Sulfonate S: A major form of forest soil organic sulfur

Summary Several forests of varying elevations, soils and vegetation were studied to evaluate the relative importance of sulfonate S, amino acid S, and ester sulfate as constituents of soil organic S. Suflonate S exceeded 40% of total S in the O1 horizon of all but one site examined, and comprised at least 50% of total S in the O2 horizons of 14 out of 18 study sites examined. Sulfonate pool sizes, on a percentage basis, tended to decrease with increasing sample depth within the mineral horizons, but sulfonate S was still a major form of organic S in the C horizon. Amino-acid S pool sizes were, as a general rule, lower than those for sulfonate in the O1 and O2 horizons, and lower than those for both ester sulfate and sulfonate when mineral soil horizons were considered. In no case did amino-acid S represent>25% of total S. Amino-acid S decreased with increasing depth at all but one site examined. Ester sulfate pool sizes were generally less than those of sulfonate S and greater than those of amino-acid S. This trend was observed with the O1, O2, and A horizons, but it was not apparent with samples from the intermediate and lowest soil horizons, where ester sulfate levels exceeded those for sulfonate S in 4 out of 8 and 5 out of 14 sites, respectively, in these latter horizons. Although there were some exceptions, collectively, the data suggest that sulfonate S is a major form of organic S in forest soils, irrespective of depth.     

Amino-acid metabolism in forest soil—Isolation and turnover of organic matter covalently labelled with s-methionine

Organic matter was extracted with pyrophosphate-NaOH buffer, pH 8.0 from the 02 horizon of a hardwood forest and exposed to ¹⁵S-methionine for 18 h to allow the incorporation of the aminoacid into the extract. Compared with other amendments, supplementation of the incorporation medium with Na₂SO₄ yielded the highest recoveries of the labelled organic matter. This material was chemically recalcitrant, requiring treatment for 6 h at 121°C with 6 NaOH for complete release of the incorporated methionine. A and B horizon soils from several forests were exposed to labelled material following dialysis to remove unlabelled components and a minor ³⁵S-labeIled component. All samples began to release ³⁵S-methionine after 48 h. Further metabolism of this amino-acid included mineralization and conversion to cysteine. A portion of the sulfate-S generated from mineralization was incorporated into organic matter and was recovered only by acid and alkali extraction. ³⁵S-labelled methionine and cysteine were also found in these latter fractions, indicating that these amino-acids had been directly incorporated into organic material during exposure.     

Importance of organic and inorganic sulfur to mineralization processes in a forest soil

Sulfur mineralization rates, changes in organic and inorganic S constituents and arylsulfatase activity were determined in four soil horizons (O2, B21h, B22hir and B23) which represent the major portion of a forest Spodosol (Becket). Biweekly, for 20 weeks, soil subsamples were leached with deionized water and analyzed for S constituents. Rates of water-soluble sulfate release were 123, 39, 34 and 18 nmol S g^?1 dry mass week^?1 for O2, B22hir, B23 and B21h horizons, respectively. Only in the organic O2 horizon did non-sulfate inorganic S (Zn-HCl-S) increase (15 nmol S g^?1) while phosphate extractable S decreased in all the mineral horizons (13, 19 and 28 nmol S g^?1 week^?1, B21h, B22hir and B23, respectively) due to desorption. Ester sulfate was mineralized in the B22hir and B23 horizons (?66 and ?22 nmol S g^?1 week^?1) and increased in the O2 (174 nmol S g^?1 week^?1). Arylsulfatase activity varied among horizons and decreased with time. Carbon-bonded S decreased in all horizons, especially those with high respiration rates (i.e. O2 and B21h), but changes were not significant. Only the B22hir horizon exhibited a significant loss of total S (128 nmol S g^?1 week^?1). The interrelationships among inorganic and organic S dynamics were outlined.     

Microbially available carbon in buried riparian soils in a glaciated landscape

Buried horizons and lenses in riparian soil profiles harbor large amounts of carbon relative to the surrounding soil horizons. Because these buried soil horizons, as well as deep surface horizons, frequently lie beneath the water table, their impact on nitrogen transport across the terrestrial–aquatic interface depends upon their frequency and spatial distribution, and upon the lability of associated organic matter. We collected samples of 51 soil horizons from 14 riparian zones Rhode Island, USA, where soil profiles are characterized by glacial outwash and alluvial deposits. These soil samples came from as deep as 2 m and ranged in carbon content from <1% to 44% in a buried O horizon 54–74 cm deep. We used these samples to: (1) determine the extent to which carbon in buried horizons, and deep surface horizons, is potentially microbially available; (2) identify spatial patterns of carbon mineralization associated with surface and buried horizons; and (3) evaluate likely relationships between soil horizon types, chemical characteristics and carbon mineralization. Carbon mineralization rates associated with buried horizons during anaerobic incubations ranged from 0.0001 to 0.0175 μmol C kg soil^?1 s^?1 and correlated positively with microbial biomass (R=0.89, P<0.0001, n=21). Excluding surface O horizons from the analysis, carbon mineralization varied systematically with horizon type (surface A, buried A, buried O, lenses, A/C, B, C) (P<0.05) but not with depth or depth x horizon interaction (overall R²=0.59, P<0.0005, n=47). In contrast to this result and to most published data sets, ¹³C-to-¹²C and ¹⁵N-to-¹⁴N ratios of organic matter declined with depth (¹³C?26.9 to ?29.3 per mil, ¹⁵N+5.6 to ?0.8 per mil). The absence of a relationship between horizon depth and C availability suggests that carbon availability in these buried horizons may be determined by the abundance and quality of organic matter at the time of horizon formation or burial, rather than by duration since burial, and implies that subsurface microbial activity is largely disconnected from surface ecosystems. Our results contribute to the emerging view that buried horizons harbor microbially available C in quantities relevant to ecosystem processes, and suggest that buried C-rich soil horizons need to be incorporated into assessments of the depth of the biologically active zone in near-stream subsurface soils.     

Factors affecting sulfate adsorption,organic sulfur formation,and mobilization in forest and grassland spodosols

Forest and grassland soils that contained varying amounts of Fe and Al were collected from England and Wales. Fractionation of free Fe and Al was accomplished to determine which components affected sulfate adsorption. Organic Fe and Al were the dominant fractions in most soil horizons and high amounts of these organically bound metals and, to some extent, crystalline Fe oxide were associated with high sulfate adsorption potentials. These adsorption potentials reflected naturally occurring amounts of absorbed sulfate and ester sulfate. Overall, the C content exhibited a positive relationship with sulfate adsorption potentials. Soils with a high C content also exhibited high rates of organic S formation. The rate of organic S mobilization was greater in soils with higher amounts of soluble sulfate. Organic S was the largest pool and, typically, sulfonate S was the most abundant constituent of the organic pool.     

Soil organic matter stabilization in acidic forest soils is preferential and soil type-specific

Minerals with large specific surface areas promote the stabilization of soil organic matter (SOM). We analysed three acidic soils (dystric, skeletic Leptic Cambisol; dystric, laxic Leptic Cambisol; skeletic Leptic Entic Podzol) under Norway spruce (Picea abies) forest with different mineral compositions to determine the effects of soil type on carbon (C) stabilization in soil. The relationship between the amount and chemical composition of soil organic matter (SOM), clay content, oxalate‐extractable Fe and Al (Fe_o; Al_o), and dithionite‐extractable Fe (Fe_d) before and after treatment with 10% hydrofluoric acid (HF) in topsoil and subsoil horizons was analysed. Radiocarbon age, ¹³C CPMAS NMR spectra, lignin phenol content and neutral sugar content in the soils before and after HF‐treatment were determined and compared for bulk soil samples and particle size separates. Changes in the chemical composition of SOM after HF‐treatment were small for the A‐horizons. In contrast, for B‐horizons, HF‐soluble (mineral‐associated) and HF‐resistant (non‐mineral‐associated) SOM showed systematic differences in functional C groups. The non‐mineral associated SOM in the B‐horizons was significantly depleted in microbially‐derived sugars, and the contribution of O/N‐alkyl C to total organic C was less after HF‐treatment. The radiocarbon age of the mineral‐associated SOM was younger than that of the HF‐resistant SOM in subsoil horizons with small amounts of oxalate‐extractable Al and Fe. However, in horizons with large amounts of oxalate‐extractable Al and Fe the HF‐soluble SOM was considerably older than the HF‐resistant SOM. In acid subsoils a specific fraction of the organic C pool (O/N‐alkyl C; microbially‐derived sugars) is preferentially stabilized by association with Fe and Al minerals. Stabilization of SOM with the mineral matrix in soils with large amounts of oxalate‐extractable Al_o and Fe_o results in a particularly stable and relatively old C pool, which is potentially stable for thousands of years.     

Cations adsorbed to soil organic matter — A regulatory factor for the release of organic carbon and hydrogen ions from soils to waters

Surface waters in northern forest ecosystems receive a substantial amount of drainage water from superficial soil horizons enriched in organic matter (SOM). Chemical reactions in the interface between the soil solution andf organic colloides will therefore affect the surface water chemistry. The mobilization of total organic carbon (TOC) and pH was studied as a function of amounts of organically adsorbed Na, Ca and Al in two O and one A horizon, which differed in the likelihood of contributing to the chemistry in runoff, in a forested watershed in northern Sweden. The samples were hydrogen ion saturated, washed and titrated with NaOH, Ca(OH)₂ and Al(OH)₃ in a constant ionic medium of 0.01 M NaCl in order to give rise to a population of manipulated samples differing in the composition of adsorbed cations. The highly humified SOM accumulated in the O_h and A_h horizons of a Gleysol close to the draining stream was stabilized by flocculating Al (95% of adsorbed metal cations), which resulted in a low release of TOC. These horizons showed a high potential of organic carbon solubility when Al was changed for di- or monovalent cations. Calculations suggested that the release of TOC would increase more than ten times if Al was exchanged for Ca upon liming to pH 6.0. The pH values of all horizons were shown to be determined mainly by the composition of adsorbed mono-,di- and trivalent cations.     

Contribution of different proton sources to pedogenetic soil acidification in forested ecosystems in Japan

The contribution of different proton sources to pedogenetic soil acidification was evaluated for three Japanese forest soils, i.e. ando soil, podzolic soil and brown forest soil in relation to the respective soil forming processes. Soil acidification rate and net proton generation were quantified based on the theory of proton budget for the respective soil horizon compartments (mainly the O, A and B horizons) by measuring fluxes of solutes entering and leaving the soil horizon compartment and vegetation uptake. Protons were produced by the dissociation of organic acids and nitrification in the O horizon and then consumed by adsorption and decomposition of organic acids and nitrate uptake by vegetation in deeper soil horizons at all plots. Excess uptake of cation over anion by vegetation was highest among proton sources in the whole soil compartment at all plots. Pedogenetic soil acidification was considered to include cation leaching from surface soil horizons due to proton generation by the dissociation of organic acids and nitrification and subsequent cation excess accumulation in wood in the growth stage of forests. In ando soil, andosolization resulted from the low contribution of net proton generation by the dissociation of organic acids as well as a lower soil acidification rate and complete acid neutralization. Dissolved organic carbon (DOC) fluxes in ando soil were lower than those in podzolic soil and brown forest soil due to high adsorption capacity of amorphous materials. In podzolic soil, podzolization resulted from intensive acidification in the O horizon, which derived from net proton generation by the dissociation of organic acids and nitrification as well as cation excess uptake by vegetation due to concentrated fine root biomass in the O horizon, and subsequent high proton efflux to subsoil. The high fluxes of DOC and Al leached from surface soil horizons were considered to contribute to eluviation of Al from surface soil and illuviation in subsoil in podzolic soil. In brown forest soil, brunification resulted from a lower DOC flux from the O horizon due to high decomposition and adsorption by oxides, where podzolization was weakened by high acid neutralization. Thus, the three representative processes involved in the pedogenesis of Japanese forest soils were well characterized by quantification of the respective proton-generating and consuming processes in each soil horizon.     

Soil Aluminum Distribution in the Near-Stream Zone at the Bear Brook Watershed in Maine

Near-stream and upslope soil chemical properties were analyzed to infer linkages between soil and surface water chemistry atthe Bear Brook Watershed in Maine [BBWM]. Organic and mineral soil samples were collected along six 20 m transects perpendicular to the stream and one 200 m transect parallel tothe stream. O horizon soils immediately adjacent to the streamhad a significantly higher pH (4.20) and lower soil organic matter percentage (54%) than upslope O horizons (3.84 and 76%,respectively). Additionally, near-stream O horizon soils hadsignificantly higher concentrations of water-soluble Al (2.7 ×),exchangeable Al (2.3 ×), and organically-bound Al (3.9 ×) andsignificantly lower concentrations of exchangeable Ca (0.4 ×) than O horizons upslope. These results suggest that Al can accumulate in non-hydric near-stream zone soils at this site. Mobilization of labile Al from near-stream zone soils duringhydrologic events could play a key role in explaining controls on Al in stream water at BBWM.     

Forest soil acidification assessment using principal component analysis and geostatistics

Soil acidification and consequent Al release is a problem particularly under forests in mountainous areas of the Czech Republic. It is controlled by a number of factors, like acid deposition, forest type, parent rock, altitude, etc. The Jizera Mountains region presents an area heavily influenced by acidification and forest decline. This paper focused on the effect of stand factors on spatial distribution of soil characteristics of the surface organic (O) and sub-surface (B) horizons from 98 sites using a combination of principal component analysis (PCA) and geostatistics. In the PCA, five principal components (PC) describing more than 70% of total variation were selected. The properties of the O and B horizons (pH, C, N, and S content, potentially dangerous Al forms) were in most cases separated, suggesting different processes and effects in each horizon. Spatial variation of PC scores was analysed using variograms, maps of their distribution were created using kriging. Spatial correlation with stand factors (altitude, slope aspect, forest type and age, soil unit, liming, and grass cover) was analysed using cross-variograms. The surface horizons are more sensitive to external influence (acid deposition, liming, grass expansion) and their spatial variation is stronger. The B horizons are more influenced by forest type (beech vs. spruce) and age, and by soil units (cambic vs. spodic horizons). The effect of stand factors is complex and often indirect. Nevertheless, used combination of pedometrical methods provided concise information about spatial variation and relationships between soil characteristics and the effect of stand factors.     

Chemical significance of aluminium extracted from three horizons of an acid forest soil, using chloride salt solutions

Aluminium is usually important in exchange reactions in acid soil, but amounts extracted are strongly affected by procedure. We examined the impact of proton release, following salt addition, on Al removal from three horizons (Oe, Oa and A₁₂) of an acid, brown forest soil. The Al was first extracted with either 1 M KC1, 1 m NH₄C1 or 0.5 m CuCl₂ on the three horizons limed to pH 5.5. For any lime addition, both the pH drop following salt addition and the amounts of extracted Al were greater with CuCl₂ than with KC1 or NH₄C1. For the Oa and A₁₂ horizons there was a single inverse linear relation between amount of Al extracted and the pH of final extract, independent of the extracting cation. In the Oe horizon CuCl₂ extracted constant amounts of Al, whereas the quantity of Al removed by KC1 or NH₄C1 increased linearly as pH declined below pH 3.6. Extra Al was mobilized following unbuffered KC1 extraction, a side effect induced by the extracting procedure itself. Al mobilization increased with increasing H⁺ additions. In the Oe horizon, equilibrium was established rapidly (≤ 24 h), and decreasing the pH of the soil-KCl suspensions to 2.0 resulted in extracted Al amounts equal to or slightly greater than those obtained with CuCl₂ solutions. Al amounts extracted with acidified KC1 solutions from the A₁₂ were close to those obtained using CuCl₂ solutions at similar pH in the extract. In the samples from the A₁₂ layer, increasing the equilibration time resulted in increasing proton consumption and equivalent release of Al ions in the extract.     

Speciation and mobilization of aluminium and cadmium in podzols and cambisols of S. Sweden

Processes governing the mobilization of Al and Cd in podzols and cambisols of S. Sweden having different tree layer vegetation (Picea abies, Fagus sylvatica, or Betula pendula) were investigated. Speciation of Al and Cd in soil solutions were performed by a column cation exchange procedure (cf. Driscoll, 1984) in combination with thermodynamic calculations. Podzols in spruce and beech stands were characterized by a high release of organic compounds from the O/Ah horizons, resulting in a high organic complexation of Al (c. 93%) in the soil solution from the E horizon (15 cm lysimeters). Organic complexes were mainly adsorbed/precipitated in the upper Bh horizon and the overall transport of Al at 50 cm depth was governed by a pH dependent dissolution of a solid-phase Al pool. In the cambisols, inorganic Al forms were predominant at both 15 and 50 cm depth, and Al solubility was closely related to solution pH. Secondary minerals like synthetic gibbsite, jurbanite, kaolinite or imogolite could generally not explain measured solution Al³⁺ activities. Results instead indicated that the relatively large organically bound solid-phase Al pools present in both soil types could do so. The column fractionation procedure could be used only qualitatively for Cd, but results strongly indicated that Cd-organo complexes contributed significantly to the overall mobilization of Cd in the podzol E horizons. In all other soil solutions, Cd²⁺ was the predominant species. Both solid-phase and solution chemistry suggests that ion exchange processes controlled the Cd²⁺ activities in these solutions. All reactive solidphase Cd was extractable by NH₄Cl and Cd²⁺ activities could in most cases effectively be modeled by the use of ion exchange equations. Solubilized Al³⁺ efficiently competed for exchange sites and played an important role for the Cd mobilization in these soils.     

Buried organic horizons represent amino acid reservoirs in boreal forest soils

We examined the composition and concentration of amino acids by soil horizon and depth on the Tanana River floodplain in interior Alaska. Soils from mid-successional stages of balsam poplar and white spruce were separated into successive forest floor (Oe/Oa), buried organic horizons (BOHs), and mineral horizons; and water-extractable amino acid composition and concentration were determined by HPLC. The number, depth, and thickness of BOHs were highly variable across the landscape and among replicates of the same stand type, reflecting differences in terrace age, flood frequency, flood intensity, river channel position, vegetation inputs, and decomposition. BOHs generally had lower pH and bulk density, higher moisture content, and greater concentrations of carbon, nitrogen, and roots than the surrounding mineral horizons. In each horizon of both successional stages, the soil amino acid pool was dominated by glutamic acid, glutamine, alanine, asparagine, aspartic acid, and histidine, which together accounted for approximately 80% of the total amino acids found. Despite the similar overall amino acid composition among the horizons, proportions of glutamine generally increased with depth and were generally greater in the mineral horizons than in the BOHs, suggesting root exudation or fine root turnover as an amino acid source. In both successional stages, amino acid concentrations were nearly always highest in the Oe/Oa horizon and rapidly decreased with depth. BOHs generally had greater amino acid concentrations than the surrounding mineral horizons in both successional stages, but amino acid concentrations in successive BOHs declined with depth in the soil profile, suggesting that although BOHs do remain as biological hot spots and potential nutrient reservoirs as far down as 60 cm depth, their importance declines over time.     

Seed storage protein deficiency improves sulfur amino acid content in common bean (Phaseolus vulgaris L.): redirection of sulfur from gamma-glutamyl-S-methyl-cysteine

The contents of sulfur amino acids in seeds of common bean ( Phaseolus vulgaris L.) are suboptimal for nutrition. They accumulate large amounts of a gamma-glutamyl dipeptide of S-methyl-cysteine, a nonprotein amino acid that cannot substitute for methionine or cysteine in the diet. Protein accumulation and amino acid composition were characterized in three genetically related lines integrating a progressive deficiency in major seed storage proteins, phaseolin, phytohemagglutinin, and arcelin. Nitrogen, carbon, and sulfur contents were comparable among the three lines. The contents of S-methyl-cysteine and gamma-glutamyl-S-methyl-cysteine were progressively reduced in the mutants. Sulfur was shifted predominantly to the protein cysteine pool, while total methionine was only slightly elevated. Methionine and cystine contents (mg per g protein) were increased by up to ca. 40%, to levels slightly above FAO guidelines on amino acid requirements for human nutrition. These findings may be useful to improve the nutritional quality of common bean.     

Determination of aluminium complexes of low molecular organic acids in soil solution from forest soils using ultrafiltration

Ultrafiltration (<1000 D) was evaluated as an analytical method for determination of Al bound to low molecular weight organic acids (LMWOA) in soil solutions from podzolised forest soils. The results were compared to those obtained by two chemical equilibrium models and a size exclusion chromatography method. The percentage of Al bound to LMWOAs was highest in the O horizon solutions (15–44%) and decreased in the deeper horizons. Citric acid was found to be the most important complex former. Generally the ultrafiltration method and modelling showed acceptable agreement. The method showed acceptable precision and good recovery of Al in the samples.     

The influence of land management on concentrations of dissolved organic carbon and its effects on the mobilization of aluminium and iron in podzol soils in Mid-Wales

Abstract. The aluminium (Al), iron (Fe) and Dissolved Organic Carbon (DOC) contents of the soil solution were monitored in two upland grassland and afforested podzol soils in Mid-Wales. Al organo-metallic complexes predominated in the O horizon leachates of the grassland soil, whereas inorganic monomeric Al forms dominated in the lower mineral horizons. Dissolved organic matter determines the chemistry, solubility, and transport of Al and Fe in the O horizon, and these are under strong biological control. The distributions of organic-Al, Fe and DOC within the soil profile were consistent with traditional podzolization theory. Observed increases in the molar ratios of Al:DOC in solution in the lower soil horizons may be responsible for the small solubility of Al organo-metallic complexes in those horizons. Afforestation increased the concentrations of organic-Al and Fe in the soil solution as compared with the concentrations observed for the grassland soil. Clearcutting further significantly mobilized Al and Fe from the upper soil horizon, primarily by increasing the DOC concentration in the soil water.     

Factors controlling accumulation and decomposition of organic carbon in humus horizons of Andosols

Andosols often accumulate soil organic matter (SOM) in large amounts. To investigate the factors controlling the stability and lability of organic carbon (OC) in humus horizons of Andosols, we selected 19 A horizon samples (surface and subsurface horizons) from the Field Station of Tohoku University including areas where benchmark soil profiles of non-allophanic Andosols are distributed. We determined the soil properties possibly controlling the OC accumulation, such as pH(H₂O), 1 M KCl-extractable aluminum (KCl-Al), pyrophosphate-extractable Al and iron (Al_p, Fe_p), acid oxalate-extractable silicon (Si_o), total OC, water-extractable OC, and humified OC. To evaluate the OC mineralization, we measured the soil respiration rates in a laboratory for non-treated, neutralized (CaCO₃, Ca(OH)₂ and NaOH), and nutrient applied (KH₂PO₄, (NH₄)₂SO₄) soil samples. Statistical analyses, including a path analysis, showed that the Al_p and pH(H₂O) values are directly related to the OC concentration (P?<?0.01 and P?<?0.05, respectively). There was a significant negative correlation (P?<?0.01) between the soil respiration rates of the non-treated samples and the ratios of the humified OC to total OC, showing that the humification of the SOM was definitely related to the OC stability. Effects of the chemical treatments to the soil respiration rates were greater in the surface horizon samples with an abundant labile OC than those in the subsurface samples. Neutralization affected the soil respiration rates more significantly than the nutrient application. Among the neutralization treatments, the liming materials more effectively increased the respiration rates. This was probably due to an increase in the lability of the humified OC by liming.     

Mechanisms controlling the mobility of dissolved organic matter, aluminium and iron in podzol B horizons

The processes governing the (im)mobilization of Al, Fe and dissolved organic matter (DOM) in podzols are still subject to debate. In this study we investigated the mechanisms of (im)mobilization of Al, Fe and organic matter in the upper and lower B horizons of two podzols from the Netherlands that are in different stages of development. We equilibrated batches of soil material from each horizon with DOM solutions obtained from the Oh horizon of the corresponding soil profiles. We determined the amount of (im)mobilized Al, Fe and DOM after addition of Al and Fe at pH 4.0 and 4.5 and initial dissolved organic carbon (DOC) concentrations of 10 mg C litre^?1 or 30 mg C litre^?1, respectively. At the combination of pH and DOC concentrations most realistic for the field situation, organic matter was retained in all horizons, the most being retained in the lower B horizon of the well‐developed soil and the least in the upper B horizon of the younger profile. Organic matter solubility seemed to be controlled mainly by precipitation as organo‐metal complexes and/or by adsorption on freshly precipitated solid Al‐ and Fe‐phases. In the lower B horizons, at pH 4.5, solubility of Al and Fe appeared to be controlled mainly by the equilibrium with secondary solid Al‐ and Fe‐phases. In the upper B horizons, the solubility of Al was controlled by adsorption processes, while Fe still precipitated as inorganic complexes as well as organic complexes in spite of the prevailing more acidic pH. Combined with a previous study of eluvial horizons from the same profiles, the results confirm the important role of organic matter in the transport of Al and Fe to create illuvial B horizons initially and subsequently deepening and differentiating them into Bh and Bs horizons.     

Amino acid cycling in three cold-temperate forests of the northeastern USA

Amino acids play a critical role in soil-N cycling. Much of the current research on amino acid cycling has been conducted in arctic, alpine, boreal, and temperate grassland ecosystems. There are no comparable data for temperate forests. We quantified the concentration and production of amino acid N and inorganic N in three forests varying in parent material and tree species composition in Connecticut, USA. Soil samples were collected on three sample dates in 2001 and 2002. At all three sites, a pool of free amino acids was present in soil on all sample dates. Among-site differences in the production of amino acids were related to variations in the activity of proteolytic enzymes, the sensitivity of proteolytic enzymes to the availability of protein substrate, and the presence or absence of a surface organic horizon. Among-site differences in amino acid turnover appeared to be at least partially related to soil C-to-N ratios and their effect on C vs. N limitation to microbial function. Amino acid concentrations in the top 15 cm of mineral soil in these study sites fell within the range of reported values for ecosystems spanning a wide latitudinal gradient, including ecosystems in which amino acids are thought to contribute substantively to plant-N nutrition. The concentration of amino acid N in the organic horizons of these study sites was considerably higher than those reported in the literature. The implications of the results for N capture by temperate forest trees are discussed.     

Organic matter retention in an upland humic podzol; the effects of pH and solute type

This paper discusses the effects of different horizons and soil solution compositions on dissolved organic matter retention in a moorland podzol and compares the results with previous studies of forest podzols. Adsorption isotherms were constructed for each of the major horizons of a freely draining, upland, moorland, humic podzol from north-east Scotland, to investigate processes of retention and release of dissolved organic matter (DOM). Carbon retention of a range of solute types was studied, and phthalate was chosen as a model compound to measure carbon retention at three different pH values (3, 4.5 and 6). Retention and release of DOM was related to chemical, physical and mineralogical characteristics of the different soil horizons. All the mineral horizons retained DOM, with the Bs horizon most retentive. Solution pH did not significantly affect DOM retention in the O and A horizons. At pH 3 and 4.5 organic matter was weakly retained in the Bhs horizon, but strongly retained in the Bs and the Cx horizons. At pH 6 reversal of surface charge occurred in the Bs and Cx horizons resulting in the release of similar amounts of organic matter to that released from the O horizon at the same pH. The results demonstrate how podzols act as a ‘valve’ in controlling the input of dissolved organic compounds into surface and ground water, and how sensitive the controlling mechanisms are to pH change.