15N isoscapes in a subtropical savanna parkland: spatial-temporal perspectives

Spatial patterns of soil delta N-15 reflect variation in rates of N-cycling processes across landscapes. However, the manner in which soil delta N-15 is affected by vegetation and topoedaphic properties under non-steady state conditions is understood poorly. Here we propose and evaluate a conceptual model that explains how soil delta N-15 values will respond to changes in disturbance regimes (intensification of grazing and removal of fire) and the resultant invasion of a subtropical grassland by woody vegetation dominated by Prosopis glandulosa (honey mesquite), a N-fixing tree legume. Spatially-specific sampling along a catena (hill-slope) gradient where woody plants are known to have displaced grasses over the past 100 years revealed a positive relationship between soil delta N-15 and delta C-13, and a negative relationship between NDVI and soil delta N-15 on upland portions of the landscape, indicating that plant cover is a critical determinant of delta N-15 spatial patterns. Because the dominant woody invader is a N-fixer, its invasion has increased N input and reduced soil delta N-15. However, while honey mesquite also invaded and came to dominate lowland portions of the landscape, soil delta N-15 values in woodlands of intermittent drainages were significantly elevated relative to those in uplands. This is likely attributable to higher soil moisture, clay content, and total N in the lower portions of the catena gradient, which create conditions favoring more rapid N-transformation rates, higher preferential 14 N losses (e. g., gaseous), and thus enrichment of N-15. Thus, while spatial and temporal variation of soil delta N-15 has the potential to be an indicator of disturbance-induced changes in the net N balance, its sensitivity is compromised in topoedaphic settings with where rates of N-transformation are high. Continued improvements in our understanding of controls over the spatial variability of soil delta N-15 at the landscape-scale will enhance our ability to use delta N-15 as a diagnostic tool for inferring N dynamics under both steady-state and disturbed conditions.