Collaborative Research: Biogeochemical Transfers Among Terrestrial and Aquatic Biospheres and the Atmosphere in Forests and Pastures of Eastern Amazonia

在生长在高度风化土壤上的热带森林中，植被中存在着相当大一部分的生态系统必需植物营养物质。砍伐森林会带走这些养分储备，可能会使养分贫乏的土壤无法支持可持续的农业做法或旺盛的森林再生。另一方面，热带土壤和植被的许多特性，包括pH值依赖的可变电荷，大的有效生根深度，和mycorrhlzal协会，可以提取土壤养分在低浓度下，可能会使这些生态系统的抗干扰能力。拟议研究的目标是研究亚马逊河流域东部典型的原生林和次生林以及多产和退化的牧场的生态地球化学通量，项目组已经测量了土壤、植被、雨水、土壤溶液和溪流中的营养物质，以及这些土地使用的土壤中的气体排放。在解决许多生态地球化学通量的变化的同时，这项研究也提出了新的问题：当退化的牧场，耗尽了植物有效氮的土壤池，被遗弃时，氮从哪里来支持次生演替？许多退化的牧草地通过施磷和重新种植新品种的牧草而被“改造”，但是牧草从哪里获得氮来支持它们旺盛的生产力呢？在这些生态系统中，共生固氮和联合固氮的比率是多少？大部分肥料磷是被草吸收，吸附在土壤中的铁和铝氧化物上，还是流失到溪流中？本地先锋演替物种提取土壤池的P比那些确定为“植物可用”的农艺土壤测试指标？在森林砍伐和燃烧后，接受灰烬输入的泥炭和次生林土壤最终会被艾德酸化，但是是通过什么机制和在什么时间尺度上被酸化的呢？这些合作调查将把养分流失和积累的机制研究与植物大量营养素的地球化学通量的实地测量联系起来。水文调查，包括井断面，测压计巢，土壤湿度测量，并在每一个土地使用的能量平衡计算，将量化通过土壤剖面和流的水的流动路径。将把对溪流排水量的测量以及对地下水流量和非饱和流量的估计与对溶液浓度的测量相结合，以计算土壤剖面内以及陆地和水生系统之间的营养物流动。生物固氮率将估计在草地和次生林先锋物种种植的田间小区使用创新的组合~sN池稀释和xsN质量平衡。~ 4 N：~sN同位素比值的变化将为这些系统的大气输入提供一个灵敏的测量。温室生物测定磷有效性的质量平衡将确定可提取的磷池的土壤样品从不同的深度所采取的生长幼苗。此外，这些土壤磷的物理化学库的丰度将通过测量生物量磷库在再生植被在外地的植物需求进行比较。对养分积累、损失和保持的关键机制的这些调查将有助于解释对养分分布和转化的实地观察。将这些生态研究与水文流动路径和流速的调查相结合，将使人们能够量化和更好地了解土地利用变化这一复杂景观中的地球化学通量。

ABSTRACTDaniel Markewitz DEB-9807633In tropical forests growing on highly weathered soils, a significant fraction of ecosystem stocks of essential plant nutrients exists in the vegetation. Deforestation removes these nutrient stocks, perhaps leaving a nutrient-poor soil incapable of supporting sustainable agricultural practices or vigorous forest regrowth. On the other hand, many properties of tropical soils and vegetation, including pH-dependent variable charge, large effective rooting depths, and mycorrhlzal associations that can extract soil nutrients at low concentrations, may render these ecosystems resilient to disturbance. The objectives of the proposed research are to study these biogeochemical fluxes in a mosaic of primary and secondary forests and productive and degraded cattle pastures that is typical of the eastern Amazon Basin.The project group have already measured nutrients in soils, vegetation, rain water, soil solutions, and stream water, and emissions of gases from the soils of these land uses. While addressing many of the changes in biogeochemical fluxes, this research has also raised new questions: Where does the nitrogen come from to support secondary succession when degraded pastures, which have depleted soil pools of plant-available N, are abandoned? Many of these degraded pastures arc being "reformed" by fertilizing with P and replanting in new varieties of pasture grasses, but where do the grasses obtain N to support their vigorous productivity? What are the rates of symbiotic and associative N fixation in these ecosystems? Is most of the fertilizer P taken up by the grasses, adsorbed onto iron and aluminum oxides in the soil, or lost to stream water? Can native pioneer successional species extract soil pools of P other than those identified as "plant-available" by agronomic soil test indices? Pasture and secondary forest soils that receive ash inputs following forest clearing and burning arc eventually reacidi~ed, but by what mechanisms and over what time scales?These collaborative investigations will link mechanistic studies of nutrient loss and accumulation with field measurements of biogeochemical fluxes of plant macronutrients. Hydrologic investigations, including well transects, piezometer nests, soil moisture measurements, and energy balance calculations in each land use, will quantify flow paths of water through soil profiles and to the stream. Measures of stream water discharge and estimates of groundwater flow and unsaturated flow will be combined with measures of solution concentrations to calculate nutrient fiuxes within soil profiles and between the terrestrial and aquatic systems. Rates of biological nitrogen fixation will be estimated in field plots planted in grasses and in secondary forest pioneer species using an innovative combination of ~sN pool dilution and xsN mass balance. Changes in the isotope ratios of ~4N:~sN will provide a sensitive measure of atmospheric inputs to these systems. A mass balance of a greenhouse bioassay of P availability will identify the extractable P pools of soils sampled from various depths that are taken up by growing seedlings. In addition, the abundance of these physio-chernical pools of soil P will be compared to plant demands by measuring biomass P pools in regrowing vegetation in the field. These investigations of key mechanisms of nutrient accumulation, loss, and retention will help explain field observations of nutrient distributions and transformations. Coupling these ecological studies to investigations of hydrologic flow paths and flow rates will permit quantification and improved understanding of the biogeochemical fluxes in this complicated landscape of changing land use.