Plant carbon investment in fine roots and arbuscular mycorrhizal fungi: A cross-biome study on nutrient acquisition strategies

Comparing the belowground allocation of assimilated carbon (C) to roots and mycorrhizal fungi across biomes can reveal specific plant nutrient acquisition strategies in ecosystems and allows to predict consequences of environmental changes. Three natural ecosystems (arid shrubland, coastal matorral, humid-temperate forest) distinct in annual precipitation and vegetation cover and compositions were selected to conduct a 13CO2 pulse labeling of natural woody vegetation to chase the allocation of assimilated C to arbuscular mycorrhizal (AM) fungi and fine roots. Further, nitrogen (N) and phosphorus (P) availability, root traits, root colonization, and the extraradical AM fungal mycelium (PLFA and NLFA 16:1ω5c) were analyzed to evaluate the efficiency of nutrient acquisition strategies. AM fungal colonization decreased with increasing aridity by up to 55% intraradical and by up to 90% extraradical. High root tissue densities – indicating longevity of roots – and low specific root lengths – indicating a low nutrient uptake capacity – pointed to a slow and resource conservative acquisition strategy of plants in the arid shrubland. Plants in the matorral, on the contrary, had lower root tissue densities but higher specific root lengths and higher root N contents, pointing to a fast nutrient acquisition strategy. The expression of abundant acquisitive fine roots of plants in the matorral, however, comes at the cost of larger C investment, shown by high 13C incorporation into root tissue. High root tissue densities and greater root diameter indicated that plants in the humid-temperate forest followed a resource-conservative strategy and outsource their nutrient acquisition to AM fungi. This outsourcing provides an efficient pathway to compensate a low uptake capacity of thick and dense roots. These ecosystem-specific acquisition strategies and distinct mutualism with AM fungi across the biomes will likely affect the sensitivity of plants to abiotic and biotic stressors and, thus, ecosystem responses to future climatic and environmental changes.