Conceptualizing soil organic matter into particulate and mineral-associated forms to address global change in the 21st century.

Managing soil organic matter (SOM) stocks to address global change challenges requires well-substantiated knowledge of SOM behavior that can be clearly communicated between scientists, management practitioners, and policy makers. However, SOM is incredibly complex and requires separation into multiple components with contrasting behavior in order to study and predict its dynamics. Numerous diverse SOM separation schemes are currently used, making cross-study comparisons difficult and hindering broad-scale generalizations. Here, we recommend separating SOM into particulate (POM) and mineral-associated (MAOM) forms, two SOM components that are fundamentally different in terms of their formation, persistence, and functioning. We provide evidence of their highly contrasting physical and chemical properties, mean residence times in soil, and responses to land use change, plant litter inputs, warming, CO2 enrichment, and N fertilization. Conceptualizing SOM into POM versus MAOM is a feasible, well-supported, and useful framework that will allow scientists to move beyond studies of bulk SOM, but also use a consistent separation scheme across studies. Ultimately, we propose the POM versus MAOM framework as the best way forward to understand and predict broad-scale SOM dynamics in the context of global change challenges and provide necessary recommendations to managers and policy makers

Intensive grassland management disrupts below-ground multi-trophic resource transfer in response to drought

Modification of soil food webs by land management may alter the response of ecosystem processes to climate extremes, but empirical support is limited and the mechanisms involved remain unclear. Here we quantify how grassland management modifies the transfer of recent photosynthates and soil nitrogen through plants and soil food webs during a post-drought period in a controlled field experiment, using in situ 13C and 15N pulse-labelling in intensively and extensively managed fields. We show that intensive management decrease plant carbon (C) capture and its transfer through components of food webs and soil respiration compared to extensive management. We observe a legacy effect of drought on C transfer pathways mainly in intensively managed grasslands, by increasing plant C assimilation and 13C released as soil CO2 efflux but decreasing its transfer to roots, bacteria and Collembola. Our work provides insight into the interactive effects of grassland management and drought on C transfer pathways, and highlights that capture and rapid transfer of photosynthates through multi-trophic networks are key for maintaining grassland resistance to drought

Land management shapes drought responses of dominant soil microbial taxa across grasslands

Soil microbial communities are dominated by a relatively small number of taxa that may play outsized roles in ecosystem functioning, yet little is known about their capacities to resist and recover from climate extremes such as drought, or how environmental context mediates those responses. Here, we imposed an in situ experimental drought across 30 diverse UK grassland sites with contrasting management intensities and found that: (1) the majority of dominant bacterial (85%) and fungal (89%) taxa exhibit resistant or opportunistic drought strategies, possibly contributing to their ubiquity and dominance across sites; and (2) intensive grassland management decreases the proportion of drought-sensitive and non-resilient dominant bacteria-likely via alleviation of nutrient limitation and pH-related stress under fertilisation and liming-but has the opposite impact on dominant fungi. Our results suggest a potential mechanism by which intensive management promotes bacteria over fungi under drought with implications for soil functioning

Drought decreases incorporation of recent plant photosynthate into soil food webs regardless of their trophic complexity

Theory suggests that more complex food webs promote stability and can buffer the effects of perturbations, such as drought, on soil organisms and ecosystem functions. Here, we tested experimentally how soil food web trophic complexity modulates the response to drought of soil functions related to carbon cycling and the capture and transfer below‐ground of recent photosynthate by plants. We constructed experimental systems comprising soil communities with one, two or three trophic levels (microorganisms, detritivores and predators) and subjected them to drought. We investigated how food web trophic complexity in interaction with drought influenced litter decomposition, soil CO2 efflux, mycorrhizal colonization, fungal production, microbial communities and soil fauna biomass. Plants were pulse‐labelled after the drought with 13C‐CO2 to quantify the capture of recent photosynthate and its transfer below‐ground. Overall, our results show that drought and soil food web trophic complexity do not interact to affect soil functions and microbial community composition, but act independently, with an overall stronger effect of drought. After drought, the net uptake of 13C by plants was reduced and its retention in plant biomass was greater, leading to a strong decrease in carbon transfer below‐ground. Although food web trophic complexity influenced the biomass of Collembola and fungal hyphal length, 13C enrichment and the net transfer of carbon from plant shoots to microbes and soil CO2 efflux were not affected significantly by varying the number of trophic groups. Our results indicate that drought has a strong effect on above‐ground–below‐ground linkages by reducing the flow of recent photosynthate. Our results emphasize the sensitivity of the critical pathway of recent photosynthate transfer from plants to soil organisms to a drought perturbation, and show that these effects may not be mitigated by the trophic complexity of soil communities, at least at the level manipulated in this experiment.info:eu-repo/semantics/publishedVersio

Combatting global grassland degradation

Grasslands are under severe threat from ongoing degradation, undermining their capacity to support biodiversity, ecosystem services and human well-being. Yet, grasslands are largely ignored in sustainable development agendas. In this Perspective, we examine the current state of global grasslands and explore the extent and dominant drivers of their degradation. Socio-ecological solutions are needed to combat degradation and promote restoration. Important strategies include: increasing recognition of grasslands in global policy; developing standardized indicators of degradation; using scientific innovation for effective restoration at regional and landscape scales; and enhancing knowledge transfer and data sharing on restoration experiences. Stakeholder needs can be balanced through standardized assessment and shared understanding of the potential ecosystem service trade-offs in degraded and restored grasslands. The integration of these actions into sustainability policy will aid in halting degradation and enhancing restoration success, and protect the socio-economic, cultural and ecological benefits that grasslands provide

Advancing understanding of the formation and stability of soil organic matter in a changing environment

2015 Spring.Includes bibliographical references.Soil is one of our most precious natural resources. It plays a key role in maintaining soil fertility and water quality, and represents a major reservoir in both the global carbon (C) and nitrogen (N) cycles. Soils contain more C and reactive N than the atmosphere and all vegetation combined, the majority of which is found in soil organic matter (SOM). Despite its considerable significance, little is known about the factors that control the formation of SOM, and its stability in the environment. Key questions pertain to whether environmental changes will increase the production of CO₂ during SOM formation and decomposition, forming a large positive feedback to climate change. Answering those questions required a better understanding of how various mechanisms that confer SOM stability are affected by environmental change. My dissertation research aimed to address some of these key questions, and to advance our overall understanding of SOM formation, SOM stability, and the response of stable SOM to changes in the environment. First, I conducted two soil incubation experiments using isotopically labeled (¹³C and ¹⁵N) plant material, which allowed me to track the incorporation of plant-derived C and N into SOM, and efflux of plant-derived C in CO₂. In one soil incubation, I tested the effects of plant litter quality and on the rate and efficiency of SOM formation (a measure of the amount of SOM formed versus the amount of CO₂ lost in the process) by comparing SOM formation from leaves versus roots. I found that plant litter chemistry (C/N ratio) was a reliable predictor of SOM formation after the initial stage of decomposition, with low C/N ratios resulting in more SOM formation and higher formation efficiencies overall. In the second soil incubation, I tested the effect of warming on the rate and efficiency of SOM formation, as well as the rate of destabilization of stable SOM. I found that warming generally led to lower formation efficiencies, causing greater CO₂ production per unit of SOM formed. Warming also led to higher rates of destabilization of stable SOM throughout the experiment. Next, I aimed to investigate the effect of warming on SOM in the field, using soils from two multi-factor climate change experiments. Results from that study suggested that while warming increased the rate of turnover of SOM in some cases, any resulting losses of SOM were offset by increased inputs of SOM, so that total SOM stocks were unchanged. Last, I investigated the persistence of pyrogenic SOM, which is thermally transformed by fire, in the face of land use change at three agricultural sites across the US. I found that pyrogenic SOM was present in all three soils, and had persisted to a greater extent than other SOM with land use change. Many studies of SOM dynamics do not account for pyrogenic SOM, and the results of my work suggest that this lack of accounting can preclude us from fully understanding the mechanisms behind SOM stability. Overall, my work advances our understanding of stable SOM in terms of how it is formed, and whether it will persist in the face of environmental change. Changes in plant litter quality and temperature may lead to changes fluxes of CO₂ to the atmosphere during SOM formation, and while some SOM (pyrogenic SOM) is highly stable in the environment, other SOM is susceptible to loss with warming and land use change. However, in the case of warming, increased plant inputs may offset increased rates of SOM decomposition

Are researchers following best storage practices for measuring soil biochemical properties?

It is widely accepted that the measurement of organic and inorganic forms of carbon (C) and nitrogen (N) in soils should be performed on fresh extracts taken from fresh soil samples. However, this is often not possible, and it is common practice to store samples (soils and/or extracts), despite a lack of guidance on best practice. We utilised a case study on a temperate grassland soil taken from different depths to demonstrate how differences in soil and/or soil extract storage temperature (4 or −20 ∘C) and duration can influence sample integrity for the quantification of soil-dissolved organic C and N (DOC and DON), extractable inorganic nitrogen (NH4+ and NO3-) and microbial biomass C and N (MBC and MBN). The appropriateness of different storage treatments varied between topsoils and subsoils, highlighting the need to consider appropriate storage methods based on soil depth and soil properties. In general, we found that storing soils and extracts by freezing at −20 ∘C was least effective at maintaining measured values of fresh material, whilst refrigerating (4 ∘C) soils for less than a week for DOC and DON and up to a year for MBC and MBN and refrigerating soil extracts for less than a week for NH4+ and NO3- did not jeopardise sample integrity. We discuss and provide the appropriate tools to ensure researchers consider best storage practice methods when designing and organising ecological research involving assessments of soil properties related to C and N cycling. We encourage researchers to use standardised methods where possible and to report their storage treatment (i.e. temperature, duration) when publishing findings on aspects of soil and ecosystem functioning. In the absence of published storage recommendations for a given soil type, we encourage researchers to conduct a pilot study and publish their findings.</p

Land management shapes drought responses of dominant soil microbial taxa across grasslands

Soil microbial communities are dominated by a relatively small number of taxa that may play outsized roles in ecosystem functioning, yet little is known about their capacities to resist and recover from climate extremes such as drought, or how environmental context mediates those responses. Here, we imposed an in situ experimental drought across 30 diverse UK grassland sites with contrasting management intensities and found that: (1) the majority of dominant bacterial (85%) and fungal (89%) taxa exhibit resistant or opportunistic drought strategies, possibly contributing to their ubiquity and dominance across sites; and (2) intensive grassland management decreases the proportion of drought-sensitive and non-resilient dominant bacteria—likely via alleviation of nutrient limitation and pH-related stress under fertilisation and liming—but has the opposite impact on dominant fungi. Our results suggest a potential mechanism by which intensive management promotes bacteria over fungi under drought with implications for soil functioning