Natural ¹⁵N abundance of ammonium and nitrate in soil profiles: New insights into forest ecosystem nitrogen saturation

Abstract

Assessment of nitrogen (N) saturation of forests is critical to the evaluation of the manner in which ecosystems will respond to current and future global changes such as N deposition. However, quantifying N saturation remains a challenge. We developed a conceptual model of N saturation stages in forest ecosystems based on (1) a hypothetical relative rate of ammonification, nitrification, and denitrification, (2) concentrations of ammonium and nitrate in the soil, and (3) 15 N enrichment pattern of bulk soil N, ammonium, and nitrate in the soil profile. We tested the hypotheses using data from forests located at five sites across eastern Asia. The fraction of nitrate in total inorganic N indicated that the sites represent an N saturation gradient with one boreal forest at stage 1 (least saturated), three temperate forests at stage 2, and one tropical forest at stage 3 (most saturated). The δ 15 N of bulk soil N increased from topsoil to subsoil more sharply at N-limited sites than at the N-rich sites along the N deposition gradient. We also found distinct 15 N enrichment patterns of bulk soil N, ammonium, and nitrate in the soil profile across the study sites. At stage 1, nitrate was more 15 N-depleted than ammonium only in the organic soil

Within‐site difference in nitrogen status between mixed forests and larch plantations: Evidence from multiple indicators

“…In contrast, plant competition for N in the mixed forest may deplete soil N (Thomas et al, 2020). The similar (or even more) 15 N‐enrichment of nitrate than ammonium in the soil profile in the larch forest at MES (Appendix S1: Figure S2) indicates the effects of fractionation during denitrification (Gurmesa et al, 2022; Houlton et al, 2006).…”

“…The spatial patterns of soil inorganic N across sites suggest differences in the relative importance of soil N transformation rates (e.g., mineralization and nitrification), primarily controlled by N deposition and climate (MAT and MAP). For example, the dominancy of soil inorganic N by ammonium at the DXAL site (Figure 4a–c) indicates that mineralization is the dominant process, which is characteristic of N‐limited forests (Gurmesa et al, 2022). Nitrification at DXAL is likely limited because of strong demand for N by plants and soil microbes under high C/N conditions (C‐rich soil) (Booth et al, 2005).…”

“…We hypothesized that (1) the mixed forest would have higher N availability than the larch forest because the intensive management in the larch forest would deplete soil N pool through the removal of nutrients with the biomass and alteration of soil microbial environments, and (2) soil N availability of the sites would follow the north–south climatic and N deposition gradients, being lowest at DXAL, moderate at MES, and highest at QY. Data on the concentrations of ammonium and nitrate and their δ 15 N in the soil profile in the mixed forests have been recently reported (Gurmesa et al, 2022) while the data for the adjacent larch forests are reported here for the first time.…”

“…Several ecosystem variables related to plant, soil, or ecosystems, including foliar %N (Pardo et al, 2006), C/N (Gundersen et al, 1998), soil N transformation rates, and pool sizes of inorganic N (Aber et al, 1998), N leaching (Dise & Wright, 1995), and plant and soil δ 15 N (Gurmesa et al, 2022; Hobbie & Ouimette, 2009; Pardo et al, 2006) have been identified as key indicators of N status of forest ecosystems over different temporal and spatial scales. Recently, the fraction of NO 3 − in inorganic N pool, together with the relative 15 N‐enrichment pattern of bulk soil N, ammonium, and nitrate in the soil profile have been shown to indicate N status across wide environmental and climate gradients (Gurmesa et al, 2022). However, diagnosing the N status for a given forest remains difficult because each indicator reflects only a specific aspect of N availability, resulting in inconsistent conclusions about the overall N status of the ecosystem.…”

“…Forest N status, initially defined as the degree of N saturation (Currie, 1999; MacDonald et al, 2002), can be assessed using various measures of N availability such as indexes of plant nutritional status and N losses from ecosystems. Several ecosystem variables related to plant, soil, or ecosystems, including foliar %N (Pardo et al, 2006), C/N (Gundersen et al, 1998), soil N transformation rates, and pool sizes of inorganic N (Aber et al, 1998), N leaching (Dise & Wright, 1995), and plant and soil δ 15 N (Gurmesa et al, 2022; Hobbie & Ouimette, 2009; Pardo et al, 2006) have been identified as key indicators of N status of forest ecosystems over different temporal and spatial scales. Recently, the fraction of NO 3 − in inorganic N pool, together with the relative 15 N‐enrichment pattern of bulk soil N, ammonium, and nitrate in the soil profile have been shown to indicate N status across wide environmental and climate gradients (Gurmesa et al, 2022).…”