The effect of soil water status on fertiliser, topsoil and subsoil phosphorus utilisation by wheat

Background and aims

Crop phosphorus (P) content is controlled by P uptake from both banded P fertiliser and from P throughout the soil profile. These P supply factors are in turn controlled by soil, climatic and plant factors. The aim of this experiment was to measure the contribution of fertiliser, topsoil and subsoil P to wheat plants under wet and dry growing season conditions.

Methods

An isotopic tracer technique was used to measure P uptake from fertiliser at seven agricultural field sites under wet and dry growing season conditions. At three of these sites a dual isotopic technique was used to distinguish between wheat uptake of P from fertiliser, topsoil (0–15 cm) and subsoil (below 15 cm).

Results

The amount of P fertiliser used by wheat was in the order of 3–30% of the P applied and increased with increasing rainfall. Topsoil P was the most important P source, but when sufficient P was present in the subsoil, P fertiliser addition stimulated the use of subsoil P.

Conclusions

Most crop P uptake was from the topsoil, however P fertiliser banded below the seed increased plant P uptake and stimulated the use of subsoil P in one soil type in a decile 7 (above average rainfall) growing season.     

Uptake of zinc and phosphorus by plants is affected by zinc fertiliser material and arbuscular mycorrhizas

Background and Aims

Water solubility of zinc (Zn) fertilisers affects their plant availability. Further, simultaneous application of Zn and phosphorus (P) fertiliser can have antagonistic effects on plant Zn uptake. Arbuscular mycorrhizas (AM) can improve plant Zn and P uptake. We conducted a glasshouse experiment to test the effect of different Zn fertiliser materials, in conjunction with P fertiliser application, and colonisation by AM, on plant nutrition and biomass.

Methods

We grew a mycorrhiza-defective tomato genotype (rmc) and its mycorrhizal wild-type progenitor (76R) in soil with six different Zn fertilisers ranging in water solubility (Zn sulphate, Zn oxide, Zn oxide (nano), Zn phosphate, Zn carbonate, Zn phosphate carbonate), and supplemental P. We measured plant biomass, Zn and P contents, mycorrhizal colonisation and water use efficiency.

Results

Whereas water solubility of the Zn fertilisers was not correlated with plant biomass or Zn uptake, plant Zn and P contents differed among Zn fertiliser treatments. Plant Zn and P uptake was enhanced when supplied as Zn phosphate carbonate. Mycorrhizal plants took up more P than non-mycorrhizal plants; the reverse was true for Zn.

Conclusions

Zinc fertiliser composition and AM have a profound effect on plant Zn and P uptake.     

Contrasting effects of manure and green waste biochars on the properties of an acidic ferralsol and productivity of a subtropical pasture

Background and Aim

We hypothesised that amending an acidic ferralsol with biochar would improve the productivity of a subtropical dairy pasture via reducing soil acidity related constraints and result in improved nitrogen use efficiency. We examined two contrasting biochars with different carbon, nutrient content and acid neutralising values.

Methods

Field plots were amended with one of three biochar treatments (Nil, feedlot manure biochar [FM], green waste biochar [GW]) in combination with presence or absence of NPK fertiliser and presence or absence of liming. The FM and GW biochars had a carbon content of 44 and 76 %, available phosphorous of 5,960 and 93 mg kg^?1, and liming values of 13 and 5.6 %, respectively. The pasture was managed to supply year round high quality feed for dairy production.

Results

The FM biochar increased total pasture productivity by 11 % and improved the agronomic nitrogen use efficiency by 23 %. It also reduced soil acidity but did not significantly affect the pH dependent soil cation exchange capacity. The GW biochar did not improve pasture productivity. Both biochars resulted in an increase in the soil carbon density.

Conclusions

The high available phosphorous content of FM biochar makes it an effective amendment for acidic ferralsols. Greenwaste biochar did not have sufficient acid neutralising capacity or phosphorous content to reduce soil acidity constraints. Both biochars enhance soil carbon storage in pasture systems on ferralsol.     

Fate of fertiliser N applied to wheat on a coarse textured highly calcareous soil under simulated semi-arid conditions

Background and Aims

Quantitative information on the fate and efficiency of nitrogen (N) fertiliser applied to coarse textured highly calcareous soils in semi-arid farming systems is scarce but, as systems intensify, is essential to support sustainable agronomic management decisions

Method

A glasshouse study was undertaken to trace the fate of N fertiliser applied to wheat (Triticum aestivum L. cv Yitpi) grown on a reconstructed profile (0 to 600 mm) of a grey highly calcareous (>35% CaCO₃) sandy loam soil. Two watering treatments were applied (drier and wetter) equivalent to low (decile 2, 179 mm) and medium (decile 5, 234 mm) growing season rainfall for a location with typical semi-arid environment in southern Australia. 15 N-labelled urea fertiliser (35.4 mg N/pot) was applied in a split application - at sowing and 70 days after sowing, followed by immediate watering or watering after delay of 1 week.

Results

Recovery of N fertiliser in grain (30 to 52%) was greater for the wetter treatment, and when water was applied immediately following fertiliser application. It was also similar for N applied at sowing and N applied during crop growth. Overall, more than 40% of the urea fertiliser N remained in the soil at anthesis, largely in the top 100 mm, indicating little movement of fertiliser N down the profile even with application of water. Losses of urea fertiliser N (13-24%) were considered relatively small given the highly calcareous nature of the soil; and were significantly greater from N applied during growth compared to at sowing, particularly where watering did not immediately follow application. There was no effect of fertiliser N on grain yield due to sufficient available N in soil at sowing (139mgN/pot), but N concentration and DM of stubbles was increased. Watering treatment did not affect shoot dry matter production up to anthesis, although root weight was higher in the wetter treatment, and grain yield was 9% greater.

Conclusions

It is concluded that the potential for N losses from urea applied to highly calcareous coarse textured soils in semi-arid agricultural areas appears relatively low. Further, where there are relatively large amounts of plant available N present at sowing, a strategy of delayed or withheld applications of N to manage economic risk may have minimal effects on grain production in seasons with drier than average rainfall.     

The fate of fertiliser P in soil under pasture and uptake by subterraneum clover – a field study using <Superscript>33</Superscript>P-labelled single superphosphate

Background and aims

Single superphosphate (SSP) is a major source of phosphorus (P) used in grazing systems to improve pasture production. The aim of this experiment was to determine the fate of fertiliser P in clover pastures under field conditions.

Methods

A procedure was developed to radiolabel SSP granules with a ³³P radiotracer, which was then applied to the soil surface (equivalent to ~12 kg P ha^?1) of a clover pasture. Recovery of fertiliser P was determined in clover shoots, fertiliser granules and soil fractions (surface layer: 0–4 cm and sub-surface layer: 4–8 cm).

Results

The P diffusion patterns of the ³³P-labelled SSP granules were not significantly different to those of commercial SSP granules (P?>?0.05). Recovery of fertiliser P in clover shoots was 30–35 %. A considerable proportion of the fertiliser P (~28 %) was recovered in the surface soil layer and was largely inorganic P.

Conclusions

Recovery of fertiliser P by clover plants was up to 35 % in the year of application. Much of the fertiliser P in soil fractions was inorganic P, which highlights the importance of inorganic P forms and dynamics in soils under clover pasture on a single season timeframe at these sites.

     

The critical soil P levels for crop yield, soil fertility and environmental safety in different soil types

Background and aims

Sufficient soil phosphorus (P) is important for achieving optimal crop production, but excessive soil P levels may create a risk of P losses and associated eutrophication of surface waters. The aim of this study was to determine critical soil P levels for achieving optimal crop yields and minimal P losses in common soil types and dominant cropping systems in China.

Methods

Four long-term experiment sites were selected in China. The critical level of soil Olsen-P for crop yield was determined using the linear-plateau model. The relationships between the soil total P, Olsen-P and CaCl₂-P were evaluated using two-segment linear model to determine the soil P fertility rate and leaching change-point.

Results

The critical levels of soil Olsen-P for optimal crop yield ranged from 10.9 mg kg^?1 to 21.4 mg kg^?1, above which crop yield response less to the increasing of soil Olsen-P. The P leaching change-points of Olsen-P ranged from 39.9 mg kg^?1 to 90.2 mg kg^?1, above which soil CaCl₂-P greatly increasing with increasing soil Olsen-P. Similar change-point was found between soil total P and Olsen-P. Overall, the change-point ranged from 4.6 mg kg^?1 to 71.8 mg kg^?1 among all the four sites. These change-points were highly affected by crop specie, soil type, pH and soil organic matter content.

Conclusions

The three response curves could be used to access the soil Olsen-P status for crop yield, soil P fertility rate and soil P leaching risk for a sustainable soil P management in field.     

Field inoculation with arbuscular-mycorrhizal fungi overcomes phosphorus and zinc deficiencies of linseed (Linum usitatissimum) in a vertisol subject to long-fallow disorder

Background and aims

Long-fallow disorder is expressed as exacerbated deficiencies of phosphorus (P) and/or zinc (Zn) in field crops growing after long periods of weed-free fallow. The hypothesis that arbuscular-mycorrhizal fungi (AMF) improve the P and Zn nutrition, and thereby biomass production and seed yield of linseed (Linum usitatissimum) was tested in a field experiment.

Methods

A factorial combination of treatments consisting of ± fumigation, ±AMF inoculation with Glomus spp., ±P and ±Zn fertilisers was used on a long-fallowed vertisol. The use of such methods allowed an absolute comparison of plants growing with and without AMF in the field for the first time in a soil disposed to long-fallow disorder.

Results

Plant biomass, height, P and Zn concentrations and contents, boll number and final seed yield were (a) least in fumigated soil with negligible AMF colonisation of the roots, (b) low initially in long-fallow soil but increased with time as AMF colonisation of the roots developed, and (c) greatest in soil inoculated with AMF cultures. The results showed for the first time in the field that inflows of both P and Zn into linseed roots were highly dependent on %AMF-colonisation (R²?=?0.95 for P and 0.85 for Zn, P?<?0.001) in a soil disposed to long-fallow disorder. Relative field mycorrhizal dependencies without and with P+Zn fertiliser were 85 % and 86 % for biomass and 68 % and 52 % for seed yield respectively.

Conclusions

This research showed in the field that AMF greatly improved the P and Zn nutrition, biomass production and seed yield of linseed growing in a soil disposed to long-fallow disorder. The level of mycorrhizal colonisation of plants suffering from long-fallow disorder can increase during the growing season resulting in improved plant growth and residual AMF inoculum in the soil, and thus it is important for growers to recognise the cause and not terminate a poor crop prematurely in order to sow another. Other positive management options to reduce long fallows and foster AMF include adoption of conservation tillage and opportunity cropping.     

Growth and phosphorus nutrition of rice when inorganic fertiliser application is partly replaced by straw under varying moisture availability in sandy and clay soils

Background and aims

The combined effects of (1) reduced soil moisture availability, (2) reduced application of inorganic fertilisers while incorporating straw, (3) soil type, and their effects on growth, root system plasticity, phosphorus (P) nutrition of rice, and soil P dynamics are poorly known, but very important when aiming to increase the efficiency of water and P use.

Methods

Using large pots a three-factor factorial experiment was conducted with two moisture treatments (i.e. continuous flooding, and draining of top soil after flowering while subsoil was kept moist through capillary action), three fertilisation treatments; with (P1) and without (P0) applications of inorganic P fertilisers, and 25 % of inorganic fertilisers reduced while incorporating straw (5 t ha^?1), and soil type (i.e. clay and sandy soils with 15 and 9 mg P kg^?1 soil, respectively in P0). Shoot and root growth, root system plasticity, P nutrient status and soil P dynamics were measured.

Key results

Straw incorporation with reduced inorganic fertiliser application ensured a higher shoot dry weight and yield only in flooded clay soil as compared with P0 and P1, and a similar shoot dry weight and yield to P1 under drained clay soil. A positive growth response was facilitated by an increased water-use efficiency and rate of photosynthesis in shoots, and increased root system plasticity through the production of greater root length, more roots in deep soil layers, and an increased fraction of fine roots. Straw enhanced P extractability in soil. Drained soil reduced P uptake (15–45 %) and increased P-use efficiency. In addition to the re-translocation of P from senescing leaves and stems under both moisture conditions, the P concentration in green leaves under drained condition was also reduced (41–72 %).

Conclusion

Growth benefits of straw incorporation were observed in clay soil under both moisture conditions, and this was facilitated by the improved P availability, increased P uptake, and greater root system plasticity with the production of deeper and finer roots, compared with that in sandy soil, and inorganic fertiliser applications alone. As P uptake was reduced under drained soil, P re-translocation and % P allocated to panicles increased.     

Soil test measures of available P (Colwell, resin and DGT) compared with plant P uptake using isotope dilution

Background and aims

Recent research has demonstrated the high accuracy of a new method for assessment of plant available P in soil called diffusive gradients in thin-films (DGT). The process of P released by additions of bicarbonate to soil samples simulating common soil P tests is yet to be assessed by the new method (DGT). The aim of this study was to identify the pools of soil P extracted by soil test methods (DGT, Colwell and resin) by comparing, in ³²P–labelled soils, the specific activity (SA) of phosphorus extracted by common soil test extracts with the SA of wheat plants grown in a range of agricultural soils from southern Australia.

Methods

Wheat (cv. Frame) was grown for 4 weeks in 14 soils that were labelled uniformly with carrier-free ³²P. The specific activity (SA) of P (MBq ³²P kg ³¹P^?1) in each soil test extract was compared to the SA of P in the wheat plants.

Results

The SA of P in plants were similar to P extracted by the Colwell extractant in only 4 of the 14 soils; while SA in plants and extractants corresponded in 10 of the soils for the resin method and in 12 of the soils for the DGT method. Phosphorus in the Colwell and resin extract solutions had significantly lower SAs compared to P in the plants for 10 and 4 of the soils, respectively, indicating greater extraction of non-labile P sources (unlabelled ³¹P). Phosphorus in the DGT extractant had significantly lower SA than the plants for 1 soil and in 1 soil the SA was higher. Overall, across all soils, 25 % of P extracted by the Colwell method was non labile compared to 9 % and 2 % for the resin and DGT methods, respectively.

Conclusion

The new DGT method for extraction of soil P has the potential to accurately predict occurrences of P deficiency because it generally extracts the same pool of labile soil P accessed by wheat plants, while methods using bicarbonate solution (e.g. Colwell, Olsen) or water (resin) at wide soil:solution ratios are more likely to measure more non-labile forms of P in soil.     

Wheat leaf properties affecting the absorption and subsequent translocation of foliar-applied phosphoric acid fertiliser

Background and aims

Although foliar fertilisation using liquid forms of phosphorus (P) is not a new concept, its adoption has been hindered by a limited understanding of the variability in performance of fluid forms of foliar P applied to broadacre crops. There is a need to identify how the surface structure of leaves influences the absorption and subsequent translocation of foliar-applied P in above ground plant parts.

Methods

This study examined the surface properties of wheat leaves using scanning electron microscopy and measured the recovery of foliar-applied fertiliser that was labelled with either ³²P or ³³P from both the adaxial (upper) and abaxial (lower) leaf sides into untreated plant parts.

Results

We found that the adaxial leaf surface absorbed and translocated more foliar-applied P away from the treated leaf than the abaxial surface, likely related to the higher abundance of trichomes and stomata present on that side of the leaf. The recovery of the foliar-applied fertiliser varied with rate and timing of application; ranging from <30 % to as much as 80 % of the adaxial-applied fertiliser translocated from the treated leaf into the wheat ear.

Conclusions

This study demonstrated that the differences in surface morphological features between leaf sides influenced the combined absorption and subsequent translocation of foliar-applied P in the above ground plant parts. This is due to a direct effect on the foliar pathway and/or due to differences in wettability affecting both the leaf coverage and drying time of fertilisers on the leaves. Although foliar fertilisation in this study contributed less than 10 % of the total P in the plant, it provided a more efficient pathway for P fertilisation than soil-applied P.     

Interaction between root growth allocation and mycorrhizal fungi in soil with patchy P distribution

Aims and Background

Many plants preferentially grow roots into P-enriched soil patches, but little is known about how the presence of arbuscular mycorrhizal fungi (AMF) affects this response.

Methods

Lotus japonicus (L.) was grown in a low-P soil with (a) no additional P, (b) homogeneous P (28 mg pot^?1), (c) low heterogeneous P (9.3 mg pot^?1), and (d) high heterogeneous P (28 mg pot^?1). Each P treatment was combined with one of three mycorrhiza treatments: no mycorrhizae, Glomus intraradices, indigenous AMF. Real-time PCR was used to assess the abundance of G. intraradices and the indigeneous AMF G. mosseae and G. claroideum.

Results

Mycorrhization and P fertilization strongly increased plant growth. Homogeneous P supply enhanced growth in both mycorrhizal treatments, while heterogeneous P fertilization increased biomass production only in treatments with indigenous AMF inoculation. Preferential root allocation into P-enriched soil was significant only in absence of AMF. The abundance of AMF species was similar in P-enriched and unfertilized soil patches.

Conclusion

Mycorrhization may completely override preferential root growth responses of plants to P- patchiness in soil. The advantage of this effect for the plants is to give roots more freedom to forage for other resources in demand for growth and to adapt to variable soil conditions.     

Root hairs explain P uptake efficiency of soybean genotypes grown in a P-deficient Ferralsol

Background and aims

Incorporating soybean (Glycine max) genotypes with a high nitrogen fixation potential into cropping systems can sustainably improve the livelihoods of smallholder farmers in Western Kenya. Nitrogen fixation is, however, often constrained by low phosphorus (P) availability. The selection of soybean genotypes for increased P efficiency could help to overcome this problem. This study investigated the contribution of different root traits to variation in P efficiency among soybean genotypes.

Methods

Eight genotypes were grown in a Ferralsol amended with suboptimal (low P) and optimal (high P) amounts of soluble P. Root hair growth was visualized by growing plants in a novel agar system where P intensity was buffered by Al₂O₃ nanoparticles.

Results

In the pot trial, P uptake was unaffected among the genotypes at high P but differed about 2-fold at low P. The genotypes differed in P uptake efficiency but not in P utilization efficiency. Regression analysis and mechanistic modeling indicated that P uptake efficiencies were to a large extent related to root hair development (length and density) and, to a lower extent, to colonization by mycorrhizal fungi.

Conclusion

Breeding for improved root hair development is a promising way to increase P uptake efficiency in soybean.     

Coupled soil-availability and tree-limitation nutritional shifts induced by N deposition: insights from N to P relationships in Abies pinsapo forests

Background and aims

Interacting effects of atmospheric N deposition on the degree to which tree demand for other nutrients is met by soil supply has seldom been explored in Mediterranean-type ecosystems. We hypothesized that patterns for the relative availability of N and P in soils will be matched by variations in process rates related to soil organic P cycling and by shifts from N to P limitation of tree growth.

Methods

We examined N/P relationships in Mediterranean-fir (Abies pinsapo) forests from two nearby regions differing in N deposition levels.

Results

N pools and transformation rates and the contribution of organic fractions to the labile P pool in soils showed increasing trends toward the pollution source. Phosphomonoesterase activity (PME) in bulk soils, root PME per unit biomass (but not per unit soil volume) and biomass accumulation in P-fertilized root-in-growth cores incubated in situ were also the highest at the sites receiving elevated N deposition, indicating P limitation. In contrast, forest stands in the region farther from the pollutant source were N-limited (preferential root growth in N-rich soil microsites) and showed lower PME activities and higher total fine root biomass.

Conclusions

In the forests under elevated N deposition, higher values for an overall indicator of soil N status matched with indications of an accelerated soil organic P subcycle and P-limitation of tree growth.     

Modelling root plasticity and response of narrow-leafed lupin to heterogeneous phosphorus supply

Background & Aims

Searching for root traits underpinning efficient nutrient acquisition has received increased attention in modern breeding programs aimed at improved crop productivity. Root models provide an opportunity to investigate root-soil interactions through representing the relationships between rooting traits and the non-uniform supply of soil resources. This study used simulation modelling to predict and identify phenotypic plasticity, root growth responses and phosphorus (P) use efficiency of contrasting Lupinus angustifolius genotypes to localised soil P in a glasshouse.

Methods

Two L. angustifolius genotypes with contrasting root systems were grown in cylindrical columns containing uniform soil with three P treatments (nil and 20 mg P kg^?1 either top-dressed or banded) in the glasshouse. Computer simulations were carried out with root architecture model ROOTMAP which was parameterized with root architectural data from an earlier published hydroponic phenotyping study.

Results

The experimental and simulated results showed that plants supplied with banded P had the largest root system and the greatest P-uptake efficiency. The P addition significantly stimulated root branching in the topsoil, whereas plants with nil P had relatively deeper roots. Genotype-dependent root growth plasticity in response to P supply was shown, with the greatest response to banded P.

Conclusions

Both experimental and simulation outcomes demonstrated that 1) root hairs and root proliferation increased plant P acquisition and were more beneficial in the localised P fertilisation scenario, 2) placing P deeper in the soil might be a more effective fertilisation method with greater P uptake than top dressing, and 3) the combination of P foraging strategies (including root architecture, root hairs and root growth plasticity) is important for efficient P acquisition from a localised source of fertiliser P.     

Effects of two contrasting canopy manipulations on growth and water use of London plane (Platanus x acerifolia) trees

Aims

The objectives were to investigate (i) the forms and release pattern of P from an ash-rich biochar-amended sandy soil; (ii) the transformation of biochar P in a soil-plant system.

Methods

Several methodologies (a bioassay test, soluble P extractions, a sequential P fractionation and successive P extractions via resin strips) were used to study the bioavailability and transformation of P in a sandy soil fertilised with either conventional P fertilisers [Ca(H₂PO₄)₂ (CaP) and Sechura phosphate rock (SPR)] or biochars produced from cattle manure (MAe) and alum-treated biosolids (BSe) at four temperatures (250, 350, 450, and 550 °C).

Results

Biochar P mainly contributed to increase soil resin-extractable P- and inorganic NaOH-extractable P-fractions, and thus to plant available P. The decrease in P concentrations of those fractions was caused by the uptake of P by plants rather than their transformations into more stable forms. P release rates diminished following the order: CaP > MAe > BSe > SPR, which indicates a decline in P availability from these P sources.

Conclusions

Phosphorus-rich biochar can be used as a slow-release fertiliser. It is necessary to determine available P (either soil or fertiliser tests) in biochars prior to its application to soil, so that dose, frequency and timing of application are correctly established.     

The impact of changing moisture conditions on short-term P availability in weathered soils

Background and Aims

Bioavailable phosphorus (P) represents a primary constraint on productivity in many ecosystems on highly-weathered soils. Soil moisture can be important to determining P bioavailability and net primary productivity in these systems. However, hydrologic controls on P availability remain poorly understood.

Methods

We used “resins” (anion-exchange membranes) to quantify the response of labile P, an estimate of bioavailable P, to soil moisture conditions in two highly-weathered soils (rendzina, ultisol). The resins were either incubated in soil or shaken with a soil-water slurry.

Results

Resin incubations in aerobic soil effectively quantified labile P in soils under changing moisture conditions, extracting significant amounts of labile P while avoiding the disturbance imposed by slurries. Wetting field-moist soils resulted in pulsed labile P, with lagged peaks occurring days after the largest moisture additions. Re-wetting air-dried soils enhanced labile P immediately, with the largest amounts observed at the highest moisture levels; labile P steadily declined following the moisture addition.

Conclusions

Soil moisture levels and history strongly impacted labile P, indicating the importance of both variables when interpreting labile P measurements. These results also suggest that P availability is linked to both the amount and timing of rainfall, with implications for plant productivity in regions exposed to changing moisture regimes.     

Efficacy of zinc oxides as fertilisers

Background and aims

Human zinc (Zn) deficiency is prevalent in developing countries and Zn biofortification of grains is used to increase the Zn content of food staples. Agronomic interventions to biofortify grain involve fertiliser selection and management. The usefulness of a zinc compound as a fertiliser will depend on its solubility, bioavailability and the effect of its distribution in the soil profile.

Methods

Various sources of Zn oxide and Zn sulfate fertiliser were characterised for nutrient content, morphology, solubility, and fertiliser recovery when applied to the surface, banded near the seed or uniformly mixed.

Results

Compared with Zn sulfates, Zn oxide fertilisers had very low water solubility and slow dissolution rates, because of a higher dissolution pH. This did not translate to a diminished ability to supply Zn to plants when both sources of Zn were mixed through the soil, but there was significantly less fertiliser recovery from Zn oxides than from Zn sulfates when the fertiliser was banded near the seed.

Conclusions

All sources will be equally effective if uniformly mixed through the profile. In no-till systems where fertiliser is banded near seed, Zn sulfate is superior to Zn oxide.     

C:N:P stoichiometry in Australian soils with respect to vegetation and environmental factors

Aims

We estimate organic carbon (C): total nitrogen (N): total phosphorus (P) ratios in soils under Australia’s major native vegetation groups.

Methods

We use digital datasets for climate, soils, and vegetation created for the National Land and Water Resources Audit in 2001. Analysis-of-variance is used to investigate differences in nutrient ratios between ecosystems. Linear discriminant analysis and logistic regression are used to investigate the relative importance of climatic variables and soil nutrients in vegetation patterns.

Results

We find that the N:P and C:P ratios have a greater range of values than the C:N ratio, although major vegetation groups tend to show similar trends across all three ratios. Some apparently homeostatic groupings emerge: those with very low, low, medium, or high N:P and C:P. Tussock grasslands have very low soil N, N:P, and C:P, probably due to frequent burning. Eucalypt woodlands have low soil N:P and C:P ratios, although their total P level varies. Rainforests and Melaleuca forests have medium soil N:P and C:P ratios, although their total P level is different. Heathlands, tall open eucalypt forests, and shrublands occur on soils with low levels of total P, and high N:P and C:P ratios that reflect foliar nutrient ratios and recalcitrant litter.

Conclusions

Certain plant communities have typical soil nutrient stoichiometries but there is no single Redfield-like ratio. Vegetation patterns largely reflect soil moisture but for several plant communities, eucalypt communities in particular, soil N and P (or N:P) also play a significant role. Soil N:P and the presence of Proteaceae appear indicative of nutrient constraints in ecosystems.     

Phosphorus pools and other soil properties in the rhizosphere of wheat and legumes growing in three soils in monoculture or as a mixture of wheat and legume

Background and aims

Phosphorus and nitrogen availability and forms are affected by soil properties as well as by plant species and further modulated by soil microbes. Additionally, close contact of the roots of two plant species may affect concentrations and forms of N and P. The aim of this study was to assess properties related to N and P cycling in the rhizosphere of wheat and legumes grown in monoculture or in wheat/legume mixtures in three soils differing in pH.

Methods

Faba bean, white lupin and wheat were grown in three soils differing in pH (4.8, 7.5 and 8.8) in monoculture or in mixed culture of wheat and legumes. Rhizosphere soil was collected at flowering and analyzed for P pools by sequential fractionation, available N as well as community structure of bacteria, fungi, ammonia oxidizers, N₂-fixers and P mobilizers by polymerase chain reaction (PCR)—denaturing gradient gel electrophoresis (DGGE).

Results

Soil type was the major factor determining plant growth, rhizosphere nutrient dynamics and microbial community structure. Among the crop species, only faba bean had a significant effect on nitrification potential activity (PNA) in all three soils with lower activity compared to the unplanted soil. Soil type and plant spieces affected the community composition of ammonia-oxidizing archaea (AOB), ammonia-oxidizing archaea (AOA), N₂-fixers (nifH), P mobilizers (ALP gene) and fungi, but not that of bacteria. Among the microbial groups, the AOA and nifH community composition were most strongly affected by crop species, cropping system and soil type, suggesting that these groups are quite sensitive to environmental conditions. All plants depleted some labile as well as non-labile P pools whereas the less labile organic P pools (NaOH extractable P pools, acid extractable P pools) accumulated in the rhizosphere of legumes. The pattern of depletion and accumulation of some P pools differed between monoculture and mixed culture as well as among soils.

Conclusions

Plant growth and rhizosphere properties were mainly affected by soil type, but also by crop species whereas cropping system had the least effect. Wheat and the legumes depleted less labile inorganic P pools in some soils whereas less labile organic P pools (NaOH extractable P, acid extractable P) accumulated in the rhizosphere of legumes.