Gross N mineralization-immobilization rates in heterogeneous intact soil cores can be estimated without marked error

Gross rates of N mineralization - immobilization turnover (MIT) and gross nitrification in soil can be determined by use of a (NH4+)-N-15 pool dilution technique, under the assumption that native (NH4+)-N-14, applied (NH4+)-N-14 and microbial processes are uniformly distributed in the soil. In a laboratory investigation we compared gross N fluxes obtained from two labelling techniques applied to an arable sandy loam: ( 1) injection of (NH4+)-N-15 solution into intact soil cores, and ( 2) mixing (NH4+)-N-15 solution into disturbed soil. It was assumed that MIT obtained with the mixing technique reflected the true rates, since the assumption of uniformity was thought to be satisfied by this technique. MIT from the two techniques were not significantly different, thus non-uniform distribution of native (NH4+)-N-14, injected (NH4+)-N-15 and microbial processes in the intact-core technique did not cause a marked error in the MIT rates. In contrast the gross nitrification rates were twofold higher with the mixing technique than with the intact-core technique. Gross nitrification rates are likely to increase with the mixing technique because mixing redistributes nitrifiers and added (NH4+)-N-15, and thereby increases the contact between NH4+ and nitrifiers.