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

Abstract

Gross rates of N mineralization–immobilization turnover (MIT) and gross nitrification in soil can be determined by use of a ¹⁵NH₄⁺ pool dilution technique, under the assumption that native ¹⁴NH₄⁺, applied ¹⁴NH₄⁺ 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 ¹⁵NH₄⁺ solution into intact soil cores, and (2) mixing ¹⁵NH₄⁺ 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 ¹⁴NH₄⁺, injected ¹⁵NH₄⁺ 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 ¹⁵NH₄⁺, and thereby increases the contact between NH₄⁺ and nitrifiers.