The nitrogen (N) cycle is series of biogeochemical transformations involving soil microbes and plants that are modulated by environmental conditions. Because N limits primary production, understanding interactions among these factors is central to predicting soil resource availability to primary producers and the global distribution of terrestrial biomass carbon (C). Phosphorus (P), another limiting nutrient, and C, further affect microbial N transformations and N bioavailability through the stoichiometric requirements of plant and microbial biomass. Natural environmental gradients coupled with field manipulations are investigative tools that have helped to elucidate controls on C, N and P interactions, particularly under conditions of environmental change. This work considered the influences of increasing mean annual temperature (MAT) and the relative availabilities of soil N and P on belowground plant activity, microbial functional groups, and N cycling rates in tropical and temperate forests. Past research showed that belowground C flux and soil respiration increased with warming across an elevation/MAT gradient (13-18.2 °C) in a tropical montane wet forest. Across the same gradient, this study found that soil N bioavailability and the abundance of nitrifying archaea also increased with warming, suggesting that MAT drives nitrifier populations and resultant N availability. The present study also showed that warming reduced fine root proliferation into soil microsites enriched with P or a combination of N and P across the gradient. This suggests that fine root growth is more co-limited by N and P at low MAT where bulk soil N is more limiting than in warm conditions where N is abundant. Additionally, arbuscular mycorrhizal colonization of fine roots increased with warming, possibly indicating that nutrient acquisition is performed by fungal symbionts more than fine roots in warm, N-rich environments. Using a stand fertilization experiment in a northern hardwood forest, this work showed that litter N:P and gross N cycling declined in the presence of elevated P, while microbial biomass N increased with elevated P. The results show that litter chemistry is strongly related to soil N and P limitation, and may control gross N cycling through litter substrate limitation. Together, these studies highlight the complex interplay among temperature, soil P availability, and plant/microbial nutrient uptake that shape the N cycle in different forest ecosystems.