Plant induced alteration in the rhizosphere and the utilisation of soil heterogeneity

Plant-soil interactions result in a special rhizosphere soil chemistry, differing from that of the bulk soil found only a few mm from the root. The aim of this study was to investigate adaptation mechanisms of herbs growing in acid soils through studying their rhizosphere chemistry in a greenhouse experiment and in a field study. Ten herbs were grown in acid soil (pH 4.2 in the soil solution) in the greenhouse. The concentrations of NO₃ ^-, SO₄ ^2-, phosphates, Ca²⁺, Mg²⁺, Mn²⁺, K⁺, Na⁺, NH₄ ⁺ and pH were analysed in soil solutions obtained by centrifugation. The general pattern found was a depletion of nutrients in the rhizosphere compared with their concentrations in the bulk soil. The pH increase (up to 0.7 units) in the rhizosphere soil appeared to be caused by plant uptake of NO₃ ^- (r²=0.88). The ion concentrations in the soil solution of the rhizosphere were dependent on plant species and biomass increase. Although species with a larger biomass and higher growth rates showed a higher degree of ion depletion (except for Na⁺, SO₄ ^2-) in the rhizosphere, there were also species specific responses. A field study of five herbs at five oak forest sites in Southern Sweden (Scania) was also carried out. In addition to the soil solution concentrations, the loss on ignition (LOI) and the concentrations of 0.1 M BaCl₂ extractable K⁺, Mg²⁺, Mn²⁺, Ca²⁺, and Al ions were measured. The amount of soil solution Al was determined as free ionic (quickly reacting) Al. For all species and sites, the LOI and the concentrations of exchangeable cations were higher in the rhizosphere than in the bulk soil, apparently due to the roots preferably growing at organic-rich microsites. The concentrations of the ions as measured in the centrifuged soil solution, were either higher in the rhizosphere than in the bulk soil or were the same in both, except for NO₃ ^- and quickly reacting Al. The lower concentrations of quickly reacting Al in the rhizosphere, compared with the bulk soil could indicate the uptake of Al by the plant or the exudation of complexing substances. The pH differences were only small and mostly non-significant. Plant-soil interactions and the ability of plants to utilise heterogeneity of the soil appear to be more important for plant growth in acid soils than recognised heretofore. Rhizosphere studies provide an important means of understanding plant strategies in acid soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of elevated CO2 on nutrient cycling in a sweetgum plantation

The effects of elevated CO₂ on nutrient cycling and selected belowground processes in the closed-canopy sweetgum plantation were assessed as part of a free-air CO₂ enrichment (FACE) experiment at Oak Ridge, Tennessee. We hypothesized that nitrogen (N) constraints to growth response to elevated CO₂ would be mitigated primarily by reduced tissue concentrations (resulting in increased biomass production per unit uptake) rather than increased uptake. Conversely, we hypothesized that the constraints of other nutrients to growth response to elevated CO₂ would be mitigated primarily by increased uptake because of adequate soil supplies. The first hypothesis was not supported: although elevated CO₂ caused reduced foliar N concentrations, it also resulted in increased uptake and requirement of N, primarily because of greater root turnover. The additional N uptake with elevated CO₂ constituted between 10 and 40% of the estimated soil mineralizeable N pool. The second hypothesis was largely supported: elevated CO₂ had no significant effects on tissue concentrations of P, K, Ca, or Mg and caused significantly increased uptake and requirement of K, Ca, and Mg. Soil exchangeable pools of these nutrients are large and should pose no constraint to continued growth responses. Elevated CO₂ also caused increased microbial biomass, reduced N leaching and increased P leaching from O horizons (measured by resin lysimeters), reduced soil solution NH ₄ ⁺ , SO ₄ ^2– , and Ca²⁺ concentrations, and increased soil solution pH. There were no statistically significant treatment effects on soil nutrient availability as measured by resin capsules, resin stakes, or in situ incubations. Despite significantly lower litterfall N concentrations in the elevated CO₂ treatment, there were no significant treatment effects on translocation or forest floor biomass or nutrient contents. There were also no significant treatment effects on the rate of decomposition of fine roots. In general, the effects of elevated CO₂ on nutrient cycling in this study were not large; future constraints on growth responses imposed by N limitations will depend on changes in N demand, atmospheric N deposition, and soil mineralization rates.     

Hillslope Nutrient Dynamics Following Upland Riparian Vegetation Disturbance

We investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic carbon (DOC), K⁺, Na⁺, Ca²⁺, Mg²⁺, NO₃ ⁻-N, NH₄ ⁺-N, PO₄ ³⁻-P, and SO₄ ²⁻ were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations of NO₃ ⁻-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO₃ ⁻-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO₃ ⁻-N increased four fold, and streamwater NO₃ ⁻-N doubled. Significant changes also occurred in soilwater pH, DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope. Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient uptake by canopy trees is a key control on NO₃ ⁻-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes significant nutrient loss to streams. Received 30 January 2001; accepted 25 July 2002.     

Alleviation of cadmium toxicity on maize seedlings by calcium

The rate of germination, radicle and plumule length, fresh and dry mass of maize seedlings were increased as Ca²⁺ was added to the nutrient solution, which contained different levels of Cd²⁺, especially at low concentration of Ca²⁺ (5 mM) and high concentrations of Cd²⁺ (1.4 and 1.8 mM). The biosynthesis of pigments, respiration rate and content of soluble saccharides in endosperm were reduced sharply as the concentration of Cd²⁺ in the medium increased. This effects was alleviated by Ca²⁺ addition. Cd²⁺ content in seedlings was increased as the Cd²⁺ concentration in medium was increased and decreased sharply as Ca²⁺ was present in the culture medium. The study suggests liming of soil with CaCO₃ to improve the yield of many crops.     

Effect of soil salinity, soil drought, and their combined action on the biochemical characteristics of cotton roots

Stress caused by soil salinity and soil drought limits cotton productivity in China. To determine the tolerance levels of cotton, we assessed the effects of soil salinity and soil drought on the biochemical characteristics of the roots of two cotton cultivars (CCRI-44, salt-tolerant; Sumian 12, salt-sensitive). Specifically, we analyzed root biomass, fatty acid composition, antioxidative enzyme activity, lipid peroxidation, H⁺-ATPase and Ca²⁺-ATPase activities. The cotton root biomass of the two cultivars declined significantly under conditions of soil salinity, soil drought, and the two stressors combined. The antioxidant enzyme activity of the roots also decreased markedly, which caused lipid peroxidation to increase, and changed the composition of the fatty acid membrane. H⁺-ATPase, Ca²⁺-ATPase and antioxidant enzyme activity decreased more under the two stressors combined. However, H₂O₂ content and O₂ ^? generation increased under the two stressors combined, compared to each stressor separately. Overall, the combination of soil salinity and drought has a greater inhibitory effect and more harmful impact on root growth than each stressor separately. The higher tolerance of CCRI-44 to soil salinity and drought stress than Sumian 12 might be explained by differences in cotton root antioxidative enzyme activity. The lipid peroxidation levels of cotton roots might represent an important biochemical trait for stress tolerance.     

Growth,net photosynthesis and leaf nutrient status of <Emphasis Type="Italic">Fagus crenata</Emphasis> seedlings grown in brown forest soil acidified with H<Subscript>2</Subscript>SO<Subscript>4</Subscript> or HNO<Subscript>3</Subscript> solution

To obtain basic information for evaluating critical loads of acid deposition for protecting Japanese beech forests, growth, net photosynthesis and leaf nutrient status of Fagus crenata seedlings grown for two growing seasons in brown forest soil acidified with H₂SO₄ or HNO₃ solution were investigated. The whole-plant dry mass of the seedlings grown in the soil acidified by the addition of H₂SO₄ or HNO₃ solution was significantly less than that of the seedlings grown in the control soil not supplemented with H⁺ as H₂SO₄ or HNO₃ solution. However, the degrees of reduction in the whole-plant dry mass and net photosynthetic rate of the seedlings grown in the soil acidified by the addition of H⁺ as H₂SO₄ solution at 100 mg l^–1 on the basis of air-dried soil volume (S-100 treatment) were greater than those of the seedlings grown in the soil acidified by the addition of H⁺ as HNO₃ solution at 100 mg l^–1 (N-100 treatment). The concentrations of Al and Mn in the leaves of the seedlings grown in the S-100 treatment were significantly higher than those in the N-100 treatment. A positive correlation was obtained between the molar ratio of (Ca+Mg+K)/(Al+Mn) in the soil solution and the relative whole-plant dry mass of the seedlings grown in the acidified soils to that of the seedlings grown in the control soil. Based on the results, we concluded that the negative effects of soil acidification due to sulfate deposition are greater than those of soil acidification due to nitrate deposition on growth, net photosynthesis and leaf nutrient status of F. crenata, and that the molar ratio of (Ca+Mg+K)/(Al+Mn) in soil solution is a suitable soil parameter for evaluating critical loads of acid deposition in efforts to protect F. crenata forests in Japan.     

Calcium Additions and Microbial Nitrogen Cycle Processes in a Northern Hardwood Forest

Evaluating, and possibly ameliorating, the effects of base cation depletion in forest soils caused by acid deposition is an important topic in the northeastern United States. We added 850 kg Ca ha⁻¹ as wollastonite (CaSiO₃) to an 11.8-ha watershed at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA, in fall 1999 to replace calcium (Ca) leached from the ecosystem by acid deposition over the past 6 decades. Soil microbial biomass carbon (C) and nitrogen (N) concentrations, gross and potential net N mineralization and nitrification rates, soil solution and stream chemistry, soil:atmosphere trace gas (CO₂, N₂O, CH₄) fluxes, and foliar N concentrations have been monitored in the treated watershed and in reference areas at the HBEF before and since the Ca addition. We expected that rates of microbial C and N cycle processes would increase in response to the treatment. By 2000, soil pH was increased by a full unit in the Oie soil horizon, and by 2002 it was increased by nearly 0.5 units in the Oa soil horizon. However, there were declines in the N content of the microbial biomass, potential net and gross N mineralization rates, and soil inorganic N pools in the Oie horizon of the treated watershed. Stream, soil solution, and foliar concentrations of N showed no response to treatment. The lack of stimulation of N cycling by Ca addition suggests that microbes may not be stimulated by increased pH and Ca levels in the naturally acidic soils at the HBEF, or that other factors (for example, phosphorus, or Ca binding of labile organic matter) may constrain the capacity of microbes to respond to increased pH in the treated watershed. Possible fates for the approximately 10 kg N ha⁻¹ decline in microbial and soil inorganic pools include components of the plant community that we did not measure (for example, seedlings, understory shrubs), increased fluxes of N₂ and/or N storage in soil organic matter. These results raise questions about the factors regulating microbial biomass and activity in northern hardwood forests that should be considered in the context of proposals to mitigate the depletion of nutrient cations in soil.     

Effects of CO2 and nitrogen fertilization on vegetation and soil nutrient content in juvenile ponderosa pine

This paper summarizes the data on nutrient uptake and soil responses in opentop chambers planted with ponderosa pine (Pinus ponderosa Laws.) treated with both N and CO₂. Based upon the literature, we hypothesized that 1) elevated CO₂ would cause increased growth and yield of biomass per unit uptake of N even if N is limiting, and 2) elevated CO₂ would cause increased biomass yield per unit uptake of other nutrients only by growth dilution and only if they are non-limiting. Hypothesis 1 was supported only in part: there were greater yields of biomass per unit N uptake in the first two years of growth but not in the third year. Hypothesis 2 was supported in many cases: elevated CO₂ caused growth dilution (decreased concentrations but not decreased uptake) of P, S, and Mg. Effects of elevated CO₂ on K, Ca, and B concentrations were smaller and mostly non-significant. There was no evidence that N responded in a unique manner to elevated CO₂, despite its unique role in rubisco. Simple growth dilution seemed to explain nutrient responses in almost all cases.There were significant declines in soil exchangeable K⁺, Ca²⁺, Mg²⁺ and extractable P over time which were attributed to disturbance effects associated with plowing. The only statistically significant treatment effects on soils were negative effects of elevated CO₂ on mineralizeable N and extractable P, and positive effects of both N fertilization and CO₂ on exchangeable Al³⁺. Soil exchangeable K⁺, Ca²⁺, and Mg²⁺ pools remained much higher than vegetation pools, but extractable P pools were lower than vegetation pools in the third year of growth. There were also large losses of both native soil N and fertilizer N over time. These soil N losses could account for the observed losses in exchangeable K⁺, Ca²⁺, Mg²⁺ if N was nitrified and leached as NO ₃ ^– .     

Increased cucumber salt tolerance by grafting on pumpkin rootstock and after application of calcium

Self-grafted and pumpkin rootstock-grafted cucumber plants were subjected to the following four treatments: 1) aerated nutrient solution alone (control), 2) nutrient solution with 10 mM Ca(NO₃)₂ (Ca), 3) nutrient solution with 90 mM NaCl (NaCl), and 4) nutrient solution with 90 mM NaCl + 10 mM Ca(NO₃)₂ (NaCl+Ca). The NaCl treatment decreased the plant dry mass and content of Ca²⁺ and K⁺ but increased the Na⁺ content in roots and shoots. Smaller changes were observed in pumpkin rootstock-grafted plants. Supplementary Ca(NO₃)₂ ameliorated the negative effects of NaCl on plant dry mass, relative growth rate (RGR), as well as Ca²⁺, K⁺, and Na⁺ content especially for pumpkin rootstock-grafted plants. Supplementary Ca(NO₃)₂ distinctly stimulated the plasma membrane (PM) H⁺-ATPase activity which supplies the energy to remove excess Na⁺ from the cells. The expressions of gene encoding PM H⁺-ATPases (PMA) and gene encoding a PM Na⁺/H⁺ antiporter (SOS1) were up-regulated when Ca(NO₃)₂ was applied. The pumpkin rootstock-grafted plants had higher PM H⁺-ATPase activity as well as higher PMA and SOS1 expressions than the self-grafted plants under NaCl + Ca treatment. Therefore, the addition of Ca²⁺ in combination with pumpkin rootstock grafting is a powerful way to increase cucumber salt tolerance.     

Cu-ions mediated changes in growth,chlorophyll and other ion contents in a Cu-tolerantKoeleria splendens

The effects of Cu²⁺ on growth, chlorophyll and other ion contents ofKoeleria splendens originated from Cu-contaminated soil have been investigated in nutrient solution. The most evident Cu²⁺ effects concern the root growth, especially the root length. Since in plants grown under lower Cu²⁺ concentrations (4 and 8 μM) root elongation, biomass, chlorophyll, Mg²⁺, Fe²⁺, Ca²⁺ and K⁺ content were increased compared with the control, the development of an adaptive mechanism ofK. splendens to Cu²⁺ is suggested. High Cu²⁺ concentration (160 μM) caused a significant reduction in root length and biomass as well as a decreased rate of chlorophyll biosynthesis. The reduction of growth can be correlated with the toxic effect of Cu²⁺ on photosynthesis, root respiration and protein synthesis in roots. 160 μM Cu²⁺-treatment had a negative influence on the concentrations of Ca²⁺, Fe²⁺, Mg²⁺ and K⁺ and a positive influence on the Cu²⁺ concentration in the plant tissues. Loss of nutrients similar to the senescence response suggests that excess of Cu²⁺ leads to the progressive senescence of the plants. Our results demonstrate the existence of an adaptive mechanism ofK. splendens under low Cu²⁺ concentrations, while high Cu²⁺ quantities cause disturbances in plant function.     

Changes in nutrient distribution in forests and soils of Walker Branch watershed,Tennessee, over an eleven-year period

Changes in vegetation, litter, and soil nutrient content were measured in selected plots on Walker Branch watershed, Tennessee, from 1972–73 to 1982. The watershed has been allowed to revert to forest since 1942, before which it consisted of small subsistence farms and woodland pastures. Changes in Ca status were of particular interest because initial nutrient cycling characterizations indicated that net Ca accumulation in vegetation could have caused large decreases in soil exchangeable Ca²⁺ within 20 years.Decreases in forest floor and subsoil (45–60 cm) N, exchangeable Ca²⁺, and Mg²⁺ content were noted in several plots from 1972 to 1982. Surface soils (0–15 cm) showed either no change or, in some cases (e.g., N and exchangeable K⁺ in certain plots), increases over the 11-year period. Reductions in forest floor and subsoil exchangeable Ca²⁺ and exchangeable Mg²⁺ on cherty, upper slope oak-hickory and chestnut oak forests were most striking. The changes in Ca²⁺ are thought to be due primarily to high rates of Ca²⁺ incorporation into woody tissues of oak and hickory species. Reductions in forest floor and subsoil exchangeable Mg²⁺ could not be accounted for by woody increment; leaching may have played a major role in causing these decreases. Changes in P and exchangeable K⁺ were variable, with both increases and decreases.There were significant increases in exchangeable Al³⁺ in both subsoils and surface soils of certain plots, but these were not accompanied by decreases in exchangeable base cations or consistent decreases in pH. Dissolution of interlayer Al from 2:1 clays may be the cause of the exchangeable Al³⁺ increases.These results suggest a general decline in fertility, especially with regard to Ca and Mg in those forests with low soil Ca and Mg supplies. Monitoring of further changes (if any) in these ecosystems will continue as the currently aggrading forests approach steady state.     

Ecophysiological responses of Japanese forest tree species to ozone, simulated acid rain and soil acidification

In this review, I summarized the results obtained from experimental studies on the ecophysiological responses of Japanese forest tree species to O₃, simulated acid rain and soil acidification. Based on the studies conducted in Japan, exposure to ambient levels of O₃ below 100 nl·l⁻¹ (ppb) for several months is sufficient to inhibit dry matter production and net photosynthesis of sensitive Japanese forest tree species such as Siebold's beech and Japanese zelkova. On the other hand, exposure to simulated acid rain with a pH of 4.0 or above for several months cannot induce any adverse effects on dry matter production and physiological functions of Japanese forest tree species. However, when the pH of simulated rain or fog is lowered below 4.0, negative effects appear on dry matter production and physiological functions such as transpiration in several sensitive Japanese forest tree species such as Japanese fir and Nikko fir. Based on limited information, it may be concluded that (1) Al dissolved into soil solution is the most important limiting factor for dry matter production, physiological functions and nutrient status of Japanese forest tree species grown in acidic soil, (2) the (Ca+Mg+K)/Al molar ratio in soil solution is a useful indicator to evaluate and predict the effects of soil acidification due to acid deposition on whole-plant dry matter production of Japanese forest tree species at the present time and in the future, and (3) Japanese coniferous tree species such as Japanese cedar and red pine are relatively sensitive to a reduction in (Ca+Mg+K)/Al molar ratio in soil solution compared with European forest tree species such as Norway spruce.     

凋落物和根系输入对南亚热带季风常绿阔叶林土壤养分的影响

植被凋落物和根系输入在调节森林土壤元素生物地球化学循环中扮演着关键作用。目前仍然不清楚凋落物和根系输入对热带原始林土壤主要元素含量的调控作用。针对该研究现状,以中国南亚热带季风常绿阔叶林为研究对象,通过开展凋落物与根系输入改变的控制试验（6个处理,每处理4次重复：对照、凋落物加倍、凋落物去除、断根、断根+凋落物加倍、断根+去除凋落物）,探讨了凋落物和断根处理对土壤可溶性离子、土壤酸中和能力（ANC）和阳离子交换量（CEC）的短期影响。凋落物与根系处理半年后的结果显示：（1）凋落物去除与加倍处理都显著增加了0-40 cm土壤NO₃^-含量,并且凋落物去除效应大于添加效应;去除凋落物增加了表层土壤（0-20 cm） Ca²⁺、Mg²⁺、Na⁺的含量。（2）断根处理显著增加0-40 cm土壤NO₃^-和表层土壤Ca²⁺、Mg²⁺含量。（3）断根和去除凋落物交互处理显著增加了0-40 cm土壤NO₃^-和表层土壤Ca²⁺、Mg²⁺、K⁺含量,产生了叠加效应。（4）凋落物和断根处理并没有改变土壤pH,但降低了土壤酸中和能力（除凋落物加倍外）,其降低的原因主要与阳离子交换量的降低和NO₃^-含量的增加有关。这些结果表明,土壤养分离子的可利用性（尤其是NO₃^-和Ca²⁺、Mg²⁺）和酸缓冲能力对凋落物和根系输入改变响应敏感,森林植物及其凋落物对土壤养分保留和缓冲性能具有重要调节作用。在人为干扰和气候变化加剧背景下,该研究可为森林生态系统可持续管理提供重要的理论参考。此外,植被凋落物和根系输入改变引起的长期生态学效应仍值得进一步关注。     

Forest soil solutions: Acid/base chemistry and response to calcite treatment

Soil solution chemistry was investigated at a forested watershed draining into Woods Lake. N.Y. as part of the Experimental Watershed Liming Study (EWLS). The objective of this study was to assess the response of soil water to watershed treatment of calcite (CaCO₃). This material was applied in an effort to mitigate the effects of acidic atmospheric deposition. Soil solutions draining Oa and Bs horizons in reference subcatchments were characterized by low pH and acid neutralizing capacity (ANC) due to elevated concentrations of SO ₄ ^2– , NO ₃ ^– and organic anions relative to the sum of base cation (C_B Ca²⁺, Mg²⁺, Na⁺, K⁺) concentrations. Seasonal and spatial variation of pH andANC in soil solutions appeared to belargely controlled by variations in the concentrations of dissolved organic acids which, in turn, were regulated by reactions of Al with soil organic matter. Nitrate was positively correlated and SO²⁺ was negatively correlated with Ca²⁺ and Al concentrations in reference soil solutions, indicating that changes in NO ₃ ^– influences spatial and seasonal variations in Ca²⁺ and Al concentrations. On this basis, NO ₃ ^– appears to be important in soil acidification and the dynamics of drainage water acidity. Comparison of our results with historical data for the site showed declines in concentrations of SO ₄ ^2– , which are consistent with decreases in emissions of SO₄, in the eastern U.S. and atmospheric deposition of SO ₄ ^2– , to the Adirondack region. Mineral soil solutions have shown large increases in concentrations of NO ₃ ^– . Declines in concentrations of C_B and increases in concentrations of Al have occurred over the last ten years, suggesting depletion of soil pools of exchangeable basic cations and increased sensitivity to acidic deposition. Calcite (CaCO₃) treatment of 6.89 Mg/ha resulted in a significant increase of Ca²⁺, ANC and pH in both Oa and Bs horizon soil solutions. Soil water response to CaCO₃ addition was most evident during the first year after treatment, apparently due to macropore transport of particulate and dissolved CaCO₃ However, increases in ANC and pH in the mineral soil waters were not sustained and appeared insufficient to result in substantial improvement in surface water quality over the 43 month study period.     

Simulated effects of atmospheric sulfur deposition on nutrient cycling in a mixed deciduous forest

The effects of three S deposition scenarios — 50% reduction, no change, and 100% increase — on the cycles of N, P, S, K, Ca, and Mg in a mixed deciduous forest at Coweeta, North Carolina, were simulated using the Nutrient Cycling model (NuCM). The purpose of this exercise was to compare NuCM's output to observed soil and streamwater chemical changes and to explore NuCM's response to varying S deposition scenarios. Ecosystem S content and SO₄ ^2– leaching were controlled almost entirely by soil SO₄ ^2– adsorption in the simulations, which was in turn governed by the nature of the Langmuir isotherm set in the model. Both the simulations and the 20-year trends in streamwater SO₄ ^2– concentration suggest that the ecosystem is slowly becoming S saturated. The streamwater data suggest S saturation is occurring at a slower rate than indicated by the simulations, perhaps because of underestimation of organic S retention in the model. Both the simulations and field data indicated substantial declines in exchangeable bases in A and BA soil horizons, primarily due to vegetation uptake. The correspondence of model output with field data in this case was a result of after-the-fact calibration (i.e. setting weathering rates to very low values) rather than prediction, however. Model output suggests that soil exchangeable cation pools change rapidly, undergoing annual cycles and multi-decade fluctuations.Varying S deposition had very little effect upon simulated vegetation growth, nutrient uptake, or N cycling. Varying S deposition strongly affected simulated Ca²⁺. Mg²⁺, K⁺, and P leaching but caused little change in soil exchangeable pools of Ca²⁺ K⁺, or P because soil exchangeable pools were large relative to fluxes. Soil exchangeable Mg²⁺ pools were reduced by high rates of S deposition but remained well above levels sufficient for tree growth. Although the total soil pools of exchangeable Ca²⁺ and K⁺ were only slightly affected by S deposition, there was a redistribution of Ca²⁺ and K⁺ from upper to lower horizons with increasing S deposition, causing increased base saturation in the deepest (BC) horizon. The 100% increased S deposition scenario caused increasing peaks in simulated Al³⁺ concentrations in A horizons after 25 years as a result of large seasonal pulses of SO₄ ^2– and lowered base saturation. Simulated soil solution Al³⁺ concentrations remained well below toxicity thresholds for selected tree species at the site.     

Equilibrium solution composition and exchange properties of disturbed and undisturbed soil samples from an acid forest soil

Microscalic heterogeneity of soil chemical properties caused by soil structure has been reported for several soils. We investigated exchange properties and soil solution composition of disturbed and undisturbed samples of an acid forest soil lacking visible structure. Cation concentrations in the soil solution resulting from two extraction procedures and two analytical methods were compared. The effective cation exchange capacity (CEC_e) of the undisturbed sample represented 56–69% of the bulk soil CEC_e. Base saturation of undisturbed samples equalled that of disturbed samples for EA, Bhs, and Bsh horizons, and was higher for the Bw horizon. Contradicting the results of other authors, soil pore solution obtained by percolating soil cores under conditions of low water tension offered more favourable conditions for plant roots when compared to the equilibrium soil solution of the bulk soil sample in all except the Bsh horizon. Ca²⁺/Al³⁺ molar ratios were higher and fractions of H⁺ + Al³⁺ on total cationic charge were lower in the soil pore solution. These results were obtained employing soil: solution ratios of about 1:0.5 during the extraction of soil pore solution, and by determination of free cations. Other authors used a water extraction with soil:solution ratios up to 1:2 and took total metal for ion concentrations. The combination of the latter extraction and analytical method in our study, too, led to unfavourable Ca²⁺/Al³⁺ ratios and high tractions of H⁺ + Al³⁺. The choice of analytical and extraction method are thus decisive for the valuation of the soil solution composition in view of plant nutrition.     

Involvement of calcium-mediated effects on ROS metabolism in the regulation of growth improvement under salinity

Salinity reduces Ca²⁺ availability, transport, and mobility to growing regions of the plant and supplemental Ca²⁺ is known to reduce salinity damages. This study was undertaken to unravel some of the ameliorative mechanisms of Ca²⁺ on salt stress at the cellular and tissue levels. Zea mays L. plants were grown in nutrient solution containing 1 or 80 mM NaCl with various Ca²⁺ levels. Measurements of growth and physiological parameters, such as ion imbalance, indicated that the Ca²⁺-induced alleviation mechanisms differed between plant organs. Under salinity, H₂O₂ levels increased in the leaf-growing tissue with increasing levels of supplemental Ca²⁺ and reached the levels of control plants, whereas superoxide levels remained low at all Ca²⁺ levels, indicating that Ca²⁺ affected growth by increasing H₂O₂ but not superoxide levels. Salinity completely abolished apoplastic peroxidase activity. Supplemental Ca²⁺ increased its activity only slightly. However, under salinity, polyamine oxidase (PAO) activity was shifted toward the leaf base probably as an adaptive mechanism aimed at restoring normal levels of reactive oxygen species (ROS) at the expansion zone where NADPH oxidase could no longer provide the required ROS for growth. Interestingly, addition of Ca²⁺ shifted the PAO-activity peak back to its original location in addition to its enhancement. The increase in PAO activity in conjunction with low levels of apoplastic peroxidase is supportive of cellular growth via nonenzymatic wall loosening derived by the increase in H₂O₂ and less supportive of the peroxidase-mediated cross-linking of wall material. Thus extracellular Ca²⁺ can modulate ROS levels at specific tissue localization and developmental stages thereby affecting cellular extension.     

Nutrient budgets in a dry heathland watershed in northeastern Spain

Dissolved nutrient inputs in bulk precipitation and outputs in streamwater were measured during 3 years of contrasting hydrological conditions in a 6.3-ha, grazed heathland watershed on schists in the Montseny mountains (NE Spain), drained by an intermittent stream. On average, 39% of the precipitation became streamflow. Bulk precipitation delivered positive net alkalinity (mean 0.22 keq/ha/yr), sulphate input was moderate (9.0 kg SO₄-S/ha/yr), and the mean input of inorganic N was not exceptionally high (6.6 kg/ha/yr). Ion concentrations were relatively low in streamwater; SO₄ ^2- was the dominant anion. Most concentrations in streamwater varied seasonally, with maxima in late summer or early autumn and minima in spring. This pattern probably resulted from increased availability of ions for leaching due to decomposition of organic matter and chemical weathering during the warm period. Nitrate concentrations were relatively high in winter and dropped sharply in early spring, probably because of biological uptake. Annual element outputs in streamwater varied between years and seemed to be controlled by both the amount of annual streamflow and its seasonal distribution. Annual inputs exceeded outputs for dissolved inorganic N. The watershed accumulated H⁺ and Ca²⁺, had net losses of Na⁺ and Mg²⁺, and was close to steady state for K⁺, SO₄ ^2-, Cl^- and alkalinity. The chloride budgets gave no evidence of substantial dry deposition in this system. The cationic denudation rate was negative (-0.14 keq/ha/yr) because Ca²⁺ retention was higher than net exports of Na⁺ and Mg²⁺ from silicate weathering. Low nutrient export and little production of alkalinity suggest that this watershed has a low buffering capacity.     

The effects of acid deposition on the biogeochemical cycles of major nutrients in miniature red spruce ecosystems

As part of an experimental study of air pollution effects on tree growth and health, we combined process studies with an ecosystem approach to evaluate the effects of acidic deposition on soil acidification, nutrient cycling and proton fluxes in miniature red spruce ecosystems. Ninety red spruce saplings were transplanted into 1-m diameter pots containing reconstructed soil profiles and exposed to simulated acid rain treatments of pH 3.1, 4.1 and 5.1 for four consecutive growing seasons. All the principal fluxes of the major elements were measured. During the first year of treatments, the disturbance associated with the transplanting of the experimental trees masked any treatment effects by stimulating N mineralization rates and consequent high N0₃ ⁻ cation, and H⁺ flux through the soil profile. In subsequent years, leaching of base cations and labile Al was accelerated in the most intensive acid treatment and corresponding declines in soil pH and exchangeable pools of Ca and Mg and increases in exchangeable Al concentrations were observed in the organic horizon. Leaching of Ca²⁺ and Mg²⁺ also was significantly higher in the pH 4.1 than in the pH 5.1 treatment. Flux of Ca from foliage and soil was increased in response to strong acid loading and root uptake increased to compensate for foliar Ca losses. In contrast, K cycling was dominated by root uptake and internal cycling and was relatively insensitive to strong acid inputs. Cation leaching induced by acidic deposition was responsible for the majority of H⁺ flux in the pH 3.1 treatment in the organic soil horizon whereas root uptake accounted for most of the H⁺ flux in the pH 4.1 and 5.1 treatments. Although no measurable effects on tree nutrition or health were observed, base cation leaching was significantly accelerated by acidic deposition, even at levels below that observed in the eastern U.S., warranting continued concern about acid deposition effects on the soil base status of forested ecosystems.     

The experimental watershed liming study: Comparison of lake and watershed neutralization strategies

The Experimental Watershed Liming Study (EWLS) was initiated to evaluate the application of CaCO₃ to a forested watershed in an effort to mitigate the acidification of surface water. The objective of the EWLS was to assess the response of the Woods Lake watershed to an experimental addition of CaCO₃. During October 1989, 6.89 Mg CaCO₃/ha was applied by helicopter to two subcatchments comprising about 50% (102.5 ha) of the watershed area. The EWLS involved individual investigations of the response of soil and soil water chemistry, forest and wetland vegetation, soil microbial processes, wetland, stream and lake chemistry, and phytoplankton and fish to the CaCO₃ treatment. In addition, the Integrated Lake/Watershed Acidification (ILWAS) model was applied to the site to evaluate model performance and duration of the treatment. The results of these studies are detailed in this volume. The purposes of this introduction and synthesis paper are to: 1) present the overall design of the EWLS, 2) discuss the linkages between the individual studies that comprise the EWLS, and 3) summarize the response of the lakewater chemistry to watershed addition of CaCO₃ and compare these results to previous studies of direct lake addition. An analysis of lake chemistry revealed the watershed treatment resulted in a gradual change in pH, acid neutralizing capacity (ANC) and Ca²⁺ in the water column. This pattern was in contrast to direct lake additions of CaCO₃ which were characterized by abrupt changes following base addition and subsequent rapid reacidification. Over the three-year study period, the supply of ANC to drainage waters was largely derived from dissolution of CaCO₃ in wetlands. Relatively little dissolution of CaCO₃ occurred in freely draining upland soils. The watershed treatment had only minor effects on forest vegetation. The watershed treatment eliminated the episodic acidification of streamwater and the near-shore region of the lake during snowmelt, a phenomenon that occurred during direct lake treatments. Positive ANC water in the near-shore area may improve chemical conditions for fish reproduction, and allow for the development of a viable fish population. The watershed CaCO₃ treatment also decreased the transport of Al from the watershed to the lake, and increased the concentrations of dissolved organic carbon (DOC) and dissolved silica (H₄SiO₄) in stream and lakewater. The watershed treatment appeared to enhance soil nitrification, increasing concentrations of NO₃ ^– in soilwater and surface waters. However, the acidity associated with this NO₃ ^– release was small compared to the increase in ANC due to CaCO₃ addition and did not alter the acid-base status of Woods Lake. Acid neutralizing capacity (ANC) budgets for 12-month periods before and after the watershed treatment showed that the lake shifted from a large source of ANC to a minor source due to retention of SO₄ ^2–, NO₃ ^–, Al and the elevated inputs of Ca²⁺ associated with the watershed CaCO₃ application. In contrast to the direct lake treatments, Ca²⁺ inputs from the watershed application were largely transported from the lake.