Canopy nitrogen application greatly enhances forest productivity

we established a large-scale field experiment in 2013 comparing the effects of CAN with UAN on forest productivity at three N-addition rates (0, 25 or 50 kg N ha–1 yr–1). This experiment consists of 20 circular plots (four per unique treatment) each with 34 m diameter, and has been going on with N additions for a decade in a temperate deciduous forest located in Dabie Mountain, central China (details in Materials and Methods). To directly evaluate how canopy N application affect forest productivity, we measured leaf litterfall plus branch, stem wood, and fine root production during the N treatment periods. We conducted 9260 leaf litterfall measurements using litter traps (1 m × 1 m) between 2017 and 2022. Leaf litterfall production was monitored weekly over four consecutive years (2017–2020) to assess temporal dynamics. We measured fine root production using the ingrowth core method every three months from May 2017 to March 2019 across a total of 720 locations. In each plot, we measured the diameter at breast height (DBH) and height of all trees with DBH ≥ 3 cm in 2012, 2017, and 2022. Using region-specific allometric equations, we then calculated the annual net primary productivity (NPP) over the ten-year period. We calculated ∆C/∆N by dividing the difference in NPP between N addition plots and control plots by the amount of added N. Additionally, we employed a 15N tracer (99.3 atom% 15NH415NO3) to track the fate and redistribution of added N within the forest ecosystem, including leaves, branches, stem wood, fine roots, mineral soil, and organic soil.