Organic matter amendments have been proposed as a means to enhance soil carbon (C) stocks on degraded soils. However, only few data exist on rates of soil C sequestration or the fate of added C in ...grassland soils, which are generally thought to have high C storage potential. We measured changes in the amount of C and nitrogen (N) in soils and in the composition of soil organic matter (SOM) following a single application of composted organic matter in two annual grasslands from different bioclimatic zones (coastal and inland valley). There was a significant increase in bulk soil organic C content at the valley grassland, and a similar but non-significant trend at the coastal grassland. Physical fractionation of soil three years after organic matter amendment revealed increases in C and N in the free- and occluded light fractions in both the valley and coastal grasslands. Amendments resulted in a greater relative increase in the N stored in light soil fractions compared to C, leading to lower C:N ratios. Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy showed an increase in the ratio of carboxyl and carbonyl functional groups to aliphatic methyl and methylene groups in the free- and occluded light fractions. These data show that the organic matter amendment was incorporated in the free light and occluded light fractions over three years. Our results indicate that a single application of compost to grassland soils can increase soil C and N storage in labile and physically protected pools over relatively short time periods and contribute to climate change mitigation.
•A single organic matter addition increased soil C storage over 3 y in grasslands.•Organic matter amendment increased C and N in the free and occluded light fractions.•The increase in occluded C and N indicates enhanced organic matter stabilization.•Rapid C incorporation into protected pools contributes to climate change mitigation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Abstract
Soil is the largest terrestrial reservoir of organic carbon and is central for climate change mitigation and carbon-climate feedbacks. Chemical and physical associations of soil carbon with ...minerals play a critical role in carbon storage, but the amount and global capacity for storage in this form remain unquantified. Here, we produce spatially-resolved global estimates of mineral-associated organic carbon stocks and carbon-storage capacity by analyzing 1144 globally-distributed soil profiles. We show that current stocks total 899 Pg C to a depth of 1 m in non-permafrost mineral soils. Although this constitutes 66% and 70% of soil carbon in surface and deeper layers, respectively, it is only 42% and 21% of the mineralogical capacity. Regions under agricultural management and deeper soil layers show the largest undersaturation of mineral-associated carbon. Critically, the degree of undersaturation indicates sequestration efficiency over years to decades. We show that, across 103 carbon-accrual measurements spanning management interventions globally, soils furthest from their mineralogical capacity are more effective at accruing carbon; sequestration rates average 3-times higher in soils at one tenth of their capacity compared to soils at one half of their capacity. Our findings provide insights into the world’s soils, their capacity to store carbon, and priority regions and actions for soil carbon management.
Numerous studies have demonstrated that fertilization with nutrients such as nitrogen, phosphorus, and potassium increases plant productivity in both natural and managed ecosystems, demonstrating ...that primary productivity is nutrient limited in most terrestrial ecosystems. In contrast, it has been demonstrated that heterotrophic microbial communities in soil are primarily limited by organic carbon or energy. While this concept of contrasting limitations, that is, microbial carbon and plant nutrient limitation, is based on strong evidence that we review in this paper, it is often ignored in discussions of ecosystem response to global environment changes. The plant‐centric perspective has equated plant nutrient limitations with those of whole ecosystems, thereby ignoring the important role of the heterotrophs responsible for soil decomposition in driving ecosystem carbon storage. To truly integrate carbon and nutrient cycles in ecosystem science, we must account for the fact that while plant productivity may be nutrient limited, the secondary productivity by heterotrophic communities is inherently carbon limited. Ecosystem carbon cycling integrates the independent physiological responses of its individual components, as well as tightly coupled exchanges between autotrophs and heterotrophs. To the extent that the interacting autotrophic and heterotrophic processes are controlled by organisms that are limited by nutrient versus carbon accessibility, respectively, we propose that ecosystems by definition cannot be ‘limited’ by nutrients or carbon alone. Here, we outline how models aimed at predicting non‐steady state ecosystem responses over time can benefit from dissecting ecosystems into the organismal components and their inherent limitations to better represent plant–microbe interactions in coupled carbon and nutrient models.
An ecosystem as a whole cannot be ‘limited’ by nutrients because soil microbes are carbon limited. The growth of microbes in the soil is primarily limited by carbon availability, while autotrophic plants are primarily limited by nutrient availability. These contrasting limitations support whole ecosystem carbon cycling, and explicit recognition of the carbon limitation of soil microbes can improve predictions of whole ecosystem responses to non‐steady state conditions.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We provide observational evidence that land‐atmosphere coupling is underestimated by a conventional metric defined by the correlation between soil moisture and surface evaporative fraction (latent ...heat flux normalized by the sum of sensible and latent heat flux). Land‐atmosphere coupling is 3 times stronger when using leaf area index as a correlate of evaporative fraction instead of soil moisture, in the Southern Great Plains. The role of vegetation was confirmed using adjacent flux measurement sites having identical atmospheric forcing but different vegetation phenology. Transpiration makes the relationship between evaporative fraction and soil moisture nonlinear and gives the appearance of weak coupling when using linear soil moisture metrics. Regions of substantial coupling extend to semiarid and humid continental climates across the United States, in terms of correlations between vegetation metrics and evaporative fraction. The hydrological cycle is more tightly constrained by the land surface than previously inferred from soil moisture.
Key Points
Evaporative fraction is often better correlated with vegetation phenology than with soil moisture
Vegetation controls on evaporative fraction can be separated from atmospheric forcing
Vegetation metrics imply stronger land‐atmosphere coupling than soil moisture metrics
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Several states and countries have adopted targets for deep reductions in greenhouse gas emissions by 2050, but there has been little physically realistic modeling of the energy and economic ...transformations required. We analyzed the infrastructure and technology path required to meet California's goal of an 80% reduction below 1990 levels, using detailed modeling of infrastructure stocks, resource constraints, and electricity system operability. We found that technically feasible levels of energy efficiency and decarbonized energy supply alone are not sufficient; widespread electrification of transportation and other sectors is required. Decarbonized electricity would become the dominant form of energy supply, posing challenges and opportunities for economic growth and climate policy. This transformation demands technologies that are not yet commercialized, as well as coordination of investment, technology development, and infrastructure deployment.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Climatic, atmospheric, and land-use changes all have the potential to alter soil microbial activity, mediated by changes in plant inputs. Many microbial models of soil organic carbon (SOC) ...decomposition have been proposed recently to advance prediction of climate and carbon (C) feedbacks. Most of these models, however, exhibit unrealistic oscillatory behavior and SOC insensitivity to long-term changes in C inputs. Here we diagnose the source of these problems in four archetypal models and propose a density-dependent formulation of microbial turnover, motivated by community-level interactions, that limits population sizes and reduces oscillations. We compare model predictions to 24 long-term C-input field manipulations and identify key benchmarks. The proposed formulation reproduces soil C responses to long-term C-input changes and implies greater SOC storage associated with CO
-fertilization-driven increases in C inputs over the coming century compared to recent microbial models. This study provides a simple modification to improve microbial models for inclusion in Earth System Models.
In recent years, the role of soil erosion on terrestrial carbon sequestration had been the focus of a growing number of studies. However, relatively little attention has been paid so far to the role ...of erosion on the lateral distribution of soil nitrogen (N) and the role of geomorphic processes on soil N dynamics. Here, we present primary data on the stock of nitrogen in soil and its rate of erosion at a relatively undisturbed, zeroorder watershed in northern California. Erosion transports 0.26–0.47 g N m⁻² year⁻¹ from eroding slope positions (Summit and Slope), and about two-thirds of the eroded N enters depositional landform positions (Hollow and Plain). Our results show that depositional-position soil profiles contain up to 3 times more N than soil profiles in the eroding positions. More than 92% of all soil nitrogen was chemically bound to soil minerals in all the landform positions, compared to 2–4% each found in the free light and occluded light fractions. Nitrogen associated with the free light fraction in topsoil is particularly susceptible to loss by soil erosion. By comparison, soil N associated with the aggregate-protected occluded light fractions and the mineral-associated dense fractions is likely to be protected from gaseous and dissolved losses. On average, we found that soil N has mean residence time of 694 years in eroding landform positions, compared to 2951 years in depositional landform positions. Our results also show that microbial processing of organic matter exerts strong control on overall soil N storage and N stabilized through sorptive interactions with soil minerals only in poorly drained depositional landform positions. Soil erosion exerts important control on stock, distribution, and long-term fate of soil N in dynamic landscapes.
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BFBNIB, DOBA, EMUNI, FZAB, GEOZS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NMLJ, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for ...millennia whereas other SOM decomposes readily--and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming.
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DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Intergovernmental Panel on Climate Change (IPCC) Special Report on Global Warming of 1.5°C points to the need for carbon neutrality by mid‐century. Achieving this in the United States in only ...30 years will be challenging, and practical pathways detailing the technologies, infrastructure, costs, and tradeoffs involved are needed. Modeling the entire U.S. energy and industrial system with new analysis tools that capture synergies not represented in sector‐specific or integrated assessment models, we created multiple pathways to net zero and net negative CO2 emissions by 2050. They met all forecast U.S. energy needs at a net cost of 0.2–1.2% of GDP in 2050, using only commercial or near‐commercial technologies, and requiring no early retirement of existing infrastructure. Pathways with constraints on consumer behavior, land use, biomass use, and technology choices (e.g., no nuclear) met the target but at higher cost. All pathways employed four basic strategies: energy efficiency, decarbonized electricity, electrification, and carbon capture. Least‐cost pathways were based on >80% wind and solar electricity plus thermal generation for reliability. A 100% renewable primary energy system was feasible but had higher cost and land use. We found multiple feasible options for supplying low‐carbon fuels for non‐electrifiable end uses in industry, freight, and aviation, which were not required in bulk until after 2035. In the next decade, the actions required in all pathways were similar: expand renewable capacity 3.5 fold, retire coal, maintain existing gas generating capacity, and increase electric vehicle and heat pump sales to >50% of market share. This study provides a playbook for carbon neutrality policy with concrete near‐term priorities.
Plain Language Summary
We created multiple blueprints for the United States to reach zero or negative CO2 emissions from the energy system by 2050 to avoid the most damaging impacts of climate change. By methodically increasing energy efficiency, switching to electric technologies, utilizing clean electricity (especially wind and solar power), and deploying a small amount of carbon capture technology, the United States can reach zero emissions without requiring changes to behavior. Cost is about $1 per person per day, not counting climate benefits; this is significantly less than estimates from a few years ago because of recent technology progress. Models with more detail than used in the past revealed unexpected synergies, counterintuitive results, and tradeoffs. The lowest‐cost electricity systems get >80% of energy from wind and solar power but need other resources to provide reliable service. Eliminating fossil fuel use altogether is possible but higher cost. Restricting biomass use and land for renewables is possible but could require nuclear power to compensate. All blueprints for the United States agree on the key tasks for the 2020s: increasing the capacity of wind and solar power by 3.5 times, retiring coal plants, and increasing electric vehicle and electric heat pump sales to >50% of market share.
Key Points
The United States can reach zero net CO2 emissions from energy and industry in 2050 at a net cost of 0.2–1.2% of GDP, not counting climate benefits
Multiple feasible pathways exist, all based on energy efficiency, clean electricity, electrification, and carbon capture for use or storage
Least‐cost electricity systems obtain >80% of their energy from wind and solar, with existing types of thermal generation for reliability
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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