable isotope probing methodologies and analyses, which in turn can be used to link microbial genes and genomes to in situ carbon cycling activity. Overall, this work advances our knowledge of, and ability to study the ecological and biological controls of bacterially mediated soil carbon cycling.

Soil Carbon Storage: Modulators, Mechanisms and Modeling takes a novel approach to the issue of soil carbon storage by considering soil C sequestration as a function of the interaction between biotic (e.g. microbes and plants) and abiotic (climate, soil types, management practices) modulators as a key driver of soil C. These modulators are central to C balance through their processing of C from both plant inputs and native soil organic matter. This book considers this concept in the light of state-of-the-art methodologies that elucidate these interactions and increase our understanding of a vitally important, but poorly characterized component of the global C cycle. The book provides soil scientists with a comprehensive, mechanistic, quantitative and predictive understanding of soil carbon storage. It presents a new framework that can be included in predictive models and management practices for better prediction and enhanced C storage in soils. Identifies management practices to enhance storage of soil C under different agro-ecosystems, soil types and climatic conditions Provides novel conceptual frameworks of biotic (especially microbial) and abiotic data to improve prediction of simulation model at plot to global scale Advances the conceptual framework needed to support robust predictive models and sustainable land management practices

Several textbooks and edited volumes are currently available on general soil fertility but‚ to date‚ none have been dedicated to the study of “Sustainable Carbon and Nitrogen Cycling in Soil.” Yet this aspect is extremely important, considering the fact that the soil, as the ‘epidermis of the Earth’ (geodermis)‚ is a major component of the terrestrial biosphere. This book addresses virtually every aspect of C and N cycling, including: general concepts on the diversity of microorganisms and management practices for soil, the function of soil’s structure-function-ecosystem, the evolving role of C and N, cutting-edge methods used in soil microbial ecological studies, rhizosphere microflora, the role of organic matter (OM) in agricultural productivity, C and N transformation in soil, biological nitrogen fixation (BNF) and its genetics, plant-growth-promoting rhizobacteria (PGPRs), PGPRs and their role in sustainable agriculture, organic agriculture, etc. The book’s main objectives are: (1) to explain in detail the role of C and N cycling in sustaining agricultural productivity and its importance to sustainable soil management; (2) to show readers how to restore soil health with C and N; and (3) to help them understand the matching of C and N cycling rules from a climatic perspective. Given its scope, the book offers a valuable resource for educators, researchers, and policymakers, as well as undergraduate and graduate students of soil science, soil microbiology, agronomy, ecology, and the environmental sciences. Gathering cutting-edge contributions from internationally respected researchers, it offers authoritative content on a broad range of topics, which is supplemented by a wealth of data, tables, figures, and photographs. Moreover, it provides a roadmap for sustainable approaches to food and nutritional security, and to soil sustainability in agricultural systems, based on C and N cycling in soil systems.

Understanding the response of soil microbial communities and decomposition to global environmental changes is central to our ability to accurately forecast future terrestrial carbon (C) storage and atmospheric CO2 levels. Increases in the frequency and severity of disturbance events are one element of global change in terrestrial ecosystems. The goal of this dissertation was to measure the response of soil microbial communities and decomposition to disturbance events and to examine the mechanisms underlying post-disturbance changes in decomposition. In the first part of my dissertation work I explored these questions within the context of wildfires in boreal forests. Chapter 1 characterized soil microbial communities and the rate of decomposition across a fire chronosequence in interior Alaska. I found that boreal forest fires reduced soil microbial abundance, altered fungal community composition, and suppressed litter decomposition. Chapter 2 investigated whether soil microbial responses to boreal forest fires differ as a function of fire severity. I demonstrated that higher severity fires elicited greater reductions in soil microbial biomass and larger shifts in fungal community composition than lower severity fires. Chapter 3 tested the mechanisms through which boreal forest fires alter decomposition processes. I discovered that decomposition rates were slower in recently burned forests because of post-fire reductions in soil moisture and C substrate quality. In the second part of my dissertation I expanded my findings to other types of disturbance events using meta-analysis. Chapter 4 reviewed the response of soil microbial biomass to fires. I found that soil microbial biomass was significantly lower in recently burned ecosystems, but the response of microbial biomass to fire differed by fire type and biome. Chapter 5 examined soil microbial responses to abiotic (fire, harvesting, storms) and biotic (insect infestation, pathogen outbreaks) disturbances in forests. I observed that abiotic disturbances significantly reduced soil microbial biomass, while changes in microbial biomass were non-significant following biotic disturbance events. Collectively, these findings suggest that reductions in soil microbial biomass and decomposition rates following abiotic disturbances are likely to slow the transfer of C from soils to the atmosphere and provide a negative feedback to rising atmospheric CO2 concentrations and global change.

This book presents a comprehensive overview of nutrient cycling processes and their importance for plant growth and ecosystem sustainability. The book combines fundamental scientific studies and devised practical approaches. It contains contributions of leading international authorities from various disciplines resulting in multidisciplinary approaches, and all chapters have been carefully reviewed. This volume will support scientists and practitioners alike.