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) and nitrate (NO) 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 and NO 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 and NO concentrations increased with the age of the dune, whereas coefficients of variation for NH and NO concentrations decreased with the age of the dune. Soil NO concentration showed strong spatial structure, but soil NH concentration was not spatially structured. Plant-available NH and NO showed relatively little spatial structure: only NO availability in the second sampling period had significant patch structure. Spatial maps of NH and NO availability changed greatly over time, and there were few significant correlations among soil nitrogen availability at different points in time. NO availability in the second sampling period was highly correlated (r = 0.90) with the initial soil NO concentrations, providing some evidence that patches of plant-available NO may reappear at the same spatial locations at irregular points in time.