Rates of weathering and soil formation

Abstract

A new method for evaluation of rates of weathering and soil formation is proposed. For the elements derived exclusively from rocks, geochemical mass balances were assumed to exist between parent rocks, soils and river waters on both global and regional scales. Seven geochemical mass balance equations, which include the rate of weathering and soil formation as unknown quantities, were set up using the mean contents of seven major elements (Al, Fe, Ca, K, Mg, Na, and Si) in the earth's crust, soils and river waters, respectively. Solutions of 21 sets of simultaneous equations made from the seven equations gave 21 sets of values of rates of rock weathering and soil formation. Reliability of the results were checked by the criterion that the rate of rock weathering is higher than that of soil formation and both of them have to be positive values. The ranges of results which satisfied the criterion were 520–1310 (mean = 1100) kg ha −1 yr −1 for the rate of rock weathering and 370–1290 (mean = 700) kg ha −1 yr −1 for the rate of soil formation on a global scale. The comparison with the reported value of the mean rate of erosion, 906 kg ha − yr −1, suggested that the results of the calculations were reliable. By applying this method, we could calculate the annual rate of soil formation not only on a global scale but also on a regional scale. Some regional examples in small watersheds were tested. The rate of soil formation was 569 kg ha −1 yr −1 in Hubbard Brook watershed, New Hampshire, USA and 2700–5700 kg ha −1 yr −1 in Iu river watersheds in southwestern Japan. The rates of weathering and soil formation in watersheds are basic items of information for establishing sustainable watershed management practices to protect soil from degradation, to control desertification, and to minimize the effects of acid precipitation.