Abstract
The 15N isotope pool dilution (IPD) technique is the only available method for measuring gross ammonium (NH4+) production and consumption rates. Rapid consumption of the added 15N-NH4+ tracer is commonly observed, but the processes responsible for this consumption are not well understood. The primary objectives of this study were to determine the relative roles of biotic and abiotic processes in 15N-NH4+ consumption and to investigate the validity of one of the main assumptions of IPD experiments, i.e., that no reflux of the consumed 15N tracer occurs during the course of the experiments. We added a 15N-NH4+ tracer to live and sterile (autoclaved) soil using mineral topsoil from a beech forest and a grassland in Austria that differed in NH4+ concentrations and NH4+ consumption kinetics. We quantified both biotic tracer consumption (i.e. changes in the concentrations and 15N enrichments of NH4+, dissolved organic N (DON), NO3− and the microbial N pool) and abiotic tracer consumption (i.e., fixation by clay and/or humic substances). We achieved full recovery of the 15N tracer in both soils over the course of the 48 h incubation. For the forest soil, we found no rapid consumption of the 15N tracer, and the majority of tracer (78%) remained unconsumed at the end of the incubation period. In contrast, the grassland soil showed rapid 15N-NH4+ consumption immediately after tracer addition, which was largely due to both abiotic fixation (24%) and biotic processes, largely uptake by soil microbes (10%) and nitrification (13%). We found no evidence for reflux of 15N-NH4+ over the 48 h incubation period in either soil. Our study therefore shows that 15N tracer reflux during IPD experiments is negligible for incubation times of up to 48 h, even when rapid NH4+ consumption occurs. Such experiments are thus robust to the assumption that immobilized labeled N is not re–mobilized during the experimental period and does not impact calculations of gross N mineralization.
Highlights
Nitrogen (N), in its inorganic forms ammonium (NH4+) and nitrate (NO3−), is often considered to be the limiting nutrient for plants in terrestrial ecosystems (Falkowski et al, 2008)
We found complete 15N recovery from live grassland and forest soils in the combined fixed and labile N pool, the labile N pool representing the sum of the extractable N pool (NH4+, NO3− and dissolved organic N (DON)) plus the microbial N pool (Table 2)
Time had a significant effect on total 15N recoveries in both soils (Table 2) but mean values were indistinguishable from 100%, given the large variance around the mean which arises from the propagation of measurement errors for concentration and atom%15N from six different
Summary
Nitrogen (N), in its inorganic forms ammonium (NH4+) and nitrate (NO3−), is often considered to be the limiting nutrient for plants in terrestrial ecosystems (Falkowski et al, 2008). A powerful tool for the determination of soil N transformation processes is the isotope pool dilution (IPD) technique (Barraclough, 1991; Di et al, 2000; Kirkham and Bartholomew, 1954; Wanek et al, 2010), which allows to estimate both rates of gross production and gross consumption of major plant nutrients in soil This technique has been used across a wide range of natural and agricultural systems to study N transformation rates in soil (e.g., Booth et al, 2005, 2006; Hart et al, 1994; Murphy et al, 2003), and is recognized as the recommended method to obtain estimates on soil N dynamics (Hart et al, 1994). Depending on tracer application approaches e.g. to intact soil-plant systems in situ or to sieved soils, plant mediated processes are included such as root uptake of inorganic N or tracer dynamics only reflect microbial processes such as in sieved soils (Murphy et al, 2003; Rütting et al, 2011)
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