USING ANION-EXCHANGE MEMBRANES TO MEASURE SOIL NITRATE AVAILABILITY AND NET NITRIFICATION

There are few methods that provide adequate integrative measures of soil N availability to plants. We evaluated a new ion-exchange membrane (IEM) technique for measuring soil NO3 availability and nitrification by burying commercially-available anion-exchange membranes (IEMs) in a silt-loam Luvisol (fine, mixed, mesic Typic Fragiudalf) and in the same soil amended with either wheat straw, legume leaves or both materials. Soil was incubated in the laboratory for periods ranging from 1 to 27 days before removing the IEMs and determining membrane-bound NO3 and soil inorganic N concentrations. The soil amendments led to large differences in soil N dynamics and rates of net N mineralization and nitrification among treatments. In all soil treatments, rates of NO3-N uptake by IEMs were rapid initially, but slowed after 7 days. On all dates, significantly less NO3-N was recovered from IEMs in the amended soils than from those in the control soil, probably because of greater microbial immobilization of NO3 due to the added organic substrates. The IEMs did not act as infinite sinks for NO3, since under strongly N-immobilizing conditions in the wheat straw-amended soil membrane-bound NO3-N declined between 3-14 days. Across all soil treatments and sampling dates, membrane-bound NO3-N was significantly correlated with soil NO3-N concentrations and net soil nitrification (r(2) = 0.53 and 0.86, respectively), even when net nitrification was negative. Correlations improved when data from the initial membrane equilibration period (days 1 and 3) were excluded (r(2) = 0.85 and 0.96, respectively). The presence of IEMs significantly reduced soil NO3-N concentrations in 3 of the 4 soil treatments, and in all of the soil treatments, net soil nitrification and N mineralization were significantly greater in the presence of the IEMs. Results from our correlative study suggest that the IEM technique can be a useful tool for assessing soil nutrient availability and mineralization processes. However, more work is needed to develop this technique before reliable interpretations can be made under a wide variety of field and laboratory conditions.