排水沟蓄水条件下农田与排水沟水盐监测

为了查明盐碱地排水沟蓄水条件下农田与排水沟之间的水盐交换,基于两年现场观测试验数据,分析了农田与排水沟的水位响应以及电导率变化规律。结果发现:研究区排水沟蓄水条件下,相邻排水沟水位与农田地下水位变化基本一致,作物生长期较为强烈的蒸发蒸腾作用进一步降低了田间地下水位;排水沟水位变化可以直接或间接地影响周围农田的地下水位和水质状况,在无外界来水干扰的情况下,农田地下水和排水沟水样的电导率变化趋势一致,当排水沟受到外来淡水补给时,沟内浓缩的盐分得到稀释,电导率明显低于农田地下水。研究结果可为类似地区盐碱地治理和生态环境保护提供参考依据。     

江苏省太湖流域产业结构的水环境污染效应

通过研究太湖流域(江苏部分)的产业结构污染负荷和水环境的空间分布特点,得出了产业结构污染负荷的水环境空间响应状况.根据小流域划分技术,得出该区域水环境质量空间分布特点:劣于Ⅴ类水质的区域占32％,Ⅴ类水质的占30％,仅有27％的区域水质低于Ⅳ类.另一方面,对应水环境质量空间分布研究,构建了综合反映产业结构水环境污染负荷的经济社会指标体系,分析了流域农田污染因子和工业污染因子的空间分布特点.通过主成分分析,提取了水环境污染因素的四个主成分,分别为土地利用强度、三次产业结构、农业面源和工业点源.基于主成分载荷,解析了四个水环境污染因素的贡献率.在产业结构方面,研究得出工业化仍是区域水环境污染最大影响因素,但区域水环境恶化是各种因素共同交织的结果.又对工业点源污染和农业面源污染等级进行分区,对各区的空间分布特点做了详细的分析.研究了流域水环境综合污染的空间分布和市县区域分解特点,发现县级市比市区在产业发展上对水环境影响更大,主要是由于县级市工业发展相对较为粗放,水环境监管的区域差异性.为控制流域产业结构的水环境污染,综合提出了在流域内整合产业结构调整和产业优化布局,调整重污染工业结构和控制农业面源污染的对策,及相应的产业结构污染负荷区域差别化削减的建议.     

Soil Carbon and Nitrogen Responses to Nitrogen Fertilizer and Harvesting Rates in Switchgrass Cropping Systems

The environmental sustainability of bioenergy cropping systems depends upon multiple factors such as crop selection, agricultural practices, and the management of carbon (C), nitrogen (N), and water resources. Perennial grasses, such as switchgrass (Panicum virgatum L.), show potential as a sustainable bioenergy source due to high yields on marginal lands with low fertilizer inputs and an extensive root system that may increase sequestration of C and N in subsurface soil horizons. We quantified the C and N stocks in roots, free particulate, and mineral-associated soil organic matter pools in a 4-year-old switchgrass system following conversion from row crop agriculture at the W.K. Kellogg Biological Station in southwest Michigan. Crops were fertilized with nitrogen at either 0, 84, or 196 kg N ha^?1 and harvested either once or twice annually. Twice-annual harvesting caused a reduction of C and N stocks in the relatively labile roots and free-particulate organic matter pools. Nitrogen fertilizer significantly reduced total soil organic C and N stocks, particularly in the stable, mineral-associated C and N pools at depths greater than 15 cm. The largest total belowground C stocks in biomass and soil occurred in unfertilized plots with annual harvesting. These findings suggest that fertilization in switchgrass agriculture moderates the sequestration potential of the soil C pool.     

农田施氮对水质和氮素流失的影响

基于ISI Web of Science数据库,利用文献计量学方法分析了自1957年以来各国在农田氮流失领域的研究发展态势,综述了农田氮流失特征及氮流失防控措施。结果表明,目前各国对农田氮流失的研究主要集中在施氮对水体水质的污染和监测方法上,涉及的关键词主要有Groundwater、Water quality、Surface water、Nitrate pollution、Eutrophication、Contamination、Nonpoint source pollution、Lysimeter、Runoff、Subsurface drainage等。中国、美国和加拿大等农业大国的研究机构在这一领域的研究成果最多发文量最多的期刊主要分布在荷兰、美国和中国。文献分析表明,受降水、地形、土壤、施肥等诸多管理措施的影响,不同区域的农田氮流失量差别很大,中国各类农田的氮流失量(13.7—347 kg/hm~2)明显高于欧美国家(4—107 kg/hm~2)。我国单位面积化肥(357.3 kg/hm~2)和氮肥(165.1 kg/hm~2)施用量均远高于世界平均用量(87.5 kg/hm~2和52.9 kg/hm~2),当季氮肥利用率(17%)却明显低于世界平均水平(58%),表明氮肥施用过量且利用率过低是造成氮流失的关键因素。综合分析农田氮流失防控措施发现,从源头控制氮流失是最有效的措施,优化农艺管理措施和氮迁移过程拦截等分别可减少15%—92%、46%—77%的氮素流失,其中针对农田适宜施氮量的研究最多。然而,面对粮食生产需求与资源短缺、水体水质持续恶化的现状,未来的研究重点应从简单的表观平衡向整个农田生态系统的氮素循环过程转变,更为迫切的是加快探索以水质保护为目标的化肥氮施用阈值(造成环境污染的临界施氮量),并推广示范这些有效的氮流失防控措施。     

基于水量平衡下灌区农田系统中氮素迁移及平衡的分析

本文以于河套灌区典型灌域（前旗北疙堵乡塔布村）为研究对象,通过分析农田系统中水量平衡和氮素平衡,建立农田系统中氮素平衡和水分平衡的联立模型,并用此模型分析典型区域内农田系统中氮素随水分迁移的规律。分析表明：应用水量和氮平衡模型分析灌区农田系统中氮迁移及平衡,可以反映出农业面源污染物（氮）在整个农田系统中的去向,化肥使用量的降低和灌水量的减少,将有效的减少农田系统中氮素的输入量;秋灌—秋浇期间,玉米、番茄和葵花作物对氮的吸收率分别为17%、18%和32%,3种土壤中残留量分别为57%、60%和58%。依据河套灌区年施肥60万t计算,土壤残留量达到17.2万t。在秋灌-秋浇期间从黄河引入的氮总量达到1.21万t,随农田退水进入沟道的量为840 t。     

Drainage ditching at the catchment scale affects water quality and macrophyte occurrence in Swedish lakes

1. We have limited knowledge of the effects of land use in general and of drainage ditching in particular on macrophyte communities in lakes. I quantified catchment land use, including drainage ditching, as well as water quality and the number of macrophyte species in 17 Swedish lakes in summer 2005. 2. Land use within 1 km of the studied lakes was analysed in a geographic information system. The following variables were included: areas of forests, mires, agricultural land and urbanization, length of drainage ditches (classified according to the use of the land they drained), and shortest distance from lake to an urban area. To extract and analyse general trends in the data sets, redundancy analysis was used. 3. Water quality was explained mainly by three land‐use related variables: the lengths of agricultural, forest and mire ditches. The length of agricultural ditches was positively correlated with lake water conductivity, total dissolved solids, Ca, N and total organic carbon (TOC). The lengths of forest and mire ditches were positively correlated with lake water characteristics, especially TOC. 4. The number of species representing different macrophyte life forms was explained by three environmental variables: conductivity, and lengths of forest and agricultural ditches. The numbers of isoetids, nymphaeids, elodeids and total obligate hydrophytes were negatively correlated with length of forest ditches. In contrast, the number of lemnids and helophytes was positively correlated with conductivity and length of agricultural ditches. Furthermore, the number of isoetids was exponentially related (negatively) to lengths of agricultural and forest ditches, indicating a threshold response to drainage ditch length. 5. The results suggest that effects on water quality and macrophyte communities result from drainage ditching in the lake catchments rather than from land use itself. Given the total area of drainage‐ditched land worldwide (millions of ha in Scandinavia alone), drainage ditching should be considered when evaluating environmental impacts on lake water quality and macrophyte occurrence and when determining reference conditions for catchment land use.     

Linkages among land-use, water quality, physical habitat conditions and lotic diatom assemblages: A multi-spatial scale assessment

We assessed the importance of spatial scales (catchment, stream network, and sample reach) on the effects of agricultural land-use on lotic diatom assemblages along a land-use gradient in the agricultural Willamette Valley Ecoregion of Oregon. Periphyton, water chemistry, and physical habitat conditions were characterized for 25 wadeable streams during a dry season (July to September, 1997). Additional water chemistry samples were collected in the following wet season (February 1998) to assess seasonal effects of land-use on stream water chemistry. Percent agricultural land-use in the study catchments ranged from 10% to 89% with an average of 52%. Partial canonical correspondence analysis (CCA) with the first axis constrained by % agricultural land-use showed that % agricultural land-use at 3 spatial scales explained between 3.7%–6.3% of variability in the diatom species dataset. Monte Carlo Permutation tests indicated that the variance explained by % agricultural land-use was only significant at the spatial scale of the stream network with 10- and 30-m band width (p<0.05, 999 permutations). In addition to the effects of % agricultural land-use, partial CCAs with a forward selection option showed that water chemistry (e.g., SiO₂), reach-scale stream channel dimensions (e.g., width, depth, and slope), reach-scale in-stream habitats (substrates and filamentous algal cover in stream beds), and riparian vegetative buffer were all important with relation to diatom species assemblages. Percent of obligately nitrogen-heterotrophic taxa was the only diatom autecological metric that showed a significant but weak correlation with % agricultural land-use along the stream network (r=0.50), but not at catchment or sample reach scale. Correlation between % agricultural land-use and water chemistry variables varied among the spatial scales and between seasons. Physical habitat variables (log₁₀ erodible substrate diameters and stream reach slope) were significantly correlated with % agricultural land-use along the stream network but not at catchment or sample reach scale. Our data suggest that spatial scales are important in assessing effects of land-use on stream conditions but the spatial scale effects may vary between seasons. Direct linkages between agricultural land-use and lotic diatom assemblages were weak during summer base-flow time regardless of the spatial scales. Summer sampling may underestimate the effects of catchment land-use on stream conditions in areas where seasonal patterns are so distinctive as in the Willamette Valley.     

非点源污染输出对土地利用和社会经济变化响应的案例研究

为研究三峡库区(重庆段)自2006年10月蓄水(156m)以来,非点源污染输出对土地利用及社会经济背景变化响应的时空分异特征,综合考虑库区典型土地利用变化模式及社会经济背景,选取三峡库区腹地奉节县、忠县、库尾江津区及主城沙坪坝区为研究对象,使用动态输出系数模型法,以2007年、2010年和2013年3期遥感影像为数据源,分析了研究区2007年到2013年非点源污染TN、TP负荷量随土地利用类型改变及社会经济的发展而产生的时空变化特征。研究表明,6年间,研究区污染物TN从2007年的12126.9t上升到2013年的12692.3t,TP从2007年的394.6t上升到2013年的407.2t。就各典型区县而言,城市发展新区江津区非点源污染负荷呈上升趋势,TN、TP负荷量涨幅分别达到了28.6%和24.1%;而库尾城市快速扩展的沙坪坝区则呈现下降趋势,非点源污染TN、TP负荷量分别下降了48.8%和68.9%;仍保持传统农业的奉节县和忠县变化不大。     

有机类肥料对土壤养分含量的影响

绿色、可持续是我国发展现代农业的必然要求,肥料投入是关键之一。基于有机肥培肥土壤、生态友好的优势,发展和推广有机肥已经成为我国农业生产的一项基本国策。我国有机肥来源广泛、种类繁多,农业生产条件千差万别。本文分析了我国有机肥产品种类及标准现状、理化性质差异、碳氮矿化特性差异及影响因素,评述了施用有机肥对提高耕地基础地力的作用。因来源、制作工艺不同,不同有机肥的有机碳组分含量、全氮和活性氮含量、碳氮比等基本性质差异明显,这四者是影响有机肥碳氮矿化率、碳氮矿化量、碱解氮释放量等碳氮供应特性的主要内在因素;加之土壤环境等外界因素的影响,不同有机肥表现出不同的养分供应特性。总体上,有机肥一方面对提高土壤有机质含量,尤其是活性有机质含量和碳库管理指数效果明显,另一方面可增加土壤氮磷钾养分容量,减少作物生长消耗带来的土壤养分亏损,促进土壤质量良性发展。今后,应加强有机肥在耕地培肥中的应用,并加强有机肥肥效理论的基础研究和安全高效有机肥新产品的开发,促进有机肥资源的高效循环利用,使其成为重要的农业资源。     

Simulation of watershed hydrology and stream water quality under land use and climate change scenarios in Teshio River watershed,northern Japan

Quantitative prediction of environmental impacts of land-use and climate change scenarios in a watershed can serve as a basis for developing sound watershed management schemes. Water quantity and quality are key environmental indicators which are sensitive to various external perturbations. The aim of this study is to evaluate the impacts of land-use and climate changes on water quantity and quality at watershed scale and to understand relationships between hydrologic components and water quality at that scale under different climate and land-use scenarios. We developed an approach for modeling and examining impacts of land-use and climate change scenarios on the water and nutrient cycles. We used an empirical land-use change model (Conversion of Land Use and its Effects, CLUE) and a watershed hydrology and nutrient model (Soil and Water Assessment Tool, SWAT) for the Teshio River watershed in northern Hokkaido, Japan. Predicted future land-use change (from paddy field to farmland) under baseline climate conditions reduced loads of sediment, total nitrogen (N) and total phosphorous (P) from the watershed to the river. This was attributable to higher nutrient uptake by crops and less nutrient mineralization by microbes, reduced nutrient leaching from soil, and lower water yields on farmland. The climate changes (precipitation and temperature) were projected to have greater impact in increasing surface runoff, lateral flow, groundwater discharge and water yield than would land-use change. Surface runoff especially decreased in April and May and increased in March and September with rising temperature. Under the climate change scenarios, the sediment and nutrient loads increased during the snowmelt and rainy seasons, while N and P uptakes by crops increased during the period when fertilizer is normally applied (May through August). The sediment and nutrient loads also increased with increasing winter rainfall because of warming in that season. Organic nutrient mineralization and nutrient leaching increased as well under climate change scenarios. Therefore, we predicted annual water yield, sediment and nutrient loads to increase under climate change scenarios. The sediment and nutrient loads were mainly supplied from agricultural land under land use in each climate change scenario, suggesting that riparian zones and adequate fertilizer management would be a potential mitigation strategy for reducing these negative impacts of land-use and climate changes on water quality. The proposed approach provides a useful source of information for assessing the consequences of hydrology processes and water quality in future land-use and climate change scenarios.     

农地整理中路沟渠生态化设计研究进展

农地整理是人类对农田生态系统的强烈干扰.传统农地整理中路沟渠设计的主要技术经济指标是方便生产通行和调配水资源以及提高水的利用率,尚缺少系统化、生态化考量,已经影响了农田物种扩散,导致生物栖息地环境的退化,并对农田生态系统的生物竞争机制、缓冲和补偿能力、抵御虫害能力产生了显著的负效应.本文系统总结了路沟渠生态化的功能,着重介绍了近年来生态化路沟渠的形式与结构设计、路沟渠生态工法参数的试验以及路沟渠生态材料选择的技术和方法.未来的相关研究应根据各级道路、灌排沟渠的不同功能要求和规格,按照田间不同生物的习性和栖息要求,精确设计能够满足大多数生物需求的农田路沟渠形态、结构和控制性尺寸,切实解决农地整理带来的上述问题.     

宁夏灵武地区农田排水沟植物物种多样性

为探索宁夏灵武地区农田排水沟植物物种组成和多样性特征,选择24条农田排水沟,分别于2009和2010年8月对植物物种和群落结构进行调查,并对物种多样性特征进行分析.结果表明,排水沟植物由17科36属43种组成,分为以芦苇(Phragmites australis)为主要建群种的6种群落类型和1个沉水植物群落,其中水烛+芦苇(Typha angustifolia+Phragmites australis)群落生物量(平均鲜重达2326.2 g· m-2)显著高于其他群落,2010年排水沟植物群落生物量较2009年小;边坡较沟底的物种多样性高,其中Shannon指数、丰富度指数差异达到显著水平,且2009年沟底物种多样性高于2010年.排水沟水文条件、水质和人为干扰可能是影响农田排水沟植物分布、群落组成及多样性特征的主要因素.     

秸秆管理和施肥方式对绿洲棉田土壤有机碳库的影响

通过监测绿洲滴灌棉田不同秸秆管理和施肥方式下土壤有机碳库及碳库组分的变化,可揭示农田管理措施对棉田土壤有机碳库的调节机制,为干旱区提高农田土壤生产力以及农业固碳减排措施的制定提供科学依据.试验采用裂区设计,以秸秆还田(S)和秸秆不还田(NS)2种秸秆管理方式为主区,4种施肥处理为副区:包括不施肥(CK)、单施氮磷钾化肥(NPK)、单施有机肥(OM)和氮磷钾化肥与有机肥混施(NPK+OM).结果表明: 施肥和秸秆还田均显著增加了土壤有机碳库,提高了有机碳(C_T)、易氧化有机碳(C_L)、微生物生物量碳(C_MB)、水溶性有机碳(C_WS)、热水溶性有机碳(C_HWS)的含量和有机碳累计矿化量(C_TM)及碳库管理指数(CMI).秸秆还田较秸秆不还田土壤有机碳库提高了20.6%;处理NPK、OM、NPK+OM分别较CK提高了7.8%、29.5%、37.7%.不同施肥处理下C_T、C_L、C_MB、C_WS、C_HWS均表现为NPK+OM>OM>NPK>CK.秸秆还田较秸秆不还田C_TM提高了5.9%;NPK、OM、NPK+OM处理较CK分别提高了32.7%、59.5%、97.3%.对CMI与SOC及其组分间的相关性分析表明,CMI与C_T、C_MB、C_L、C_WS、C_HWS、C_TM、C库、固碳潜力均呈极显著相关关系,因此, CMI是评价绿洲棉田管理措施对土壤质量影响的重要指标.在干旱区建设高标准绿洲农田,发展棉花生产,采用秸秆还田和有机无机肥配施等农业技术措施,不仅能增加土壤有机碳及活性组分的含量,培肥地力,而且能促进土壤固碳,有利于农业资源高效利用和可持续发展.     

Abundance,Activity and Community Structure of Denitrifiers in Drainage Ditches in Relation to Sediment Characteristics,Vegetation and Land-Use

Drainage ditches are ubiquitous yet understudied features of the agricultural landscape. Nitrogen pollution disrupts the nutrient balance of drainage ditch ecosystems, as well as the waterbodies in which they drain. Denitrification can help ameliorate the impact of N-fertilization by converting reactive nitrogen into dinitrogen gas. However, factors affecting denitrification in drainage ditches are still poorly understood. In this study, we tested how within-ditch and regional environmental conditions affect denitrifier activity, abundance, and community structure, to understand controls on denitrification at multiple scales. To this end, we quantified in situ denitrification rates and denitrifier abundance in 13 drainage ditches characterized by different types of sediment, vegetation and land-use. We determined how denitrification rates relate to denitrifier abundance and community structure, using the presence of nirS, nirK and nosZ genes as a proxy. Denitrification rates varied widely between the ditches, ranging from 0.006 to 24 mmol N m^?2 h^?1. Ditches covered by duckweed, which contained high nitrate concentrations and had fine, sandy sediments, were denitrification hotspots. We found highest rates in ditches next to arable land, followed by those in grasslands; lowest rates were observed in peatlands and nature reserves. Denitrification correlated to nitrate concentrations, but not to nirK, nirS and nosZ abundance, whereas denitrifier-gene abundance correlated to organic matter content of the sediment, but not to nitrate concentrations. Our results show a mismatch in denitrification regulators at its different organizational scales. Denitrifier abundance is mostly regulated at within-ditch scales, whereas N-loads, regulated by landscape factors, are most important determinants of instantaneous denitrification rates.     

天然常绿阔叶林改造为板栗林对土壤有机碳库的影响

天然林改造为人工林后,由于植被覆盖类型和经营管理措施发生改变,从而显著影响土壤有机碳库的特征.测定浙江省临安市相邻的天然常绿阔叶林和板栗林(板栗林由常绿阔叶林改造而来,集约经营10年)表层(0～20 cm)和亚表层(20～40 cm)土壤有机碳储量和不同形态活性有机碳库,用固态核磁共振方法分析土壤有机碳的化学结构特征,研究天然常绿阔叶林改造为板栗林对土壤有机碳库的影响.结果表明: 常绿阔叶林改造为板栗林后,土壤表层有机碳储量、水溶性有机碳、热水溶性有机碳、微生物生物量碳和易氧化碳含量分别下降19.7%、34.4%、25.8%、30.4%和25.2%,土壤亚表层的各指标分别下降13.5%、38.4%、19.8%、34.1%和22.2%.土壤表层烷氧碳含量、芳香碳含量以及芳香度显著降低,而烷基碳含量、羰基碳含量以及A/O-A值均显著增加;土壤亚表层烷氧碳含量显著降低,而烷基碳含量和A/O-A值显著增加,而芳香碳含量、羰基碳含量以及芳香度无显著变化.天然常绿阔叶林改造为板栗林并长期集约经营后,土壤有机碳储量和活性有机碳库均显著下降,有机碳的化学结构发生显著变化.     

农田氮素非点源污染控制的生态补偿标准——以江苏省宜兴市为例

农田氮素流失引起的非点源污染已成为国内外农田生态环境资源可持续利用及农业可持续发展的瓶颈,对其进行有效控制愈显迫切.对非点源污染的控制不能简单地照搬点源污染的方法,须针对非点源污染本身具有的强烈外部性及复杂、广泛、不易监测等特征,探究与非点源污染特征规律相对应的对策.生态补偿作为应对全球生态危机和环境污染的一种公共政策工具,对于内化外部效应具有良好的效果.以农户减少一定程度的氮肥施用量获得政府补偿为切入点,论证了农户减少氮肥用量到最佳生态经济施氮量是获得补偿的依据;以宜兴市为实证对象,运用意愿调查评估法和成本-收益法相结合的方式测算了农户参与农田氮素非点源污染控制的生态补偿标准.研究表明:(1)宜兴市主要粮食生产的最佳生态经济施氮量为375.6 kg/hm2,农户参与农田氮素非点源污染控制的补偿额度理论值为620.0-7098.0元/hm2;(2)68.3％的受访农户愿意接受补偿而减少氮肥用量,受偿意愿与种田经验、受教育程度等因素正相关;(3)愿意接受补偿的农民中50.7％选择氮肥量减少到最佳生态经济施氮量,农田氮素非点源污染控制的补偿标准为620.0元/hm2.     

Cyanobacteria and eukaryotic algae in sports turf and amenity grasslands: a review

A review is presented on the occurrence and role of cyanobacteria and eukaryotic algae in sports turf, especially golf curses, and on the problems arising when these organisms become abundant. The literature depends largely on observations in only a few countries, mainly Canada, New Zealand, U.K. and U.S.A., but problem growths are probably widespread. The genera reported to be conspicuous at times include Nostoc, Phormidium, Coccomyxa, Cosmarium, Cylindrocystis, Klebsormidium, Mesotaenium and Zygogonium. Conspicuous surface growths are probably most often related to unsatisfactory drainage or irrigation practices, but other factors such as fertilizer treatment (especially excess ammonium sulphate) have been implicated. These surface growths sometimes incorporate copious mucilaginous ‘slime’, which can be a serious hazard by causing people to slip. U.K. observations suggest that this is especially likely under acidic conditions, where Coccomyxa, Cylindrocystis and Mesotaenium are among the probable culprits. Some literature indicates that cyanobacteria are also associated with a subsurface black layer, which can cause serious problems for turf management where sandy soils are subject to unsatisfactory drainage. It seems likely, however, that cyanobacteria are largely, if not entirely, absent from the subsurface layer, but may form a dark layer at the turf surface overlying the position of the subsurface black layer. The dark surface layer is probably due largely to narrow filamentous cyanobacteria, whose growth may enhance the poor drainage and thus reinforce the conditions favouring the black subsurface layer associated with anoxic conditions. The soil algal vegetation of sports turf may also be expected to exhibit beneficial effects known to occur in soils of other types of community, such as nitrogen fixation by some cyanobacteria and the binding of particles. However, little study on such effects has been directed specifically to sports turf.     

Effects of nutrient loadings from catchments on Asajino mire, a small coastal ombrotrophic mire in northernmost Japan

The relationship between water chemistry and vegetation was studied in a coastal ombrotrophic mire in northern Hokkaido, Japan. The distributions of Sphagnum and Phragmites communities were separated clearly by the pH and ion concentration of the peat surface-pore water. The drainage ditches along the road across the center of the mire had a high pH and ion concentration, as did the peat water in the western part of the mire. It was found that fields used for livestock farming on a hill to the west of the mire leached materials into the mire through drainage ditches, surface runoff, and probably also through ground water, and thus influenced the water chemistry of the mire. Management of the water, including that in the catchment of the mire, should be introduced before biological buffering capacity against excess nutrient loading caused by human activity is exceeded and the mire loses its ombrotrophic status.     

Aquatic Ecosystem Impacts of Land Sharing Versus Sparing: Nutrient Loading to Southeast Asian Rivers

As world food demand continues to increase, two broad strategies for agricultural production have been widely discussed: land sparing and land sharing. Reflecting tradeoffs between the extent and intensity of agricultural use, land sparing maximizes the ratio of conserved to agricultural land, whereas land sharing allows more extensive, nature-friendly transformation of the agroecological matrix. Freshwater ecosystems are rarely considered in weighing these strategies, despite being strongly affected by land-use change. Here we analyze how shifting from extensive to intensive agricultural practices is altering dry season riverine nutrient availability and algal growth in upland Southeast Asia, which is experiencing rapid conversion from swidden—or shifting—cultivation to fertilized row crops. In situ algal growth assays and nutrient concentration data from eleven catchments representing a spectrum of land use and population densities show that intensive agriculture is associated with significantly elevated nitrate concentrations but no shift in phosphorus. As a result, nitrogen to phosphorus ratios increases dramatically, shifting algal growth toward colimitation by nitrogen and phosphorus. Geochemical analysis of suspended solids suggested comparable geological inputs across basins, but dissolved rare earth elements reveal that inorganic fertilizers are changing water chemistry in parallel with land use. Taken together, our analyses suggest that the transition from low-input land-sharing management (swidden) to high-input land-sparing practices (intensive row-crop agriculture) underway in Southeast Asia has profound consequences for river ecosystems. Such nutrient loading can affect river productivity and food webs, suggesting that land-use conservation strategies for Southeast Asia need to account for impacts on freshwater ecosystems.     

Changes in Soil Carbon and Nitrogen after Contrasting Land-use Transitions in Northeastern Costa Rica

Northeastern Costa Rica is a mosaic of primary and secondary forests, tree plantations, pastures, and cash crops. Many studies have quantified the effects of one type of land-use transition (for example, deforestation or reforestation) on soil properties such as organic carbon (C) storage, but few have compared different land-use transitions simultaneously. We can best understand the effects of land-use change on regional and global ecosystem processes by considering all of the land-use transitions that occur in a landscape. In this study, I examined the changes in total soil C and nitrogen (N) pools (to 0.3 m) that have accompanied different land-use transitions in a 140,000-ha region in northeastern Costa Rica. I paired sites that had similar topography and soils but differed in recent land-use history. The following land-use transitions were represented: 12 conversions of primary forests to banana plantations, 15 conversions of pastures to cash crops, and four conversions of pastures to Vochysia guatemalensis tree plantations. The conversion of forests to bananas decreased soil C concentrations and inventories (Mg C ha^–1) in the surface soil by 37% and 16.5%, respectively. The conversion of pastures to cash crops reduced soil C concentrations and inventories to the same extent that forest-to-banana cropping did. Furthermore, young Vochysia plantations do not appear to increase soil C storage, at least over the 1st decade. When data from all land-use transitions were pooled, the difference in root biomass and leaf litter pools between land-use pairs explained 50% of the differences in soil C concentrations and 36% of the differences in soil C inventories. Thus, reduced productivity or C inputs to the soil is one mechanism that could explain the losses in soil C pools with land-use change. In this landscape, losses of soil C due to cultivation are rapid, whereas re- accumulation rates are slow. Total soil N pools (0–10 cm) were also reduced after the conversion of forests to banana plantations or the conversion of pastures to crops, despite fertilization of the cropped soils. This suggests that the added N fertilizer is not retained but instead is exported via produce, N gas emissions, and hydrologic processes.