Changing perspectives on terrestrial nitrogen cycling: The importance of weathering and evolved resource‐use traits for understanding ecosystem responses to global change

Abstract

Our understanding of terrestrial nitrogen (N) cycling is changing as new processes are uncovered, including the sources, turnover and losses of N from ecosystems. We integrate recent insights into an updated N‐cycling framework and discuss how a new understanding integrates eco‐evolutionary dynamics with nutrient cycling. These insights include (a) the significance of rock weathering as a biologically meaningful N source to plants and microbes; (b) the lack of consistent N limitation of organic matter decomposition by soil microbes; (c) species‐specific variation in plant N limitation; and (d) how fire effects on soil N shift with ecosystem properties. Using an eco‐evolutionary framework and revised knowledge of N cycling, we describe how (a) rock N weathering could have contributed more strongly to gradients in soil N availability than previously recognized, (b) evolution and co‐evolution of plant and soil microbial resource‐use traits underlie whether decomposition and production are N‐limited, and (c) the effects of fire on soil N pools are mediated by composition of plant species and time‐scale. Our revised framework of N cycling provides a way forward for improving biogeochemical models to more accurately estimate rates of plant production and decomposition, and total soil N. A free Plain Language Summary can be found within the Supporting Information of this article.

Global Nitrogen Cycle: Critical Enzymes, Organisms, and Processes for Nitrogen Budgets and Dynamics

“…453,501 However, species-specific responses to N fertilization are diverse, with certain plant species showing biomass increases and others showing decreases within the same ecosystem. 314 There is strong evidence for the stimulatory effects of N deposition on above ground storage in forest biomes, for example, see refs 327, 328, and 502. N deposition can also induce greater levels of below-ground C storage as soil organic matter by stimulating the production of litter and fine roots, the production of more recalcitrant plant biomass, or altering soil microbial activities.…”

“…313 Differences in resource use traits appear to play a large role in shaping the form and function of species responses to N deposition. 314 A relatively long and variable time lag for the recovery of ecosystems to the biodiversity levels of their pre-N saturated states is suggested by recent forest studies. 315,316 1.3.3.…”

“…512 On the other hand, mild C gains have been observed for wet sedge systems exposed to anthropogenic N. 513 Understanding how N fertilization affects the function of plant and microbes with different resource use traits will be essential to constraining the terrestrial C sink. 314 Taken together, anthropogenic N has had a modest fertilizing effect in terrestrial ecosystems, slowing the accumulation of anthropogenic CO 2 in the atmosphere. Given predictions that insufficient nutrient availability will limit the continued function of the land C sink, particularly in boreal and temperate forests, 418,514 it is likely that anthropogenic N will continue to aid biological C sequestration in the 21st century.…”

CO2, nitrogen deposition and a discontinuous climate response drive water use efficiency in global forests

“… 35 – 38 ). A cautionary note is that N deposition impacts are often greatest at relatively low rates for a range of ecosystem functions 39 , 40 , as seen in the low N Zones in the Southern Hemisphere (see Fig. 4 and Supplementary Fig.…”

Storage of soil samples leads to over-representation of the contribution of nitrate to plant-available nitrogen

“…Nitrogen (N) in soils is a highly dynamic nutrient, rapidly converting between different forms and moving through the soil profile. Nitrogen pool dynamics are driven by the chemical, physical and biological characteristics of the soil, and can drastically change with a stress applied to any of these conditions, such as during waterlogging and anoxia (Kuypers et al 2018;Wooliver et al 2019). This presents a considerable challenge for quantification of soil N pools, where disturbance and removal from the in situ conditions are often unavoidable.…”