Soil erosion from agricultural fields removes valuable nutrients and organic matter from them; and, if severe enough, it can lead to significant decreases in both the productivity of the land and its ability to sequester carbon. That is why various soil conservation practices have historically been employed to reduce the severity of this phenomenon. There will, however, always be some erosion from nearly all agricultural fields; and a new study suggests that a modest amount of soil erosion may actually be beneficial, as it enhances the totality of carbon sequestration on the fields and the wetlands that border the streams that receive their sediments, which phenomenon thereby slows the rate of rise of the air's CO2 content and tempers its tendency to induce global warming.
How was this counterintuitive concept developed? Two inquisitive scientists with the USDA's Agricultural Research Service - G.W. McCarty and J.C. Ritchie - focused on the fact that soil erosion is but the "front end" of a greater redistribution process, which transfers a large portion of the eroded soil from one place to another within a larger segment of the landscape. What is one area's carbon loss, therefore, is another area's carbon gain.
At first blush, there may appear to be nothing to be gained by this type of carbon transfer; but McCarty and Ritchie (2002) proved otherwise. Working on a small watershed in Maryland, they meticulously measured soil carbon (C) contents at 25-meter grid-points across the area's agricultural fields, as well as within the riparian ecosystem associated with a first-order stream that meandered through it. On exposed farmland hilltops, they found soil C contents to average about 1%; but in the riparian ecosystem bordering the stream, they measured soil C contents on the order of fully 20%.
Although much of this higher-elevation to lower-elevation soil C content increase was indeed the result of C redistribution (and, thus, represented no net carbon gain for the watershed), McCarty and Ritchie noticed several things which indicate that this erosion-transport-storage process is responsible for much more than mere C redistribution.
First, they report that yearly-recurring growth and decay processes in the watershed's agricultural erosion areas "promote formation of new organic C in zones of soil loss," as predicted by Stallard (1998) and observed by Harden et al. (1999). This phenomenon results in the continual generation of new C on the erosion-prone agricultural fields and thereby provides a steady source of C for subsequent transfer to storage sites along the stream.
Second, the researchers note that carbon may be stored for much longer periods of time in the anoxic soils of the riparian ecosystem than in the aerobic soils of the watershed's farmland. On agricultural soils operated under so-called best management practices, for example, storage rates of about 0.2 tons C per hectare per year are typically observed, while rates of 1.6 to 2.2 tons C per hectare per year - a full order of magnitude larger - were found to prevail in the wetland soils the scientists studied.
Third, McCarty and Ritchie note that wetlands often acquire extra phosphorous from the sediments they receive from agricultural lands, and that they likewise acquire extra nitrogen via that which infiltrates the ground as a consequence of excess fertilizer applications. Both of these nutrient inputs help to boost the net primary production of the riparian ecosystem to a much higher level than what it otherwise would be able to sustain (Aerts et al., 1999; Bedford et al., 1999); and, hence, they thereby increase its active participation in the carbon sequestration process.
These three phenomena are what are primarily responsible for the scientists' observation that rates of carbon sequestration in the riparian zone of the watershed "are much higher than rates that have occurred over the pre-modern history of the wetland," although they also dutifully - and correctly - mention the auxiliary role of "CO2 fertilization increasing the biomass accumulation."
The findings of this new study are highly welcome, as they put a bright new face on what was long believed to be a phenomenon of absolutely no virtue whatsoever, i.e., soil erosion. They also provide a wholly new reason for protecting earth's wetlands, i.e., preserving an important sink for atmospheric CO2. In fact, the research of McCarty and Ritchie has shown that the carbon sequestration capacity of wetlands is anywhere from 4 to 7 times greater than what was previously believed, which is quite an accomplishment for a single study. Hats off to these perceptive scientists!
| Dr. Sherwood B. Idso | Dr. Craig D. Idso |
References
Aerts, R., Verhoeven, J.T.A. and Whigham, D.F. 1999. Plant-mediated controls on nutrient cycling in temperate fens and bogs. Ecology 80: 2170-2181.
Bedford, B.L., Walbridge, M.R. and Aldous, A. 1999. Patterns in nutrient availability and plant diversity of temperate North American wetlands. Ecology 80: 2151-2169.
Harden, J.W., Sharpe, J.M., Parton, W.J., Ojima, D.S., Fries, T.L., Huntington, T.G. and Dabney, S.M. 1999. Dynamic replacement and loss of soil carbon on eroding cropland. Global Biogeochemical Cycles 13: 885-901.
McCarty, G.W. and Ritchie, J.C. 2002. Impact of soil movement on carbon sequestration in agricultural ecosystems. Environmental Pollution 116: 423-430.
Stallard, R.F. 1998. Terrestrial sedimentation and the carbon cycle: Coupling weathering and erosion to carbon burial. Global Biogeochemical Cycles 12: 231-257.