Although amino sugars represent a major component of soil organic nitrogen (ON), the assimilation of nitrate (NO3 −) and ammonium (NH4 +) into amino sugars (AS) by soil bacteria and fungi represents a neglected aspect of the global N cycle. A deeper knowledge of AS responses to N fertiliser addition may help enhance N use efficiency (NUE) within agricultural systems. Our aim was to extend a sensitive compound-specific 15N-stable isotope probing (SIP) approach developed for amino acids (AAs) to investigate the immobilization of inorganic N into a range of amino sugars (muramic acid, glucosamine, galactosamine, mannosamine). Laboratory incubations using 15N-ammonium and 15N-nitrate applied at agriculturally relevant rates (190 and 100 kg N ha−1 for 15NH4 + and 15NO3 −, respectively) were carried out to obtain quantitative measures of N-assimilation into the AS pool of a grassland soil over a 32-d period. Using gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) we found that δ15N values for individual AS reflected differences in routing of the applied ammonium and nitrate. The contrasting N-assimilation dynamics of bacterial and fungal communities were demonstrated through determinations of percentage 15N incorporation into diagnostic AS. N-assimilation dynamics of the bacterial community were altered with the applied substrate whilst fungal N-assimilation dynamics were unaffected. Rates and fluxes of the applied N-substrates into the bacterial AS pool reflected known biosynthetic pathways for AS, with fungal glucosamine appearing to be biosynthetically further from the applied substrates than bacterial glucosamine due to different turnover rates. This sensitive and specific compound-specific 15N-SIP approach using AS, building on existing approaches with AAs, enables differentiation of N-assimilation dynamics within the microbial community and assessment of microbial NUE with agriculturally relevant fertilisation rates.
A method was devised in which plant roots can be easily and uniformly radiolabelled with 14C for use in soil decomposition studies. The roots were labelled from an exogenous sugar solution for a total period of 48 hours after which root decomposition studies could be performed. The method offers distinct advantages over the existing constant 14C‐CO2 atmosphere labelling method.
How weathering affects the physiochemical properties of biochar and its long-term carbon (C) sequestration potential remains unclear. In this study, we mea
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Warmer winters in Arctic regions may melt insulating snow cover and subject soils to more freeze–thaw cycles. The effect of freeze–thaw cycles on the microbial use of low molecular weight, dissolved organic carbon (LMW-DOC) is poorly understood. In this study, soils from the Arctic heath tundra, Arctic meadow tundra and a temperate grassland were frozen to −7.5 °C and thawed once and three times. Subsequently, the mineralisation of 3 LMW-DOC substrates types (sugars, amino acids and peptides) was measured over an 8-day period and compared to controls which had not been frozen. This allowed the comparison of freeze–thaw effects between Arctic and temperate soil and between different substrates. The results showed that freeze–thaw cycles had no significant effect on C mineralisation in the Arctic tundra soils. In contrast, for the same intensity freeze–thaw cycles, a significant effect on C mineralisation was observed for all substrate types in the temperate soil although the response was substrate specific. Peptide and amino acid mineralisation were similarly affected by FT, whilst glucose had a different response. Further work is required to fully understand microbial use of LMW-DOC after freeze–thaw, yet these results suggest that relatively short freeze–thaw cycles have little effect on microbial use of LMW-DOC in Arctic tundra soils after thaw.
No abstract is provided for this article.
The difference in the ability of lytic microorganisms to bring about dissolution of the cell walls of the soil yeasts Cryptococcus albidus and C. terreus has been examined. It was found that the composition of the cell walls, which varied according to the cultural conditions employed, determined the extent to which the walls were lysed. Thus walls from cells of C. albidus grown under conditions favourable for growth contained α- and β-glucans and chitin as major components and were lysed by two Streptomyces spp. but not by a nonfruiting myxobacterium Cytophaga johnsonii. Significant lysis of C. albidus walls by the myxobacterium as well as by the Streptomyces sp. occurred, however, when the α-glucan component was considerably reduced by growing the yeast under unfavourable conditions. Ultrastructural studies showed the absence of definite layers in the wall. The chitinous residue after chemical extraction of the walls retained the general shape of the cell and was composed of microfibrils, in contrast to the granular chitinous residues from other yeasts e.g. Saccharomyces spp.
The pollination of the orchid Microtis parviflora by ants, in southern Victoria, is described. The ants are attracted to the inflorescence by a sweet perfume and feed on an abundance of nectar secreted by the labellum. The pollinarium is deposited on the frons of the ant and is so structured that layers of pollen are stripped off by contact with a sticky stigma. The orchid is autogamous but not parthenocarpic and 75 per cent of the flowers are pollinated by ants within 3 days of anthesis.
