Distinct effects of canopy vs understory and organic vs inorganic N deposition on root resource acquisition strategies of subtropical Moso bamboo plants.

Atmospheric nitrogen (N) deposition inevitably alters soil nutrient status, subsequently prompting plants to modify their root morphology (i.e., adopting a do-it-yourself strategy), mycorrhizal symbioses (i.e., outsourcing strategy), and root exudation (i.e., nutrient-mining strategy) linking with resource acquisition. However, how N deposition influences the integrated pattern of these resource-acquisition strategies remains unclear. Furthermore, most studies in forest ecosystems have focused on understory N and inorganic N deposition, neglecting canopy-associated processes (e.g., N interception and assimilation) and the impacts of organic N on root functional traits. In this study, we compared the effects of canopy vs understory, organic vs inorganic N deposition on eight root functional traits of Moso bamboo plants. Our results showed that N deposition significantly decreased arbuscular mycorrhizal fungi (AMF) colonization, altered root exudation rate and root foraging traits (branching intensity, specific root area, and length), but did not influence root tissue density and N concentration. Moreover, the impacts of N deposition on root functional traits varied significantly with deposition approach (canopy vs. understory), form (organic vs. inorganic), and their interaction, showing variations in both intensity and direction (positive/negative). Furthermore, specific root area and length were positively correlated with AMF colonization under canopy N deposition and root exudation rate in understory N deposition. Root trait variation under understory N deposition, but not under canopy N deposition, was classified into the collaboration gradient and the conservation gradient. These findings imply that coordination of nutrient-acquisition strategies dependent on N deposition approach. Overall, this study provides a holistic understanding of the impacts of N deposition on root resource-acquisition strategies. Our results indicate that the evaluation of N deposition on fine roots in forest ecosystems might be biased if N is added understory.

Extraradical hyphae alleviate nitrogen deposition-induced phosphorus deficiency in ectomycorrhiza-dominated forests.

The continuous imbalance between nitrogen (N) and phosphorus (P) deposition is expected to shift many ecosystems from N- to P limitation. Extraradical hyphae of ectomycorrhizal (ECM) fungi play important roles in plant nutrient acquisition under nutrient deficiency. However, whether and how ECM hyphae enhance soil P availability to alleviate N-induced P deficiency remains unclear. We investigated the impacts of ECM hyphae on transformations among different soil P fractions and underlying mechanisms under N deposition in two ECM-dominated forests. Ectomycorrhizal hyphae enhanced soil P availability under N addition by stimulating mineralization of organic P (Po) and desorption and solubilization of secondary mineral P, as indicated by N-induced increase in positive hyphal effect on plant-available P pool and negative hyphal effects on Po and secondary mineral P pools. Moreover, ECM hyphae increased soil phosphatase activity and abundance of microbial genes associated with Po mineralization and inorganic P solubilization, while decreasing concentrations of Fe/Al oxides. Our results suggest that ECM hyphae can alleviate N-induced P deficiency in ECM-dominated forests by regulating interactions between microbial and abiotic factors involved in soil P transformations. This advances our understanding of plant acclimation strategies via mediating plant-mycorrhiza interactions to sustain forest production and functional stability under changing environments.

Differential aboveground-belowground adaptive strategies to alleviate N addition-induced P deficiency in two alpine coniferous forests.

Increasing atmospheric nitrogen (N) deposition has resulted in phosphorus (P) limitation in multiple terrestrial ecosystems, yet how plants coordinate aboveground and belowground strategies to adapt to such P deficiency remains unclear. In this study, we conducted a field N fertilization experiment in two alpine coniferous plantations (Picea asperata Mast. and Pinus armandii Franch.) with different soil N availability on the eastern Tibetan Plateau of China, to examine N addition effects on plant nutrient limiting status and plant adaptive strategies corresponding to aboveground P conservation and belowground P acquisition. The results showed that N addition aggravated P deficiency in both plantations, as indicated by decreased needle P concentrations and increased N:P ratios, and that plant strategies for addressing such P deficiency differed in the two plantations with different initial soil N availabilities. In the P. asperata plantation with relatively high N availability, significantly enhanced needle phosphatase activity and shifts in P fraction allocation (downregulation of the structural P fraction and increased allocation to the residual P fraction) co-occurred with increased rhizosphere effects on phosphatase activity under N addition, indicating a synergistic strategy of aboveground P conservation and belowground P mining to alleviate P deficiency. In the P. armandii plantation with relatively low N availability, however, N addition only enhanced phosphatase activity and increased allocation to residual P fraction in the aboveground but had little effect on belowground P acquisition-associated traits, suggesting a decoupling relationship between aboveground P conservation and belowground P acquisition. This study highlights the vital significance of initial soil nutrient availability in regulating the coordination of aboveground and belowground strategic alternatives, emphasizing the need to integrate soil nutrient conditions for a holistic understanding of forest adaptation to anthropogenic N enrichment.