Print PageEcosystems are dynamic and respond to environmental perturbation. Studying ecosystem changes in structure and function along environmental gradients provides one approach to predicting ecosystem response to climate change. These changes can considerably influence nutrient cycling, microbial stoichiometry, soil organic matter decomposition, and plant-microbial interactions in terrestrial ecosystems. In addition, trans-ecosystem subsidies, especially allochthonous inputs and the downhill transport of nutrients, have been shown to influence aquatic biogeochemical cycling. Understanding spatial and temporal patterns of C, N and P pools along environmental gradients is critical in elucidating the link between terrestrial and aquatic ecosystems. Our objective in this project was to characterize resource availability and microbial biomass along transects spanning a topographic gradient connecting upland prairies to low-lying wetlands.
We measured labile C, extractable N and P, and microbial C, N and P in soil samples collected along two topographic gradients in a restored prairie in central Minnesota. Each transect consisted of two to three upland prairie sites and five wetland sites arrayed across a moisture gradient from transitional marginal zones to permanent wetland. The western transect was connected to an ephemeral pond, and the eastern transect was connected to a permanent wetland. Labile pools of C, N, and P were estimated using standard extraction protocols, and microbial pools were estimated using the chloroform fumigation direct extraction technique.
Results/Conclusions
We found little spatial variation in nutrient availability within transects, but clear differences in nutrient availability between transects. Soil analyses revealed higher labile P and lower microbial N:P ratios in the western transect compared to the eastern transect. The presence of N-fixing legumes (Petalostemum purpureum) along the eastern topographical gradient may have released the eastern transect from N-limitation (N:P>7).
In October labile nutrient pools and microbial stoichiometric ratios in the wetlands reflected the prairies. In November, the eastern and western transects were still significantly different, but nutrient availability decreased in the prairies and increased in the wetlands, and microbial N:P showed the opposite trend. Microbial processing in the prairies may have depleted labile nutrient pools following plant dormancy. In contrast, the wetland water column remained unfrozen, facilitating primary production and insulating microbial communities.
Our results indicate that resource availability is driven by environmental gradients and exhibits seasonal change. The flow of nutrients from terrestrial sources appears to strongly influence aquatic biogeochemical cycling. Assessing the influence of topography on the spatial movement of nutrients will inform models and improve our understanding of ecosystem linkages.
