The combined controls of land use legacy and earthworm activity on soil organic matter chemistry and particle association during afforestation

Abstract

The chemistry and physical association of soil organic matter in the patchwork of successional forest stands in the eastern US is strongly controlled by past land use. Invasive earthworm activity in these same systems, however, may impart a chemical and physical disturbance exceeding that of land use legacy. We established eight plots within forests of the Smithsonian Environmental Research Center (SERC) (Edgewater, MD), to compare sites with no record of significant agricultural disturbance or earthworm activity and successional mixed hardwood forests recovering from past agriculture (60–132yr) that contained both native and non-native earthworms. Soils (0–15cm) were separated into physical fractions by size (microaggregates) and density (light and heavy particulate organic matter) and investigated for organic carbon (C) and nitrogen (N) partitioning. In addition, molecular composition was analyzed using FTIR spectroscopy and lignin phenol and substituted fatty acid (SFA) extraction.Even after 132yr of recovery, the successional forests were nearly devoid of Oa+e horizons; a condition we attribute to high activity of invasive earthworms. Additionally, soil organic carbon (SOC) concentration profiles, and 14C derived mean residence times indicated mixing of the surface soils and fresh input of carbon to 10cm, distinct from the undisturbed, mature sites. The proportion of microaggregated particulate organic matter (iPOM) and silt+clay (iSC) was significantly higher in successional than undisturbed forests, which we attribute to the combined influence of past agricultural land use and high earthworm activity. Among the successional sites, older forests exhibited a significant decrease in the proportion of C and N in iSC but an increase in their proportion in iPOM, suggesting selective incorporation of iPOM with earthworm activity over great periods of time. In addition, continual consumption and mixing activities of the earthworm population could also be a primary control of the higher concentration and less oxidized lignin phenols as well as a higher proportion of lignin phenols to SFA in all soil fractions in the successional sites. Using partial least squares (PLS) regression of FTIR spectra, we also demonstrated a strong correlation between soil C physical distribution (microaggregated vs. non-microaggregated) and chemical aspects of specific FTIR regions which confirmed our findings from the lignin and SFA and showed distinct chemical dominance among the different sites. Our results indicated that while past agricultural practice may have been the primary initial influence on C and N stock and soil physical distribution in the successional sites, the prolonged legacy and trajectory of recovery from the past land disturbance can be controlled by the nature of the invasive and native earthworm activity during afforestation.