کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
6408656 | 1629466 | 2015 | 11 صفحه PDF | دانلود رایگان |
- Density fractions are mixtures of younger and older OC, and thus, not homogeneous.
- Temporal changes in â14C confirm fast cycling OC in all density fractions at 0-5Â cm depth.
- Soil at 10-20Â cm depth is probably still accumulating bomb-derived OC.
- â14C changes below 30Â cm soil depth suggest the presence of active OC in subsoils.
- Turnover times based on single time point â14C analyses underestimate the presence of active OC.
Radiocarbon measurements are frequently used to model the turnover of soil organic carbon (OC) fractions. The assumption of homogeneous turnover in these fractions is typically stated, but consequences of its violation have not been tested. We used samples of a repeated soil inventory of a German beech forest from the litter layer to 50Â cm depth in 2004 and 2009 to determine the suitability of short-term repeated radiocarbon inventories for estimating the turnover times of OC in soil fractions. Samples from 0-5 and 10-20Â cm soil depth were density separated into a free light fraction (fLF), an occluded light fraction (oLF), and a heavy, mineral-associated fraction (HF). Samples were analysed for radiocarbon (â14C), OC, and total nitrogen (TN) contents.Similar portions of OC were stored in the HF, but contributions of fLF and oLF varied between study years, probably due to interannual variations or methodological constraints. Following declining atmospheric CO2-14C, also â14C values at 0-5Â cm depth declined significantly between 2004 and 2009. Exchange of old for new OC was largest in the fLF and smallest in the HF, which confirms slow turnover of OC associated with minerals. Model results revealed that turnover time estimates based on single-pool models were not in agreement with observed changes in any of the fractions, suggesting all of them to be mixtures, to varying degrees, of fast and slow cycling pools. While single-pool models suggest average turnover times of 115Â years for HF-OC at 0-5Â cm depth, thus being a stable fraction, fitting a two-pool model to the two-point measurements of radiocarbon suggested the presence of a fast cycling pool of 15-25Â years of turnover time. It was however only possible to constrain the portion of this fast pool as being between 50 and 85% of total HF-OC. Increasing â14C in bulk soil and density fractions between study years at 10-20Â cm depth suggest that OC enters deeper soil layers with a longer time lag than topsoil layers, e.g., by slow transport, and 5Â years was not enough to induce significant changes.Even 40Â years after the bomb peak, radiocarbon time series are still suitable to detect OC fractions of decadal turnover and hint at time lags and translocation processes. Nevertheless, they do not allow for fixing the portions of fast and slow cycling OC in two-pool models and their turnover times.
Repeated radiocarbon inventory in a German beech forest.
Journal: Geoderma - Volumes 239â240, February 2015, Pages 168-178