Enzyme activities and nutrient dynamics in arctic soil (2021-2023)

This study was conducted to investigate how nutrient dynamics and microbial and plant enzyme activities vary across arctic tundra hillslopes with changing precipitation and temperature events. Samples were collected during the plant-growing season between 2021 and 2023 in two different hillslope positions with contrasting pH and different glaciation histories (older acidic tundra and younger non-acidic tundra areas). Study plots were established in both areas along hillslope transects characterized by increasing soil moisture from top to bottom. Three replicate 5 meter (m) x 5m plots were established in each the dry, mesic, and hydric sites to collect soil, plant and porewater samples for nutrient and enzyme assays.
Freshly collected soil samples from all plots were separately extracted with water, 0.5M NaHCO3 (Olsen), or 0.5M K2SO4 to determine extractable nutrient concentrations. Enzyme assays were also performed on freshly collected soils. Hydrolytic enzyme activities were determined fluorometrically and oxidative enzyme activities were determined colorimetrically. Hydrolytic enzyme activities were also assayed fluorometrically on freshly collected live roots irrespective of plant species from each site and zone.
To determine mobile nutrient concentrations in soil, porewater samples were collected from each site using 10 centimeter (cm) rhizon samplers (dry and mesic) and custom-made perforated strong Teflon tubes (commonly known as sippers) with variable lengths based on intended depths. Porewater samples were collected from different soil depths (0cm for surface water, 5cm, 10cm, 20cm) during 2021 and 2022 plant-growing seasons. In 2023, porewater samples were collected from different soil horizons – O horizon, A horizon, and O/A horizon. Porewater samples were acidified or frozen (based on analytical needs) immediately after collection to preserve for later measurements of carbon and nutrients, and metal concentrations.
Additionally, to determine how nutrients move downslope at our sites, we setup one transect per area, comprised of 13 approximately equally spaced sampling points, starting at the dry sites at the tops of the hillslopes, through the mid- and downslopes (i.e., mesic and hydric zones, respectively). Soil porewater was collected from these transect locations as described above. Furthermore, anion and cation exchange resin strips were installed at each transect point and left in soils over summers (June/July-August) or winters (August-June/July) as measures of cumulative soil nutrient availability. Upon collection, resin strips were extracted with 2M KCl, and the extracts were frozen and later analyzed for inorganic nitrogen and phosphorus.