Effects of nitrogen deposition on ectomycorrhizal structure and enzyme activity of four tree species

The increasing levels of nitrogen deposition have varying impacts on forest ecosystems. Excessive input of available nitrogen leads to changes in plant root strategies for nutrient acquisition, consequently altering the structure and ecological functions of important soil microorganisms such as ectomycorrhizal (ECM) communities associated with tree roots. Understanding and determining the threshold of changes in root-associated microbial community activity in response to nitrogen deposition are of crucial importance for studying nutrient cycling characteristics and sustainable management practices in forests.

The present experiment employed an indoor pot experiment, selecting four tree species, namely Pinus massoniana, Pinus armandii, Pinus elliottii and Pinus taeda. Five gradients of nitrogen addition (0, 15, 30, 60, 150 kg·hm^-2·a^-1) were applied to study the dynamic changes in the structure and enzymatic activity of ECM communities under different nitrogen deposition levels. The nutrient content of seedlings and ECM root tip enzyme activity were analyzed to investigate the variations in the ECM community structure and enzymatic activity among the different pine species in response to nitrogen deposition levels.

1) The extracellular enzyme activity of most pine seedlings reached a threshold at 30 kg·hm^-2·a^-1. β-D-Glucosidase, mainly involved in cellulose decomposition, continues to increase with increasing N concentration in slash pine, loblolly pine, and China Armand pine. Even when N application reached 10 times the local N deposition level (150 kg·hm^-2·a^-1), the enzyme activity still did not reach the threshold; 2) The analysis of the ECM community revealed that the genus Tomentella was a dominant species in all four tree species, while the abundance of the genera Rhizopogon and Phialocephala varies depending on the host species; 3) There was no significant difference in the enzyme activities of ECM mycorrhizal communities of P. armandii and P. taeda at different N concentrations, indicating the ecological redundancy.

In the scenario of a long-term increase in N input levels, the composition of the ECM community undergoes adjustments in response to changes in both host plants and N deposition levels. Determining the threshold for these changes serves as a criterion for assessing this process. This process holds crucial academic significance in terms of guiding the understanding of carbon, nitrogen, and phosphorus cycles within soil nutrient dynamics and forest management practices.