長期氮施肥對溪頭人工針葉林之樹木生長、林分發展與土壤性質影響

Abstract

現今大氣氮(N)沉降量相較於20世紀之前有明顯增加，增加的氮沉降可能經由多種不同方式影響森林生態系統。然而，我們對於氮沉降增加對臺灣森林生態的影響，仍不清楚。為了探討人為氮輸入對森林生態系統的影響，我們於臺灣中部溪頭地區進行為期12年的氮施肥實驗(自2011至2022年)，施肥樣區包含四個不同的針葉樹人工林林分，並以硝酸銨 (NH₄NO₃)的形式進行每年每公頃40公斤氮肥的施用。我們利用樹木生長量測帶(dendrometers)測量個別樹木與整體林分生長，並透過土壤採樣與水分滲漏儀(lysimeters)收集土壤與土壤溶液樣本，以評估長期氮添加的對森林生態的影響。此外，我們也進行了一項盆栽試驗，利用三種闊葉樹幼苗，進行氮(N)、磷(P)及氮磷添加施肥處理，用以了解土壤肥力概況。實驗結果顯示，經過12年的氮肥添加，林分密度在所有試驗地點均下降不超過15%，然而施肥並未引起存活樹木及林分的生長提升，施肥與未施肥樣區之土壤有機碳(SOC)和全氮(TN)並未呈現明顯差異，施肥樣區土壤pH值及可交換性鉀(K⁺)、鎂(Mg²⁺)、鈣(Ca²⁺)離子濃度也未出現顯著降低。不過，土壤溶液中可以觀測到施用氮肥後的差異，導致施肥樣區土壤溶液之硝酸鹽氮濃度及電導度(EC)明顯上升，同時有2個林分之土壤溶液呈現pH顯著下降。盆栽試驗亦顯示，不同處理之間的幼苗生長並無顯著差異。以上結果推測樣區中林木生長未因施肥而增加的主要原因，可能為土壤氮飽和。

Atmospheric nitrogen (N) deposition have greatly increased compared to the levels before the 20th century. Increased N deposition can affect forest ecosystems through various ways. However, the knowledge of the effects of elevated N deposition on forest ecosystem is scarce in Taiwan. To evaluate the influence of anthropogenic N input on the forest ecosystem, we established a 12-year N-addition experiment from 2011 to 2022 in Xitou, central Taiwan. Annual N fertilizers in the form of ammonium nitrate (NH4NO3) at 40 kg N ha-1 yr-1 was added in 4 different coniferous stands. We monitored tree growth (with dendrometers) and collected soil samples and soil solutions (with lysimeters) to examine the impact of chronic N addition. Furthermore, a pot experiment (using three kinds of broadleaf seedlings) was conducted with treatments involving N, phosphorus (P), and their combined application to test soil fertility. The results showed that stand density declined by no more than 15% in all sites after 12 years of N addition. Still, the growth enhancement of living trees and stands caused by fertilization did not occur, nor did the increases in soil organic carbon (SOC) and total nitrogen (TN). Moreover, there was no remarkable reduction in soil pH and exchangeable potassium (K+), magnesium (Mg2+), and calcium ion (Ca2+) in the fertilized plots. Input N seemed to appear in soil solution since the N-nitrate concentration and electrical conductivity (EC) rose significantly in all fertilized plots, accompanied by the solution pH decreased in 2 out of 4 stands. No notable difference in seedling volume increments was found among treatments after 10 months of cultivation for the pot experiment. This indicates that the main reason for the no growth-enhancing effect was soil N saturation.