Abstract

The 'Espinal' agroforestry system of the Mediterranean zone of central Chile, which covers an area of 2000 000 ha, is in various stages of degradation due to human activities. The objective of our study was: (i) to determine the effects of the canopy cover of Acacia caven ('Espino') on total soil organic carbon (SOC), soil respiration and the labile components of soil organic matter (microbial biomass, and light fraction); and (ii) to determine the influence of ecosystem degradation on total and labile components of SOC. Soils of the study area are classified as fine, mixed, active, mesic Ultic Palexeralfs, typical of the Mediterranean-type environment. We investigated sites according to the percentage coverage of A. caven canopy: (i) well-preserved Espinal (WPE), 80-51% cover; (ii) good Espinal (GE), 50-26% cover; (iii) degraded Espinal (DE), 25-11% cover; and (iv) very degraded Espinal (VDE), < 10% cover. In addition, a site under native forest (NF) was included to characterize the original state of the zone. Soil samples were taken under and outside the canopy of A. caven at two depths, 0-5 and 5-10 cm. We conclude that the microbial biomass carbon (C mic), and total and labile components of SOC are influenced by the presence of the A. caven tree, with greater values under than outside its canopy. Under the tree canopy, to a depth of 10 cm, C mic was less under all the agroforestry systems than in NF (46 and 30% less for WPE and GE, respectively, and 67 and 57% less for DE and VDE). However, there was no clear trend for less C mic with increased ecosystem degradation, especially outside the canopy. However, the respiration of microbial communities was affected by ecosystem degradation for both soil depths under the tree canopy, e.g. soil respiration in VDE ecosystems was about 50% greater than that found in WPE ecosystems. Increasing the coverage of the A. caven tree in the semiarid ecosystems of central Chile, e.g. changing from VDE to WPE, would result in an eventual, long-term (over several centuries) increase in soil organic C of approximately 50%. © 2006 British Society of Soil Science.