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In this chapter, we discuss the current understanding of internal N cycling, or the flow of N through plant and soil components, in the Niwot Ridge alpine ecosystem. We consider the internal N cycle largely as the opposing processes of uptake and incorporation of N into organic form and mineralization of N from organic to inorganic form. We will outline the major organic pools in which N is stored and discuss the transfers of N into and from those pools. With a synthesis of information regarding the various N pools and relative turnover of N through them, we hope to provide greater understanding of the relative function of different components of the alpine N cycle. Because of the short growing season, cold temperatures, and water regimes tending either toward very dry or very wet extremes, the alpine tundra is not a favorable ecosystem for either production or decomposition. Water availability, temperature, and nutrient availability (N in particular) all can limit alpine plant growth (chapter 9). Cold soils also inhibit decomposition so that N remains bound in organic matter and is unavailable for plant uptake (chapter 11). Consequently, N cycling in the alpine often is presumed to be slow and conservative (Rehder 1976a, 1976b; Holzmann and Haselwandter 1988). Nonetheless, studies reveal large spatial variation in primary production and N cycling in alpine tundra across gradients of snowpack accumulation, growing season water availability, and plant species composition (May and Webber, 1982, Walker et al., 1994, Bowman, 1994, Fisk et al. 1998; chapter 9). Furthermore, evidence for relatively large N transformations under seasonal snowcover (Brooks et al., 1995a, 1998) and maintenance of high microbial biomass in frozen soils (Lipson et al. 1999a) provide a complex temporal component of N cycling on Niwot Ridge. Our discussion of N cycling on Niwot Ridge will focus on two main points: first, the spatial variation in N turnover in relation to snowpack regimes and plant community distributions; and second, the temporal variability of N transformations during both snow-free and snow-covered time periods.

Alpine tundra is an important indicator system of environmental change (Grabherr et al. 1996; Beniston and Fox 1996). This ecosystem occurs at all latitudes, with its lower altitudinal limit at timberline inversely related to latitude (Woodward 1993). Thus, the global distribution of alpine tundra and the fact that this system exists near the limits of vascular plant tolerance to temperature, moisture, and growing season duration makes it an excellent system for monitoring environmental change across the globe (e.g., Körner and Larcher 1988). In addition, the functional integrity of this system is critical to lower elevation ecosystems because substantial amounts of water and elements are intercepted by the alpine, filtered, and transported to lower elevations (Williams et al. 1996; chapter 4). Alpine tundra has been spared most of the large-scale disturbances associated with human development and resource extraction that have occurred in lower altitudinal ecosystems. This is probably due to its climatic severity and lack of renewable resources that can be exploited (e.g., trees, fast-growing forage for grazing). Some high-altitude sites have been impacted by recreational development (e.g., ski areas, trails, roads), mining, and grazing. In addition, changes in native herbivore populations, particularly elk and deer, due to extirpation of predators by humans (chapter 12), may have significantly influenced tundra vegetation (chapter 14). However, the indirect effects of human activities associated with the burning of fossil fuels are of greater current and future concern as the dominant anthropogenic influences on alpine tundra ecosystems. At a global scale, climate change is of concern for all ecosystems, whereas at a more-regional scale, there is concern for the impact of N and acid deposition near centers of industrial and urban growth (Galloway et al. 1995). Niwot Ridge, and possibly much of the Colorado Front Range, has experienced significant increases in the rate of N deposition (Sievering et al. 1996; chapter 3) and precipitation (Fig. 16.1; chapter 2) in the past several decades. Local sources of NOX compounds that potentially contribute to the elevated rates of N deposition in the Front Range include power plants in western Colorado and automobile and industrial emissions in the urban corridor at the base of the mountains (Fort Collins-Boulder-Denver-Colorado Springs megalopolis), which is home to around four million people.

Sustainable agriculture can be defined in many different ways. In industrial nations, sustainable agriculture means improving energy use efficiency, reducing environmental pollution, and increasing and sustaining profitability. For millions of small-holder farmers throughout the tropics, sustainable agriculture means providing basic food needs for the farming family, improving the farmer’s ability to replenish soil nutrients and control soil degradation, and optimizing crop yield per unit area of land. Soil utilization for agricultural production in the tropics during the past two centuries, to a large extent, has been influenced by the technological and economic changes in temperate regions. Research and development for agriculture during the colonial era were mainly focused on the needs of industrial nations, while the production of food crops for the indigenous inhabitants was largely left in the hands of the traditional slash-and-burn cultivators. Large and small cash crop plantations were developed on fertile, high-base-status allophanic and oxidic soils for coffee, cocoa, banana, and sugarcane production throughout the humid and subhumid tropics. Cotton was cultivated on smectitic soils and high-base-status kaolinitic soils in the subhumid and semiarid regions of Africa for the textile industries in temperate regions. In tropical America, cattle ranching, a production system introduced by European immigrants, still occupies most of the fertile flat land today, while food grains are usually cultivated on less fertile land or in shallow soils on steep slopes. In tropical Africa and Latin America, a wide range of food crops, such as maize and beans, potato, cowpea, sorghum, millet, cassava, and yam are mostly produced under the traditional slash-and-burn system of cultivation on less fertile kaolinitic soils. In tropical Asia, the indigenous intensive rice-based agriculture on wet smectitic soil has been practiced over many centuries and has been able to meet the basic food needs for the increasing population in the region. Generally, upland food crop production in the tropics has not kept pace with human population growth in the tropics during the past century. It was not until the 1950s and 1960s, following the independence of many nations in tropical Asia and Africa, that more attention was given to the research and development of food crop production.

Smectitic soils of the tropics are medium- to fine-textured alluvial soils containing moderate to large amounts (20% or more) of smectite, a shrinking and swelling clay mineral, in the clay fraction. Small to moderate amounts of other layer silicate minerals, such as illite, chlorite, vermiculite, and kaolinite, are also present in the clay fraction. Smectitic soils have moderate to high values of CEC (10-50 cmol/kg of soil), high base saturation, and high water-retention capacity. These soils are usually developed on alluvial materials rich in basic cations, especially Mg. Smectitic soils commonly occur on alluvial plains in river valleys and deltas as well as in inland depressions. In the wetter tropics, large areas of smectitic soils are found in tropical Asia, especially Vietnam, Thailand, and Myanmar (Burma). These young alluvial soils are rich in nutrient-bearing weatherable minerals, such as micas, feldspars, and hornblende. Smectitic soils on the alluvial plains and inland valleys have a shallow groundwater table, and some soils are flooded during the rainy season. Thus, they are best suited for rice cultivation. For example, in the flood plains along the Mekong and Chao Phraya rivers of the Indo- China peninsula, mineral-rich deposits from annual flooding are able to maintain relatively high rice yields with little or no additional nutrient inputs. Smectitic soils occurring in seasonally flooded coastal mangrove swamps are known as acid sulfate soils. These soils are used for cultivation of swamp rice and floating rice during the rainy season, depending upon the depth of flooding by fresh water. In drier regions, clayey smectitic soils (mainly Vertisols) often exhibit large cracks during the dry season and become very sticky and difficult to work with during the rainy season. In the drier tropics, large areas of clayey smectitic soils are found in central India, central Sudan, southern Ghana, and in the Lake Chad region of central Africa. Clayey smectitic soils are usually found in the inland depressions scattered throughout the drier regions of West, East and Central Africa. Because of their high chemical fertility, these soils are important soils for cropping and grazing in the drier tropics.