low-P Soils Research Articles

Large Crown Root Number Improves Topsoil Foraging and Phosphorus Acquisition.

Suboptimal phosphorus (P) availability is a primary constraint to plant growth on Earth. We tested the hypothesis that maize (Zea mays) genotypes with large crown root number (CN) will have shallower rooting depth and improved P acquisition from low-P soils. Maize recombinant inbred lines with contrasting CN were evaluated under suboptimal P availability in greenhouse mesocosms and the field. Under P stress in mesocosms, the large-CN phenotype had 48% greater root respiration, 24% shallower rooting depth, 32% greater root length density in the topsoil, 37% greater leaf P concentration, 48% greater leaf photosynthesis, 33% greater stomatal conductance, and 44% greater shoot biomass than the small-CN phenotype. Under P stress in the field, the large-CN phenotype had 32% shallower rooting depth, 51% greater root length density in the topsoil, 44% greater leaf P concentration, 18% greater leaf photosynthesis, 21% greater stomatal conductance, 23% greater shoot biomass at anthesis, and 28% greater yield than the small-CN phenotype. These results support the hypothesis that large CN improves plant P acquisition from low-P soils by reducing rooting depth and increasing topsoil foraging. The large-CN phenotype merits consideration as a selection target to improve P capture in maize and possibly other cereal crops.

Plant responses to fertilization experiments in lowland, species-rich, tropical forests.

We present a meta-analysis of plant responses to fertilization experiments conducted in lowland, species-rich, tropical forests. We also update a key result and present the first species-level analyses of tree growth rates for a 15-yr factorial nitrogen (N), phosphorus (P), and potassium (K) experiment conducted in central Panama. The update concerns community-level tree growth rates, which responded significantly to the addition of N and K together after 10yr of fertilization but not after 15yr. Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora. Under these circumstances, we expect muted responses to fertilization, and we predicted species associated with low-P soils would respond most slowly. The data did not support this prediction, species-level tree growth responses to P addition were unrelated to species-level soil P associations. The meta-analysis demonstrated that nutrient limitation is widespread in lowland tropical forests and evaluated two directional hypotheses concerning plant responses to N addition and to P addition. The meta-analysis supported the hypothesis that tree (or biomass) growth rate responses to fertilization are weaker in old growth forests and stronger in secondary forests, where rapid biomass accumulation provides a nutrient sink. The meta-analysis found no support for the long-standing hypothesis that plant responses are stronger for P addition and weaker for N addition. We do not advocate discarding the latter hypothesis. There are only 14 fertilization experiments from lowland, species-rich, tropical forests, 13 of the 14 experiments added nutrients for five or fewer years, and responses vary widely among experiments. Potential fertilization responses should be muted when the species present are well adapted to nutrient-poor soils, as is the case in our experiment, and when pest pressure increases with fertilization, as it does in our experiment. The statistical power and especially the duration of fertilization experiments conducted in old growth, tropical forests might be insufficient to detect the slow, modest growth responses that are to be expected.

Quantitative trait loci associated with soybean seed weight and composition under different phosphorus levels.

Seed size and composition are important traits in food crops and can be affected by nutrient availability in the soil. Phosphorus (P) is a non-renewable, essential macronutrient, and P deficiency limits soybean (Glycine max) yield and quality. To investigate the associations of seed traits in low- and high-P environments, soybean recombinant inbred lines (RILs) from a cross of cultivars Fiskeby III and Mandarin (Ottawa) were grown under contrasting P availability environments. Traits including individual seed weight, seed number, and intact mature pod weight were significantly affected by soil P levels and showed transgressive segregation among the RILs. Surprisingly, P treatments did not affect seed composition or weight, suggesting that soybean maintains sufficient P in seeds even in low-P soil. Quantitative trait loci (QTLs) were detected for seed weight, intact pods, seed volume, and seed protein, with five significant QTLs identified in low-P environments and one significant QTL found in the optimal-P environment. Broad-sense heritability estimates were 0.78 (individual seed weight), 0.90 (seed protein), 0.34 (seed oil), and 0.98 (seed number). The QTLs identified under low P point to genetic regions that may be useful to improve soybean performance under limiting P conditions.

Linking root traits to superior phosphorus uptake and utilisation efficiency in three Fabales in the Core Cape Subregion, South Africa.

In the low-P soil of the fynbos biome, plants have evolved several morphological and physiological P acquisition and use mechanisms, leading to variable uptake and use efficiencies. We expected that plants grown in low-P soils would exhibit greater P acquisition traits and hypothesised that Aspalathus linearis (Burm. f.) R. Dahlgren, a cluster-root-forming species adapted to drier and infertile soils, would be the most efficient at P acquisition compared with other species. Three fynbos Fabales species were studied: A. linearis and Podalyria calyptrata (Retz.) Willd, both legumes, and Polygala myrtifolia L., a nonlegume. A potted experiment was conducted where the species were grown in two soil types with high P (41.18mgkg-1) and low P (9.79mgkg-1). At harvest, biomass accumulation, foliar nutrients and P acquisition mechanisms were assessed. Polygala myrtifolia developed a root system with greater specific root length, root hair width and an average root diameter that exuded a greater amount of citrate and, contrary to the hypothesis, exhibited greater whole-plant P uptake efficiency. However, P. calyptrata had higher P use efficiency, influenced by N availability through N2 fixation. Specific root length, root length and root:shoot ratio were promising morphological traits for efficient foraging of P, whereas acid phosphatase exudation was the best physiological trait for solubilisation of P.

Root-associated fungal microbiota of nonmycorrhizal Arabis alpina and its contribution to plant phosphorus nutrition

Most land plants live in association with arbuscular mycorrhizal (AM) fungi and rely on this symbiosis to scavenge phosphorus (P) from soil. The ability to establish this partnership has been lost in some plant lineages like the Brassicaceae, which raises the question of what alternative nutrition strategies such plants have to grow in P-impoverished soils. To understand the contribution of plant-microbiota interactions, we studied the root-associated fungal microbiome of Arabis alpina (Brassicaceae) with the hypothesis that some of its components can promote plant P acquisition. Using amplicon sequencing of the fungal internal transcribed spacer 2, we studied the root and rhizosphere fungal communities of A. alpina growing under natural and controlled conditions including low-P soils and identified a set of 15 fungal taxa consistently detected in its roots. This cohort included a Helotiales taxon exhibiting high abundance in roots of wild A. alpina growing in an extremely P-limited soil. Consequently, we isolated and subsequently reintroduced a specimen from this taxon into its native P-poor soil in which it improved plant growth and P uptake. The fungus exhibited mycorrhiza-like traits including colonization of the root endosphere and P transfer to the plant. Genome analysis revealed a link between its endophytic lifestyle and the expansion of its repertoire of carbohydrate-active enzymes. We report the discovery of a plant-fungus interaction facilitating the growth of a nonmycorrhizal plant under native P-limited conditions, thus uncovering a previously underestimated role of root fungal microbiota in P cycling.

Bioavailability of phosphorus from composts and struvite in acid soils

ABSTRACT The objective of this study was to assess the type and fractions of phosphorus (P) forms in composts and struvite and how these P forms affect the bioavailability of P in the soil. P fertilization was performed with compost from sewage sludge (CSS), compost from poultry litter (CPL) and struvite (SV) and compared with single superphosphate (SSP). P forms were quantified through a sequential fractionation scheme. The first extraction was performed with H2O, the second with 0.5 M NaHCO3, the third with 0.1 M NaOH and the fourth with 1 M HCl. The release of P over time, after soil P fertilization, was assessed by incubating the fertilizers with a low-P acid soil. P bioavailability was assessed through a micro-pot experiment with the incubated soils in a growth chamber using rye plants (Secale cereale L.). Inorganic P forms in the first two fractions represented ~50% (composts), 32% (SV) and 86% (SSP) of the total P; and in the HCl fraction, ~40% (composts), 26% (SV) and 13% (SSP) of the total P. Despite the variability of the P form fractions in the composts and struvite, the P release and bioavailability were similar among the fertilized treatments. The acidic nature of the soil, which improve solubility of Ca-P forms, and the high efficiency of rye, which favors P uptake, were factors that contributed to these results.

Phosphorus mobilization in low-P arable soils may involve soil organic C depletion

Organic farming systems often show negative nutrient balances that may compromise the availability of phosphorus over the mid-term. Calcareous soils with low organic matter generally show low or very low phosphorus availability. Under these conditions, P retained in soil organo-mineral complexes -either in organic or inorganic forms-after its solubilization and -in the case of organic P compounds-subsequent mineralization may be a source of P to plants and soil organisms. By studying the changes in soil retained and soluble P pooloccurring during the growth of two legumes in two scenarios of contrasting P availability we aimed to describe the release of soil retained P, organic and inorganic, into the soluble pools and to relate it to changes in organic C during crop growth.The experimental design was a split-plot of two nearby fields with contrasting P availability. In each field, two species of legumes, chickpea and bitter vetch, were planted and manure was added in alternating plots. During the legume growth we monitored soil P by analyzing soluble inorganic and organic P (NaHCO3-Pi and NaHCO3-Po), the soil retained inorganic and organic P pools (H2SO4-P before and after ashing), and acid and alkaline phosphatase activities in each plot at sowing time and at the late flowering stage.All P forms were higher in the high-P field than in the low-P field, except for soluble Po, which was higher in the low-P field at sowing time. In the high-P field during legume growth we detected an increase of the soluble Po and soluble Pi pools and of the soil retained Po that may have originated from a reduction of the soil retained Pi pool. In contrast, in the low-P field, the decrease of the soil retained Pi pool coincided with a decrease of the soluble Po pool, while the soil retained Po did not show any significant change. In low-P soils, the soil retained Po pool appeared to be the main source of soluble Pi. Changes in the soil retained Pi pool during legume growth occurred in all soils and were much larger than the amount of P required by plants. Likewise, the most likely P transformations in our soils involved changes between inorganic and organic forms suggesting that these changes were mainly mediated by soil microbiota. P transformations in low-P soils reduced soil organic C and the C/Po ratio, thus suggesting that crops growing in low-P soils may deplete organic matter from protected mineral-organic associations in low organic matter arable soils likely by promoting organic acid exudation by roots and soil microbiota.

Phosphorus Over-Fertilization and Nutrient Misbalance of Irrigated Tomato Crops in Brazil.

Over the past 20 years, the use of center-pivot irrigation has increased tomato (Solanum lycopersicum L.) yields in Brazil from 42 Mg ha−1 to more than 80 Mg ha−1. In the absence of field trials to support fertilizer recommendations, substantial amounts of phosphorus (P) have been applied to crops. Additional P dosing has been based on an equilibrated nutrient P budget adjusted for low-P fertilizer-use efficiency in high-P fixing tropical soils. To document nutrient requirements and prevent over-fertilization, tissue samples and crop yield data can be acquired through crop surveys and fertilizer trials. Nevertheless, most tissue diagnostic methods pose numerical difficulties that can be avoided by using the nutrient balance concept. The objectives of this study were to model the response of irrigated tomato crops to P fertilization in low- and high-P soils and to provide tissue diagnostic models for high crop yield. Three P trials, arranged in a randomized block design with six P treatments (0–437 kg P ha−1) and three or four replications, were established on a low-P soil in 2013 and high-P soils in 2013 and 2014, totaling 66 plots in all. Together with crop yield data, 65 tissue samples were collected from tomato farms. We found no significant yield response to P fertilization, despite large differences in soil-test P (coefficient of variation, 24%). High- and low-yield classes (cutoff: 91 Mg fruits ha−1) were classified by balance models with 78–81% accuracy using logit and Cate–Nelson partitioning models. The critical Mahalanobis distance for the partition was 5.31. Tomato yields were apparently not limited by P but were limited by calcium. There was no evidence that P fertilization should differ between center-pivot-irrigated and rain-fed crops. Use of the P budget method to arrive at the P requirement for tomato crops proved to be fallacious, as several nutrients should be rebalanced in Brazilian tomato cropping systems.

Generation mean analysis of phosphorus-use efficiency in freely nodulating soybean crosses grown in low-phosphorus soil

Freely nodulating soybean genotypes vary in their phosphorus (P) uptake and P-use efficiency (PUE) in low-P soils. Understanding the genetic basis of these genotypes’ performance is essential for effective breeding. To study the inheritance of PUE, we conducted crosses using two high-PUE genotypes, two moderate-PUE genotypes and two inefficient-PUE genotypes, and obtained F1, F2, BC1 and BC2 populations. The inheritance of PUE was evaluated using a randomized complete block design. A generation mean analysis of phenotypic data showed that PUE was heritable, with complex inheritance patterns and the presence of additive, dominance and epistatic gene effects. Seed P, shoot P, root P, P-incorporation efficiency and PUE were largely quantitatively inherited traits. There were dominance, additive × additive and dominance × dominance gene effects on PUE, grain yield, shoot dry weight, 100-seed weight, root dry weight and shoot dry matter per unit P for populations grown under low-P conditions. Dominance effects were generally greater than additive effects on PUE-related indices. These PUE indices can be used to select P-efficient soybean genotypes from segregating populations.

Rhizosphere Organic Anions Play a Minor Role in Improving Crop Species' Ability to Take Up Residual Phosphorus (P) in Agricultural Soils Low in P Availability.

Many arable lands have accumulated large reserves of residual phosphorus (P) and a relatively large proportion of soil P is less available for uptake by plants. Root released organic anions are widely documented as a key physiological strategy to enhance P availability, while limited information has been generated on the contribution of rhizosphere organic anions to P utilization by crops grown in agricultural soils that are low in available P and high in extractable Ca, Al, and Fe. We studied the role of rhizosphere organic anions in P uptake from residual P in four common crops Triticum aestivum, Avena sativa, Solanum tuberosum, and Brassica napus in low- and high-P availability agricultural soils from long-term fertilization field trials in a mini-rhizotron experiment with four replications. Malate was generally the dominant organic anion. More rhizosphere citrate was detected in low P soils than in high P soil. B. napus showed 74–103% increase of malate in low P loam, compared with clay loam. A. sativa had the greatest rhizosphere citrate concentration in all soils (5.3–15.2 μmol g−1 root DW). A. sativa also showed the highest level of root colonization by arbuscular mycorrhizal fungi (AMF; 36 and 40%), the greatest root mass ratio (0.51 and 0.66) in the low-P clay loam and loam respectively, and the greatest total P uptake (5.92 mg P/mini-rhizotron) in the low-P loam. B. napus had 15–44% more rhizosphere acid phosphatase (APase) activity, ~0.1–0.4 units lower rhizosphere pH than other species, the greatest increase in rhizosphere water-soluble P in the low-P soils, and the greatest total P uptake in the low-P clay loam. Shoot P content was mainly explained by rhizosphere APase activity, water-soluble P and pH within low P soils across species. Within species, P uptake was mainly linked to rhizosphere water soluble P, APase, and pH in low P soils. The effects of rhizosphere organic anions varied among species and they appeared to play minor roles in improving P availability and uptake.

Influence of phosphorus application on seed yield, quality and phosphorus uptake pattern of Indian mustard (Brassica juncea) cultivars

A field experiment was conducted during rabi 2012-13 at the Punjab Agricultural University, Ludhiana on low P soil to study the response of Indian mustard [Brassica juncea (L.) Czern and Coss] cultivars to applied phosphorus. Application of phosphorus up to 15 kg/ha of P2O5 significantly increased the concentration and uptake of P in plants at all the growth stages, in seed and stover at harvest, and seed yield, oil content, oil yield and protein yield whereas similar increase with application of 30 over 15 kg/ha of P2O5 was significant for P concentration in seed and stover, P uptake at 90 and 120 DAS, stover and total P uptake (seed+ stover) and oil content. Seed yield (1766 kg/ha), oil yield (625 kg/ha) and protein yield (291 kg/ha) produced with application of 30 kg/ha of P2O5 was 9.9, 11.2 and 8.9 per cent, respectivelymore than that produced without application of phosphorus. Among cultivars, NRCHB 601 has significantly out performed all other cultivars for P concentration and uptake, seed-, oil- and protein-yield. Total P uptake at maturity by NRCHB 601 (21.52 kg/ha) was significantly more (18.8-54.8%) than rest of the cultivars. Cultivar NRCDR 2 contained significantly higher oil content (40.2%) than rest of the test cultivars, but, its oil yield was significantly lower than that of NRCHB 601.

Effect of uneven root allocation on shoot Zn uptake by crop plants growing on soil with heterogeneous P and Zn distribution

Many plants are known to preferentially allocate root growth to soil patches with elevated availability of growth-limiting nutrients. Little is known about how this influences the uptake of other nutrients such as trace metals. Here we investigated zinc (Zn) uptake by different crop plants in soil with heterogeneous phosphorus (P) and Zn distribution. Triticum aestivum L. (spring wheat), Cucumis sativus L. (cucumber) and Lupin albus L. (white lupin) were grown in rhizoboxes on low-P soil treated with local P and Zn amendments. A treatment with homogeneous P amendment was used as control. Root system development was monitored over 55days using neutron radiography and Winrhizo. The treatments had no effect on total root and shoot biomass, but root growth responded positively to local soil P enrichment but not to local Zn enrichment in all three plants. Wheat and cucumber responded with preferential allocation of root length, while in white lupin cluster root (CR) allocation was shifted to the P-enriched soil patches. Shoot Zn uptake was increased in wheat and cucumber when Zn- and P-enrichment were co-located and decreased when they were disjunct, indicating that shoot Zn uptake was related to root length growth in these two plants. In contrast, preferential CR allocation in lupin was not associated with a treatment effect on shoot Zn uptake.

Arbuscular mycorrhizal fungi contribute to overyielding by enhancing crop biomass while suppressing weed biomass in intercropping systems

Intercropping often produces more biomass than the average of the corresponding monocultures, particularly in sustainable, low-input agriculture. Characterizing plant-plant interactions is fundamental for a better understanding of the ecological functioning of intercropping. However, there is little information on whether mycorrhizal networks exhibit different effects upon intra- and interspecific plant interactions in intercropping systems. Microcosms with one inner and three outer compartments were used. The inner compartment was separated from the outer compartments by a 30-μm nylon mesh or a solid barrier or remained open to allow full interactions between plants grown in the different compartments, creating three treatments, namely −Roots−CMNs (solid barrier), −Roots+CMNs (mesh), and +Roots+CMNs (no barrier). Either faba bean (Vicia faba) or maize (Zea mays) was grown as the donor plant in the inner compartment. Maize was combined with maize, wheat (Triticum aestivum) or the weed species foxtail (Setaria viridis) as the neighboring plant species. Faba bean was combined with faba bean, maize, or foxtail. The low-P soil was inoculated with the arbuscular mycorrhizal fungus (AMF) Funneliformis mosseae or remained uninoculated. In the absence of AMF, plant growth was restricted by P deficiency and root interactions did not significantly affect plant-plant interactions. The presence of AMF enhanced plant biomass (maize by 104.5 %, faba bean by 61.6 %) and nutrient (N and P) uptake. Significant overyielding occurred in treatments where roots and hyphal networks of all plant species intermingled. The weed (foxtail) was suppressed by interspecific interactions while maize and faba bean benefitted from a mycorrhizal network and the presence of intermingling roots. In low-P soils, mycorrhizal networks contribute to overyielding in intercropping by enhancing the biomass and nutrient uptake of the mycorrhiza-dependent plant species. The biomass depression of weed species by AMF and/or root interactions may have ecological implications in weed control in intercropping systems.

Shift from complementarity to facilitation on P uptake by intercropped wheat neighboring with faba bean when available soil P is depleted.

Rhizosphere processes stimulate overyielding and facilitative phosphorus (P) uptake in cereal/legume intercropping systems. However, little is known about when and how rhizosphere alteration of legumes plays a role in improving P uptake by cereals. Wheat was grown isolated, monocropped or intercropped with faba bean in pots with low-P soil. The biomass, P content, carboxylates and phosphatases activity were measured in 15 destructive samplings. Intraspecific competition of the biomass and P uptake of monocropped wheat was not significant before 40 and 36 days after sowing (DAS), whereas there was interspecific competition of biomass of intercropped wheat before 66 DAS. However, afterwards, the increments of the biomass and P uptake of the intercropped wheat were 1.3–1.9 and 1.9–2.3 times of increment of monocropped wheat. Meanwhile, the concentrations of malate and citrate and the acid phosphatase activity in the rhizospheres of intercropped wheat were significantly increased, which suggested that wheat/faba bean intercropping is efficient in P utilization due to complementary P uptake in the early growth stage and the positive interactions of the rhizosphere processes when the soil P was depleted.

The rise and fall of arbuscular mycorrhizal fungal diversity during ecosystem retrogression.

Ecosystem retrogression following long-term pedogenesis is attributed to phosphorus (P) limitation of primary productivity. Arbuscular mycorrhizal fungi (AMF) enhance P acquisition for most terrestrial plants, but it has been suggested that this strategy becomes less effective in strongly weathered soils with extremely low P availability. Using next generation sequencing of the large subunit ribosomal RNA gene in roots and soil, we compared the composition and diversity of AMF communities in three contrasting stages of a retrogressive >2-million-year dune chronosequence in a global biodiversity hotspot. This chronosequence shows a ~60-fold decline in total soil P concentration, with the oldest stage representing some of the most severely P-impoverished soils found in any terrestrial ecosystem. The richness of AMF operational taxonomic units was low on young (1000's of years), moderately P-rich soils, greatest on relatively old (~120000years) low-P soils, and low again on the oldest (>2000000years) soils that were lowest in P availability. A similar decline in AMF phylogenetic diversity on the oldest soils occurred, despite invariant host plant diversity and only small declines in host cover along the chronosequence. Differences in AMF community composition were greatest between the youngest and the two oldest soils, and this was best explained by differences in soil P concentrations. Our results point to a threshold in soil P availability during ecosystem regression below which AMF diversity declines, suggesting environmental filtering of AMF insufficiently adapted to extremely low P availability.

Phosphorus Availability in Low-P and Acidic Soils as Affected by Liming and P Addition

Acidic soils typically suffer from high phosphorus (P) retention, a problem that can be dealt with using greater P fertilization, soil liming, or both. The aim of this work was to examine which of these practices bears the more beneficial result for Lolium perenne L. growth. In a pot experiment, five acidic soils were treated as follows: L0P0 (unamended control), L1P0 (liming only), L0P1 (P addition only), and L1P1 (both liming and P addition). We found that P amendment alone was sufficient to increase plant P levels when the initial soil P concentrations were low. Liming without P addition increased plant P satisfactorily only in the high-P soil. We conclude that P addition alone is a better practice than liming alone for improved plant growth conditions in acidic, low-P soils, unless there is relatively high P content in soil, in which case liming alone may be sufficient to increase P availability.

No need to breed for enhanced colonization by arbuscular mycorrhizal fungi to improve low-P adaptation of West African sorghums

Western Africa (WA) sorghums are predominantly cultivated under low plant available phosphorus (P) soil conditions with a diverse population of arbuscular mycorrhizal fungi (AMF) present. This study aims to determine whether sorghum breeding programs should target higher colonization by AMF through understanding the genotypic variation of sorghum for AMF-root colonization (AMF-RC) under different P-fertility conditions at different growth stages and assessing the genetics underlying AMF-RC using genome-wide association study (GWAS). A sorghum diversity panel of 187 WA genotypes was grown in low-P soil in a pot trial for 38 days and a subset of 13 genotypes was grown in a low- and high-P field until maturity at ICRISAT-Samanko in Mali, WA. Root samples were taken at 38 days from the pot trial plants and at flowering time in the field trials. Shoot biomass was analyzed for P concentration and dry matter yield. GWAS was conducted for shoot-P-content and AMF-RC. Significant genotypic variation was observed for AMF-RC, but the repeatability estimates were only low (w2 = 0.15 at 38 days) to moderate (w2 = 0.54–0.56 at flowering time). AMF-RC was significantly higher in low-P versus high-P field conditions. Large residual variation was observed for AMF-RC in both pot and field trials. None of the genotypic groups, contrasting for selection history, race and grain yield performance across multiple field trials, differed significantly for AMF-RC. AMF-RC showed no or negative relationships to shoot-P-content and grain yield, irrespective of soil-P level or plant developmental stage. AMF-RC at 38 days was significantly correlated (r = 67**) to AMF-RC at flowering. However, GWAS did not detect significant genomic regions for AMF-RC but did for shoot-P content. Although genetic differences for AMF-RC were detected, the trait appears to be highly polygenic. Genotypic selection for higher AMF-RC in WA sorghums is not promising due to the low heritability and the lack of positive relationships with P acquisition.

Improving Sensitivity of Solution 31P NMR Analysis in Australian Xeralfs

Phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy is widely used to identify and quantify phosphorus (P) forms in soil. This study aimed to determine whether narrowing the soil to extractant sodium hydroxide–ethylenediaminetetraacetic acid (NaOH-EDTA) ratio from 1:20 to values as low as 1:4 would improve sensitivity of solution 31P NMR spectroscopy without degrading resolution or quantitation. Four Australian soils were tested using four ratios. The narrowest ratio of 1:4 gave the best quality NMR spectra in terms of signal-to-noise ratio. Peak resolution was not degraded on narrowing the ratio. There was no clear effect of narrowing the extraction ratio on extraction efficiency or the distribution of signal among chemical shift regions (orthophosphate, monoester P, diester P, and pyrophosphate). We conclude that a ratio of 1:4 improved NMR analysis for these particular soils and should be considered for other soils, particularly low-P soils, where NMR sensitivity is limiting.

Phosphate response of Trifolium uniflorum compared with T. repens and some T. repens×T. uniflorum hybrids

Introgression of genes from Trifolium uniflorum L. into T. repens L. (white clover) is being investigated as a method to improve phosphorus (P) use efficiency in white clover; however, little is known about the edaphic adaptations or P physiology of T. uniflorum. Growth responses to added P of T. uniflorum, T. repens and some T. repens × T. uniflorum hybrids were determined in a glasshouse experiment in pots of soil. Trifolium uniflorum showed traits consistent with adaptation to low-P soils: slow growth rate, small leaves, relatively high leaf-tissue P concentrations, and sequestration of P in its roots when soil P levels were increased. The response of Kopu II, one of the hybrid backcross parents, was quite different; it showed high growth rate, large leaves, much lower leaf P concentrations, and a large decrease in root : shoot P allocation as soil P increased. Tahora, the other backcross parent, exhibited several characteristics that were intermediate between Kopu II and T. uniflorum, probably reflecting its breeding origins from New Zealand hill-country ecotypes. This study confirms the potential for interspecific hybridisation with T. uniflorum to increase the tolerance of white clover to low soil P levels, through incorporation of traits related to edaphic adaptations. Variation among the hybrid families in their response to changing soil P confirmed previously published conclusions about the need to screen widely in hybrid material.

The regime and P availability of omitting P fertilizer application for rice in rice/wheat rotation in the Taihu Lake Region of southern China

In the Taihu Lake Region (TLR) of China, farmers’ injudicious and excessive use of phosphorus (P) fertilizer has led to a dramatic spike in P accumulation. In view of that, the water flooding practice can increase soil P release and enhance P availability in rice season, compared with the strong P fixation in wheat season; it seems possible to save P fertilizer in rice season with the aim of reducing P loads without any crop yield declines. To validate this possibility, a 4-year pot experiment encompassing eight rice/wheat seasons and using four paddy soils with varying Olsen-P contents (6.16 to 40.95 mg kg−1) was conducted to compare rice/wheat yield, inorganic and organic P accumulation under four different P regimes, P fertilization for both rice and wheat (PR + W; conventional practice), P fertilization only for wheat (PW), P fertilization only for rice (PR), and no P fertilization for both seasons (Pzero). Compared with conventional PR + W treatment, PR treatment significantly decreased wheat yields, especially in medium- and low-P soils, with an Olsen-P concentrate decline of 34.4–62.8 %. In contrast, PW treatment showed no significant difference in the rice/wheat yields over 4 years irrespective of high-, medium-, and low-P-concentrated soils, despite the soil Olsen-P concentration declining by 34.9–64.4 %. This highlights the feasibility of omitting P fertilizer application to flooded rice for at least 4 years in rice/wheat cropping paddy fields while maintaining crop yields and reducing environmental risk. In four paddy soils, available inorganic P was the dominant effective P source and increased with the concentration of Olsen-P. Without P fertilization over time, the concentration of soil inorganic P fractions declined and organic P remained relatively constant. According to the P supply capacity of different soils under the regime of omitting P fertilization for rice, how to utilize the bioavailability of P in different P supply capacity soils when P fertilization is omitted for rice crops will be required in future work.