program would involve monitoring of N
availability metrics such as N concentrations
in plant tissues, net N mineralization in soils,
N concentrations in aquatic ecosystems, and
herbivore dietary quality. Systematic collec-
tion of satellite hyperspectral remote sens-
ing data will soon help facilitate assessment
of foliar [N] across broader spatial scales
than those possible with current airborne
instrumentation or field sampling. Continental-scale monitoring of N deposition has provided
important information about N supply, but
complex spatial patterns of rising, falling,
and stabilizing trends ( 2 , 3 ) justify expanded
instrumentation.Masonet al.,Science 376 , eabh3767 (2022) 15 April 2022 7 of 11
Fig. 5. Addressing the root cause of declining N implies reducing CO 2
emissions, whereas ecosystem assessments and continued research are
needed to inform management actions.Variation in N availability trends
over space and time can be elucidated through field monitoring campaigns,
reconstruction of historical records, and creation of maps via hyperspectral remote
sensing techniques. Continued experimentation is required to better understand
the processes driving, and resulting from, reductions in N availability. Incorporating
this knowledge in ESMs will clarify how declining N may affect the ability of
ecosystems to buffer CO 2 emissions. [Image credits (counterclockwise from top
left): h080/CC BY-SA 2.0; US Forest Service, Northern Research Station; B. Kasman;
S.Ollinger;M.Kirk/CC BY-SA 4.0; US Department of Energy, Oak Ridge National
Laboratory/CC BY 2.0;L.Lamsa/CC BY 2.0; and N. Tonelli/CC BY 2.0]Fig. 4. Impacts of declines in foliar N concentrations on herbivore performance.Reduction in forage quality (AandD) may result in reduced herbivore body size
and/or development rate (BandE) and reproduction (CandF) because herbivore growth rates and populations are often limited by protein availability.
RESEARCH | REVIEW