Transition in the isotopic signatures of fatty-acid soil biomarkers under changing land use: Insights from a multi-decadal chronosequence.

The effects of climate warming on soil erosion in upland ecosystems will be disproportionately higher than for lowlands due to steeper topography and higher predicted rainfall. Soil erosion may be enhanced by climate warming and upslope shifts in agriculture as conditions for plant growth improve. Identification of eroded-soil sources will inform land management practices that mitigate soil loss and impacts on aquatic receiving environments. Isotopic signatures of plant-derived fatty acid (FA) soil biomarkers can discriminate sediment sources and will detect shifts in land use and natural vegetation toposequences. Accounting for these isotopic shifts requires knowledge of the magnitude and time scale for transition in biomarker signatures. We examined a 30-year chronosequence to quantify the transition in isotopic values of bulk nitrogen, carbon and FA biomarkers following a change from pine forestry to pastoral agriculture in the central North Island of New Zealand. We found the transition in soil biomarker isotopic values was complete within 6 years, with substantial increases in both organic carbon (1% yr) and total N (0.13% yr) of top soils. Subsequent changes were negligible (i.e., <0.04% yr), indicative of a new steady state. Similar patterns were observed in the isotopic signatures of bulk δC and δN values and FA δC values (i.e., ±0.5-0.6‰ yr). Bulk C and N properties and the FAs C14:0, C16:0, C18:2, C24:0 and C26:0 displayed clear transitions from harvested pine to mature pasture. We found evidence that mycorrhizal fungi could disperse and influence soil FA isotopic signatures. This highlights the need to consider both harvested and mature forests in source-tracing studies. Finally, our study shows that near-instantaneous changes in land use associated with agriculture can alter the isotopic signatures of plant biomarkers in soils. This produces a step change that can be readily detected in sedimentary records.