Fine-root respiration rates were measured at nineteen National Ecological Observatory Network (NEON) forest sites across the continental US, including Harvard Forest. The data was combined with available NEON root biomass and litter nutrient data to assess the hypothesis that, at the ecosystem level, carbon allocated to root system respiration was strongly correlated with annual nutrient uptake – the work required of the root system. For forests, annual nutrient uptake demand can be approximated by quantifying the nutrients lost annually as plant litter is shed, as this same quantity of nutrients must be obtained to replace the shed tissues (primarily foliage) for the next growing season. As a result, ecosystem fine-root respiration (specific respiration rate times root biomass) should be strongly related to the product of litter mass multiplied by nutrient concentration, in a relationship that should hold across ecosystems both within and across biomes.
From July through October of 2024, field measurements of specific fine-root respiration at ambient soil temperature were completed at the nineteen NEON sites. Fine roots are very active roots, less than 0.5 mm in diameter. Samples were then frozen pending further laboratory analysis of sample biomass and nitrogen concentration. To date, these analyses have been completed for fourteen of the sites, including Harvard Forest. The correlation between ecosystem fine-root respiration measured at ambient soil temperature and foliar litterfall N content itself is not significant (P = 0.12), however the sites differ in growing season length, which also must be accounted for. When NEON data for green days at each site is multiplied by ecosystem fine-root respiration, there is a statistically significant correlation (r = 0.57, P = 0.04) between litterfall N content (g N m-2 y-1) and growing season ecosystem root respiration (g C m-2 y-1). Further, our field data is for a single point in time, but soil temperature varies over the growing season. When respiration rates are adjusted to mean growing season temperature, the relationship strengthens (r = 0.60, P = 0.03). Finally, two of the fourteen sites were experiencing extreme drought at the time of measurement, which is known to severely reduce root respiration rates. When those two sites are removed from the data, the relationship becomes even stronger (r = 0.87, P