Microbial Mechanisms Underlying the Reduction of Soil Mineral Nitrogen Availability by Ectomycorrhizal Tree Introduction in Cedar Plantations

Forest mycorrhizal type and soil acidity strongly influence soil nitrogen (N) availability. Arbuscular mycorrhizal pure cedar plantations are expected to exhibit higher soil mineral N availability than ectomycorrhizal (ECM) forests, particularly under low soil acidity. We hypothesized that mixing ECM-associated tree species into such pure cedar plantations would reduce soil mineral N, ultimately lowering the risk of nitrate leaching, in sites with relatively low soil acidity. We investigated belowground microbial communities and N cycling across three microhabitats—roots, rhizosphere soils, and bulk soils—in six forest ecosystems with varying ECM tree abundance and soil acidity. These included pure cedar plantations, cedar plantations mixed with ECM-associated trees, and ECM-dominated forests, located at two types of sites differing in soil acidity (low acidity: pH > 5, high acidity: pH < 5) but both characterized by relatively low mineral soil C to N ratios. Higher ECM fungal abundance and higher soil acidity were associated with lower abundance of ammonia-oxidizing archaea (AOA) and lower nitrate content. Notably, ECM fungi appeared to suppress AOA abundance independently of ECM-mediated soil acidification. Moreover, competitive interactions between ECM fungi and AOA may suppress nitrate N contents in low acidity sites. Meanwhile, cedar trees appeared to exhibit a relatively high capacity for nitrate uptake, as evidenced by a negative rhizosphere effect on nitrate. Mixing ECM-associated tree species into pure cedar plantations could help mitigate nitrate leaching risks by reducing AOA abundance through enhancing ECM fungal activity, while simultaneously taking advantage of the cedar trees’ high nitrate uptake capacity, especially under low soil acidity.