Scaling soil respiration dynamics across regional land-use and climate gradients in southern California, USA

Background/Question/Methods

              Human land-use change alters ecosystems through modifications of surface and air temperature, biota, hydrologic routing, and carbon and nitrogen cycling. Soil-atmosphere CO₂ flux (R_s) is a dominant process influencing ecosystem functioning. To better understand the effects of land-use change on ecosystem processes, we evaluated two complementary questions. First, what regulates the spatial and temporal variation of R_s across mixed-use lands? Second, how do soil temperature and moisture influence the spatial and temporal variation of R_s? We measured R_s during the summer and winter months across three major land-cover types in this region—mesic residential, citrus agriculture, and wildland—at three sites spanning a coastal to desert climate gradient in southern California. Analysis across multiple scales—microsite, land-cover, site, and region—was used to examine scaling relations associated with the variation in R_s. Temporal influences on R_s for each land-cover were measured by sampling in the months of July through September, and December through February (n= 78 land-cover^-1 x 3 land-covers x 3 sites x 2 seasons).

Results/Conclusions

              The spatial and temporal variability of R_s in the mesic residential-agriculture-wildland mosaic was dependent on the site and the scale considered.  For summer and winter measurements, the coastal site had the highest R_s for wildland land-covers, and desert site had the largest R_s for agricultural land-covers.  Wildland land-covers experienced, on average, an order of magnitude or more increase in R_s from summer to winter. Soil temperature and moisture had characteristic influences on the variability of R_s. Between land-covers, the variation in R_s was positively related to temperature, and negatively related to soil moisture for summer and winter measurements. The variation in R_s within most land-covers decreased from summer to winter, with the highest variation occurring in the wildlands and the lowest in the mesic residential land-covers. Between land-cover variation was the highest in the desert site and lowest at the coastal site. In conclusion, human land-use change in this system alters soil moisture and temperature, decreasing variance of R_s within land-covers, and between sites along the climate gradient, while increasing the variance of R_s within the sites.