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Response to heavy nitrogen applications in fertilizer experiments in British forests 总被引:1,自引:0,他引:1
Increasing concern over the level of nitrogen inputs to forests in polluted rain has led to a number of suggestions of possible adverse consequences under the general heading of nitrogen saturation. A saturated ecosystem may be one (a) in which the trees show no growth response to the addition of further nitrogen, (b) in which addition of further nitrogen leads to growth disturbances or reduction, or (c) in which elevated nitrogen inputs lead to increased losses of nitrate in streamwater. Experience gained from forest fertilizer experiments is used to examine each of these suggestions. A definition involving a lack of growth response (a) is shown to be based on a lack of understanding of the continuing changing patterns of nitrogen demand and mineralization in even-aged forests. Similarly, using growth disturbances (b) is unsatisfactory because, it is suggested, these are either secondary deficiencies that appear once growth accelerates with added nitrogen or are a consequence of changing growth with increasing size. A definition based on increased loss of nitrate is, by analogy with the situation for sulphate, at least superficially attractive. However, the fact that nitrate retention is predominantly biological, rather than chemical, makes for difficulties and the limited evidence available suggests that many exceptions and variations may exist. Whilst experience with forest fertilizers might not be entirely apposite, for example foliar uptake from polluted rain may be a factor, it is urged that at least any hypothesis put forward should be compatible with information gained from fertilizer trials. 相似文献
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Nonpoint Source Pollution Responses Simulation for Conversion Cropland to Forest in Mountains by SWAT in China 总被引:1,自引:0,他引:1
Several environmental protection policies have been implemented to prevent soil erosion and nonpoint source (NPS) pollutions
in China. After severe Yangtze River floods, the “conversion cropland to forest policy” (CCFP) was carried out throughout
China, especially in the middle and upper reaches of Yangtze River. The research area of the current study is located in Bazhong
City, Sichuan Province in Yangtze River watershed, where soil erosion and NPS pollution are serious concerns. Major NPS pollutants
include nitrogen (N) and phosphorus (P). The objective of this study is to evaluate the long-term impact of implementation
of the CCFP on stream flow, sediment yields, and the main NPS pollutant loading at watershed level. The Soil and Water Assessment
Tool (SWAT) is a watershed environmental model and is applied here to simulate and quantify the impacts. Four scenarios are
constructed representing different patterns of conversion from cropland to forest under various conditions set by the CCFP.
Scenario A represented the baseline, i.e., the cropland and forest area conditions before the implementation of CCFP. Scenario
B represents the condition under which all hillside cropland with slope larger than 25° was converted into forest. In scenario
C and D, hillside cropland with slope larger than 15° and 7.5° was substituted by forest, respectively. Under the various
scenarios, the NPS pollution reduction due to CCFP implementation from 1996–2005 is estimated by SWAT. The results are presented
as percentage change of water flow, sediment, organic N, and organic P at watershed level. Furthermore, a regression analysis
is conducted between forest area ratio and ten years’ average NPS load estimations, which confirmed the benefits of implementing
CCFP in reducing nonpoint source pollution by increasing forest area in mountainous areas. The reduction of organic N and
organic P is significant (decrease 42.1% and 62.7%, respectively) at watershed level. In addition, this study also proves
that SWAT modeling approach can be used to estimate NPS pollutants’ impacts of land use conversions in large watershed. 相似文献