Effect of girdling on soil respiration and root composition in a sweet chestnut forest

Rhizosphere respiration is a significant component of the total carbon dioxide (CO,) efflux from forest soils. Since the root-derived CO, is not part of soil C loss, it is worth estimating this fraction and determining its driving forces. In order to determine the dependency of the root respiration on recent photosynthates in a European chestnut (Castanea sativa Mill.) stand in Southern Switzerland, we girdled 104 tree stems on an area of 625 m(2) at the end of August. The response of forest girdling on the below-ground C cycle was assessed by measuring soil respiration, soil microbial carbon biomass and root composition during the 37 days of experiment. Recent photosynthates contributed significantly to the soil respiration, which declined by 22 and up to 36% on average after girdling. The difference in soil respiration between girdled and control plots was significant only during a short time period (9-20 days after girdling). A significant decline in soil respiration was observed up to a distance of 4 in from the stems. Fine roots from girdled trees were strongly depleted in starch (-90%), although they were functional as shown by the dehydrogenase test. We assume that a large fraction of the starch loss was most likely respired and contributed to the soil respiration or there was a decline in new starch supply due to reduced carbon allocation to the roots. (c) 2006 Elsevier B.V. All rights reserved.