本篇文章共分 4 段，從雨林土壤缺乏礦物質與有機化合物為出發點，說明生氣蓬勃的雨林植物如何在貧瘠土壤環境下仍能獲得充分養分的環境與運作機制。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Rain Forest Soils 雨林土壤

1. 雨林土壤缺乏礦物質

   On viewing the lush plant growth of a tropical rain forest, most people would conclude that the soil beneath it is rich in nutrients. However, although rain forest soils are highly variable, they have in common the fact that abundant rainfall washes mineral nutrients out of them and into streams. This process is known as leaching. Because of rain leaching, most tropical rain forest soils have low to very low mineral nutrient content, in dramatic contrast to mineral-rich grassland soils. Tropical forest soils also often contain particular types of clays that, unlike the mineral-binding clays of temperate forest soils, do not bind mineral ions well. Aluminum is the dominant cation (positively charged ion) present in tropical soils; but plants do not require this element, and it is moderately toxic to a wide range of plants. Aluminum also reduces the availability of phosphorus, an element in high demand by plants.

   當看到熱帶雨林中鬱鬱蔥蔥的植物生長時，大多數人都會得出結論，認為其下的土壤富含營養。然而，儘管熱帶雨林的土壤變化很大，但它們有一個共通的事實，即大量的降雨將礦物質養分從土壤中沖出，流入河川。這個過程被稱為瀝濾。由於雨水的沖刷，大多數熱帶雨林土壤的礦物質養分含量較低或非常低，與富含礦物質的草原土壤形成鮮明的對比。熱帶雨林土壤還經常含有特殊類型的粘土，與溫帶森林土壤的礦物結合粘土不同，它們不能有效地結合礦物離子。鋁是熱帶土壤中最主要的陽離子（帶正電的離子）；但植物不需要這種元素，而且它對多種植物有中等毒性。鋁還會減少磷的供應，而磷是植物有高度需求的元素。

2. 雨林土壤缺乏有機化合物

   High moisture and temperatures speed the growth of soil microbes that decompose organic compounds, so tropical soils typically contain far lower amounts of organic materials (humus) than do other forest or grassland soils. Because organic compounds help loosen compact clay soils, hold water, and bind mineral nutrients, the relative lack of organic materials in tropical soils is deleterious to plants. Plant roots cannot penetrate far into hard clay soils, and during dry periods, the soil cannot hold enough water to supply plant needs. Because the concentration of dark-colored organic materials is low in tropical soils, they are often colored red or yellow by the presence of iron, aluminum, and manganese oxides; when dry, these soils become rock hard. The famous Cambodian temples of Angkor Wat, which have survived for many centuries, were constructed from blocks of such hard rain forest soils.

   高濕度和高溫度加速了土壤中分解有機化合物的微生物的生長，因此熱帶土壤中的有機物質（腐殖質）含量通常比其他森林或草原土壤低得多。由於有機化合物有助於疏鬆緊實的粘土，保持水分，並結合礦物質養分，熱帶土壤中相對缺乏的有機材料對植物是有害的。植物的根系無法深入到堅硬的粘土中，在乾旱時期，土壤無法保持足夠的水分來滿足植物的需要。由於熱帶土壤中深色有機物的濃度很低，它們通常因鐵、鋁和錳氧化物的存在而被染成紅色或黃色；乾燥時，這些土壤變得堅硬如石。聞名遐邇的柬埔寨吳哥窟寺廟就是用這種堅硬的雨林土壤建造的，它們已經存活了許多個世紀。

3. 雨林植物的菌根夥伴關係

   Given such poor soils, how can lush tropical forests exist? The answer is that the forest's minerals are held in its living biomass—the trees and other plants and the animals. In contrast to grasslands, where a large proportion of plant biomass is produced underground, that of tropical forests is nearly all aboveground. Dead leaves, branches, and other plant parts, as well as the wastes and bodies of rain forest animals, barely reach the forest floor before they are rapidly decayed by abundant decomposers—bacterial and fungal. Minerals released by decay are quickly absorbed by multitudinous shallow, fine tree feeder roots and stored in plant tissues. Many tropical rain forest plants (like those in other forests) have mycorrhizal (fungus-root) partners whose delicate hyphae spread through great volumes of soil, from which they release and absorb minerals and ferry them back to the host plant in exchange for needed organic compounds. The fungal hyphae are able to absorb phosphorus that plant roots could not themselves obtain from the very dilute soil solutions, and fungal hyphae can transfer mineral nutrients from one forest plant to another. Consequently, tropical rain forests typically have what are known as closed nutrient systems, in which minerals are handed off from one organism to another with little leaking through to the soil. When mineral nutrients do not spend much time in the soil, they cannot be leached into streams. Closed nutrient systems have evolved in response to the leaching effects of heavy tropical rainfall. Evidence for this conclusion is that nutrient systems are more open in the richest tropical soils and tightest in the poorest soils.

   既然土壤如此貧瘠，那麼茂盛的熱帶雨林又怎麼會存在呢？答案是，森林的礦物質存在於其生物質中—樹木和其他植物以及動物。與草原相比，其大部分植物生物量都在地下產生，而熱帶森林的生物量幾乎都在地上。枯葉、樹枝和其他植物部分，以及雨林動物的排泄物和屍體，在被豐富的分解者（細菌和真菌）迅速腐爛之前，幾乎沒有到達森林地面。腐爛所釋放的礦物質迅速被眾多的淺層細樹根吸收並儲存在植物組織中。許多熱帶雨林植物（像其他森林中的植物一樣）都有菌根（真菌-根）夥伴，它們精緻的菌絲在大量的土壤中傳播，從中釋放和吸收礦物質，並將它們運回宿主植物以換取所需的有機化合物。真菌菌絲能夠吸收植物根系無法從非常稀薄的土壤溶液中獲得的磷，而且真菌菌絲可以將礦物營養從一種森林植物轉移到另一種。因此，熱帶雨林通常有所謂的封閉式營養系統，其中礦物質從一個生物體轉移到另一個生物體，很少滲漏到土壤中。當礦物質養分在土壤中停留時間不長時，它們就不能被浸入溪流。封閉式營養系統是為了應對熱帶大雨的瀝濾作用而進化的。這一結論的證據是，營養系統在最豐富的熱帶土壤中更加開放，而在最貧瘠的土壤中最緊密。

4. 固氮細菌的功能

   The growth of organisms is dependent on the availability of nutrients, none of which is more important than nitrogen. Although there is an abundant supply of nitrogen in Earth’s atmosphere, it cannot be absorbed by plants unless it is “fixed,” or combined chemically with other elements to form nitrogen compounds. Nitrogen-fixing bacteria help tropical rain forest plants cope with the poor soils there by supplying them with needed nitrogen. Many species of tropical rain forest trees belong to the legume family, which is known for associations of nitrogen-fixing bacteria within root nodules. Also, cycads (a type of tropical plant that resembles a palm tree) produce special aboveground roots that harbor nitrogen-fixing cyanobacteria. By growing above the ground, the roots are exposed to sunlight, which the cyanobacteria require for growth. Nitrogen fixation by free-living bacteria in tropical soils is also beneficial.

   生物體的生長取決於營養物質的供應，其中最重要的莫過於氮。儘管地球大氣中存在著豐富的氮供應，但它不能被植物吸收，除非它被「固化」，亦即與其他元素進行化學結合，形成氮化合物。固氮細菌通過為熱帶雨林植物提供所需的氮，幫助它們應對那裡貧瘠的土壤。熱帶雨林的許多樹種都屬於豆科，該科以根瘤內的固氮細菌組織而聞名。此外，蘇鐵（一種類似於棕櫚樹的熱帶植物）產生特殊的地上根系，其中藏有固氮的藻青菌。透過在地面上生長，根部暴露在陽光下，而藻青菌的生長需要陽光。熱帶土壤中自由生活的細菌的固氮作用也是有益的。