The function of root mat fungal communities: Changes in response to pH and phosphorus addition

Abstract

Acid deposition affects forests in eastern North America, Europe, and eastern Asia, and reduces nutrient availability for forest trees. Reduced nutrient availability can reduce tree growth and forest productivity. Our research suggests that trees may rely on fungi, including mycorrhizal fungi, that colonize root mats near the soil surface for nutrient acquisition in forests subjected to acid-induced nutrient limitation. These fungi may increase acquisition of nutrients from organic matter. Forest management practices that limit surface soil disturbance and damage to root mats, especially during timber harvesting activities, may be important for conserving forest productivity in regions subjected to acid deposition. Summary: Many temperate forest soils are affected by a legacy of acid deposition, which can mobilize soil aluminum and reduce phosphorus (P) availability. These chemical changes are mostly manifest in mineral soil. However, many temperate trees have extensive shallow roots, or root mats, which are found on the surface of mineral soil. These root mats are in contact with the O horizon where substantial organic nutrients exist. But the role of root mats in accessing nutrients in nutrient limited, acidic soil is not well understood. We examined root mat fungal communities and their function using an existing 6-year ecosystem-level pH and P manipulation experiment, which artificially increased pH by more than 1.5 units and increased available P threefold. Dominant fungal taxa on roots were identified using Sanger sequencing and broad community changes were assessed using DNA fragment analysis. Nutrient acquisition within root mats was assessed by measuring the extracellular enzyme activity in rhizosphere soil. Fungal community structure changed with soil pH, while P availability altered extracellular enzyme production of phosphomonoesterase, β-1,4-glucosidase, and leucine aminopeptidase. Quercus–Fagus root mats contained ectomycorrhizal fungi and Acer roots contained both ectomycorrhizal and arbuscular mycorrhizal fungi. Interestingly, both Acer and Quercus–Fagus root mats contained ericoid mycorrhizal fungi, which were less abundant at higher pH. Our results suggest an important role of root mats in nutrient acquisition from organic sources in temperate forests. These root mats could assist plants in overcoming the legacy of acid deposition in temperate forests.