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Soil organic carbon contents for 9 million acres of agricultural soils used for rice and their non-agricultural predecessors have been tabulated. For the top soil layer (0 to 15 cm depth) approximately 0.7% carbon has been lost; for the next layer (15-30 cm) the agricultural soil has 0.3% more carbon than the nonagricultural soil. Thus the net carbon loss as a result of agricultural activities in these soils to a depth of approximately 0.3 meters is 0.4% or 5 tons per acre. It appears that the agricultural soils used for rice (and soybeans) in the Mississippi River Basin (Arkansas and Louisiana) are not the reservoir for the "missing" carbon. In fact, the conversion of these soils (some originally forested) to rice/soybean production has been a (small) net source of C02 to the atmosphere over the last 50 years. Introduction Global climate change is thought to be induced by anthropogenic activities that add greenhouse gases (such as carbon dioxide and methane ) to the atmosphere in excess the "natural" steady state (IPCC 1994). Actually the steady-state has changed with time as shown by measurement of C02 and CH4 in ice cores with ages as old as 100,000 years (Raynaud and others, 1993). The cycles of these greenhouse gases have been studied extensively but their exact non-atmospheric reservoirs are still somewhat uncertain. This is in part because there are so many varied sources and sinks for these gases. It is also because the carbon contents in soils and the biosphere that are their sources and sinks are averages of artificially designated groups rather than a continuium of the real world. Organic carbon stored (fixed) in soils that were covered/glaciated with ice has changed (increased) since the last ice age (Harden and others, 1993). Computer/mathematical modelling and dynamics of C02 transfer (uptake and emission) between the atmosphere, oceans, and biosphere gives results that are out of balance by 1.8±1.4 G tons (1 Gt=lPg, G=109, P=>1015 ) (Sundquist 1993) which amounts to 1% of the total budget. Using C02 and Carbon-13 isotopic measurements (Keeling et al, 1995) in the models seems to indicate that this "missing" (unaccounted) carbon may be in the northern terrestrial reservoir; that is, stored in northern temperate latitudes in soils or biomass (forests) (Deming and others, 1995; Sarmiento, 1993) while others are not sure if it is an oceanic or terrestrial sink (Francey and others, 1995). This Report gives the data for agricultural rice soils in the Mississippi River basin in order to see if they might be part of the sink for this "missing" carbon. Carbon in soils has changed because of the extensive agricultural activities and other anthropogenic activities such as destruction (filling and draining) of natural wetlands. The effects of agriculture are usually to decrease the amount of carbon stored in soils due to exposure of organic matter to oxidation from plowing and also due to increased erosion. For the last century, the practices of modern agriculture have resulted in the loss of carbon stored in soils. Schlesinger (1995) summarizes this work and reports losses of 20 to 40% of the soil carbon when virgin lands are converted to agriculture. This loss is greatest during the first years of cultivation and slower after about 20 years and depends on how refractory the soil carbon is (Harrison and others, 1993). Current rates of loss are estimated to be 0.8 Pg per year (mostly in the tropics) (Schlesinger 1995; Houghton, 1995, Eswaran and others, 1995). However, because rice soils are flooded during part of the growth cycle, it was considered possible that this general loss of soil carbon might be reduced or might be different for these soils. This report concerns the extensive acreage where rice is grown in the Mississippi River Basin in Arkansas and Louisiana.

u.s. geological survey open file report/open-file report

Soil organic carbon content in rice soils of Arkansas and Louisiana and a comparison to non-agricultural soils; including a bibliography for agricultural soil carbon