Aboveground and Belowground Responses to Nutrient Additions and Herbivore Exclusion in Arctic Tundra Ecosystems in Northern Alaska

The Exploitation Ecosystem Hypothesis (EEH) describes how food chains develop along aboveground net primary productivity (ANPP) gradients. As ANPP increases, plant biomass increases until a threshold is reached where herbivore populations can be sustained. As ANPP increases further, plant biomass stabilizes and the additional productivity is consumed by herbivores until there is -sufficient herbivore biomass to support predators. We tested the predictions of EEH in two common low productivity Alaskan tundra communities, dry heath (DH) and moist acidic tundra (MAT), subject to manipulations of soil nutrients and mammalian herbivory for eleven years. We took an integrated multi-trophic level approach, incorporating simultaneous measures of changes in vegetation, mammalian activity, soil fauna, and microbial communities as well as soil physical characteristics.

In 1996 a factorial design was implemented in both DH and MAT to determine how mammals and nutrients affected vegetation and soils. Control and +NP (10 g m² yr^-1 as NH₄NO₃ and 5 g m² yr^-1 as P₂O₅ applied annually)  plots of 5 x 20 m were replicated within four blocks at MAT and three blocks at DH.  Half of each 5 x 20 m plot in each block was left unfenced (+H), while the remaining 5x10 m was fenced with large mesh fence to exclude caribou. Half of this enclosure area (5 x 5 m) was additionally enclosed by a smaller mesh fence buried in the soil at least 10 cm at construction to exclude small mammals.  In 2006 we sampled the control, +NP, -H, and +NP-H (small fence –H plots only) from both MAT and DH. We quantified aboveground plant biomass, including rhizomes, of each tissue type by species and root biomass by plant group. We collected organic soil and quantified the biomass of the major soil foodweb groups including bacteria, fungi, protozoa, nematodes, and arthropods, further separating each group into trophic levels. We modeled carbon flux rates through the soil food web using established turnover rates for each group.

Nutrient addition and herbivore exclusion each resulted in changes in vegetation structure with transitions from a diverse community  to one dominated by shrubs in MAT, and a transition from evergreen shrubs and lichens to grasses and shrubs in DH. MAT exhibited a significant increase in NPP while DH exhibited a significant increase in root biomass in response to +NP. Herbivore exclusion increased total plant biomass in DH, as well as NPP in MAT. Belowground responses to the treatments were dependent on ecosystem type, but exposed alterations in trophic structure that included changes in microbial biomass, the establishment of microbivorous enchytreaids, the occurrence of root-feeding nematodes in +NP and –H treatments,  and declines in top predators. The models revealed shifts in the activities within and between the fungal and bacterial energy pathways, with the treatments inducing a bottleneck in activity at the microbe-microbivore trophic levels within both pathways. Our results confirm the complex nature of aboveground and belowground linkages and interactions in defining ecosystem structure and function. Drivers that enhanced plant production and altered plant community structure had direct and indirect effects on the structure and activity within the belowground foodweb, the nature of which can help inform the mechanisms behind C dynamics in response to climate change in the Arctic.