The primary requirement for heterotrophic denitrifiers is an easily available carbon source, which will fulfill two roles: (1) simple organic carbon compounds provide a carbon source for growth of heterotrophic denitrifiers and (2) in an aerobic environment labile organic carbon stimulates the respiration of O2 which in turn will reduce the O2 concentration surrounding the respiring bacteria. The denitrifiers among them eventually are under pressure to switch from aerobic to anaerobic respiration using

NO 3. Many studies have shown an increase in denitrification rate when a labile carbon source was added to soils in the laboratory or in the field. The importance of labile organic matter was shown in studies on intact soil cores, and subsequently on dissecting these into smaller and smaller subcores. Between 25% and 85% of the denitri-fication activity could be attributed to a single decaying leaf or worm. This study and many other field studies have shown that denitrification is a very opportunistic behavior, leading to spatial and temporal hotspots of the production of N2O and N2.

Generally, the rhizosphere, the soil layer rich in root exudates and therefore high microbial activity, provides an ideal environment of high C and low O2 concentrations for denitrification to take place. The dense root mat of grasslands is particularly active and denitrification rates are larger than those in arable soils or forests.

Mineralization of complex carbon compounds in soil increase the pool of simple carbon compounds. The most well-studied processes that stimulate the mineralization of organic carbon are plowing, freeze-thaw events, and wetting of very dry soil (pulsing effect). Plowing aerates the soil and thereby stimulates mineralization of organic carbon compounds in parts of the soil previously not receiving the O2 concentrations required for mineralization to take place. Increased mineralization stimulated by freeze-thaw events or the pulsing effects are governed by the same principle. Extreme cold and extreme dry conditions kill a large part of the microbial community and reduce the activity of the survivors. When conditions become more tolerable, due to a thaw or rainfall, the remaining microbial population become activated and feed off the nutrients supplied by the dead microbial biomass and any nutrients deposited in the soil during periods of drought or freeze. These three events are usually accompanied by increased rates in soil respiration and NO and N2O emissions. Increased NO, N2O, and CO2 emissions are observed for a few days only and fade away when the supply of nutrient has diminished. Subsequent pulsing, freeze-thaw cycles produce smaller emission peaks, due to less substrate stored, compared to the first event. The dominant N gas emitted depends on the soil properties and meteorological conditions, with low aeration stimulating N2O/N2 emission, while high aeration stimulating NO emission. Emissions can be very high and these very sporadic events can significantly contribute to the overall annual NO or N2O flux.

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