Instead of containing stable and chemically unique ‘humic substances’, as has been widely accepted, soil organic matter is a mixture of progressively decomposing organic compounds; this has broad implications for soil science and its applications. The exchange of nutrients, energy and carbon between soil organic matter, the soil environment, aquatic systems and the atmosphere is important for agricultural productivity, water quality and climate. Long-standing theory suggests that soil organic matter is composed of inherently stable and chemically unique compounds. Here we argue that the available evidence does not support the formation of large-molecular-size and persistent ‘humic substances’ in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds. We discuss implications of this view of the nature of soil organic matter for aquatic health, soil carbon–climate interactions and land management. Soil organic matter contains a large portion of the world's carbon and plays an important role in maintaining productive soils and water quality. Nevertheless, a consensus on the nature of soil organic matter is lacking. Johannes Lehmann and Markus Kleber argue that soil organic matter should no longer be seen as large and persistent, chemically unique substances, but as a continuum of progressively decomposing organic compounds.

The contentious nature of soil organic matter

This Perspective looks at how microbial anabolism and the soil microbial carbon pump control microbial necromass accumulation and stabilization; the ‘entombing effect’.

The importance of anabolism in microbial control over soil carbon storage

Human security has and will continue to rely on Earth's diverse soil resources. Yet we have now exploited the planet's most productive soils. Soil erosion greatly exceeds rates of production in many agricultural regions. Nitrogen produced by fossil fuel and geological reservoirs of other fertilizers are headed toward possible scarcity, increased cost, and/or geopolitical conflict. Climate change is accelerating the microbial release of greenhouse gases from soil organic matter and will likely play a large role in our near-term climate future. In this Review, we highlight challenges facing Earth's soil resources in the coming century. The direct and indirect response of soils to past and future human activities will play a major role in human prosperity and survival.

/pdf/soil-and-human-security-in-the-21st-century-3wppnsj44i.pdf

Soil and human security in the 21st century

Human appropriation of land for agriculture has greatly altered the terrestrial carbon balance, creating a large but uncertain carbon debt in soils. Estimating the size and spatial distribution of soil organic carbon (SOC) loss due to land use and land cover change has been difficult but is a critical step in understanding whether SOC sequestration can be an effective climate mitigation strategy. In this study, a machine learning-based model was fitted using a global compilation of SOC data and the History Database of the Global Environment (HYDE) land use data in combination with climatic, landform and lithology covariates. Model results compared favorably with a global compilation of paired plot studies. Projection of this model onto a world without agriculture indicated a global carbon debt due to agriculture of 133 Pg C for the top 2 m of soil, with the rate of loss increasing dramatically in the past 200 years. The HYDE classes “grazing” and “cropland” contributed nearly equally to the loss of SOC. There were higher percent SOC losses on cropland but since more than twice as much land is grazed, slightly higher total losses were found from grazing land. Important spatial patterns of SOC loss were found: Hotspots of SOC loss coincided with some major cropping regions as well as semiarid grazing regions, while other major agricultural zones showed small losses and even net gains in SOC. This analysis has demonstrated that there are identifiable regions which can be targeted for SOC restoration efforts.

https://www.pnas.org/content/pnas/114/36/9575.full.pdf

Soil carbon debt of 12,000 years of human land use

No-till agriculture is generally considered good for soils, and probably also beneficial in relation to climate change adaptation. However, this Perspective argues that the potential for climate change mitigation through soil carbon sequestration that is possible from a change to no-till agriculture has been widely overstated.

Limited potential of no-till agriculture for climate change mitigation

The knowns, known unknowns and unknowns of sequestration of soil organic carbon

Summary
Eucalyptus regnans grows rapidly from seed after wildfires, out-competing other species, thereby forming pure stands of mature forests that rank amongst the world's most carbon dense. By global standards, these forests grow on infertile soils. It is unclear how E. regnans is able to obtain large amounts nitrogen (N) from these infertile soils to support its rapid growth after fire.
We measured carbon (C) and N stored in plant biomass and photosynthetic rates of E. regnans 2 years after a wildfire and examined whether E. regnans stimulated its own N supply through root-induced increases in microbial decomposition and N mineralization. We compared microbial biomass, gross N mineralization rates and soil C in trenched and rooted plots.
Photosynthetic rates of E. regnans seedlings were high and comparable to photosynthetic rates observed in fertilized crops. Presence of roots of E. regnans and allied microflora enhanced gross N mineralization more than fivefold compared to soil without roots present. Soil microbial biomass was more than doubled by root presence. The soil N pulse caused by the fire and N mineralization rates in the absence of roots were too small to account for the large amount of N stored in E. regnans 2 years after the fire.
Our results suggest that E. regnans facilitated its rapid growth by enhancing microbial activity and N mineralization. This enhanced microbial activity also contributed to a substantial loss of soil C (˜62% of carbon gained in plant biomass was concurrently lost from soil).
Synthesis. At the ecosystem scale, the synergistic effects of plant growth and soil N mineralization need to be carefully assessed against costs to soil C for forests regenerating after disturbance.

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2745.12663

Enhanced decomposition and nitrogen mineralization sustain rapid growth of Eucalyptus regnans after wildfire

Terrestrial ecosystems are expected to experience multiple changes in climate in the future. Global changes including rising concentrations of atmospheric carbon dioxide (CO2), increases in temperature and rates of nitrogen deposition, and severity of precipitation extremes can directly or indirectly influence biological processes in soil such as root and microbial respiration. For example, rising atmospheric [CO2] may alter belowground carbon supply and enhance soil water availability by reducing stomatal conductance; climate warming may increase organic matter mineralisation and soil nitrogen supply; and changes in precipitation and nitrogen deposition may alter soil respiration in unexpected directions by modulating the CO2 fertilisation and warming effects. These processes in turn will influence soil health and may cause positive or negative feedbacks to climate.

Vivien de Rémy de Courcelles

Papers

The knowns, known unknowns and unknowns of sequestration of soil organic carbon

Enhanced decomposition and nitrogen mineralization sustain rapid growth of Eucalyptus regnans after wildfire

Soil Respiration in Future Global Change Scenarios