Weathering and soil formation rates based on geochemical mass balances in a small forested watershed under acid precipitation in subtropical China

Accurate weathering and soil formation rates in natural environment and their quantitative dependences on environmental factors remain poorly understood, despite their significance in the understanding of biogeochemical cycling and for the development of sustainable land-use strategies. In the present study, rates of weathering and soil formation on granite and their dependences on acid precipitation were studied in a small forested watershed, Fengxingzhuang (FXZ), in subtropical China using geochemical mass balance equations and multiple regression analysis. Atmospheric input from wet and dry deposition, and stream output through runoff were monitored from March, 2007 to February, 2010. The physical and chemical properties of soil and granite rock were also determined. The results show that acid precipitation is very severe in the FXZ forested watershed by bringing in H+ both directly from strong acids (741 mol ha− 1 yr− 1) and indirectly from nitrogen and sulfur transformations (831 mol ha− 1 yr− 1), which serves as an important driving force for weathering and soil formation. The FXZ forested watershed currently remains as a net sink for hydrogen ion, inorganic nitrogen, sulfur, potassium, and aluminum, with a mean of 0.07, 1.60, 1.48, 0.10, and 0.44 g m− 2 yr− 1, respectively, and a net source for dissolved silicon, sodium, calcium, and magnesium, with a mean of 5.33, 2.95, 1.04, and 0.34 g m− 2 yr− 1, respectively. Based on geochemical mass balance equations, the weathering rate of granite (R) in the FXZ forested watershed is 1.04 ± 0.65 t ha− 1 yr− 1, but varying according to the changes in environmental conditions such as rainfall amount, air temperature, and H+ input within different seasons. Correspondingly, soil formation rate (S) is 0.95 ± 0.69 t ha− 1 yr− 1, which equals to 0.066 ± 0.048 mm yr− 1 of soil depth, suggesting a base for establishing the soil loss tolerance value in the granitic region of subtropical China. Acid precipitation significantly promotes weathering and soil formation by the input of rainfall and H+, although air temperature effects occur simultaneously. A proposed model that quantitatively describes weathering and soil formation rates is a combined product of rainfall amount (x1), H+ input (x2) and air temperature (x3) resulting from multiple regressions: R = 0.00045 ∗ x1 + 0.026 ∗ e0.0022x2 + 0.012 ∗ e0.0459x3; S = 0.00035 ∗ x1 + 0.029 ∗ e0.002x2 + 0.016 ∗ e0.0424x3, which improves the prediction from simple linear regression.

► Small watershed scale source-sink characteristics of elements over a 3-year period
► Field weathering and soil formation rates with consideration of dry deposition in mass balance
► An improved quantitative model that coupled effects of H+ input, rainfall and temperature