Soil Organic Carbon
The most prevalent form of carbon in the soil is organic carbon, and its dynamics influence global biogeochemical cycles
Together with the atmosphere, land, water and vegetation cover, soil is a fundamental building block of terrestrial ecosystems and plays an essential role in global biogeochemical cycles. Soil performs numerous ecosystem functions and services that include storage, filtering and transformation of substances such as water, nitrogen and carbon that are crucially important for the existence of life on Earth. Land productivity is conditioned by soil fertility and nutrient exchange. Carbon is the soil’s energy pool. Carbon-related ecosystem services include biomass and soil carbon sequestration. These services suffer when natural systems are converted to agroforestry, pasture, no-tillage cropping, conventional tillage agriculture and industrial land-use systems.
A key property of soils is its soil organic matter (SOM). The amount of organic matter in soil depends on a number of factors, including the rates of input and decomposition of organic constituents (e.g. soil biota, root exudates, plant and animal tissues, animal residues) and climate. SOM affects numerous physical, chemical and biological properties of soils, including the retention of water and nutrients (and thus directly impacts biological processes like plant biomass production and photosynthesis), and improves soil structure and reduces erosion (which can lead to improved quality of ground- and surface waters).
A major proportion of SOM is carbon, which is termed soil organic carbon (SOC). Globally, stores of SOC contain twice the amount of carbon present in the atmosphere and three times that present in terrestrial vegetation. As shown on the map, SOC stocks vary among biomes and eco-regions due to differences in climate, soil type, physiography, vegetation and land use. The total carbon contained in terrestrial ecosystems of the globe is estimated to be about 3 170 Gt (1 Gt = 1 billion metric tonnes) of which about 80 % is found in soils as either organic (SOC=1 550 GT) or inorganic carbon (950 GT, which is mainly elemental carbon and carbonate materials that don't play a role in soil atmosphere carbon fluxes).
Since SOC is a key part of global biogeochemical cycling, disturbances or degradation processes that lead to carbon losses have major implications. For example, the conversion of natural ecosystems to agricultural systems initially depletes SOC, which can have a negative impact on productivity and contributes to climate change. The annual flux of carbon dioxide (CO2) between soil and the atmosphere is estimated to be seven times that derived from the burning of fossil fuels. Emissions from land-use change are estimated to account for up to 20 % of atmospheric CO2 through loss of biomass and SOM. It is known that the seasonal amplitude of atmospheric CO2 in the northern hemisphere is due to a higher carbon uptake by high productive agricultural systems during summer and their greater release of CO2 during winter compared to natural ecosystems. There is growing evidence of a positive feedback land–climate loop where increased climate warming accelerates the breakdown of SOC, with the subsequent release of CO2 and methane serving as a feedback that can accelerate climate change.
Through adaptive conservation and restoration agriculture and land management practices, sequestration of organic carbon in the soil can be increased. Soil carbon sequestration is higher in cooler, or wetter climates and clayey soils. Hence, changes in tillage and cropping practices can increase soil organic carbon storage at different rates, as has been observed in tropical moist (largest) and temperate dry conditions (smallest). Also, high-input cropping systems were found to be less constraining on SOC storage in moist climates compared to dry climates. Abandonment of cultivated land can increase carbon Sequestration but can cause other problems related to erosion or invasive species. In tropical areas it can take decades or centuries to recapture the carbon lost by clearing.
Covering around 41 % of the global land area, dryland ecosystems play an important role in the inter-annual variability of global carbon sinks, however these are strongly controlled by temperature and precipitation variations. Anthropogenic landuse- related soil carbon loss in the past created a sink capacity that now could be filled by carbon sequestration through restoration and adaptive land management practices. These include perennial cropping systems, increasing fallow periods, erosion control, low stocking rates with controlled grazing and conservation tillage.
Global biogeochemical cycles
Global biogeochemical cycles are processes of nutrient cycling in ecosystems by which a chemical substance moves through the Earth’s abiotic and biotic components. Oxygen, water, carbon, nitrogen and phosphorus are important cycles, and their quantities regulate atmosphere, climate and life conditions on Earth.
Soil at the centre of the "critical zone" concept
The Earth “critical zone” concept incorporates the interfaces of the atmosphere, biosphere, pedosphere and lithosphere. Considering the role of soil as part of the Earth’s critical zone allows for a better understanding of the interactions of a wide variety of environmental, ecological, geological, agricultural and natural resource issues. Soil-related issues that are important for land degradation assessment include carbon emission and sequestration, nutrient cycling and water quality and quantity, all of which should be considered, not only within the landscape context, but also as a critical component of global biogeochemical cycles.
Change in carbon stocks due to cropland conversion compared to potential natural vegetation carbon stocks. The average carbon loss from cropland expansion per unit area converted is nearly twice as high in the tropics as in temperate regions. Clearing tropical rainforest for cropland use can locally provoke substantial carbon loss up to −440 tonnes C ha. Conversion from grassland to cropland can cause vast carbon losses depending on the original sink. Rates of loss have been estimated at -4 % – 22 % for Mongolian grasslands, up to 50-55 % in Central USA and Northern China respectively.
Source: West et al. 2010.
Long-term conventional cultivation causes soil carbon loss – adaptive management practices can improve carbon sequestration.
Source: Redrawn after Ogle, S., 2005.