Microbial reduction of nitrate in irrigated soil after wastewater application

• Main Author: Ben Faraj, Khereya Ahmed
• Published: University of Edinburgh 2004
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.735416

Nitrogen is an essential nutrient for plant growth and is routinely added in the fonn of fertilizer or via irrigation water for optimal crop production. Nitrogen losses through denitrification have an environmental and economical impact. The end product of topsoil denitrification is likely to be N2O rather than N2, because in topsoil, N 2O readily diffuses into the atmosphere before further reduction to N2. This increases concerns over N2O emissions into the atmosphere, since N20 is a greenhouse gas and also causes depletion of the ozone layer. Furthermore, if the N added with the fertilization practice is being reduced to gaseous forms through the denitrification process, this has a negative impact on crop production and is not economically valuable. It is therefore important to estimate the N-gaseous losses through denitrification in the irrigated topsoil. It has been common practice since the 1950s to irrigate arid and semi-arid agricultural land with treated wastewater since this optimizes water use in water-limited environments and adds a considerable amount of N to the soil. Although using wastewater for irrigation purposes represents a good source of water in arid and semi-arid regions and also provides a considerable amount of some essential nutrients (e.g. N) which are required for plant growth, it also contains high levels of soluble sodium and exhibits a high Biological Oxygen Demand (BOD5), which can potentially increase gaseous N losses. Therefore, it is particularly important to study N losses through topsoil denitrification in topsoil of wastewater-irrigated arid and semi-arid land. Intact and repacked soil cores from two sites located in the Thessaloniki plain in Northern Greece were used in order to study the effect of irrigation water quality, soil texture and structure, soil moisture and oxidizable organic carbon on total topsoil denitrification rates. These two sites were separated by a distance of 10 km. The first site was the Galicos River Wastewater Treatment project, where soil cores of clay loam and sandy silt loam were obtained from two areas, each irrigated either w ith secondary wastewater or fresh water (well water) supplied by a furrow for four years. The second site consisted of fields located in the Axios Delta region, irrigated naturally by the flooding of the Axios River. Clay, clay loam and sandy loam soil cores were collected from these fields. The microbial reduction of nitrate in the top layer of irrigated soils was studied in laboratory topsoil incubations using the acetylene inhibition method. The use of acetylene to inhibit further microbial reduction of nitrous oxide to dinitrogen allows estimates of total denitrification to be obtained by gas chromatography analysis of nitrous oxide. Incubations of repacked soil cores from the Galicos River site which were amended with carbon in the form of glucose were found to have a total denitrification rate ranging from 1.41 to 2603 pg N kg' 1 dry soil day'1. Unamended repacked soil had total denitrification rates ranging from 1.1 to 2.3 pg N kg' 1 dry soil day'1. Total denitrification rates measured in incubations of intact soil cores from the same field site and amended with carbon were found to range from 1.21 to 92.90 pg N kg' 1 dry soil day'1. Unamended intact soil cores had total denitrification rates ranging from 1.08 to 1.33 pg N kg' 1 dry soil day'1. The differences in total denitrification rates in amended and unamended repacked soil cores were statistically significant and indicate that available carbon may be a factor limiting topsoil denitrification in repacked soil cores from this site. In addition, the differences in total denitrification rates between wastewater and well water irrigated soil cores from this site were statistically significant and indicate that soil quality after irrigation was probably a factor regulating the topsoil denitrification in repacked and intact soil cores. A crust layer produced on the surface of repacked and intact soil cores had no significant effect on total denitrification rates. Total denitrification rates in soil core incubations from the Axios Delta site ranged from 0.71 to 178 pg N kg' 1 dry soil day' 1 under field capacity moisture status and natural carbon concentrations. N 20 gaseous emissions in the soil core incubations from these fields were found to increase exponentially in the initial phase, after which N 20 concentrations decreased, probably due to the consumption of acetylene (C2H2) by micro-organisms allowing the reduction of N 20 to N2 to take place. This exponential increase in N20 emission indicates that a decrease in 0 2 diffusion into soil aggregates and into denitrification sites is the main factor limiting topsoil denitrification in the Axios field site. Results from the investigation of the effect of soil texture on denitrification rates disagreed with the general view that higher denitrification rates are expected to occur in fine-textured soils. In order to evaluate the accuracy of the N2O measurements, a 15N balance for the soil core incubations from the Galicos River site was calculated. This balance indicated that the 15N unrecovered from soil analysis at the end of the incubation time averaged 55 and 36 % of the nitrogen applied to repacked soil cores from wastewater and well water irrigated plots, respectively, and that unrecovered l5N accounted for an average of 14.43 and 12.07 % of the total nitrogen applied to intact soil cores irrigated with wastewater and well water, respectively. of these values, emitted N20 gas accounted for an average of 35 and 28 % of the nitrogen applied to repacked soil cores from wastewater and well water irrigated plots, respectively, and for an average of 1.02 and 0.07 % of the total nitrogen applied to intact soil cores irrigated with wastewater and well water, respectively. The unaccounted nitrogen losses at the end of incubation constituted an average of 2 0 and 8 % of the total nitrogen added to repacked cores under wastewater and well water irrigated plots, respectively, and an average of 13.4 and 12 % of the total nitrogen added to the intact cores. These unaccounted 1:,N losses indicate that there are additional sinks for added nitrogen besides those measured in the incubations, and that part of the N 20 escaped measurement because of its further reduction to N2 when incomplete acetylene inhibition of N 20 reductase occurred.