N uptake

Dodds WK, L6pez AJ, Bowden WB, Gregory SV, Grimm NB, Hamilton SK, Hershey AE, Marti E, McDowell WH, Meyer JL, Morrall D, Mulholland PJ, Peterson BJ, Tank JL, Valett HM, Webster JR, Wollheim WM (2002) N uptake as a function of concentration in streams. J N Am Benthol Soc 21 206—220... 

Furthermore, when part of the receiver s root system was placed in nutrient solution (split-root technique), the receiver grew much better. The authors concluded that quackgrass caused an allelopathic inhibition of K+ and N uptake by maize (12). 

Field trials to evaluate the N efficiency in terms of comparative yield or N uptake, respectively, relative to control or mineral fertilized plots... 

To calculate the inflow, Cta must be found from the concentration in the soil bulk taking into account rates of transport through the soil. Kirk and Solivas (1997) have done this for N uptake by rice growing in flooded soil and used the resulting model to assess the relative importance of root uptake properties and transport through the soil. Their results are summarized in the following. 

Kirk and Solivas measured the time course of N uptake by soil-grown rice plants and the simultaneous changes in soil solution NH4+ and root length density, and then compared the results with the calculated minimum root length densities required to explain the uptake. The calculation was based on the following picture of events. 

This is somewhat surprising in dryland crops the total root length is generally far larger than necessary to account for N uptake. An important difference is that, as a result of NH4+ adsorption on the soil solid, unlike for NOs , the concentration of NH4+ in the soil solution is less than for high affinity NH4+ transporters in the root and so V < Vmax and a larger root length is required. 

Figure 6.8 The time-course of (a) N uptake, (b) soil solution NH4, and (c) and (d) root length density in pots of flooded soil planted with rice with (O) and without ( ) added N. In (a), lines are fitted logistic curves, slopes of which give values of dU/dt in Equation (6.10). In (b), solid horizontal lines are Cl broken lines Cls calculated with Equation (6.11). In (c) and (d), the lines indicate the minimum root length densities required to explain uptake calculated with the measured Cl values se (full lines) and Cl derived from exchangeable NH4+ values se (Kirk and Solivas, 1997). Reproduced by permission of Blackwell Publishing...

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake of citrus was studied by Serna et al. (1992). In short-term water culture experiments with 15N labelled NH/ or N03 at various concentrations, citrus seedlings absorbed... 

Norkus, E.P, Bryce, G.F., and Bhagavan, H.N., Uptake and bioconversion of alpha-tocopheryl acetate to alpha-tocopherol in skin of hairless mice, Photochem. Photobiol., 57, 613, 1993. 

Based on Bronk (2002) with an uncertainty of about 30%. Turnover rate based on her estimate that about 40% of N uptake by phytoplankton ends up as DON. 

Another consideration when calculating f-ratios is the diurnal variabUity of N versus carbon (C) metaboUsm. When relating new production to C export (sensu Eppley and Peterson, 1979), stoichiometric calculations of C export from N uptake can be in error when C and N uptake are not evaluated over appropriate timescales. Diel variability in uptake rates of various N compounds has been demonstrated (e.g., Cochlan and Harrison, 1991b Cochlan et al., 1991a Tremblay et al., 2000 Wilkerson and Grunseich, 1990) and faUure to integrate daUy uptake rates over an entire diel cycle may seriously bias estimates of f-ratios. 

In addition to complicating our view of the marine N cycle, high rates of inorganic N uptake by bacteria and organic N uptake by phytoplankton may compromise stoichiometric calculation of C assimilation from N uptake (or vice versa) using the Redfield C N ratio in the euphotic zone (Wheeler and Kirchman, 1986). Further, bacterial uptake of NH4+ and NOs can confound computation of the f-ratio leading to over- or underestimates of new production. Consequently, estimating C export from N-based estimates of new production needs to be reevaluated for a variety of reasons. 

A variety of phytoplankton are known to possess cell surface oxidases (Palenik and Morel, 1990a,b) and extracellular oxidation of amino acids has been shown to occur in nature (Pantoja and Lee, 1994 Mulholland et al., 1998, 2002a). Direct uptake of amino acid-derived N from this process represented up to 4% of the observed NFI4+ uptake in a mid-Adantic estuary (Mulholland et al., 2003). Recently, a cell surface protein expressed under N-limitation was identified as a deaminase suggesting that these enzymes are regulated by cellular N status as for other pathways of N uptake and metabolism (Palenik and Koke, 1995). Failure to account for alternative pathways for mobilization of DON might result in underestimates of its utilization in nature. 

Light is a key environmental factor having an impact on nearly aU phytoplankton physiological processes N uptake is no different. Nutrient uptake versus irradiance curves often show a hyperbolic relationship that have been mathematically fit by... 

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