Turnover of S-containing amino acids (cysteine and methionine) in grassland soils and their availability to plants

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  • Deying Wang

    Research areas

  • Cysteine, Methionine, Biodegradation, Mineralization, Grassland productivity gradient, Free amino acid, Plant-microbial competition, Radioisotope tracers, Rhizosphere, Root exudation, Maize, Isotope pool dilution method, Labile organic S, S turnover

Abstract

Sulphur (S) is one of the most important elements in nature with large quantities cycling through the biosphere, atmosphere and geosphere annually. In recent decades an increased frequency of S deficiency has been observed in soils from many regions of the world. Consequently, in agricultural systems with low S inputs from fertilizers and atmospheric deposition, plants must rely heavily on the release of S from soil organic matter. Typically, less than 10% of the soil S pool occurs as inorganic sulphate with most soil S held in an organic form. Sulphate, together with simple organic S compounds (low molecular weight organic S compounds such as cysteine (Cys) and methionine (Met)), are regarded as potentially being plant-available. However, little information is available on the mineralization and availability of soil organic S, and this has severely limited our capacity to understand the factors that regulate its persistence, bioavailability, and movement in soil. Therefore, the overall aims of this thesis were to explore the bioavailability and biodegradation of Cys and Met in grassland soils. This was achieved by first investigating the intrinsic dissolved organic sulphur (DOS) and inorganic sulphate concentrations in grassland soils (Chapter 3). The results revealed that DOS is the dominant fraction of dissolved S in grassland soils. These results enabled us to optimise the design of the following experimental chapters. The following experiment (Chapter 4) quantified microbial uptake and mineralization of Cys and Met using 14C labelling. My results revealed that 14C-labelled Cys and Met were directly and rapidly assimilated by soil microbes, with assimilation rates being an order of magnitude (or more) faster than microbial mineralisation rates measured via 14CO2 evolution. The considerable delay between uptake and mineralization indicates that the turnover of Cys and Met in soil solution was largely biotically mediated. In a subsequent experiment (Chapter 7), I investigated the concurrent mineralization of S, C and N from Cys and Met, as well as the influence of available C, N and S on this process. My results revealed that while a large proportion of added Cys-C and Met-C were used for microbial respiration and microbial biomass incorporation, N and S were excreted as by-products into soil (NH4+ and SO42-). We assume this was due to the low C/N ratio of these amino acids (C/N ratio = 3 for Cys, and 5 for Met). This assumption was supported by the results that glucose addition promoted a more complete utilization of both amino acids, whereas nutrient addition had less effect. In Chapter 6, I quantified the gross S mineralization rates and the size of the total labile S pools in a closed incubation experiment (70 days) using a 35S isotope-pool dilution-based method. Another critical question is whether plants can acquire S containing amino acids from soil solution, and if so, how does this compare with the uptake rate of inorganic sulphate? To answer this question, I measured the root uptake of three individual S compounds, namely Cys, Met, and sulphate labelled with 14C or 35S, over short time periods (24 h) at an ecologically relevant concentration (100 µM; Chapter 8). Our results revealed that sulphate is preferred to S-containing amino acids by maize plants. A large proportion of the exogenously applied Cys (66%) and Met (73%) could have been taken up rapidly intact by the roots under sterile hydroponic conditions, indicating that they may supply a significant proportion of S to plants when microbial populations are low. I then studied the microbial-plant competition for S-containing amino acids in a rhizosphere context (Chapter 5). My results revealed that < 10% of the added amino acid-14C was captured by the plant, while the rhizosphere microbial community assimilated > 75%. The addition of inorganic N and S, not C, reduced the uptake of Cys and Met from soil by the maize plants, indicating that amino acid utilization may be regulated by inorganic N and S availability. In conclusion, this research provides a more detailed understanding of the turnover of Cys and Met in soils and their availability to maize plants. In addition, it demonstrated the importance of DOS, its rapid turnover in soils, and the intense microbial and plant competition which exists for this resource. The results will also help develop more accurate models of S cycling in soils.  

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Original languageEnglish
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Award date8 Dec 2021