Soils hold the largest biogeochemically active terrestrial carbon pool on Earth and are critical for stabilizing atmospheric CO2 concentrations. Nonetheless, global pressures on soils continue from changes in land management, including the need for increasing bioenergy and food production
Soil organic matter (SOM) anchors global terrestrial productivity and food and fiber supply. SOM retains water and soil nutrients and stores more global carbon than do plants and the atmosphere combined. SOM is also decomposed by microbes, returning CO2, a greenhouse gas, to the atmosphere. Unfortunately, soil carbon stocks have been widely lost or degraded through land use changes and unsustainable forest and agricultural practices.
To understand its structure and function and to maintain and restore SOM, we need a better appreciation of soil organic carbon (SOC) saturation capacity and the retention of above- and belowground inputs in SOM. Our analysis suggests root inputs are approximately five times more likely than an equivalent mass of aboveground litter to be stabilized as SOM. Microbes, particularly fungi and bacteria, and soil faunal food webs strongly influence SOM decomposition at shallower depths, whereas mineral associations drive stabilization at depths greater than ∼30 cm. Global uncertainties in the amounts and locations of SOM include the extent of wetland, peatland, and permafrost systems and factors that constrain soil depths, such as shallow bedrock. In consideration of these uncertainties, we estimate global SOC stocks at depths of 2 and 3 m to be between 2,270 and 2,770 Pg, respectively, but could be as much as 700 Pg smaller. Sedimentary deposits deeper than 3 m likely contain > 500 Pg of additional SOC. Soils hold the largest biogeochemically active terrestrial carbon pool on Earth and are critical for stabilizing atmospheric CO2 concentrations. Nonetheless, global pressures on soils continue from changes in land management, including the need for increasing bioenergy and food production
Excerpts on future directions:
2.1. Emerging Research Questions for Plant Production, Allocation, and SOM:
1. What is the relative contribution of roots compared with that of litter inputs to the accumulation of SOM under different vegetation types, soil conditions, land uses, and climates?
2. Is the higher CUE of root litter compared with that of aboveground litter explained by differences in chemical composition or root-soil interactions?
3. What is the fate of nutrients such as nitrogen and phosphorus from aboveground and belowground organic matter respired during decomposition, and what is their role in SOM formation?
4. In consideration of trade-offs with production, how feasible is it to manage plant allocation patterns in managed landscapes to sequester SOM but maintain growth and yield?
3.1. Emerging Research Questions for Belowground Food Webs and Soil Ecology
Although it is well established that microbes and soil fauna exert strong controls on the rates and pathways of plant litter decay, their role in soil carbon stabilization is less clear. Mycorrhizae have a strong role in carbon stabilization in many ecosystems, but the relative role of fungi in soil carbon stabilization, compared with that of bacteria, is not well characterized. Several questions deserve particular attention:
1. How critical is understanding microbial physiology to predicting future changes in soil carbon stocks with climate change?
2. Will microbial CUE be altered by global warming, will thermal adaptation occur, or will broad changes in the microbial community lead to unexpected changes in soil carbon stabilization patterns?
3. How will changes in future vegetation patterns affect detrital inputs to soil and the stabilization of these inputs?
4. Do soil fauna need to be added to models of SOM that include microbes?
4.1. Emerging Research Questions for Biotic–Abiotic Interactions and SOM
1. How will interactions between biotic processes (e.g., NPP, detrital inputs, and microbial activity) and carbon retention on mineral surfaces be altered by climate change?
2. Do soil minerals and their interactions with biotic processes need to be included in future SOM models?
3. How can abiotic and biotic factors be incorporated into land surface and Earth system models to reduce future uncertainty?
5.3. Emerging Research Questions for Global SOM Stocks, Distributions, and Controls:
Answers to the following important research questions could help close the data gap:
1. How can we better constrain the distributions of peatland and permafrost systems, the amount of SOC and SON they contain, and their vulnerability to a warming climate?
2. How can computational approaches enhance our understanding of depth distributions for SOM and their biotic and abiotic controls?
3. How can we best improve and verify estimates of bedrock depth and its influence on the global content of SOC and SON?
Soil carbon is vulnerable to oxidation and release to the atmosphere through a variety of human activities (Figure 1), including land use disturbance and the effects of climate change. The greatest human-induced loss of SOC has come from the conversion of native forests and grasslands to annual crops (Paustian et al. 1997, Lal 2004). Understanding the role of agricultural management on SOC stocks is therefore critical both for predicting future carbon fluxes and for devising best-management strategies to mitigate and reverse soil loss…. Mitigating and even reversing these land use effects, however, are both possible and desirable (Minansy et al. 2017)….. The initial status of the land is critical to the interpretation of afforestation studies. A degraded system often gains SOM with afforestation or other management; a healthy, native ecosystem may sometimes lose it….
….The adoption of soil conservation practices such as reduced tillage, improved residue management, reduced bare fallow, and conservation reserve plantings has stabilized, and partially reversed, SOC loss in North American agricultural soils (Paustian et al. 2016)….Improved grazing management, fertilization, sowing legumes, and improved grass species are additional ways to increase soil carbon by as much as 1 Mg C ha−1year−1 (Conant et al. 2017)…
….Ecosystem and Earth system models can improve their representations of SOM by adding modifiers and microbial attributes that influence SOM formation and stabilization across scales….
….Over the next century, most projected land use change is expected to arise from repurposing existing agricultural land rather than clearing native forests (Watson et al. 2014). Emerging land use activities that combine carbon sequestration with crop production offer great promise to increase global SOM while sustainably meeting food and fiber production for an increasing human population (Francis et al. 2016).
….anthropogenic greenhouse gas emissions have altered the planet’s climate, including
temperatures, precipitation, and vapor pressure deficit, and will continue to do so. Additional changes are apparent in the patterns and extremes of weather and in the frequency, intensity, and severity of disturbances. All the factors, knowledge, and skill illustrated through the examples in this review will be needed to project the effects of climate change on SOM. Global pressures on soils are coming from continuing changes in land management, such as the need for increasing bioenergy and food production. For these reasons and more, furthering progress in experiments, synthesis, and modeling of SOM will remain a research priority for decades..
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