Contributions to gross N mineralization from N-15-labelled soil macroorganic matter fractions during laboratory incubation

We examined the role of macroorganic matter contributions to rates of gross nitrogen mineralization measured during a laboratory incubation at 15 degrees C. Two N-15-labelling techniques were employed. The first, referred to as the direct method, involved a mirror-image approach whereby labelled or unlabelled macroorganic matter from two different soils was incorporated into soil, and the contribution of this material to gross mineralization was measured using a combination of isotope dilution and pool enrichment procedures. Macroorganic matter was extracted from a sandy loam (Sonning soil) and an organic loam (Black soil). The second technique, a difference method, involved comparing measured gross rates of mineralization in soil with or without the incorporated unlabelled macroorganic matter. During the 66 day incubation, approx. 2.4 and 10.8% of the incorporated macroorganic matter was released from the Sonning and Black soil macroorganic matter fractions, respectively. Following correction to the amounts of macroorganic matter normally recovered in the soils, it was estimated that the Black macroorganic matter fraction contributed from 3 to 21.9% of the gross N mineralization, with a mean of 12%. The contribution of the Sonning macroorganic matter ranged from 0% initially to approx. 14% at the end of the incubation, with a mean of 4.5%. Agreement between the two techniques was reasonable, although the variation encountered in the difference method was considerably less than that observed in the direct method.