Mississippi Basin Carbon Project Science Plan

Background

Humans have influenced many aspects of the Earth-surface environment and have markedly altered the biogeochemical and physical cycling of many materials. The Mississippi Basin Carbon Project (MBCP) of the U.S. Geological Survey (USGS) is an effort to examine interactions among some of the most profound human effects on the land surface. Humans have fundamentally changed the global carbon cycle by burning fossil fuels and by modifying a significant fraction of the world’s photosynthetic production for forestry and agriculture. Human activities have also radically transformed the land surface itself by converting large areas for human use and by drastically altering the transport of sediments and nutrients across the land and to the oceans. Interactions among these changes in the land surface and carbon cycle are central to many global environmental problems.

Human influences on the carbon cycle and on the nature of the land surface are inherently interrelated. One of the most fundamental connections is through climate. The relationships between global climate and atmospheric carbon dioxide (CO₂) are a worldwide public concern and a principal issue in global environmental policy deliberations. There is a broad consensus among climate scientists that global warming and changes in rainfall patterns will result from the enhanced greenhouse effect of increasing atmospheric CO₂ levels, augmented by the effects of increasing concentrations of other trace gases including methane (CH₄) and carbon monoxide (CO) (IPCC, 1996). Although the principal source of anthropogenic CO₂ is the burning of fossil fuels, atmospheric concentrations of CO₂, CH₄, and CO are significantly influenced by human alterations of the land surface. In turn, changes in atmospheric chemistry are thought to influence carbon cycling and other processes on the land surface through a complex system of climatic and biogeochemical feedbacks.

Fluxes and transformations of terrestrial carbon are also critical aspects of other global problems associated with human land use. On a global scale agricultural productivity is threatened by increasing erosion and declining fertility of soils, which comprise the largest carbon reservoir on the land surface. Natural organic compounds play a critical role in the transport and storage of contaminants in waters and sediments. Carbon compounds account for most of the oxygen depletion associated with anoxia and eutrophication in lakes, streams, and coastal waters. An improved understanding of interactions between changes in the land surface and changes in the global carbon cycle is essential to predicting and coping with global environmental change.

The need for this improved understanding is perhaps best illustrated by our inability to balance the present-day global CO₂ budget. The amount of CO₂ produced by burning fossil fuels and changing land use (especially by deforestation) appears to exceed the amount accumulating in the atmosphere and oceans. The carbon needed to balance the CO₂ budget (sometimes termed the "missing" carbon, amounting to about 1 to 2 x 10⁹ tonnes yr^-1) is probably absorbed by land plants and ultimately deposited in soils or sediments. Increasing evidence points toward carbon absorbed on land in the Northern Hemisphere. However, the specific identities of the needed CO₂ sinks have not yet been documented (IPCC, 1996). Thus, efforts to balance the global CO₂ budget focus particular attention on the terrestrial and coastal components of the global carbon cycle, and especially on the behavior of these components in our own North American "backyard."

The net carbon budget of land plants and soils has been a subject of intense debate and study for decades within the CO₂ research community (see, for example, Bolin and Eriksson, 1959). Questions persist about the balance of effects of deforestation and reforestation, the extent of fertilization due to increased atmospheric CO₂ concentrations, the response of soils to cultivation, and the influence of enhanced nutrient levels derived from wastes, rainfall, and chemical fertilizers. Possible carbon sinks can be calculated, but none have been directly observed that can account for the imbalance in the global CO₂ budget. New perspectives are needed to advance our understanding of carbon on the land surface.

In their summary of the effects of human activities on sediment yields of North American rivers, Meade and others (1990) observed that agricultural land use typically accelerates erosion 10- to 100-fold, and that "ninety percent of the sediment presently being eroded off the land surface of the conterminous United States is being stored somewhere in the river systems between the uplands and the sea." Stallard (1995a) calculated that this storage amounts to about 3 x 10⁹ tonnes of sediment within the conterminous United States and perhaps 30 x 10⁹ tonnes globally. Much of this sediment is stored in channels, behind dams, as alluvium, and as colluvium near sites of erosion. If this sediment contains 1.5% carbon (Stallard, in press), this would be a carbon sink amounting to 0.45x10⁹ tonnes yr^-1. Although this calculation is a crude approximation, it illustrates that significant quantities of organic carbon are buried in sediments influenced by human activities. The burial of this carbon does not in itself imply a net removal of CO₂ from the atmosphere. However, it points toward the potential for a significant and demonstrable net CO₂ flux caused by the effects of human land use on the interactions among terrestrial erosion, sedimentation, and soil development.

Soil erosion has an enormous impact on the balance between carbon inputs and losses in agroecosystems. Erosion appears to be particularly significant in the loss of organic matter from soils that have been cultivated for a period of decades (Wander and Traina, 1996). Although soil conservation practices may greatly reduce erosion, soil organic matter inputs are concentrated at the soil surface, and hence eroded materials may be enriched in carbon (Rasmussen and Collins, 1991). Most models of soil organic carbon (SOC) dynamics have not considered the effects of erosion or sediment deposition on carbon storage. These effects may occur not only through removal and transport of soil organic matter, but also because erosion associated with tillage repeatedly renews the substrates for soil organic matter formation. In most areas, the formation of soil organic matter occurs more rapidly in relatively fresh substrates (Schlesinger, 1986).

Previous studies of the movement of sediment and carbon across landscapes have emphasized either small upland watersheds or continent-to-ocean transfer. Measurement of carbon and sediment fluxes has been included in some studies of small watersheds, such as the USGS Water, Energy, and Biogeochemical Budget (WEBB) sites and the National Science Foundation (NSF) Long-Term Ecological Research (LTER) sites. Unfortunately, few if any of these sites are representative of terrain subject to agriculture or other intensive human modifications. Several syntheses have examined carbon and sediment fluxes in the discharge of relatively large rivers (Meybeck, 1982; Ittekkot and Laane, 1991; Degens, 1982) and continental contributions to the global marine carbon budget (Sarmiento and Sundquist, 1992). However, these studies have not addressed the importance of the central feature of terrestrial sediment transport: that vast quantities of sediment are stored on land before getting to the ocean. A few previous studies have examined the potential for significant carbon deposition in terrestrial sediments (Mulholland, 1981; Mulholland and Elwood, 1982; Kempe, 1984). Although these studies suggested global alluvial deposition of up to 11 x 10⁹ tonnes C yr^-1 (Mulholland, 1981), they did not attempt an assessment of relative rates of deposition and oxidation in both soils and sediments. Stallard (in press) attempted to bridge this gap using an ensemble of plausible sediment and soil scenarios to estimate rates of carbon burial on land. Rates of carbon burial between 0.6 and 1.5 x 10⁹ tonnes C yr^-1 appear to be entirely plausible, with about half being derived from upland erosion and half from autochthonous production in reservoirs, lakes, and agricultural wetlands. Moreover, most of this carbon burial results from human activities and occurs in northern temperate latitudes (Figure 1). Stallard (in press) suggested that these calculations suggest a plausible contribution to the so-called "missing" carbon.

By investigating these influences on carbon in soils and sediments of the Mississippi Basin, we hope to take a significant step toward understanding the global significance of human effects on terrestrial carbon cycling.

	Contents
	Summary
	Approach