Comparison of N-15 labelling methods to measure gross nitrogen mineralisation

A laboratory study was conducted to compare methods of labelling the soil N H4+ pool with N-15 as a prerequisite to the measurement of gross N minerali sation. Composite soil (0-12 cm) was collected from 12 sites differing in l and use and soil texture. Three methods were used to apply N-15 to the soil : (i) addition of a (NH3)-N-15-air gas mixture to the headspace above the s oil (gas jar), (ii) injection of a (NH3)-N-15-air gas mixture into re-packe d soil cores (gas injector) and (iii) addition of a ((NH4)-N-15)(2)SO4 solu tion pipetted onto the soil surface (solution), N-15 isotopic dilution was determined between 24 and 72 h after N-15 application. Of the applied N-15 52 to 93% was recovered as inorganic N 24 h after application. Less N-15 wa s recovered in the clay loam compared to the sandy loam and silty clay loam for all methods. Recovery from the gas injector was lower than from the ga s jar or solution treatments in the sandy loam and silty clay loam. No diff erence in the recovery of applied N-15 between methods was observed in the clay loam. N-15 isotopic dilution was less in the gas jar compared to the g as injector and solution treatments. This resulted in significantly lower e stimates of gross N mineralisation rates and NH4+ consumption for the gas j ar treatment. The non-uniform distribution of N-15 through the soil probabl y caused these differences as a consequence of (NH3)-N-15 dissolving quickl y in the surface of the soil. In seven of the 12 soils, gross rates of N mi neralisation were not significantly different when N-15 was applied as solu tion or by gas injection. In the remaining five soils, estimates of gross N mineralisation rates were greater in the solution treatment. Ammonium cons umption was significantly different between the solution and gas injector t reatments in three soils, Where there were differences in values for gross N mineralisation and NH4+ consumption rates between methods, the effects we re not associated with a particular soil texture or land use. Greater nitri fication of (NH4+)-N-15 to (NO3-)-N-15 and shorter pseudo-residence rimes o f the NH4+ pool were observed in the solution treatment compared to gas inj ector, This suggests a difference in the distribution of N-15 within soil m icrosites, with the N-15 applied in the solution treatment being more acces sible for microbial consumption. Nevertheless, application of N-15 as an NH 4+ solution, or injection of (NH3)-N-15 gas into soil, resulted in estimate s of a similar magnitude for gross N mineralisation rates and identified th e same differences between soil type and land use. (C) 1999 Elsevier Scienc e Ltd, All rights reserved.