Temperature effects on N mineralization: changes in soil solution composition and determination of temperature coefficients by TDR

We studied the changes in composition of the soil solution following mineralization of N at different temperatures, with a view to using TDR to calculate temperature coefficients for the mineralization of N. Mineralization from soil organic nitrogen was measured during aerobic incubation under controlled conditions at six temperatures ranging from 5.5 to 30degreesC, and at constant water content in a loamy sand soil. We also monitored during the incubation the concentrations of SO4 (2-) , Cl- , HCO3 (-) , Ca2+ , K+ , Mg2+ and Na+ , and the pH and the electrical conductivity in 1:2 soil:water extracts. Zero-order N mineralization rates ranged between 0.164 at 5.5degreesC and 0.865 mg N kg(-1) soil day(-1) at 30degreesC. There was a significant decrease in soil pH during incubation, of up to 0.6 pH units at the end of the incubation at 30degreesC. The electrical conductivity of the soil extracts increased significantly at all temperatures (the increase between the start and the end of the incubation was 4-fold at 30degreesC) and was strongly correlated with N mineralization. The ratio of bivalent to monovalent cations increased markedly during mineralization (from 2.2 to 5.9 at 30degreesC), and this increase influenced the evolution of the electrical conductivity of the soil solution through the differences in molar-limiting ion conductivity between mainly Ca2+ and K+ . Zero-order mineralization rate constants, k , for NO3 (-) concentrations calculated from TDR varied between 0.070 (at 5.5degreesC) and 0.734 mg N kg(-1) soil day(-1) (at 30degreesC), which were slightly smaller, but in the same range, as the measured rates. Underestimation of the measured N mineralization rates was due, at least in part, to differences in cation composition of the soil solution between calibration and mineralization experiments. A temperature-dependence model for N mineralization from soil organic matter was fitted to both the measured and the TDR-calculated mineralization rates, k and k (TDR) , respectively. There were no significant differences between the model parameters from the two. Our results are promising for further use of TDR to monitor soil organic N mineralization. However, the influence of changing cation ratios will also have to be taken into account when trying to predict N mineralization from measured electrical conductivities.