Soil Hydrological Conditions Research Articles

Quantification of Chemical Transport Processes from the Soil to Surface Runoff

There is a good conceptual understanding of the processes that govern chemical transport from the soil to surface runoff, but few studies have actually quantified these processes separately. Thus, we designed a laboratory flow cell and experimental procedures to quantify the chemical transport from soil to runoff water in the following individual processes: (i) convection with a vertical hydraulic gradient, (ii) convection via surface flow or the Bernoulli effect, (iii) diffusion, and (iv) soil loss. We applied different vertical hydraulic gradients by setting the flow cell to generate different seepage or drainage conditions. Our data confirmed the general form of the convection-diffusion equation. However, we now have additional quantitative data that describe the contribution of each individual chemical loading process in different surface runoff and soil hydrological conditions. The results of this study will be useful for enhancing our understanding of different geochemical processes in the surface soil mixing zone.

不同干扰对黄土区典型草原物种多样性和生物量的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 不同干扰对黄土区典型草原物种多样性和生物量的影响 DOI: 10.5846/stxb201201140085 作者: 作者单位: 中国科学院水利部水土保持研究所,中国科学院水利部水土保持研究所,国家林业局中南林业调查规划设计院,西北农林科技大学,中国科学院水利部水土保持研究所,中国科学院水利部水土保持研究所,西北农林科技大学 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金重点项目(41230852);中国科学院战略性先导科技专项项目(XDA05050202);农业部 "现代农业产业技术体系建设专项资助(CARS-35-40) Effects of different disturbances on diversity and biomass of communities in the typical steppe of loess region Author: Affiliation: Institute of Soil and Water Conservation,Chinese Academy of Sciences and Ministry of Water Resources,Yangling Shaanxi,Institute of Soil and Water Conservation,Chinese Academy of Sciences and Ministry of Water Resources,Yangling Shaanxi,Central South Forest Inventory and Planning Institute of State Forestry Administration,Changsha Hunan,Northwest Agricultural Forestry University,Yangling Shaanxi,,Institute of Soil and Water Conservation,Chinese Academy of Sciences and Ministry of Water Resources,Yangling Shaanxi,Northwest Agricultural Forestry University,Yangling Shaanxi Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:对黄土区典型草原进行封育+施肥(EF)、封育+火烧(EB)、封育(E)和放牧(G)处理,实地调查分析群落盖度、高度、密度、地上现存量和物种多样性,以研究不同干扰对黄土区典型草原群落物种多样性和生物量的影响.结果表明:在4种干扰类型中,施肥+封育草地群落盖度和地上生物量最高,且优势度指数最高,这与禾本科草占优势地位有关,群落均匀度指数和多样性指数最低,符合"生态位理论";放牧地群落高度、盖度、密度和地上现存量最低,群落丰富度指数和多样性指数最高,支持"中度干扰理论";封育地密度和均匀度指数最高;具体表现为:4种干扰类型地上生物量的变化趋势为封育+施肥＞封育+火烧＞封育＞放牧;说明长时间的封育对草地是一种严重干扰.群落丰富度指数(R和Ma)的排列顺序为放牧＞封育+施肥＞封育+火烧＞封育,群落物种多样性指数(H'和D)的排列顺序为放牧＞封育＞封育+火烧＞封育+施肥,优势度指数与多样性指数相反,群落均匀度指数(Jsw和Ea)的排列顺序为封育＞放牧＞封育+火烧＞封育+施肥.不同干扰样地群落生产力与Shannon-Wiener和Simpson多样性指数间呈负相关关系,这个结论可以用地上/地下竞争的相互作用来解释. Abstract:Typical steppe is a type of grassland which forms in inland temperate semi-arid climatic conditions, where xerophytic perennial bush grasses, especially Stipa spp., dominate. Grassland ecosystems are one of the largest terrestrial ecosystems affected by human activities. It is important to study the impact of different types of disturbance on the biodiversity and biomass of grasslands ecosystems during restoration and reconstruction work. Exclosures, grazing, fertilization and burning are four major types of anthropogenic disturbance occurring in grassland ecosystems, and have very important impacts on them. To provide a scientific basis for the protection of grassland biodiversity and the sustainable use of grasslands, the influences of different disturbances on the characteristics of above-ground vegetation were studied in a typical steppe in the loess region of China based on the field investigation and monitoring. We established plots representing four different types of treatments: exclosure (E), exclosure plus fertilization (EF), exclosure plus burning (EB) and grazing (G). All plots had the same soil type, topography and hydrological conditions. During the entire experiment, we determined total coverage, vegetation height, individual density, aboveground biomass and diversity of the plant community in each plot. Our results show the dominant vegetation community type of G plots is Agropyron cristatum+Potentilla acaulis+Artemisia sacrorum, a community which is the result of long-term grazing. The dominant vegetation community type of the E plots is Stipa grandis+Poa sphondylodes+Medicago ruthenica, a result of fencing. The dominant vegetation community type of the EF plots is Carex duriuscula+Stipa bungeana+Stipa grandis. The dominant vegetation community type of the EB plots is Artemisia sacrorum+Stipa bungeana+Stipa grandis. The different disturbance regimes strongly influenced community diversity and biomass. To be more specific, total coverage, above-ground biomass and the dominance indices were all at the highest level in the EF treatments in four types of treatments. This may attributed to grass species which play a dominant role in this community. Also, with low evenness and diversity indices, this result was consistent with niche theory which states differences among species in their environmental tolerances, niche requirements, and competitive abilities determine coexistence. Total coverage, height and aboveground biomass were all lowest for the G treatment, while community richness and diversity indices were higher for G than other three treatments. This result supports the intermediate disturbance hypothesis which states that biodiversity will be greatest in communities subjected to moderate levels of disturbance. Only the individual density and evenness index was higher with the E treatment than with the other treatments. Thus we can document the trend for biomass as EF > EB > E > G. These demonstrate long-term exclosure is a severe type of disturbance in grassland ecosystems. Also, the community richness indices (R and Ma) showed a certain tendency within these four treatments: G > EF > EB > E and community evenness indices (Jsw and Ea) could be sequenced in the order of E > G > EB > EF. Meanwhile, the order of diversity indices (H' and D) was as follows: G > E > EB > EF, while the dominance index was opposite the order of diversity index. Finally, there was negative correlation between community productivity and Shannon-Wiener and Simpson indices, which can be explained by interactions between root and shoot competition. 参考文献 相似文献 引证文献

Factors related to Bh-horizon depth for artificial and natural E–Bh sequences

Aquods occur extensively in the coastal plain of the SE USA, where podzolization is mainly restricted to fluctuating water table conditions. Depths to upper boundaries of Spodic (Bh) horizons vary from a few cm to ≥75cm even under similar drainage conditions. Hence, seasonal water table depth parameters do not explain the wide variation in boundary depth. One objective of this study was to experimentally test the following hypotheses: (H1) Depth to the E–Bh boundary relates inversely to concentration of metal oxides, silt, and clay content of initial (“parent”) material for similar hydrologic conditions and organic acid activity; (H2) depth to that boundary is greater under higher organic activity for a given soil material and hydrologic condition. Another objective was to statistically evaluate how the composition of >270 natural Spodic horizons collected during the Florida Cooperative Soil Survey Program related to upper boundary depth. Thirteen sandy coastal plain soil materials of varying noncrystalline-metal-oxide- (Al and Fe), silt-, and clay contents were packed in columns and subjected to daily cycles of fluctuating water tables and dilute organic acid applications. Artificial E–Bh boundaries of variable depths were generated in the columns. Boundaries tended to be abrupt but topographically irregular. Clay- and extractable-noncrystalline Al and Fe contents were inversely related to final depth of artificial Bh horizons for the same water-table conditions. Higher oxalic acid concentrations produced deeper boundaries in the same materials; hence, local biogeochemical factors related to organic acid production are also likely to influence the ultimate Aquod morphology on the landscape. Silt-, clay-, and metal-oxide contents were significantly higher in shallower than in deeper natural Florida Spodic horizons.

Effectiveness of vegetated filter strips in retention of Escherichia coli and Salmonella from swine manure slurry

Vegetated filter strips (VFS) are commonly recommended as a best management practice to prevent manure-borne microorganisms from reaching surface water resources. However, relatively little is known about the efficacy of VFS in mitigating bacterial runoff from land-applied swine manure. A field lysimeter study was designed to evaluate the effect of surface soil hydrologic conditions and vegetation on the retention of swine manure-borne Escherichia coli and Salmonella under simulated rainfall conditions. Experimental plots (6.5 m × 3.9 m) were set on a 5% slope lysimeter with loamy topsoil, clay loam or loam subsoil and a controllable groundwater level. Three small flow-intercepting miniflumes were installed 4.5 m from the plot's top, while all remaining runoff was collected in a gutter at the bottom. Plots were divided into bare soil and grass vegetation and upper surface soil moisture before rainfall events was controlled by the subsurface groundwater level. Swine manure slurry inoculated with E. coli and Salmonella, and with added bromide tracer, was applied on the top of the plots and simultaneously initiated the simulated rainfall. Runoff was collected and analyzed every 5 min. No substantial differences between retention of E. coli and Salmonella were found. In initially wet soil surface conditions, there was limited infiltration both in bare and in vegetated plots; almost all bromide and about 30% of bacteria were recovered in runoff water. In initially dry soil surface conditions, there were substantial discrepancies between bare and vegetated plots. In bare plots, recoveries of runoff water, bromide and bacteria under dry conditions were comparable to wet conditions. However, in dry vegetated plots, from 50% to 75% of water was lost to infiltration, while bromide recoveries ranged from 14 to 36% and bacteria recovery was only 5%. Substantial intraplot heterogeneity was revealed by the data from miniflumes. GIS analysis of the plot microtopography showed that miniflumes located in the zones of flow convergence collected the majority of bacteria. Overall, the efficiency of VFS, with respect to the retention of swine manure bacteria, varied dramatically depending upon the hydrologic soil surface condition. Consequently, VFS recommendations should account for expected amounts of surface soil water saturation as well as the relative soil water storage capacity of the VFS.

Effects of afforestation on soil respiration in an arid shrubland in Chile

We studied the effects of afforestation on soil respiration (Rs) in an arid shrubland in northern Chile. We sampled Rs monthly for three years in two adjacent plots, one left as control in natural condition, and one afforested with Acacia saligna. Sampling occurred 3–6 years after afforestation. Soil temperature (Ts) was a good predictor of Rs on certain periods of the year, although we could not relate this to specific levels of soil moisture (θ). Seasonal Rs variation related more to θ than to Ts, which could be explained by extremely low annual precipitation (138mm), concentrated during winter months. The correlation between Ts and Rs was higher when measured under plants compared to bare ground. The afforested site had in average 17% lower annual Rs (p=0.06) than the control site. During some months of the third year of measurements, the afforested site showed greater Rs than the control, suggesting that in the near future, Rs might be higher for the afforested site than for the natural condition, depending on the effect that trees may have on soil carbon stability, temperature and hydrological conditions.

Land use changes on the slopes of Mount Elgon and the implications for the occurrence of landslides

A reconstruction of land use changes and the implications thereof for landslide occurrence on critical slopes of Mount Elgon in Eastern Uganda were undertaken. Aerial photographs taken in 1960 formed the benchmark for the analysis of respective land use changes between 1995 and 2006, using 30 m Landsat TM and 20 m SPOT MS images. Landslide sites were mapped using a MobileMapper, and terrain parameters were derived using a 15 m Digital Elevation Model. A supervised classification approach was employed to generate land-cover maps, from which the areas of three land-cover classes (agricultural fields, woodlands and forests) were calculated. A post-classification comparison change-detection technique revealed different trends in land-cover change between the periods 1960–1995 and 1995–2006. Whereas there were minimal land use changes between 1960 and 1995, the period 1995–2006 marked a considerable loss of woodlands and forest cover, particularly on steep concave slopes (36°–58°) of the National Park. The encroachment onto the critical slopes was noted to have induced a series of shallow and deep landslides in the area. All the mapped landslides were noted to lie on steep concave slopes of a northerly orientation, which had been opened up for cultivation. Deforestation and cultivation alter the soil hydrological conditions on steep concave slopes, rendering them susceptible to saturation. This may trigger debris flows during rainfall events. There is a need to restore forest cover on the fragile steep slopes and restrain local communities from opening up new areas for cultivation on critical slopes, particularly within the protected area.

Evaluation of the EUROSEM model for predicting the effects of erosion-control blankets on runoff and interrill soil erosion by water

The European Soil Erosion Model (EUROSEM, Morgan et al., 1998) is an event-based soil erosion model which predicts runoff and sediment discharge for different environmental conditions. Applying geotextiles or erosion-control blankets (ECB’s) on the soil surface significantly affects surface seal formation and topsoil properties and therefore controls runoff and soil erosion rates during a rainfall event. Since these within-storm changes of soil surface characteristics and hydrological conditions are not incorporated in EUROSEM, errors in runoff and soil erosion predictions may occur for soil surfaces covered with ECB’s. Therefore, the objective of this paper is to evaluate and improve the performance of a research version of the physically-based erosion model EUROSEM (EUROSEM-2010; Borselli and Torri, 2010) for simulating the effects of ECB’s on runoff and interrill soil erosion by water during intense simulated rainfall events. Results of model simulations are compared with experimental results of interrill erosion using biological (i.e. natural) ECB’s and simulated rainfall. Because ECB’s applied on the soil surface retard seal formation, the differences between observed and predicted runoff rates and sediment discharges are rather high during the first 20–30 min of the simulated rainstorm. Therefore, a simple approach is proposed to cope with the dynamic evolution of some soil characteristics, i.e. saturated hydraulic conductivity, soil erodibility and soil cohesion, during an intense rainfall event. This time-dependent approach improves the predictions of runoff rate and sediment discharge during the first 20–30 min of a rainfall event and increases the model efficiency (i.e. a measure for the goodness of fit) from 0.84 to 0.98 and from 0.48 to 0.68 for the total runoff volume and soil loss, respectively. For most conditions, the predicted final sediment discharge is still considerably larger than the observed values, which can be partly attributed to the deposition of sediment in the bare soil patches (i.e. inter-weave open areas) of the ECB’s, which is not simulated by EUROSEM in this study. This model approach increases our understanding of the effects of ECB’s on within-storm changes in hydrological conditions and soil surface characteristics.

Within‐Field Variability of Bare Soil Evaporation Derived from Eddy Covariance Measurements

Bare soil evaporation was measured with the eddy‐covariance method at the Selhausen field site. The site has a distinct gradient in soil texture, with a considerably higher stone content at the upper part of the field. We investigated the effect of different soil properties in the upper and lower parts of the field on evaporation using eddy covariance (EC) measurements that were combined with a footprint model. Because only one EC station was available, simultaneous evaporation measurements from the two field parts were not available. Therefore, measurements were put into the context of meteorologic and soil hydrologic conditions. Meteorologic conditions were represented by the potential evaporation, i.e., the maximum evaporation that is determined by the energy available for evaporation. The influence of precipitation and soil hydrologic conditions on the actual evaporation rate was represented by a simple soil evaporation model. The amount of water that could be evaporated at the potential rate from the lower part of the field was found to be large and considerably larger than from the upper part of the field. The difference in evaporation led to threefold larger predicted percolation or runoff in the upper than the lower part of the field. Simulations using the Richards equation were able to reproduce the differences in evaporation between the lower and upper parts of the field and relate them to the different groundwater table depths in the two parts of the field.

Location of Agricultural Drainage Pipes and Assessment of Agricultural Drainage Pipe Conditions Using Ground Penetrating Radar

Methods are needed to not only locate buried agricultural drainage pipe, but to also determine if the pipes are functioning properly with respect to water delivery. The primary focus of this research project was to confirm the ability of ground penetrating radar (GPR) to locate buried drainage pipe, and then determine if GPR provides insight into drain line water conveyance functionality. Ground penetrating radar surveys using [Formula: see text] transmitter/receiver antennas were conducted at a specially designed test plot under drained, moderately wet soil conditions (water table below drain lines) and undrained, extremely wet soil conditions (water table above drain lines). The test plot contained four drain lines: one a clay tile to corrugated plastic tubing (CPT) drain line that was completely open to flow; one comprised of CPT with an isolated obstruction near the midpoint, completely preventing through-flow of water; one comprised of CPT but filled with soil; and one comprised of CPT but severed near its midpoint, producing a partial obstruction to water flow. Subsequent GPR computer modeling simulations were employed to assist with interpretation of the GPR field data. Results of the GPR field surveys indicate that given suitable shallow hydrologic conditions, GPR not only finds drainage pipes, but can also determine the position along a drain line where there is an isolated obstruction that completely blocks water flow. However, results show that a partial pipe obstruction is difficult to locate using GPR. Surprisingly, the soil-filled drain line was clearly detectable under both soil hydrologic conditions tested. The GPR computer modeling simulations indicate that soil had likely settled within the pipe, and that the GPR responses obtained at the test plot for the soil-filled pipe were responses representative of a pipe that was in fact only partially filled with soil. Overall, these research results provide valuable information for those contemplating the use of GPR to locate agricultural drainage pipes and then determine their functionality.

Phosphate status of chernozem-like hydromorphic soils in the northern Tambov Plain

The phosphate status of chernozem-like soils in the northern forest steppe of the Tambov Lowland depends on soil waterlogging and hydrological conditions. Due to surface waterlogging and free effluent seep-age in podzolized, chernozem-like soils of open watershed depressions, the removal of bases and iron decrease the total phosphorus content by 10–15% because of the decrease in active mineral phosphates. Organic matter acts as a buffer preventing phosphorus from leaching. In podzolized, chernozem-like and podzolic, gleyic soils of closed watershed depressions, significant amounts of iron phosphates are accumulated in fine earth and ortsteins due to surface waterlogging and difficult effluent seepage. Under ground waterlogging, calcium phosphates prevail in the composition of active mineral phosphorus in the gleyed, gleyic, and gley chernozem-like soils of above-floodplain terraces.

Anoxic versus oxic sample pretreatment: Effects on the speciation of sulfur and iron in well-aerated and wetland soils as assessed by X-ray absorption near-edge spectroscopy (XANES)

For a toposequence with increasing groundwater influence (Cambisol, Stagnosol, Histosol) and with different groundwater regimes (Histosols 1 and 2) in a forested watershed in the Fichtelgebirge (Germany), the speciation of sulfur (S) and iron (Fe) in the soils was assessed by X-ray absorption near-edge spectroscopy (XANES) after anoxic and conventional oxic sample pretreatments. For samples with anoxic pretreatment, the contribution of reduced inorganic S compounds (monosulfide, pyrite) to total S increased with soil depth for the Cambisol and the Stagnosol, but decreased for the Histosols; the opposite trend was noticed for the contribution of reduced organic S (organic mono- and disulfides, thiols). The contribution of reduced S to the soil S pool increased and the contribution of oxidized S compounds decreased in the sequence Cambisol–Stagnosol–Histosol 1 (permanently anoxic). Histosol 2 (seasonally oxic) showed a markedly larger contribution of oxidized and intermediate S compounds to total S than Histosol 1. The dominating Fe-bearing phases in the Cambisol were Fe(III) oxyhydroxides; the contribution of sulfide-bound Fe was < 5% of total Fe in all horizons. In Histosol 1, the contribution of sulfide-bound Fe increased with soil depth up to 50% in the Cr horizon, whereas in Histosol 2 Fe(III) phases strongly dominated in all horizons. After conventional oxic sample pretreatment, the contribution of reduced inorganic S to total S was markedly decreased in all soils. In the organic surface horizons, the contribution of reduced organic S was increased to the same extent; the contribution of oxidized S (sulfate) remained more or less unchanged. In the mineral soil, the contribution of sulfate and the mean oxidation state of sulfur (MOS) were strongly increased after oxic sample preparation. In Histosol 1, oxic sample pretreatment resulted in oxidation of labile Fe(II) compounds, probably sulfides or Fe(II)–S-org-complexes, to Fe(III). Our study shows that for anoxic wetland soils which contain inorganic sulfide and/or divalent Fe, the exclusion of O 2 during the entire period between sampling and analysis is crucial for a correct S and Fe speciation. Only after appropriate sample preparation, clear relationships between the mean oxidation states of S and Fe (MOFe) on one hand and soil hydrological conditions on the other become evident: a concomitant systematic decrease of MOS and MOFe from the well-aerated Cambisol to the permanently anoxic Histosol 1, and larger MOS and MOFe in the seasonally oxic Histosol 2 than in Histosol 1 indicate a close coupling of S and Fe cycling in the soils. Finally, the results of our study suggest that in organic horizons of wetland soils inorganic sulfide S is overestimated and reduced organic S is underestimated by S K-edge XANES, if a significant portion of the thiol groups in reduced organic S is complex-bound to Fe 2+ or other chalcophilic metal cations. This is supported by the observation that synthetic organic compounds (cysteine; 1,3,5-trimer-captotriazine [TMT]; ferredoxin) after addition of Fe show spectra with pre-edge peaks at energies < 2472 eV that are typical for inorganic sulfide.

Episodic discharge of lead, copper and antimony from a Norwegian small arm shooting range

Small arm shooting ranges are major deposits of lead (Pb), copper (Cu), antimony (Sb), and zinc (Zn) from use of ammunition. Metals and metalloids from the ammunition residues may leach into the soil and surrounding watercourses and may pose a threat to exposed wildlife and humans. Discharge of elements is dependent on several factors such as soil properties, hydrological conditions, precipitation and time. In order to identify strategies to limit spread of ammunition residues from shooting ranges it is of importance to gain knowledge of the mobility of the elements from the spent bullets. Here we have monitored the levels of Pb, Cu and Sb in a small drainage stream from a Norwegian military small arm shooting range in 2001 and 2006. The first campaign in 2001 was initiated ahead of the snow-melting period in the spring and continued two months in order to quantify discharge and discharge patterns of the selected elements in the drainage stream. Two shorter campaigns were performed in autumn 2001 during an excavation, and in summer 2006. Mean levels of Pb, Cu and Sb in the stream during the first monitoring period in 2001 were 14 microg/l, 39 microg/l and 9 microg/l respectively. High flow, following precipitation, led to an approximately fourfold increase in the concentration of Pb and threefold increase in the level of Cu and Sb compared to low discharge concentrations. An estimation of discharge of the metals showed that the snow melting period and precipitation events constituted to a large proportion of the total release. A sudden increase in the levels can induce more stress and reduce survival of exposed aquatic animals due to the short time available for adaptation.

Soil hydrological properties and conditions, site preparation, and the long-term performance of planted Scots pine (&lt;em&gt;Pinus sylvestris&lt;/em&gt; L.) on upland forest sites in Finnish Lapland

Dissertation. University of Helsinki, Faculty of Agriculture and Forestry, Department of Forest Ecology. 2009

Estimation of global SCS curve numbers using satellite remote sensing and geospatial data

The Soil Conservation Service Curve Number (SCS CN) method is an efficient and widely used method for determining the direct runoff (effective rainfall) from a storm event for flood disaster assessment (rainfall‐runoff modelling). The CN can be estimated based on the area's hydrologic soil group (HSG), land use/cover, and hydrologic condition. The two former factors are of greater importance in determining the CN value. This study reports an attempt to derive a global CN map. First, HSG was classified from digital soil maps. Second, CN was estimated as a function of HSG, land‐cover classification, and hydrologic conditions according to USDA (1986) and NEH‐4 (1997) standard lookup tables. Potential applications of this CN map may include real‐time global flood assessment by incorporating an operational multisatellite precipitation estimation system (e.g. http://trmm.gsfc.nasa.gov).

The effect of toposequence position on soil properties, hydrology, and yield of rainfed lowland rice in Southeast Asia

A large proportion of rainfed lowland rice in Southeast Asia is grown in gently sloping areas along toposequences with differences in elevation of a few meters. These small differences in elevation can lead to differentiation in soil properties and hydrological conditions, which in turn may affect crop performance and yield. It may be appropriate to replace blanket crop management recommendations in rainfed areas with toposequence-specific management recommendations. However, thorough statistical analyses of the relationships between toposequence position and field and crop conditions are lacking. In this paper, we statistically analysed the effect of toposequence position on soil properties, hydrological conditions, yield, and yield increase due to weed control and/or fertilizer management in rainfed areas in four villages in Indonesia and Thailand each in 2000–2002. Differences were substantial in field hydrology (average depth of ponded surface water and of groundwater, number of days without ponded surface water), exchangeable K, organic C, and clay content depending on toposequence position. There were also differences in other soil properties, including N, P, CEC, pH, sand, silt, bulk density, yield, and the magnitude of yield increase due to intensive weed control and/or recommended fertilizer application, but these effects were not consistent across countries, seasons, and years. The hypothesis that toposequence position would be a useful recommendation domain for weed control and fertilizer recommendations was not supported by our statistical results. The reasons why toposequence position has an inconsistent statistical effect could be (1) that the variability of the field conditions is larger among villages than among toposequence positions, and/or (2) that farmers already respond to differences in field conditions in their prevalent management practices, thus masking the effects of toposequence-specific variation on yield. Our findings suggest that despite the large toposequence effects on soil nutrient and water availability, weed and fertilizer management recommendations should be field-specific and time-specific rather than toposequence-specific.

(89) Abiotic Factors Possibly Contributing to Reset Failure of Citrus Trees in a Commercial Grove in Florida

Reset/replant problems and/or failure are recognized as a problem in fruit tree production throughout the world. Problems in establishing or reestablishing fruit trees have been documented in apple, peach, grape, and citrus. Research to date has not identified a single casual agent responsible for reset failure, but rather there is evidence that such problems are related to a complex interaction of biotic and abiotic factors. In this study, we compared abiotic factors related to soil texture and soil nutrients in healthy and reset failure areas of a commercial citrus grove in Florida. A significant difference in soil texture was found in healthy sand, silt, and clay (94%, 4%, and 2%, respectively) vs. failed (99% sand and &lt;1% silt and clay) reset areas in the citrus grove block. Mehlich III extractions were performed on soil samples and levels of Ca, Cu, Fe, K, Mg, Mn, P, and Zn in the extract solution were significantly higher in healthy compared to failed reset areas of the citrus grove block. These data are consistent with what has been observed in other fruit crops where soil texture, which directly relates to soil hydrologic conditions, and soil nutrient levels have been identified as contributing factors related reset/replant failure in fruit tree crops.

Compound-specific δ13C and δ2H analyses of plant and soil organic matter: A preliminary assessment of the effects of vegetation change on ecosystem hydrology

Here we present δ 13C and δ 2H data of long-chained, even-numbered (C 27–C 31) n-alkanes from C3 (trees) and C4 (grasses) plants and from the corresponding soils from a grassland–woodland vegetation sequence in central Queensland, Australia. Our data show that δ 13C values of the C4 grassland species were heavier relative to those of C3 tree species from the woodland ( Acacia leaves) and woody grassland ( Atalaya leaves). However, n-alkanes from the C4 grasses had lighter δ 2H values relative to the Acacia leaves, but showed no significant difference in δ 2H values when compared with C3 Atalaya leaves. These results differ from those of previous studies, showing that C4 grasses had heavier δ 2H values relative to C3 grasses and trees. Those observations have been explained by C4 plants accessing the more evaporation-influenced and isotopically heavier surface water and tree roots sourcing deeper, isotopically lighter soil water (“Two-layered soil–water system”). By comparison, our data suggest that ecosystem changes (vegetation “thickening”) can significantly alter the soil hydrological characteristics. This is shown by the heavier δ 2H values in the woodland soil compared with lighter δ 2H values in the grassland soil, implying that the recent vegetation change (increased tree biomass) in the woodland had altered soil hydrological conditions. Estimated δ 2H values of the source-water for vegetation in the grassland and woodland showed that both trees and grasses in open settings accessed water with lighter δ 2H values (avg. −46‰) compared with water accessed by trees in the woodland vegetation (avg. −7‰). These data suggest that in semi-arid environments the “two-layer” soil water concept might not apply. Furthermore, our data indicate that compound-specific δ 2H and δ 13C analyses of n-alkanes from soil organic matter can be used to successfully differentiate between water sources of different vegetation types (grasses versus trees) in natural ecosystems.

Interpretation of digital soil photographs using spatial analysis: II. Application

Colour variation in digital photographs of soils was quantified and classified meaningfully, using ArcView Spatial Analyst. This procedure holds great promise for the unbiased and quantitative determination of soil colour properties and statistical correlation with related soil hydrological conditions. The analysis of digital photographs from ten profiles in the Weatherley catchment indicated that diagnostic grey and yellow-brown colours were dominant in all the profiles described for this study. The occurrence of grey increased with depth, at the expense of yellow-brown. This pointed to a wetter subsoil water regime. Diagnostic red dominated in the Bloemdal profile. The occurrence of diagnostic red in profiles of the Longlands, Tukulu, Kroonstad and Katspruit soil forms was associated with the occurrence of Fe oxide mottles. Black classification was used mainly for the elimination of shaded areas from the photographs.

Phosphorus release and retention by soils of natural isolated wetlands

Hydrological restoration of historically isolated wetlands may mitigate phosphorus (P) loss. The objectives of this study were to quantify P in soil, and to determine the effect of (1) soil characteristics on P release, and (2) antecedent soil hydrological conditions on P dynamics. Humic/fulvic acid bound P and residual P accounted for majority of P (>78%) in surface soils. Soils with highest nutrient status and labile P fractions released most P during initial flooding. Phosphorus dynamics during additional flooding were dependent on soil characteristics, antecedent soil hydrological conditions, and P levels in the water. Phosphorus retention varied between 0.3 and 8 mg m-2 d-1.

Spatially distributed storm runoff depth estimation using Landsat images and GIS

The use of geographic information systems (GISs) and remote sensing to facilitate the estimation of runoff from watershed and agricultural fields has gained increasing attention in recent years. This is mainly due to the fact that rainfall-runoff models include both spatial and geomorphologic variations. The US Department of Agriculture, Natural Resources Conservation Service Curve Number (USDA-NRCS-CN) method was used in this study for determining the runoff depth. Runoff curve number was determined based on the factors of hydrologic soil group, land use, land treatment, and hydrologic conditions. GIS and remote sensing were used to provide quantitative measurements of drainage basin morphology for input into runoff models so as to estimate runoff response. The study was conducted on the S-65A sub-basin of the Kissimmee River basin in south Florida. Land use from Landsat images for 1980, 1990 and 2000 were considered in the study. The process of determining spatially distributed runoff curve numbers from Landsat images is presented in this study using GIS and image processing software. Spatially distributed runoff curve numbers and runoff depth were determined for the watershed for different land use classes. Results of the study show that land use changes determined from Landsat images are useful in studying the runoff response of the basin. It is shown that the S-65A sub-basin has undergone land use and runoff response changes over the 20 years period of time. The area covered by water and wetlands in 2000 is higher than in 1980 and 1990. In 2000 areas having CN of greater than 90 accounted for 3% compared to 0.9 and 0.6% in 1980 and 1990 respectively. This was due to the increase in wetlands and water covered areas attributed to the Kissimmee River restoration work, which started in 1997 and aimed at restoring lost wetlands and floodplains.