Evidence, causes, and consequences of declining nitrogen availability in terrestrial ecosystems

Abstract

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 (CO 2 ) 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.

Will The Grass Be Greener On The Other Side Of Climate Change?

“…Future reductions in the forage quality of sourveld and sweetveld will depend on how other major climate change drivers (temperature and precipitation) interact with the carbon fertilisation effect (CFE) to alter the balance between plant growth-driven demand for, and soil supply of, nutrients, primarily N. 6,7,12 The CFE would be most pronounced when resources and environmental conditions do not restrict plant growth. 19 However, the effects of multi-way interactions between climate drivers on plants and soils, particular on forage quality, are poorly understood; these interactions can be complex, multiplicative 20,21 , and species-specific 22 .…”

“…Research is required across a sweet-mixed-to-sourveld gradient to understand patterns and mechanisms of seasonal nutrient flows between plant parts – our knowledge of these is still surprisingly rudimentary 10 , – species-specific responses to interactions between multiple climate change drivers 22 , the potential effects on plant growth and quality of ongoing atmospheric nitrogen deposition 20 , and the extent to which the CFE effect on plant quality could be attenuated by downregulation of photosynthesis through acclimation and progressive N limitation over time 1,7,8 . We also recommend resampling sites with historical plant data, such as those presented here (Supplementary tables 1 & 2), to establish the extent to which CFE-driven shifts in forage quality may have already occurred over the last few decades, and to monitor regularly, widely, and seasonally shifts in leaf stoichiometry (at minimum C : N ratios 7,13 ) to establish the degree and extent of climate-driven oligotrophication 8 .…”

“…6 Diminishing foliar [N] could cascade through ecosystems, slowing protein flow from plants to insect and mammalian herbivores. 7 Even small decreases in the protein content and digestibility of forage would adversely affect animal health, reproduction, and weight gains. 7,8…”

“…7 Even small decreases in the protein content and digestibility of forage would adversely affect animal health, reproduction, and weight gains. 7,8…”

“…6 Diminishing foliar [N] could cascade through ecosystems, slowing protein flow from plants to insect and mammalian herbivores. 7 Even small decreases in the protein content and digestibility of forage would adversely affect animal health, reproduction, and weight gains. 7,8 The potential impacts of elevated [C0 2 ] (eCO2)and other climate change drivers on forage quality will occur across a well-recognised and agronomically important spatiotemporal gradient in South Africa, from sweetveld through mixedveld to sourveld.…”