Soil feedback on plant growth in a sub-arctic grassland as a result of repeated defoliation

In the long term, defoliation of plants can be hypothesized to decrease plant carbon supply to soil decomposers and thus decrease decomposer abundance and nutrient mineralization in the soil. To test whether defoliation creates changes in soil that can feedback to plant growth, we collected soil from sub-arctic grassland plots that had been either defoliated or non-defoliated for three years and followed the growth of different plant species combinations in these soils in greenhouse conditions. Plant N acquisition and plant growth were lower in the soil collected from the defoliated field plots than in the soil collected from the non-defoliated plots. This response did not depend on the species composition or richness of the tested plant community. In the field, defoliation decreased net nitrogen mineralization. Despite the negative effect of decreased nutrient mineralization rate on plant growth and N accumulation in the greenhouse test, the aboveground abundance of most plant species was not affected by defoliation in the field. This indicates that plants in these sub-arctic grasslands can at least temporarily overcome defoliation-induced decrease in soil nutrient availability. To our knowledge, our results are the first direct evidence that defoliation can induce changes in the soil that negatively feedback to plant growth and N accumulation. This finding indicates that, especially in arctic and sub-arctic grasslands where nutrient mineralization rates are inherently low, soil feedbacks can have an important role in the outcome of herbivore–plant interactions.