Water chemistry and nutrient budgets in an undisturbed evergreen rainforest of southern Chile

In a pristine evergreen rainforest of Nothofagus betuloides, located at the Cordillera de los Andes in southern Chile (41 °S), concentrations and fluxes of nutrients in bulk precipitation, cloud water, throughfall water, stemflow water, soil infiltration and percolation water and runoff water were measured. The main objectives of this study were to investigate canopy-soil-atmosphere interactions and to calculate input-output budgets. From May 1999 till April 2000, the experimental watershed received 8121 mm water (86% incident precipitation, 14% cloud water), of which the canopy intercepted 16%. Runoff water volume amounted 9527 mm. Bulk deposition of inorganic (DIN) and organic (DON) nitrogen amounted 3.6 kg ha^–1 year^–1 and 8.2 kg ha^–1 year^–1 respectively. Occult deposition (clouds + fog) contributes for 40% to the atmospheric nitrogen input (bulk + occult deposition) of the forest. An important part of the atmospheric ammonium deposition is retained within the canopy or converted to nitrate or organic nitrogen by epiphytic bacteria or lichens. Also the export of inorganic (0.9 kg ha^–1 year^–1) and organic (5.2 kg ha^–1 year^–1) nitrogen via runoff is lower than the input to the forest floor via throughfall and stemflow water (3.2 kg DIN ha–1 year^–1 and 5.6 kg DON ha^–1 year^–1). The low concentrations of NO ₃ ^– and NH ₄ ⁺ under the rooting depth suggest an effective biological immobilization by vegetation and soil microflora. Dry deposition and foliar leaching of base cations (K⁺, Ca²⁺, Mg²⁺) was estimated using a canopy budget model. Bulk deposition accounted for about 50% of the total atmospheric input. Calculated dry and occult deposition are both of equal value (about 25%). Foliar leaching of K⁺, Ca²⁺, and Mg²⁺ accounted for 45%, 38% and 6% of throughfall deposition respectively. On an annual basis, the experimental watershed was a net source for Na⁺, Ca²⁺ and Mg²⁺.     

Water-use efficiency of one C3 and two C4 grasses in response to varying soil moisture and herbage-removal levels in a seasonally dry tropical region

In a seasonally dry tropical region the water use efficiency (WUE) of three grasses (C₃ winter annualPolypogon monspeliensis, C₄ perennialDichanthium annulatum and C₄ warm seasonal annualEchinochloa colonum) was evaluated during summer and winter under nine experimental conditions (3 soil moisture×3 herbage removal). Generally leaf water status and transpiration rate decreased with soil moisture stress and increased with clipping intensity. During winter the transpiration rate of Dichanthium was much lower than that of Polypogon and its own rate in summer. Both soil moisture stress and clipping intensity increased the WUE in all instances. Despite differences in photosynthetic type, growing season and life form, these grasses exhibited broadly similar positive relationships, across nine treatments for WUE: soil moisture stress, and water consumption: production. The range of WUE (g. mm^–1) calculated on TNP through the nine treatments was: summer—Dichanthium 2.9–10.0, Echinochloa 2.0–6.7; winter—Dichanthium 4.3–36.3, Polypogon 1.9–12.0.     

Nitrogen fixation and metabolism by groundwater-dependent perennial plants in a hyperarid desert

The Central Asian Taklamakan desert is characterized by a hyperarid climate with less than 50 mm annual precipitation but a permanent shallow groundwater table. The perched groundwater (2–16 m) could present a reliable and constant source of nitrogen throughout the growing season and help overcome temporal nitrogen limitations that are common in arid environments. We investigated the importance of groundwater and nitrogen fixation in the nitrogen metabolism of desert plants by assessing the possible forms and availability of soil N and atmospheric N and the seasonal variation in concentration as well as isotopic composition of plant N. Water availability was experimentally modified in the desert foreland through simulated flooding to estimate the contribution of surface water and temporally increased soil moisture for nutrient uptake and plant–water relations. The natural vegetation of the Taklamakan desert is dominated by plants with high foliar nitrogen concentrations (2–3% DM) and leaf nitrate reductase activity (NRA) (0.2–1 mol NO₂^– g^–1 FW h^–1). There is little evidence that nitrogen is a limiting resource as all perennial plants exhibited fast rates of growth. The extremely dry soil conditions preclude all but minor contributions of soil N to total plant N so that groundwater is suggested as the dominant source of N with concentrations of 100 M NO₃^–. Flood irrigation had little beneficial effect on nitrogen metabolism and growth, further confirming the dependence on groundwater. Nitrogen fixation was determined by the ¹⁵N natural abundance method and was a significant component of the N-requirement of the legume Alhagi, the average contribution of biologically fixed nitrogen in Alhagi was 54.8%. But nitrogen fixing plants had little ecological advantage owing to the more or less constant supply of N available from groundwater. From our data we conclude that the perennial species investigated have adapted to the environmental conditions through development of root systems that access groundwater to satisfy demands for both water and nutrients. This is an ecologically favourable strategy since only groundwater is a predictable and stable resource.     

Change in water quality during the passage through a tropical montane rain forest in Ecuador

We studied five 20-m transects onthe lower slope under tropical lower montanerain forest at 1900–2200 m above sea level. We collectedsamples of soil and of weekly rainfall,throughfall, litter leachate, and stream waterbetween 14 March 1998 and 30 April 1999 anddetermined the concentrations of Al, totalorganic C (TOC), Ca, Cl^–, Cu, K, Mg, Mn,NH₄ ⁺-N, NO₃ ^–-N, total N (TN), Na, P, S, and Zn. The soils were shallowInceptisols; pH ranged 4.4–6.3 in the Ohorizons and 3.9–5.3 in the A horizons, totalCa (6.3–19.3 mg kg^–1) and Mgconcentrations (1.4–5.4) in the O horizon weresignificantly different between the transects.Annual rainfall was 2193 mm; throughfall variedbetween 43 and 91% of rainfall, cloud waterinputs were 3.3 mm a^–1 except forone transect (203). The volume-weighted mean pHwas 5.3 in rainfall and 6.1–6.7 in throughfall.The median of the pH of litter leachate andstream water was 4.8–6.8 and 6.8, respectively.The concentrations of Ca and Mg in litterleachate and throughfall correlatedsignificantly with those in the soil (r =0.76–0.95). Element concentrations inthroughfall were larger than in rainfallbecause of leaching from the leaves (Al, TOC,Ca, K, Mg), particulate dry deposition (TOC,Cu, Cl^–, NH₄ ⁺-N), and gaseousdry deposition (NO₃ ^–-N, total N, S).Net throughfall (= throughfall-rainfalldeposition) was positive for most elementsexcept for Mn, Na, and Zn. High-flow eventswere associated with elevated Al, TOC, Cu, Mn,and Zn concentrations.     

Carbon, nitrogen and phosphorus dynamics in creek water of a southeast Asian mangrove forest

Water column dynamics of carbon, nitrogen and phosphorus in the main creek of the Bangrong mangrove forest, Phuket Island, Thailand, were examined during the dry season. Water sampled from the upper and lower reaches of the creek throughout entire neap and spring tide periods was incubated under saturated irradiation and in the dark. The activity of microbial primary producers and heterotrophs were evaluated from changes in O₂, TCO₂, DOC, DIN, DON and PO₄ ^3- concentrations. Gross primary production was moderate (1.6–2.4 M C h^–1) with no pronounced spatial and temporal variations. A large fraction of the assimilated inorganic carbon and nitrogen was released in the form of DOC (50–90%) and DON (50–60%) indicating that primary producers were under stress or nutrient depleted. PO₄ ^3- and occasionally DIN appeared to be the limiting nutrients. The pelagic heterotrophic community was supported by DOC (50–100%) and DON (40–90%) excreted by primary producers when exposed to light. However, rapid light attenuation in the turbid creek water rendered the entire water column strongly heterotrophic at all times (1.1–2.8 M C h^–1). The microbial heterotrophs are therefore largely supported by particulate and dissolved substrates derived from tidal resuspension, mangrove root exudates and leachates from fallen leaves on the forest floor. The relatively high concentrations of metabolites (e.g. CO₂, NH₄ ⁺) in creek water were primarily supplied by microbial mineralization. Water seeping from creek banks, which was only enriched in TCO₂ (2 times) and PO₄ ^3- (20–100 times) compared with creek water, is not considered an important source of solutes in the waterways of the Bangrong mangrove forest. Although the results obtained here are only strictly representative for the dry season, water column processes in the wet season are not expected to differ much due to the limited seasonal variations with respect to physical, chemical and biological parameters in the Bangrong area.     

Nitrogen transformations and nitrous oxide flux in a tropical deciduous forest in México

Summary Emissions of nitrous oxide and soil nitrogen pools and transformations were measured over an annual cycle in two forests and one pasture in tropical deciduous forest near Chamela, México. Nitrous oxide flux was moderately high (0.5–2.5 ng cm^–2 h^–1) during the wet season and low (<0.3 ng cm^–2 h^–1) during the dry season. Annual emissions of nitrogen as nitrous oxide were calculated to be 0.5–0.7 kg ha^–1 y^–1, with no substantial difference between the forests and pasture. Wetting of dry soil caused a large but short-lived pulse of N₂O flux that accounted for <2% of annual flux. Variation in soil water through the season was the primary controlling factor for pool sizes of ammonium and nitrate, nitrogen transformations, and N₂O flux.     

Root distribution in relation to soil nitrogen availability in field-grown tomatoes

Tomato root growth and distribution were related to inorganic nitrogen (N) availability and turnover to determine 1) if roots were located in soil zones where N supply was highest, and 2) whether roots effectively depleted soil N so that losses of inorganic N were minimized. Tomatoes were direct-seeded in an unfertilized field in Central California. A trench profile/monolith sampling method was used. Concentrations of nitrate (NO₃ ^-) exceeded those of ammonium (NH₄ ⁺) several fold, and differences were greater at the soil surface (0–15 cm) than at lower depths (45–60 cm or 90–120 cm). Ammonium and NO₃ ^- levels peaked in April before planting, as did mineralizable N and nitrification potential. Soon afterwards, NO₃ ^- concentrations decreased, especially in the lower part of the profile, most likely as a result of leaching after application of irrigation water. Nitrogen pool sizes and rates of microbial processes declined gradually through the summer.Tomato plants utilized only a small percentage of the inorganic N available in the large volume of soil explored by their deep root systems; maximum daily uptake was approximately 3% of the soil pool. Root distribution, except for the zone around the taproot, was uniformly sparse (ca. 0.15 mg dry wt g^-1 soil or 0.5 cm g^-1 soil) throughout the soil profile regardless of depth, distance from the plant stem, or distance from the irrigation furrow. It bore no relation to N availability. Poor root development, especially in the N-rich top layer of soil, could explain low fertilizer N use by tomatoes.     

Temporal dynamics of soil spatial heterogeneity in sagebrush-wheatgrass steppe during a growing season

Variability in five soil resources essential for plant growth (NH₄ ⁺, NO₃ ^-, P, K and soil moisture) was quantified using univariate, multivariate and geostatistical techniques in a sagebrush-grass steppe ecosystem at three times (early April, June and August) during the 1994 growing season. Samples were collected every meter in a 10 × 10-m ‘macrogrid’, every 20 cm within nested 1 × 1-m ‘minigrids’, and every 3 cm within additionally nested 15 × 15-cm ‘microgrids’. Strong autocorrelation for all variables in the three sample periods was only found over distances less than 2 m, indicating that patches of high internal uniformity in this soil were smaller than 2 m during the growing season. Differences in semivariograms between sample periods were most pronounced for NO₃ ^-, NH₄ ⁺ and soil moisture, variables that we consider to primarily limit plant growth in this system. The distance over which sample points were autocorrelated for NO₃ ^-, NH₄ ⁺ and soil moisture increased from April to June. In contrast P and K, which are relatively more abundant at the study site, exhibited relatively constant semivariance patterns over the three sample periods. Weak correlation was found between samples collected in the three sample periods for N and soil moisture indicating that the spatial pattern of these limiting resources changed between sample periods. However, P and K had highly significant correlations (p<0.00001) among sample periods, indicating that the distributional patterns of these relatively more abundant resources remained rather constant. There were strong negative correlations between P and K and distance from the base of shrubs for all sample times (p<0.001), indicating an increase in P and K close to shrubs. Similar strong negative correlations were not found between distance from the shrubs and levels of NH₄ ⁺, NO₃ ^-, or soil moisture, nor for any soil variable and distance from perennial tussock grasses. Changes in patterns of nutrient and soil moisture variability within a growing season suggest that not only must plants acquire soil resources that vary in time and space, but that they may also have to adjust to different scales of resource patchiness during the season.     

Simulation study of nutrient uptake by plants from soilless cultures as affected by salinity buildup and transpiration

Soilless plant growth systems are widely used as a means to save irrigation water and to reduce groundwater contamination. While nutrient concentrations in the growth medium are depleted due to uptake by the plants, salinity and toxic substances accumulate due to transpiration. A theoretical model is suggested, to simulate nutrient uptake by plants grown in soilless cultures with recycled solutions. The model accounts for salinity accumulation with time and plant growth, and its effects on uptake of the different nutrients by means of interaction with Na and Cl ions. The sink term occurs due to uptake by a growing root system. Influx as a function of the ion concentration is according to Michaelis–Menten active mechanisms for K⁺, NO₃ ^–-N, NH₄ ⁺-N, PO₄-P, Ca²⁺, Mg²⁺ and SO₄ ^2-, whose influx parameters are affected by Na and Cl^–, but not with time (age). Sodium influx is passive above a critical concentration. Sum of cations–anions concentrations is balanced by Cl^– to maintain electro-neutrality of the growth solution. Salinity (by means of Na concentration) suppresses root and leaf growth, which further effect uptake and transpiration. The model accounts for instantaneous transpiration losses, during daytime only and its effect on uptake of nutrients and plant development due to salt accumulation. The model was tested against NO₃ ^– and K⁺ uptake by plants associated with cumulative transpiration and with different NaCl salinity levels. Deviations from observed K⁺ uptake should be attributed to the salinity tolerance of the plants. In a study with data obtained from published literature, the model indicated that nutrient depletion and salinity buildup might be completely different with fully grown-up plants (that do not grow) and plants that grow with time. Depletion of different nutrients are according to their initial concentration and plant uptake rate, but also affected by their interactions with Na and Cl ions.     

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)     

Composition and deposition of throughfall in a flooded forest archipelago

The sources of spatial and temporal variation and rates of nutrient deposition via throughfall were studied for 9 months in the Anavilhanas archipelago of the Negro River, Brazil. A total of 30 events was sampled individually for rain and throughfall chemistry in a 1-ha plot of flooded forest. Throughfall samples were collected in 40 collectors distributed in five parallel transects in the study plot, while rain was collected in 4 collectors in an adjacent channel. Volume-weighted mean (VWM) concentrations of solutes in rain were consistently lower than in throughfall, except for H⁺, NO ₃ ^– and NH ₄ ⁺ . Ratios of VWM concentrations of rain to throughfall indicated that K⁺, followed by Mg²⁺ and PO ₄ ^3– , were the most enhanced solutes as rain passed through the forest canopy. The deposition of solutes varied significantly among transects, except for Na⁺ and Ca²⁺, and was significantly correlated with maximum flooding depth, foliar nutrient content, soil fertility and canopy closure for most solutes. The concentrations of PO ₄ ^3– and most major ions were higher in throughfall compared to those in rain due to canopy exchange and dry deposition. In contrast, NO ₃ ^– , NH ₄ ⁺ and H⁺ were retained due to immobilization by leafy canopy and ion exchange processes. Solute inputs via throughfall (not including stemflow) to a floodplain lake (Lake Prato) of the archipelago accounted for 30 to 64% of the total for most solutes in the lake at high water, which indicates that throughfall is an important source of nutrients to the aquatic ecosystem of the Anavilhanas archipelago.     

The seasonal dynamics of amino acids and other nutrients in Alaskan Arctic tundra soils

Past research strongly indicates the importance of amino acids in the N economy of the Arctic tundra, but little is known about the seasonal dynamics of amino acids in tundra soils. We repeatedly sampled soils from tussock, shrub, and wet sedge tundra communities in the summers of 2000 and 2001 and extracted them with water (H₂O) and potassium sulfate (K₂SO₄) to determine the seasonal dynamics of soil amino acids, ammonium (NH₄⁺), nitrate (NO₃^–), dissolved organic nitrogen (DON), dissolved organic carbon (DOC), and phosphate (PO₄^2–). In the H₂O extractions mean concentrations of total free amino acids (TFAA) were higher than NH₄⁺ in all soils but shrub. TFAA and NH₄⁺ were highest in wet sedge and tussock soils and lowest in shrub soil. The most predominant amino acids were alanine, arginine, glycine, serine, and threonine. None of the highest amino acids were significantly different than NH₄⁺ in any soil but shrub, in which NH₄⁺ was significantly higher than all of the highest individual amino acids. Mean NO₃^– concentrations were not significantly different from mean TFAA and NH₄⁺ concentrations in any soil but tussock, where NO₃^– was significantly higher than NH₄⁺. In all soils amino acid and NH₄⁺ concentrations dropped to barely detectable levels in the middle of July, suggesting intense competition for N at the height of the growing season. In all soils but tussock, amino acid and NH₄⁺ concentrations rebounded in August as the end of the Arctic growing season approached and plant N demand decreased. This pattern suggests that low N concentrations in tundra soils at the height of the growing season are likely the result of an increase in soil N uptake associated with the peak in plant growth, either directly by roots or indirectly by microbes fueled by increased root C inputs in mid-July. As N availability decreased in July, PO₄^2– concentrations in the K₂SO₄ extractions increased dramatically in all soils but shrub, where there was a comparable increase in PO₄^2– later in the growing season. Previous research suggests that these increases in PO₄^2– concentrations are due to the mineralization of organic phosphorus by phosphatase enzymes associated with soil microbes and plant roots, and that they may have been caused by an increase in organic P availability.     

Fluxes and transformations of nitrogen in a high-elevation catchment,Sierra Nevada

The fluxes and transformations of nitrogen (N) were investigated from 1985 through 1987 at the Emerald Lake watershed (ELW), a 120 ha high-elevation catchment located in the southern Sierra Nevada, California, USA. Up to 90% of annual wet deposition of N was stored in the seasonal snowpack; NO ₃ ^– and NH ₄ ⁺ were released from storage in the form of an ionic pulse, where the first fraction of meltwater draining from the snowpack had concentrations of NO ₃ ^– and NH ₄ ⁺ as high as 28 eq L^–1 compared to bulk concentrations of <5 eq L^–1 in the snowpack. The soil reservoir of organic N (81 keq ha^–1) was about ten times the N storage in litter and biomass (12 keq ha^–1). Assimilation of N by vegetation was balanced by the release of N from soil mineralization, nitrification, and litter decay. Mineralization and nitrification processes produced 1.1 keq ha^–1 yr^–1 of inorganic N, about 3 1/2 times the loading of N from wet and dry deposition. Less than 1% of the NH ₄ ⁺ in wet and dry deposition was exported from the basin as NH ₄ ⁺ . Biological assimilation was primarily responsible for retention of NH ₄ ⁺ in the basin, releasing one mode of H⁺ for every mole of NH ₄ ⁺ retained and neutralizing about 25% of the annual acid neutralizing capacity produced by mineral weathering in the basin. Nitrate concentrations in stream waters reached an annual peak during the first part of snowmelt runoff, with maximum concentrations in stream water of 20 eq L^–1, more than 4 times the volume-weighted mean annual concentrations of NO ₃ ^– in wet deposition. This annual peak in stream water NO ₃ ^– was consistent with the release of NO ₃ ^– from the snowpack in the form of an ionic pulse; however soil processes occurring underneath the winter snowpack were another potential source of this NO ₃ ^– . Concentrations of stream water NO ₃ ^– during the summer growing season were always near or below detection limits (0.5 eq L^–1).     

Foliar and root absorption of Na+ and Cl− in maize and barley: Implications for salt tolerance screening and the use of saline sprinkler irrigation

Above-canopy sprinkler irrigation with saline water favours the absorption of salts by wetted leaves and this can cause a yield reduction additional to that which occurs in salt-affected soils. Outdoor pot experiments with both sprinkler and drip irrigation systems were conducted to determine foliar ion accumulation and performance of maize and barley plants exposed to four treatments: nonsaline control (C), salt applied only to the soil (S), salt applied only to the foliage (F) and salt applied to both the soil and to the foliage (F+S). The EC of the saline solution employed for maize in 1993 was 4.2 dS m^–1 (30 mM NaCl and 2.8 mM CaCl₂) and for barley in 1994, 9.6 dS m^–1 (47 mM NaCl and 23.5 mM CaCl₂). The soil surface of all pots was covered so that in the F treatment the soil was not salinized by the saline sprinkling and drip irrigation supplied nutrients in either fresh (treatments C and F) or saline water (treatments S and F+S).Saline sprinkling increased leaf sap Na⁺ concentrations much more than did soil salinity, especially in maize, even though the saline sprinkling was given only two or three times per week for 30 min, whereas the roots of plants grown in saline soil were continuously exposed to salinity. By contrast, leaf sap Cl^– concentrations were increased similarly by saline sprinkling and soil salinity in maize, and more by saline sprinkling than saline soil in barley. It is concluded that barley leaves, and to a greater extent maize leaves, lack the ability to selectively exclude Na⁺ when sprinkler irrigated with saline water. Moreover, maize leaves selectively absorbed Na⁺ over Cl^– whereas barley leaves showed no selectivity. When foliar and root absorption processes were operating together (F+S treatment) maize and barley leaves accumulated 11–14% less Na⁺ and Cl^– than the sum of individual absorption processes (treatment F plus treatment S) indicating a slight interaction between the absorption processes. Vegetative biomass at maturity and cumulative plant water use were significantly reduced by saline sprinkling. In maize, reductions in biomass and plant water use relative to the control were of similar magnitude for plants exposed only to saline sprinkling, or only to soil salinity; whereas in barley, saline sprinkling was more detrimental than was soil salinity. We suggest that crops that are salt tolerant because they possess root systems which efficiently restrict Na⁺ and Cl^– transport to the shoot, may not exhibit the same tolerance in sprinkler systems which wet the foliage with saline water. ei]T J Flowers     

Solution chemistry profiles of mixed-conifer forests before and after fire

Solution chemistry profiles of mixed-conifer forests in granitic catchments of the Sierra Nevada were measured for three years before (1987–1990) and three years after (1990–1993) prescribed fire. Wet deposition, throughfall and soil solution samplers were installed in both white-fir and giant-sequoia dominated forest stands underlain by poorly developed inceptisols. Stream water chemistry was monitored as part of an ongoing study of catchment outputs. Calcium, NO ₃ ^– and Cl^– were the major ions in precipitation. Canopy leaching increased mean concentrations of all major ions, especially K⁺ and Ca²⁺. Water flux through the soil occurred largely during spring snowmelt. Forest floor leachate represented the most concentrated solutions of major ions. Interaction with the mineral soil decreased mean concentrations of most species and the average composition of soil solutions closely resembled stream water at baseflow. Bicarbonate alkalinity, Ca²⁺, Mg²⁺, and Na⁺ were enriched in stream water relative to precipitation whereas inputs of H⁺, NH ₄ ⁺ , NO ₃ ^– and SO ₄ ^2– were retained within the catchments.Burning of the forest understory and litter layer increased solute concentrations in soil solution and stream water. Mean soil solution Ca²⁺, Mg²⁺ and K⁺ concentrations increased more than 10 fold, but the relative predominance of these cations was not affected by burning. Sulfate concentration, which was very low in soil solutions of undisturbed stands (<25 mmol_c m^–3), increased more than 100 times following fire. Ammonium concentration exhibited a rapid, short-term increase and then a decrease below pre-burn levels. Changes in soil solution chemistry were reflected in catchment outputs.Corresponding author.     

Brief pre- and post-irrigation sprinkling with fresh water reduces foliar salt uptake in maize and barley sprinkler irrigated with saline water

Brief pre- and post-irrigation sprinkling treatments using freshwater were tested to determine if these practices could reduce the uptake of salts through leaves when saline water is used to sprinkler irrigate crops. Maize and barley were sprinkler irrigated 2 to 3 times per week for 30 min with saline water (4.2 dS m^–1, 30 mmol L^–1 NaCl and 2.8 mmoles L^–1 CaCl₂ for maize and 9.6 dS m^–1, 47 mmoles L^–1 NaCl and 23.5 mmoles L^–1 CaCl₂ for barley) in separate experiments with plants grown in pots outdoors. The soil surface of all pots was covered to prevent salinization of the soil by the sprinkling water. One half of the sprinkled plants was grown in nonsaline soil to study the effects of pre-wetting and post-washing when ion uptake was primarily through leaves. The other half of the sprinkled plants was grown in soil salinized by drip irrigation, in order to evaluate the effects of pre-wetting and post-washing when Na⁺ and Cl^- uptake was through both leaves and roots.Post-washing with freshwater (5 min) reduced the leaf sap concentrations of Cl^- in saline-sprinkled plants from 56 to 43 mmol L^–1 in maize and from 358 to 225 mmol L^–1 in barley (averages for plants grown in nonsaline and saline soil). Na⁺ concentrations in leaf sap were reduced from 93 to 65 mmoles L^–1 (maize) and from 177 to 97 mmoles L^–1 (barley) by the post-washing. Pre-wetting had a small effect on ion uptake through leaves, the only significant reduction in seasonal means being in leaf Na⁺ concentrations for plants grown in nonsaline soil. Pre-wetting and post-washing, when combined, reduced leaf Cl^- concentrations to levels similar to those of nonsprinkled plants grown in saline soil; however, Na⁺ concentrations in leaves remained 3.5 times (maize) and 1.5 times (barley) higher than those of nonsprinkled plants. When pre-wetting and post-washing were not applied, sprinkled barley plants grown in saline soil had grain yields which were 58% lower than nonsprinkled plants grown in saline soil, but the reduction in grain yield was only 17% when the freshwater treatments were given. We conclude that a brief period of post-washing with freshwater is essential when saline water is employed in sprinkler irrigation. By comparison, the benefits from pre-wetting were small in these experiments. ei]T J Flowers     

Resource limitations to nitric oxide emissions from a sagebrush-steppe ecosystem

We monitored soil emissions of NO, NO₂, N₂O, and CO₂ throughout the summer dry season at a remote North American sagebrush-steppe ecosystem following application of several resources, including water, NH ₄ ⁺ , NO ₃ ^– and sucrose. Despite low levels of soil NH ₄ ^– (5.60±0.95 mg NH ₄ ^– -N per kg soil, mean ± S.E.), and NO ₃ ^– -N (1.34±0.20 mg NO ₃ ^– -N per kg soil), NO emissions ranged from about 0.2 to 2.8 ng NO-N m^–2 s^–1, comparable to rates measured from many agricultural, tropical, and other undisturbed ecosystems. Soil wetting increased NO emissions as much as 400-fold when initial gravimetric soil moisture contents were less than about 50 mg kg _soil ^–1 and soil temperature was greater than or equal to 20 °C. Wetting treatments with 20 mg NH ₄ ⁺ -N kg _soil ^–1 raised NO emission rates to a level that was nearly an order of magnitude higher than that observed after water addition alone. Wetting treatments with 20 mg NO ₃ ^– -N kg _soil ^–1 , 240 mg sucrose-C kg _soil ^–1 , or NO ₃ ^– plus sucrose had no statistically significant effect upon NO emissions. Soil denitrifying enzyme activity was low at this site, and N₂O emissions in the field were below detection limits. Soil nitrifying enzyme activity was extremely high at this site, indicating that the NH ₄ ⁺ released by ammonification would be consumed at least once every 1.7 days. These observations indicate that NO emissions from this undisturbed ecosystem were likely a consequence of high nitrification activity, and that sagebrush-steppe ecosystems may be a more important NO source than has been previously assumed.     

Dynamics of water and ion transport driven by corn canopy in the Yellow River basin

Water and ion balance in a corn field in the semi-arid region of the upper Yellow River basin (Inner Mongolia, China) was analyzed with special reference to transpiration stream and selective nutrient uptake driven by the crop canopy. During the crop development stage (June 7 to July 17, 2005), crop transpiration and soil evaporation were evaluated separately on a daily basis, and concentrations of NO ₃ ^? , PO ₄ ^3? , K⁺, Na⁺, Ca²⁺, Mg²⁺ and Cl^? ions in the Yellow River water, irrigation water, ground water, soil of the root zone and xylem sap of the crop were analyzed.The crop transpiration accounted for 83.4% of the evapotranspiration during the crop development stage. All ions except for Na⁺ were highly concentrated in the xylem sap due to the active and selective uptake of nutrients by roots. In particular, extremely high concentrations of the major essential nutrients were found in the nighttime stem exudate, while these concentrations in the river water, the irrigation water, the ground water and the root-zone soil were lower. On the other hand, Na⁺, which is not the essential element for crop growth, was scarcely absorbed by roots and was not highly concentrated in the xylem sap. Consequently, Na⁺ remained in the ground water and the root-zone soil at higher concentrations. These results indicate that during the growing season, crop transpiration but not soil evaporation induces the most significant driving force for mass flow (capillary rise) transporting the ground water toward the rhizosphere, where the dynamics of ion balance largely depends on the active and selective nutrient uptake by roots.     

Atmospheric deposition and sulphur cycling in chalk grasslandA mechanistic model simulating field observations

Sulphate fluxes in bulk deposition, throughfall and soil solution were monitored during two years, and integrated within a model describing the cycling of S in a chalk grassland ecosystem. Throughfall fluxes were strongly determined by interceptive properties of the grassland canopy. Seasonal variation in Leaf Area Index resulted in dry deposition velocities for SO₂ varying between 0.1 cm.s^–1 (snow cover, almost no aerodynamic resistance) to 0.9–1.8 cm.s^–1 in periods with a fully developed canopy. On an annual basis net canopy exchange (assimilation of SO₂ minus foliar leaching) was estimated to be –15% of net throughfall. Simulated soil solution concentrations, being the result of throughfall input, leaching, adsorption, biomass uptake and mineralization, closely fitted actual values (r > 0.92; p > 0.001). Actual and simulated leaching were 1.74 ± 0.03 and 2.00 keq.-ha^–1.yr^–1, respectively. Sulphur budgets for the soil showed net accumulation from April to October and net losses from October to April. Annual budgets for the ecosystem showed atmospheric input (2.02keq.ha^–1.yr^–1) and actual output (2.05keq.ha^–1.yr^–1) to be almost balanced. Apart from increased soil solution concentrations, additional input of sulphate (3.55 keq.ha^–1.yr^–1) to experimental plots resulted in additional accumulation in the ecosystem of 0.62 keq.ha^–1.yr^–1     

Major ions,nutrients and primary productivity in volcanic neotropical streams draining rainforest and pasture catchments at Los Tuxtlas,Veracruz, Mexico

Six streams in the Los Tuxtlas region, a volcanic area in southeastern Mexico, were characterized chemically and biologically. Temperature, pH, conductivity, ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, CaCO^– ₃ and SO^2- ₄), nutrients (NO^– ₃, NH⁺ ₄, total P and PO^–3 ₄), and chlorophyll a from epilithon were measured every other month from September 1996 to July 1997. The streams studied had a consistent pattern of cation dominance (Na⁺>Ca²⁺>Mg²⁺>K⁺), and ionic concentrations varied little during the year of study; nutrients, however, showed strong temporal variability. The ion chemistry of the streams was influenced by bedrock weathering according to the Gibbs Model. The streams are chiefly mesotrophic, but their primary production may be limited by nitrogen based on the N:P ratio. Streams differed in chlorophyll a concentrations and their productivity changed temporally. They were among the most mineral-rich tropical streams, and both their ion concentration levels and cationic patterns coincided with other neotropical volcanic streams. Although there was a pattern in which ion concentrations of the streams were negatively related to the proportion of conserved vegetation and positively related to the proportion of pastures and croplands, the relationships were not statistically significant. We concluded that differences in the major ions of the streams studied were caused by the great heterogeneity in geology and soil types, as well as by geothermal activity in the area. Temporal changes in nutrients were related to biological processes in the streams that influenced primary productivity. Moreover, the influence of land use might be hidden by the strong effect of this heterogeneity on the streams studied.