Low N use Efficiency in A Long-Term N-Fertilized Cunninghamia Lanceolata Plantation Ecosystem in Subtropical China: A 14-Year Case Study

Abstract Background and aims Forests host among the most important N pools of all terrestrial ecosystems. Influences of N application on forest N cycle have received increasing concern, which is particularly problematic given the increasing atmospheric N deposition in recent decades. However, accurate assessments of N storage and recovery rates in forest ecosystems remain elusive. We selected Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantation ecosystem to explore how long-term N fertilization affected the N storage and recovery rate. Methods Plots in the field have been fertilized continuously for 14 years (2004–2017) with urea at rates of 0 (N0, control), 60 (N60, low-N), 120 (N120, medium-N) and 240 (N240, high-N) kg N hm− 2a− 1. Data collected in the field include N content and biomass on various plant organs (i.e., leaves, branches, stems, roots, and bark), understorey layer and litter in the ecosystem as well as soil N content and density at different depths (0–20, 20–40 and 40–60 cm). Key results The total N storage of ecosystem in the N-fertilized treatments was 1.1–1.4 times higher than that in the unfertilized treatment after 14 years of N fertilization. About 12.36% of the total ecosystem N was stored in vegetation (plant, litter, and understorey layer) and 87.64% was stored in soil (0–60 cm). N storage varied among ecosystem components and plant organs; and the plant organs, litter, and soil had higher N storage than that in understorey layer. Significantly higher Chinese fir N uptake was found in the medium-N (1.2 times) and high-N (1.4 times) treatments than that in the control. The N recovery rate of understorey layer in the N-fertilized treatments was negative, and less than that in the control. Conclusions Application of long-term N fertilizer to this stand led to a low N recovery rate (averagely 11.39%) while high loss of N (averagely 91.86%) which indicate low N use efficiency in the Chinese fir plantation ecosystem.