Negligible Response of Transpiration to Late-Summer Nitrogen Fertilization in Japanese Oak (Quercus crispula)

Nao Nagano,Kyoichi Otsuki,Tomonori Kume,Masaaki Chiwa,Yasuhiro Utsumi,Nobutada Tashiro

doi:10.3390/nitrogen3010006

Abstract

Increased atmospheric nitrogen (N) deposition, caused by anthropogenic activities, has various effects on forest ecosystems. Some reports have investigated the responses in tree transpiration to N addition, but few studies have measured the short-term response of mature tree transpiration to N fertilization. This study aimed to clarify the short-term transpiration response in 27-year-old deciduous hardwood trees to an increase in N availability. We established two plot types (control and N-fertilized plots) in Quercus crispula plantation stands in Hokkaido, Northern Japan. We measured sap flow density (SFD; cm3 m−2 s−1) using a thermal dissipation method for three months during the growing season. In the N-fertilized plot, we added 50 kg N ha−1 yr−1 of ammonium nitrate (NH4NO3) to the forest floor in the middle of the measurement periods. For daily mean SFD, we did not find a significant difference between the control and the N-fertilized plots. Leaf N contents did not differ between treatments, implying a negligible difference in physiological responses and transpiration rates. The slight difference between treatments could be because the trees had already foliated before applying the N fertilizer to our deciduous hardwood trees. The present results indicate that the potential increase in N deposition during the growing season does not immediately alter tree transpiration.

Highlights

Atmospheric nitrogen (N) deposition has increased due to anthropogenic activities, such as fossil fuel combustion and the fertilization of farmlands, and deposition will likely continue to rise in the future [1,2]
We evaluated the effect of N fertilization on transpiration using the change in sap flow density (SFD), before and after N fertilization, in each plot
The diurnal pattern of SFD in tree individuals corresponded to the meteorological conditions (VPD and photosynthetic active radiation (PAR)), which is similar to findings in previous studies [26,35] (Figure 2)

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Atmospheric nitrogen (N) deposition has increased due to anthropogenic activities, such as fossil fuel combustion and the fertilization of farmlands, and deposition will likely continue to rise in the future [1,2]. Previous studies have revealed that elevated atmospheric N deposition has changed both below-ground and above-ground forest ecosystems. Examples of below-ground changes include increasing soil respiration [3], promoting soil acidification, and leaching exchangeable cations [4]. Above-ground changes include increasing the stem volume of trees [5] and the degradation of vegetation diversity [6]

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Discussion

Conclusion