Aims
The influence of soil edaphic factors on recruitment and composition of bacteria in the legume nodule is unknown. Typically, low (acidic) pH soils have a negative effect on the plant‐rhizobia ...symbiosis and thereby reduce clover growth. However, the specific relationship between soil pH and the ecology of rhizobia is unknown, in either their free‐living or nodule‐inhabiting states. We used New Zealand pasture systems with soils of different pH, and white (WC) and subterranean (SC) clovers, to examine the relationship between soil pH and the diversity of bacteria that inhabit the nodules.
Methods and Results
Amplicon sequencing (16S rRNA) assessed the bacterial community in 5299 nodules recovered from both legume species grown in 47 soils of different edaphic (including pH) properties. Fewer nodules were formed on both clovers at low soil pH. As expected, rhizobia comprised ∼92% of the total reads in both clovers, however 28 non‐rhizobia genera were also present. Soil pH influenced the community structure of bacteria within the nodule, and this was more evident in non‐Rhizobium taxa than Rhizobium. Host strongly influenced the diversity of bacteria in the nodules. The alpha diversity of nodule microbiome in SC nodules was higher than in WC nodules and SC nodules also harbored a higher relative abundance of non‐Rhizobium bacteria than WC. Beta diversity of Rhizobium and non‐Rhizobium bacteria was influenced more by clover species rather than edaphic factors.
Conclusions
The results indicate that these clover species modified their nodule biomes in response to pH‐stress.
Significance and Impact of the Study
The non‐Rhizobium bacteria may have some functional significance (such as improved clover persistence in low pH soils) in legume nodules.
•We grew four maize hybrids with different ‘stay-green’ ratings and applied different nitrogen (N) levels.•The ‘stay green’ trait was exhibited in the maintenance of green leaf area.•Hybrids of low ...‘stay green’ rating remobilized more of their pre-silking leaf N to grain.•Grain N content was lower in hybrids with a high ‘stay green’ rating.
The decline in dry matter accumulation in maize (Zea mays L.) during kernel filling, as N is remobilised, has elicited interest in the evaluation of traits to sustain leaf dry matter during this period. This study at Lincoln University, New Zealand, examined the post-silking nitrogen economy of maize hybrids that differed in their ‘stay-green’ rating (sgr). The objective was to quantify contributions to dry matter and kernel yield that accrue from the ‘stay-green’ trait and also understand its physiological basis. The response of ‘P39K38’ (sgr 6), ‘P38V12’ (sgr 7), ‘P38F70’ (sgr 8), and ‘P38G43’ (sgr 9) to 0 or 270kg nitrogen (N) ha−1 and nil or full irrigation was quantified in 2008/09. Only ‘P39K38’ (sgr 6), and ‘P38G43’ (sgr 9) were examined in 2009/10 when all crops were fully irrigated. Total accumulated dry matter and N content were determined from sequential harvests. In 2008/09 total accumulated dry matter did not differ (P=0.509) amongst the hybrids but ‘P39K38’ (sgr 6) yielded 2 tha−1 more (P=0.013) than ‘P38G43’ (sgr 9) in 2009/10. Total crop N did not differ among the hybrids at silking in both seasons. However, throughout kernel filling, the lower ‘stay-green’ rated hybrids remobilised more N from their leaves (∼20kgNha−1 at physiological maturity in 2008/09). In both seasons, the lower ‘stay-green’ rated hybrids also had more N accumulated in their kernels. The expression of the ‘stay-green’ trait was shown by the retention of more leaf N with no consequence kernel yield, but a decrease in kernel N content.
Atmospheric carbon‐dioxide concentration (CO2) is increasing rapidly, but its interactions with nitrogen (N) and phosphorus (P) fertiliser on wheat grain quality are not well understood. We ...investigated the effects of ambient CO2 (aCO2; ∼410 ppm) and elevated CO2 (eCO2; 760 ppm) on crop harvest index (CHI), nutrient harvest index (NuHI), shoot macro‐nutrient content and grain macro‐nutrient concentration of wheat grown under two contrasting amounts of N (0.5 and 6 mol m−3 NO3− N) and P (10 and 250 mmol P m−3) fertiliser supply (low and optimum, respectively). Our results highlighted interactions between CO2 and N and P fertiliser supply for the shoot biomass at anthesis and straw biomass at harvest maturity. This was because biomass yield did not respond to CO2 level when fertiliser was deficient. However, shoot and straw yield increased (10.0–‐34.0%) with increasing CO2 at optimum fertiliser rates. Across experiments, grain yield increased (15.6%) with increasing CO2, which resulted in grain nutrient concentration decreasing (3.0–‐13.0%) with increasing CO2. This was attributed to nutrient 'dilution' due to increased carbohydrate content in the grain. Overall, fertiliser supply impacted crop responses more than CO2 treatments, and the impact was greater under N than P deficiency. This was reflected through conservative values for CHI, thousand grain weight and NuHIs suggesting plants allocated biomass and nutrients at similar rates for vegetative and reproductive organs independent of CO2.
Results highlighted interactions between CO2 and N and P fertiliser supply for the shoot biomass at anthesis and straw biomass yield at harvest maturity, as the biomass yield did not respond to CO2 levels when fertiliser was deficient. Across experiments, grain yield increased (15.6%) with increasing CO2, which resulted in grain macro‐nutrient concentration decreasing (3.0%–13%) with increasing CO2. This was attributed to nutrient ‘dilution’ due to increased carbohydrate content in the grain.
Aims and methods Lucerne and Caucasian clover dry matter were measured in response to recommended lime and capital P inputs for six years in an acidic soil in the New Zealand high country. The ...initial three years of the field experiment indicated successful establishment and persistence of both legumes. Lucerne dry matter (DM) yield was up to 4 t/ha/yr in this period and higher than Caucasian clover yields. However, a lack of persistence of lucerne was apparent from this point forward compared with Caucasian clover which produced 7.7 t DM/ha in Year 6. An experiment using tubes of soil was used to investigate whether differences in root traits, nodulation and nodule occupancy were responsible for the differences observed in field persistence over time. Results These showed that when rhizobia inoculant was added, the fine root length of Caucasian clover was unaffected (R2 = 0.14) by aluminium (Al) content of the soil. In contrast, fine root growth of lucerne was suppressed (R2 = 0.79) by the soil Al content. Nodulation of Caucasian clover was unaffected by soil pH or Al when the rhizobia inoculant was provided which suggests the viability of the commercial genotype ICC148 in this soil with a pH of 5.5 and AI ca. 7 mg/kg soil. For lucerne, the maximum nodulation score of 7.3 occurred with 2 t/ha of lime added (soil pH ca.6, Al ca. 0.3 mg/kg) plus inoculant. Conclusions This suggests an Al toxic threshold of <1.0 mg Al/kg soil for effective lucerne nodulation. From the lucerne nodules, eight naturalized strains of Ensifer meliloti were identified. In contrast, only one R. leguminosarum strain was detected in the Caucasian clover nodules. The competition between those rhizobia genotypes may negatively affect the efficiency of biological nitrogen fixation in lucerne. Therefore, the lack of genetic diversity of R. leguminosarum bv. trifolii in New Zealand soils might be an advantage especially if the commercial strain is acid soil tolerant.
Aim
The aim of this work was to develop a tool to investigate the influence of soil factors on carbon utilization activity of single micro‐organisms.
Methods and Results
The assay for Rhizobium ...leguminosarum bv. trifolii in γ‐irradiated soil, using the MicroResp™ system, was optimized for sterility, incubation time, and moisture level. The optimized method was validated with experiments that assessed (i) differences in C utilization of different rhizobia strains and (ii) how this was affected by soil type. Carbon utilization differed among strains of the same species (and symbiovar), but some strains were more responsive to the soil environment than others.
Conclusions
This novel modification of the MicroResp™ has enabled the scope of carbon‐utilization patterns of single strains of bacteria, such as Rh. leguminosarum bv. trifolii, to be studied in soil.
Significance and Impact of the Study
The system is a new tool with applications in microbial ecology adaptable to the study of many culturable bacterial and fungal soil‐borne taxa. It will allow measurement of a micro‐organism's ability to utilize common C sources released in rhizosphere exudates to be measured in a physical soil background. This knowledge may improve selection efficiency and deployment of commercial microbial inoculants.
This study used results from field experiments to calibrate the Agricultural Production Systems Simulator (APSIM) model for ‘Grasslands Kaituna’ lucerne (Medicago sativa L.) crops grown in the cool ...temperate climate of New Zealand. The field data allowed refinement of functions related to the seasonality of crop phenology and above- and below-ground growth. The model structure was adequate to accommodate the calibration of reproductive development in response to temperature and photoperiod in both release and calibrated versions (root mean squared deviation RMSD from 14% to 3% of the mean, respectively). Similarly, vegetative development, characterised by the rate of appearance of main-stem nodes, was simulated with acceptable accuracy (RMSD from 28% to 25% of the mean). Analysis showed a lack of current field data to accurately represent the number of main-stem nodes at the beginning of each regrowth cycle, which is required to further improve the accuracy of leaf appearance simulations. The simulation of leaf area index (LAI) was slightly more accurate when using a site-specific empirical canopy modelling approach for ‘Grassland Kaituna’ (RMSD = 34% of the mean) than with the more mechanistic approach in the release version used for Australian cultivars (RMSD = 40% of the mean). This is particularly important before canopy closure, when LAI has the greatest influence on light interception and biomass accumulation. ‘Grassland Kaituna’ showed a seasonal pattern of change in leaf size and specific leaf area that could not be parameterised using the approach in the release version. Finally, the model structure of the calibrated version was adapted to explicitly simulate the seasonal flow of biomass to shoots and roots. For that, the ‘shoot’ radiation use efficiency (RUE) was modified to a ‘whole plant’ RUE of 1.6 g/MJ total solar radiation and a seasonal rate of below-ground biomass turnover (i.e. respiration plus senescence) was incorporated to correct for the initial offset observed between simulated and observed root biomass. These changes improved the RMSD from 53% to 38% for shoot biomass and 29% to 18% of the mean for roots. This calibration illustrates the heuristic aspect of applying crop models to identify knowledge gaps for guiding future experimentation, including time of node appearance on basal buds and leaf area index dynamics of contrasting lucerne cultivars.
Six dryland pastures were established at Lincoln University, Canterbury, New Zealand, in February 2002. Production and persistence of cocksfoot pastures established with subterranean, balansa, white ...or Caucasian clovers, and a perennial ryegrass‐white clover control and a lucerne monoculture were monitored for nine years. Total annual dry‐matter (10.0–18·5 t DM ha⁻¹) and sown legume yields from the lucerne monoculture exceeded those from the grass‐based pastures in all but one year. The lowest lucerne yield (10 t ha⁻¹ yr⁻¹) occurred in Year 4, when spring snow caused ungrazed lucerne to lodge and senesce. Cocksfoot with subterranean clover was the most productive grass‐based pasture. Yields were 8·7–13·0 t DM ha⁻¹ annually. Subterranean clover yields were 2·4–3·7 t ha⁻¹ in six of the nine years which represented 26–32% of total annual production. In all cocksfoot‐based pastures, the contribution of sown pasture components decreased at a rate equivalent to 3·3 ± 0·05% per year (R² = 0·83) and sown components accounted for 65% of total yield in Year 9. In contrast, sown components represented only 13% of total yield in the ryegrass‐white clover pastures in Year 9, and their contribution declined at 10·1 ± 0·9% per year (R² = 0·94). By Year 9, 79% of the 6.6 t ha⁻¹ produced from the ryegrass‐white clover pasture was from unsown species and 7% was dead material. For maximum production and persistence, dryland farmers on 450–780 mm yr⁻¹ rainfall should grow lucerne or cocksfoot‐subterranean clover pastures in preference to ryegrass and white clover. Inclusion of white clover as a secondary legume component to sub clover would offer opportunities to respond to unpredictable summer rainfall after sub clover has set seed.
After seed potatoes (Solanum tuberosum L.) are harvested, they may be stored in sheds, in low temperature coolers or left un-lifted in the ground. This research describes the response of tuber yield ...and distribution (formed by the weight grade of each potato) of 'Bondi' and 'Fraser' crops planted from seed potatoes at different physiological ages generated from different combinations of these storage regimes applied in the Early and Late phases of storage. De-sprouting half of the potatoes prior to planting was used to accelerate the rate of physiological ageing, synchronise the planting material and increase the range of treatments. Total yield and number of potatoes produced were unaffected by any of the storage treatments and reflected a constant pattern of canopy development, radiation use efficiency and harvest index. The largest tubers were attained by higher rates of tuber growth which was inversely related to the number of stems per plant.
Shade cloth is commonly used in agroforestry research. It produces a continuous, uniform reduced light environment. Shade cloth and a slatted structure were compared in relation to the inability to ...represent the light regime and plant responses of an agroforestry system. The split-split-plot randomised block experiment had main plots as covering status (with or without radiata pine trees), subplots as artificial shade (none, shade cloth or wooden slats) and sub-subplots as growth rotation, over sown alfalfa, in three replicates. The quantity of light transmittance was 49% under trees, 41% under cloth and 44% under slats. Temporal changes and spectral composition under trees were more accurately reproduced under the slats than shade cloth. The red to far red ratio was 0.64 under tree shade and 0.74 during the shaded period under slats. This compared with 1.31 in open pasture, 1.28 under shade cloth in open and 1.26 under slats during sunny periods. To compensate for low light quantity and quality, alfalfa had elongated stems and internodes. In open pasture and under cloth in the open, it produced short stems. The mean dry matter yield under trees was 68% of the 30.3 t ha⁻¹ in open pasture, 56% under cloth and 57% under slats. The slats induced similar morphological responses in alfalfa to those in the agroforestry system. The magnitude of changes had little effect on growth and yield responses. The artificial slatted structure approximated the intermittent light environment and consequent plant responses observed in an agroforestry system.
First‐year crops of diploid perennial ryegrass (cvs. Meridian, Bronsyn and Grasslands Impact) were sown on 1 April and 14 May 2008. Applications of trinexapac ethyl (TE) plant growth regulator at 0, ...200 and 400 g a.i. ha−1 were used to shorten stems to examine the impact of seed growth. Seed filling followed a consistent sigmoidal growth pattern with a lag phase of 127°C days, and linear duration of 390°C days. Time to 95% of final seed weight was 517°C days. Seed yield increases from TE were from higher numbers of first‐grade seeds m−2, achieved by a higher rate of seed filling during the linear phase of 0·115 mg per °C day per spike. For all cultivars, the maximum stem dry weight occurred at 310–400°C days post‐anthesis, which suggest the stem was a strong sink. As seeds developed, their demand for assimilate increased and they drew more from the stem. At harvest, stem weights from TE treatments were 25% heavier than at anthesis, while untreated ‘Bronsyn’ and ‘Grasslands Impact’ stems were similar to those at anthesis. Thus, stems treated with TE contributed assimilates to increase seed yield but were still a net sink with assimilates in the stem at harvest. Trinexapac ethyl rate induced an inverse relationship between seed yield and stem height. This showed that competition for assimilate between stems and growing seeds limited the seed yield. Management or genetic factors that reduce stem height are likely to increase seed yields of perennial ryegrass.
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