Abstract
Australian native species grow competitively in nutrient limited environments, particularly in nitrogen (N) limited soils; however, the mechanism that enables this is poorly understood. Biological nitrification inhibition (BNI), which is the release of root exudates into the plant rhizosphere to inhibit the nitrification process, is a hypothesized adaptive mechanism for maximizing N uptake. To date, few studies have investigated the temporal pattern and components of root exudates by Australian native plant species for BNI. This study examined root exudates from two Australian native species, Hibiscus splendens and Solanum echinatum, and contrasted with exudates of Sorghum bicolor, a plant widely demonstrated to exhibit BNI capacity. Root exudates were collected from plants at two, four, and six weeks after transplanting to solution culture. Root exudates contained three types of organic acids (OAs), oxalic, citric and succinic acids, regardless of the species. However, the two Australian natives species released larger amount of OAs in earlier development stages than S. bicolor. The total quantity of these OAs released per unit root dry mass was also seven-ten times greater for Australian native plant species compared to S. bicolor. The root exudates significantly inhibited nitrification activity over six weeks’ growth in a potential nitrification assay, with S. echinatum (ca. 81% inhibition) > S. bicolor (ca. 80% inhibition) > H. splendens (ca. 78% inhibition). The narrow range of BNI capacity in the study plants limited the determination of a relationship between OAs and BNI; however, a lack of correlation between individual OAs and inhibition of nitrification suggests OAs may not directly contribute to BNI. These results indicate that Australian native species generate a strongly N conserving environment within the rhizosphere up to six weeks after germination, establishing a competitive advantage in severely N limited environments.
Highlights
Nitrogen (N) and phosphorous (P) are generally the most commonly limiting nutrients in terrestrial ecosystems, depending on ecosystem development and input processes (Agren, Wetterstedt & Billberger, 2012; Menge, Hedin & Pacala, 2012)
Two native Australian plant species, Hibiscus splendens and Solanum echinatum, were selected because of their adaptation to slightly acidic, light textured soils with low soil N content, as determined by the CSIRO soil map (National Land & Water Resources Audit, 2001). These soil properties have been previously correlated with plants which exhibited biological nitrification inhibition (BNI) capacity (Subbarao et al, 2012; Zhu et al, 2012), leading to the supposition that Australian native plant species adapted to similar environments may release root exudates that inhibit soil nitrification processes
Over six weeks of growth, nitrification rates were limited to a range of 0.0145–0.0440 mg NO3-N L−1 h−1 in the presence of S. bicolor exudates; 0.0183–0.0539 mg NO3-N L−1 h−1 with H. splendens exudates; and 0.0047–0.0581 mg NO3-N L−1 h−1with S. echinatum exudates
Summary
Nitrogen (N) and phosphorous (P) are generally the most commonly limiting nutrients in terrestrial ecosystems, depending on ecosystem development and input processes (Agren, Wetterstedt & Billberger, 2012; Menge, Hedin & Pacala, 2012). A key adaptation strategy for plants in N-limited soils is biological nitrification inhibition (BNI) mediated by root exudates from plants to increase the residence time of N in soil. Biological nitrification inhibition has been identified in a range of plant species, including grasses, weeds, and agricultural crops (Subbarao et al, 2007a; Sun et al, 2016; O’Sullivan et al, 2016; O’Sullivan et al, 2017). Plants exhibiting BNI function are often species originated and/or adapted to N poor soils, suggesting that BNI in plants is an adaptive mechanism for growth in N-limited environments (Lata et al, 2004; Subbarao et al, 2007a). Lolium rigidum, Bromus driandrus, Raphinus raphinastrum, and Avena fatua, have demonstrated BNI, which is associated with their vigorous growth (O’Sullivan et al, 2017)
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