title: Sampling spatial and temporal variation in soil nitrogen availability.
abstract: 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+4and NO-3concentrations 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+4and NO-3concentrations increased with the age of the dune, whereas coefficients of variation for NH+4and NO-3concentrations decreased with the age of the dune. Soil NO-3concentration showed strong spatial structure, but soil NH+4concentration was not spatially structured. Plant-available NH+4and NO-3showed relatively little spatial structure: only NO-3availability in the second sampling period had significant patch structure. Spatial maps of NH+4and NO-3availability changed greatly over time, and there were few significant correlations among soil nitrogen availability at different points in time. NO-3availability in the second sampling period was highly correlated (r = 0.90) with the initial soil NO-3concentrations, providing some evidence that patches of plant-available NO-3may reappear at the same spatial locations at irregular points in time.
authors.affiliation: Department of Biology, New Mexico State University, Las Cruces, NM 88003, USA e-mail: mcain@nmsu.edu; Fax: +1-505-6465665, , , , , , MX.
authors.forename: Michael L
authors.initials: ML
authors.lastname: Cain
authors.affiliation: The Ohio State University Soil Ecology Laboratory, Columbus, OH 43210, USA, , , , , , US.
authors.forename: Scott
authors.initials: S
authors.lastname: Subler
authors.affiliation: Department of Biology, The University of the South, Sewanee, TN 37383, USA, , , , , , US.
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Evans
authors.affiliation: Département de biologie, Université de Sherbrooke, Sherbrooke, Québec, J1K 2R1, Canada, , , , , , CA.
authors.forename: Marie-Josée
authors.initials: MJ
authors.lastname: Fortin
keywords: Ion exchange membrane spikes; Key words Coastal dune ecosystems; Soil nitrogen availability; Soil resource heterogeneity; Spatial statistics