Erosion’s Contribution to Greenhouse Gases

Jan. 1, 2002
Although current global warming caused by fossil-fuel burning is debatable, the well-documented rise in greenhouse gases (GHGs) culminating in the past couple of decades is not. Carbon dioxide (CO2) has increased 30% since the last century. Currently, the rate of emissions of methane (CH4) is more than double that estimated for the preindustrial period and is estimated to rise 1.1% per year. Nitrous oxide (N2O) has also increased 15% during the same period (Schlesinger, 1997). These atmospheric gases have risen dramatically, but how?This article proposes that the organic fraction of soil, through the process of enhanced erosion caused by cultivation and deforestation, is a large contributor of N2O, a significant factor in CH4, and a modest source of CO2. This ranking is based on the size of the various pools, competing reaction, and isotopic studies that suggest the origin of the additional gases. Because of the difficulty of measuring this eroded organic pool, a correctable source of these atmospheric gases has yet to be adequately recognized, described, or modeled (Lal, 1998).Erosion and Soil StructureSoils can form over thousands of years. One storm can punctuate the steady state condition and initiate irreparable depletion. Although erosion is a component in the geological cycle of the Earth, the ubiquitous nature of agriculture has accelerated the rate of natural erosion. The ever-increasing demand for agricultural land has created an “ecology of scale”–more cultivation, more erosion. Based on measurements of the sediment content of the world’s rivers, total global erosion is estimated to be approximately 20 billion tons per year (Milliman and Meade, 1983). Because most eroded soil does not immediately flow into a stream or catchment basin, but is deposited downslope, any erosion totals based on sediment loads of rivers remain highly speculative and conservative (Walling, 1990). Enhanced global erosion could have finally reached a level that might be significantly impacting GHG levels.Erosion is a symptom of poor soil structure. Soil structure is the arrangement of soil particles and the pore spaces between them. Organic-inorganic interactions are integral to the stability of soil structure. Soil organic matter improves structure by binding clay, silt, and sand together into clusters called aggregates (Allison, 1968).Tisdall and Oades (1982) recognized that different processes are dominant at different-size classes of aggregates. Soil crumbs larger than 0.2 mm are stabilized by a network of fine roots and fungal hyphae. The size class from 0.2 to 0.02 mm is bound mainly by amorphous cementing agents including resistant organic matter. Bacterial capsules and fragments of fungal cell walls are found to bind particles between 0.02 and 0.002 mm. The smaller-than-0.002-mm class consists of flocculated clay plates held together by hydrogen bonding, van der Waals forces, and coulombic attractions. Aggregation is therefore a product of many forces, including transient mechanical binding by roots and fungal hyphae, temporary adhesive properties by products of microbial synthesis and decay, and persistent cementing action by resistant organic components that provide long-term soil stability but can be disrupted by cultivation and erosion.Soil Organic MatterSoil organic matter (SOM) contains three times more carbon than vegetation and twice that of the atmosphere. The organic carbon content within the fine surface fraction, which is more susceptible to erosion, is about 1-2%. Soil also contains enormous nitrogen reserves. The stable SOM nitrogen has an approximate ratio of one per 10 carbons. The term soil organic matter is as much a composite of concepts as SOM itself is a composite of compounds. Because SOM is a complex matrix, a polycondensate, it has often been necessary to describe it by separation schemes. Chemical fractionation, the classical method of describing SOM, gave rise to the terms humic acids and fulvic acids. Soils are mixed with a dilute alkali solution, and the solids are removed. Humic acids are a mixture of compounds that precipitate from the solution when acid is added to lower the pH to 2. The organic fraction that remains in the solution is defined as fulvic acids. These have a higher proportion of carboxyl and hydroxyl functional groups and a higher percentage of oxygen than humic acids. The latter contain a higher percentage of carbon and usually range between 2,000 and 100,000 molecular weight (Schnitzer, 1982).Rather than define SOM by extraction, Jenkinson and Rayner defined it by function: decomposable plant material, resistant plant material, soil microbial biomass, physically stabilized organic matter, and chemically stabilized organic matter (Jenkinson and Rayner, 1977). The first two categories are determined by the rate of decay of plant material, while soil microbial biomass represents the living organic fraction. Physically stabilized organic matter is associated with sand, silt, and clay that become inaccessible to microbes. It is this portion of SOM that is the primary reservoir of carbon and nitrogen, which can be released through the mechanical action of water erosion. Chemically stabilized organic matter is believed to be derived from partially decomposed components of plants (phenyl-propanoid units of lignin) and microbes (polyphenols and amino acids), which combine, through the formation of quinones, to form compounds that are resistant to further decomposition (Martin and Haider).SOM stabilizes soil structure, but the reverse is also true: Good structure physically protects SOM. A brief overview of soil organic and inorganic associations helps illustrate how erosion releases SOM from microbial protection that the inorganic fraction confers. Several mechanisms often are involved in the sorption of organic compounds to each other and to clay minerals that physically or chemically protect SOM. These include hydrogen bonding, van der Waals forces (physical adsorption), and cation bridging. Nonpolar portions of organic polymers, such as alkanes and alkenes, can form short-range reversible adsorption to surfaces through hydrophobic (solvent-induced) bonding. These functional groups of SOM not only control clay-polymer behavior but also influence macrostructural characteristics, such as aggregation. During the erosion process, these organic attachments become exposed to environmental conditions that allow for microbial decomposition.DecompositionStable SOM (humus), which can have a mean residence time of centuries, becomes a “liable” source of GHGs through a series of biochemical transformations initiated by the physical process of erosion. Erosion enhances SOM decomposition at two locations: the eroded surface of the land and the eroded “in transit” soil. Erosion creates a new pool of mineralizable organic matter that is different from the stable organic matter remaining. This soil organic fraction that is carried away is no longer under the same physical and environmental conditions that allowed the organic matter to stabilize initially (Jenny, 1980).SOM decomposition often is limited because of the wide distribution of soil pores, which hinders the diffusion of soluble substrates, extracellular enzymes, bacteria, and nutrients, as well as the diffusion of oxygen into and metabolic products out of intact soil. Aggregates can also protect SOM by physically excluding bacteria and oxygen from the interior micropores. Water erosion eliminates two of the usual rate-limiting factors to decomposition of SOM: substrate diffusion and moisture. Erosion turbulently mixes microorganisms and organic matter and transports the soil under near-saturated conditions from a site of stabilization, even if just a few meters away. It is well established that when topsoil is disturbed and aerated, whether tilled in the field or sieved in the laboratory, there is a flush of SOM decomposition. This temporal enhanced rate of decomposition is primarily dependent on soil temperature and moisture. Once the physical protection conferred by the aggregate is removed, these fluctuating conditions allow the eroded organic fraction to decompose much more rapidly to CH4, CO2, and N2O in anaerobic conditions and to CO2 and H2O in aerobic environments.Cultivation removes the protection that plants give to the soil and often leaves the surface bare, susceptible to the disruptive impact of rain. The initial site of structural degradation is commonly the surface aggregates. The aggregates below the air-soil interface are physically protected by those above. Cultivation causes a decline in organic carbon content through the physical disruption of aggregates, which exposes previously inaccessible organic matter to enhanced decomposition. Most cultivation practices disrupt soil structure, decrease SOM, and enhance erosion. Cultivation can cause a 30-50% loss of SOM over decades in temperate regions. This is not erosion but enhanced SOM decomposition in situ. Tillage weakens aggregates, but water erosion completely destroys them. Because erosion is much more invasive than cultivation, it releases more SOM and at a greater rate.Greenhouse GasesWithout denying that fossil-fuel burning is the dominant contributor to the modern spike of CO2, we can say that erosion might be a significant contributor to other GHGs, such as methane and nitrous oxide. The combination of methane and nitrous oxide is estimated to contribute up to 20% of the increased global warming. Wetlands are the largest single natural source of methane in the atmosphere, accounting for 20% of the total (Matthews, 1993). Eroded soil could be considered a “transient wetland” with its similar nutrient dynamics.The aerobic nitrification process is a source of atmospheric N2O. After organic nitrogen is metabolized to NH4+, but before chemoautotrophic bacteria convert it to NO3, a fraction of the nitrogen is “leaked” as N2O. Denitrification can also release N2O when NO3is anaerobically converted to N2. Both anaerobic and aerobic reactions can occur in unsaturated soils. Aerobic reactions predominate in large, well-aerated pores and anaerobic activity in the smaller pores that can be found inside aggregates. Anaerobic reactions, such as methanogenesis and denitrification, increase with soil moisture content.Data suggest that such disturbance as cultivation or erosion also might decrease methane uptake as compared with that of undisturbed soil (Mosier et al., 1991). Increased moisture content also decreases CH4 uptake to soil, especially immediately after precipitation, while it increases N2O release (Vitousek and Matson, 1993).Tropical soils do not have a high percentage of SOM, but because they are often deep–more than a meter–they are significant reservoirs of the world’s stable soil organic carbon and nitrogen. The SOM buried deep in the soil profile remains stable unless exposed to the surface, such as during an erosion event. When organic matter becomes available, decomposition is favored with moderately high moisture content and high temperature. For this reason, tropical erosion caused by land clearing has the potential to change atmospheric gas content over a relatively brief time frame of decades. ConclusionErosion is a controllable element in the rise of certain atmospheric gases (N2O, CH4, and CO2). The organic matter contained in eroded topsoil is decomposed at an accelerated rate because of cultivation and land clearing. The enhanced amount of erosion and the associated decomposition rate of SOM are difficult to estimate because this pool of organic carbon and nitrogen defined by the process of erosion is such a “moving target.” The eroded organic matter mineralizes to CO2, CH4, and N2O at rates far in excess of its noneroded counterpart because of the loss of physical protection and the changes in environmental conditions caused by turbulent mixing during dislodgment and transport.

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