When considering N2O production and its subsequent emission from the soil, it is worth to realize that N2O is mostly a product of various microbial activities in the soil. Classical idea on N2O production suggests respiratory denitrification (i.e. conversion of nitrate or nitrite to NO, N2O and N2) as the major microbial process responsible for most of N2O produced in terrestrial ecosystems, although autotrophic nitrification (i.e. conversion of NH4+ to NO3-) and dissimilatory nitrate reduction to ammonium (i.e. conversion of NO3- to NH4+) (Fazollari et al., 1990) as a source of N2O cannot be omitted in various conditions and ecosystems. Moreover, certain amounts of N2O can be evolved during other reactive N transformations (compare Figure 1), perhaps also due to heterotrophic nitrification carried out by many heterotrophic bacteria and fungi. In this process, reduced nitrogen can be either inorganic (similarly to autotrophic nitrification) and organic. Although there are reports of occurrence and importance of heterotrophic nitrification in various ecosystems including acidic soils, its ecological importance and the proportion of N2O possibly evolved during the process is largely unknown. Despite many uncertainties, however, it is clear that N2O production is regulated by a number of factors and conditions controlling either denitrification and nitrification rates and other N-transformations as well.
Atmospheric molecular nitrogen is assimilated by di-nitrogen fixing microorganisms in nitrogen fixation process (f) (or industrially); it has been hypothesized that during nitrogen fixation, certain amounts of N can be evolved as N2O, although the idea is not widely accepted. Ammonium form of nitrogen NH4+ is assimilated (a) into the biomass (R-NH2), fixed by clay minerals in the soil (j), volatilized as gaseous ammonia (v) and/or nitrified (n) and anaerobically oxidized in anammox reaction to N2 with NO2- as the electron acceptor (am). Nitrification is a source of N2O and NO as well. Resulting nitrite nitrogen (NO2-) can be chemically destroyed to several N gases (at rather low pH values < 4.0). Normally it is further oxidized in the second step of nitrification (n), and nitrate nitrogen (NO3-) is formed. Fate of nitrate is various: it can be assimilated (a), leached out of soil (l), or reduced to nitrite (NO2-)
due to several different processes and organisms: denitrified in respiratory denitrification by true denitrifiers (d) respired by nitrate respirers (i) or utilized for dissimilative nitrate reduction to ammonium (k).
Both in nitrate and nitrite reductions, NO and N2O are produced too. In respiratory denitrification first NO, then N2O and finally N2 are formed (d), while reduction of NO2- can finish in formation of NH+ (dnra). In many ecosystems and environmental conditions, respiratory denitrification is the major source of N2O (produced via nitrite and nitric oxide reductases), although under specific conditions, other processes can be significant. Nitrogen accumulated in biomass is evolved in mineralization processes (m) producing mostly NH4+, but also NO3- and NO2-, producing small amounts of N gases, too.
Denitrification occurs in general if organic carbon is available and nitrate (or nitrite) is present in a vicinity of denitrifying microorganisms in conditions of lowered partial pressure of molecular oxygen. Therefore increased denitrification can be expected in grazed grasslands receiving relatively unevenly distributed N and C in animal excreta, while soil is impacted by the presence of animals and animal trampling in a way often leading to creation of anoxic microsites in the soil matrix. Although experimental data on the direct effect of soil compaction (caused by the animal treading) on N2O fluxes are limited, it can be easily hypothesized that such simple relationship exists. For example, Menneer et al. (2005) found that dairy cows treading increased substantially denitrification rate due to reduction of soil aeration through soil physical damage and due to decreased soil N utilization by plants leading to temporary accumulation of mineral nitrogen in the soil. Which is even more important, grazing changes spatial distribution of nitrogen in a grazed ecosystem: nitrogen distributed over the pasture in plant biomass in relatively low concentrations now concentrates in animal excreta. This occurs because grazing ruminants utilize relatively little of the nitrogen consumed, excreting thus most of nitrogen ingested (de Klein and Ledgard, 2005). Nitrogen concentrated in urine and dung patches can not be fully utilized by grass stand and therefore it is available for microbial transformations (which are often characterized by significant "leaks" due to nitrogen gas formation in and subsequent emission from the soil system) as well as it is lost by leaching or via ammonia volatilization. By this mechanism, nitrogen is not only relocated in pasture ecosystem, but its significant proportion is easily lost, which leads to subsequent undesirable impacts of grazing agroecosystems.
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