冻融交替对河岸缓冲带土壤无机氮和土壤微生物量氮的影响

全球气候变化引起的中高纬度地区积雪覆盖和降雪格局变化,造成该区域土壤冻融交替强度和频次变化,是土壤氮循环的重要影响因素。冻融温差和冻融循环次数影响微生物数量和群落的变化,进而影响土壤氮素生物地球化学循环。以大伙房水库实验林场小流域的河岸缓冲带生态系统为研究对象,通过分析冻融交替对河岸缓冲带土壤无机氮和土壤微生物量氮的影响,阐明冻融交替对土壤无机氮含量变化的影响机制,为评估小流域氮素流失风险提供依据。结果表明：随着冻融循环次数的增加,土壤无机氮含量呈增加趋势;不同温差的冻融循环处理对土壤无机氮影响不同,冻融条件为-5/+5℃和-20/+5℃时土壤无机氮含量在冻融循环10次之后分别为34.9±0.9 mg/kg和37.2±0.8 mg/kg,是处理前的1.21和1.41倍;冻融温差和冻融循环次数对土壤NH₄⁺–N含量有极显著影响(P<0.01),土壤冻融10次后土壤NH₄⁺–N含量是对照处理的4-10倍;冻融循环次数对土壤NO₃^－–N含量有显著影响(P<0.05),冻融温差对NO₃^－–N含量无显著影响(P>0.05);土壤微生物量氮含量对冻融循环的响应显著(P<0.01)。可见,冻融交替显著增加了土壤无机氮含量,由于早春季节植被对无机氮吸收较少,可能增大土壤氮素随冰雪融化的淋溶流失风险。     

黄河湿地孟津段不同植物群落类型土壤有机质含量变化特征研究

通过对黄河湿地孟津段不同湿地植被群落类型土壤进行采样分析,探讨了黄河小浪底水库修建之后调水调沙对下游滨河湿地不同植被群落类型土壤沉积特征的影响,并系统研究了滨河湿地土壤有机质的空间分异特征。研究结果表明：受上游小浪底水库建设的影响,滨河湿地不同植被群落类型土壤沉积层次和沉积厚度变化较大,近岸湿地植被向陆生演替的特征明显,土壤沉积加速、并不断向河道推移;滨河湿地不同植物群落类型、不同土壤层次的土壤有机质含量差异显著;典型湿地特征的植被群落表层土壤有机质含量明显高于其它深部沉积土壤;滨河湿地土壤有机质含量的显著差异性和受外部条件的显著控制作用,充分反映了滨河湿地生态系统的脆弱性。     

Rural property size drives patterns of upland and riparian forest retention in a tropical deforestation frontier

Deforestation in Brazilian Amazonia accounts for a disproportionate global scale fraction of both carbon emissions from biomass burning and biodiversity erosion through habitat loss. Here we use field- and remote-sensing data to examine the effects of private landholding size on the amount and type of forest cover retained within economically active rural properties in an aging southern Amazonian deforestation frontier. Data on both upland and riparian forest cover from a survey of 300 rural properties indicated that 49.4% (SD = 29.0%) of the total forest cover was maintained as of 2007, and that property size is a key regional-scale determinant of patterns of deforestation and land-use change. Small properties (≤150 ha) retained a lower proportion of forest (20.7%, SD = 17.6) than did large properties (>150 ha; 55.6%, SD = 27.2). Generalized linear models showed that property size had a positive effect on remaining areas of both upland and total forest cover. Using a Landsat time-series, the age of first clear-cutting that could be mapped within the boundaries of each property had a negative effect on the proportion of upland, riparian, and total forest cover retained. Based on these data, we show contrasts in land-use strategies between smallholders and largeholders, as well as differences in compliance with legal requirements in relation to minimum forest cover set-asides within private landholdings. This suggests that property size structure must be explicitly considered in landscape-scale conservation planning initiatives guiding agro-pastoral frontier expansion into remaining areas of tropical forest.     

Vegetation recruitment in an enhanced floodplain: Ancillary benefits of salmonid habitat enhancement

Widespread loss and degradation of riverine habitats due to dams, diversions, levees, and human development have led to an increase in river habitat enhancement projects in recent decades. These projects typically focus on improving either terrestrial (e.g., riparian vegetation) or aquatic (e.g., fish spawning and rearing) habitats, and do not commonly address the relationship between the two systems. However, there is abundant evidence that fundamental linkages exist between terrestrial and aquatic ecosystems, and anthropogenic impacts such as urban expansion, agricultural activities, and river impoundment can synergistically degrade both systems. This study examines the effects of adult and juvenile salmonid habitat restoration on recruitment, density, and composition of riparian vegetation in an area heavily impacted by mining and flow regulation. For a year following in-channel coarse sediment placement and floodplain construction in an area previously covered with coarse mine tailings, we compared the abundance, richness and diversity of vegetation across four treatments: the newly constructed floodplain, isolated mine tailings, mine tailings near an access road, and a remnant riparian area that was less impacted by mining. Richness and diversity were higher in the floodplain than in any of the other treatments; we identified a total of 15 plant families in the floodplain treatments, as compared to three to five families in the other treatments. We observed significant differences in plant assemblage composition between treatments, with higher richness of primarily obligate or facultative wetland plant taxa in the floodplain treatment. This study demonstrates that restoring hydrological linkages between aquatic and terrestrial habitats, and redistribution of sediment size classes altered by mining, can create conditions that promote rapid wetland plant colonization, enhancing biodiversity and improving ecosystem function.     

The conversion of natural riparian forests into agricultural land affects ecological processes in Atlantic forest streams

Agricultural practices affect the integrity of riparian areas of small streams. In this study we tested the hypothesis that the increase of agricultural activities influences negatively the functional conditions of the low order streams in the Atlantic forest of southern Brazil. Litter bags with leaves of Nectandra megapotamica (Spreng.) Mez were located in eight streams with different amounts of woody vegetation and agriculture land uses in their riparian zones. After 7, 15 and 30 days, the litter bags were removed for identification of associated invertebrates and determination of decomposition rate. Decomposition rates were negatively influenced by agriculture in the riparian zone while primary production was positively influenced. On the other hand, the decomposition mediated by microorganisms did not vary along the degradation gradient. The abundance of collectors increased in streams adjacent to agricultural land while the abundance of shredders was decreased. Our results showed that algae biomass and leaf decomposition were sensitive to the replacement of native vegetation by agricultural use. However, the trophic structure of invertebrates was moderately sensitive to agricultural land use.     

Litter breakdown of invasive alien plant species in a pond environment: Rapid decomposition of Solidago canadensis may alter resource dynamics

Alien plant invasions of riparian zones can trigger bottom-up effects on freshwater ecosystems through changes in leaf litter supply. Riparian zones of ponds are often invaded by alien species, and although these habitats are common, the effect of invasive alien species on ponds has rarely been studied. We performed a leaf litter experiment in a pond and compared within- and between-species variation in the breakdown rates of three native species (Alnus glutinosa, Phragmites australis and Typha angustifolia) and two aggressive alien invaders of riparian zones (Fallopia japonica and Solidago canadensis). The litter of S. canadensis decomposed faster than the litter of the other plants; more than 50 % of the S. canadensis biomass decomposed within a week. This contradicts the home-field advantage hypothesis, and we argue that the quality rather than the origin of litter might be the key factor driving breakdown rates. We also reported considerable intra-specific variation; old leaves (collected in spring after a partial aerial breakdown on stems) decomposed two to seven times slower than senescent leaves (collected in autumn just after abscission). The continuous seasonal supply of leaves of different quality into freshwaters may be disrupted by terrestrial invasions, especially if an invader forms monoculture stands and produces a highly palatable litter, as is the case with S. canadensis. This may fundamentally alter the resource dynamics in the pond environment through a rapid depletion of litter mass before the next litterfall.     

FEUWAnet: a multi-box water level and lateral exchange model for riparian wetlands

Riparian wetlands as typical aquatic-terrestrial interfaces control, in a very specific way, nonpoint water and related chemical fluxes exchanging between catchment areas to their respective water systems (streams, lakes). The existing groundwater and soilwater flow models reveal gaps in dealing with the complex behaviour of processes and the considerable spatial and temporal heterogeneity of riparian wetlands. Based on long-term experience gained through field observations and the interpretation of model produced data, a multi-box aggregation of processes which determines lateral as well as vertical flows and, as a whole, water balance, is used to discretise a generic riparian wetland transect situated between an upland aquifer and a receiving water body.

The resulting mathematical model, FEUWAnet, endowed also with an original methodology to adapt parameters, has been applied to a riparian alder wetland adjacent to Lake Belau (northern Germany). Results of simulations illustrate a good fit between calculated water levels and observed values and an accordance of calculated water balance to previous independent evaluations. This confirms that the sound simplifications of real situations performed by the FEUWAnet mathematical model are a promising way to deal with hydrological complexity of riparian zones. Moreover, FEUWAnet permits, to a certain extent, one to unravel the spatial heterogeneity and temporal variation of lateral (from catchment area to water systems) and vertical (from canopy to groundwater zone) water fluxes typical of riparian ecosystems: this is the necessary step to undertake when developing integrated models capable of assessing the effectiveness of riparian systems in controlling the fluxes of nonpoint pollution discharging in the open water bodies.     

Riparian alder fens — Source or sink for nutrients and dissolved organic carbon? — 1. Effects of water level fluctuations

The biogeochemistry of riparian alder wetlands was studied from 1995 to 1997. Nutrient and DOC chemistry was related to water level changes. The spatial and temporal patterns of nutrients (P and N) and dissolved organic carbon (DOC) were measured in the surface water flowing through a riparian alder fen and in the adjacent creek. Nutrient and DOC concentrations were extremely variable temporally but not spatially within the wetland. In the wetland and the adjacent creek concentrations of NO₃-N, PO₄-P and DOC were homogenous during high-flow periods and frozen conditions. After low-flow conditions water bodies were isolated from the creek. The concentration of NH₄-N, PO₄-P and DOC in these isolated water bodies was significantly higher than in the adjacent creeks due to low oxygen levels.

Enclosures of different sizes were installed in the wetland to study possible release rates. A large enclosure experiment in the flooded alder fen showed the same concentrations as after high-flood conditions except for DOC. The DOC concentrations were enriched in the large enclosure after decomposition from leaf litter during fall season. Small enclosures with low oxygen levels confirmed data obtained from low-flow conditions. The release rates were calculated for low-flow conditions from small enclosure experiments for 2 months a year when the alder fen is not flooded. The rates for July and August were 11.6 kg/ha NH₄-N, 8.6 kg/ha PO₄-P and 57.6 kg/ha DOC. The DOC concentrations for fall estimated from the large enclosure-experiments were 168.2 kg/ha for the months September and October.

This means possible output rates of N, P and DOC during the summer and DOC during fall in the adjacent river system. This can cause eutrophication and organic pollution depending on the length of the low-flow conditions and the size of the alder fen. Water level changes must be regarded as important for the management of riparian wetlands such as alder fens. The riparian alder system may vary from a nutrient sink to a nutrient source at different times of a year depending on high or low water levels.     

Channel response to a new hydrological regime in southwestern Australia

The Kent River flows from semi-arid headwaters in the agricultural (wheatbelt) region of Western Australia to a wetter and forested lower-catchment. It is set in an atypical fluvial environment, with rainfall decreasing inland towards a low-relief upper catchment. Replacement of native deep-rooted perennial vegetation with shallow-rooted seasonal crops has altered the hydrology of the upper catchment. Clearing for agriculture has also increased recharge of regional groundwater systems causing groundwater to rise and mobilise salt stores. This has increased stream salinity which has degradation riparian vegetation and decreased flow resistance. Elevated groundwater has also affected streamflow, increasing flow duration and annual discharge. The altered hydrological regime has affected geomorphic stability, resulting in channel responses that include incision and removal of uncohesive material. Channel response is variable, showing a high dependence on channel morphotype, channel boundary material and severity of salinity (degree of vegetation degradation). Response in confined reaches bounded by sandy material has been characterised by minor lateral bank erosion. In the fine-grained, wider, low-gradient reaches, mid-channel islands have been stripped of sandy sediment where vegetation has degraded. Following an initial period of high erosion rates in these reaches, the channel is now slowly adjusting to a new set of boundary conditions. The variable response has significant implications for management of salt affected rivers in southwestern Australia.