Background and aims Growth, morphological traits, and mycorrhizal colonization of fine roots show high degree of plasticity in response to changes in nutrient availability, causing shifts in root nutrient-foraging strategy. However, little is known about how this plasticity associated with root branching orders respond to atmospheric nitrogen (N) deposition in subtropical coniferous forests. Methods We used soil block sampling method to examine the responses of six key root functional parameters (including three morphological traits (specific root length (SRL), root tissue density (RTD), and root diameter), two growth indices (total root length (TRL) and biomass) on an areal basis across five root orders, and ectomycorrhizal (EM) tip colonization) to different doses and species of N addition in a slash pine (Pinus elliottii) plantation in subtropical China. Results TRL, root biomass in all root orders, and EM tip colonization increased significantly with N addition. However, SRL, RTD, and root diameter did not change in any root orders. In comparison to low doses of N input, high doses of N input exerted greater effects on lower-order roots. In regard to species of N added, stronger responses in lower-order roots were observed under ammonium-based than nitrated-based N input. Foliar P content was significantly decreased and stoichiometric N:P ratio was markedly increased in response to high dose of ammonium-based N input. Conclusions Fine root growth and EM tip colonization displayed higher degree of plasticity than morphological traits in response to N addition. The plastic responses were not root-order dependent, but dependent on both N dose and species, especially for ephemeral lower-order roots that are mostly like to be the main nutrient acquisition structures. Our results imply that while N limitation was alleviated by exogenous N input, P limitation may persist or even be exacerbated, thus causing an increase of absorptive root length, biomass, and dependence on ectomycorrhizae for nutrient acquisition in subtropical slash pine plantation forests.

Purpose Understanding how process-specific nitrogen (N) transformations in natural forest soils are modified by N deposition and fertilization is critically important to gain mechanistic insights on the links between global N deposition and N enrichment and loss in forest soils. Materials and methods Here we identify the general characteristics and the main mechanisms of N deposition-and fertilization-induced modifications in multiple N transformations, including N immobilization, N mineralization, nitrification (autotrophic nitrification and heterotrophic nitrification), and denitrification, in forest soils by literature survey. Results and discussion Overall, N status, soil C/N ratios, C availability, and soil pH are key factors separately and/or interactively affecting the effects of N deposition and fertilization on forest soil N transformations. In the N-limited stage, N deposition and fertilization can act as a stimulator of N mineralization by removing microbial N limitation and reducing the C/N ratios of the substrate being decomposed. In the Nunlimited stage, N added to forest ecosystems can retard N mineralization, which may primarily be a result of decreased microbial activity due to soil acidification and low C availability. The changes in N mineralization may drive a corresponding change in N immobilization, autotrophic nitrification, and denitrification. Despite the fact that ammonia-oxidizing archaea (AOA) has a higher affinity than ammonia-oxidizers (AOB) for low-concentration ammonia (NH 3 ), low NH 3 availability may still limit the rate of ammonia oxidation (autotrophic nitrification) in acidic forest soils even in the case of high NH 4 + input. Heterotrophic nitrification, however, may be favored if soil C/N ratios and pH decrease with N deposition and fertilization. The responses of denitrification and N 2 O emission to N deposition and fertilization in forests may be nonlinear, with a trend of stimulation in the short term but a decline over time, partly because soil pH has a contrast effect on denitrification capacity and N 2 O emission. Conclusions There are various effects of N deposition and fertilization on forest soil N transformations; thus, their responses to N deposition are still not well characterized and understood. N deposition-and fertilization-induced modifications in soil N transformations have important implications for N enrichment, N loss, and soil acidification in forest ecosystems. In the future, more research is required to investigate on dissimilatory nitrate reduction to ammonium (DNRA) process and link microbial community characteristics and functions of microbial extracellular enzymes with these rate processes in forest soils to narrow the uncertainty in evaluating and predicting ecosystem responses to global N deposition.

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