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

Daniel Markewitz DEB-9807633在生长在高度风化土壤上的热带森林中，植被中存在着生态系统中相当大一部分必需的植物营养物质。森林砍伐移除了这些养分储备，也许会使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，包括依赖于pH的可变电荷，较大的有效根深，以及能够在低浓度下提取土壤养分的真菌群落，可能使这些生态系统对干扰具有弹性。这项拟议研究的目标是研究亚马逊盆地东部典型的原生林和次生林以及多产和退化的牧场镶嵌中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、土壤溶液和溪水中的养分，以及这些土地利用的土壤中的气体排放。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，支持次生演替的氮来自哪里？退化的牧场已经耗尽了土壤中的植物有效氮。许多退化的牧场正在通过施磷肥和重新种植新品种的牧草来“改造”，但这些牧草从哪里获得氮来支持它们旺盛的生产力？在这些生态系统中，共生固氮和缔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，还是流失到溪水中？当地先锋演替物种是否可以提取农学土壤测试指标所确定的“植物可利用”磷以外的土壤磷？在森林砍伐和焚烧后接受灰分输入的牧场和次生林土壤最终会发生酸化，但通过什么机制和在什么时间尺度上进行呢？这些合作研究将把养分流失和积累的机制研究与植物大量营养素生物地球化学通量的野外测量联系起来。水文调查，包括水井横断面、测压仪巢穴、土壤水分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。将把河流水量的测量、地下水流量和非饱和流量的估计与溶液浓度的测量结合起来，以计算土壤剖面内以及陆地和水生系统之间的养分流动。将使用~SN池稀释和XSN质量平衡的创新组合，在种植在草类和次生林先锋物种的田间地块中估计生物固氮率。~4N：~SN同位素比值的变化将为这些系统提供大气输入的灵敏测量。温室生物磷有效性分析的质量平衡将确定从不同深度采样的土壤中可提取的磷库，这些土壤被生长的苗木吸收。此外，将通过测量田间再生植被中的生物量磷库，将这些土壤磷的物理-化学库的丰度与植物需求进行比较。这些对养分积累、流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田间观测。将这些生态学研究与水文流动路径和流速的调查结合起来，将使我们能够量化和更好地了解这一不断变化的土地利用的复杂景观中的生物地球化学通量。

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.