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.