Small-scale spatial variation of soil organic matter pools generated by cork oak trees in Mediterranean agro-silvo-pastoral systems

The aim of this study was to assess the role of cork oak (Quercus suber L.) trees on the small-scale variation of soil organic matter (SOM) pools in an agro-silvo-pastoral system under Mediterranean semi-arid conditions in northeastern Sardinia, Italy. Six cork oak trees were selected in a wooded grassland (30% tree ground cover). For each tree, and along two opposite transects (NE and SW), floor litter and soil (20-cm depth) were sampled at five points starting next to the trees' trunk and ending beyond the tree crown projection. Soil organic matter quality was characterized by measuring the content of water extractable organic matter (WEOM), free particulate SOM (POMf), occluded aggregates particulate SOM (POMo), and mineral-associated SOM (MOM). Carbon (C) input from floor litter was larger in the sampling points under the tree crown projection than in those beyond the tree crown. The C content of SOM pools differed among sampling positions regardless of the transect orientation, decreasing from the trunk to the positions beyond the tree crown projection, from 24.1 to 15.7 g C kg^− 1 for MOM, and from 9.9 to 5.7 g C kg^− 1 for the sum of WEOM, POMf and POMo. The C in MOM next to the tree trunk was above the saturation level sensu Hassink and Whitmore (1997), but below saturation beyond the tree crown projection. The nitrogen (N) distribution showed a similar trend. These results indicate that in these agro-silvo-pastoral systems, oak trees generate hot spots of soil C storage, by controlling the rate of C inputs via litterfall. Hence, conservation strategies designed to maintain cork oak trees in grasslands will also contribute to maintain a high stock of C stored and a resilient, multipurpose system.