Masonet al.,Science 376 , eabh3767 (2022) 15 April 2022 2 of 11
Fig. 1. Changes in the N cycle can be detected by monitoring eco-
system N inputs, internal soil N cycling and plant N status, and N
losses.In contrast to the well-established monitoring of global atmospheric
CO 2 , for example, tracking N availability requires observing a comprehensive
set of metrics that are often highly variable in space and time and the
measurement of which involves considerable effort. Lower N input rates
(A), smaller pools and fluxes of plant-available soil N (B), decreased plant
N status (C), and lower N losses (DandE) over time may indicate
reductions in N availability, whereas N stable isotope ratios (F) provide
a measure that integrates over several determinants of N availability.
For (A) to (F), respectively, example time series of N availability indicators,
here primarily taken from forest ecosystems, are adapted from ( 3 , 99 ), ( 28 ),
( 23 ), ( 28 ), ( 85 ), and ( 29 ). [Photo credits: (A) M. Jonard; (B) Natural
Resources Conservation Service/CC BY 2.0; (C) milomingo/CC BY-NC-ND 2.0;
(D) A. Contosta; (E) US Forest Service, Northern Research Station;
and (F) B. Kasman]RESEARCH | REVIEW