When carbon inputs from photosynthesis exceed C losses, SOC levels increase over time. When carbon inputs and outputs are in balance with one another, there is no net change in SOC levels. Other ecosystem processes that can lead to carbon loss include soil erosion and leaching of dissolved carbon into groundwater. Plant debris is less recalcitrant, resulting in a much shorter residence time in soil. Humus is highly recalcitrant, and this resistance to decomposition leads to a long residence time in soil. These various forms of SOC differ in their recalcitrance, or resistance to decomposition. Decomposition of biomass by soil microbes results in carbon loss as CO 2 from the soil due to microbial respiration, while a small proportion of the original carbon is retained in the soil through the formation of humus, a product that often gives carbon-rich soils their characteristic dark color (Fig. For example, many plants form symbiotic associations between their roots and specialized fungi in the soil known as mycorrhizae the roots provide the fungi energy in the form of carbon while the fungi provide the plant with often-limiting nutrients such as phosphorus. Soil C results both directly from growth and death of plant roots, as well as indirectly from the transfer of carbon-enriched compounds from roots to soil microbes. SOC input rates are primarily determined by the root biomass of a plant, but also include litter deposited from plant shoots. Photosynthesis is the fixation of atmospheric CO 2 into plant biomass. SOC levels result from the interactions of several ecosystem processes, of which photosynthesis, respiration, and decomposition are key. Soil organic carbon (SOC) levels are directly related to the amount of organic matter contained in soil and SOC is often how organic matter is measured in soils. SOM is made of organic compounds that are highly enriched in carbon. SOM is a heterogeneous mixture of materials that range in stage of decomposition from fresh plant residues to highly decomposed material known as humus. Proper restoration techniques can help restore terrestrial ecosystem functions.įundamentals of Soil Organic CarbonSoil organic matter is composed of soil microbes including bacteria and fungi, decaying material from once-living organisms such as plant and animal tissues, fecal material, and products formed from their decomposition. Restoration of organic matter levels in soil requires an understanding of the ecological processes important for SOM storage. Soil disturbance also leads to increased erosion and nutrient leaching from soils, which have led to eutrophication and resultant algal blooms within inland aquatic and coastal ecosystems, ultimately resulting in dead zones in the ocean (Fig. For example, destruction of rainforests that hold a significant amount of the carbon stored in terrestrial ecosystems contributes significantly to rising atmospheric carbon dioxide (CO 2) levels linked to climate change, while reductions in SOM levels from soil disturbance from mining can impact infiltration of rainfall and the storage of soil moisture important for flood mitigation. Only in recent history has the understanding of soil productivity been tied to SOM levels, with the depletion of SOM stocks often leading to large-scale impacts on whole ecosystems as well as the entire planet. Since the beginnings of recorded history, societies have understood that human activities can deplete soil productivity and the ability to produce food (McNeill and Winiwarter 2004). SOM improves soil structure and reduces erosion, leading to improved water quality in groundwater and surface waters, and ultimately to increased food security and decreased negative impacts to ecosystems. Benefits of soil organic matter (SOM) include improvement of soil quality through increased retention of water and nutrients, resulting in greater productivity of plants in natural environments and agricultural settings. Organic matter is a key component of soil that affects its physical, chemical, and biological properties, contributing greatly to its proper functioning on which human societies depend.
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