Soil acidification and loss of base cations in a subtropical agricultural watershed

Soil acidification along with base cations loss degrades soil quality and is a major environmental problem, especially in agroecosystems with extensive nitrogen (N) fertilization. So far, the rates of proton (H⁺) production and real soil acidification (loss of base cations) remain unclear in subtropical agricultural watersheds. To assess the current status and future risk of soil acidification in subtropical red soil region of China, a two-year monitoring was conducted in a typical agricultural watershed with upland, paddy fields, and orchards where high N fertilizers are applied (320 kg N ha^-1 yr^-1). H⁺ production, neutralization and base cations losses were quantified based on the inputs (rainwater, inflow of water, and fertilizer) and outputs (outflow of water, groundwater drainage, and plant uptake) of major elements (K⁺, Ca²⁺, Na⁺, Mg²⁺, Al³⁺, NH₄⁺, NO₃^-, SO₄^2-, Cl^-, and H⁺). The result showed that total H⁺ production in the watershed was 5152 mol_c ha^-1 yr^-1. N transformation was the most important H⁺ source (68%), followed by excess plant uptake of cations (25%) and H⁺ deposition (7%). Base cations exchange and weathering of minerals (3842 mol_c ha^-1 yr^-1) dominated H⁺ neutralization, followed by SO₄^2- adsorption (1081 mol_c ha^-1 yr^-1), while H⁺ and Al³⁺ leaching amounted to 431 mol_c ha^-1 yr^-1, only. These results state clearly that despite significant soil acidification, the acidification of surface waters is minor, implying that soils have buffered substantially the net H⁺ addition. As a result of soil buffering, there was abundant loss of base cations, whose rate is significantly higher than the previously reported weathering rate of minerals in red soils (3842 vs 230-1080 mol_c ha^-1 yr^-1). This suggests that the pool of exchangeable base cations is being depleted in the watershed, increasing the vulnerability of the watershed, and posing a serious threat to future recovery of soils from acidification.