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

   https://doi.org/10.1126/science.abh3767  

   Mason, Rachel E. ; Craine, Joseph M. ; Lany, Nina K. ; Jonard, Mathieu ; Ollinger, Scott V. ; Groffman, Peter M. ; Fulweiler, Robinson W. ; Angerer, Jay ; Read, Quentin D. ; Reich, Peter B. ; et al ( April 2022 , Science)

   BACKGROUND The availability of nitrogen (N) to plants and microbes has a major influence on the structure and function of ecosystems. Because N is an essential component of plant proteins, low N availability constrains the growth of plants and herbivores. To increase N availability, humans apply large amounts of fertilizer to agricultural systems. Losses from these systems, combined with atmospheric deposition of fossil fuel combustion products, introduce copious quantities of reactive N into ecosystems. The negative consequences of these anthropogenic N inputs—such as ecosystem eutrophication and reductions in terrestrial and aquatic biodiversity—are well documented. Yet although N availability is increasing in many locations, reactive N inputs are not evenly distributed globally. Furthermore, experiments and theory also suggest that global change factors such as elevated atmospheric CO 2 , rising temperatures, and altered precipitation and disturbance regimes can reduce the availability of N to plants and microbes in many terrestrial ecosystems. This can occur through increases in biotic demand for N or reductions in its supply to organisms. Reductions in N availability can be observed via several metrics, including lowered nitrogen concentrations ([N]) and isotope ratios (δ 15 N) in plant tissue, reduced rates of N mineralization, and reduced terrestrial N export to aquatic systems. However, a comprehensive synthesis of N availability metrics, outside of experimental settings and capable of revealing large-scale trends, has not yet been carried out. ADVANCES A growing body of observations confirms that N availability is declining in many nonagricultural ecosystems worldwide. Studies have demonstrated declining wood δ 15 N in forests across the continental US, declining foliar [N] in European forests, declining foliar [N] and δ 15 N in North American grasslands, and declining [N] in pollen from the US and southern Canada. This evidence is consistent with observed global-scale declines in foliar δ 15 N and [N] since 1980. Long-term monitoring of soil-based N availability indicators in unmanipulated systems is rare. However, forest studies in the northeast US have demonstrated decades-long decreases in soil N cycling and N exports to air and water, even in the face of elevated atmospheric N deposition. Collectively, these studies suggest a sustained decline in N availability across a range of terrestrial ecosystems, dating at least as far back as the early 20th century. Elevated atmospheric CO 2 levels are likely a main driver of declines in N availability. Terrestrial plants are now uniformly exposed to ~50% more of this essential resource than they were just 150 years ago, and experimentally exposing plants to elevated CO 2 often reduces foliar [N] as well as plant-available soil N. In addition, globally-rising temperatures may raise soil N supply in some systems but may also increase N losses and lead to lower foliar [N]. Changes in other ecosystem drivers—such as local climate patterns, N deposition rates, and disturbance regimes—individually affect smaller areas but may have important cumulative effects on global N availability. OUTLOOK Given the importance of N to ecosystem functioning, a decline in available N is likely to have far-reaching consequences. Reduced N availability likely constrains the response of plants to elevated CO 2 and the ability of ecosystems to sequester carbon. Because herbivore growth and reproduction scale with protein intake, declining foliar [N] may be contributing to widely reported declines in insect populations and may be negatively affecting the growth of grazing livestock and herbivorous wild mammals. Spatial and temporal patterns in N availability are not yet fully understood, particularly outside of Europe and North America. Developments in remote sensing, accompanied by additional historical reconstructions of N availability from tree rings, herbarium specimens, and sediments, will show how N availability trajectories vary among ecosystems. Such assessment and monitoring efforts need to be complemented by further experimental and theoretical investigations into the causes of declining N availability, its implications for global carbon sequestration, and how its effects propagate through food webs. Responses will need to involve reducing N demand via lowering atmospheric CO 2 concentrations, and/or increasing N supply. Successfully mitigating and adapting to declining N availability will require a broader understanding that this phenomenon is occurring alongside the more widely recognized issue of anthropogenic eutrophication. Intercalibration of isotopic records from leaves, tree rings, and lake sediments suggests that N availability in many terrestrial ecosystems has steadily declined since the beginning of the industrial era. Reductions in N availability may affect many aspects of ecosystem functioning, including carbon sequestration and herbivore nutrition. Shaded areas indicate 80% prediction intervals; marker size is proportional to the number of measurements in each annual mean. Isotope data: (tree ring) K. K. McLauchlan et al. , Sci. Rep. 7 , 7856 (2017); (lake sediment) G. W. Holtgrieve et al. , Science 334 , 1545–1548 (2011); (foliar) J. M. Craine et al. , Nat. Ecol. Evol. 2 , 1735–1744 (2018) 

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2. Robust observations of land-to-atmosphere feedbacks using the information flows of FLUXNET

   https://doi.org/10.1038/s41612-019-0094-4  

   Gerken, Tobias ; Ruddell, Benjamin L. ; Yu, Rong ; Stoy, Paul C. ; Drewry, Darren T. ( October 2019 , npj Climate and Atmospheric Science)

   Feedbacks between atmospheric processes like precipitation and land surface fluxes including evapotranspiration are difficult to observe, but critical for understanding the role of the land surface in the Earth System. To quantify global surface-atmosphere feedbacks we use results of a process network (PN) applied to 251 eddy covariance sites from the LaThuile database to train a neural network across the global terrestrial surface. There is a strong land–atmosphere coupling between latent (LE) and sensible heat flux (H) and precipitation (P) during summer months in temperate regions, and betweenHandPduring winter, whereas tropical rainforests show little coupling seasonality. Savanna, shrubland, and other semi-arid ecosystems exhibit strong responses in their coupling behavior based on water availability. Feedback couplings from surface fluxes toPpeaks at aridity (P/potential evapotranspiration ET_p) values near unity, whereas coupling with respect to clouds, inferred from reduced global radiation, increases asP/ET_papproaches zero. Spatial patterns in feedback coupling strength are related to climatic zone and biome type. Information flow statistics highlight hotspots of (1) persistent land–atmosphere coupling in sub-Saharan Africa, (2) boreal summer coupling in the central and southwestern US, Brazil, and the Congo basin and (3) in the southern Andes, South Africa and Australia during austral summer. Our data-driven approach to quantifying land atmosphere coupling strength that leverages the global FLUXNET database and information flow statistics provides a basis for verification of feedback interactions in general circulation models and for predicting locations where land cover change will feedback to climate or weather.

    

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3. Rainfall drives variation in rates of change in intrinsic water use efficiency of tropical forests

   https://doi.org/10.1038/s41467-019-11679-8  

   Adams, Mark A. ; Buckley, Thomas N. ; Turnbull, Tarryn L. ( August 2019 , Nature Communications)

   Rates of change in intrinsic water use efficiency (W) of trees relative to those in atmospheric [CO₂] (c_a) have been mostly assessed via short-term studies (e.g., leaf analysis, flux analysis) and/or step increases inc_a(e.g., FACE studies). Here we use compiled data for abundances of carbon isotopes in tree stems to show that on decadal scales, rates of change (dW/dc_a) vary with location and rainfall within the global tropics. For the period 1915–1995, and including corrections for mesophyll conductance and photorespiration,dW/dc_afor drier tropical forests (receiving ~ 1000 mm rainfall) were at least twice that of the wettest (receiving ~ 4000 mm). The data also empirically confirm theorized roles of tropical forests in changes in atmospheric¹³C/¹²C ratios (the¹³C Suess Effect). Further formal analysis of geographic variation in decade-to-century scaledW/dc_awill be needed to refine current models that predict increases in carbon uptake by forests without hydrological cost.

    

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4. Tree Canopies Reflect Mycorrhizal Composition

   https://doi.org/10.1029/2021GL092764  

   Sousa, Daniel ; Fisher, Joshua B. ; Galvan, Fernando Romero ; Pavlick, Ryan P. ; Cordell, Susan ; Giambelluca, Thomas W. ; Giardina, Christian P. ; Gilbert, Gregory S. ; Imran‐Narahari, Faith ; Litton, Creighton M. ; et al ( May 2021 , Geophysical Research Letters)

   Mycorrhizae alter global patterns of CO₂fertilization, carbon storage, and elemental cycling, yet knowledge of their global distributions is currently limited by the availability of forest inventory data. Here, we show that maps of tree‐mycorrhizal associations (hereafter “mycorrhizal maps”) can be improved by the novel technology of imaging spectroscopy because mycorrhizal signatures propagate up from plant roots to impact forest canopy chemistry. We analyzed measurements from 143 airborne imaging spectroscopy surveys over 112,975 individual trees collected across 13 years. Results show remarkable accuracy in capturing ground truth observations of mycorrhizal associations from canopy signals across disparate landscapes (R² = 0.92,p < 0.01). Upcoming imaging spectroscopy satellite missions can reveal new insights into landscape‐scale variations in water, nitrogen, phosphorus, carotenoid/anthocyanin, and cellulose/lignin composition. Applied globally, this approach could improve the spatial precision of mycorrhizal distributions by a factor of roughly 10⁴and facilitate the incorporation of dynamic shifts in forest composition into Earth system models.

    

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5. Wetting‐induced soil CO2 emission pulses are driven by interactions among soil temperature, carbon, and nitrogen limitation in the Colorado Desert

   https://doi.org/10.1111/gcb.16669  

   Andrews, Holly M. ; Krichels, Alexander H. ; Homyak, Peter M. ; Piper, Stephanie ; Aronson, Emma L. ; Botthoff, Jon ; Greene, Aral C. ; Jenerette, G. Darrel ( March 2023 , Global Change Biology)

   Warming‐induced changes in precipitation regimes, coupled with anthropogenically enhanced nitrogen (N) deposition, are likely to increase the prevalence, duration, and magnitude of soil respiration pulses following wetting via interactions among temperature and carbon (C) and N availability. Quantifying the importance of these interactive controls on soil respiration is a key challenge as pulses can be large terrestrial sources of atmospheric carbon dioxide (CO₂) over comparatively short timescales. Using an automated sensor system, we measured soil CO₂flux dynamics in the Colorado Desert—a system characterized by pronounced transitions from dry‐to‐wet soil conditions—through a multi‐year series of experimental wetting campaigns. Experimental manipulations included combinations of C and N additions across a range of ambient temperatures and across five sites varying in atmospheric N deposition. We found soil CO₂pulses following wetting were highly predictable from peak instantaneous CO₂flux measurements. CO₂pulses consistently increased with temperature, and temperature at time of wetting positively correlated to CO₂pulse magnitude. Experimentally adding N along the N deposition gradient generated contrasting pulse responses: adding N increased CO₂pulses in low N deposition sites, whereas adding N decreased CO₂pulses in high N deposition sites. At a low N deposition site, simultaneous additions of C and N during wetting led to the highest observed soil CO₂fluxes reported globally at 299.5 μmol CO₂ m⁻² s⁻¹. Our results suggest that soils have the capacity to emit high amounts of CO₂within small timeframes following infrequent wetting, and pulse sizes reflect a non‐linear combination of soil resource and temperature interactions. Importantly, the largest soil CO₂emissions occurred when multiple resources were amended simultaneously in historically resource‐limited desert soils, pointing to regions experiencing simultaneous effects of desertification and urbanization as key locations in future global C balance.

    

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