Spatial variability of soil metabolic rate along a dryland elevation gradient

Abstract

A general framework of ecosystem hotspots suggests variation in soil metabolic activity can be understood through the relative distribution and intensity of patches of disproportionately high ecosystem process rates. To better understand the causes of soil metabolic spatial variability and the variation in ecosystem hotspots we quantified soil respiration (R) spatial heterogeneity across a network of seven sites spanning a 2,489 m elevation gradient in the Santa Rosa Mountains of Southern California. At each site, soil samples were collected from 0–5 and 5–15 cm soil depths at 2 m intervals along three 100 m transects. Each soil sample was analyzed for organic matter content (SOM) and was incubated at 40% water holding capacity for 20 days. R was measured at days 5 and 20. Strong contrasts were observed between the relationships of soil physical variables and R at scales of individual landscapes and the whole region. Notably, the relationship between SOM and R was positive within individual landscapes and negative across the entire region. Plant canopy microenvironments were associated with elevated SOM and R relative to the interspaces. This microenvironment effect on R was reduced by elevation, incubation interval, and soil depth. Geostatistical analyses conducted individually for each site identified an increasing range of autocorrelation from 2 to 10 m and a decreasing proportion of variation that was included in this range with elevation. These results suggest hotspots increase in size but decrease in intensity with elevation thereby creating a maximum hotspot effect at middle elevations.