Evaluation of an in situ net soil nitrogen mineralization method in dryland agroecosystems

Direct quantitative measurement of soil net N mineralization in agricultural soils under field conditions has not been widely used. A potential method of in situ net N mineralization was investigated in the fallow phase of a 3-yr no-till crop rotation at two sites. Undisturbed soil cores (5 by 15 cm) with anion- and cation-exchange resins (Sybron Ionac ASB-1P and C-249) at the bottom were incubated in situ. Nitrate-N plus NH4+-N extracted from soil was added to extracted amounts from resin bags to determine net N mineralized during each of three incubation periods (3-4 wk each). Total net N mineralization was 33.7 and 26.5 kg N ha(-1) during 84 and 75 d of incubation at Sterling and Stratton, respectively. Relative amounts of resin did not affect N captured but cores placed midway between old corn (Zea Mays L.) rows tended to accumulate more (P > F = 0.13) N than cores placed in rows. This in situ method appears to be a reliable method for measuring net N mineralization in the field; however, variation is large and many observations are required to obtain net N mineralization rates within an acceptable confidence interval.