Sampling spatial and temporal variation in soil nitrogen availability

There are few studies in natural ecosystems on how spatial maps of soil att ributes change within a growing season. In part, this is due to methodologi cal difficulties associated with sampling the same spatial locations repeat edly over time. We describe the use of ion exchange membrane spikes, a rela tively 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 (NH4+) and nitrate (NO3-) availability in a mid-successio nal coastal dune for four periods of time during the growing season. For a single point in time, we also measured soil NH4+ and NO3- concentrations fr om soil cores collected from the mid-successional dune and from an early an d a late successional dune. Soil nitrogen concentrations were low and highl y variable in dunes of all ages. Mean NH4+ and NO3- concentrations increase d with the age of the dune, whereas coefficients of variation for NH4+ and NO3- concentrations decreased with the age of the dune. Soil NO3- concentra tion showed strong spatial structure, but soil NH4+ concentration was not s patially structured. Plant-available NH4+ and NO3- showed relatively little spatial structure: only NO3- availability in the second sampling period ha d significant patch structure. Spatial maps of NH4+ and NO3- availability c hanged greatly over time, and there were few significant correlations among soil nitrogen availability at different points in time. NO3- availability in the second sampling period was highly correlated (r = 0.90) with the ini tial soil NO3- concentrations, providing some evidence that patches of plan t-available NO3- may reappear at the same spatial locations at irregular po ints in time.