Browsing by Author "Keitel, C"

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    Measured deuterium in water vapour concentration does not improve the constraint on the partitioning of evapotranspiration in a tall forest canopy, as estimated using a soil vegetation atmosphere transfer model
    (Elsevier, 2011-06-15) Haverd, V; Cuntz, M; Griffith, DWT; Keitel, C; Tadros, CV; Twining, JR
    Partitioning the evapotanspiration (ET) flux in a forest into its component fluxes is important for understanding the water and carbon budgets of the ecosystem. We use non-linear parameter estimation to determine the vertical profile of the Lagrangian timescale (T(L)) and partitioning of ET that simultaneously optimise agreement between modelled and measured vertical profiles of temperature, water vapour, carbon dioxide concentrations, and deuterated water vapour for a two-week period in November 2006. High precision real-time trace gas measurements were obtained by FTIR spectroscopy. Modelled temperature and concentration profiles are generated using a Lagrangian dispersion theory combined with source/sink distributions of HDO, H(2)O, sensible heat, and CO(2). These distributions are derived from an isotopically enabled multilayer Soil Vegetation Atmospheric Transfer (SVAT) model subject to multiple constraints. The soil component of the model was tested in isolation using measured deuterium content of soil chamber evaporate, while the leaf component was tested using isotopic analyses of leaf and xylem water, combined with leaf-level gas exchange measurements. Optimisation of T(L) and the partition of ET was performed twice: once using only temperature, H(2)O and CO(2) profiles and a second time including HDO as well. The modelled vertical concentration profiles resulting from inclusion of HDO in the cost function demonstrate our ability to make consistent estimates of both the scalar source distributions and the deuterium content of the water vapour sources. However, introducing measurements of deuterium in water vapour does not significantly alter resulting estimates of normalised T(L) (0.4 +/- 0.1 at canopy top) and the partition of ET(85 +/- 2% transpiration), suggesting that the additional data and modelling required to use deuterium are not warranted for the purpose of partitioning ET using the framework presented here. Crown Copyright (C) 2011 Published by Elsevier B.V.
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    Plant and microbial uptake of nitrogen and phosphorus affected by drought using 15N and 32P tracers
    (Elsevier, 2015-03) Dijkstra, FA; He, MZ; Johansen, MP; Harrison, JJ; Keitel, C
    Competition for nutrients between plants and microbes is an important determinant for plant growth, biodiversity and carbon cycling. Perturbations such as drought affect the availability of nitrogen (N) and phosphorus (P), and may cause shifts in uptake of N and P between plants and microbes. Competitiveness for these nutrients may depend on how flexible plants and microbes are in taking up N and P. We used a novel dual isotope labelling technique (15N and 32P) to assess short-term uptake of N and P by plants and microbes affected by drought in two different plant–soil systems. Mesocosms were extracted from two grassland sites differing in soil nutrient availability and plant species. Half of the mesocosms were subjected to drought one week prior to injection of 15N (as KNO3) and 32P (as H3PO4) tracers. Uptake rates of NO3- and P in plants and microbes were estimated based on average source pool enrichment during the labelling period and on plant and microbial recovery of 15N and 32P measured after 4 days of labelling. Overall competition for N and P was reduced with drought as less NO3- and P was taken up in plants and microbes. However, plant NO3-uptake of was more sensitive to drought than microbial NO3- uptake, while microbial P uptake was more sensitive than plant P uptake. These different sensitivities to drought by plants and microbes may decouple the N and P cycle with increased drought conditions. © 2015 Elsevier Ltd
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    Plant uptake of nitrogen and phosphorus among grassland species affected by drought along a soil available phosphorus gradient
    (Springer Nature, 2020-01-10) Mariotte, P; Cresswell, T; Johansen, MP; Harrison, JJ; Keitel, C; Dijkstra, FA
    Aims Here we assessed N and P uptake of four grassland species grown together in response to a short-term drought event along a soil P gradient. Methods We used 15N and 32P tracers to examine uptake of N and P by the grasses Bothriochloa macra, Themeda triandra, Lolium perenne and Microlaena stipoides grown together in pots with initial available P levels of 3, 8, 12, 20 mg P kg−1 soil. Soil moisture in half the pots was reduced from 60 to 30% water holding capacity during a 7-day period to simulate drought. Results Plant P uptake was strongly reduced by drought in all species across all P levels, much more so than N uptake, indicating decoupling in N and P uptake. Soil available P (Bray method) was not affected by drought, suggesting that the reduced P uptake with drought was due to reduced soil P mobility. Plant competition for N and P changed with drought and soil P levels, where relatively more N and P was taken up with drought by M. stipoides at the lowest soil P level. Conclusions We showed that greater reductions in P than in N uptake and shifts in N and P uptake among species caused by a short-term drought have strong consequences for plant growth. © Springer Nature Switzerland AG 2020
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    Plant-microbe competition for nitrogen and phosphorous affected by drought
    (The University of Western Australia, 2014-08-05) Dijkstra, FA; He, MZ; Johansen, MP; Harrison, JJ; Keitel, C
    Competition for nutrients between plants and microbes is an important determinant for plant growth, biodiversity and carbon cycling. Perturbations such as drought affect plant-microbe competition for nitrogen (N) and phosphorus (P). Despite the importance of these nutrients in most ecosystems, plant-microbe competition for N and P remains poorly understood. We used a novel dual isotope labelling technique (15N and 32P) to assess plant-microbe competition for N and P affected by drought in two different plant-soil systems. Mesocosms were extracted from a grassland site where plants were strongly limited by N (N-limiting system) and from a grassland site that showed strong soil P adsorption (P-adsorbing system). Half of the mesocosms were subjected to drought one week prior to injection of the tracers. Stable 15N (as KNO3) and radio-labelled 32P (as H3PO4) were injected, and measured in the plant and microbial biomass 72 hrs later. Microbial uptake of 32P was strongly reduced by drought (on average by 89%), while microbial 15N uptake was not. In contrast, drought reduced plant uptake of 15N (by 28%), but not of 32P. Microbial 15N uptake was much larger in the N-limiting system than in the P-adsorbing system (by 491%), while plant 32P uptake was much larger in the P-adsorbing system than in the Nlimiting system (by 703%). Both plants and microbes showed large flexibility in taking up 15N and 32P with the largest uptake of the nutrient that was in greatest demand. Our results suggest that under drought conditions, plants lose in terms of N uptake, but win in terms of P uptake when competing for these nutrients with microbes. These different sensitivities to drought by plants and microbes may enhance decoupling of the N and P cycle with increased drought conditions, depending on if plants and microbes are N or P limited.
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    Re-allocation of nitrogen and phosphorus from roots drives regrowth of grasses and sedges after defoliation under deficit irrigation and nitrogen enrichment
    (John Wiley & Sons, Inc, 2021-09-04) Wang, RZ; Cresswell, T; Johansen, MP; Harrison, JJ; Jiang, Y; Keitel, C; Cavagnaro, TR; Dijkstra, FA
    1. Re-allocation of nutrients from roots to shoots is essential for plant regrowth in grasslands, particularly in nutrient-poor conditions. However, the response of root nutrient re-allocation to changes in nitrogen (N) and water availability remains largely unknown. 2. Using a novel 15N and 32P labelling technique, we quantified the contribution of N and phosphorus (P) to shoot regrowth from either root re-allocation or direct soil uptake for perennial grasses exposed to high-frequency deficit irrigation (HFDI) and N addition. 3. Without N addition, HFDI showed no impact on uptake and re-allocation of N and P, likely due to unaffected soil N availability and a greater diffusion barrier offsetting increased accumulation in plant-available soil P. With N addition, HFDI increased plant N rather than P uptake, because of increasing soil N availability instead of P under combined HFDI and N addition. The HFDI decreased both N and P re-allocation with N addition, possibly due to exhaustion of nutrient reserves in roots that were re-allocated aboveground. Re-allocation contributed 48-97% of N and 58-79% of P required during the first two weeks of shoot regrowth. 4. Synthesis. Our results highlight the importance of N and P re-allocation from roots to buffer against changes in soil N and P availability and to maintain N:P ratio in shoot regrowth. © 2021 British Ecological Society