drivers of soil respiration of root and microbial ... - Unitus DSpace

oxidation of carbohydrates with subsequent evolution of CO2 (Aslam and Huffacker, 1982;
Ninomyia and Sato, 1984).
Depending on the species, the site of NO3 - reduction could be located in shoots or roots
(Andrews, 1986; Oaks and Hirel, 1985; Pate and Layzell, 1990; Schilling et.al., 2006; Silveira et al.,
2001; Vuylsteker et al., 1997). By this property, plants are divided into three groups: species
reducing NO3 - predominantly in roots, species reducing NO3 - predominantly in shoots, and those
that do both. The C costs for reduction of NO3 - to NH4 + depend on the site of nitrate reduction in
plants.
Nevertheless, the contribution of roots and shoots to whole plant nitrate reduction remains
also uncertain. The location of nitrate reduction site seems to be even not species- and cultivars-
dependant (Schilling et al., 2006; Silveria et al., 2001), but could vary within a single plant.
Previous studies confirm that environmental conditions, plant growing stage and the quantity of N
supply could influence and change the reduction site location within a plant, although the major part
of these findings were done in nutrient solution studies, which do not reflect the real plant uptake
rates, root and microbial respiration under true soil conditions. It was found that NO3 - translocation
from the root to the shoot depends on the N concentration in soil solution. At low NO3 -
concentration the reduction occurs mainly in roots, at higher concentrations storage and then
transport is normally adjusted (Atkins et al. 1979, 1980; Oscarson & Larsson 1986; Agrell, et al.,
1997). The nitrate uptake and reductase capacity is not equally distributed along a root axis and not
identical in roots of different age and ontogeny (Lazof et al., 1994; Cruz et al., 1995; Di Laurenzio
et al., 1996). Siebrecht, et al. (1995) and Pan et al. (1985) located a high uptake rate together with a
highest reductase activity in the root apical region. Older root parts were more active in root uptake,
but nitrate reductase was low, indicating a possible NO - 3 translocation from these root parts to the
shoots. Gojon et al. (1986) have found that roots participate actively in the reduction of incoming
nitrates during induction process, shortly after the fertilization, after that the root contribution
decrease sufficiently.
Based on these results, obtained mainly from the experiments in nutrient solutions, we
hypothesized that the form of N nutrition may have a significant effect on the amount of CO2
released by root respiration under true soil conditions. Additionally, we expected that the effect of
the form of N nutrition may change between the species with different location of nitrate reduction
site and during the growth of a single plant.
Summarizing the above findings and uncertainties, the aims of the study were: a) to evaluate
the effect of N form (NO3 - vs. NH4 + ) on CO2 efflux from soil and root respiratory losses for species
with different location of nitrate reduction site b) to verify if the relative contribution of roots to the
whole plant nitrate reduction process remains stable during various growing stages of a single
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