Effects of acidity on mineralization: pH-dependence of organic matter mineralization in weakly acidic soils

The literature is ambiguous regarding the influence of acidity on mineralization of soil organic matter. Although mineralization is often regarded as being relatively insensitive to acidity, reports of agronomically-significant increases in N mineralization after liming of acid soils are common. We analyzed 61 soils (pH 5.1–7.9), representing all agro-ecological zones of Saskatchewan, Canada, to determine the pH-dependence of N mineralization. Mineralization was measured by aerobic incubation. There was no statistical relationship between the parameters of the first-order kinetic equation [i.e. the rate constant (k) and potentially mineralizable N (N0)] used to describe the incubation data and soil pH. However, when pH of two slightly acid (pH 5.7 and 5.8) soils was raised using Ca(OH)2, mineralization of N and C was stimulated. Initially, the rate of CO2 evolution from soils treated with Ca(OH)2, to raise pH to 7.3–7.4, was 2–3 times that from the unamended soils. Rate of CO2 evolution from Ca(OH)2-treated soil declined rapidly after about 7–10 d. During the entire 100-d incubation, Ca(OH)2-treated soils at pH 7.3–7.4 produced 37% and 67% more CO2C than their untreated counterparts. We observed comparable increases in N mineralization. The effect of Ca(OH)2 was attributed to release of labile organic matter when pH was increased. Dissolved organic matter in saturated paste extracts was well correlated with C and N mineralized. A model consisting of two simultaneous first-order equations was needed to describe mineralization in Ca(OH)2-treated soil. Application of Ca(OH)2 increased the labile pool of mineralizable C from 18 to 157 mg kg−1 in one soil and from 45 to 301 mg kg−1 in the other. We showed that the phosphate-borate buffer test for mineralizable N is pH-dependent because of the effect of pH on organic N solubility. In contact with the buffer, soil pH is raised to 11.2 (i.e. buffer pH), resulting in release of organic N, which is then susceptible to hydrolysis. Organic N extracted using an unbuffered extractant, hot 2 m KCl, was independent of soil pH.