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Grasslands store approximately one third of the global terrestrial carbon stocks and can act as an important soil carbon sink. Recent studies show that plant diversity increases soil organic carbon (SOC) storage by elevating carbon inputs to belowground biomass and promoting microbial necromass contribution to SOC storage. Climate change affects grassland SOC storage by modifying the processes of plant carbon inputs and microbial catabolism and anabolism. Improved grazing management and biodiversity restoration can provide low-cost and/or high-carbon-gain options for natural climate solutions in global grasslands. The achievable SOC sequestration potential in global grasslands is 2.3 to 7.3 billion tons of carbon dioxide equivalents per year (CO 2 e year -1 ) for biodiversity restoration, 148 to 699 megatons of CO 2 e year -1 for improved grazing management, and 147 megatons of CO 2 e year -1 for sown legumes in pasturelands.

Soil organic matter (SOM) supports the Earth's ability to sustain terrestrial ecosystems, provide food and fiber, and retains the largest pool of actively cycling carbon. Over 75% of the soil organic carbon (SOC) in the top meter of soil is directly affected by human land use. Large land areas have lost SOC as a result of land use practices, yet there are compensatory opportunities to enhance productivity and SOC storage in degraded lands through improved management practices. Large areas with and without intentional management are also being subjected to rapid changes in climate, making many SOC stocks vulnerable to losses by decomposition or disturbance. In order to quantify potential SOC losses or sequestration at field, regional, and global scales, measurements for detecting changes in SOC are needed. Such measurements and soil-management best practices should be based on well established and emerging scientific understanding of processes of C stabilization and destabilization over various timescales, soil types, and spatial scales. As newly engaged members of the International Soil Carbon Network, we have identified gaps in data, modeling, and communication that underscore the need for an open, shared network to frame and guide the study of SOM and SOC and their management for sustained production and climate regulation., (© 2017 The Authors. Global Change Biology Published by John Wiley & Sons Ltd.)

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.

The opportunity of agricultural management practices to sequester soil organic carbon (SOC) is recognized as an important strategy for mitigating climate change. However, there is low confidence when it comes to understanding the magnitude of the climate benefit we can expect from SOC sequestration or how best to achieve it. Several issues are often confounded when it comes to the mitigation potential of SOC sequestration and greenhouse gas (GHG) reductions from agriculture, creating confusion and making it difficult to clearly identify the knowns, unknowns and risks to implementing policy and practice recommendations. Here, we identify and explain four major areas of uncertainty: (1) the expected changes in soil carbon or GHG emissions resulting from agricultural management practice changes; (2) the extent to which social, environmental and economic factors constrain mitigation potential; (3) the ability to execute reliable measurement, monitoring, reporting and verification (MMRV) frameworks; and (4) the perception of risk associated with different ways of promoting practice adoption (e.g., voluntary carbon markets fueled by the private sector, pay-for-practice programs funded by public investment). We aim to pinpoint knowledge gaps and areas of disagreement to help right-size expectations and guide effective investment in GHG removals and reductions from agriculture. [ABSTRACT FROM AUTHOR]

Biochar has been widely researched as an important technology for climate smart agriculture, yet work is still necessary to identify the magnitude of potential greenhouse gas ( GHG) mitigation and mechanisms involved. This study measured slow-pyrolysis wood-derived biochar's impact on GHG efflux, mineral N dynamics, and soil organic C in a series of two incubations across fertilized and unfertilized agricultural soils and soil moisture regimes. This research explored the magnitude of biochar's full GHG mitigation potential and drivers of such impacts. Results of this incubation indicate slow-pyrolysis wood-derived biochar has potential to provide annual emission reductions of 0.58-1.72 Mg CO2-eq ha−1 at a 25 Mg ha−1 biochar application rate. The greatest GHG mitigation potential was from C sequestration and nitrous oxide (N2O) reduction in mineral N fertilized soils, with minimal impacts on N2O emissions in unfertilized soils, carbon dioxide ( CO2) emissions, and methane ( CH4) uptake. Analysis of mineral N dynamics in the bulk soil and on biochar isolates indicated that neither biochar impacts on net mineralization and nitrification nor retention of ammonium ( [ABSTRACT FROM AUTHOR]

Climate change is predicted to increase climate variability and frequency of extreme events such as drought, straining water resources in agricultural systems. Thus, limited irrigation strategies and soil amendments are being explored to conserve water in crop production. Biochar is the recalcitrant, carbon-based coproduct of biomass pyrolysis during bioenergy production. When used as a soil amendment, biochar can increase soil water retention while enhancing soil properties and stimulating food webs. We investigated the effects of coupled biochar amendment and limited irrigation on belowground food web structure and function in an irrigated maize agroecosystem. We hypothesized that soil biota biomass and activity would decrease with limited irrigation and increase with biochar amendment and that biochar amendment would mitigate the impact of limited irrigation on the soil food web. One year after biochar addition, we extracted, identified, and estimated the biomass of taxonomic groups of soil biota (e.g., bacteria, fungi, protozoa, nematodes, and arthropods) from wood-derived biochar-amended (30 Mg ha−1) and nonamended soils under maize with limited (two-thirds of full) and full irrigation. We modeled structural and functional properties of the soil food web. Neither biochar amendment nor limited irrigation had a significant effect on biomass of the soil biota groups. Modeled soil respiration and nitrogen mineralization fluxes were not different between treatments. A comparison of the structure and function of the agroecosystem soil food web and a nearby native grassland revealed that in this temperate system, the negative impact of long-term conventional agricultural management outweighed the impact of limited irrigation. One year of biochar amendment did not mitigate nor further contribute to the negative effects of historical agricultural management. [ABSTRACT FROM AUTHOR]

Biochar has been advocated as a method of sequestering carbon while simultaneously improving crop yields and agro-ecosystem sustainability. It can be produced from a wide variety of biomass feedstocks using different thermochemical conversion technologies with or without the recovery of energy coproducts, resulting in chars of differing quality and a range of overall system greenhouse gas ( GHG) mitigation outcomes. This analysis expands on previous sustainability studies by proposing a mechanistic life cycle GHG and economic operating cost assessment model for the coproduction of biochar and bioenergy from biomass residue feedstocks, with a case study for north-central Colorado presented. Production is modeled as a continuous function of temperature for slow pyrolysis, fast pyrolysis, and gasification systems. Biochar environmental benefits ( C sequestration, N2O suppression, crop yield improvements) are predicted in terms of expected liming value and recalcitrance. System-level net GHG mitigation is computed, and net returns are estimated that reflect the variable economic costs of production, the agronomic value of biochar based on agricultural limestone or fertilizer displacement, and the value of GHG mitigation, with results compared to the alternate use of char for energy production. Case study results indicate that slow pyrolysis systems can mitigate up to 1.4 Mg CO2eq/ Mg feedstock consumed, provided a favorable feedstock is utilized, production air pollutant emissions are mitigated, and energy coproducts are recovered. The model suggests that while financial returns are generally greater when char is consumed for energy (biocoal) than when used as a soil amendment (biochar), chars produced through high-temperature conversion processes will have greater GHG-mitigation value as biochar. The biochar scenario reaches economic parity at carbon prices as low as $50/ Mg CO2eq for optimal scenarios, despite conservative modeling assumptions. This model is a step toward spatially explicit assessment and optimization of biochar system design across different feedstocks, conversion technologies, and agricultural soils. [ABSTRACT FROM AUTHOR]

Fractionation of soil organic carbon (SOC) is crucial for mechanistic understanding and modeling of soil organic matter decomposition and stabilization processes. It is often aimed at separating the bulk SOC into fractions with varying turnover rates, but a comprehensive comparison of methods to achieve this is lacking. In this study, a total of 20 different SOC fractionation methods were tested by participating laboratories for their suitability to isolate fractions with varying turnover rates, using agricultural soils from three experimental sites with vegetation change from C3 to C4 22–36 years ago. Enrichment of C4-derived carbon was traced and used as a proxy for turnover rates in the fractions. Methods that apply a combination of physical (density, size) and chemical (oxidation, extraction) fractionation were identified as most effective in separating SOC into fractions with distinct turnover rates. Coarse light SOC separated by density fractionation was the most C4-carbon enriched fraction, while oxidation-resistant SOC left after extraction with NaOCl was the least C4-carbon enriched fraction. Surprisingly, even after 36 years of C4 crop cultivation in a temperate climate, no method was able to isolate a fraction with more than 76% turnover, which challenges the link to the most active plant-derived carbon pools in models. Particles with density >2.8 g cm −3 showed similar C4-carbon enrichment as oxidation-resistant SOC, highlighting the importance of sesquioxides for SOC stabilization. The importance of clay and silt-sized particles (<50 μm) for SOC stabilization was also confirmed. Particle size fractionation significantly outperformed aggregate size fractionation, due to the fact that larger aggregates contain smaller aggregates and organic matter particles of various sizes with different turnover rates. An evaluation scheme comprising different criteria was used to identify the most suitable methods for isolating fractions with distinct turnover rates, and potential benefits and trade-offs associated with a specific choice. Our findings can be of great help to select the appropriate method(s) for fractionation of agricultural soils. [ABSTRACT FROM AUTHOR]

Carbon sequestration in agricultural soils has been promoted as a means to reduce atmospheric concentrations of greenhouse gases (GHG) whilst improving soil productivity. Although there is broad agreement on practices that increase carbon (C) stocks, uncertainty remains on how agricultural management affects the stability of these gains. The fate of above-ground residue into soil organic matter (SOM) was tracked using isotopically labelled ( 13 C and 15 N) residue over 12 months in a pasture soil in sub-tropical Australia. Agricultural residue management was simulated by (1) altering the rate of residue input and (2) incorporating residue with topsoil or leaving on soil surface. Increased input and incorporation of residue increased residue-derived SOM content, with the majority of residue-derived SOM accumulating as particulate organic matter (POM) (65%) with more modest gains in mineral-associated fractions. Rapid accumulation of residue-derived SOM in the mineral-associated fractions in the initial stages of decomposition, coinciding with a high loss of labile residue components, indicate an important role for soluble OM inputs in providing an immediate and long-term sink for C and N. However, this must be considered alongside high rates of accumulation in the more readily mineralised POM fraction, particularly when a soil is approaching saturation, which is likely to lead to greater mineralisation of SOM. [ABSTRACT FROM AUTHOR]

Carbon sequestration in agricultural soils has been promoted as a means to reduce atmospheric concentrations of greenhouse gases (GHG) whilst improving soil productivity. Although there is broad agreement on practices that increase carbon (C) stocks, there is a lack of understanding on the stability of these gains and how changes in soil organic carbon (SOC) pools can influence GHG fluxes. We tracked the fate of above-ground residues into functionally different SOC pools and GHG fluxes using isotopically labelled residues ( 13 C and 15 N) over 12 months in a pasture soil in sub-tropical Australia. Agricultural residue management was simulated by: (1) altering the rate of residue input and, (2) mixing residue with topsoil. GHG fluxes were significantly greater at high residue input levels due to the priming of existing SOC and elevated N 2 O losses, fuelled by a greater availability of labile substrate. There was evidence of an asymptotic relationship between C input and residue-derived C accumulation in stable soil C pools at higher input levels, indicating that the soil was reaching its protective capacity. Mixing of residues contributed to a 40% increase in GHG fluxes in comparison to surface applied treatment, most notably from residue-derived C and N. This can be attributed to (i) the physical disruption of soil, particularly that of aggregates, which changed the microenvironment stimulating microbial activity, and (ii) greater residue-soil contact. Greater residue-soil contact through mixing also contributed to a 2 fold increase in the residue-derived C recovered in the mineral soil with the majority (56%) in the active C pool. Over a 12 month period, C sequestration was outweighed by GHG fluxes at high rates of input and when residues were mixed with the topsoil. C sequestration policies and associated management approaches must be assessed holistically under a range of conditions and in the long-term to ensure that detrimental practices are not promoted. [ABSTRACT FROM AUTHOR]