Sampling spatial and temporal variation in soil nitrogen availability

There are few studies in natural ecosystems on how spatial maps of soil attributes change within a growing season. In part, this is due to methodological difficulties associated with sampling the same spatial locations repeatedly over time. We describe the use of ion exchange membrane spikes, a relatively nondestructive way to measure how soil resources at a given point in space fluctuate over time. We used this method to examine spatial patterns of soil ammonium (NH⁺₄) and nitrate (NO^-₃) availability in a mid-successional coastal dune for four periods of time during the growing season. For a single point in time, we also measured soil NH⁺₄ and NO^-₃ concentrations from soil cores collected from the mid-successional dune and from an early and a late successional dune. Soil nitrogen concentrations were low and highly variable in dunes of all ages. Mean NH⁺₄ and NO^-₃ concentrations increased with the age of the dune, whereas coefficients of variation for NH⁺₄ and NO^-₃ concentrations decreased with the age of the dune. Soil NO^-₃ concentration showed strong spatial structure, but soil NH⁺₄ concentration was not spatially structured. Plant-available NH⁺₄ and NO^-₃ showed relatively little spatial structure: only NO^-₃ availability in the second sampling period had significant patch structure. Spatial maps of NH⁺₄ and NO^-₃ availability changed greatly over time, and there were few significant correlations among soil nitrogen availability at different points in time. NO^-₃ availability in the second sampling period was highly correlated (r = 0.90) with the initial soil NO^-₃ concentrations, providing some evidence that patches of plant-available NO^-₃ may reappear at the same spatial locations at irregular points in time.