Soil erosion contributes to the removal of soil organic carbon (SOC) from cultivated soils and its entrapment in terrestrial depressions. The fate of SOC entrapped in terrestrial deposits is largely unknown, but there has been speculation that such entrapment could lead to C sequestration, thereby playing a role in global C cycling. It has been hypothesized that the fate of eroded C in these deposits depends on SOC quality (bioavailability) and the environmental conditions at the depositional site. The SOC storage and dynamics were studied in cultivated, forested, and deposited soils at sites with and without subsurface tile-drainage. Microbial biomass carbon (MBC), readily mineralizable C (MinC), and basal soil respiration (BSR) rates were used as SOC quality indicators and were assessed in field-moist soil aggregates of four sizes: 2-3 mm, 1-2 mm, 0.5-1 mm and <0.5 mm. Soil organic carbon inventory (mass of C over the depth sampled) was significantly different (P < 0.01) among the land uses and was in the order: deposit > forest > cropland. It was also significantly (P < 0.1) different between the two deposits, amounting to 14.6 and 17.1 kg C m−2 in the tile-drained and undrained deposits, respectively. Over that same depth, the total SOC pool in the forest soil was 9.5 kg C m−2. Overall, the various aggregate sizes did not differ significantly in terms of their total SOC content, but the SOC quality indicators (MBC, MinC, and BSR) were generally higher in the larger than in the smaller aggregates. These indices were also higher in the forest and depression sites than in the croplands. The data indicated that cultivation and erosion resulted in depletion of both total and labile SOC, but the labile pools were depleted at rates 1.5 to 3 times faster. Conversely, there was an enrichment in both clay (1.4 to 2 times) and SOC (1.3 to 1.6 times) in the depression areas, indicating removal of fine particle-associated SOC from the cultivated fields and its entrapment in the deposits. However, the levels of labile C (MBC, MinC) in the entrapped materials were 20 to 46% lower than would be anticipated based on their total C contents. These reductions suggest that, compared with the forest and cropland, a relatively greater proportion of the C retained in the deposits is in the slow and passive pools, and that distribution is favorable to sequestration of C in these landscape positions.
The soils of the world play a crucial role in global C circulation, not only because of the size of the soil C reservoir (1576 pg C; Eswaran et al., 1993), but also because of the dynamic character of some soil organic carbon (SOC) fractions. Although soils can be a net sink for C, available data indicate that, as a whole, managed soils have historically been a net source of atmospheric CO2 (Schlesinger and Andrews, 2000), contributing to more than 20% of the annual increase of CO2 (3.2 pg C; Watson et al., 1992) in the atmosphere. A reduction in the rate of CO2 accumulation in the atmosphere can be achieved through enhancement of long-term C storage or sequestration in the terrestrial biosphere.
The interactions of SOC with the soil primary particles and its distribution among the soil structural units are dominant controllers of SOC accumulation and decomposition (Oades, 1988). The hierarchical concept (Oades, 1984) proposes that soil is an organized system in which, through various bonding mechanisms, smaller structural units are assembled into larger units or macroaggregates. Available evidence suggests that the macroaggregates are held together by recently deposited organic matter (Puget et al., 1995; Jastrow et al., 1996; Angers and Giroux, 1996). This view is consistent with the observations that macro-aggregate-C is more labile (Miller and Dick, 1995; Angers and Giroux, 1996) and more sensitive to disturbances such as tillage (Elliott, 1986) than the C associated with microaggregates.
Water erosion results in the breakdown of impacted soil aggregates and the release of SOC previously protected within these aggregates. Erosion selectively removes the lighter soil particles, which tend to contain some of the most labile SOC fractions (Tiessen and Stewart, 1983). Some of the eroded materials are carried to rivers and streams, but the bulk of these materials is deposited in depressions downslope of the eroding landscape. As much as 90% of eroded soils can be retained in terrestrial deposits (Walling, 1983).
The fate of eroded C trapped in terrestrial deposits is largely unknown. Gregorich et al. (1998) suggested that accumulation of eroded C in deposits could lead to long-term SOC storage. Arguing that eroded SOC is more likely to remain immobilized rather than to undergo mineralization, Stallard (1998) proposed that terrestrial sedimentation could act as a C sink. van Noordwijk et al. (1997) advanced a similar view and contended that relocation of SOC by erosion could have a positive effect on C sequestration. They asserted further that erosion control measures may be counter to C sequestration objectives. A fundamental premise of these contentions is that increased clay and moisture contents in deposits create an oxygen-poor environment that favors SOC accumulation over mineralization. These views have not yet been evaluated in field studies carried over a range of environmental conditions. It is possible, however, that eroded C is entrapped in upland deposits, downslope of an actively eroding landscape, rather than translocated to acidic swamps, freshwater, and marine sediments as many have assumed (Schimel et al., 1985; van Noordwijk et al., 1997). Contrary to these reports, data from Anderson et al. (1986) indicate that deposited soils can even be SOC-depleted relative to native and cultivated soils if active mineralization of C occurs at the sites of deposition.
To determine whether entrapment of eroded C in terrestrial deposits can lead to its sequestration, it is necessary to evaluate the quality of the C retained in these deposits. Soil organic carbon quality is defined here as a measure of the SOC availability to microbial degradation (Agren and Bosatta, 1999). Numerous chemical and biochemical properties of soils have been used as indicators of soil organic matter quality, including (i) microbial biomass carbon (MBC), the engine of organic matter decomposition in soils, and (ii) basal soil respiration (BSR) and readily mineralizable C (MinC), which can be viewed, respectively, as indices of decomposers performance and of the overall quality of the organic matter available for microbial degradation. It is hypothesized that the fate of SOC trapped in terrestrial sediments depends on the biochemical quality of deposited SOC as well as on environmental factors controlling aeration status and biological activity at the sites where it is deposited. Clay content and drainage conditions at the depositional sites, because of their role in the physical protection of SOC and in the regulation of moisture and oxygen availability, could be major controllers of the fate of eroded SOC trapped in terrestrial deposits.
A large portion of cropland in the North Central region of the United States lacks adequate internal drainage and requires installation of tile-drainage infrastructures for removal of excess water to allow timely implementation of farming activities. A 1982 survey by the USDA-Natural Resources Conservation Service (NRCS) estimated that 20.8 × 106 ha of agricultural lands in that region are drained. The same survey also indicated that about half (3 × 106 ha) of all cropland in Ohio is tile-drained (USDA-NRCS, 1987). When present, drainage infrastructure is likely to alter the storage and dynamics of eroded SOC at depositional sites, primarily through its effect on soil aeration.
This research was undertaken to study the dynamics of SOC at depositional sites under two contrasting drainage regimes: tile and no-tile drainage. Indicators of SOC quality were also evaluated in samples taken from corresponding upland cultivated fields and from an adjacent hardwood forest. Data obtained were analyzed to understand the fate of eroded C in terrestrial deposits and to evaluate the impact of soil erosion on the global C cycle.