No abstract

Pyromorphite (Pb5(PO4)3Cl), the most stable lead mineral under a wide range of geochemical conditions [1], can form in urban and industrially contaminated soils [2] [3] [4] [5]. It has been suggested that the low solubility of this mineral could reduce the bioavailability of lead, and several studies have advocated pyromorphite formation as a remediation technique for lead-contaminated land [3] [5] [6], if necessary using addition of phosphate [6]. Many microorganisms can, however, make insoluble soil phosphate bioavailable [7] [8] [9] [10], and the solubilisation of insoluble metal phosphates by free-living and symbiotic fungi has been reported [11] [12] [13] [14] [15]. If pyromorphite can be solubilised by microbial phosphate-solubilising mechanisms, the question arises of what would happen to the released lead. We have now clearly demonstrated that pyromorphite can be solubilised by organic-acid-producing fungi, for example Aspergillus niger, and that plants grown with pyromorphite as sole phosphorus source take up both phosphorus and lead. We have also discovered the production of lead oxalate dihydrate by A. niger during pyromorphite transformation, which is the first recorded biogenic formation of this mineral. These mechanisms of lead solubilisation, or its immobilisation as a novel lead oxalate, have significant implications for metal mobility and transfer to other environmental compartments and organisms. The importance of considering microbial processes when developing remediation techniques for toxic metals in soils is therefore emphasised.

Sea-water level rise leads to increased saltwater intrusion causing soil salinity on arable land with negative effects on soil microbial processes. Organic amendments are known to reduce the effects of salinity on soil microorganisms, therefore positively influencing microbial activity and nutrient cycling. However, the extent of this effect in paddy rice soils under aerobic compared to anaerobic conditions is unknown. Consequently, benefits of organic matter addition on carbon (C) and nitrogen (N) mineralisation under saline conditions were evaluated in a short-term laboratory incubation experiment. Two soils from Bangladesh were incubated with rice straw, manure or a manure-rice straw mixture at 50 and 100% water holding capacity (25 • C, 27 days). In addition, NaCl was added to half of the samples, which resulted in a set of non-saline (EC e = 1.1-1.3 dS m −1 ) and saline (EC e = 24.0-32.4 dS m −1 ) soils. Soil respiration (CO 2 -release) was measured throughout the experiment. At the end of the experiment, dissolved organic C, inorganic N, microbial biomass as well as bacterial, archaeal, and fungal domains were determined. Overall, effects of substrate addition overruled effects caused by salinity and water content. Microbial activity and biomass in particular fungi increased most strongly after rice straw addition and resulted in N immobilisation independent of moisture level. Rice straw and manure alleviated the effects of salinity on microorganisms; these were therefore mainly detectable in the non-amended soils. The reason for this is likely a higher C availability for soil microorganisms after amendment of organic materials, which allows them to produce osmolytes, counteracting the osmotic effects of increased salinity. However, the microbial communities of the two soils under investigation showed different response patterns to salinity reflected by a substantially higher fungi-bacteria ratio in soil B prone to salinity at 50% WHC as compared to soil A. Likewise, the metabolic quotient Wichern et al. Salinity Alleviation and Soil Microorganismswas higher in soil B and not affected by any of the short term treatments, revealing a soil legacy. Our results highlight the importance of organic amendments, such as rice straw to paddy soils under saline conditions to reduce negative effects on soil microbial processes, allowing them to maintain their major functions.

The aim of this study was to ascertain whether metal resistance in willow (Salix) clones grown in a hydroponics screening test correlated with data from the same clones grown independently in a field trial. If so, results from a short-term, glasshouse-based system could be extrapolated to the field, allowing rapid identification of willows suitable for planting in metal-contaminated substrates without necessitating longterm field trials. Principal Components Analysis was used to show groups of clones and to assess the relative importance of the parameters measured in both the hydroponics system and the field; including plant response factors such as increase in stem height, as well as metal concentrations in plant tissues. The clones tested fell into two distinct groups. Salix viminalis clones and the basket willow Black Maul (S. triandra) were less resistant to elevated concentrations of heavy metals than a group of hardier clones, including S. burjatica 'Germany,' S.x dasyclados, S. candida and S. spaethii. The more resistant clones produced more biomass in the glasshouse and field, and had higher metal concentrations in the wood. The less resistant clones had greater concentrations of Cu and Ni in the bark, and produced less biomass in the glasshouse and field. Significant relationships were found between the response of the same clones grown the in short-term glasshouse hydroponics system and in the field.