Abstract
Nitrification is a major process within the nitrogen (N) cycle leading to global losses of N, including fertiliser N, from natural and agricultural systems and producing significant nitrous oxide emissions. One strategy for the mitigation of these losses involves nitrification inhibition by plant-derived biological nitrification inhibitors (BNIs). Cultivation-based studies of BNIs, including screening for new compounds, have predominantly investigated inhibition of a single ammonia-oxidising bacterium (AOB), Nitrosomonas europaea, even though ammonia oxidation in soil is usually dominated by ammonia-oxidising archaea (AOA), especially in acidic soils, and AOB Nitrosospira sp., rather than Nitrosomonas, in fertilised soils. This study aimed to assess the sensitivity of ammonia oxidation by a range of AOA and AOB pure cultures to BNIs produced by plant roots (methyl 3-(4-hydroxyphenyl) propionate, sakuranetin and 1,9-decanediol) and shoots (linoleic acid, linolenic acid and methyl linoleate). AOA were generally more sensitive to BNIs than AOB, and sensitivity was greater to BNIs produced by shoots than those produced by roots. Sensitivity also varied within AOA and AOB cultures and between different BNIs. In general, N. europaea was not a good indicator of BNI inhibition, and findings therefore highlight the limitations of use of a single bioassay strain and suggest the use of a broader range of strains that are more representative of natural soil communities.
Highlights
The global N cycle is largely driven by soil microbial N transformations within which nitrification, the oxidation of ammonia (NH3) to nitrate (NO3−), is a key process
A greater abundance of ammonia-oxidising archaea (AOA) than ammonia-oxidising bacterium (AOB) has been reported in the rhizosphere of several plants, including rice (Chen et al 2008; Ke et al 2013), wheat (Ai et al 2013), maize (Wattenburger et al 2020) and grasses (Thion et al 2016). These findings suggest that meaningful assessment of the efficiency of biological nitrification inhibitors (BNIs) should focus on a larger and more representative set of soil ammonia oxidisers than N. europaea, and lead to the following hypotheses: (H1) AOA are more sensitive than AOB to BNIs; (H2) relative inhibition of AOA and AOB will depend on the source of BNIs, with greater sensitivity of AOB to root-derived BNIs and similar responses of AOA and AOB to those derived from shoots. (H3) N. europaea is not an appropriate model AO for bioassay of BNI inhibition of soil AO
Hypothesis H1 predicted that AOA would be more sensitive than AOB to BNIs, based on reported greater sensitivity of AOA in soil planted with some B. humidicola genotypes (Subbarao et al 2009)
Summary
The global N cycle is largely driven by soil microbial N transformations within which nitrification, the oxidation of ammonia (NH3) to nitrate (NO3−), is a key process. Nitrification involves initial oxidation of NH3 to nitrite (NO2−) by ammonia-oxidising archaea and bacteria (AOA and AOB), which is oxidised to NO3− by nitrite-oxidising bacteria (NOB), while comammox can perform both steps. A key factor driving the soil N cycle, and global agricultural production, is the application of N fertilisers, which comprise more than 50% of N input. Significant N2O emissions are associated directly with the activity of AOB and/or AOA, through nitrifier denitrification, incomplete oxidation of hydroxylamine and nonenzymatic conversion of nitrification products and intermediates (Prosser et al 2019). Strategies for the control of nitrification in agroecosystems are required to reduce the N footprint, reduce N2O emissions and increase fertiliser nitrogen use efficiency (NUE)
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