The productivity of ecosystems and their capacity to support life depends on access to reactive nitrogen (N). Over the past century, humans have more than doubled the global supply of reactive N through industrial and agricultural activities. However, long-term records demonstrate that N availability is declining in many regions of the world. Reactive N inputs are not evenly distributed, and global changes—including elevated atmospheric carbon dioxide (CO2) levels and rising temperatures—are affecting ecosystem N supply relative to demand. Declining N availability is constraining primary productivity, contributing to lower leaf N concentrations, and reducing the quality of herbivore diets in many ecosystems. We outline the current state of knowledge about declining N availability and propose actions aimed at characterizing and responding to this emerging challenge.
Bibliographical noteFunding Information:
This work was supported by the National Socio-Environmental Synthesis Center (SESYNC) under funding received from the National Science Foundation (NSF) DBI-1639145 (R.E.M., Q.D.R., and A.J.E.). Long-term nitrogen studies at the Hubbard Brook Experimental Forest have been supported by the NSF Long-Term Ecological Research program since 1988 (P.M.G.). Work on this paper was also supported, in part, by the US Department of Agriculture, Agricultural Research Service (J.A.); US Department of Agriculture, Forest Service, Northern Research Station (N.K.L.); NSF grants 1638688, 1920908, and 832210 (S.V.O.); and RI Sea Grant (R.W.F.).
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