Greater root phosphatase activity in nitrogen‐fixing rhizobial but not actinorhizal plants with declining phosphorus availability

Abstract

Summary The abundance of nitrogen (N)‐fixing plants in ecosystems where phosphorus (P) limits plant productivity poses a paradox because N fixation entails a high P cost. One explanation for this paradox is that the N‐fixing strategy allows greater root phosphatase activity to enhance P acquisition from organic sources, but evidence to support this contention is limited. We measured root phosphomonoesterase (PME) activity of 10 N‐fixing species, including rhizobial legumes and actinorhizal Allocasuarina species, and eight non‐N‐fixing species across a retrogressive soil chronosequence showing a clear shift from N to P limitation of plant growth and representing a strong natural gradient in P availability. Legumes showed greater root PME activity than non‐legumes, with the difference between these two groups increasing markedly as soil P availability declined. By contrast, root PME activity of actinorhizal species was always lower than that of co‐occurring legumes and not different from non‐N‐fixing plants. The difference in root PME activity between legumes and actinorhizal plants was not reflected in a greater or similar reliance on N fixation for N acquisition by actinorhizal species compared to co‐occurring legumes. Synthesis. Our results support the idea that N‐fixing legumes show high root phosphatase activity, especially at low soil P availability, but suggest that this is a phylogenetically conserved trait rather than one directly linked to their N‐fixation capacity.

Nutrient acquisition strategies augment growth in tropical N2‐fixing trees in nutrient‐poor soil and under elevated CO2

“…For example, Cernusak et al (2011) showed variation in the capacity for nodulation among tropical N 2 fixers, Nasto et al (2017) showed that differences in nutrient acquisition strategies between non-N 2 and N 2 fixers were driven by individual species belonging to each functional group, Zalamea et al (2016) and Png et al (2017) suggested that root phosphatase activity is phylogenetically constrained and not related to functional group or N 2 fixation, and Soper et al (2018) demonstrated a lack of a relationship between N 2 fixation, foliar N, and P acquisition among a variety of N 2 -fixing legumes, non-N 2 -fixing legumes, and non-legumes. For example, Cernusak et al (2011) showed variation in the capacity for nodulation among tropical N 2 fixers, Nasto et al (2017) showed that differences in nutrient acquisition strategies between non-N 2 and N 2 fixers were driven by individual species belonging to each functional group, Zalamea et al (2016) and Png et al (2017) suggested that root phosphatase activity is phylogenetically constrained and not related to functional group or N 2 fixation, and Soper et al (2018) demonstrated a lack of a relationship between N 2 fixation, foliar N, and P acquisition among a variety of N 2 -fixing legumes, non-N 2 -fixing legumes, and non-legumes.…”

How belowground interactions contribute to the coexistence of mycorrhizal and non-mycorrhizal species in severely phosphorus-impoverished hyperdiverse ecosystems

“…Various legume species in south-western Australia form both AM and ECM along with N 2 -fixing nodules (Albornoz et al 2016a;Brundrett 2009;Png et al 2017), giving rise to the possibility for an elaborate guild of structures for nutrient exchanges between plants that have never been documented (Fig. 4).…”

“…These N 2 -fixing plants are P sinks at this early stage, and may meet their P requirements with the help of scavenging mycorrhizal hyphae and/or the intermingling of roots around cluster roots of neighbouring P-mining species ). Such scavenging may represent an efficient strategy for obtaining P in extremely P-impoverished soils; legumes also show higher root phosphatase activity than co-occurring non-legume species, suggesting that they are particularly good at acquiring organic P (Png et al 2017). Both AM and EM fungi could access newly mobilised P from cluster root 'burst' microsites, but EM may be favoured, since they possess highaffinity P transporters at their mycelia front (Cairney 2011).…”

“…There are more than 350 plant species along the Jurien Bay chronosequence with contrasting nutrient-acquisition strategies (Zemunik et al A trade-off between P-acquisition efficiency and pathogen defence in combination with facilitation of nutrient acquisition (Muler et al 2014) may not be the only mechanism that maintains hyperdiverse ecosystems. This trade-off may explain why some species exhibit both a cluster-root strategy and mycorrhizas, e.g., Casuarinaceae, including Casuarina cunninghamiana (Reddell et al 1997) and Allocasuarina humilis (Png et al 2017), and some Fabaceae, e.g., Viminaria juncea (de Campos et al 2013). It may not be a case of 'belt and braces', i.e.…”

Nitrogen fixation and foliar nitrogen do not predict phosphorus acquisition strategies in tropical trees

“…Houlton et al () and other studies that have contrasted fixing and nonfixing plants have generally found that phosphatase enzyme production is elevated in the roots and rhizospheres of N 2 ‐fixing legumes (Houlton et al, ; Nasto et al, ; Png et al, ) potentially because they have more N available to invest in enzyme synthesis. Alternatively, there is evidence that phosphatase enzyme production is not dependent on N supply from fixation, but rather may be a phylogenetically conserved trait between groups or species (Png et al, ).…”