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Direct Amino Acid Uptake by Plants related to Grassland Diversity - methodological and ecological Investigations
(2009)
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Leopold Sauheitl
- Uptake of intact amino acids by plants has been identified as an alternative nitrogen (N) source for plants in a number of ecosystems and soil types. Up to now it is assumed that this uptake strategy is of particular relevance in ecosystems with low mineral N (Nmin) contents due to insignificant microbial activity or due to poorly developed soils. However, it has also been discussed that amino acid uptake might enable plants to lower intracspecific competition for mineral N and shortcut the microbial mineralization of organic N in systems were competition is exceptionally high. The positive effect of plant diversity on plant productivity is known to induce conditions of intense N competition and thus amino acid uptake might explain how plant communities enable higher productive with increasing diversity. However, the ecological importance of organic N uptake has also been questioned due to the high competitive power of microbes in soils of the temperate zone and due to a number of flaws in the commonly used method to proof and quantify direct amino acid uptake. In this, dual labelled (13C and 15N) amino acids are injected into the soil and direct tracer uptake is quantified via bulk isotope measurement of 13C and 15N enrichment in plant tissues, which recently has been challenged to exclusively reflect direct amino acid uptake. The first objective therefore was to identify and reduce methodological influences on the direct amino acid uptake by plants. Thus the effect of changed amino acid concentrations on amino acid uptake was investigated by application of different tracer amounts. Next, the accuracy and precision of commonly used bulk isotope measurements were compared to compound specific measurements with respect to the determination of direct amino acid uptake. It was shown that the use of high tracer amounts accompanied by high Nmin release reduces direct amino acid uptake via plant internal down regulation of amino acid transporters. This corroborates the importance of minimizing tracer amounts and suggests that plants can actively increase amino acid uptake when N availability in soil is low. Bulk measurements turned out to overestimate direct amino acid uptake by a factor of up to six, as they were not able to separate uptake of intact tracer molecules from uptake of tracer fragments or inorganic carbon. At the same time compound specific isotope measurements proofed to be an accurate and precise tool to demonstrate and quantify uptake of intact amino acids. Using these optimized methods, the importance of amino acid uptake for the N-nutrition of plants with respect to changing plant diversity was investigated. The uptake of amino acids and mineral N by plants as well as the competition between plants and microbes for amino acid N was investigated in grassland communities with 1 to 16 grassland species. Microbes were superior competitors for amino acid derived nitrogen, irrespective of plant diversity and took up 54% of the applied amino acid N in average within 24 h. In contrast, plants only incorporated 2.7% of the applied N and were thus less effective by a factor of 20 in short term N acquisition than microbes. In addition, plant mineral N uptake decreased with increasing plant diversity while uptake of intact amino acids increased. Thus the contribution of amino acid uptake to the overall plant N nutrition increased from 1.5 to 7.0% in which amino acid uptake was mainly controlled by plant N concentration shoot biomass and rooting density while mineral N uptake was controlled by microbial competition. In detail amino acid uptake increased with decreasing plant N concentration while mineral N uptake decreased with increasing microbial abundance and microbial N uptake. Thus, the boosted importance of amino acid uptake for plant N nutrition has to be seen as a reaction on increased N competition with increasing plant diversity. Additionally, plant diversity stimulated microbial diversity which was most likely due to the bottom up effect of increased root exudation and litter input caused by increasing N competition and plant productivity, respectively. While the microbial community was dominated by bacteria (54.7%) the abundance of litter and soil organic matter decomposing gram positive bacteria and fungi as well as protozoan abundance increased with increasing plant diversity. Protozoa are known to stimulate turnover of bacteria which was indicated by higher tracer incorporation of this microbial group and an overall increase of deaminase activity with increasing plant diversity. As higher microbial turnover is associated with increased loss of microbial N to plants, we have to expect higher N availability for plants in the long term. The positive feedback of a plant-induced higher microbial turnover rate on N availability in soil together with an increased amino acids uptake might therefore be an important model to explain the positive effect of plant diversity on plant productivity.
