Deep within soil profiles, organic matter (OM) inputs are derived from root growth and root exudates of deeply-rooted species, and organic compounds percolating through the profile from more shallow horizons. Severe disturbance “orphans” these deep roots, which eventually decay in place. When annual crops replace long-lived, deeply rooted vegetation, sustained organic inputs into deep soil volumes are limited to those that percolate down from the overlying horizons. Multiple studies suggest that even after forests are re-planted, it can take more than a century for deep roots to become re-established.

We thus ask if past disturbance and subsequent changes across depth in OM inputs (both content and form) have a contemporary influence on transformations and fate of deep, ancient soil OM and associated biogeochemical fluxes and microbial communities. If so, this phenomenon would suggest a far-reaching nature of the biogeochemical legacies of past disturbance, both in vertical space (down deep) and time (~60 y after forest re-establishment at the Calhoun experimental forest). Discerning and quantifying any such effect would add an additional dimension to existing disturbance-related literature, and helps address some of the questions raised in the Calhoun Critical Zone Observatory proposal.

Our objective was to quantify the influence of past land use history and current land cover on deep soil biogeochemical processes linked to OM transformations. For this, we collected soil samples from the Calhoun CZO, from three replicate sites of old-grown hardwood forests, from three replicate sites of old-field pine forests, and from one current crop field (“dovefield”), from depths of 40-50 cm and 400-500 cm by hand augering.

Once the shipped soil samples arrived at the University of Kansas, Lawrence, we incubated subsamples in mason jars and prepared them for gas sampling. Gas samples from the incubation jars were at time 0 and time 1, and CO2, CH4 and N2O concentrations in the sampled gas were analyzed with a gas chromatograph. This allowed us to compute rates of soil microbial CO2, CH4 and N2O production. Determining δ13C of the sampled gases with a 13CO2/12CO2 gas analyzer allowed us to assess the δ13C of the CO2 respired by the soil microbes. At the same sampling points during the soil incubations, we also took aliquots of the samples for flourometric enzyme assays. This aimed at quantifying the activities of the microbial extracellular enzymes β-glucosidase, β-N-acetyl glucosaminidase, acid phosphatase, and phenol oxidase.

All other soil parameters were analyzed on subsamples of the soils immediately after their arrival at the University of Kansas.
Date Range Comments: irregular collections