Exchangeable Base Cation Concentrations Research Articles

Evaluating soil acidification risk and its effects on biodiversity–ecosystem multifunctionality relationships in the drylands of China

BackgroundSoil acidification caused by anthropogenic activities may affect soil biochemical cycling, biodiversity, productivity, and multiple ecosystem-related functions in drylands. However, to date, such information is lacking to support this hypothesis. MethodsBased on a transect survey of 78 naturally assembled shrub communities, we calculated acid deposition flux in Northwest China and evaluated its likely ecological effects by testing three alternative hypotheses, namely: niche complementarity, mass ratio, and vegetation quantity hypotheses. Rao's quadratic entropy and community-weighted mean traits were employed to represent the complementary aspect of niche complementarity and mass ratio effects, respectively. ResultsWe observed that in the past four decades, the concentrations of exchangeable base cations in soil in Northwest China have decreased significantly to the extent of having faced the risk of depletion, whereas changes in the calcium carbonate content and pH of soil were not significant. Acid deposition primarily increased the aboveground biomass and shrub density in shrublands but had no significant effect on shrub richness and ecosystem multifunctionality (EMF), indicating that acid deposition had positive but weak ecological effects on dryland ecosystems. Community weighted mean of functional traits (representing the mass ratio hypothesis) correlated negatively with EMF, whereas both Rao's quadratic entropy (representing the niche complementarity hypothesis) and aboveground biomass (representing the vegetation quantity hypothesis) correlated positively but insignificantly with EMF. These biodiversity–EMF relationships highlight the fragility and instability of drylands relative to forest ecosystems. ConclusionsThe findings from this study serve as important reference points to understand the risk of soil acidification in arid regions and its impacts on biodiversity–EMF relationships.

Evaluation of simple and multivariate linear regression models for exchangeable base cation conversion between seven measurement techniques on South African soils

Conversions of exchangeable base cation concentrations between different analytical methods are required for comparisons of soil test results with norm values developed using specific methods. Different methods vary in extraction efficiency due to underlying chemical mechanisms of cation exchange. There is a lack of knowledge pertaining to the combined effects of soil physicochemical properties on the accuracy and parsimony of conversion models. Thus, the aim of this study was to develop equations for conversion of exchangeable base cation (Ca, Mg, K and Na) concentrations between seven different methods of measurement for soils dominated by moderate-activity clays. Additional objectives included the discussion of chemical mechanisms influencing nutrient extraction efficiency, development of root mean square error (RMSE) thresholds for model selection, and evaluation of the effects of soil physicochemical properties on model performance. The ammonium acetate (1 M and 0.2 M, pH = 7), compulsive exchange, Ambic-I, Mehlich-III, Bray-II and Citric acid (1%) soil test methods were performed on 57 soils from a Mediterranean climatic zone. Simple linear regressions (SLR) as well as multivariate linear regressions (MLR) incorporating soil physicochemical properties as parameters were developed. The accuracy of cross-validated models was comparable to accuracy of predictions for samples included in calibration. MLR equations (RMSECV = 1.4–1.6 cmolc kg−1, 0.4 cmolc kg−1, 16–20 mg kg−1 and 25–40 mg kg−1 for Ca, Mg, Na and K, respectively) were generally more accurate than SLR models. Relationships between Ca content extracted by different methods depended on soil alkalinity. Use of the developed SLR or MLR models to convert nutrient quantities between different methods should be chosen based on contextual accuracy requirements.

Long-term response of tropical Andisol properties to conversion from rainforest to agriculture

Short-term changes in tropical rainforest soil properties and their impact on C cycling following land-use conversion to agriculture have received intensive study. However, long-term, land-use changes have not been explored for tropical Andisols, whose high carbon stocks and several distinctive properties may differ in their response to land-use conversion. Thus, the primary objective of this study was to assess changes in selected soil properties of Andisols in response to long-term (>100 years) changes of land use from tropical rainforests to agriculture. Soils were sampled by horizon to a depth of 110–140 cm in pine forests (PF), tea plantation (TP) and horticultural crops with either intensive cultivation (IH) or bare fallow (FH) cropping systems. Selected physical, chemical and biological soil properties were characterized, including microbial biomass carbon (MBC) and laboratory CO2 mineralization rates. Results showed that land-use change from rainforest to agriculture resulted in increased soil bulk density and meso/micropores that contributed to increased plant-available water retention capacity. Soil carbon and nitrogen stocks in the upper 1 m of soil were higher in agricultural soils (25–29 kg C m−2; 1.7 – 2.3 kg N m−2) than pine forest soil (17 kg C m−2; 1 kg N m−2) with a redistribution of organic matter from topsoil to subsoil horizons. Organic matter quality was also affected by land-use conversion with the horticultural soils having lower rates of carbon mineralization per unit soil carbon (PF > TP > IH > FH) and lower microbial biomass, especially in topsoil horizons. The MBC sharply decreased in the topsoil horizon due to land-use change from forest (330 mg kg−1) to agricultural production (<118 mg kg−1). The intensive horticultural soil receiving recent additions of horse manure had higher extractable mineral N (especially NO3), reduced P fixation, increased available P, higher pH, and higher concentrations of exchangeable base cations (Ca2+, Mg2+ and K+) and micronutrients (Zn, Mn and Cu). The Andisols in this study demonstrated strong resilience to long-term degradation of soil properties following conversion from rainforest to agronomic land use. Further, this study demonstrates the ability of these Andisols to sequester additional C upon conversion to selected agriculture practices, thereby providing a positive impact on C mitigation.

Changes in basic soil properties and enzyme activities along an afforestation series on the dry Aral Sea Bed, Kazakhstan

Afforestation of the desiccated Aral Sea Bed is needed for the rapid introduction of vegetation and rehabilitation of the soil environment. The present study aimed to detect the soil amelioration effect by afforestation of the Aral Sea Bed with respect of changes in topsoil properties and enzyme activities. In August and November 2018, soils were sampled from the barren areas and from areas afforested in 1991, 2005, 2009, 2010, and 2013. The exchangeable base cation concentrations (Ca2+, K+, Mg2+, and Na+), cation exchange capacity (CEC), plant-available P concentration, electrical conductivity, pH, and enzyme activities (acid phosphatase, β-glucosidase, and N-acetyl-glucosaminidase) were analyzed in the surface soil (0–10 cm). Base cation concentrations, CEC, and electrical conductivity decreased following afforestation possibly because of root absorption. The observed increase in soil pH could be affected by both root absorption and decomposition of plant residues. Enzyme activities, which are early indicators of soil recovery, might have been increased by afforestation through the release of nutrients from litter and root exudates. Our findings indicate that the establishment of vegetation through afforestation can provide supportive microenvironments for plants and microorganisms by decreasing soil salinity and activating soil microbial enzymes; these effects of afforestation are amplified over time.

Relic DNA is abundant in soil and obscures estimates of soil microbial diversity.

Extracellular DNA from dead microorganisms can persist in soil for weeks to years1-3. Although it is implicitly assumed that the microbial DNA recovered from soil predominantly represents intact cells, it is unclear how extracellular DNA affects molecular analyses of microbial diversity. We examined a wide range of soils using viability PCR based on the photoreactive DNA-intercalating dye propidium monoazide4. We found that, on average, 40% of both prokaryotic and fungal DNA was extracellular or from cells that were no longer intact. Extracellular DNA inflated the observed prokaryotic and fungal richness by up to 55% and caused significant misestimation of taxon relative abundances, including the relative abundances of taxa integral to key ecosystem processes. Extracellular DNA was not found in measurable amounts in all soils; it was more likely to be present in soils with low exchangeable base cation concentrations, and the effect of its removal on microbial community structure was more profound in high-pH soils. Together, these findings imply that this 'relic DNA' remaining in soil after cell death can obscure treatment effects, spatiotemporal patterns and relationships between microbial taxa and environmental conditions.

Assessing forest soil base cation status and availability using lake and stream sediment geochemistry: A case study in Quebec (Canada)

Spatial information about forest soil base cations is important because of its implications for forest health and resilience to disturbance and management.The relevance of using geochemical data from lake and stream sediments collected for mining prospection purposes was evaluated to assess on a relative basis the forest soil base cation status and availability across the Quebec (Canada) forest landbase, an area for which information on soil properties is scarce.The relevance of the acid extraction of the fine fraction (<177μm) method, used to determine element composition of sediments, was tested as an indicator of forest soil fertility and exchangeable base cation status.Values of element concentrations of sediments were then extrapolated spatially from their sampling points to the whole of the Quebec forest landbase, based on their geographical location and on the topographical features of their position using the k-nearest neighbour imputation method. The acid extraction used to determine the geochemical composition of sediments yielded concentrations of Ca, Mg and K that were well correlated with the observed exchangeable base cation concentrations, effective cationic exchange capacity and presence of clay-sized particles of soil samples. Also, spatial imputation of the geochemical signature of sediments to the forest landbase produced values that successfully represented variations in acid-extractable K and Mg in forest soils, as validated with two independent forest soil datasets, but it failed to capture variations in acid-extractable Ca concentrations. Nevertheless, the spatialized values broadly illustrated the gradient of forest soil exchangeable base cation reserves, soil clay content and site richness at a coarse scale across landscapes.

Response of soil chemistry to forest dieback after bark beetle infestation

We evaluated changes in the chemistry of the uppermost soil horizons in an unmanaged spruce forest (National Park Bohemian Forest, Czech Republic) for 3 years after dieback caused by a bark beetle infestation, and compared these changes with a similar undisturbed forest area. The soils below the disturbed forest received 2–6 times more elements via litter fall compared to the unaffected plot. The subsequent decomposition of litter and reduced nutrient uptake by trees resulted in a steep increase in soil concentrations of soluble N (NH4-N, organic-bound N) and P forms in the disturbed plot. The average concentrations of NH4-N and soluble reactive P increased from 0.8 to 4.4 mmol kg−1 and from 0.04 to 0.9 mmol kg−1, respectively, in the uppermost soil horizon. Decomposition of litter at the disturbed plot elevated soil concentrations of Ca2+, Mg2+ and K+, which replaced Al3+ and H+ ions from the soil sorption complex. Consequently, soil concentrations of exchangeable base cations increased from 120 to 200 meq kg−1, while exchangeable Al3+ and H+ decreased 66 and 50 %, respectively, and soil base saturation increased from 40 to 70 %. The Al3+ liberation did not elevate concentrations of ionic Al in the soil solution, because most of the liberated Al3+ was rapidly complexed by dissolved organic carbon (DOC) and transformed to DOC–Al complexes. The chemical parameters investigated at the unaffected plot remained stable during the study.

Soil Depletion of Ca, Mg and K Due to Vicinal Intensive Hog Farming Operation Located in East Mediterranean

One of the main environmental impacts of concentrated animal feeding operations is soil degradation in the vicinity of the livestock breeding facilities due to substances such as ammonia emitted from the various stages of the process. In this research, the soil degradation effects of an intensive hog farming operation (IHFO) located at a Mediterranean limestone soil coastal area have been investigated. Soil samples of the upper mineral soil were taken in various distances and directions from the IHFO boundaries. Thirteen experimental cycles were carried out in the duration of 1.5 years starting in March 2009 until October 2010. The soil samples were analysed on total, exchangeable and water-soluble Ca, Mg and K as well as water-soluble ammonium concentrations. Significantly lower concentrations of the exchangeable and water-soluble base cations were observed on soil samples at increasing proximity downwind from the farm (south). Southern soil average concentrations of exchangeable base cations ranged between 78.6 and 128.52 mmol Ca2+ kg−1 soil, 8.42–21.39 mmol Mg2+ kg−1 soil and 4.25–8.1 mmol K+ kg−1 soil, respectively. Southern soil average concentrations of water-soluble base cations ranged between 0.57 and 2.17 mmol Ca2+ kg−1 soil, 0.16–0.89 mmol Mg2+ kg−1 soil and 0.48–0.95 mmol K+ kg−1 soil, respectively.

Soil fertility and fine root dynamics in response to 4 years of nutrient (N, P, K) fertilization in a lowland tropical moist forest, Panama

AbstractThe question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studiedin situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m−2 year−1), phosphorus (P as superphosphate, 5 g P m−2 year−1) and potassium (K as KCl, 5 g K m−2 year−1) within 38 ha of old‐growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images. We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg−1), low concentrations of Bray‐extractable phosphate (PO4 = 2.2 mg kg−1), and modest concentrations of KCl‐extractable nitrate (NO3 = 5.0 mg kg−1) and KCl‐extractable ammonium (NH4 = 15.5 mg kg−1). Added N increased concentrations of KCl‐extractable NO3and acidified the soil by one pH unit. Added P increased concentrations of Bray‐extractable PO4and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m−2. The turnover coefficient of fine roots &lt;1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species‐rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (&lt;140 days), and control of fine root production by nutrient availability in tropical forests deserves more study.

Spatial heterogeneity of soil chemical properties in a lowland tropical moist forest, Panama

We evaluated spatial heterogeneity for pH and a comprehensive set of nutrient and trace elements in surface (0–0.1 m depth) and subsurface (0.3–0.4 m depth) soils across 26.6 ha of old-growth, lowland, tropical moist forest, established on a highly weathered soil in Panama. Little is known about spatial heterogeneity patterns of soil properties in tropical forest soils. Soil was moderately acidic (pH 5.28) with low concentrations of exchangeable base cations (13.4 cmolc/kg), Bray-extractable PO4 (2.2 mg/kg), KCl-extractable NO3 (5.0 mg/kg), and KCl-extractable NH4 (15.5 mg/kg). The coefficient of variation for soil properties ranged from 24% to &gt;200%, with a median value of 84%. Geostatistical analysis revealed spatial dependence at a scale of 10–100 m for most of the soil properties; however, pH, NH4, Al, and B had spatial dependence at a scale up to 350 m. Best-fit models to individual variograms included random, exponential, spherical, Gaussian, linear, and power functions, indicating many different spatial patterns among the set of soil properties. Correlation among individual elements was poor, indicating independent patterns. Our results show complex spatial patterns in soil chemical properties and provide a basis for future investigations on soil–plant relationships and soil nutrient niche differentiation.

Stream acidification and base cation losses with grassland afforestation

Afforestation of natural grasslands with fast‐growing pine and eucalyptus species is increasing globally, but little is known about its effect on ecosystems and watersheds and, ultimately, the quality of water resources. To investigate the biogeochemical and hydrological consequences of this land use change, we sampled stream water in paired watersheds in Uruguay and Argentina. In watersheds planted with pine, we found no change in stream pH following afforestation, while in watersheds planted with eucalyptus, pH was 0.7 units lower on average than in streams draining grasslands. To further investigate the mechanism behind the decrease in pH, we sampled soils and streams of eucalypt catchments in Uruguay and analyzed exchangeable base cation concentrations, alkalinity, and dissolved inorganic carbon (DIC). At these sites, Ca, Mg, and Na concentrations were &gt;30% lower in afforested soils than in grassland soils, and pH was significantly lower below 10 cm depth. Stream measurements taken over three years illustrate that these soil changes were also manifested in stream water chemistry. In the eucalypt watersheds, base cation concentrations were &gt;40% lower, and alkalinity and DIC were halved in stream water. A test with data from additional sites where both pines and eucalypts were planted nearby showed that eucalyptus has a stronger acidifying effect than pine. Overall, our data suggest that repeated harvesting cycles at some locations could negatively impact the soil store of base cations and reduce downstream water quality. Our results can be used to help minimize negative impacts of this land use and to inform policy in this and other regions targeted for plantation forestry.

Factors controlling soil water and stream water aluminum concentrations after a clearcut in a forested watershed with calcium-poor soils

The 24 ha Dry Creek watershed in the Catskill Mountains of southeastern New York State USA was clearcut during the winter of 1996–1997. The interactions among acidity, nitrate (NO 3 − ), aluminum (Al), and calcium (Ca2+) in streamwater, soil water, and groundwater were evaluated to determine how they affected the speciation, solubility, and concentrations of Al after the harvest. Watershed soils were characterized by low base saturation, high exchangeable Al concentrations, and low exchangeable base cation concentrations prior to the harvest. Mean streamwater NO 3 − concentration was about 20 μmol l−1 for the 3 years before the harvest, increased sharply after the harvest, and peaked at 1,309 μmol l−1 about 5 months after the harvest. Nitrate and inorganic monomeric aluminum (Alim) export increased by 4−fold during the first year after the harvest. Alim mobilization is of concern because it is toxic to some fish species and can inhibit the uptake of Ca2+ by tree roots. Organic complexation appeared to control Al solubility in the O horizon while ion exchange and possibly equilibrium with imogolite appeared to control Al solubility in the B horizon. Alim and NO 3 − concentrations were strongly correlated in B-horizon soil water after the clearcut (r 2 = 0.96), especially at NO 3 − concentrations greater than 100 μmol l−1. Groundwater entering the stream from perennial springs contained high concentrations of base cations and low concentrations of NO 3 − which mixed with acidic, high Alim soil water and decreased the concentration of Alim in streamwater after the harvest. Five years after the harvest soil water NO 3 − concentrations had dropped below preharvest levels as the demand for nitrogen by regenerating vegetation increased, but groundwater NO 3 − concentrations remained elevated because groundwater has a longer residence time. As a result streamwater NO 3 − concentrations had not fallen below preharvest levels, even during the growing season, 5 years after the harvest because of the contribution of groundwater to the stream. Streamwater NO 3 − and Alim concentrations increased more than reported in previous forest harvesting studies and the recovery was slower likely because the watershed has experienced several decades of acid deposition that has depleted initially base-poor soils of exchangeable base cations and caused long-term acidification of the soil.

Acid‐base Characteristics of Soils in the Adirondack Mountains, New York

It is believed that atmospheric deposition of S and N in the Adirondack Mountains of New York has depleted soil‐base cation pools, reduced soil base saturation (BS), and contributed to enhanced acidification of soils and surface waters. However, data to determine changes in soil characteristics are generally lacking. It is expected that soil acid‐base status will improve as acidic deposition declines in response to atmospheric emissions controls. We studied edaphic characteristics at 199 locations within 44 statistically selected Adirondack lake‐watersheds, plus 26 additional watersheds that are included in long‐term lakewater monitoring programs. The statistically selected watersheds were chosen to be representative of Adirondack watersheds containing lakes larger than 1 ha and deeper than 1 m that have lakewater acid neutralizing capacity (ANC) less than or equal to 200 μmolc L−1 Results of soil analyses were extrapolated to the watersheds of 1320 low ANC lakes. In general, the concentrations of exchangeable base cations, base saturation, and soil pH were low. More than 75% of the target lakes received drainage from watersheds having average B horizon exchangeable Ca concentrations &lt; 0.52 cmolc kg−1, base saturation &lt; 10.3%, and pH (H2O) &lt; 4.5. Variations in the effective cation exchange capacity in both O and B horizons were closely correlated with soil organic matter content. These data provide a baseline against which to compare future changes in regional soil chemistry, and provide input data for aquatic and terrestrial effects models intended to project future changes in surface water chemistry, biological conditions, and forest health.

Forest regrowth as the controlling factor of soil nutrient availability 75 years after fire in a deciduous forest of Southern Quebec

The effects of secondary succession on soil nutrient availability following fire in the 1920s was investigated in a hardwood forest of southern Quebec by correlation analyses between soil and solution chemistry, tree species composition, tree growth, litterfall nutrient fluxes and presence of charcoal monitored in six 300 m 2 plots between 1994 and 1998. The results suggests that the pioneer stand of largetooth aspen, paper birch and yellow birch that followed fire enriched the forest floor and upper mineral soil with its K-rich litter, but lowered solution NH4, NO3 and Mg concentrations through its high nutrient demand. High woody biomass primarily observed in the maple-dominated stands was associated with low exchangeable base cation concentrations in the forest floor, suggesting that nutrient immobilisation in trees is also a significant process leading to spatial variation in base cation availability in the forest floor. Finally, charcoal was positively correlated with exchangeable base cations in the forest floor which leads to believe that charcoal has a high affinity for base cations and that it can increase base cation availability decades after forest fire disturbance.

Sulfate, chloride and fluoride retention in Andosols exposed to volcanic acid emissions

The continuous emissions of SO 2, HCl and HF by Masaya volcano, Nicaragua, represent a substantial source of atmospheric S-, Cl- and F-containing acid inputs for local ecosystems. We report on the effects of such acid depositions on the sulfate, chloride and fluoride contents in soils (0–40 cm) from two distinct transects located downwind from the volcano. The first transect corresponds to relatively undifferentiated Vitric Andosols, and the second transect to more weathered Eutric Andosols. These soils are exposed to various rates of volcanogenic acid addition, with the Vitric sites being generally more affected. Prolonged acid inputs have led to a general pH decrease and reduced exchangeable base cation concentrations in the Andosols. The concentrations of 0.5 M NH 4F- and 0.016 M KH 2PO 4-extractable sulfate (NH 4F–S and KH 2PO 4–S, respectively) indicate that volcanic S addition has increased the inorganic sulfate content of the Vitric and Eutric soils at all depths. In this process, the rate of sulfate accumulation is also dependent on soil allophane contents. For all soils, NH 4F extracted systematically more (up to 40 times) sulfate than KH 2PO 4. This difference suggests sulfate incorporation into an aluminum hydroxy sulfate phase, whose contribution to total inorganic sulfate in the Vitric and Eutric Andosols is estimated from ∼34 to 95% and ∼65 to 98%, respectively. The distribution of KH 2PO 4-extractable chloride in the Vitric and Eutric Andosols exposed to volcanic Cl inputs reveals that added chloride readily migrates through the soil profiles. In contrast, reaction of fluoride with Al and Fe oxyhydroxides and allophanes is an important sink mechanism in the Masaya Andosols exposed to airborne volcanic F. Fluoride dominates the anion distribution in all soil horizons, although F is the least concentrated element in the volcanic emissions and depositions. The soil anion distribution reflects preferential retention of fluoride over sulfate and chloride, and of sulfate over chloride. The primary acidifying agent of the Andosols subject to the volcanic acid inputs is HCl.

The effect of lake-water infiltration on the acidity and base cation status of forest soil

In this article, the effects of the artificialrecharging of groundwater by infiltrating surface water throughforest soil, i.e. sprinkling infiltration, on the acidity andbase cation status of the soil are described. The study wascarried out in the Ahvenisto esker area, Hameenlinna,southern Finland, during 1996–1998. The sample plots werelocated in a 110- to 160-yr-old Scots pine (Pinussylvestris L.) stand. The site was classified as the Oxalis-Maianthemum type. The soil consisted of a mixture oftill and glaciofluvial sediments. The pH of the organic layerincreased from 4.7 to >6.5 soon after the start of irrigationon the infiltration plot. The pH of the 0–10 cm mineral soillayer also increased from 4.9 to 6.4 as a result ofinfiltration. Sprinkling infiltration increased theexchangeable Ca and Mg concentrations in the organic anduppermost mineral soil layers. The output of Ca and Mg inpercolation water from the 0–100 cm thick layer was lower thanthe input to the soil surface via irrigation in 1996. Theretention of Ca and Mg on cation exchange sites took placewithin a relatively short period of time, since retention wasobserved only in 1996 but no longer in 1997 or 1998 indicatingsaturation of the cation exchange sites by base cations. Lakewater infiltration leads to the neutralisation of forest soilacidity, and increases the capacity of the soil to withstandacidic inputs by increasing the concentrations of exchangeablebase cations on cation exchange sites in the soil.

Acid-base status of soils in groundwater discharge zones — relation to surface water acidification

Critical load calculations have suggested that groundwater at depth of 2 m in Sweden is very sensitive to acid load. As environmental isotope studies have shown that most of the runoff in streams has passed through the soil, there is a risk in the near future of accelerated acidification of surface waters. To assess the importance of the last soil horizon of contact before discharge, the upper 0–0.2m of soils in seven discharge zones were analysed for pools of base cations, acidity and base saturation. The sites were about 3–4 m 2 in size and selected from two catchments exposed to different levels of acid deposition. The soils in the seven sites had high concentrations of exchangeable base cations and consequently high base saturation. The high correlation ( r 2 = 0.74) between base saturation in the soils of the discharge zones and mean pH of the runoff waters suggested that the discharge zone is important for surface water acidification. The high pool of exchangeable base cations will buffer initially against the acid load. As the cation exchange capacity (meq dm −3) and base saturation were lower in the sites from the catchment receiving lower deposition, these streams may be more vulnerable to acidification in the near future. The high concentration of base cations in non-exchangeable fractions may also buffer against acidification as it is likely that some of these pools will become exchangeable with time.

Simulated effects of atmospheric deposition, harvesting, and species change on nutrient cycling in a loblolly pine forest

The effects of atmospheric deposition (current, 50% sulfur (S) deposition, and 200% nitrogen (N) deposition), harvest, and species change (from loblolly pine ( Pinus taeda L.) to mixed oak) on nutrient cycling were simulated for Duke Forest, North Carolina, using the Nutrient Cycling Model. Based upon the literature, we hypothesized that: (1) changes in S and N deposition would cause only minor changes in leaching rates and soil exchangeable cation pools because of ecosystem buffering of S and N; (2) replacement of loblolly pine by hardwoods would cause depletion of exchangeable Ca 2+, reduced Ca 2+ leaching and increased Mg 2+ leaching because of high rates of Ca uptake by hardwoods; (3) total Ca export via leaching and whole-tree harvesting in the hardwood and loblolly pine stands would be similar because lower Ca removal in harvest would be offset by greater Ca 2+ loss by leaching in the loblolly pine stand. Hypothesis I was supported by the results of the simulations: atmospheric deposition scenarios caused minor (typically < 5%) changes in leaching rates and soil exchangeable cation pools. However, changes in atmospheric deposition did cause large changes in simulated soil solutions and exchangeable cation concentrations in surface horizons, where total pool sizes were relatively small. Thus, although the simulation results suggest that the changes in S and N deposition will have little effect upon total leaching losses and nutrient pool sizes, they do not imply that atmospheric deposition has negligible effects upon exchangeable base cation concentrations. Hypothesis II was supported by the simulation results. At the end of the 20 year simulations, soil exchangeable Ca 2+ and Ca 2+ leaching were greatly reduced by hardwoods. However, Hypothesis III was not supported: cumulative Ca 2+ leaching in the hardwood stand was only slightly (16%) less than in the loblolly stand, and total Ca export via leaching and whole-tree harvesting was considerably greater in the hardwood stand than in the loblolly pine stand. The patterns in Ca and Mg distribution and leaching in the simulations were very similar to the field results from near Oak Ridge, Tennessee. The simulation results suggest that nutrient budget analysis using only a few years of leaching data extrapolated over a full rotation can be very misleading because of changes in leaching rate with time.

A Critical Appraisal of Field Evidence From A Regional Survey for Acid Deposition Effects On Scottish Moorland Podzols

Abstract Samples have been collected from major horizons of 34 podzol profiles distributed throughout Scotland, all developed from granite or granitic tills and under Calluna moorland. the pH in water and calcium chloride pastes, exchangeable cations and cation exchange capacity, and extractable aluminium of the soils collected have been measured, and the results studied in relation to reported atmospheric deposition of H+, non-marine sulphur and nitrate. for all horizons, significant positive correlations were found between soil pH and rainfall mean pH, as might be expected when the critical load of H+ deposition is exceeded. Acidifying pollutant deposition also apparently increased soil extractable aluminium concentrations in the B and C horizons. However, exchangeable base cation concentrations tended to increase, rather than decrease, with increasing precipitation acidity. This effect was attributed to increases in biogeochemical cycling of base cations, increases in leaching inputs of base cations fr...

Effects of enhanced supplies of nitrogen and sulphur on rhizosphere and soil chemistry in a Norway spruce stand in SW Sweden

Rhizophere and bulk soil chemistry were investigated in a Norway spruce stand in SW Sweden. The rhizosphere and bulk soil chemistry in water extracts in control plots (C) and plots repeatedly treated with ammonium sulphate (NS) were compared. Treatment regime was started in 1988. Cylindrical core samples of the LFH-layer and mineral soil layers were collected in 1992 and used for water extract analyses. Samples of soil from LFH-layer and mineral soil layers were taken in 1991 and 1993 for determination of CEC and base saturation. Soil pH and NH4-N, NO3-N and SO4-S, Al, Ca, K and Mg concentrations in water extracts were measured for rhizosphere and bulk soils. The pH-values of bulk and rhizosphere soils in NS plots decreased compared with those in control plots, whereas concentrations of NH4-N, NO3-N, SO4-S, base cations and Al in water extract increased. In both bulk and rhizosphere soils the concentration of NH4-N was much higher than that of NO3-N. A significant difference in the pH and Mg concentration of bulk and rhizosphere soil between the treated and control plots was found only in the 0–10 cm layer. For all layers, there was a significant difference in NH4-N concentrations in the bulk and rhizosphere soil between the NS treatment and control plots. Concentrations of exchangeable base cations and the base saturation level in the LFH-layer decreased in the NS plots. The concentration of extractable SO4-S increased in the NS plots. The NS treatment enhanced the amount of litter in L-layer, owing to increases in needle biomass and litterfall but led to losses of base cations, mainly K and Mg, from LFH-layer. It was concluded that the NS treatment displaced cations from exchangeable sites in the LFH-layer leading to higher concentrations of these elements in both rhizosphere and bulk soil.