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

5.1. Introduction
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Nitrogen (N) alongside carbon (C) is the element needed in greatest abundance by lower and
higher plants. Due to losses by leaching and microbial consumption, N is often a limiting factor for
plants, especially in soils with temperate and humid climates. Therefore, plants have developed
mechanisms to cope with the low N supply. These include very sensitive uptake systems and
possibility to grow on various N sources. The main N sources include ammonium (NH + 4) and
nitrate (NO - 3) (Tischner, 2000) however, in some environments amino acids are considered as
significant organic N source (Nasholm et al., 2000; Xu et al., 2006).
Utilization of N in the form of either nitrate (NO - 3) or ammonium ions (NH4 + ) may affect the
carbohydrate metabolism and energy economy of the plant (Blacquiere, 1987). NH4 + is fixed by
GS/GOGAT pathway into amino acids (glutamine/glutamate), this incorporation occurs
immediately after N uptake in roots and no significant amount of ammonium have ever been
discovered in the xylem sap (Tischner, 2000). In the case of NO3 - as N-source is well established
that it is taken up by roots and its reduction occur in both roots and shoots of higher plants
(Beevers, 1980). Reduction of NO - 3 to NH4 + , catalyzed by nitrate and nitrite reductase enzymes,
together with subsequent assimilation of NH4 + is among the most energy-intensive processes in
plants and in some cases is accompanied by additional respiration.
However, there are still active debates on the effect of the N source on root respiration, as
attempts to explain it experimentally have led to arguable results supporting different hypotheses.
Some authors suggest that, when compared to NO3 - nutrition, NH4 + nutrition stimulates the rate of
root respiration, attributing this increase to the stimulation of alternative pathway activity (Barneix
et al. 1984; Blacquière 1987, Lasa et al., 2002). There are two pathways involved in respiration: the
phosphorylating cytochrome and the nonphosphorylating alternative pathway. The physiological
role of the latter is not clear but several authors suggest that this alternative pathway could avoid the
overreduction of the electron transport chain and the subsequent production of reactive oxygen
species (Purvis and Shewfelt 1993). Thus, this pathway could allow oxidation of TCA cycle
reductant, maintaining TCA cycle carbon flow for provision of biosynthetic intermediates for NH4 +
ion assimilation.
On the other hand, NO3 - coming to the plant before assimilation have to be firstly reduced to
NH4 + , and this process, together with assimilation, is among the most energy-intensive processes in
plants, in some cases followed by an additional CO2 evolution (Atkins et al., 1979; Aslam and
Huffacker, 1982; Ninomyia and Sato, 1984; Warner and Kleinhofs, 1992; Blacquiere, 1987;
Tischner, 2000). The process proceeds in two steps: conversion of NO3 - to NO2 - and the following
conversion of NO2 - to NH4 + . In illuminated leaves, both processes are coupled to photosynthetic
electron transport. However, in roots and during darkness, reducing equivalents are generated by