Adsorption and desorption dynamics of citric acid anions in soil

The functional role of organic acid anions (e.g. citrate, oxalate, malonate, etc) in soil has been intensively investigated with special focus either on (i) microbial respiration and soil carbon dynamics, (ii) nutrient solubilization, or (iii) metal detoxification. Considering the potential impact of sorption processes on the functional significance of these effects, comparatively little is known about the adsorption and desorption dynamics of organic acid anions in soils. The aim of this study therefore was to experimentally characterize the adsorption and desorption dynamics of organic acid anions in different soils using citrate as a model carboxylate. Results showed that both adsorption and desorption processes were fast, reaching a steady state equilibrium solution concentration within approximately 1 hour. However, for a given total soil citrate concentration(ctot) the steady state value obtained was critically dependent on the starting conditions of the experiment (i.e. whether most of the citrate was initially present in solution (cl) or held on the solid phase (cs)). Specifically, desorption-led processes resulted in significantly lower equilibrium solution concentrations than adsorption led processes indicating time-dependent sorption hysteresis. As it is not possible to experimentally distinguish between different sorption pools in soil (i.e. fast, slow, irreversible adsorption/desorption), a new dynamic hysteresis model was developed that relies only on measured soil solution concentrations. The model satisfactorily explained experimental data and was able to predict dynamic adsorption and desorption behaviour. To demonstrate its use we applied the model to two relevant scenarios (exudation and microbial degradation), where the dynamic sorption behaviour of citrate occurs. Overall, this study highlights the complex nature of citrate sorption in soil and concludes that existing models need to incorporate both a temporal and sorption hysteresis component to realistically describe the role and fate of organic acids in soil processes

Rock-eating mycorrhizas: their role in plant nutrition and biogeochemical cycles

A decade ago, tunnels inside mineral grains were found that were likely formed by hyphae of ectomycorrhizal (EcM) fungi. This observation implied that EcM fungi can dissolve mineral grains. The observation raised several questions on the ecology of these ¿rock-eating¿ fungi. This review addresses the roles of these rock-eating EcM associations in plant nutrition, biogeochemical cycles and pedogenesis. Research approaches ranged from molecular to ecosystem level scales. Nutrient deficiencies change EcM seedling exudation patterns of organic anions and thus their potential to mobilise base cations from minerals. This response was fungal species-specific. Some EcM fungi accelerated mineral weathering. While mineral weathering could also increase the concentrations of phytotoxic aluminium in the soil solution, some EcM fungi increase Al tolerance through an enhanced exudation of oxalate. Through their contribution to Al transport, EcM hyphae could be agents in pedogenesis, especially podzolisation. A modelling study indicated that mineral tunnelling is less important than surface weathering by EcM fungi. With both processes taken together, the contribution of EcM fungi to weathering may be significant. In the field vertical niche differentiation of EcM fungi was shown for EcM root tips and extraradical mycelium. In the field EcM fungi and tunnel densities were correlated. Our results support a role of rock-eating EcM fungi in plant nutrition and biogeochemical cycles. EcM fungal species-specific differences indicate the need for further research with regard to this variation in functional traits

Heavy metal distribution in some French forest soils: evidence for atmospheric contamination

This study is one of very few dealing with the distribution and the origin of heavy metals in French soils from a priori non-polluted forest areas. The abundance of heavy metals measured in these soils decreases as follows: Cr) Zn)Pb)Ni)Cu)Co4Cd. Total concentrations of Pb, Cr and Ni in some soils exceed the European thresholds for non-polluted soils and even the French association of normalization critical values for sludge spreading. The lowest heavy metal contents are observed in acid soils while the highest concentrations are in the calcaric cambisol and in the mollic andosol, which is rather scarce as compared with the other French forest soils. With the exception of the podzol, Cr and Ni concentrations increase with depth in all soil profiles. The distribution pattern of Co, Cu, Zn depends on the soil characteristics. In some acid soils, however, Cu and Zn decrease with depth. Pb and Cd are accumulated in the upper soil horizons. Heavy metals accumulate in deep soil horizons in relation to important clay content in the dystric planosol and stagnic luvisol. The concentration of each heavy metal is always controlled by different parameters (soil pH, iron and aluminum oxide content, clay content, organic matter and cation exchange capacity), which are heavy metal specific. This study highlights the metal-trapping character of andosol and calcaric soil, the weak heavy metal retention in acid soils, the leaching and trapping character in leached clayed soils, and the migration of heavy metals in the podzol. Pb and Cr concentrations indicate a significant enrichment in surface horizons from various soils in areas which receive significant acid atmospheric pollution. Particularly, the highest Pb content is observed in a soil located in the N-NE part of France. Lead isotope ratios measured in the cambic podzol and the calcaric cambisol, exhibit the importance of the anthropogenic sources and particularly the influence of global atmospheric inputs from leaded gasoline compared to regional and local industrial emissions. The anthropogenic Pb contribution is estimated to 83, 30 and 11%, respectively, for surface, intermediate and deep horizons of the cambic podzol located in the northern part of France, and to 68% in surface horizon of the calcaric cambisol located in the Alps

An integrated chemical and micromorphological investigation of humic substances in three podzolized soils of the Netherlands.

The humic substances in a 'Holt' leptic podzol, a 'Haar' humic podzol and a 'Veld' gleyic podzol from the Netherlands were analysed chemically and micromorphologically. The absorption spectra of Na-humates in visible light and infrared spectra were determined for the humic acids. The translocation and distribution of the principal humus compounds occurred by mechanical illuviation in the 'Holt' podzol, and by translocation in solution in the 'Haar' podzol and the 'Veld' podzol. The micromorphological images were closely correlated with the chemical composition of the humic substances. Illuviation cutans occurring in the B horizons of the 'Haar' podzol and the 'Veld' podzol appeared to consist mainly of fulvic acids. The dark humus micro-aggregates characteristic of the 'Haar' podzol and the Ap horizon of the 'Veld' podzol were composed mainly of bitumens, humins and humic acids of high molecular weight, while the brown micro-aggregates of the 'Holt' podzol and the lower horizons of the 'Veld' podzol consisted mainly of fulvic acids and humic acids of low molecular weight. The stability of micro-aggregates was correlated with their composition. (Abstract retrieved from CAB Abstracts by CABI’s permission

Analysis of prehistoric brown earth paleosols under the podzol soils of Exmoor, UK

The deforestation of the upland landscapes in southwest Britain during prehistory is an established archaeological narrative, documenting human impacts on the environment and questioning the relationship of prehistoric societies to the upland landscapes they inhabited. Allied to the paleoenvironmental analyses of pollen sequences, which have provided the evidence of this change, there has been some investigation of prehistoric paleosols fossilized under principally Bronze Age archaeological monuments. These analyses identified brown earth soils that were originally associated with temperate deciduous woodland, on occasion showing evidence of human impacts such as tilling. However, the number of analyses of these paleosols has been limited. This study presents the first analysis of a series of pre‐podzol brown earth paleosols on Exmoor, UK, two of which are associated with colluvial soil erosion sediments before the formation of peat. This study indicates these paleosols are spatially extensive and have considerable potential to inform a more nuanced understanding of prehistoric human impacts on the upland environments of the early‐mid Holocene and assess human agency in driving ecosystem change

Simultaneous quantification of depolymerization and mineralization rates by a novel 15N tracing model

The depolymerization of soil organic matter, such as proteins and (oligo-)peptides, into monomers (e.g. amino acids) is currently considered to be the rate-limiting step for nitrogen (N) availability in terrestrial ecosystems. The mineralization of free amino acids (FAAs), liberated by the depolymerization of peptides, is an important fraction of the total mineralization of organic N. Hence, the accurate assessment of peptide depolymerization and FAA mineralization rates is important in order to gain a better process-based understanding of the soil N cycle. In this paper, we present an extended numerical 15N tracing model Ntrace, which incorporates the FAA pool and related N processes in order to provide a more robust and simultaneous quantification of depolymerization and gross mineralization rates of FAAs and soil organic N. We discuss analytical and numerical approaches for two forest soils, suggest improvements of the experimental work for future studies, and conclude that (i) when about half of all depolymerized peptide N is directly mineralized, FAA mineralization can be as important a rate-limiting step for total gross N mineralization as peptide depolymerization rate; (ii) gross FAA mineralization and FAA immobilization rates can be used to develop FAA use efficiency (NUEFAA), which can reveal microbial N or carbon (C) limitation

Heavy metals partioning in three French forest soils by sequential extraction procedure

To know the concentration of heavy metal associated to chemical compounds is important to evaluate the environmental risks on soils, particularly regarding ion bioavailability. The relative mobility and strength of binding of heavy metals to the soil components can be studied using a sequential extraction procedure which provides a meaningful comparison between different soil profiles. The heavy metals partitioning bas been identified in three different french forest soils : one cambic podzol, one calcaric cambisol and one mollic andosol, using a new sequential extraction method. Results show that metal fractionation is metal and site specific. The water leaching phase does not contain any metals. The proportion of heavy metal leached in the exchangeable and the acid — soplhuabslee is significant for Cd. The residual phase is important for Cr, Pb, and to a lesser extend for Ni. The organic matter fraction is dominant for Zn and Cu. Thus, the considered heavy metals are mainly bounded to iron oxides, to the organic matter and to the mineral residue. The order of heavy metals availability in the three soils would be : Zn>Cd>Cu>Pb>NiL&egatd;C r.i sotopes in some extracts of the extraction procedure corroborate the anthropogenic inputs for two soils. These both infomations allow to trace the origin, the mobility and the distribution of Pb in the soil

Aqua regia extractable selenium concentrations of some Scottish topsoils measured by ICP-MS and the relationship with mineral and organic soil components

The definitive version is available at www3.interscience.wiley.com.Peer reviewedPreprin

Responses of ectomycorrhizal fungi to mineral substrates

Boreal forest soils are complex, heterogeneous growth substrates where organic and mineral components provide nutrient resources for soil organisms and plants. Mineral nutrients are cycled between living and dead organic components of the forest soil and weathering of soil minerals provides an important input of new resources, compensating for losses from the ecosystem. Predicting soil responses to changing climate and management practices is important to determine their effect on forest production. Models for this purpose are largely based on the concept of the soil solution as the interface controlling soil processes such as weathering and nutrient uptake by plants, whereas soil microbiology recognises microbial processes as the driving force in soil nutrient cycling. In boreal forests most tree root tips are colonised by ectomycorrhizal fungi. The mycelia of these symbiotic fungi mediate nutrient uptake by their tree hosts. These fungi are abundant in the organic layer of forest soils and ectomycorrhizal research has therefore largely focused on nutrient uptake from this horizon. Minerals in the soil may, however, also serve as nutrient resources for ectomycorrhizal fungi. Through combined chemical and physical processes fungi can affect nutrient availability by weathering minerals. This thesis describes a field experiment investigating the distribution of different ectomycorrhizal fungi in organic and mineral forest soil horizons, in vitro studies of fungal acidification of artificial substrates with different mineral element composition, microcosm studies of growth and carbon allocation in intact ectomycorrhizal systems colonising heterogeneous mineral substrates and a preliminary investigation of changes in surface micro-topography of minerals colonised by ectomycorrhizal hyphae. Half of the fungal species identified in the forest soil occurred exclusively in the mineral horizons. Mycelial growth, carbon allocation and substrate acidification by fungi colonising different mineral substrates in vitro and in microcosms appeared to be influenced by mineral element composition. Interpretation of possible modification of mineral surface micro-topography is more difficult but together the results obtained suggest that ectomycorrhizal fungi may contribute to the development of microenvironments on colonised mineral surfaces, where increased weathering can take place. Processes regulating nutrient availability in such microenvironments are different from those estimated from the bulk soil solution

Spatial organisation of groundwater dynamics and streamflow response from different hydropedological units in a montane catchment

Funding was provided by the Leibniz Association (SAW-2012-IGB 4167) within the International Leibniz Graduate School: Aquatic boundaries and linkages- Aqualink. We would like to thank the NRI staff for their help during field work.Peer reviewedPostprin