Abstract
Plants can stimulate bacterial nitrogen (N) removal by secretion of root exudates that may serve as carbon sources as well as non-nutrient signals for denitrification. However, there is a lack of knowledge about the specific non-nutrient compounds involved in this stimulation. Here, we use a continuous root exudate-trapping system in two common aquatic duckweed species, Spirodela polyrrhiza (HZ1) and Lemna minor (WX3), under natural and aseptic conditions. An activity-guided bioassay using denitrifying bacterium Pseudomonas fluorescens showed that crude root exudates of the two species strongly enhanced the nitrogen-removal efficiency (NRE) of P. fluorescens (P < 0.05) under both conditions. Water-insoluble fractions (F) obtained under natural conditions stimulated NRE to a significant extent, promoting rates by about 30 %. Among acidic, neutral and basic fractions, a pronounced stimulatory effect was also observed for the neutral fractions from HZ1 and WX3 under both conditions, whereas the acidic fractions from WX3 displayed an inhibitory effect. Analysis of the active fractions using gas chromatography/mass spectrometry (GC/MS) revealed that duckweed released fatty acid methyl esters and fatty acid amides, specifically: methyl hexadecanoate, methyl (Z)-7-hexadecenoate, methyl dodecanoate, methyl-12-hydroxystearate, oleamide, and erucamide. Methyl (Z)-7-hexadecenoate and erucamide emerged as the effective N-removal stimulants (maximum stimulation of 25.9 and 33.4 %, respectively), while none of the other tested compounds showed stimulatory effects. These findings provide the first evidence for a function of fatty acid methyl esters and fatty acid amides in stimulating N removal of denitrifying bacteria, affording insight into the “crosstalk” between aquatic plants and bacteria in the rhizosphere.
Highlights
Nutrients from non-point sources have become a major source of water contamination in many parts of the world
Methyl (Z)-7-hexadecenoate and erucamide emerged as the effective N-removal stimulants, while none of the other tested compounds showed stimulatory effects. These findings provide the first evidence for a function of fatty acid methyl esters and fatty acid amides in stimulating N removal of denitrifying bacteria, affording insight into the “crosstalk” between aquatic plants and bacteria in the rhizosphere
During a 72-h incubation, the nitrogen-removal efficiency (NRE) amended with duckweed root exudates was higher than that of CH2Cl2 control and differed across treatments, with the exception of the acidic fractions of WX3 under the two conditions and the basic fraction of WX3 under aseptic conditions, where the efficiencies were lower than the control
Summary
Nutrients from non-point sources have become a major source of water contamination in many parts of the world. One ecological approach used to treat polluted agricultural and industrial wastewater containing high nitrate is the use of plants with high nitrate-removal capabilities (Siddiqi et al 1998) or the stimulation of bacterial denitrification in afflicted soils and water bodies (Mulvaney et al 1997; Constantin and Fick 1997). We reported the acceleration of N removal by duckweed, a small floating aquatic plant within the family Lemnaceae, which is widely distributed in paddy fields all over the world (Zhou et al 2010). Most of the studies investigating the effect of root exudates on N removal have focused on the structure and activity of the soil-resident denitrifying communities (Baudoin et al 2003; Henry et al 2008; Shi et al 2011) and much less on the plant component(s). The release of compounds that may stimulate bacterial denitrification seemed plausible, and is investigated here
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