Threshold and multiple indicators for nitrogen saturation in subtropical forests

Abstract

The influence of nitrogen (N) deposition on forest ecosystems largely depend on the N status. Developing threshold and practical indicators for N saturation in subtropical forests, with extremely high N deposition, would both enhance forest management and the assessments of global N balance and carbon (C) sequestration. Here, we quantified the N mass balance and assessed current N status at a number of subtropical forest sites in South China, using both N content, C/N ratio, and ¹⁵N natural abundance (δ¹⁵N) as potential indicators of N saturation. Among the studied sites, N deposition ranged from 13.8 to 113 kg N ha^-1 yr^-1 in throughfall, and was dominated by ammonium (NH₄⁺). The threshold for N leaching in subtropical forest was first found to be 26-36 kg N ha^-1 yr^-1, which was 160% higher than in temperate forest (based on prescribed minimum). This indicates that critical parameter inputs in global models of the impact of N deposition are in need of revision, based on specific ecosystem characteristics. We found a critical C/N ratio of 20 for the O/A horizon as indicator of N saturation. Foliar N content and δ¹⁵N were positively correlated with N deposition and were well suited to indicate regional N status. The δ¹⁵N enrichment factor (Ɛ_foli/So2, δ¹⁵N_foliage - δ¹⁵N_Soil2) was between -10‰ and -1‰, and had similar trend to those obtained from other regions with increasing N deposition. These suggest that the enrichment factor could be used to investigate the influence of N deposition in forest ecosystems, regardless of spatial heterogeneity in δ¹⁵N of N input, soil N availability and geomorphology.

Keywords: Enrichment factor; N deposition; N isotope; N saturation; Subtropical forests.