Microbial Necromass in Soils—Linking Microbes to Soil Processes and Carbon Turnover

The organic matter of living plants is the precursor material of the organic matter stored in terrestrial soil ecosystems. Although a great deal of knowledge exists on the carbon turnover processes of plant material, some of the processes of soil organic matter (SOM) formation, in particular from microbial necromass, are still not fully understood. Recent research showed that a larger part of the original plant matter is converted into microbial biomass, while the remaining part in the soil is modified by extracellular enzymes of microbes. At the end of its life, microbial biomass contributes to the microbial molecular imprint of SOM as necromass with specific properties. Next to appropriate environmental conditions, heterotrophic microorganisms require energy-containing substrates with C, H, O, N, S, P, and many other elements for growth, which are provided by the plant material and the nutrients contained in SOM. As easily degradable substrates are often scarce resources in soil, we can hypothesize that microbes optimize their carbon and energy use. Presumably, microorganisms are able to mobilize biomass building blocks (mono and oligomers of fatty acids, amino acids, amino sugars, nucleotides) with the appropriate stoichiometry from microbial necromass in SOM. This is in contrast to mobilizing only nutrients and consuming energy for new synthesis from primary metabolites of the tricarboxylic acid cycle after complete degradation of the substrates. Microbial necromass is thus an important resource in SOM, and microbial mining of building blocks could be a life strategy contributing to priming effects and providing the resources for new microbial growth cycles. Due to the energy needs of microorganisms, we can conclude that the formation of SOM through microbial biomass depends on energy flux. However, specific details and the variability of microbial growth, carbon use and decay cycles in the soil are not yet fully understood and linked to other fields of soil science. Here, we summarize the current knowledge on microbial energy gain, carbon use, growth, decay, and necromass formation for relevant soil processes, e. g. the microbial carbon pump, C storage, and stabilization. We highlight the factors controlling microbial necromass contribution to SOM and the implications for soil carbon use efficiency (CUE) and we identify research needs for process-based SOM turnover modelling and for understanding the variability of these processes in various soil types under different climates.

Non-Specific Interactions of Rhizospheric Microbial Communities Support the Establishment of Mimosa acutistipula var. ferrea in an Amazon Rehabilitating Mineland

Mimosa acutistipula var. ferrea (Fabaceae) is endemic to ferruginous tropical rocky outcrops in the eastern Amazon, also known as canga. Canga are often associated with mining activities and are the target of protection and rehabilitation projects. M. acutistipula stands out in this biodiversity hotspot with high growth rates, even in rehabilitating minelands (RMs). However, little is known about the diversity of soil microorganisms interacting with M. acutistipula in canga and RMs. This study analyzed the rhizosphere-associated bacterial and fungal microbial communities associated with M. acutistipula growing in an RM and a native shrub canga. The fungal phylum Ascomycota was the dominant taxa identified in the rhizosphere of the canga (RA: 98.1) and RM (RA: 93.1). The bacterial phyla Proteobacteria (RA: 54.3) and Acidobacteria (RA: 56.2) were the dominant taxa identified in the rhizosphere in the canga and RM, respectively. Beneficial genera such as Bradyrhizobium, Rhodoplanes, and Paraconiothyrium were identified in the rhizosphere of M. acutistipula in both areas. However, the analyses showed that the fungal and bacterial diversity differed between the rhizosphere of the canga and RM, and that the microbial taxa adapted to the canga (i.e., Rasamsonia, Scytalidium, Roseiarcus, and Rhodomicrobium) were lacking in the RM. This influences the microbe-mediated soil processes, affecting long-term rehabilitation success. The results showed that M. acutistipula established non-specific interactions with soil microorganisms, including beneficial taxa such as nitrogen-fixing bacteria, mycorrhizal fungi, and other beneficial endophytes, well known for their importance in plant adaptation and survival. High levels of microbe association and a plant’s ability to recruit a wide range of soil microorganisms help to explain M. acutistipula’s success in rehabilitating minelands.

Bridging Microbial Functional Traits With Localized Process Rates at Soil Interfaces

In this review, we introduce microbially-mediated soil processes, players, their functional traits, and their links to processes at biogeochemical interfaces [e.g., rhizosphere, detritusphere, (bio)-pores, and aggregate surfaces]. A conceptual view emphasizes the central role of the rhizosphere in interactions with other biogeochemical interfaces, considering biotic and abiotic dynamic drivers. We discuss the applicability of three groups of traits based on microbial physiology, activity state, and genomic functional traits to reflect microbial growth in soil. The sensitivity and credibility of modern molecular approaches to estimate microbial-specific growth rates require further development. A link between functional traits determined by physiological (e.g., respiration, biomarkers) and genomic (e.g., genome size, number of ribosomal gene copies per genome, expression of catabolic versus biosynthetic genes) approaches is strongly affected by environmental conditions such as carbon, nutrient availability, and ecosystem type. Therefore, we address the role of soil physico-chemical conditions and trophic interactions as drivers of microbially-mediated soil processes at relevant scales for process localization. The strengths and weaknesses of current approaches (destructive, non-destructive, and predictive) for assessing process localization and the corresponding estimates of process rates are linked to the challenges for modeling microbially-mediated processes in heterogeneous soil microhabitats. Finally, we introduce a conceptual self-regulatory mechanism based on the flexible structure of active microbial communities. Microbial taxa best suited to each successional stage of substrate decomposition become dominant and alter the community structure. The rates of decomposition of organic compounds, therefore, are dependent on the functional traits of dominant taxa and microbial strategies, which are selected and driven by the local environment.

New earthworm records from the Caspian Hyrcanian Forests of Iran (Oligochaeta: Megadrili)

Earthworms are the most important soil invertebrates worldwide, in terms of biomass and effects on soil processes. In this study, 21 earthworm species, including four new records were identified from Caspian Hyrcanian Forests (North of Iran). Four species; Criodrilus lacuum, Lumbricus rubellus, Metaphire californica, and Octodrilus transpadanus and the family Criodrilidae are reported for the first time. Previous studies have identified 31 earthworm species belonging to 14 genera and three families (Lumbricidae, Acanthodrilidae, and Megascolecidae) in Iran; therefore, these new records increase the number of earthworm species to a total of 35.

Remotely detected plant function in two midwestern prairie grassland experiments reveals belowground processes

Imaging spectroscopy provides the opportunity to incorporate leaf and canopy optical data into ecological studies, but the extent to which remote sensing of vegetation can enhance the study of belowground processes is not well understood. In grassland systems, aboveground and belowground vegetation quantity and quality are coupled, and both influence belowground microbial processes and nutrient cycling, providing a potential link between remote sensing and belowground processes. We hypothesized that ecosystem productivity, and the chemical, structural and phylogenetic-functional composition of plant communities would be detectable with remote sensing and could be used to characterize belowground plant and soil processes in two grassland biodiversity experiments—the BioDIV experiment at Cedar Creek Ecosystem Science Reserve in Minnesota and the Wood River Nature Conservancy experiment in Nebraska. Specifically, we tested whether aboveground vegetation chemistry and productivity, as detected from airborne sensors, predict soil properties microbial processes and community composition. Imaging spectroscopy data were used to map aboveground biomass and green vegetation cover, functional traits and phylogenetic-functional community composition of vegetation. We examined the relationships between the image-derived variables and soil carbon and nitrogen concentration, microbial community composition, biomass and extracellular enzyme activity, and soil processes, including net nitrogen mineralization. In the BioDIV experiment—which has low overall diversity and productivity despite high variation in each—belowground processes were driven mainly by variation in the amount of organic matter inputs to soils. As a consequence, soil respiration, microbial biomass and enzyme activity, and fungal and bacterial composition and diversity were significantly predicted by remotely sensed vegetation cover. In contrast, at Wood River, where plant diversity and productivity were consistently higher, remotely sensed functional, chemical and phylogenetic composition of vegetation predicted belowground extracellular enzyme activity, microbial biomass, and net nitrogen mineralization rates, while aboveground biomass did not. The strong, contrasting associations between the quantity and chemistry of aboveground inputs with belowground soil processes and properties provide a basis for using imaging spectroscopy to understand belowground processes across productivity gradients in grassland systems. However, a mechanistic understanding of how above and belowground components interact among different ecosystems remains critical to extending these results broadly.

Ecological memory of recurrent drought modifies soil processes via changes in soil microbial community

Climate change is altering the frequency and severity of drought events. Recent evidence indicates that drought may produce legacy effects on soil microbial communities. However, it is unclear whether precedent drought events lead to ecological memory formation, i.e., the capacity of past events to influence current ecosystem response trajectories. Here, we utilize a long-term field experiment in a mountain grassland in central Austria with an experimental layout comparing 10 years of recurrent drought events to a single drought event and ambient conditions. We show that recurrent droughts increase the dissimilarity of microbial communities compared to control and single drought events, and enhance soil multifunctionality during drought (calculated via measurements of potential enzymatic activities, soil nutrients, microbial biomass stoichiometry and belowground net primary productivity). Our results indicate that soil microbial community composition changes in concert with its functioning, with consequences for soil processes. The formation of ecological memory in soil under recurrent drought may enhance the resilience of ecosystem functioning against future drought events.