Long-term ecological research and the COVID-19 anthropause: A window to understanding social-ecological disturbance.

The period of disrupted human activity caused by the COVID-19 pandemic, coined the "anthropause," altered the nature of interactions between humans and ecosystems. It is uncertain how the anthropause has changed ecosystem states, functions, and feedback to human systems through shifts in ecosystem services. Here, we used an existing disturbance framework to propose new investigation pathways for coordinated studies of distributed, long-term social-ecological research to capture effects of the anthropause. Although it is still too early to comprehensively evaluate effects due to pandemic-related delays in data availability and ecological response lags, we detail three case studies that show how long-term data can be used to document and interpret changes in air and water quality and wildlife populations and behavior coinciding with the anthropause. These early findings may guide interpretations of effects of the anthropause as it interacts with other ongoing environmental changes in the future, particularly highlighting the importance of long-term data in separating disturbance impacts from natural variation and long-term trends. Effects of this global disturbance have local to global effects on ecosystems with feedback to social systems that may be detectable at spatial scales captured by nationally to globally distributed research networks.

Immobilization of a 15N-labeled nitrate addition by decomposing forest litter.

Effects of chronic HNO3 and H2SO4 additions on decomposition of senesced birch leaf, beech leaf, spruce needle, and wood chip litters were examined. Litters were incubated for up to 4 years in fiberglass mesh (1 mm) bags on experimental plots in a mixed-species forest near the Bear Brooks Watershed Manipulation (BBWM) site in eastern Maine, United States. Plot treatments included HNO3 additions at 28 and 56 kg N·ha-1·year-1, H2SO4 additions at 128 kg S·ha-1·year-1, and a combined HNO3 and H2SO4 treatment at 28 kg N and 64 kg S ·ha-1·year-1. The 15N content of all NO3 added was artificially increased to 344% δ15N. Litter bags were collected each fall and analyzed for organic matter loss, nitrogen concentration, and 15N abundance throughout the 4-year experiment. Extractive (non-polar-soluble+water-soluble), cellulose (acid-soluble), and lignin (acid-insoluble) fractions were analyzed for the first 2 years. In wood chips, nitrogen additions increased mass loss and N concentration, but not the mass of N after 4 years. Neither N nor S additions had large effects on mass loss, N concentration, or N content of leaf litters. All litters immobilized and mineralized N simultaneously, but we were able to place a lower bound on gross N immobilization by mass balancing 15N additions. Birch and spruce litters showed net mineralization, while beech leaf and wood chip litters showed net immobilization. Net immobilizing litters were those with the highest initial cellulose concentration (wood chips=80% beech leaves=54%), and we attribute the higher capacity for immobilization to more readily available carbon. Lignin mass increased initially in all litter types except spruce needles. Also, extractives in net immobilizing litters either increased initially (wood chips) or decreased at a slower rate than bulk litter (beech leaves). We calculate the potential of decomposing litter to immobilize exogenous nitrate in this system to be 1-1.5 kg N·ha-1·year-1, which is about half of the usual NO3 deposition at this site, but only a small fraction of the experimental addition.

The fate of 15N-labelled nitrate additions to a northern hardwood forest in eastern Maine, USA.

We followed the movements of 15N-labelled nitrate additions into biomass and soil pools of experimental plots (15×15 m each) in a mid-successional beech-maple-birch-spruce forest in order to identify sinks for nitrate inputs to a forest ecosystem. Replicate plots (n=3) were spray-irrigated with either 28 or 56 kg N ha-1 year-1 using 15N-labelled nitric acid solutions (δ15N = 344‰ ) during four successive growing seasons (April-October). The 15N contents of foliage, bolewood, forests floor and mineral soil (0-5 cm) increased during the course of treatments. Mass balance calculations showed that one-fourth to one-third of the nitrate applied to forest plots was assimilated into and retained by above ground plant tissues and surface soil horizons at both rates of nitrate application. Plant and microbial assimilation were of approximately equal importance in retaining nitrate additions to this forest. Nitrate use among tree species varied, however, with red spruce showing lower rates of nitrate assimilation into foliage and bolewood than American beech and other deciduous species.