Amazon deforestation alters small stream structure, nitrogen biogeochemistry and connectivity to larger rivers
Amazon deforestation alters small stream structure, nitrogen biogeochemistry and connectivity to larger rivers
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DOI：
10.1007/s10533-010-9540-4
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发表时间：
2011-09-01
期刊：
影响因子：
4
通讯作者：
de Moor, Emily
中科院分区：
文献类型：
--
作者：
Deegan, Linda A.;Neill, Christopher;de Moor, Emily
改变土地复盖的人类活动可以改变小溪的结构和生物地球化学，但在森林砍伐率高的潮湿热带大片地区，人们对这些影响知之甚少。我们研究了亚马逊低地热带森林转变为牧场对小溪的物理和化学结构、有机质储量和N循环的影响。我们结合了对小溪的地区性地面调查，并在三个有代表性的溪流中使用N-15添加剂对营养循环进行了深入研究：二级森林溪流、二级牧场溪流和三级牧场溪流。这三条溪流彼此相距几公里，而且土壤相似。以牧场取代森林减少了溪流栖息地的复杂性，将溪流从带有林叶碎屑(50%盖度)的流动和池塘水道改变为具有缓慢流动的水流的草满水道(63%盖度)。在调查中，与类似大小的森林溪流相比，牧场溪流的溶解氧和硝酸盐(NO3(-))浓度一直较低。稳定同位素分析表明，二级牧场溪流对NH4(+)的吸收长度较短，对有机质组分的吸收速率较高，(NH4)-N-15(+)的停留时间较短。森林溪流硝化作用显著(占施加量(NH4)-N-15(+)的19%)，而二级牧场(0%)和三级牧场(6%)的硝化作用不明显。森林溪流保留了7%的增加的N-15在有机质隔间，输出了53%((NH4)-N-15(+)=34%；(NO3)-N-15(-)=19%)。相反，二级牧草径流保留了75%的添加N-15，主要是牧草(69%)，只有4%以(NH4)-N-15(+)的形式输出。示踪剂N-15在三级牧场溪流中的去向更接近于森林溪流，保留了5%的添加氮，输出了26%的氮((NH4)-N-15(+)=9%；(NO3)-N-15(-)=6%)。这些发现表明，在退耕还草地区的小河流中，草的广泛填充极大地增加了一级和二级河流中无机氮的滞留，这些河流约占亚马逊盆地流域总河道长度的四分之三。这一现象的重要性及其对跨越亚马孙盆地更大区域的更大河流的N运输的影响，将取决于对草填充河流的程度和规模的更好评估，但我们的分析表明，这种现象是普遍存在的。
Human activities that modify land cover can alter the structure and biogeochemistry of small streams but these effects are poorly known over large regions of the humid tropics where rates of forest clearing are high. We examined how conversion of Amazon lowland tropical forest to cattle pasture influenced the physical and chemical structure, organic matter stocks and N cycling of small streams. We combined a regional ground survey of small streams with an intensive study of nutrient cycling using N-15 additions in three representative streams: a second-order forest stream, a second-order pasture stream and a third-order pasture stream. These three streams were within several km of each other and on similar soils. Replacement of forest with pasture decreased stream habitat complexity by changing streams from run and pool channels with forest leaf detritus (50% cover) to grass-filled (63% cover) channel with runs of slow-moving water. In the survey, pasture streams consistently had lower concentrations of dissolved oxygen and nitrate (NO3 (-)) compared with similar-sized forest streams. Stable isotope additions revealed that second-order pasture stream had a shorter NH4 (+) uptake length, higher uptake rates into organic matter components and a shorter (NH4)-N-15 (+) residence time than the second-order forest stream or the third-order pasture stream. Nitrification was significant in the forest stream (19% of the added (NH4)-N-15 (+)) but not in the second-order pasture (0%) or third-order (6%) pasture stream. The forest stream retained 7% of added N-15 in organic matter compartments and exported 53% ((NH4)-N-15 (+) = 34%; (NO3)-N-15 (-) = 19%). In contrast, the second-order pasture stream retained 75% of added N-15, predominantly in grasses (69%) and exported only 4% as (NH4)-N-15 (+). The fate of tracer N-15 in the third-order pasture stream more closely resembled that in the forest stream, with 5% of added N retained and 26% exported ((NH4)-N-15 (+) = 9%; (NO3)-N-15 (-) = 6%). These findings indicate that the widespread infilling by grass in small streams in areas deforested for pasture greatly increases the retention of inorganic N in the first- and second-order streams, which make up roughly three-fourths of total stream channel length in Amazon basin watersheds. The importance of this phenomenon and its effect on N transport to larger rivers across the larger areas of the Amazon Basin will depend on better evaluation of both the extent and the scale at which stream infilling by grass occurs, but our analysis suggests the phenomenon is widespread.