Intensive agriculture alters headwater streams, but our understanding of its effects is limited in tropical regions where rates of agricultural expansion and intensification are currently greatest. Riparian forest protections are an important conservation tool, but whether they provide adequate protection of stream function in these areas of rapid tropical agricultural development has not been well studied. To address these gaps, we conducted a study in the lowland Brazilian Amazon, an area undergoing rapid cropland expansion, to assess the effects of land use change on organic matter dynamics (OM), ecosystem metabolism, and nutrient concentrations and uptake (nitrate and phosphate) in 11 first order streams draining forested (n = 4) or cropland (n = 7) watersheds with intact riparian forests. We found that streams had similar terrestrial litter inputs, but OM biomass was lower in cropland streams. Gross primary productivity was low and not different between land uses, but ecosystem respiration and net ecosystem production showed greater seasonality in cropland streams. Although we found no difference in stream concentrations of dissolved nutrients, phosphate uptake exceeded nitrate uptake in all streams and was higher in cropland than forested streams. This indicates that streams will be more retentive of phosphorus than nitrogen and that if fertilizer nitrogen reaches streams, it will be exported in stream networks. Overall, we found relatively subtle differences in stream function, indicating that riparian buffers have thus far provided protection against major functional shifts seen in other systems. However, the changes we did observe were linked to watershed scale shifts in hydrology, water temperature, and light availability resulting from watershed deforestation. This has implications for the conservation of tens of thousands of stream kilometers across the expanding Amazon cropland region.

中文翻译：

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土地利用变化影响亚马逊低地盆地源头溪流中的生态系统功能

集约化农业改变了水源流，但我们对其影响的了解在目前农业扩张和集约化速度最快的热带地区是有限的。河岸森林保护是一种重要的保护工具，但它们是否能在热带农业快速发展的这些地区为河流功能提供足够的保护还没有得到很好的研究。为了解决这些差距，我们在巴西亚马逊低地进行了一项研究，该地区正在经历快速的农田扩张，以评估土地利用变化对有机物质动态 (OM)、生态系统代谢以及养分浓度和吸收（硝酸盐和磷酸盐）的影响) 在 11 条一级河流中排干森林 ( n = 4) 或农田 ( n= 7) 具有完整河岸森林的流域。我们发现溪流具有相似的陆地凋落物输入，但农田溪流中的有机质生物量较低。总初级生产力低且在土地利用之间没有差异，但生态系统呼吸和净生态系统产量在农田溪流中表现出更大的季节性。尽管我们发现河流中溶解养分的浓度没有差异，但所有河流中磷酸盐的吸收量都超过了硝酸盐的吸收量，而且农田中的比森林河流中的更高。这表明河流比氮更能保持磷，如果肥料氮到达河流，它将在河流网络中输出。总的来说，我们发现了流函数的相对细微的差异，表明迄今为止，河岸缓冲区已为防止其他系统中出现的重大功能转变提供了保护。然而，我们确实观察到的变化与流域森林砍伐导致的水文、水温和光照可用性的流域规模变化有关。这对在不断扩大的亚马逊农田地区保护数以万计的河流公里具有影响。