Flooding Relay Denitrification Mechanism In The Soil

Denitrification in soil is mainly mediated by microorganisms. For a long time, people have considered the denitrifiers as the main contributors in soil denitrification, while the action of other microorganisms in soil is neglected. Many data indicate that the denitrifiers are not the dominant bacterial flora that are able to reduce nitrate or nitrite in the soil. In 1987-1990, when people studied the effect of minimal tillage on soil microorganisms, they enumerated the populations of nitrate reducers and dinitrifiers for 21 times in continuous 3 years and found the ratio of denitrifiers to total nitrate reducers was only 4.4%~37.1%. Moreover, the activity of denitrifiers as pure culture was found to be insignificantly different to that of nitrate reducers. Therefore it is postulated that the denitrification in soil is likely to be the cooperation of denitrifiers and other microorganisms.Based on this point of view, we hypothesized that denitrification hi soil is not only participated by denitrifiers, many other partial denitrifiers also participate the process at the same time. Product produced by one partial denitrifier may serve as another's substrate and finally complete the whole denitrification. This mechanism of denitrification is termed successive denitrification. The purpose of this paper is to isolate some partial denitrifiers from the soil and to demonstrate the presence of this mechanism in the soils.Soil used in present work was sampled from the submerged paddy field of Changshu, where the Changshu Agroecological Experimental Station, Institute of Soil Science, Chinese Academy of Sciences, has been established since 1987. Strains were isolated from a soil that had been enriched by supplying N2O as sole electron acceptor and soil extract as substrate. When purified, the isolated strains were characterized for their reduction of different nitrogenous oxides. In this work, we have isolated 7 strains which were complete denitrifiers, and two strains which could reduce NO2'to N2 only, 39 strains which were able to reduce NO3- to NO2- only, and 1 strain which could reduce NO2- to N2O only. When strain 21-6-9-2 (one strain which is able to reduce NO3- to NO2- only) and strain 1-9-5-3 (one strain which is able to reduce NO2- to N2O only)were mixed into sterile soil with proper ratio and without adding any additional carbon substrate, NO3- was reduced to N2O, demonstrating that partial denitrifiers could cooperate in substrate uses and play similar role as that by typical denitrifiers, thereby support the hypothesis of successive-denitrification mechanism.The above-mentioned hypothesis could be used to explain ,at least in part, N2O production in different soils. To do this, the MPN procedures were used for determining the patterns of N2O production and consumption across the serially diluted soil suspensions. This method has been widely used for estimating culturable microbial populations in soils and other environmental samples. The final concentration of N2O in soil was determined by both N2O producers and N2O consumers. Nitrous oxide produced by N2O producers might be consumed by N2O consumers, the final main product of denitrification in soil was N2O when the N20 consumers were the dominant bacteria while when the N2O producers were the dominant bacteria, the final main product of denitrification in soil was not N2O or N2O was very small. This conclusion had proved that N2O consumers in soil could consume N2O which was produced by N2O producers and reduce it to N2. Therefore, different partial denitrifiers in soil were able to cooperate and play similar role as that by typical denitrifiers, successive-denitrification mechanism in soil was approved again.Because we didn't add any other organic carbon in the substrate in culturing, the carbon originally existed in soil can be used as electron donor in successive-denitrification (one of the necessary factor in denitrification). In this experimental condition, there was some NO2- accumulated and some NO3- remained, it showed that the electron donor...