Rooting depth,water availability,and vegetation cover along an aridity gradient in Patagonia

Above-and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44–45°S). Sites, ranging from those with Nothofagus forest with high annual rainfall (770 mm) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 mm), were chosen to test whether rooting depth compensates for low rainfall. Along this gradient, both mean above-and belowground biomass and leaf area index decreased, but average carbon isotope ratios of sun leaves remained constant (at-27), indicating no major differences in the ratio of assimilation to stomatal conductance at the time of leaf growth. The depth of the soil horizon that contained 90% of the root biomass was similar for forests and grasslands (about 0.80–0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2–3 m depth. The depth profile of oxygen and hydrogen isotope ratios of soil water corresponded inversely to volumetric soil water contents and showed distinct patterns throughout the soil profile due to evaporation, water uptake and rainfall events of the past year. The isotope ratios of soil water indicated that high soil moisture at 2–3 m soil depth had originated from rainy periods earlier in the season or even from past rainy seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from recent rain events in the topsoil and not from water-saturated soils at greater depth. However, this study cannot explain the vegetation zonation along the transect on the basis of water supply to the existing plant cover. Although water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not fully utilized by the current plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling establishment on the use of water by fully developed plant cover, are discussed in relation to climate change and vegetation modelling.     

北京山区典型植被土壤水分对次降雨的响应

为了揭示北京山区不同植被类型土壤水分对不同强度降雨的响应过程,选取北京山区内侧柏、荆条灌丛、荒草地为研究对象,基于2022年6—10月降雨和土壤水分的连续观测数据,分析土壤水分对不同降雨事件的响应特征。结果表明：(1)观测期内研究区降雨事件主要由小雨构成,小雨事件的总降雨量占总降雨量的14.78%,小雨事件对土壤水分的响应深度可达到40—60 cm土层;中雨、大雨、暴雨事件的总降雨量占总降雨量的85.52%,中雨、大雨、暴雨事件对土壤水分的响应深度均可达到60—80 cm土层;大降雨事件对土壤水分的补给作用更明显,降雨量越大,降雨能补给的土层深度越深,土壤水分补给效果越好。(2)三种植被平均土壤水分补给速率大小依次为荒草地>侧柏>荆条灌丛,说明降雨对荒草地的补给效果最好;土层活跃程度对植被土壤水分有影响,土层越活跃,土壤水分波动越大,三种植被平均土壤水分变异系数大小依次为荒草地>荆条灌丛>侧柏,而三种植被平均土壤储水量大小则依次为侧柏>荒草地>荆条灌丛,说明侧柏的土壤水分最为稳定。(3)荒草地、荆条灌丛剖面上各层土壤水分逐渐减少,侧柏则增加,说明土壤...     

平茬对半干旱黄土丘陵区柠条林地土壤水分的影响

半干旱黄土丘陵区多年生柠条人工林地发生土壤旱化,研究柠条林平茬对土壤水分影响对于防治土壤旱化具有重要意义。采用中子仪测定土壤水分,对未平茬和平茬柠条林地土壤水分进行测定,分析了平茬对土壤水分的影响。结果表明:未平茬和平茬柠条林地降雨补给量(R₁,R₂)同降雨量(P)显著正相关(P<0.05)。定义降雨耗损量(林冠截留量和地表径流之和)占降雨量的百分比为降雨耗损率,未平茬林地降雨损耗率(L₁)和平茬柠条林地降雨损耗率(L₂)分别与其降雨前土壤表层(0-20 cm)含水量(S₁,S₂)呈明显指数关系(P<0.05):L₁=2.54exp(0.22S₁),L₂=2.40exp(0.27S₂),表层含水量相同时,平茬林地降雨损耗率明显高于未平茬林地。平茬后,林地降雨最大入渗深度减小,土壤水分利用深度减小;短时间内(2个月左右)林地20-160 cm含水量增加,之后平茬林地土壤含水量与未平茬林地土壤含水量接近;丰水年和丰水年后的第一年,平茬林地含水量低于未平茬林地,0-400 cm土壤储水量比未平茬林地最多低45.9 mm。平茬后200-400 cm土层土壤水分有少量增加,但是0-200 cm土层土壤含水量损失更严重。平茬3a后,平茬对柠条林地土壤水分的影响减弱。     

Hydraulic lift: consequences of water efflux from the roots of plants

Hydraulic lift is the passive movement of water from roots into soil layers with lower water potential, while other parts of the root system in moister soil layers, usually at depth, are absorbing water. Here, we review the brief history of laboratory and field evidence supporting this phenomenon and discuss some of the consequences of this below-ground behavior for the ecology of plants. Hydraulic lift has been shown in a relatively small number of species (27 species of herbs, grasses, shrubs, and trees), but there is no fundamental reason why it should not be more common as long as active root systems are spanning a gradient in soil water potential (Ψ_s) and that the resistance to water loss from roots is low. While the majority of documented cases of hydraulic lift in the field are for semiarid and arid land species inhabiting desert and steppe environments, recent studies indicate that hydraulic lift is not restricted to these species or regions. Large quantities of water, amounting to an appreciable fraction of daily transpiration, are lifted at night. This temporary partial rehydration of upper soil layers provides a source of water, along with soil moisture deeper in the profile, for transpiration the following day and, under conditions of high atmospheric demand, can substantially facilitate water movement through the soil-plant-atmosphere system. Release of water into the upper soil layers has been shown to afford the opportunity for neighboring plants to utilize this source of water. Also, because soils tend to dry from the surface downward and nutrients are usually most plentiful in the upper soil layers, lifted water may provide moisture that facilitates favorable biogeochemical conditions for enhancing mineral nutrient availability, microbial processes, and the acquisition of nutrients by roots. Hydraulic lift may also prolong or enhance fine-root activity by keeping them hydrated. Such indirect benefits of hydraulic lift may have been the primary selective force in the evolution of this process. Alternatively, hydraulic lift may simply be the consequence of roots not possessing true rectifying properties (i.e., roots are leaky to water). Finally, the direction of water movement may also be downward or horizontal if the prevailing Ψ_s gradient so dictates, i.e., inverse, or lateral, hydraulic lift. Such downward movement through the root system may allow growth of roots in otherwise dry soil at depth, permitting the establishment of many phreatophytic species. Received: 2 June 1997 / Accepted: 24 September 1997     

Using stable hydrogen and oxygen isotopes to study water movement in soil-plant-atmosphere continuum at Poyang Lake wetland,China

Water movement in the soil-plant-atmosphere continuum (SPAC) has a significant effect on the biogeochemical process in wetlands. This study investigated the water movement in the SPAC in Poyang Lake wetland, which is a protected area with an important ecological function within the Yangtze River basin, under different water-level conditions by analyzing the responses of river, groundwater, soil and plants to precipitation using stable hydrogen and oxygen isotopes. The results show that the stable hydrogen and oxygen isotopic compositions (δ¹⁸O and δD) of soil water decrease with increasing depth due to the near surface evaporation. During the dry season the water-level in Poyang Lake is low, when it rains the influencing depth of precipitation and evaporation on soil water isotopic signatures was 20 cm below the ground surface. The rain water infiltrates into the soil, recharges groundwater and flows to the river. When the water-level in Poyang Lake is low, the Xiu River is recharged by the groundwater, which recharges the soil water by capillary rise. During the flood season, the water-level is high and the water in Poyang Lake reaches or covers the meadows, recharges the groundwater and soil water. In the meantime, the water in Poyang Lake can be recharged by rain water when it rains. During the dry season when it doesn’t rain, plants mainly use groundwater, but soil water is preferred and plants don’t use rainwater directly when it rains. When the lake water-level is extremely low, the plants in Poyang Lake wetland may suffer from water stress, which is harmful for plant growth.     

祁连山北坡青海云杉林下苔藓层对土壤水分空间差异的影响

苔藓层是青海云杉林(Picea crassifolia forest)下的一个重要层片,它通过截持降水和减少土壤蒸发,对土壤水分产生影响。以祁连山北坡排露沟小流域的一个阴坡(海拔2700 m,植被为青海云杉林)作为样坡,于2010—2012年生长季对青海云杉林下有、无苔藓层覆盖地点的土壤水分进行对比观测。结果表明在祁连山青海云杉林下,苔藓层覆盖能减少土壤水分的空间差异。主要表现为:(1)无苔藓覆盖各观测点土壤含水量极差达62.2 mm,空间变异系数(CV)为17.3%;有苔藓层覆盖观测点土壤含水量空间变异系数为2.3%,仅为无苔藓覆盖地点的1/7.5,空间差异极显著(sig.0.001)。(2)在持续无雨、小雨后和连阴雨天气下,有苔藓覆盖地点土壤含水量空间差异均显著小于无苔藓覆盖地点(sig.0.05)。在持续无雨的情况下,无苔藓层覆盖各观测点土壤含水量空间变异系数平均值为19.8%,有苔藓覆盖地点土壤含水量空间变异系数平均值为6.6%,仅为无苔藓覆盖地点的1/3。小雨后,无苔藓覆盖各观测点土壤含水量空间变异系数平均值为15.2%,有苔藓覆盖地点土壤水分空间变异系数平均为5.1%,为无苔藓覆盖地点的1/3。连阴雨后,无苔藓覆盖各观测点土壤含水量空间变异系数平均为15.4%,有苔藓覆盖地点土壤水分空间变异系数平均为4.6%,为无苔藓覆盖地点的1/3.3。(3)持续无雨的情况下,苔藓层减小土壤水分空间差异的作用主要反映在土壤表层0—15 cm,对深层的作用不显著。而小雨和连阴雨后,苔藓层对15—80 cm深层土壤影响显著,而表层0—15 cm没有明显规律。     

Hydraulic redistribution in three Amazonian trees

About half of the Amazon rainforest is subject to seasonal droughts of 3 months or more. Despite this drought, several studies have shown that these forests, under a strongly seasonal climate, do not exhibit significant water stress during the dry season. In addition to deep soil water uptake, another contributing explanation for the absence of plant water stress during drought is the process of hydraulic redistribution; the nocturnal transfer of water by roots from moist to dry regions of the soil profile. Here, we present data on patterns of soil moisture and sap flow in roots of three dimorphic-rooted species in the Tapajós Forest, Amazônia, which demonstrate both upward (hydraulic lift) and downward hydraulic redistribution. We measured sap flow in lateral and tap roots of our three study species over a 2-year period using the heat ratio method, a sap-flow technique that allows bi-directional measurement of water flow. On certain nights during the dry season, reverse or acropetal flow (i.e.,in the direction of the soil) in the lateral roots and positive or basipetal sap flow (toward the plant) in the tap roots of Coussarea racemosa (caferana), Manilkara huberi (maçaranduba) and Protium robustum (breu) were observed, a pattern consistent with upward hydraulic redistribution (hydraulic lift). With the onset of heavy rains, this pattern reversed, with continuous night-time acropetal sap flow in the tap root and basipetal sap flow in lateral roots, indicating water movement from wet top soil to dry deeper soils (downward hydraulic redistribution). Both patterns were present in trees within a rainfall exclusion plot (Seca Floresta) and to a more limited extent in the control plot. Although hydraulic redistribution has traditionally been associated with arid or strongly seasonal environments, our findings now suggest that it is important in ameliorating water stress and improving rain infiltration in Amazonian rainforests. This has broad implications for understanding and modeling ecosystem process and forest function in this important biome.     

Influence of mulching on the pattern of growth and water use by spring wheat and moisture storage on a fine textured soil

Eight tonnes ha^–1 of stubble were used to mulch spring wheat (Triticum aestivum) on a fine textured soil with the aim of controlling both transpiration and soil evaporation during the wet pre-anthesis phase to increase moisture supply during grain filling in the eastern wheatbelt of Western Australia. Mulching reduced leaf area per plant by reducing the culm number; consequently the green area index was reduced. Reduced culm number was associated with low soil temperature which at 50 mm depth averaged 7°C lower under the mulched crop relative to the control crop in mid-season. The smaller canopies of the mulched crop used 15 mm less water than those of the control before anthesis; this difference in water-use was due equally to reduced transpiration and soil evaporation. However, the mulched crop was unable to increase ET during grain filling, a response associated with the persistence of low soil temperature for most of the growth period. Hence, total ET for the season was significantly lower (18 mm) under the mulched crop than the control crop. At harvest, mulching did not have significant effects on total above-ground dry matter and grain yields, but it increased water use efficiency for grain yield by 18%, grain weight by almost 17% and available moisture in both uncropped and cropped plots by an average of 43 mm.To determine whether there was any residual effects of soil treatment on moisture storage during the summer fallow period, soil moisture was monitored both in cropped plots and uncropped plots, that were either mulched or unmulched during the growing season, from harvest in October 1988 until next planting in June 1989. Available moisture at next planting was correlated with moisture storage at harvest despite the differences in run-off, soil evaporation and fallowing efficiency (increase in moisture storage as a percentage of rainfall) between treatments during fallowing. Therefore, the mulched treatments had more moisture available (30 mm), mostly as a result of less water use during cropping in the previous growing season, than the unmulched treatment.The study shows that mulching may be used to restrain both transpiration and soil evaporation early in the season to increase availability of soil moisture during grain filling. Secondly, mulching during the previous growing season had little effect on soil moisture during the summer fallow period, however, the moisture saved by mulching during cropping was conserved for the following season. These results indicate the importance of evaluating mulching of winter crops in terms of crop yield in the subsequent growing season as well as in the current season in which the soil was treated.Abbreviations D through drainage - DAS days after sowing of the crop on 31 May 1988 - DM dry matter produced in the above-ground portion of the crop (kg ha^–1) - E₀ evaporation from Class A pan (mm) - E_s evaporation from uncropped soil (mm) - E_sc evaporation from soil beneath the wheat canopy (mm) - ET evapotranspiration (mm) - FE fallowing efficiency (gain in soil moisture storage/rainfall) - GAI green area index (area of green vegetation per unit land area) - GWUE water-use efficiency for grain production (grain yield/total ET, kg ha^–1mm^–1) - K extinction coefficient (see equation 1) - RO run-off of moisture from soil surface during/following rainfall (mm) - SM available soil moisture (mm) at harvest (SMh) or at planting (SMp) - WUE water-use efficiency for total above-ground dry matter yield (see GWUE)     

Growth Characteristics of Cowpea (Vigna unguiculata) Seedlings as Affected by Depth to Water Table

Seedlings of two Nigerian varieties of cowpea (IT82D and IT84E716)were grown together from seeds on loamy soil for 8 weeks intwo identical sloping moisture gradient boxes established outdoorsduring the dry season. Gradients of 13 cm to 79 cm depth towater table were maintained. For all growth characters determined,differences between the two varieties did not differ statisticallythough variety IT84E716 showed marginal improvement over varietyIT82D. Depth to water table had significant effects on all growthcharacteristics determined except for number of lateral shoots.No significant depth to water table x variety interactions wererecorded for any characters examined. Nodules and nodulatedroots showed maximum development in the wettest soils wherethey were relatively thick and large. The main pattern of growththat emerged was limited growth in very wet soil, maximum growthin wet soil and increasingly limited growth as the soil becamedrier. The experiment supported the hypothesis that depth towater table may provide a major edaphic control of cowpea unlessthere is frequent rainfall or irrigation. Cowpea (Vigna unguiculata cylindrica (L.) Walp.), growth characteristics, moisture gradient boxes, phenotypic plasticity, water table     

Contrasting effects of repeated summer drought on soil carbon efflux in hydric and mesic heathland soils

Current predictions of climate change include altered rainfall patterns throughout Europe, continental USA and areas such as the Amazon. The effect of this on soil carbon efflux remains unclear although several modelling studies have highlighted the potential importance of drought for carbon storage. To test the importance of drought, and more importantly repeated drought year-on-year, we used automated retractable curtains to exclude rain and produce repeated summer drought in three heathlands at varying moisture conditions. This included a hydric system limited by water-excess (in the UK) and two mesic systems with seasonal water limitation in Denmark (DK) and the Netherlands (NL). The experimental rainfall reductions were set to reflect single year droughts observed in the last decade with exclusion of rain for 2–3 months of the year resulting in a 20–26% reduction in annual rainfall and 23–38% reduction in mean soil moisture during the drought period. Unexpectedly, sustained reduction in soil moisture over winter (between drought periods) was also observed at all three sites, along with a reduction in the maximum water-holding capacity attained. Three hypotheses are discussed which may have contributed to this lack of recovery in soil moisture: hydrophobicity of soil organic matter, increased water use by plants and increased cracking of the soil. The responses of soil respiration to this change in soil moisture varied among the sites: decreased rates were observed at the water-limited NL and DK sites whilst they increased at the UK site. Reduced sensitivity of soil respiration to soil temperature was observed at soil moisture contents above 55% at the UK site and below 20% and 13% at the NL and DK sites, respectively. Soil respiration rates recovered to predrought levels in the NL and DK sites during the winter re-wetting period that indicates any change in soil C storage due to changes in soil C efflux may be short lived in these mesic systems. In contrast, in the hydric UK site after 2 years of drought treatment, the persistent reduction in soil moisture throughout the year resulted in a year-round increase in soil respiration flux, a response that accelerated over time to 40% above control levels. These findings suggest that carbon-rich soils with high organic matter content may act as a significant source of CO₂ to the atmosphere following repeated summer drought. Nonrecovery of soil moisture and a persistent increase in soil respiration may be the primary mechanism underlying the reported substantial losses of soil carbon from UK organic soils over the last 20 years. These findings indicate that the water status of an ecosystem will be a critical factor to consider in determining the impact of drought on the soil carbon fluxes and storage.     

Plants Mediate the Sensitivity of Soil Respiration to Rainfall Variability

Soil respiration from grasslands plays a critical role in determining carbon dioxide (CO₂) feedbacks between soils and the atmosphere. In these often mesic systems, soil moisture and temperature tend to co-regulate soil respiration. Increasing variance of rainfall patterns may alter aboveground–belowground interactions and have important implications for the sensitivity of soil respiration to fluctuations in moisture and temperature. We conducted a set of field experiments to evaluate the independent and interactive effects of rainfall variability and plant–soil processes on respiration dynamics. Plant removal had strong effects on grassland soils, which included altered CO₂ flux owing to absence of root respiration; increased soil moisture and temperature; and reduced availability of dissolved organic carbon (DOC) for heterotrophic respiration by microorganisms. These plant-mediated effects interacted with our rainfall variability treatments to determine the sensitivity of soil respiration to both moisture and temperature. Using time-series multiple regression, we found that plants dampened the sensitivity of respiration to moisture under high variability rainfall treatments, which may reflect the relative stability of root contributions to total soil respiration. In contrast, plants increased the sensitivity of respiration to temperature under low variability rainfall treatment suggesting that the environmental controls on soil CO₂ dynamics in mesic habitats may be context dependent. Our results provide insight into the aboveground–belowground mechanisms controlling respiration in grasslands under variable rainfall regimes, which may be important for predicting CO₂ dynamics under current and future climate scenarios.     

The influence of crop management on the water balance of lupin and wheat crops on a layered soil in a Mediterranean climate

The influence of time of sowing and sowing density on evapotranspiration and drainage loss beneath wheat (Triticum aestivum cv. Spear) and lupin (Lupinus angustifolius cv. Gungurru) crops grown on a layered soil was investigated for three seasons in a Mediterranean climate in Western Australia. The aim of the study was to investigate whether managing crops to maximise their canopy growth would increase their water use and minimise groundwater recharge contributing to dryland salinity. A soil water balance approach was used to estimate evapotranspiration, with changes in soil water content measured with a neutron water meter. The study was carried out on a layered soil typical of agricultural soils in the region with variable depth to clay (0.22–0.38 m) and a marked contrast in hydraulic properties between the topsoil and subsoil. As a result of the low permeability subsoil, a perched water table occurred in the sandy topsoil in each of the three seasons under study during winter when rainfall was high and potential evaporation low. Perched water tables persisted for 2–3 months, with hydraulic gradients consistently downward causing drainage losses to occur. Although crop management had a large influence on shoot and root development, evapotranspiration from the different treatments was generally similar. Drainage losses were not influenced by either crop type, time of sowing or sowing density, because potential evaporation and hence evapotranspiration was low during the period when drainage losses occurred. The total drainage loss measured in each season was different, with losses ranging from 20.1 to 22.2 mm in 1990, from 40.4 to 46.7 mm in 1991 and from 49.4 to 66.6 mm in 1992. The increase in drainage loss from 1990 to 1992 was a result of progressively more seasonal rainfall in 1990, 1991 and 1992. It was concluded that there was little scope to increase water use and decrease deep drainage through crop management for sites with climatic conditions where winter rainfall exceeds potential evaporation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrate distribution in tropical soils

Summary The downward movement of the nitrate ion in the top 5 ft of Tippera clay loam was followed by applying sodium nitrate at the rate of 2000 pounds per acre to bare fallow soil after different rain intervals during the 1957/58 wet season. At the end of the season soil samples were collected at 6-inch intervals and compared with samples from adjacent plots that received an equivalent anion quantity of sodium chloride after the same rain intervals. After 23.7 inches of rain the distribution of nitrate and chloride anions the soil profile was nearly identical and it was concluded that the anions are equally mobile in this soil.The mean movement of the anion was 1.075 inches for each inch of rainfall. A high positive correlation of 0.946 was obtained between mean movement and rainfall. The downward movement of both anions out of the topsoil appeared to be enhanced by channels left by partly decomposed roots.Application of sodium chloride on sandier soils revealed a much higher mean movement of anions on Blain sand than on Tippera clay loam after equal amounts of rain, but on Florina sand with a high silt content in the topsoil the mean anion movement approached that of the clay soil. The difference is explained in terms of low infiltration rate into the Florina soil.The practical implications of leaching of nitrate in Tippera soil are briefly discussed.     

Precipitation Regime Shift Enhanced the Rain Pulse Effect on Soil Respiration in a Semi-Arid Steppe

The effect of resource pulses, such as rainfall events, on soil respiration plays an important role in controlling grassland carbon balance, but how shifts in long-term precipitation regime regulate rain pulse effect on soil respiration is still unclear. We first quantified the influence of rainfall event on soil respiration based on a two-year (2006 and 2009) continuously measured soil respiration data set in a temperate steppe in northern China. In 2006 and 2009, soil carbon release induced by rainfall events contributed about 44.5% (83.3 g C m⁻²) and 39.6% (61.7 g C m⁻²) to the growing-season total soil respiration, respectively. The pulse effect of rainfall event on soil respiration can be accurately predicted by a water status index (WSI), which is the product of rainfall event size and the ratio between antecedent soil temperature to moisture at the depth of 10 cm (r ² = 0.92, P<0.001) through the growing season. It indicates the pulse effect can be enhanced by not only larger individual rainfall event, but also higher soil temperature/moisture ratio which is usually associated with longer dry spells. We then analyzed a long-term (1953–2009) precipitation record in the experimental area. We found both the extreme heavy rainfall events (>40 mm per event) and the long dry-spells (>5 days) during the growing seasons increased from 1953–2009. It suggests the shift in precipitation regime has increased the contribution of rain pulse effect to growing-season total soil respiration in this region. These findings highlight the importance of incorporating precipitation regime shift and its impacts on the rain pulse effect into the future predictions of grassland carbon cycle under climate change.     

Short and Long-Term Soil Moisture Effects of Liana Removal in a Seasonally Moist Tropical Forest

Lianas (woody vines) are particularly abundant in tropical forests, and their abundance is increasing in the neotropics. Lianas can compete intensely with trees for above- and belowground resources, including water. As tropical forests experience longer and more intense dry seasons, competition for water is likely to intensify. However, we lack an understanding of how liana abundance affects soil moisture and hence competition with trees for water in tropical forests. To address this critical knowledge gap, we conducted a large-scale liana removal experiment in a seasonal tropical moist forest in central Panama. We monitored shallow and deep soil moisture over the course of three years to assess the effects of lianas in eight 0.64 ha removal plots and eight control plots. Liana removal caused short-term effects in surface soils. Surface soils (10 cm depth) in removal plots dried more slowly during dry periods and accumulated water more slowly after rainfall events. These effects disappeared within four months of the removal treatment. In deeper soils (40 cm depth), liana removal resulted in a multi-year trend towards 5–25% higher soil moisture during the dry seasons with the largest significant effects occurring in the dry season of the third year following treatment. Liana removal did not affect surface soil temperature. Multiple and mutually occurring mechanisms may be responsible for the effects of liana removal on soil moisture, including competition with trees, and altered microclimate, and soil structure. These results indicate that lianas influence hydrologic processes, which may affect tree community dynamics and forest carbon cycling.     

Effect of Watering and Soil Moisture on Mercury Emissions from Soils

This paper presents data from experiments that measured Mercury (Hg) flux as a function of water addition and subsequent soil drying, and maintenance of soil water content over time utilizing small dynamic gas exchange chambers and large mesocosms. When soil surfaces were dry and water was added at an amount less than that necessary to saturate the soil an immediate large (relative to dry soil flux) release of Hg occurred. Diel Hg emissions from soils, unenriched (0.02 μg g⁻¹) and enriched (3 μg g⁻¹) in Hg and wet below saturation, were significantly elevated above that occurring from dry soils (2–5 times depending on soil water content) for weeks to months. Enhancement of emissions from wet soils in direct sunlight were greater than that from soils shaded or in the dark suggesting that a synergism exists between soil moisture and light. When soils were watered to saturation Hg emissions were suppressed or remained the same depending on the degree of saturation. It is hypothesized that the addition of soil water in amounts less than that necessary to saturate the soil surface results in an immediate release of elemental Hg from soil surface as the more polar water molecule out competes Hg for binding sites. As the water moves into the soil, Hg adsorbed to soil particles is desorbed into soil gas and dissolved in the soil water. The process of evaporation facilitates movement of Hg as mass flow to the soil surface where it is made available for subsequent release. The latter is hypothesized to be an important process by which Hg is recharged at the soil–air interface.     

白刺沙包浅层土壤水分动态及其对不同降雨量的响应

以乌兰布和沙漠典型白刺沙包为研究对象,使用EC-5土壤水分传感器对其浅层(0—50 cm)土壤水分进行长期连续监测,分析了白刺沙包不同深度土层对不同降雨量的响应及整个生长季的土壤水分动态特征。结果表明:降雨是乌兰布和沙漠白刺沙包土壤水分的最重要补给源,降雨量大小是影响浅层土壤水分补给深度的决定因素。小于10 mm的降雨完全被表层(0—10 cm)土壤吸收,无法补给10 cm以下土壤水分;10—20 mm的降雨对土壤水分的补给深度达到20 cm;20—30 mm的降雨对土壤水分的补给深度达到40 cm,大于30 mm的降雨补给深度可达到50 cm,甚至更深土层。在研究区降雨量以小于20 mm降雨为主的情况下,20 cm以下土层土壤水分逐步恶化,久之将有利于浅根系草本植物的生长,不利于白刺的生长繁殖。因此,这种降雨格局将对浅层土壤水分及植被演替产生重要影响。     

Mercury translocation in and evaporation from soil. III. quantification of evaporation of mercury from podzolized soil profiles treated with Hg Cl

Mercury evaporation from undisturbed iron‐humus podzol lysimeters was measured over 3 months after treatment with HgCl₂ spiked with radioactive ²⁰³Hg. The relative evaporation rate from HgCl₂ treated soils followed the sum of two exponential functions. Because evaporation asymptotically approaches zero with time, the integral of the fit curve represents the evaporative loss in percent of atmospheric deposition. For the soil investigated, about 5% of atmospheric Hg deposition was reemitted into the atmosphere. It is hypothesized that mercury evaporation can decrease the leaching of mercury in and from soil significantly; this effect is probably increasing with decreasing rain acidity or soil acidity. Mercury deposited as soluble salt remains susceptible to reemission to air for 300 d after incorporation into the soil matrix. Indications are found that Hg evaporation from soils in geological background areas predominantly derives from recent atmospheric Hg deposition and not from geological sources.     

Does soil moisture availability explain liana seedling distribution across a tropical rainfall gradient?

Liana density tends to increase with decreasing rainfall and increasing seasonality. However, the pattern of liana distribution may be due to differences in soil water retention capacity, not rainfall and seasonality per se. We tested the effect of rainfall and soil substrate with respect to the distribution of liana seedlings in six sites across a rainfall gradient from the wet Atlantic to the dry Pacific in central Panama. Soils were either limestone, with low water‐holding capacity, or laterite, with higher water‐holding capacity. We sampled liana seedlings at each site using three 1 × 100 m transect. We found that relative liana seedling density was higher on limestone soils compared to laterite soils regardless of the amount of rainfall. Furthermore, liana community composition on limestone soils was more similar to dry forest sites than to adjacent wet and moist forest sites. Liana seedling species diversity relative to trees was significantly higher in a low‐fertility dry forest site compared to a high‐fertility forest, but did not differ from the other sites. Thus, liana seedling density and community structure may be driven more by soil type and thus by soil moisture availability than strictly by mean annual rainfall and the seasonality of rainfall.     

Mercury translocation in and evaporation from soil. II. Evaporation of mercury from podzolized soil profiles treated with HgCl2 and CH3HgCl

Mercury evaporation from soil columns of an iron humus podzol treated with various amounts of HgCl₂ and CH₃HgCl was measured over 3500 h. The effects of rain acidity, rain duration, and rain intensity were investigated. Hg evaporation from CH₃HgCl‐treated soil columns seems to be a biologically determined process. Hg evaporation from HgO₂‐treated soil probably is mainly an abiotic process, following a pseudo first‐order reaction with rapidly decreasing evaporation rate due to a decreasing amount of available Hg. The added Hg compounds were transformed to highly volatile Hg⁰ and/or (CH₃)₂Hg. The Hg evaporation rate decreased with increasing rain acidity, which may cause accumulation of Hg in the soil. No effects of rain duration and rain intensity were found.