Nitrogen (N) plays a critical and complex role in the Earth's ecosystems and is often a limiting nutrient in agriculture. The work presented here investigates four aspects of the N cycle. Chapter 1 examines interactions between pyrogenic organic matter (PyOM) and ammonia (NH3). Adsorption isotherms, spectroscopy, and stoichiometric analyses show that PyOM’s NH3 retention capacity can exceed 180 mg N g-1 PyOM carbon. More than half of the NH3–N is retained through chemisorption, including the formation of a variety of covalent bonds. These results indicate that PyOM could exert an important and unaccounted-for control on global N cycling. Chapter 2 explores biochar’s capacity to improve N retention during composting. When N loss was calculated as a proportion of C loss to account for differences in microbial activity, relative N loss from compost with oxidized biochar was more than fivefold lower than N loss from compost with unoxidized biochar and comparable to relative N loss from the compost feedstocks alone. N retention by oxidized biochar was directly responsible for lower N loss from compost. These data show that biochar can be used to improve compost efficiency and that biochar’s physiochemical characteristics influence its performance in compost. Chapter 3 investigates multipartite plant-biotic synergies that increase plant N acquisition more than tenfold and account for half of the N that mycorrhizal plants acquire from soil organic matter. This relationship may contribute to more than 70 Tg of annually assimilated plant N, thereby playing a critical role in global nutrient cycling and ecosystem function. Chapter 4 provides evidence of subsurface plant acquisition of N from NH3 gas. Plants derived up to 34% of total daily N from NH3. Nearly 4% of N in soil organic matter traveled as NH3 gas belowground and accounted for over 9% of N acquired by plants per season. Together, the results presented here could be used to better understand the global N cycle and improve sustainable N delivery to crops.