LEAF WAX N-ALKANE SIGNALS IN SEDIMENTARY RECORDS: WHAT ARE THE KEY DRIVERS?
Modern plants and soils were obtained from 20 long-term monitoring plots distributed across Australia. n-Alkanes were extracted from the three most dominant species and the surface soil at each plot. Average chain length (ACL) and carbon preference index (CPI) were measured for each plant and used to model ACL and CPI inputs to soils, weighted by concentration and percent coverage. Modelled ACL and CPI were generally higher than actual values indicating that local and recent inputs do not dominate soil n-alkane signals. We attribute this to temporal and/or spatial averaging of inputs, with the local plant contributor signals being overwhelmed by an accumulation in the soils of past and/or distant contributors.
We explored whether post-depositional modification of n-alkanes in soils accounts for the observed model-data mismatch in an experimental study utilising incubated soil samples mixed with different organic amendments such as composts. Samples at 0 and 18 months incubation were analysed for n-alkane ACL, CPI and concentration. The concentration of n-alkanes decreased markedly with losses of up to 90% but ACL remained unchanged. CPI decreased significantly, suggesting that CPI may be a useful indicator of n-alkane degradation. Our results imply that inputs to soils have a greater influence on n-alkane ACL than post-depositional modification.