Allocation trade-off between root and mycorrhizal surface defines nitrogen and phosphorus relations in 13 grassland species

Abstract

In oligotrophic ecosystems efficient nutrient uptake mechanisms, like extensive root systems or the association with belowground symbionts (e.g. arbuscular mycorrhizal fungi, AMF), are crucial. Pursuing root- or AMF-dominated foraging may result in diverging success regarding nitrogen (N) and phosphorus (P) nutrition. In this study we identify species- and functional group-specific belowground allocation strategies and disentangle the role of root vs. hyphal allocation for N and P nutrition. Allocation patterns to both root and AM hyphal surface together with plant P- and N-relations were measured in non-mycorrhizal and mycorrhizal individuals of 13 common grassland species belonging to the functional groups of forbs, grasses, legumes and non-mycotrophic Brassicaceae. The trade-off between predominant investments into either roots or hyphae showed high species- and functional group-specificity and clearly defined plant N:P relations, with root strategists gaining larger N- and lower P-benefits than mycorrhizal strategists. Further, P-delivery by AMF was accompanied by strong fungal N-competition. Our results demonstrate high relevance of the allocation trade-off between root and mycorrhizal surface for N- and P-nutrition in grassland species. Low soil N:P ratios may only allow for positive AMF effects in mycorrhizal strategists, whereas root strategists may experience negative effects, likely being linked to N-limitation in the AM-state.