High spatial variation in population size and symbiotic performance of <i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> with white clover in New Zealand pasture soils

<div><p>Biological nitrogen fixation through the legume-rhizobia symbiosis is important for sustainable pastoral production. In New Zealand, the most widespread and valuable symbiosis occurs between white clover (<i>Trifolium repens</i> L.) and <i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> (<i>Rlt</i>). As variation in the population size (determined by most probable number assays; MPN) and effectiveness of N-fixation (symbiotic potential; SP) of <i>Rlt</i> in soils may affect white clover performance, the extent in variation in these properties was examined at three different spatial scales: (1) From 26 sites across New Zealand, (2) at farm-wide scale, and (3) within single fields. Overall, <i>Rlt</i> populations ranged from 95 to >1 x 10⁸ per g soil, with variation similar at the three spatial scales assessed. For almost all samples, there was no relationship between rhizobia population size and ability of the population to fix N during legume symbiosis (SP). When compared with the commercial inoculant strain, the SP of soils ranged between 14 to 143% efficacy. The N-fixing ability of rhizobia populations varied more between samples collected from within a single hill country field (0.8 ha) than between 26 samples collected from diverse locations across New Zealand. Correlations between SP and calcium and aluminium content were found in all sites, except within a dairy farm field. Given the general lack of association between SP and MPN, and high spatial variability of SP at single field scale, provision of advice for treating legume seed with rhizobia based on field-average MPN counts needs to be carefully considered.</p></div

Spatial variation in <i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> symbiotic potential (SP); i.e. efficacy at increasing the growth of <i>Trifolium repens</i> cv. Tribute.

<p>Assay conditions for clover growth had minimal nitrogen content, such that plant growth was directly related to N-fixation occurring through symbiosis. SP is calculated relative to growth of <i>T</i>. <i>repens</i> c.v Tribute inoculated with the current commercial <i>R</i>. <i>leguminosarum</i> bv. <i>trifolii</i> strain used in New Zealand, TA1 (= 100%). Black dots represent sampling spots based on GPS-marked points. Contour lines for Whangara and Pouawa are given; the field at Rotomanu had flat topography. Variation in SP between samples was assessed using the RBK method in ARC-GIS.</p

High spatial variation in population size and symbiotic performance of <i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> with white clover in New Zealand pasture soils - Fig 2

<p>(A) <i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> population size and (B) nitrogen fixing effectiveness (symbiotic potential relative to commercial <i>Rlt</i> strain TA1) in soils across different spatial scales. Boxes show median values and extend to the 25th to 75th percentiles. For 2B, the dashed line at 100% symbiotic potential = white clover growth when inoculated with the commercial rhizobia inoculant strain TA1. Treatments sharing the same lettering (<i>a</i> or <i>b</i>) have similar treatment means (Bonferroni comparison of means; α = 0.05).</p

High spatial variation in population size and symbiotic performance of <i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> with white clover in New Zealand pasture soils - Fig 6

<p>Venn diagrams showing similarity in properties of soils, at different spatial sampling scales, that have significant correlations with <i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> population size (MPN’s; 6A), and symbiotic potential (6B).</p

Sampling sites from across New Zealand and the Whangara farm (call out box).

<p>The Pouawa and Rotomanu sites, where spatially-dense sampling was conducted a single field, are labelled (star symbols). Location data (GPS) and soil type information can be found in the Supplementary Information, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192607#pone.0192607.s001" target="_blank">S1 Table</a>.</p

Relationship between the <i>R</i>. <i>leguminosarum bv</i>. <i>trifolii</i> population size and symbiotic potential of the legume-rhizobia symbiosis in 50 samples of soils collected from within a single field at Pouawa (hill country sheep and beef farm).

<p>Solid line is the linear regression model fit; dashed lines are the 95% confidence intervals.</p

Spatial variation in <i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> population size, (most probable number) based on nodule formation on <i>Trifolium repens</i> cv. Tribute.

<p>Black dots represent sampling spots based on GPS-marked points. Contour lines for Whangara and Pouawa are given; the field at Rotomanu had flat topography. MPN variation between samples was assessed using the RBK method in ARC-GIS.</p

Summary of Pearson’s correlations between <i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> populations size (MPN values) and symbiotic potential with soil physicochemical properties in samples with different geographic ranges.

<p>Summary of Pearson’s correlations between <i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> populations size (MPN values) and symbiotic potential with soil physicochemical properties in samples with different geographic ranges.</p