四種除草劑對土壤氮循環影響之研究

Abstract

除草劑被廣泛的應用於雜草管理，可能會造成土壤微生物生化反應受影響，使土壤氮循環失衡，因此本篇試驗目的為藉由土壤銨化作用 (Ammonification)、土壤硝化作用 (Nitrification) 和土壤尿素水解? (Soil urease) 活性做為觀察指標，評估除草劑對土壤氮循環造成的影響。本篇研究供試土壤採集於新北市坪林區茶園 (Pinglin)、國立臺灣大學農業試驗場旱田土壤 (NTU)、彰化縣員林市果園 (Yuanlin) 與彰化縣大村鄉葡萄園 (Dacun)，供試土壤經過7天前置孵育後，分別添加10倍田間推薦用量除草劑嘉磷塞 (Glyphosate)、二，四-地 (2,4-D)、施得圃 (Pendimethalin) 與達有龍 (Diuron)，評估除草劑是否會影響土壤氮循環中的土壤銨化作用、土壤硝化作用與土壤尿素水解?活性。結果顯示嘉磷塞、二，四-地、施得圃與達有龍對土壤銨化作用和土壤硝化作用並沒有顯著之影響，在土壤尿素水解?活性試驗顯示施得圃與達有龍在Pinglin及Dacun供試土壤具有顯著抑制效果，而二，四-地在Pinglin、NTU及Dacun供試土壤皆具有顯著抑制效果，因此後續試驗針對二，四-地抑制土壤尿素水解?的現象進行更深入探討。在二，四-地對產尿素水解?細菌及不同種類尿素水解?影響試驗結果中，顯示二，四-地對Sporosarcina pasteurii BCRC11596與Bacillus megaterium BCRC10608的半效應濃度 (50% Effective concentration, EC50) 分別為152.4 mg L-1與> 200 mg L-1，而二，四-地對細菌源與植物源之尿素水解?在pH 4.0 - 8.0範圍下，半抑制濃度 (50% Inhibitory concentration, IC50) 皆> 200 mg L-1。於土壤氨揮失試驗，施用10倍田間推薦用量二，四-地之處理，於四種供試土壤中皆有降低氨揮失量的現象，並能減緩氨揮失發生速率。本研究結果顯示10倍田間推薦用量二，四-地會對整體土壤尿素水解?活性造成影響，並進一步使氨揮失量下降。田間應用方面，若能先施用二，四-地進行雜草管理，再施用尿素追肥，可減緩尿素水解速率，降低氨揮失情形發生，提升氮肥利用率。

Herbicides have been widely applied for weed management that may affect biochemical reactions of soil microorganisms, resulting in an imbalance of soil nitrogen cycle. Therefore, the purpose of this experiment is to evaluate the effect of herbicides on soil nitrogen cycle by using soil ammonification, soil nitrification and soil urease activity as observation indicators. Herbicides, Glyphosate, 2,4-D, Pendimethalin and Diuron, are applied at officially ten times recommended levels, to soil samples from tea garden in Pinglin District, New Taipei City (Pinglin), National Taiwan University’s agricultural experimental fields (NTU), orchard of Yuanlin City, Changhua County (Yuanlin) and vineyard of Dacun Township, Changhua County (Dacun). Then, herbicide-treated soil samples are sampled to evaluate the effects on soil ammonification, soil nitrification and soil urease activity in the soil nitrogen cycle. Results showed that Glyphosate, 2,4-D, Pendimethalin and Diuron did not have significant effects on soil ammonification and soil nitrification. As for soil urease activity, Pendimethalin and Diuron show significant inhibitory effects in sample Pinglin and Dacun while 2,4-D show significant inhibitory effects in sample Pinglin, NTU and Dacun. Therefore, the phenomenon of inhibition of soil urease by 2,4-D was investigated in more deep detail in subsequent experiments. The effects of 2,4-D on urease-producing bacteria and different types of urease experiments are conducted. Results showed that the 50% effective concentration (EC50) of 2,4-D on Sporosarcina pasteurii BCRC11596 and Bacillus megaterium BCRC10608 were 152.4 mg L-1 and > 200 mg L-1, respectively. And 50% inhibitory concentration (IC50) of 2,4-D for bacteria-source and plant-source urease are >200 mg L-1 at pH 4.0 - 8.0. Lastly, in the experiment of soil ammonia volatilization, the amount and rate reduce in ammonia volatilization after applying ten times recommended levels of 2,4-D in all four soil samples. In summary, the results of this study demonstrate that ten times recommended levels of 2,4-D will affect the soil urease activity and reduce ammonia volatilization. For field application, if we accomplish weed management with 2,4-D first, and then apply urea as top dressing, it can slow down the rate of urea hydrolysis, reduce the occurrence of ammonia volatilization, also improve the efficiency of nitrogen fertilizer.