量測與模擬實驗尺度水稻田之氮平衡動態變化

Abstract

氮平衡在維繫大自然正常運作時扮演相當重要的角色，當中包含許多作用與機制。稻米為世界三大糧食作物，在臺灣亦為重要的主食，水稻田為臺灣農業耕種面積中最多的一種，約26萬公頃。然而農業活動中不適當的施肥，會導致水稻田中的氮素以各種形式的方法散失到環境中，對環境造成污染，進而影響動植物生長甚至危害人類健康。因此了解並量化水稻田中氮素的動態變化將有助於施肥管理及汙染控制。本研究藉由建立實驗尺度的水稻田，透過實驗量測氮平衡中重要的轉換機制。研究共進行兩次實驗，第一次於2012年到2013年未施肥、第二次於2013年到2014年施肥120 (kg N/ha)。實驗量測水體、土壤、植物之氮含量變化。研究結果顯示兩次實驗的灌溉入流與出流氮量主要受到硝酸鹽氮及有機氮所支配。灌溉入流氮量分別為22.59、29.84(kg N/ha)，出流氮量分別為8.98、22.91(kg N/ha)。比較兩次實驗的輸入與輸出總氮量分別減少71.09、84.71%，結果證明水稻田確實具有氮素移除的效果。第一次實驗期間，土壤平均總氮含量約為2751.94(kg N/ha)。植物攝取氮量約為14.71(kg N/ha)。第二次實驗期間，土壤平均總氮含量約為2831.86(kg N/ha)。植物攝取氮量約為16.55(kg N/ha)。計算分布於水體、土壤、植物內之氮百分比，發現土壤氮量占整體氮量比例最高，約占總量的99.10~99.78%，隨水稻生長有逐漸下降趨勢。氮素遺失的途徑若與施肥做比較，出流氮量約占施肥氮量的19.09%，植物攝取約占施肥氮量的13.79%。 然而實驗有其限制，未能於實驗量測的氮素轉換機制則透過模式加以量化。本研究以系統動力學的方法建構模式，模式中考量各種氮素的轉換過程，包含揮發作用、硝化作用、脫硝作用、植物攝取等作用。研究結果顯示，透過模式模擬第二次實驗在氮素遺失的途徑中，以脫硝作用為最多，約34.59(kg N/ha)，約占施肥氮量的28.83%。本研究所建立簡單的水稻田氮平衡系統動力模式，可用來量化水稻田中氮素轉換之機制。後續研究若能有更多的資料供模式驗證與加入各種不同的施肥情境條件，將可利用模式模擬不同情境下水稻的生長情況、水稻田中水質變化及各種氮素轉換機制的改變，增加氮平衡系統動力模式的使用範圍。以達到提供水稻田施肥管理、汙染控制參考之目標。

Nitrogen balance involves many mechanisms and plays an important role to maintain the function of nature. Rice is one of the main food crops in the world, and it is also the staple food in Taiwan. Paddy fields account for most agriculture cultivated area in Taiwan, which is about 0.26 million ha. However, improper fertilizer application in agriculture activity will cause a plenty of nitrogen losses from paddy field to environment, and then lead to pollution, ecological problems, and even threatening human health. Therefore, it is essential to understand and quantify the nitrogen dynamics in paddy fields for fertilizer management and pollution control. In this study, we build a pilot-scale paddy field and measure the important transformation mechanisms in nitrogen balance. The experiment was conducted two treatments: one was unfertilized in 2012 to 2013 and the other was fertilized 120(kg N/ha) in 2013 to 2014, and we simultaneously measured the nitrogen content in water, soil, and plant. The results show that the irrigation inflows and outflows were dominated by the nitrate and organic nitrogen in both two experiments. The irrigation inflows were 22.59 and 29.84(kg N/ha) and the outflows were 8.98 and 22.91(kg N/ha), respectively. In addition, the paddy field removed 71.09 and 84.71% of the total nitrogen from input, which confirmed that its purification effects. Total nitrogen in the soil were 2751.94 and 2831.86(kg N/ha), and plant uptake were 14.71 and 16.55(kg N/ha), respectively. The results also demonstrate the nitrogen content among water, soil, and plant. During the growth season, soil accounted for most proportion of total nitrogen ,which about 99.10~99.78%, but decreased gradually. In the second experiment, we compared the nitrogen losses with the fertilizer amounts, and the outflow and plant uptake accounted for 19.09 and 13.79% of application amount, respectively. Although we measured some nitrogen losses in previous experiments, some transformation mechanisms could not be obtained from the measurement. Hence, we used system dynamics approach to developed a model which considered major transformation processes of nitrogen in paddy fields (e.g. volatilization, nitrification, denitrification and plant uptake) to quantify some unknown mechanisms. The results indicate that denitrification is the main nitrogen loss from paddy field, which is 34.59(kg N/ha), and accounts for 28.83% of application amount. The research proposed a simple model which can estimate the temporal dynamics of nitrogen balance in paddy field. In future studies, more data and different kinds of fertilizer application scenarios should be added to the model to simulate the plant uptake, water quality, and changes of transformation processes in paddy fields to provide reference for future fertilizer management and pollution control.