Phosphorus mining for ecological restoration on former agricultural land

To restore species‐rich terrestrial ecosystems on ex‐agricultural land, establishing nutrient limitation for dominant plant growth is essential because in nutrient‐rich soils, fast‐growing species often exclude target species. However, N‐limitation is easier to achieve than P‐limitation (because of a difference in biogeochemical behavior), biodiversity is generally highest under P‐limitation. Commonly used restoration methods to achieve low soil P‐concentrations are either very expensive or take a very long time. A promising restoration technique is P‐mining, an adjusted agricultural technique that aims at depleting soil‐P. High biomass production and hence high P‐removal with biomass are obtained by fertilizing with nutrients other than P. A pot experiment was set up to study P‐mining with Lolium perenne L. on sandy soils with varying P‐concentrations: from an intensively used agricultural soil to a soil near the soil P‐target for species‐rich Nardus grassland. All pots received N‐ and K‐fertilization. The effects of biostimulants on P‐uptake were also assessed by the addition of arbuscular mycorrhiza (Glomus spp.), humic substances or phosphate‐solubilizing bacteria (Bacillus sp. and Pseudomonas spp.). In our P‐rich soil (111 µg P_Olsen/g), P‐removal rate was high but bioavailable soil‐P did not decrease. At lower soil P‐concentrations (64 and 36 µg P_Olsen/g), bioavailable soil‐P had decreased but the P‐removal rate had by then dropped 60% despite N‐ and K‐fertilization and despite that the target (<10 µg P_Olsen/g) was still far away. None of the biostimulants altered this trajectory. Therefore, restoration will still take decades when starting with ex‐agricultural soils unless P‐fertilization history was much lower than average.     

Phosphate uptake, efflux and deficiency in the water fern, Azolla

High phosphorus status (High-P) Azolla mexicana plants (P content 15.5 μmoles g fr wt^?1, doubling time ca. 2.2 d) and Low-P plants with early signs of P-deficiency (P content 6.2 μmoles g fr wt^?1, doubling time ca. 3.2 d) were used to study Pi uptake, efflux and deficiency. When High-P plants were transferred to medium lacking Pi, uptake capacity increased 1.5-fold within 12 h and before any detectable change in growth rate (24–48 h). When High-P and Low-P plants were compared, uptake rates from 0.3–10000 mmoles m^?3 Pi were 2.6–1.7 times higher in Low-P than High-P plants (18–1150 vs 7–665 μmoles g fr wt^?1 h^?1). The relationship of uptake rate to concentration was interpreted as arising from a combined operation of a high- and a low-affinity uptake system. Higher uptake in Low-P plants involved a 3.4-fold increase in V_max (high affinity), no change in K_m (high affinity), and a 1.5 to two-fold increase in both V_max (low affinity) and Km (low affinity). Rates of P efflux into 1–1000 mmoles m^?3 Pi were 1.7 to two times higher from High-P than Low-P plants (12–22 vs 7–11 μmoles g fr wt^?1 h^?1). Below 1 mmole m^?3 Pi, uptake and efflux rates were similar: the equilibrium concentration, at which net uptake was zero, was 0.22 mmoles m^?3 for High-P plants and 0.05 mmoles m^?3 for Low-P plants. Similar results were obtained with A. filiculoides. P transport characteristics of Azolla, a fern, are closely comparable with those of higher plants. Its high P requirement in the field arises from its ecological rather than physiological behaviour. We interpret the field behaviour by exploring the relationship between Azolla growth rate in the field, plant P concentration in the field, Pi transport rates required to support such growth, and Pi concentrations in pond waters. The transport characteristics which must operate in the field match those of Low-P plants in the laboratory, not High-P plants. Thus, Pi uptake in High-P plants should be interpreted as repressed from the normal state, instead of that in Low-P plants being induced.     

Interactions of an acid tolerant strain of phosphate solubilizing bacteria with a few acid tolerant crops

Phosphate solubilizing bacteria (PSB) were isolated from sixty soil samples of various soil classes and cropping histories in Himalayan regions of Uttar Pradesh, India by enrichment culture techniques. Phosphate solubilization and acid tolerance of each strain was estimated. A strain (PAS-2) isolated froma pasture and waste land of pH 4.8, organic matter 2.6% available N 265kg ha^-1, available P₂O₅(Bray's II) 2.3kg ha^-1 and available K₂O 353 kg ha^-1 had the highest P-solubilization (45 µg P per mL per day) and also highest acid tolerance rating 42. The strain was identified as Bacillus sp. Seed inoculation of this bacterial strain resulted in significant increases in grain and vegetative yield of fingermillet (Elosine coracana), maize (Zea mays), amaranth (Amaranthus hypochondriacus), buckwheat (Fagopyrium esculentum), frenchbean (Phaseolus vulgaris) with or without added P sources. The significant grain yield (quintol ha^-1) with phosphate and seed inoculation ranged from 33.85 in maize, 26.33 in frenchbean, 22.41 in buckwheat, 20.71 in amaranth and 19.19 in fingermillet as compared to controls. The highest response was observed with frenchbean followed by fingermillet, buckwheat, amaranth and maize. Phosphate use efficiency was highest in frenchbean followed by maize and lowest and almost at par in buckwheat, amaranth and fingermillet. Available phosphate was also highest in frenchbean cultivated plot followed by amaranth, fingermillet, buckwheat and maize. The MPN count of phosphate solubilizing bacteria were also influenced by seed inoculation of strain PAS-2. Frenchbean exerted greaterrhizosphere effect followed by pseudocereals and cereals. Likewise, phosphate nutrition of crops were also improved through seed inoculation irrespective of added P sources. The study thus demonstrated that selection of efficient strain of PSB from acid soil and its seed inoculation in selected crop genotype is beneficial in boosting up crop yield in low productive hill soil. Seed inoculation also created greater rhizosphere effect over uninoculation which improved P-nutrition of crops and also available soil P.     

Screening Perennial Warm-Season Bioenergy Crops as an Alternative for Phytoremediation of Excess Soil P

A recent alternative strategy to reduce environmental problems associated with P transport from agricultural soils is the use of bioenergy crops to remediate excess soil P. In addition to the positive impacts associated with P mitigation, harvested biomass used as a renewable energy source can also offset the cost associated with plant-based P remediation strategies. The objective of this study was to identify potential crop species that can be used for remediation of soil P and as a cellulosic feedstock for production of renewable energy in South Florida. Fifteen crop entries were investigated for their potential to remove P from a P-enriched soil. Dry matter (DM) yield varied among crop species with greatest yield observed for elephantgrass (Pennisetum purpureum Schum.) and sugarcane (Saccharum spp.) (43 and 39 Mg?ha^?1 year^?1, respectively). Similarly, greater P removal rates were observed for elephantgrass (up to 126 kg?P?ha^?1 year^?1 in 2008) followed by sugarcane (62 kg?P?ha^?1 year^?1 in 2008). Although there was no effect (P?=?0.45) of crop species on P reduction in the soil, soil P concentrations decreased linearly during the 3-year study. Because of its relatively greater DM yield and P removal rates, elephantgrass was shown to be a good candidate for remediation of excess soil P in South Florida Spodosols.     

Antimony uptake and toxicity in sunflower and maize growing in SbIII and SbV contaminated soil

Using pot experiments, we investigated the uptake of antimony (Sb) by sunflower (Helianthus annuus L. cv. Iregi), and maize (Zea mays L. cv. Magister) in two different soils, a potting mix and an agricultural soil. In one treatment Sb was added to the experimental soils as KSb(OH)₆ (“Sb^V-treatment”) and in the other as Sb₂O₃ (“Sb^III-treatment”). Soluble soil Sb concentrations were linearly related to the applied Sb rates, ranging from 0.02 (controls) to 175 mg L^?1 soil solution. Accumulation of Sb tended to be slightly higher in the Sb^V treatment in sunflower, while no difference in Sb uptake between the two Sb treatments was found in maize. The half maximal effective concentration (EC₅₀) values derived from the dose-response curves were higher for the Sb^V than for the Sb^III treatment when they were related to soluble soil Sb concentrations, but differences became insignificant when they were related to shoot Sb concentrations. Maize was substantially more sensitive to Sb toxicity than sunflower, indicating physiological differences in Sb tolerance between the two plant species. Our results show that on soils with high Sb contamination, as often found in shooting ranges, plants may suffer from Sb toxicity.     

Using a Crop Growth Model to Quantify Regional Biogas Potentials: an Example of the Model Region Biberach (South-West Germany)

Over the last decade, political framework conditions in the energy sector provoked a strong focus on biogas production in Germany. In this context, a sufficient and secure regional biomass supply is needed in order to run biogas plants economically. It is important to estimate which biomass amounts can be produced and are available for bioenergy production in a defined region. The present study focused on a model-based approach quantifying the biomass and, from this, the resulting biogas potential of the model region of Biberach (south-west Germany) using the process-oriented crop growth model DSSAT 4.0. Considering the regional soil and climate conditions of the model region, dry matter yields of maize, triticale, and a crop rotation system (CRS) of maize and triticale including different management systems (change in sowing and harvest date) were simulated. The results indicated an adequate model fit between simulated and measured yields. Dry matter yields of maize (14.7 t ha^?1), triticale (12.7 t ha^?1), and the CRS (18.1–19.2 t ha^?1) differed significantly, indicating that the chosen CRS provided the highest dry matter yields. The biomass potential of all crops was simulated considering different bioenergy scenarios depending on the available agricultural land used for bioenergy. The highest biomass potential was provided by the management system consisting of maize and triticale sown on 1 May and 15 October, respectively. Finally, an additional energy potential of 45,000 kW_el (bioenergy scenario 50/50 % of the agricultural land used for biogas production) and of 5,700 kW_el (bioenergy scenario 25/25 % of the agricultural land used for biogas production) was determined for the county of Biberach by implementing a CRS, which consisted of maize and triticale. It could be concluded that an additional biomass potential for biogas production exists in the county. Suitable areas for the location of biogas plants could be identified based on the available biomass potential.     

Conventionally Tilled and Permanent Raised Beds with Different Crop Residue Management: Effects on Soil C and N Dynamics

Conservation tillage in its version of permanent bed planting under zero-tillage with crop residue retention has been proposed as an alternative wheat production system for northwest Mexico. However, little is known about the dynamics of C and N in soils under wheat/maize on permanent beds (PB) where straw was burned, removed, partly removed or retained, as opposed to conventionally tilled beds (CTB) where straw was incorporated. We investigated the dynamics of soil C and N and normalized difference vegetative index (NDVI) crop values in zero-tilled PB and CTB after 26 successive maize and wheat crops. Organic C and total N were respectively, 1.15 and 1.17 times greater in PB with straw partly removed and with straw retained on the surface, than in CTB with straw incorporated. Organic C and total N were 1.10 times greater in soils with 300 kg N ha⁻¹ added than in unfertilized soil. Cumulative production of CO₂ was lower under CTB with straw incorporated than under PB treatments, and CO₂ production increased with increments in inorganic fertilizer. The N-mineralization rate was 1.18 times greater than in unamended soils when 150 kg inorganic N ha⁻¹ was applied, and 1.48 times greater when 300 kg inorganic N ha⁻¹ was added. The N-mineralization rate was significantly (1.66 times) greater in PB where the straw was burned or retained on the surface than in CTB where the straw was incorporated, but significantly (1.25 times) lower than in PB with straw partly removed. The NDVI values reached a maximum 56 days after planting and decreased thereafter. The NDVI for unfertilized soil were similar for CTB with straw incorporated, PB with straw partly removed, and PB with straw retained on the surface, but significantly lower for PB with straw burned and PB with straw removed. In soils to which 150 or 300 kg N ha⁻¹ was added, NDVI was significantly lower for PB with straw burned than for other treatments. Among other things, this suggests the utility of rotating maize or wheat with crops whose residues have lower C–N ratios, thus avoiding immobilization of large amounts of N for extended periods. PB with residue burning, however, is an unsustainable practice leading to low crop performance and soil and environmental degradation.     

Phytoremediation of Phosphorus-Enriched Soils

After numerous years of application of phosphorus (P) fertilizer and/or manures, many soils accumulate high levels of P, increasing the risk of P transfer via surface runoff. While current and future regulations may limit P application to soil, little effort is given to the reduction of soils already enriched with P. To evaluate the potential of phytoremediation for decreasing available P in soil, the P uptake of several crops and vegetables grown on a high P soil was studied. Plant shoot and root P contents, plant-available soil P, and potential P removal under optimum conditions were examined. None of the plants were “true” P hyperaccumulators, although all removed some P from the soil. Whole corn plants (for silage) and Indian mustard removed the highest amount of P; 114 and 108 kg ha^-1, respectively. If only the grain from corn and Indian mustard were harvested, significantly lower amounts of P were removed from the field. These results show that there is extensive variation in the uptake of P by various crop species. For soils highly enriched with P, it may be desirable to grow a plant species capable of removing above average concentrations of P and to harvest the whole plant because a large fraction of the plant P is contained within the leaves and stems.     

Maize productivity and nutrient dynamics in maize-fallow rotations in western Kenya

One-season fallows with legumes such as Crotalaria grahamiana Wight & Arn. and phosphorus (P) fertilization have been suggested to improve crop yields in sub-Saharan Africa. Assessing the sustainability of these measures requires a sound understanding of soil processes, especially transformations of P which is often the main limiting nutrient. We compared plant production, nitrogen (N) and P balances and selected soil properties during 5.5 years in a field experiment with three crop rotations (continuous maize, maize-crotalaria and maize-natural fallow rotation) at two levels of P fertilization (0 and 50 kg P ha^?1 yr^?1, applied as triple superphosphate) on a Kandiudalfic Eutrudox in western Kenya. The maize yield forgone during growth of the crotalaria fallow was compensated by higher post-fallow yields, but the cumulative total maize yield was not significantly different from continuous maize. In all crop rotations, P fertilization doubled total maize yields, increased N removal by maize and remained without effect on amounts of recycled biomass. Crotalaria growth decreased in the course of the experiment due to pest problems. The highest levels of soil organic and microbial C, N and P were found in the maize-crotalaria fallow rotation. The increase in organic P was not accompanied by a change in resin-extractable P, while H₂SO₄-extractable inorganic P was depleted by up to 38 kg P ha^?1 (1% of total P) in the 0–50 cm layer. Microbial P increased substantially when soil was supplied with C and N in a laboratory experiment, confirming field observations that the microbial biomass is limited by C and N rather than P availability. Maize-legume fallow rotations result in a shift towards organic and microbial nutrients and have to be complemented by balanced additions of inorganic fertilizers. Abbreviations: BNF – biological nitrogen fixation; COM – continuous maize; LR – long rainy season; MCF – maize-crotalaria fallow rotation; MNF – maize-natural fallow rotation; SR – short rainy season; TSP – triple superphosphate.     

Inorganic soil phosphate fractions as related to soil-test values by common methods

Summary An attempt to find the relationship among the soil inorganic phosphate fractions and the soil test values determined with various common methods, has been made in the present studies. Out of the methods tried (Olsenet al., Bray and Kurtz, Datta and Kamath, Bingham) the soil-test values obtained with only Bray and Kurtz's method no. 2 have shown a positive significant relationship with Ca-P fraction while the others have shown more or less a positive significant correlation with saloid-bound P, Al-P and Fe-P fractions. Negative significant relationships of Al-P and Fe-P with Ca-P indicate that they increase or decrease at the expense of each other by fertilisation and cropping. Application of basic slag at the rate of 120 lbs P₂O₅ per acre indicate a maximum increase in most of the soil test values and the saloid bound P and Al-P fractions suggesting probably its high residual value in the soils. The use of the method of Datta and Kamath along with that of Olsenet al. and Bray and Kurtz no. 1 has been recommended.     

Soil Greenhouse Gas Emissions in Response to Corn Stover Removal and Tillage Management Across the US Corn Belt

In-field measurements of direct soil greenhouse gas (GHG) emissions provide critical data for quantifying the net energy efficiency and economic feasibility of crop residue-based bioenergy production systems. A major challenge to such assessments has been the paucity of field studies addressing the effects of crop residue removal and associated best practices for soil management (i.e., conservation tillage) on soil emissions of carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). This regional survey summarizes soil GHG emissions from nine maize production systems evaluating different levels of corn stover removal under conventional or conservation tillage management across the US Corn Belt. Cumulative growing season soil emissions of CO₂, N₂O, and/or CH₄ were measured for 2–5 years (2008–2012) at these various sites using a standardized static vented chamber technique as part of the USDA-ARS’s Resilient Economic Agricultural Practices (REAP) regional partnership. Cumulative soil GHG emissions during the growing season varied widely across sites, by management, and by year. Overall, corn stover removal decreased soil total CO₂ and N₂O emissions by -4 and -7 %, respectively, relative to no removal. No management treatments affected soil CH₄ fluxes. When aggregated to total GHG emissions (Mg CO₂?eq ha^?1) across all sites and years, corn stover removal decreased growing season soil emissions by ?5?±?1 % (mean?±?se) and ranged from -36 % to 54 % (n?=?50). Lower GHG emissions in stover removal treatments were attributed to decreased C and N inputs into soils, as well as possible microclimatic differences associated with changes in soil cover. High levels of spatial and temporal variabilities in direct GHG emissions highlighted the importance of site-specific management and environmental conditions on the dynamics of GHG emissions from agricultural soils.     

Electrokinetic-enhanced phytoremediation of uranium-contaminated soil using sunflower and Indian mustard

Abstract

Electrokinetic-enhanced phytoremediation is an effective technology to decontaminate heavy metal contaminated soil. In this study, we examined the effects of electrokinetic treatments on plant uptake and bioaccumulation of U from soils with various U sources. Redistribution of uranium in soils as affected by planting and electrokinetic treatments was investigated. The soil was spiked with 100?mg kg^–1 UO₂, UO₃, and UO₂(NO₃)₂. After sunflower and Indian mustard grew for 60 days, 1 voltage of direct-current was applied across the soils for 9 days. The results indicated that U uptake in both plants were significantly enhanced by electrokinetic treatments from soil with UO₃ and UO₂(NO₃)₂. U was more accumulated in roots than in shoots. Electrokinetic treatments were effective on lowering soil pH near the anode region. Overall, uranium (U) removal efficiency reached 3.4–4.3% from soils with UO₃ and uranyl with both plants while that from soil with UO₂ was 0.7–0.8%. Electrokinetic remediation treatment significantly enhanced the U removal efficiency (5–6%) from soils with UO₃ and uranyl but it was 0.8–1.3% from soil with UO₂, indicating significant effects of U species and electrokinetic enhancement on U bioaccumulation. This study implies the potential feasibility of electrokinetic-enhanced phytoremediation of U soils with sunflower and Indian mustard.     

Changes in root system structure,leaf water potential and gas exchange of maize and triticale seedlings affected by soil compaction

The physiological reasons associated with differential sensitivity of C₃ and C₄ plant species to soil compaction stress are not well explained and understood. The responses of growth characteristics, changes in leaf water potential and gas exchange in maize and triticale to a different soil compaction were investigated. In the present study seedlings of triticale and maize, representative of C₃ and C₄ plants were subjected to low (L – 1.10 g cm⁻³), moderate (M – 1.34 g cm⁻³) and severe (S – 1.58 g cm⁻³) soil compaction level. Distinct differences in distribution of roots in the soil profile were observed. Plants of treatments M or S in comparison to treatment L, showed a decrease in leaf number, dry mass of stem, leaves and roots, and an increase in the shoot to root ratio. A drastic decrease in root biomass in M and S treatments in the soil profile on depth from 15 to 40 cm was observed. Any level of soil compaction did not influence the number of seminal and seminal-adventitious roots but decreased their length. The number and total length of nodal roots decreased with compaction. Changes of growth traits in M and S treatments in comparison to the L were greater for maize than for triticale and were accompanied by daily changes in water potential (ψ) and gas exchange parameters (P_N, E, g_s). Differences between M and S treatments in daily changes in ψ for maize were in most cases statistically insignificant, whereas for triticale, they were statistically significant. Differences in the responses of maize and triticale to soil compaction were found in P_N, E and g_s in particular for the measurements taken at 12:00 and 16:00. The highest correlation coefficients were obtained for the relationship between leaf water potential and stomatal conductance, both for maize and triticale, which indicates the close association between stomata behavior and changes in leaf water status.     

Promotion of AM utilization through reduced P fertilization 2. Field studies

The hypothesis of this study was that cumulative P fertilization decreases the contribution of arbuscular mycorrhiza (AM) to crop growth and nutrient uptake in Northern European field conditions. The modes of action of P fertilization were evaluated through effects on mycorrhization, crop dependence on AM, and AM fungal (AMF) community. Field studies were carried out within long-term experiments on soils with low and intermediate initial content of extractable P, where no P fertilization and 45 kg ha^–1 a^–1 P were applied for 20 years. AM effectiveness in terms of growth and nutrient uptake of flax, red clover and barley, percentage root length colonized by AMF, P response of flax, and spore densities and species composition of the AMF communities, were assessed. In the soil with low initial P supply, cumulative P fertilization decreased AM contribution to crop growth and nutrient uptake. The higher AM effectiveness in soil with no added P compensated the cumulative P fertilization (soil P_H2O 2.5 v. 9.5 mg kg^–1) for flax, but not completely for clover. In contrast, barley obtained no benefit from AM at harvest and only a slight benefit from cumulated P. In the soil with intermediate initial P supply, AM reduced growth of flax and barley, especially with no added P, and no response to AM was obtained on clover due to retarded mycorrhization. Cumulative P fertilization reduced yield losses of flax by AM (P_H2O 18.8 v. 5.4 mg kg^–1), because fertilization inhibited mycorrhization. In both soils, root colonization and spore density were decreased by cumulative P fertilization, but no changes in AMF species composition were observed.     

Buckwheat (Fagopyrum esculentum Moench) has high capacity to take up phosphorus (P) from a calcium (Ca)-bound Source

Two experiments were carried out in a growth chamber to investigate the phosphorus (P)-uptake efficiency of Fagopyrum esculentum Moench (buckwheat) and Triticum aestivum (spring wheat) from a Ca-bound form. The first experiment was based on a sand-culture system with either rock phosphate (RP) or CaHPO₄ (CaHP) as the P source and nitrate or ammonium nitrate as nitrogen source. A highly calcareous soil was used in the second experiment. Buckwheat was shown to be highly efficient in taking up Ca-bound P compared to spring wheat. When plants were supplied with nitrate, the total P uptake by buckwheat from RP was nearly 10-fold higher than that of spring wheat (20.1 compared with 2.1 mg P pot^–1). Changing nitrogen source from nitrate only to ammonium nitrate increased P uptake by spring wheat substantially, but not buckwheat. High P-uptake efficiency of buckwheat was also demonstrated using the field soil, but to a lesser extent, which may be related to the difference in Zn supply between sand culture and field soil. It is suggested that buckwheat may be included in intercropping or crop rotation systems to activate P sources in calcareous soils. The principal mechanism of P uptake efficiency of buckwheat may be its ability to acidify the rhizosphere; however, further study is needed to unravel the regulation of root excretion of H⁺ and its molecular basis in order to exploit buckwheat's genetic capability to utilise sparingly soluble P from soil.     

Arsenic accumulation of common plants from contaminated soils

A pot experiment was conducted to investigate the relationship between soluble concentrations of arsenic (As) in soil and its accumulation by maize (Zea mays), English ryegrass (Lolium perenne), rape (Brassica napus) and sunflower (Helianthus annuus) on two different soils: a calcareous Regosol (silty loam) and a non-calcareous Regosol (sandy loam). Arsenic (Na₂HAsO₄·7H₂O) was applied to obtain comparable soluble As concentrations in the two soils. In both soils, soluble As concentrations, extracted with 0.1 M NaNO₃, were found to correlate better with As concentrations in plants after 4 month of growth than total soil concentrations, extracted with 2 M HNO₃. With all four plant species, the relationship between the soluble As concentration in the soil and As that in the plants was non- linear, following Michaelis-Menten kinetics. Similar soluble As concentrations in the two soils did not result in a similar As concentration in the plants. Except for maize, arsenic transport from roots to shoots was significant, resulting in As concentrations in the leaves and grains above the Swiss tolerance limits for fodder and food crops (4 and 0.2 mg As kg^–1, respectively). Based on these results we suggest that beside As solubility, P availability and P demand, which are plant specific, have to be taken into account to predict the uptake of As by crop plants from As contaminated soils and to predict the risk of arsenic entering into the food chain.     

Adaptation to high temperature mitigates the impact of water deficit during combined heat and drought stress in C3 sunflower and C4 maize varieties with contrasting drought tolerance

Heat and drought stress frequently occur together, however, their impact on plant growth and photosynthesis (P_N) is unclear. The frequency, duration and severity of heat and drought stress events are predicted to increase in the future, having severe implications for agricultural productivity and food security. To assess the impact on plant gas exchange, physiology and morphology we grew drought tolerant and sensitive varieties of C3 sunflower (Helianthus annuus) and C4 maize (Zea mays) under conditions of elevated temperature for 4 weeks prior to the imposition of water deficit. The negative impact of temperature on P_N was most apparent in sunflower. The drought tolerant sunflower retained ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RubisCO) activity under heat stress to a greater extent than its drought sensitive counterpart. Maize exhibited no varietal difference in response to increased temperature. In contrast to previous studies, where a sudden rise in temperature induced an increase in stomatal conductance (G_s), we observed no change or a reduction in G_s with elevated temperature, which alongside lower leaf area mitigated the impact of drought at the higher temperature. The drought tolerant sunflower and maize varieties exhibited greater investment in root‐systems, allowing greater uptake of the available soil water. Elevated temperatures associated with heat‐waves will have profound negative impacts on crop growth in both sunflower and maize, but the deleterious effect on P_N was less apparent in the drought tolerant sunflower and both maize varieties. As C4 plants generally exhibit water use efficiency (WUE) and resistance to heat stress, selection on the basis of tolerance to heat and drought stress would be more beneficial to the yields of C3 crops cultivated in drought prone semi‐arid regions.     

Root-induced modifications of the exchange of phosphate ion between soil solution and soil solid phase

The uptake of phosphorus (P) by roots results in a depletion of phosphate ions (PO₄) in the rhizosphere. The corresponding decrease in PO₄ concentration in the soil solution (C_P) gives rise to a replenishment of P from the solid phase which is time- and C_P-dependent. This PO₄ exchange which reflects the buffer power of the soil for PO₄ also varies with the composition and the physico-chemical conditions of the soil. As root activity can modify these physico-chemical conditions in the rhizosphere, the question arises whether these modifications affect the ability of PO₄ bound to the soil solid phase to exchange with PO₄ in soil solution. The aim of the present work was to measure and compare the parameters which describe the amount of PO₄ bound to soil solid phase that is capable to replenish solution P for both rhizosphere and bulk soils. The soil sample was a P-enriched, calcareous topsoil collected from a long-term fertiliser trial. Rhizosphere soil samples were obtained by growing dense mats of roots at the surface of 3 mm thick soil layer for one week. Three plant species were compared: oilseed rape (Brassica napus L., cv Goeland) pea (Pisum sativum L., cv. Solara) and maize ( Zea mays L., cv. Volga). The time- and C_P-dependence of the PO₄ exchange from soil to solution were described using an isotopic dilution method. The measured C_P values were 0.165 mg P L⁻¹ for bulk soil and 0.111, 0.101 and 0.081 mg P L⁻¹ for rhizosphere soils of maize, pea and rape, respectively. The kinetics of the PO₄ exchange between liquid and solid phases of soil were significantly different between rhizosphere and bulk soils. However, when changes in C_P were accounted for, the parameters describing the PO₄ exchange with time and C_P between soil solution and soil solid phase were found to be very close for bulk and rhizosphere soils. For this calcareous and P-enriched soil, plant species differed in their ability to deplete PO₄ in solution. The resulting changes in the ability of the soil solid phase to replenish solution PO₄ were almost fully explained by the depletion of soil solution P. This revised version was published online in June 2006 with corrections to the Cover Date.     

Changes in soil phosphorus fractions following positive and negative phosphorus balances for long periods

The effect of phosphorus (P) balance (addition, in both fertilizers and farmyard manure (FYM), minus removal in crops) on eight soil P fractions determined by sequential extraction, was measured on archived soils from various long-term experiments run by Rothamsted Experimental Station in the United Kingdom. It has been established unequivocally that, for all the soils investigated, no one of the eight P fractions was increased or decreased during long periods of P addition or depletion, respectively. However, changes were mainly in the resin (24–30%) and the inorganic (P_i) component of the four fractions extracted sequentially by 0.5 M NaHCO₃, 0.1 M NaOH, 1.0 M NaOH, 0.5 M H₂SO₄ (41–60%). For the sandy loam there were also consistent changes in the organic (P_o) fraction (25%), especially that extracted by bicarbonate, presumably because the soil contained only a little clay and presumably had low sorption capacity. When the soils were cropped without P addition the largest proportional change was in the P extracted by resin, 0.5 M NaHCO₃ and 0.1 M NaOH, suggesting that the P in these fractions is readily available, or has the potential to become available, for crop growth. This was supported by changes in the overall P balance. On the heavier textured soils, 50–80% of the change in total soil P (P_T) was in these fractions; on the sandy soil this increased to more than 90%. The change in the sum of the first five fractions accounted, on average, for 90% of the P balance. However these changes in the P in the plough layer frequently left large amounts of P unaccounted for in some of the excessively P enriched soils. The amount of P_i extracted by resin and bicarbonate (P_i(r+b)) ranged between 14 and 50% of the sum of the P_i fractions. Soils with the lower percentages were those known to be most responsive to P fertilizers. P_i(r+b) accounted for an average of 70% of the P balance (negative) in P depleting soils where crop offtake was not offset or exceeded by annual P additions (positive balance). The ratio between P_i(r+b) and P_i(sum) could be a guide in defining soils deficient in P and those which are excessively enriched.     

Yield gaps, nutrient use efficiencies and response to fertilisers by maize across heterogeneous smallholder farms of western Kenya

The need to promote fertiliser use by African smallholder farmers to counteract the current decline in per capita food production is widely recognised. But soil heterogeneity results in variable responses of crops to fertilisers within single farms. We used existing databases on maize production under farmer (F-M) and researcher management (R-M) to analyse the effect of soil heterogeneity on the different components of nutrient use efficiency by maize growing on smallholder farms in western Kenya: nutrient availability, capture and conversion efficiencies and crop biomass partitioning. Subsequently, we used the simple model QUEFTS to calculate nutrient recovery efficiencies from the R-M plots and to calculate attainable yields with and without fertilisers based on measured soil properties across heterogeneous farms. The yield gap of maize between F-M and R-M varied from 0.5 to 3 t grain ha^?1 season^?1 across field types and localities. Poor fields under R-M yielded better than F-M, even without fertilisers. Such differences, of up to 1.1 t ha^?1 greater yields under R-M conditions are attributable to improved agronomic management and germplasm. The relative response of maize to N–P–K fertilisers tended to decrease with increasing soil quality (soil C and extractable P), from a maximum of 4.4-fold to ?0.5-fold relative to the control. Soil heterogeneity affected resource use efficiencies mainly through effects on the efficiency of resource capture. Apparent recovery efficiencies varied between 0 and 70% for N, 0 and 15% for P, and 0 to 52% for K. Resource conversion efficiencies were less variable across fields and localities, with average values of 97 kg DM kg^?1 N, 558 kg DM kg^?1 P and 111 kg DM kg^?1 K taken up. Using measured soil chemical properties QUEFTS over-estimated observed yields under F-M, indicating that variable crop performance within and across farms cannot be ascribed solely to soil nutrient availability. For the R-M plots QUEFTS predicted positive crop responses to application of 30 kg P ha^?1 and 30 kg P ha^?1 + 90 kg N ha^?1 for a wide range of soil qualities, indicating that there is room to improve current crop productivity through fertiliser use. To ensure their efficient use in sub-Saharan Africa mineral fertilisers should be: (1) targeted to specific niches of soil fertility within heterogeneous farms; and (2) go hand-in-hand with the implementation of agronomic measures to improve their capture and utilisation.