Water Table Treatments Research Articles

Comparative tolerances of riparian willows and cottonwoods to water-table decline

Cottonwoods (Populus sp.) and willows (Salix sp.) generally dominate riparian landscapes across western North America. To investigate their relative tolerances to water-table decline, rooted shoot cuttings (saplings) of two willows, Salix exigua and S. drummondiana, and two cottonwoods, Populus angustifolia, and P. balsamifera, were grown in rhizopods, controlled growth devices that allow water-table manipulation. Water-table-decline rates of 0 to 12 cm/d were applied and plant growth and survival were monitored. In a second study, seedlings of S. exigua, S. lutea, P. balsamifera, and P. deltoides were grown in rhizopods and exposed to water-table-decline rates of 0 to 8 cm/d. For saplings and seedlings of both genera, gradual declines of 1 or 2 cm/d promoted root elongation and often promoted shoot growth relative to the constant water-table treatment (0 cm/d). More abrupt declines (> 2 cm/d) reduced growth and survival, and there were some differences in sensitivity within as well as across the two genera. Thus, the willow and cottonwood saplings were similarly affected by abrupt water-table decline, but willow seedlings were slightly more vulnerable than cottonwood seedlings. It is recommended that gradual stream stage recession along regulated rivers of about 2.5 cm/d in the late spring would encourage the recruitment of cottonwood seedlings, but subsequently, more gradual recession of about 1 cm/d through the lower elevational streamside zones would enable recruitment of S. exigua and other willows.

Modelling nitrate losses in drainage water using DRAINMOD 5.0

The appropriateness of the DRAINMOD-N computer model for the southwestern Quebec region was assessed by comparing field data and simulated results. Four years of field data were collected from a 4.2 ha field research facility located at St. Emmanuel, Que. The experimental layout contrasted two different fertilizer and water table treatments in a split plot design. Nitrogen fertilizer was applied at rates of 120 and 200 kg N/ha. The water table was either maintained at 0.6 m by a subirrigation (SI) system or was under free drainage (FD) at a drain depth of 1.0 m. The model was calibrated using data from 1996 to 1997. Simulated output for 1998 and 1999 were compared to measured values based on the mean residual error (MRE), coefficient of efficiency ( C eff=1.0 represents a perfect match; C eff≤0 a poor match) and other statistical parameters. Except for FD plots in 1998 (MRE=−0.225 m, C eff=−0.198), water table depths (WTDs) were predicted to 0.17 m (23%) with C eff ranging from 0.035 to 0.846. DRAINMOD correctly predicted the pattern, but underestimated the magnitude of daily drainflow in both years. On a monthly basis total drain flow was underestimated by up to 5 mm in 1998 and 1.2 mm in 1999. Except for the 1998-SI plots ( C eff=−0.021) simulations were excellent ( C eff>0.8). With the exception of two FD-year-N combinations, DRAINMOD overestimated leached nitrogen by 0.01–1.09 kg ha −1 month −1 (0.02–5.01%). Nitrate leaching predictions were poor in 1998 (−0.20> C eff>−7.83), but good to excellent in 1999 (0.95> C eff>0.56). The poor performance in 1998 compared to 1999 is attributed to significant rainfall events occurring soon after surface fertilizer application in 1998, but not in 1999. DRAINMOD largely overpredicted monthly total denitrification. Predictions were poor in 1999 (−0.25> C eff) and very poor in 1998 (−139.7> C eff) and mostly attributable to poor simulation of June and July, the peak months of measured denitrification. DRAINMOD performed well in the prediction of hydrological parameters, but fared less well in terms of nitrate leaching and denitrification.

Productivity responses of Acer rubrum and Taxodium distichum seedlings to elevated CO 2 and flooding

Elevated levels of atmospheric CO 2 are expected to increase photosynthetic rates of C 3 tree species, but it is uncertain whether this will result in an increase in wetland seedling productivity. Separate short-term experiments (12 and 17 weeks) were performed on two wetland tree species, Taxodium distichum and Acer rubrum, to determine if elevated CO 2 would influence the biomass responses of seedlings to flooding. T. distichum were grown in replicate glasshouses ( n=2) at CO 2 concentrations of 350 or 700 ppm, and A. rubrum were grown in growth chambers at CO 2 concentrations of 422 or 722 ppm. Both species were grown from seed. The elevated CO 2 treatment was crossed with two water table treatments, flooded and non-flooded. Elevated CO 2 increased leaf-level photosynthesis, whole-plant photosynthesis, and trunk diameter of T. distichum in both flooding treatments, but did not increase biomass of T. distichum or A. rubrum. Flooding severely reduced biomass, height, and leaf area of both T. distichum and A. rubrum. Our results suggest that the absence of a CO 2-induced increase in growth may have been due to an O 2 limitation on root production even though there was a relatively deep (∼10 cm) aerobic soil surface in the non-flooded treatment.

Production and microtopography of bog bryophytes: response to warming and water-table manipulations.

Boreal peatlands, which contain a large fraction of the world's soil organic carbon pool, may be significantly affected by changes in climate and land use, with attendant feedback to climate through changes in albedo, fluxes of energy or trace gases, and soil carbon storage. The response of peatlands to changing environmental conditions will probably be dictated in part by scale-dependent topographic heterogeneity, which is known to interact with hydrology, vegetation, nutrients, and emissions of trace gases. Because the bryophyte community can contribute the majority of aboveground production in bogs, we investigated how microscale topography affects the response of bryophyte species production and cover to warming (using overhead infrared lamps) and manipulations of water-table height within experimental mesocosms. We removed 27 intact peat monoliths (2.1-m2 surface area, 0.5-0.7m depth) from a bog in northern Minnesota, USA, and subjected them to three warming and three water-table treatments in a fully crossed factorial design. Between 1994 and 1998, we determined annual production of the four dominant bryophyte taxa within three microtopographic zones (low, medium, and high relative to the water table). We also estimated species cover and calculated changes in topography and roughness of the bryophyte surface through time. Total production of all bryophytes, and production of the individual taxa Polytrichum strictum, Sphagnum magellanicum, and Sphagnum Section Acutifolia, were about 100% greater in low microtopographic zones than in high zones, and about 50% greater in low than in medium zones. Production of bryophytes increased along the gradient of increasing water-table heights, but in most years, total production of bryophytes was negatively correlated with height above the set water table only for the wettest water-table treatment. Although bryophyte production was unaffected by the warming treatments, the bryophyte surface flattened in proportion to the degree of warming. These results indicate that production of bryophytes is driven most strongly by the absolute and relative height of the bryophyte surface above the water table. Predicted changes in water-table height commensurate with changes in surface temperature may thus affect both production and superficial topography of bryophyte communities.

Nitrogen leaching from soil lysimeters irrigated with dairy shed effluent and having managed drainage

The leaching of nitrogen (N) from agricultural soils is undesirable for environmental and health reasons. We investigated the effects of adding dairy shed effluent (DSE), irrigated on a weekly basis during the milking season, on the amounts and forms of N leached from large undisturbed soil monolith lysimeters of a Gley Soil over a period of 2 years. Drainage was managed using a weir that maintained the water table at 3 depths: 25 (high), 50 (medium), or 75 (low) cm below the soil surface. The low water table treatment represented the usual situation for the soil when drained. If undrained, it would be usual during wet periods in the field for a perched water table to form on the slowly permeable horizon at 75 cm depth. The total amount of N irrigated onto the lysimeters in the first milking season was equivalent to a total of 511 kg N/ha.year, and up to 33.3 kg N/ha.year leached from the soil. The losses from lysimeters receiving effluent were nearly double those from lysimeters receiving an equivalent amount of water only, when the high and medium water tables were imposed. Adding effluent caused only a small increase (7 kg N/ha) in total N leached in the low drainage treatment. In the second milking season, the effluent-N loading was increased to 1518 kg N/ha.year and the pasture was managed to simulate a ‘cut and carry’ land treatment system. Under these conditions, up to 131.4 kg N/ha.year leached from the soil, which was nearly 100 kg N/ha more than lysimeters receiving only water. The total N leaching losses represented a similar proportion of added N (7% and 9%) for years 1 and 2, respectively. Most of the leached N (80—90%) was in organic N form. The managed drainage treatment in which the water table was nearest the soil surface resulted in less N being leached in the nitrate-N (NO 3 -N) form (<2.5 kg N/ha.year) than the other drainage treatments (6—12 kg N/ha.year); however, it did result in the greatest amount of organic and total N leached (33 and 131 kg N/ha for Year 1 and 2, respectively). The smaller amount of NO 3 -N leached from the high water table treatment is attributed to enhanced denitrification, and the greater amount of organic N is attributed to preferential flow. Although NO 3 -N concentrations in leachate generally remained below World Health Organisation (WHO) standards in all treatments, the large amount of N leached in organic form would suggest that inorganic N should not be the only form of N considered when measuring N leaching losses.

Response of Bog and Fen Plant Communities to Warming and Water-Table Manipulations

Large-scale changes in climate may have unexpected effects on ecosystems, given the importance of climate as a control over almost all ecosystem attributes and internal feedbacks. Changes in plant community productivity or composition, for example, may alter ecosystem resource dynamics, trophic structures, or disturbance regimes, with subsequent positive or negative feedbacks on the plant community. At northern latitudes, where increases in temperature are expected to be greatest but where plant species diversity is relatively low, climatically mediated changes in species composition or abundance will likely have large ecosystem effects. In this study, we investigated effects of infrared loading and manipulations of water-table elevation on net primary productivity of plant species in bog and fen wetland mesocosms between 1994 and 1997. We removed 27 intact soil monoliths (2.1 m2 surface area, 0.5–0.7 m depth) each from a bog and a fen in northern Minnesota to construct a large mesocosm facility that allows for direct manipulation of climatic variables in a replicated experimental design. The treatment design was a fully crossed factorial with three infrared-loading treatments, three water-table treatments, and two ecosystem types (bogs and fens), with three replicates of all treatment combinations. Overhead infrared lamps caused mean monthly soil temperatures to increase by 1.6–4.1°C at 15-cm depth during the growing season (May–October). In 1996, depths to water table averaged −11, −19, and −26 cm in the bog plots, and 0, −10, and −19 cm in the fen plots. Annual aboveground net primary production (ANPP) of bryophyte, forb, graminoid, and shrub life-forms was determined for the dominant species in the mesocosm plots based on species-specific canopy/biomass relationships. Belowground net primary production (BNPP) was estimated using root in-growth cores. Bog and fen communities differed in their response to infrared loading and water-table treatments because of the differential response of life-forms and species characteristic of each community. Along a gradient of increasing water-table elevation, production of bryophytes increased, and production of shrubs decreased in the bog community. Along a similar gradient in the fen community, production of graminoids and forbs increased. Along a gradient of increasing infrared loading in the bog, shrub production increased whereas graminoid production decreased. In the fen, graminoids were most productive at high infrared loading, and forbs were most productive at medium infrared loading. In the bog and fen, BNPP:ANPP ratios increased with warming and drying, indicating shifts in carbon allocation in response to climate change. Further, opposing responses of species and life-forms tended to cancel out the response of production at higher levels of organization, especially in the bog. For example, total net primary productivity in the bog did not differ between water-table treatments because BNPP was greatest in the dry treatment whereas ANPP was greatest in the wet treatment. The differential responses of species, life-forms, and above- and belowground biomass production to the treatments suggest that bog and fen plant communities will change, in different directions and magnitudes, in response to warming and changes in water-table elevation. Further, results of this and complementary research indicate that these peatlands may mediate their energy, carbon, and nutrient budgets through differential responses of the plant communities. Thus, predictions of the response of peatlands to changes in climate should consider differences in plant community structure, as well as biogeochemistry and hydrology, that characterize and differentiate these two ecosystems.

RESPONSE OF BOG AND FEN PLANT COMMUNITIES TO WARMING AND WATER-TABLE MANIPULATIONS

Large-scale changes in climate may have unexpected effects on ecosystems, given the importance of climate as a control over almost all ecosystem attributes and internal feedbacks. Changes in plant community productivity or composition, for example, may alter ecosystem resource dynamics, trophic structures, or disturbance regimes, with subsequent positive or negative feedbacks on the plant community. At northern latitudes, where increases in temperature are expected to be greatest but where plant species diversity is relatively low, climatically mediated changes in species composition or abundance will likely have large ecosystem effects. In this study, we investigated effects of infrared loading and manipulations of water-table elevation on net primary productivity of plant species in bog and fen wetland mesocosms between 1994 and 1997. We removed 27 intact soil monoliths (2.1 m2 surface area, 0.5–0.7 m depth) each from a bog and a fen in northern Minnesota to construct a large mesocosm facility that allows for direct manipulation of climatic variables in a replicated experimental design. The treatment design was a fully crossed factorial with three infrared-loading treatments, three water-table treatments, and two ecosystem types (bogs and fens), with three replicates of all treatment combinations. Overhead infrared lamps caused mean monthly soil temperatures to increase by 1.6–4.1°C at 15-cm depth during the growing season (May–October). In 1996, depths to water table averaged −11, −19, and −26 cm in the bog plots, and 0, −10, and −19 cm in the fen plots. Annual aboveground net primary production (ANPP) of bryophyte, forb, graminoid, and shrub life-forms was determined for the dominant species in the mesocosm plots based on species-specific canopy/biomass relationships. Belowground net primary production (BNPP) was estimated using root in-growth cores. Bog and fen communities differed in their response to infrared loading and water-table treatments because of the differential response of life-forms and species characteristic of each community. Along a gradient of increasing water-table elevation, production of bryophytes increased, and production of shrubs decreased in the bog community. Along a similar gradient in the fen community, production of graminoids and forbs increased. Along a gradient of increasing infrared loading in the bog, shrub production increased whereas graminoid production decreased. In the fen, graminoids were most productive at high infrared loading, and forbs were most productive at medium infrared loading. In the bog and fen, BNPP:ANPP ratios increased with warming and drying, indicating shifts in carbon allocation in response to climate change. Further, opposing responses of species and life-forms tended to cancel out the response of production at higher levels of organization, especially in the bog. For example, total net primary productivity in the bog did not differ between water-table treatments because BNPP was greatest in the dry treatment whereas ANPP was greatest in the wet treatment. The differential responses of species, life-forms, and above- and belowground biomass production to the treatments suggest that bog and fen plant communities will change, in different directions and magnitudes, in response to warming and changes in water-table elevation. Further, results of this and complementary research indicate that these peatlands may mediate their energy, carbon, and nutrient budgets through differential responses of the plant communities. Thus, predictions of the response of peatlands to changes in climate should consider differences in plant community structure, as well as biogeochemistry and hydrology, that characterize and differentiate these two ecosystems.

Influence of water table and nitrogen management on residual soil NO 3− and denitrification rate under corn production in sandy loam soil in Quebec

Nitrate-N (NO 3 −) effluents from agricultural ecosystems contributing to the degradation of water quality has become a serious environmental problem. A field experiment was conducted in 1996 and 1997 at St. Emmanuel, Que., Canada, to investigate the combined effects of water table management (WTM) and N fertilization on soil NO 3 − level and denitrification rates in the top soil layer (0–0.15 m). The field was planted to corn ( Zea mays L.) in both years. Treatments consisted of a factorial combination of two water table treatments, free drainage (FD) at about 1.0 m and subirrigation (SI) at 0.6 m below the soil surface, and two N fertilizer rates, 200 kg ha −1 (N 200) and 120 kg ha −1 (N 120). SI reduced NO 3 − concentration in the top soil layer by 42 and 16% in 1996 and 1997, respectively. Nitrate levels in soil were 50% lower in N 120 plots in 1996, and 20% in 1997 compared to the N 200 plots. Denitrification was higher in SI compared to FD, but not influenced by N rate. As a consequence, WTM practices have implications for both water quality and greenhouse gas emissions. Climatic conditions played a large role in regulating N dynamics in the soil. Due to drier and cooler conditions in 1997, denitrification rates were lower than in 1996, leaving higher residual NO 3 − in the soil profile following corn harvest.

Mobility of Pb inSphagnum-derived peat

One important assumption in applying210Pb-dating is that atmospherically deposited Pb is immobilized in the peat or sediment column. This assumption has been challenged widely, but has never been evaluated experimentally. We evaluated Pb mobility and the chemical forms in which Pb is stabilized in peat profiles by adding either soluble or particulate Pb to intact peat cores that were maintained under different water level regimes (permanently high, permanently low, fluctuating between high and low) and were subjected to simulated precipitation over a five month period. By analyzing the behavior of stable Pb we made inferences about the expected behavior of210Pb. Results indicate that added soluble Pb2+ was retained in the peat through physiochemical binding to organic matter, and as such Pb2+ was largely immobile in peat even under conditions of a fluctuating water table. Added particulate Pb was largely (most likely by physical entrapment), but not completely, immobilized in peat. In none of the water table treatments was there evidence to support mobility of Pb by alternating formation and oxidation of Sulfides, or by any other mechanism. The binding of Pb2+ with organic matter at the peat surface, and the absence of Pb mobility lend credence to210Pb-dating ofSphagnum-dominated peat deposits, which are over 90% organic matter throughout, and have high cation exchange capacities.

Improved Water Quality through Water Table Management in Eastern Canada

A two-year field study was conducted in eastern Ontario to evaluate the effects of water table management on the quality of subsurface drain flows. Drainage discharge volumes and NO3--N concentrations in drainage effluent were monitored during the growing seasons. The 3.5 ha experimental field was strip-cropped with corn (Zea mays L.) and soybean (Glycine max Merr.). Controlled-drainage subirrigation was used to manage water from precipitation and subirrigation. The two controlled water table treatments were 50 cm and 75 cm from the surface. There was also a conventional free-drainage treatment (FD). In 1995, mean NO3--N concentrations in drainage water were reduced by 84% and 75% by the 50 and 75 cm controlled water table depths, respectively. In 1996, drain flow volumes and NO3--N concentrations were significantly reduced relative to free drainage. Total drain flow was reduced by 42% by the 50 cm controlled water table, while NO3--N concentrations were reduced by 61% and 52% by the 50 cm and 75 cm contr...

Water-table changes and nutritional status affect trace gas emissions from laboratory columns of peatland soils

Peatlands potentially play a significant role in the global emission of three trace gases: carbon dioxide; methane; and nitrous oxide. We investigated the effect of a relatively small lowering of the water-table (10 cm) on the emission of these gases using repacked, root-free peat columns in the laboratory with peat from a nutrient-rich (eutrophic) site and a more nutrient-poor (mesotrophic) site, respectively. At a static high water-table (at the peat surface) high rates of anaerobic CO 2 production were found, which were reflected in high (> 1.18) molar ratios of CO 2 and CH 4. A static water-table of 10 cm below soil surface led to equal (eutrophic soil) or lower CO 2 emission (mesotrophic soil) compared with the high static water-table. However, at a regularly changing water-table (between 0 and 10 cm below the peat surface), CO 2 emission at a low water-table was 1.5 (mesotrophic soil) to 3 times (eutrophic soil) higher than at a high-water table. Maximum rates of CO 2 emission from the eutrophic soil exceeded those from the mesotrophic soil, except at a static high water-table. Methane emission was about one order of magnitude lower at a low static water-table compared with the static high water-table. There was no clear effect of the nutritional status of the peat on maximum methane emission in the various water-table treatments. The nutritional status of the peat had a profound influence on the effect of water-table lowering on N 2O emission: nitrous oxide emission from the eutrophic soil was strongly increased at a low water-table, whereas there was no detectable N 2O emission from the mesotrophic soil. Despite a clear effect of the water-table treatments on both trace gas emissions and on redox potentials in the soil, no significant correlation between these variables was found in either soil type.

Water Table Depth and Rainfall Timing Effect on BR − and NO3- Transport

Effect of water table depth and rainfall timing on bromide and nitrate transport was investigated using large undisturbed columns (30 cm in diameter and 90 cm long) obtained from Rossburg silt loam and Clermont silt loam soils. Three water table treatments and two time delays (the time interval between chemical and water applications) were used to simulate a range of saturated conditions in the soil profile and rainfall timing. Water was applied for 60 d at a rate of 970 mL/d, equivalent to a daily rainfall of 13.3 mm, using an automated irrigation system. Discharge from the water table control pipe at each column was sampled for bromide and nitrate concentration. Findings from this study include: (1) water table depth significantly affected bromide and nitrate breakthrough patterns and nitrate recovery; (2) water table depth and time delay interactively affected nitrate transport; (3) soil type was not a significant factor in this case; and (4) bromide and nitrate had similar breakthrough characteristics, yet the relative concentration of nitrate was consistently lower than that of bromide.

Effects of litter accumulation and water table on morphology and productivity of Phragmites australis

Phragmites australis (Common reed) occurs in the interface between water and land. The water depth gradient from deep water to dry land is inversely related to litter accumulation. Eutrophication can result in an excessive production of litter, which may have a large impact on the occurrence of P. australis in this gradient. In an outdoor pot experiment, it was investigated how water tables in combination with substrates containing variable amounts of litter affect morphology and productivity of P. australis. Vegetatively propagated P. australis was grown in pots filled with river sand, litter, and different mixtures of sand and litter (25, 50 and 75% by volume). Four water table treatments were applied; drained (–12 cm), waterlogged (0 cm), flooded (+12 cm), and weekly fluctuating drained and flooded conditions (–12/+12 cm of water relative to substrate level). When drained, no differences between substrate treatments were present. Waterlogging, flooding fluctuating water table treatments caused growth reduction in substrate containing litter. The plants formed short shoots and thin rhizomes. With increasing water table, allocation of dry matter to stems increased at the expense of leaves and rhizomes. At intermediate levels of litter in the substrate, allocation to leaves was lowest. In both instances a lower leaf weight ratio (LWR) was (partly) compensated for by a higher specific leaf area (SLA), resulting in less pronounced differences in leaf area ratio (LAR). Aquatic roots developed when plants were waterlogged or flooded, and increased when litter was present in the substrate. Aquatic roots were formed in the top soil layer when waterlogged. The percentage of aquatic roots increased with increasing amount of litter in the substrate when plants were flooded. It was concluded that the morphological responses of P. australis to litter strongly constrain its ability to maintain itself in deep water when the substrate contains litter. This might one of the explanations for the disappearance of P. australis along the waterward side of littoral zones.

Short-term effects of soil compaction on growth of Pinuscontorta seedlings

A growth chamber experiment was conducted with Pinuscontorta Dougl. ex Loud. var. latifolia Engelm. seedlings grown in soil compacted at 0.1, 2.0, 4.0, 6.0, and 8.0 MPa pressure. Three moisture regimes were applied factorially to compaction levels by watering from above or by maintaining 2- or 10-cm water tables at the base of the 40-cm soil columns. All treatments were grown at either 22:14 °C or 26:18 °C (light:dark) for 13 weeks. Soil compaction increased bulk density, penetrometer resistance, and soil CO2 and ethylene. The presence of water tables resulted in elevated soil gravimetric water content, which rose with increased compaction and resulted in reduced penetrometer resistance and soil O2. Increased compaction was associated with decreased needle lengths, root dry weights, and net photosynthesis and increased rates of shoot respiration. Compaction had a small effect on height growth, with the tallest seedlings occurring at the greatest compaction rate. Shoot concentrations of mineral nutrients also decreased as soil compaction increased. Within the water table treatments, increased gravimetric soil water content was generally paralleled by a rise in the negative effects of compaction on growth, root/shoot dry weight ratios, and shoot mineral nutrient concentrations.

Water Table Management Effects on Denitrification and Nitrous Oxide Evolution

AbstractPrevious research suggested that using controlled drainage to elevate the water table reduces NO3− contamination of surface water by enhancing denitrification. The C2H2 inhibition technique was used to study denitrification and N2O evolution using 56‐cm‐long, undisturbed cores of Cape Fear loam (clayey, mixed, thermic Typic Umbraquult) maintained in the field and subjected to three static water table levels (15, 30, and 45 cm). Results showed that intermittent C2H2 exposure did not (i) affect soil inorganic N distribution between NH4+ and NO3−, (ii) diminish inhibition of N2O reduction during subsequent C2H2 exposure, or (iii) induce C2H2 decomposition. Denitrification from 1 Nov. 1993 through 21 Apr. 1994 (172 d) was 341 kg N ha−1 for the 15‐cm water table treatment, 260 kg N ha−1 for the 30‐cm water table treatment, and 86 kg N ha−1 for the 45‐cm water table treatment. Denitrification was maximum at the lowest monitored zone (36–54 cm) for each water table treatment. Total N2O evolution was 9 kg N ha−1 for the 15‐cm water table treatment, 4 kg N ha−1 for the 30‐cm water table treatment, and 2 kg N ha−1 for the 45‐cm water table treatment. Nitrous oxide evolution was positively correlated with mean soil temperature (10‐cm depth) until low NO3− levels appeared to limit denitrification. Since steady‐state diffusion was not reached, estimates of N2 evolution, using N2O evolution in the presence of C2H2, were underestimated 12‐fold. Evolved N2O‐N represented only 2% of denitrification in the soil core (0–54 cm) for each water table treatment. Drainage control to elevate the water table enhanced denitrification and N2O evolution, reducing the potential for N transport with subsurface drainage to surface water.

Aldicarb Transport in Drained Coastal Plain Soil

The pesticide aldicarb is extremely soluble in water, which causes it to be mobile in ground water. A field study was conducted to monitor the fate of aldicarb in a poorly drained soil in the North Carolina coastal plain. The research site consisted of three experimental plots with three water-table management treatments: conventional drainage, controlled drainage, and subirrigation. Surface and subsurface drainage rates were measured continuously and water-table elevations were monitored in each plot. A total of 651 soil and water samples were collected over a six-month period. Aldicarb degraded to nontoxic compounds with a half-life of approximately 7 days. The maximum aldicarb loss through drainage outflow and surface runoff was 0.02% and 0.05% of total applied aldicarb, respectively.

Effects of Controlled Water Tables on Yield and Marketability of Field Tomatoes

In a 2-year study, tomato plants (Lycopersicon esculentum Mill., cv. New Yorker), grown in a sandy loam in field lysimeters, were subjected to controlled water table treatments (CWTT) of 0.3, 0.6, 0.8, and 1.0 m from the soil surface, factorially combined, in a central composite design, with 5 K–Ca fertilization combinations, replicated four times. Final shoot mass, no. fruit/plant (yield), percent marketability, fruit dimensions, and the incidence and severity of catfacing and sunscald were measured. In the first, drier year, the two higher, better-irrigated CWTT (0.3 and 0.6 m) gave higher yields, larger fruit, and higher final shoot mass, whereas in the second, wetter year, the two lower, better-drained CWTT (0.8 and 1.0 m) were best. In the dry year, the best-yielding CWTT showed the poorest marketability, while, in the wet year, the best-yielding CWTT showed the best marketability. Severity of catfacing and sunscald did not show consistent variations with CWTT. Fertilizer effects were generally not significant. Results will be discussed in the context of the use of subirrigation in field tomato production.

Effects of Water Table on Physiological Traits and Yield of Soybean. I. Effects of water table and rainfall on leaf chlorophyll content, root growth and yield.

The objective of this study was to obtain physiological information for the increasing yield of soybeans at drained paddy fields by controlling the water table level. We studied the effects of various water table levels using lysimeter (15cm∼100cm constant, fluctuation, no-irrigation) on leaf chlorophyll content, root growth and yield of soybeans grown on alluvial soil, which is a major soil type in the soybean producing area in Japan. Experiments were carried out in 1991 (wet year) and 1992 (dry year) at Tsukuba (1991, 1992) and Fukuyama (1992). Effects of water table on chlorophyll contents varied with leaf position on the main stem. The lower leaves contained more chlorophyll when the plants were grown at lower water table. The upper leaves responded in the same way as lower leaves in wet year, whereas the chlorophyll content was highest at a 40cm depth in dry year. The root length densities in each soil layer were also affected by water table : two peaks of root length density (upper layer and just above the water table) were observed in the 7Ocm-depth treatment, whereas the peak was observed only at the uppermost layer in the 20cm or 40cm depth treated plots. The effects of water table on yield seem to be affected by the amount of rainfall. The highest yield was brought in by 70cm-depth water table treatment in wet year, and by 40cm in dry year. Fluctuation of water table reduced yield. The results indicate that for getting a higher yield of soybeans at drained paddy field, it is important to maintain water table at a suitable level, which should be adjusted according to rainfall.

Influence of water table on carbon dioxide, carbon monoxide, and methane fluxes from Taiga Bog microcosms

Hydrological changes, particularly alterations in water table level, may largely overshadow the more direct effects of global temperature increase upon carbon cycling in arctic and subarctic wetlands. Frozen cores (n=40) of intact soils and vegetation were collected from a bog near Fairbanks, Alaska, and fluxes of CO2, CH4, and CO in response to water table variation were studied under controlled conditions in the Duke University phytotron. Core microcosms thawed to a 20‐cm depth over 30 days under a 20 hour photoperiod with a day/night temperature regime of 20/10°C. After 30 days the water table in 20 microcosms was decreased from the soil surface to −15 cm and maintained at the soil surface in 20 control cores. Outward fluxes of CO2 (9–16 g m−2 d−1) and CO (3–4 mg m−2 d−1) were greatest during early thaw and decreased to near zero for both gases before the water table treatment started. Lower water table tripled CO2 flux to the atmosphere when compared with control cores. Carbon monoxide was emitted at low rates from high water table cores and consumed by low water table cores. Methane fluxes were low (<1 mg m−2 d−1) in all cores during thaw. High water table cores increased CH4 flux to 8‐9 mg m−2 d−1over 70 days and remained high relative to the low water table cores (<0.74 mg m−2 d−1). Although drying of wetland taiga soils may decrease CH4 emissions to the atmosphere, the associated increase in CO2 due to aerobic respiration will likely increase the global warming potential of gas emissions from these soils.

PENGARUH PUPUK NITROGEN DAN TINGGI MUKA AIR TANAH TERHADAP PERTUMBUHAN BINTIL AKAR, PERTUMBUHAN DAN PRODUKSI KEDELAI (Glycine max (L.) Merrill)

Pot experiments and laboratory analysis were conducted to determine the effect of N fertilization and level of water table treatments of wet soybean culture on nodule formation and seed yield of soybean. The pot experiment was conducted in Cibungbulang and laboratorium analysis, was at three different laboratories: Soil, Agronomy, and Pasture, Bogor Agricultural University, started on August, 1993 up to February, 1 994. Three factors to be evaluated, namely varieties (Lokon and Lompobatang), N fertilizer applications of 0, 140, and 280 mg pot -1 (equivalent to 0, 25, and 50 kg N ha -1), and the level of water tables of (control, -15 cm, -10 cm, and -5 cm below the soil surface). They were arranged in randomized complete design. The highest value of nodule dry weight of Lokon variety (0.707 p. was resulted from the plant grown under -15 cm water table without N fertilizer (0 mg N pot- ). Lompobatang highest value (1. 727g) on -10 cm water table and fertilized with 140 mg N pot -1. The increases of nodule dry weight by the treatment were 0.300 g (73.7%) for Lokon and 1. 134 g (194.5%) for Lompobatang, respectively compared to control. Both varieties showed highest value of seed weight per plant if grown under -15 cm water table and fertilized with 140 mg N pot -1. Compared to control media, respective increased seed weight per plant 1.94 g (12.9%) for Lokon and 5. 40 g (25.4%) for Lompobatang.