EVALUATION OF METHODS FOR ESTIMATING NET NITROGEN MINERALIZATION IN A SEMIARID GRASSLAND

Methods for estimating net N mineralization in semiarid grasslands are not well tested. We compared three in situ incubation methods to evaluate (i) effects of root density and length of incubations on methodological artifacts involving N immobilization, (ii) effects of sieving on estimates of net N mineralization, and (iii) effects of containment on soil water dynamics. We incubated intact soils and sieved soils with or without roots for 15 to 90 d in uncovered tubes and determined changes in NO3 and NH4. Net N mineralization was transient and usually declined after 15 to 30 d. Nitrogen immobilization may have contributed to decreases in extractable N, but root abundance was not a dominant control of net N mineralization. Greater root mass under blue grama grass [Bouteloua gracilis (Willd. ex Kunth) Lagasca ex Griffiths] plants was expected to promote immobilization, but net N mineralization was higher under plants than in bare areas. Removing roots from sieved soil enhanced net mineralization, but not consistently. Sieving soil caused problematic changes in N dynamics. Under plants, for example, N concentrations were higher in sieved than intact soil after 15 d, but lower after 60 d. Effects of sieving mag reflect accelerated depletion of a small, biologically active organic matter pool during prolonged incubations. Soil in tubes was slightly wetter than bulk soil during moist periods, but water content in tubes tracked ambient conditions well. We recommend 15- to 30-d incubations of intact cores to avoid artificially high rates of net mineralization caused by soil disturbance and to capture effects of environmental variation.