Impacts of trace amounts of labile C on plant N limitation vary from hour to week timescales |
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| Authors: | Juha Mikola Stéphane Saj |
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| Institution: | 1. Department of Environmental Sciences, University of Helsinki, Niemenkatu 73, 15240, Lahti, Finland
2. Groupe ISA, 48 bd Vauban, 59046, Lille Cedex, France
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| Abstract: | The ability of terrestrial ecosystems to store carbon (C) under rising atmospheric CO2 will depend on how severely nitrogen (N) will limit plant growth. We tested whether increased C availability in the soil at elevated CO2 could affect N limitation by inducing N release from soil organic matter (SOM). We established microcosms composed of Holcus lanatus plants, field soil (containing “old” SOM) and 15?N-labeled plant litter (representing “new” SOM), simulated different levels of root C release by adding a single pulse of 0, 18, 44, or 175?μg glucose C?g?1 dry soil and recorded the effects on soil microbial biomass, microbial-feeding protozoa and nematodes and plant performance 1, 3, 9, and 32?days after C addition. The effects on H. lanatus growth and N uptake depended on the amount of added C and the time elapsed since addition. Shoot N concentration and N content were higher in pots amended with 44?μg?C g?1 soil than in other pots 1?day after C addition. Later, 9 and 32?days after C addition, the highest glucose addition reduced the dry mass, N concentration, and N content of H. lanatus shoots in comparison to other treatment levels. Microbial biomass was generally higher in soils subjected to 44?μg glucose C?g?1 soil than in control soils, and, at the last harvest, the numbers of protozoa were significantly higher in all soils with glucose amendments than in control soils. No effects on microbial-feeding nematodes were found, and plant N uptake from “old” and “new” SOM was equally affected by C addition. Our results seem to suggest that, while a low pulse of labile C can increase plant N uptake temporarily on an hour scale, higher amounts of C will intensify plant N limitation at timescales of days and weeks. Generalization of such dose and time dependent results requires great caution, but if verified in other plant–soil systems as well, they would suggest that plant N availability under elevated C availability may depend on the balance between positive and negative effects operating at different timescales and triggered by additional C pulses of varying size. |
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