Abstract
Nitrification tend to reduce the agronomic fertilizer use efficiency in cropping systems because nitrate, the end product of nitrification, has high potential to loss from soil environment due to denitrification and leaching. The population characteristics of ammonia oxidizing and nitrite oxidizing microorganisms carrying out nitrification in soils may affect the effectiveness of approaches taken to suppress nitrification. A study was conducted to assess the activity and abundance of nitrifiers in soils of intensively vegetable grown regions in Sri Lanka. Soil samples were collected from 72 locations across Nuwara Eliya, Marassana, Kalpitiya and Gannoruwa representing vegetable cultivated fields managed conventionally (n=45) and organically (n=9), and uncultivated areas (n=18). Basic soil characteristics were determined using standard procedures. Potential nitrification rate (PNR) and the abundance of ammonia oxidizers and nitrite oxidizers were estimated. Chemical characteristics of soils varied widely with pH, EC and organic C% ranging from 3.8 to 8.5, 0.04 to 0.94 dS/m and 0.9% to 4.5%, respectively. The PNR of the studied soils ranged from 0.18 to 15.80 NO3--N/kg/h. The abundance of ammonia oxidizers and nitrite oxidizers ranged from 1.96 to 5.97 log10CFU g-1 and 1.36 to 5.63 log10CFU g-1, respectively. The potential activity of nitrifiers did not correlate with the abundance of ammonia oxidizers or nitrite oxidizers. Thus, the functional and compositional diversities of nitrifying communities may be different across the soils. PNR values for studied soils are higher compared to reported values in literature. Hence, appropriate measures need to be taken to suppress nitrification as high nitrification rates could lead to reduce fertilizer use efficiency and increase risk of groundwater contamination with nitrate.
Highlights
11 % of the N-fertilizers imported to Sri Lanka is being consumed by vegetable sector, only second to tea sector (27 %) from among the systems that dominates cultivation under unsaturated soil moisture conditions
Fertilize-N is converted to NO3--N in soils at different rates depending upon the dynamics of soil nitrifiers
Presence of ammonia oxidizing archaea (AOA) in soil have been detected and several studies suggest that AOA may play an important role in nitrification under low ammonium concentrations and/or under suboptimal conditions defined for ammonia oxidizing bacteria (AOB), such as acidic pH, high electrical conductivity (EC), and low temperature (Leininger et al, 2006; Bernhard et al, 2010; Hofferle et al, 2010; Posser and Nicol, 2012; Hu and He, 2014)
Summary
11 % of the N-fertilizers (urea and ammonium sulphate) imported to Sri Lanka is being consumed by vegetable sector, only second to tea sector (27 %) from among the systems that dominates cultivation under unsaturated soil moisture conditions (personal communication with National Fertilizer Secretariat, 2017). Presence of ammonia oxidizing archaea (AOA) in soil have been detected and several studies suggest that AOA may play an important role in nitrification under low ammonium concentrations and/or under suboptimal conditions defined for ammonia oxidizing bacteria (AOB), such as acidic pH, high EC, and low temperature (Leininger et al, 2006; Bernhard et al, 2010; Hofferle et al, 2010; Posser and Nicol, 2012; Hu and He, 2014). Ammonia oxidizing bacteria seems to govern nitrification under NH4+-N rich soil conditions such as in soils that received fertilizer-N, polluted with sewage sludge, or experienced leakages from septic tanks (Leininger et al, 2006; Höfferle, et al, 2010)
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