Recent (<4 year old) leaf litter is not a major source of microbial carbon in a temperate forest mineral soil

Microbial communities in soil A horizons derive their carbon from several potential sources: organic carbon (C) transported down from overlying litter and organic horizons, root-derived C, or soil organic matter. We took advantage of a multi-year experiment that manipulated the (14)C isotope signature of surface leaf litter inputs in a temperate forest at the Oak Ridge Reservation, Tennessee, USA, to quantify the contribution of recent leaf litter C to microbial respiration and biomarkers in the underlying mineral soil. We observed no measurable difference (< similar to 40 parts per thousand, given our current analytical methods) in the radiocarbon signatures of microbial phospholipid fatty acids (PLFA) isolated from the top 10 cm of mineral soil in plots that experienced 3 years of litterfall that differed in each year by similar to 750 parts per thousand between high-(14)C and low-(14)C treatments. Assuming any difference in (14)C between the high- and low-(14)C plots would reflect C derived from these manipulated litter additions, we estimate that <similar to 6% of the microbial C after 4 years was derived from the added 1-4-year-old surface litter. Large contributions of C from litter < 1 year (or >4 years) old (which fell after (or prior to) the manipulation and therefore did not differ between plots) are not supported because the (14)C signatures of the PLFA compounds (averaging 200-220 parts per thousand) is much higher that of the 2004-5 leaf litter (115 parts per thousand) or pre-2000 litter. A mesocosm experiment further demonstrated that C leached from (14)C-enriched surface litter or the O horizon was not a detectable C source in underlying mineral soil microbes during the first eight months after litter addition. Instead a decline in the (14)C of PLEA over the mesocosm experiment likely reflected the loss of a pre-existing substrate not associated with added leaf litter. Measured PLFA Delta(14)C signatures were higher than those measured in bulk mineral soil organic matter in our experiments, but fell within the range of (14)C values measured in mineral soil roots. Together, our experiments suggest that root-derived C is the major (>60%) source of C for microbes in these temperate deciduous forest soils. (C) 2010 Elsevier Ltd. All rights reserved.