Richard H. April

SOIL CHEMICAL CHANGE DURING THREE DECADES IN AN OLD-FIELD LOBLOLLY PINE (PINUS TAEDA L.) ECOSYSTEM'

The ability of soil to sustain its supply of nutrients to a growing forest is controlled by a complex of biogeochemical processes. Forest soil data are notably absent, however, that describe sustained nutrient supply or nutrient depletion. The objective of this study was to evaluate how exchangeable nutrient cations of a previously cultivated Ultisol responded to the first three decades of pine forest development. On six occasions during the three decades, the upper 0.6 m of soil was sampled from eight permanent plots and chemically analyzed with the same procedures. During this period, KCl-exchangeable acidity (as positive charges of adsorbed H and Al ions) increased by 37.3 kmol,/ha in the upper 0.6 m of soil and positive charges of exchangeable Ca and Mg were depleted by 34.8 and 8.9 kmolc/ha (by 696 and 108 kg/ha), whereas, exchangeable K was reduced by only 0.5 kmolc/ha (19 kg/ha). Depletion of soil exchangeable Ca was on the same order of magnitude as Ca removals (i.e., Ca accumulation in biomass and forest floor plus that lost in soil leaching). Removals of soil Mg also appeared to outpace resupply from recycling, atmospheric deposition, and mineral weathering, but not to the same degree as Ca. Over the three decades, soil leaching loss of these divalent cations (from 0.6 m depth) appeared equal to cation accumulation in biomass plus forest floor, with sulfate balancing about half these cations in leachates. In contrast to Ca and Mg, total K removals from the soil exceeded reductions in soil exchangeable K by nearly 20-fold. Exchangeable K was well buffered in surface mineral soils apparently due to a combination of biological recycling via leaching of canopies and forest floor plus mineral weathering release. These nutrient dynamics may be common to many nutrient-demanding forest ecosystems supported by soils with low activity kandic or oxic horizons. Such soils (Ultisols and Oxisols) occur on many hundreds of millions of hectares in temperate and tropical zones.

Chemical weathering in two Adirondack watersheds: Past and present-day rates

Rates of chemical weathering in two forested Adirondack watersheds were determined from mineral and elemental depletion trends in soil profiles and from input/output budgets based on precipitation and surface-water chemistry. Long-term rates of weathering have averaged about 500 to 600 eq/ha·yr for both watersheds since the glaciers retreated from the region about 14,000 yr ago. Present-day denudation rates average 1679 eq/ha·yr in the Panther Lake watershed and only 198 eq/ha·yr in the Woods Lake watershed. Mineral weathering reactions in both watersheds involve primarily the dissolution of plagioclase, potassium feldspar, and hornblende. Mass balance calculations, however, indicate that hornblende weathers in disproportionately large amounts in the Panther Lake watershed relative to its abundance in soils and till. Cation exchange in the glacial till mantling the Panther Lake watershed may also play an important role in controlling surface-water chemistry in the basin. In the Panther Lake watershed, the current rate of weathering, which is about a factor of 3 greater than the long-term average, may reflect a recent adjustment for higher hydrogen ion fluxes brought about by acid deposition. The capacity of the thick till deposits in the Panther Lake watershed to neutralize acidity is ultimately reflected by the circum-neutrality of Panther Lake. In the Woods Lake watershed, the soils and thin till deposits cannot effectively neutralize current acid loadings and, therefore, Woods Lake is acidic with a pH below 5.

Rapid, plant-induced weathering in an aggrading experimental ecosystem

To evaluate whether rates of weathering of primary minerals are underestimated in watershed mass-balance studies that fail to include products of weathering accumulating in plants and in developing soil, changes in the calcium and magnesium content of vegetation and soil fractions were measured in large, monitored lysimeters (sandbox ecosystems) at Hubbard Brook Experimental Forest, New Hampshire. Weathering was evaluated over 4–8 yr in sandboxes planted with red pine (Pinus resinosa Ait.) and kept mostly free of vegetation (nonvegetated). Three mass-balance equations were used that cumulatively include (a) Ca and Mg in precipitation inputs and drainage outputs, (b) accumulation of Ca and Mg in vegetation, and (c) changes in products of weathering in soils. Soil products were evaluated with an extraction process designed to avoid removing ions from primary minerals. Relative to the input-output equation, the estimated rate of weathering increased 2.4 (Ca) and 1.8 (Mg) times when accumulation of Ca and Mg in pine biomass was accounted for, and 8 (Ca) and 23 (Mg) times when changes in soil products were also included. Weathering estimates that included accumulation in vegetation and soil products were 261 (Ca) and 92 (Mg) kg ha-1 yr-1 in the pine sandbox. These rates were 10 (Ca) and 18 (Mg) times higher than the rates in the nonvegetated sandbox, which were not significantly greater than zero. This study raises the possibility that weathering can play a significant role in the release of nutrients available to plants over short periods. Faster rates like this become extremely important where managers are trying to balance nutrients available to plants from precipitation and weathering release with outputs including harvest removals.

The relationship between surface water chemistry and geology in the North Branch of the Moose River

The chemistry of lakes and streams within the North Branch of the Moose River is strongly correlated with the nature and distrubution of geologic materials in the watershed. The dominance of thin glacial till and granitic gneiss bedrock in the region north and east of Big Moose Lake results in a geologically sensitive terrain that is characterized by surface water with low alkalinity and chemical compositions only slightly modified from ambient precipitation. In contrast, extensive deposits of thick glacial till and stratified drift in the lower part of the system (e.g. Moss-Cascade valley) allow for much infiltration of precipitation to the groundwater system where weathering reactions increase alkalinity and significantly alter water chemistry.The hypothesis that surficial geology controls the chemistry of surface waters in the Adirondacks holds true for 70 percent of the Moose River watershed. Exceptions include the Windfall Pond subcatchment which is predominantly covered by thin till, yet has a high surface water alkalinity due to the presence of carbonate-bearing bedrock. The rapid reaction rates of carbonate minerals allow for complete acid neutralization to occur despite the short residence time of water moving through the system. Another important source of alkalinity in at least one of the subcatchments is sulfate reduction. This process appears to be most important in systems containing extensive peat deposits.An analysis of only those subcatchments controlled by the thickness of surficial sediments indicates that under current atmospheric loadings watersheds containing less than 3 percent thick surficial sediments will be acidic while those with up to 12 percent will be extremely sensitive to acidification and only those with over 50 percent will have a low sensitivity.

Aluminum toxicity in forests exposed to acidic deposition: The ALBIOS results

The ALBIOS project was conducted to examine the influence of acidic deposition on aluminum transport and toxicity in forested ecosystems of eastern North America and northern Europe. Patterns of aluminum chemistry were evaluated in 14 representative watersheds exposed to different levels of sulfur deposition. Controlled studies with solution and soil culture methods were used to test interspecific differences in aluminum sensitivity for one indicator species (honeylocust - Gleditsia triacanthos L. ) and six commercial tree species (red spruce - Picea rubens Sarg., red oak - Quercus rubra L., sugar maple - Acer saccharum Marsh., American beech - Fagus grandifolia Ehrh., European beech - Fagus sylvatica, and loblolly pine - Pinus Taeda L. ). Overall, red spruce was the tree species whose growth was most sensitive to soluble aluminum, with significant biomass reductions occurring at Al concentrations of approximately 200–250 umol/L. Analyses of soil solutions from the field sites indicated that the conditions for aluminum toxicity for some species exist at some of the study areas. At these watersheds, aluminum toxicity could act as a contributing stress factor affecting forest growth.

Mineralogy of the rhizosphere in forest soils of the eastern United States

Chemical and mineralogical studies of forest soils from six sites in the northeastern and southeastern United States indicate that soil in the immediate vicinity of roots and fine root masses may show marked differences in physical characteristics, mineralogy and weathering compared to the bulk of the forest soil. Examination of rhizosphere and rhizoplane soils revealed that mineral grains within these zones are affected mechanically, chemically and mineralogically by the invading root bodies. In SEM/EDS analyses, phyllosilicate grains adjacent to roots commonly aligned with their long axis tangential to the root surface. Numerous mineral grains were also observed for which the edge abutting a root surface was significantly more fractured than the rest of the grain. Both the alignment and fracturing of mineral grains by growing roots may influence pedogenic processes within the rhizosphere by exposing more mineral surface to weathering in the root-zone microenvironment. Chemical interactions between roots and rhizosphere minerals included precipitation of amorphous aluminium oxides, opaline and amorphous silica, and calcium oxalate within the cells of mature roots and possible preferential dissolution of mineral grains adjacent to root bodies. Mineralogical analyses using X-ray diffraction (XRD) techniques indicated that kaolin minerals in some rhizosphere samples had a higher thermal stability than kaolin in the surrounding bulk forest soil. In addition, XRD analyses of clay minerals from one of the southeastern sites showed abundant muscovite in rhizoplane soil adhering to root surfaces whereas both muscovite and degraded mica were present in the immediately surrounding rhizosphere soil. This difference in mineral assemblages may be due to either K-enrichment in rhizoplane soil solutions or the preferential dissolution of biotite at the root-soil interface

The nature of vermiculite in Adirondack soils and till

The clay and bulk mineralogy of soil and till from 26 Adirondack watersheds was studied. The materials consist typically of quartz, K-feldspar, plagioclase, mica, vermiculite, and kaolinite. Talc, smectite, halloysite, and hornblende are present in some samples. The clay fraction of the soils is composed predominantly of vermiculite, likely derived from the transformation of a mica precursor, and kaolinite. The soil vermiculite commonly contains hydroxy-Al interlayers which are especially prevalent in the B-horizon samples. Despite significant variation in the type of bedrock and the composition of heavy mineral assemblages in these watersheds, the clay mineralogy is remarkably uniform. This finding supports earlier suggestions that the occurrence of vermiculite in soils is more dependent on climate than on the nature of the parent material.

Trioctahedral smectite and interstratified chlorite/smectite in Jurassic strata of the Connecticut Valley

Trioctahedral smectite and regularly interstratified chlorite/smectite in strata of the East Berlin Formation of the Connecticut Valley are largely restricted to black shale and gray mudstone deposited in alkaline, perennial lakes. The precursor of the mixed-layer clay appears to have been a smectite. Alkaline lake waters and inherited pore waters rich in magnesium favored the transformation of smectite to mixed-layer chlorite/smectite by fixation of brucitic interlayers into the smectite unit structure. Gray mudstones containing the mixed-layer chlorite/smectite are invariably underlain by magnesium-rich black shale—a possible source of Mg for the clay mineral transformations. The black shale is composed predominantly of Mg-rich trioctahedral smectite of probable authigenic origin.РезюмеТриоктаэдрический смектит и регулярно внутринапластованный хлорит/смектит в пласте формации Восточного Берлина в Долине Коннектикута ограничены, в основном, до черной сланцеватой глины и серой иловатой глины, осажденных в щелочных непересыхающих озерах. Смектит кажется предшественником смешанно-слойной глины. Воды щелочных озер и наследственные пористые воды, богатые в магний, способствовали преобразованию смектита в смещаннослойный хлорит/смектит путем фиксации бруцитовых промежуточных слоев в элементарную структуру смектита. Черные сланцеватые глины, богатые в магний—возможные источники магния, используемые для преобразования глинистых минералов—неизменно располагаются под серыми иловатыми глинами, содержающими смешанно-слойный хлорит/смектит. Черная сланцеватая глина, в основном, состоит из богатого в магний триоктеэдрического смектита, вероятно, аутигенного начала. [Е.С.]ResümeeTrioktaedrischer Smektit und regelmäßge Chlorit/Smektit-Wechsellagerungen sind in den Schichten der East Berlin Formation des Connecticut Valley hauptsächlich auf schwarzen Schieferton und grauen Tonstein beschränkt, die in alkalischen permanenten Seen abgelagert sind. Der Vorläufer der Wechsellagerung scheint ein Smektit gewesen zu sein. Alkalische Seewässer und Mg-reiche Porenwässer begänstigten die Umwandlung des Smektit in die Chlorit/Smektit-Wechsellagerung, indem brucitische Zwischenlagen in die Smektit-Struktur eingebaut wurden. Unter den grauen Tonsteinen, die die Chlorit/Smektit-Wechsellagerung enthalten, findet sich immer ein Mg-reicher schwarzer Schieferton, der eine mögliche Mg-Quelle für die Tonmineralumbildung ist. Der schwarze Schieferton besteht vor allem aus Mgreichem trioktaedrischem Smektit mit wahrscheinlich authigener Entstehung. [U.W.]RésuméLa smectite trioctaèdre et la chlorite/smectite régulièrement interstratifiée dans les lits de la formation East Berlin de la vallée du Connecticut sont pour la plupart restrientes au shale noir et à l’argilite grise déposés dans des lacs alkalins perpétuels. Le précurseur de l’argile à couches mélangres semble avoir été une smectite. Les eaux alkalines du lac et les eaux héritées des pores, riches en magnésium, ont favorisé la transformation de la smectite en chlorite/smectite à couches mélangées par la fixation d’intercouches brucitiques dans la structure unitaire de la smectite. On trouve invariablement des shales noirs riches en magnésium, une source possible de Mg pour les transformations de minéral argileux, sous des argilites grises contenant la chlorite/smectite à couches mélangées. Le shale noir est composé surtout de smectite trioctaèdre riche en Mg et probablement d’origine authigénique. [D.J.]

Chemical weathering and cation loss in a base-poor watershed

Weathering of minerals in soils provides important plant nutrients and consumes acidity, yet mineral weathering is difficult to observe and quantify. We derived present-day weathering flux estimates for soil minerals in a forested watershed in central New Hampshire using a geochemical mass balance. The Cone Pond watershed is characterized by low-alkalinity surface waters and acidic Spodosols developed on a thin mantle of locally derived till, making it susceptible to continued inputs of acid precipitation. Weathering reactions were developed on the basis of observed patterns of mineral abundance in the soil profile and measured mineral chemistry. The dissolution of approximately 171 mol ha −1 yr −1 of plagioclase feldspar is the predominant weathering reaction at Cone Pond. Weathering fluxes of hornblende, biotite, chlorite, and potassium feldspar ranged from 3 to 20 mol ha −1 yr −1 , an order of magnitude lower. When normalized to their abundances in the soil, however, hornblende and chlorite had the fastest weathering rates, as measured by cation release. Chemical weathering, mainly of plagioclase, resulted in the neutralization of only 52% of incoming acidity. Furthermore, silicate weathering could only account for the release of about 53 mol ha −1 yr −1 of Ca, compared to the observed net output (stream loss minus bulk precipitation input) of 75 mol ha −1 yr −1 . On the basis of these observations and a companion study, we conclude that current acid inputs exceed the ability of Cone Pond soils to neutralize hydrogen ion, and that depletion of approximately 22–53 mol ha −1 yr −1 of Ca from labile soil pools is occurring in this area. The rate of Ca depletion is one-third to one-eighth of the rate estimated for a more base-rich watershed nearby.

Mineralogy and Mineral Weathering

Elements such as Ca, Mg, and K, which are required for plant growth, are important components of the nutrient cycle in forested ecosystems, and by far the largest store of these elements in North American and European forests is within the minerals constituting the forest soil. Although external inputs from the atmosphere in both the dissolved and particulate load can provide a portion of these elements to a growing forest, the ultimate source of most inorganic elemental nutrients is provided through cation exchange and mineral weathering reactions that take place in the soil profile. Mineral inventories and determinations of the physical characteristics, mineralogy and chemistry of soil components, and mineral weathering reactions that occur in soils must be an integral part of any study that attempts to document the nutrient status of a forested ecosystem.