Effects of reindeer on plant and soil nutrient stoichiometry in Arctic tundra

Effects of reindeer on plant and soil nutrient stoichiometry in Arctic tundra

Project Summary

Herbivores directly and indirectly influence the structure and function of ecosystems throughout the world. Present conceptual models predict that herbivores have a positive effect on nutrient availability and primary productivity in nutrient rich environments and a negative one in nutrient poor environments. However, a recent meta-analysis did not support a positive relationship between plant nutrient availability and the effect of herbivores on nitrogen cycling in several grassland ecosystems. The reason for the shortcoming of present theories could be that they fail to incorporate the complex interactions regulating the release of nutrients from the soil organic matter by microbial decomposers.

In order to assess this shortcoming, a new stoichiometric explicit model of the nutrition of herbivores, plants and microbes was designed. In contrast to previous models, it includes delayed composition and stoichiometric constraints on decomposers similar to what we find in terrestrial ecosystems.

This project will focus on 1) testing predictions of the model in a reindeer-dominated tundra ecosystem and 2) further developing the model by adding key processes that operate in northern ecosystems (e.g. uptake of organic nutrients by plants, herbivore-mediated changes in plant community composition).

Collaborators

Johan Olofsson, Umeå University
Mehdi Cherif, Umeå University

Funding

The Kempe Foundation

Microbial use of phosphorus in soils

Microbial use of phosphorus in soils

Project Summary

Globally, phosphorus (P) together with nitrogen (N) is the most important nutrient element limiting plant growth. In boreal forest ecosystems, P limitation is found in groundwater discharge areas and recent studies also indicate that many alpine ecosystems may be P limited. In this project I will study microbial strategies to utilize phosphate and organophosphorus compounds and the effect of surface sorption. One method that will be used is measurement of microbial respiration using a respicond. This is a microbial bioassay for studying the availability of P to microorganisms.

Ongoing studies

In the first project we sampled six different South African forest soils. The soils were selected to represent a broad range of soil properties, especially concerning aluminium, iron and organic carbon content. The respiration results will be compared to the amount of P in the soil to show how much of the total P pool that microorganisms can utilise and how the microbial availability is affected by iron and aluminium concentrations in the soil. This way we can compare in detail how different soil properties interact with microbial available P determined in the bioassay.

In the second ongoing study we sampled 30 islands in Lake Hornavan and Lake Uddjaur, situated close to Arjeplog in the boreal forest of Northern Sweden. The time since the last major fire vary between the islands and the wildfires form a chronosequence which is closely correlated to island size and vegetation succession (Wardle et al. 1997). Earlier results indicate increasing phosphorus limitation over time since the last fire (Wardle et al, 2004). During this study we will compare the different P and N pools with microbial growth kinetics in relation to island size and humus depth.