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

在生长在高风化土壤上的热带森林中，植物必需养分的生态系统储量的很大一部分存在于植被中。森林砍伐使这些营养储备消失，可能导致营养贫乏的土壤无法支持可持续的农业实践或旺盛的森林再生。另一方面，热带土壤和植被的许多特性，包括ph依赖的可变电荷，大的有效生根深度，以及可以在低浓度下提取土壤养分的菌根组合，可能使这些生态系统对干扰具有弹性。拟议研究的目的是研究亚马逊盆地东部典型的原生林和次生林以及生产性和退化的牛牧场的这些生物地球化学通量。项目组已经测量了土壤、植被、雨水、土壤溶液和溪流中的养分，以及这些土地利用过程中土壤的气体排放。在解决生物地球化学通量的许多变化的同时，这项研究也提出了新的问题：当退化的牧场被遗弃时，氮从哪里来支持次生演替，这些牧场已经耗尽了植物有效氮的土壤库？许多退化的牧场正在通过施用磷肥和重新种植新品种牧草来“改造”，但是牧草从哪里获得氮来支持它们旺盛的生产力呢？这些生态系统中共生固氮和结合固氮的速率是多少？大部分的肥料P是被草吸收，被土壤中的铁和铝氧化物吸附，还是流失到溪流中？本地先驱演替物种能否提取除农艺土壤试验指标确定为“植物可利用”的磷以外的土壤库？在森林砍伐和燃烧之后，牧场和次生林土壤接受了灰的输入，最终会被还原，但通过什么机制和在什么时间尺度上还原呢？这些合作研究将把营养损失和积累的机理研究与植物宏量营养素生物地球化学通量的实地测量联系起来。水文调查，包括井的横断面、测压计巢穴、土壤湿度测量和每种土地利用的能量平衡计算，将量化水通过土壤剖面和流向溪流的流动路径。溪水排放的测量和地下水流量和不饱和流量的估计将与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养分通量。利用~sN池稀释和xsN质量平衡的创新组合，估算了草地和次生林先驱种的生物固氮速率。~4N:~sN同位素比值的变化将为这些系统的大气输入提供灵敏的测量。温室磷素有效性生物测定的质量平衡将确定从不同深度取样的土壤中被生长的幼苗所吸收的可提取磷素库。此外，通过测量田间再生植被的生物量磷库，将这些土壤磷物理-化学库的丰度与植物需求进行比较。这些对养分积累、损失和保留的关键机制的研究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道和流速的调查结合起来，将有助于量化和提高对这一土地利用变化的复杂景观中的生物地球化学通量的理解。

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.