Using time domain reflectometry for monitoring mineralization of nitrogen from soil organic matter

The mineralization of nitrogen from soil organic matter is important when one tries to optimize nitrogen fertilization and assess risks of N losses to the environment, but its measurement is laborious and expensive. We have explored the possibilities for monitoring N mineralization directly using time domain reflectometry (TDR). Net N and S mineralization were monitored over a 101-day period in two layers (0-30 and 30-60 cm) of a loamy sand soil during aerobic incubation in a laboratory experiment. At the same time electrical conductivity of the bulk soil, sigma(a), was measured by TDR. A series of calibration measurements with different amounts of KNO3 at different soil moisture contents was made with the topsoil to calculate the electrical conductivity, sigma(w), of the soil solution from sigma(a) and theta. The actual sigma(w) was determined from the conductivity of 1:2 soil:water extracts (sigma(1:2)) with a mass balance approach using measured NO3- concentrations, after correction for ions present prior to the addition of KNO3. The average N mineralization rate in the topsoil was small (0.12 mg N kg(-1) day(-1)), and, as expected, very small in the subsoil (0.023 mg N kg(-1) day(-1)). In the top layer NO3- concentrations calculated from sigma(a) determined by TDR slightly underestimated measured concentrations in the first 4 weeks, and in the second half of the incubation there was a significant overestimation of measured NO3-. Using the sum of both measured NO3- and SO42- reduced the overestimation. In the subsoil calculated NO3- concentrations strongly and consistently overestimated measured concentrations, although both followed the same trend. As S mineralization in the subsoil was very small, and initial SO42- concentrations were largely taken into account in the calibration relations, SO42- concentrations could not explain the overestimation. The very small NO3- and SO42- concentrations in the B layer, at the lower limit of the concentrations used in the calibrations, are a possible explanation for the discrepancies. A separate calibration for the subsoil could also be required to improve estimates of NO3- concentrations.