Nitrogen Deposition and Responses of Forest Structure to Nitrogen Deposition in a Cool-Temperate Deciduous Forest

Ruoming Cao,Shinpei Yoshitake,Siyu Chen,Toshiyuki Ohtsuka

doi:10.3390/f10080631

Abstract

Few studies have reported the estimation of nitrogen (N) deposition, including dissolved organic N (DON) fluxes, through water flows and the contribution of snowfall in Asia. In this study, the concentrations and fluxes of DON and dissolved inorganic N (DIN) in bulk precipitation (BP), the throughfall (TF) of trees and understory dwarf bamboo, and stemflow (SF) were evaluated in a cool-temperate forest over three years to clarify N fluxes via precipitation and responses of trees and understory canopies to N deposition. The input of N to the study site in BP was 11.1 ± 1.71 kg N ha−1 year−1, with a significant contribution from DON (78%). Snowfall fluxes contributed up to 46% of the total N input, with variations related to the amount of snowfall (2.08–5.52 kg N ha−1 year−1). The forest canopy enriched DON (2.11 ± 0.42 kg N ha−1 year−1) but consumed NO3 + NO2–N (−0.73 ± 0.19 kg N ha−1 year−1). In contrast, through the understory bamboo canopy, DON (−1.02 ± 0.55 kg N ha−1 year−1) decreased while DIN (0.35 ± 0.44 kg N ha−1 year−1) increased. This study indicates that DON and snowfall should not be neglected when evaluating total N deposition into forest ecosystems, especially in remote regions. The canopy processes related to the dissolved N in the presence of understory plants might have significant implications for the internal N cycle in forest ecosystems.

Highlights

Nitrogen (N), which is considered an essential nutrient for plants, can be deposited into forest ecosystems from the atmosphere via the hydrological pathway or dry deposition process.Anthropogenic N generated from intensive human activities amounted to more than 160 Tg N per year in the 1990s [1]
The average monthly concentration of dissolved organic N (DON) in bulk precipitation (BP) followed a clear seasonal pattern in each year, The average monthly concentration of DON in BP followed a clear seasonal pattern in each with the highest concentrations in spring (May to June) followed by a tendency to decrease over time year, with the highest concentrations in spring (May to June) followed by a tendency to decrease (Figure 3a)
Our results show that N deposition amounted to 11.1 ± 1.71 kg N ha−1 year−1, with a high contribution of DON (78%) and low contribution of dissolved inorganic N (DIN) (22%)

Summary

Introduction

Nitrogen (N), which is considered an essential nutrient for plants, can be deposited into forest ecosystems from the atmosphere via the hydrological pathway or dry deposition process.Anthropogenic N generated from intensive human activities amounted to more than 160 Tg N per year in the 1990s [1]. With increasing N pollution of the atmosphere, N deposition into forest ecosystems shows an increasing trend. It has been estimated that N deposition in 2050 will be approximately two-fold greater than that in 1990 at the global scale [2]. Atmospheric N deposition has been highlighted as a significant concern in terms of its impact on forest ecosystems [3,4,5]. Multiple studies have focused on the estimation of dissolved inorganic N (DIN) to comprehend the effects of N deposition on forest ecosystems [6,7,8,9,10]. Tree carbon storage has been estimated to increase due to N deposition [12]. N load thresholds have been reported for temperate deciduous forests, such as 10–15 kg N ha−1 year−1 across Europe [13] and Forests 2019, 10, 631; doi:10.3390/f10080631 www.mdpi.com/journal/forests

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Discussion

Conclusion