Changes in foliar chemistry and nutrient resorption in northern hardwood forests in response to long‐term experimental nitrogen and phosphorus addition

Northern hardwood forests have long been assumed to be primarily nitrogen limited, but may often be co‐limited by multiple elements. Nutrient limitation can be inferred through responses of foliar and litter chemistry to nutrient addition over time. We compared community‐level foliar and litter chemistry and resorption efficiency in a long‐term, factorial nitrogen (N) and phosphorus (P) fertilization study across 10 forest stands at three sites in New Hampshire, where N and P were added annually. We measured N, P, calcium (Ca), magnesium (Mg), and potassium (K) in foliage from codominant trees and in fresh litter in 2008–2010 (pretreatment) and again in 2014–2016 and 2021–2022. Foliar N and P concentrations indicated co‐limitation in 2014–2016 based on reduced concentrations of one nutrient following addition of the other, suggesting a dilution effect. In 2021–2022, an interactive effect of N and P addition was observed: foliar P concentrations were lower under N+P addition, consistent with dilution following a greater growth response to N + P than to P addition, which was observed by 2015–2019. Changes in litter N and P concentrations with N and P addition mirrored those in foliar N and P. Resorption efficiency of N and P decreased with addition of these respective nutrients and P resorption efficiency was higher in the N+P treatment than the P treatment. Foliar Ca and litter Ca and K decreased with N addition but increased with P addition. Results indicated N and P co‐limitation and revealed biogeochemical interactions among N, P and base cations.