High retention of ¹⁵N‐labeled nitrogen deposition in a nitrogen saturated old‐growth tropical forest

Abstract

The effects of increased reactive nitrogen (N) deposition in forests depend largely on its fate in the ecosystems. However, our knowledge on the fates of deposited N in tropical forest ecosystems and its retention mechanisms is limited. Here, we report the results from the first whole ecosystem N labeling experiment performed in a N-rich old-growth tropical forest in southern China. We added N tracer monthly as NH NO for 1 year to control plots and to N-fertilized plots (N-plots, receiving additions of 50 kg N ha yr for 10 years). Tracer recoveries in major ecosystem compartments were quantified 4 months after the last addition. Tracer recoveries in soil solution were monitored monthly to quantify leaching losses. Total tracer recovery in plant and soil (N retention) in the control plots was 72% and similar to those observed in temperate forests. The retention decreased to 52% in the N-plots. Soil was the dominant sink, retaining 37% and 28% of the labeled N input in the control and N-plots, respectively. Leaching below 20 cm was 50 kg N ha yr in the control plots and was close to the N input (51 kg N ha yr ), indicating N saturation of the top soil. Nitrogen addition increased N leaching to 73 kg N ha yr . However, of these only 7 and 23 kg N ha yr in the control and N-plots, respectively, originated from the labeled N input. Our findings indicate that deposited N, like in temperate forests, is largely incorporated into plant and soil pools in the short term, although the forest is N-saturated, but high cycling rates may later release the N for leaching and/or gaseous loss. Thus, N cycling rates rather than short-term N retention represent the main difference between temperate forests and the studied tropical forest.

Temporal changes in soil C‐N‐P stoichiometry over the past 60 years across subtropical China

“…Previous studies suggest that N concentrations are determined by biologic N fixation in ecosystems (Vitousek et al., ) and that global changes such as N deposition (Galloway, Schlesinger, Levy, Michaels, & Schnoor, ) and warming (Vitousek, ) have a positive effect on biologic N fixation. However, we did not find a temporal increase in soil N, which might be because most of the deposited N was incorporated into plants (Gurmesa et al., ). Another possible explanation could be that long‐term N deposition in subtropical China might have resulted in N‐saturated soils that are incapable of retaining additional N input, as evidenced by the insignificant relationship between soil total N concentrations and N deposition rate in our study area.…”

“…In particular, N deposition has increased dramatically since the 1980s in China (Liu et al., ), which is now among the highest deposition areas of the world (Jia et al., ; Peñuelas et al., ). Current deposition rates in southern China, range from 12 to 65 kg ha −1 year −1 (Chen & Mulder, ; Fang et al., ; Gurmesa et al., ; Jia et al., ). Although multiple global change scenarios have been shown to alter elemental C:N:P ratios in terrestrial plants under controlled conditions (Yuan & Chen, ; Yue et al., ), the temporal response of C:N:P stoichiometry to chronic global changes in natural environments remains poorly understood.…”

Competition for nitrogen between trees in a mixed-species plantation in the Solomon Islands

“…Soil was the main sink for 15 N, with 37% and 47% recovered in soil for 2-year and 4-year plots respectively, consistent with recent findings by Gurmesa et al (2016) in a tropical forest of Southern China. The higher total 15 N recovery in the 4-year plots (55%) than in the 2-year plots (42%) may be due to better developed root structure in the 4-year plots that prevented leaching and promoted 15 N uptake.…”

“…Canopy closure resulted in lower temperatures, reduced the direct fall of rain and increased forest floor biomass, which might have lessened the possibility both of denitrification and of leaching of 15 N. Denitrification is expected to be higher in under an open canopy with higher temperatures and the soil surface exposed to high rainfall and surface flow (Davidson et al 1993;Bustamante et al 2004). However, the total recovery of 15 N was lower than the recovery rates of approximately 75% found in other studies in tropical (Gurmesa et al 2016) and temperate ecosystems (Templer et al 2012). Few studies using 15 N labelling to analyse N cycling rates have been carried out in tropical forests, and more information is needed to unravel the mechanisms behind N retention and N loss in tropical ecosystems.…”

“…Few studies using 15 N labelling to analyse N cycling rates have been carried out in tropical forests, and more information is needed to unravel the mechanisms behind N retention and N loss in tropical ecosystems. The high cycling rates occurring under tropical settings may promote larger leaching rates and gaseous losses than those occurring under temperate conditions (Gurmesa et al 2016).…”