Abstract: Low phosphorus availability is a primary constraint to plant productivity in many natural and agricultural ecosystems. Plants display a wide array of adaptive responses to low phosphorus availability that generally serve to enhance phosphorus mobility in the soil and increase its uptake. One set of adaptive responses is the alteration of root architecture to increase phosphorus acquisition from the soil at minimum metabolic cost. In a series of studies with the common bean, work in our laboratory has shown that architectural traits that enhance topsoil foraging appear to be particularly important for genotypic adaptation to low phosphorus soils (‘phosphorus efficiency’). In particular, the gravitropic trajectory of basal roots, adventitious rooting, the dispersion of lateral roots, and the plasticity of these processes in response to phosphorus availability contribute to phosphorus efficiency in this species. These traits enhance the exploration and exploitation of shallow soil horizons, where phosphorus availability is greatest in many soils. Studies with computer models of root architecture show that root systems with enhanced topsoil foraging acquire phosphorus more efficiently than others of equivalent size. Comparisons of contrasting genotypes in controlled environments and in the field show that plants with better topsoil foraging have superior phosphorus acquisition and growth in low phosphorus soils. It appears that many architectural responses to phosphorus stress may be mediated by the plant hormone ethylene. Genetic mapping of these traits shows that they are quantitatively inherited but can be tagged with QTLs that can be used in plant breeding programs. New crop genotypes incorporating these traits have substantially improved yield in low phosphorus soils, and are being deployed in Africa and Latin America.

Abstract: In order to determine the relative importance of ionic toxicity versus the osmotic component of salt stress on germination in durum wheat (Triticum durum Desf.), seeds of three cultivars differing in their salt and drought resistance (Omrabi-5, drought-resistant; Belikh, salt-resistant and Cando, salt-sensitive) were incubated in various iso-osmotic solutions of NaCl, mannitol and polyethylene-glycol (PEG) (osmotic potential of −0.15 (control solution) −0.58, −1.05 or −1.57 MPa). Moderate stress intensities only delayed germination, whereas the highest concentration of NaCl and PEG reduced final germination percentages. PEG was the most detrimental solute, while mannitol had no effect on final germination percentages. All osmotica reduced endosperm starch and soluble sugars content as well as α-amylase activities recorded after 48 h of treatment while β-amylase activities were, in contrast, slightly stimulated in all cultivars. Deleterious effects of NaCl and PEG were higher on isolated embryos germinated onto an in vitro Linsmaier and Skoog (LS) medium comparatively to whole seeds. All PEG-treated embryos, however, recovered after the stress relief while NaCl-treated embryos exhibited a lower rate of recovery and some extent of abnormal germination after rinsing. It was concluded that stress inhibition of germination could not be attributed to an inhibition of mobilisation of reserves and that the main effect of PEG occurred via an inhibition of water uptake while detrimental effects of NaCl may be linked to long-term effects of accumulated toxic ions. The behaviour of the three cultivars during germination did not fully reflect their mean level of putative stress resistance in field conditions and germination is, therefore, not recommended as a reliable selection criterion for breeding purposes.

Abstract: The origins and composition of soil organic matter (SOM) are still largely uncertain. Arbuscular mycorrhizal fungi (AMF) are recognized as indirect contributors through their influence on soil aggregation, plant physiology, and plant community composition. Here we present evidence that AMF can also make large, direct contributions to SOM. Glomalin, a recently discovered glycoprotein produced by AMF hyphae, was detected in tropical soils in concentrations of over 60 mg cm−3. Along a chronosequence of soils spanning ages from 300 to 4.1 Mio years, a pattern of glomalin concentrations is consistent with the hypothesis that this protein accumulates in soil. Carbon dating of glomalin indicated turnover at time scales of several years to decades, much longer than the turnover of AMF hyphae (which is assumed to be on the order of days to weeks). This suggests that contributions of mycorrhizae to soil carbon storage based on hyphal biomass in soil and roots may be an underestimate. The amount of C and N in glomalin represented a sizeable amount (ca. 4–5%) of total soil C and N in the oldest soils. Our results thus indicate that microbial (fungal) carbon that is not derived from above- or below-ground litter can make a significant contribution to soil carbon and nitrogen pools and can far exceed the contributions of soil microbial biomass (ranging from 0.08 to 0.2% of total C for the oldest soils).

Abstract: The potential of the biocontrol agent Trichoderma harzianum strain T-203 to induce a growth response in cucumber plants was studied in soil and under axenic hydroponic growth conditions. When soil was amended with T. harzianum propagules, a 30% increase in seedling emergence was observed up to 8 days after sowing. On day 28, these plants exhibited a 95 and 75% increase in root area and cumulative root length, respectively, and a significant increase in dry weight (80%), shoot length (45%) and leaf area (80%). Similarly, an increase of 90 and 30% in P and Fe concentration respectively, was observed in T. harzianum inoculated plants. To better characterize the effect of T. harzianum during the early stages of root colonization, experiments were carried out in a gnotobiotic hydroponic system. An increased growth response was apparent as early as 5 days post-inoculation with T. harzianum, resulting in an increase of 25 and 40% in the dry weight of roots and shoots, respectively. Similarly a significant increase in the concentration of Cu, P, Fe, Zn, Mn and Na was observed in inoculated roots. In the shoots of these plants, the concentration of Zn, P and Mn increased by 25, 30 and 70%, respectively. Using the axenic hydroponic system, we showed that the improvement of plant nutritional level may be directly related to a general beneficial growth effect of the root system following T. harzianum inoculation. This phenomenon was evident from 5 days post-inoculation throughout the rest of the growth period, resulting in biomass accumulation in both roots and shoots.

Abstract: Phosphorus (P), an essential nutrient for crop and animal production, can accelerate freshwater eutrophication, now one of the most ubiquitous forms of water quality impairment in the developed world. Repeated outbreaks of harmful algal blooms (e.g., Cyanobacteria and Pfiesteria) have increased society's awareness of eutrophication, and the need for solutions. Agriculture is regarded as an important source of P in the environment. Specifically, the concentration of specialized farming systems has led to a transfer of P from areas of grain production to animal production. This has created regional surpluses in P inputs (mineral fertilizer and feed) over outputs (crop and animal produce), built up soil P in excess of crop needs, and increased the loss of P from land to water. Recent research has shown that this loss of P in both surface runoff and subsurface flow originates primarily from small areas within watersheds during a few storms. These areas occur where high soil P, or P application in mineral fertilizer or manure, coincide with high runoff or erosion potential. We argue that the overall goal of efforts to reduce P loss to water should involve balancing P inputs and outputs at farm and watershed levels by optimizing animal feed rations and land application of P as mineral fertilizer and manure. Also, conservation practices should be targeted to relatively small but critical watershed areas for P export.

Abstract: C and N mineralisation kinetics obtained in laboratory incubations during decomposition of crop residues under non-limiting nitrogen conditions were simulated using a simple dynamic model. This model includes three compartments: the residues, microbial biomass and humified organic matter. Seven parameters are used to describe the C and N fluxes. The decomposed C is either mineralised as CO2 or assimilated by the soil microflora, microbial decay producing both C humification and secondary C mineralisation. The N dynamics are governed by the C rates and the C:N ratio of the compartments which remain constant in the absence of nitrogen limitation. The model was parameterised using apparent C and N mineralisation kinetics obtained for 27 different residues (organs of oilseed rape plants) that exhibited very wide variations in chemical composition and nitrogen content. Except for the C:N ratio of the residues and the soil organic matter, the other five parameters of the model were obtained by non-linear fitting and by minimising the differences between observed and simulated values of CO2 and mineral N. Three parameters, namely the decomposition rate constant of the residues, the biomass C:N ratio and humification rate, were strongly correlated with the residues C:N ratio. Hyperbolic relationships were established between these parameters and the residues C:N ratio. In contrast, the other two parameters, i.e. the decay rate of the microbial biomass and the assimilation yield of residue-C by the microbial biomass, were not correlated to the residues C:N ratio and were, therefore, fixed in the model. The model thus parameterised against the residue C:N ratio as a unique criterion, was then evaluated on a set of 48 residues. An independent validation was obtained by taking into account 21 residues which had not been used for the parameterisation. The kinetics of apparent C and N mineralisation were reasonably well simulated by the model. The model tended to over-estimate carbon mineralisation which could limit its use for C predictions, but the kinetics of N immobilisation or mineralisation due to decomposition of residues in soil were well predicted. The model indicated that the C:N ratio of decomposers increased with the residue C:N ratio. Higher humification was predicted for substrates with lower C:N ratios. This simple dynamic model effectively predicts N evolution during crop residue decomposition in soil.

Abstract: The hypothesis that inoculation of transplants with vesicular-arbuscular mycorrhizal (VAM) fungi before planting into saline soils alleviates salt effects on growth and yield was tested on lettuce (Lactuca sativa L.) and onion (Allium cepa L.). A second hypothesis was that fungi isolated from saline soil are more effective in counteracting salt effects than those from nonsaline soil. VAM fungi from high- and low-salt soils were trap-cultured, their propagules quantified and adjusted to a like number, and added to a pasteurized soil mix in which seedlings were grown for 3–4 weeks. Once the seedlings were colonized by VAM fungi, they were transplanted into salinized (NaCl) soil. Preinoculated lettuce transplants grown for 11 weeks in the saline soils had greater shoot mass compared with nonVAM plants at all salt levels [2 (control), 4, 8 and 12 dS m−1] tested. Leaves of VAM lettuce at the highest salt level were significantly greener (more chlorophyll) than those of the nonVAM lettuce. NonVAM onions were stunted due to P deficiency in the soil, but inoculation with VAM fungi alleviated P deficiency and salinity effects; VAM onions were significantly larger at all salt levels than nonVAM onions. In a separate experiment, addition of P to salinized soil reduced the salt stress effect on nonVAM onions but to a lesser extent than by VAM inoculation. VAM fungi from the saline soil were not more effective in reducing growth inhibition by salt than those from the nonsaline site. Colonization of roots and length of soil hyphae produced by the VAM fungi decreased with increasing soil salt concentration. Results indicate that preinoculation of transplants with VAM fungi can help alleviate deleterious effects of saline soils on crop yield.

Abstract: It is generally assumed that phosphorus (P) availability for plant growth on highly weathered and P-deficient tropical soils may depend more on biologically mediated organic P (Po) turnover processes than on the release of adsorbed inorganic P (Pi). However, experimental evidence showing the linkages between Po, microbial activity, P cycling and soil P availability is scarce. To test whether land-use systems with higher soil Po are characterized by greater soil biological activity and increased P mineralization, we analyzed the partitioning of P among various organic and inorganic P fractions in soils of contrasting agricultural land-use systems and related it to biological soil properties. Isotopic labeling was used to obtain information on the turnover of P held in the microbial biomass. Soil samples were taken from grass–legume pasture (GL), continuous rice (CR) and native savanna (SAV) which served as reference. In agreement with estimated P budgets (+277, +70 and 0 kg P ha−1 for CR, GL and SAV, respectively), available P estimated using Bray-2 and resin extraction declined in the order CR > GL > SAV. Increases in Bray-2 and resin Pi were greater in CR than GL relative to total soil P increase. Organic P fractions were significantly less affected by P inputs than inorganic fractions, but were a more important sink in GL than CR soils. Extractable microbial P (Pchl) was slightly higher in GL (6.6 mg P kg−1) than SAV soils (5.4 mg P kg−1), and significantly lowest in CR (2.6 mg P kg−1). Two days after labeling the soil with carrier free 33P, 25, 10 and 2% of the added 33P were found in Pchl in GL, SAV and CR soils, respectively, suggesting a high and rapid microbial P turnover that was highest in GL soils. Indicators of P mineralization were higher in GL than CR soils, suggesting a greater transformation potential to render Po available. Legume-based pastures (GL) can be considered as an important land-use option as they stimulate P cycling. However, it remains to be investigated whether crops planted in pasture–crop rotations could benefit from the enhanced Po cycling in grass–legume soils. Furthermore, there is need to develop and test a direct method to quantify Po mineralization in these systems.

Abstract: Low input legume-based agriculture exists in a continuum between subsistence farming and intensive arable and pastoral systems. This review covers this range, but with most emphasis on temperate legume/grass pastures under grazing by livestock. Key determinants of nitrogen (N) flows in grazed legume/grass pastures are: inputs of N from symbiotic N2 fixation which are constrained through self-regulation via grass/legume interactions; large quantities of N cycling through grazing animals with localised return in excreta; low direct conversion of pasture N into produce (typically 5–20%) but with N recycling under intensive grazing the farm efficiency of product N: fixed N can be up to 50%; and regulation of N flows by mineralisation/immobilisation reactions. Pastoral systems reliant solely on fixed N are capable of moderate-high production with modest N losses e.g. average denitrification and leaching losses from grazed pastures of 6 and 23 kg N ha−1 yr−1. Methods for improving efficiency of N cycling in legume-based cropping and legume/grass pasture systems are discussed. In legume/arable rotations, the utilisation of fixed N by crops is influenced greatly by the timing of management practices for synchrony of N supply via mineralisation and crop N uptake. In legume/grass pastures, the spatial return of excreta and the uptake of excreta N by pastures can potentially be improved through dietary manipulation and management strategies. Plant species selection and plant constituent modification also offer the potential to increase N efficiency through greater conversion into animal produce, improved N uptake from soil and manipulation of mineralisation/immobilisation/nitrification reactions.

Abstract: Root hairs are presumably important in the acquisition of immobile soil resources such as phosphorus. The density and length of root hairs vary substantially within and between species, and are highly regulated by soil phosphorus availability, which suggests that at high nutrient availability, root hairs may have a neutral or negative impact on fitness. We used a root-hairless mutant of the small herbaceous dicot Arabidopsis thaliana to assess the effect of root hairs on plant competition under contrasting phosphorus regimes. Wildtype plants were grown with hairless plants in a replacement series design at high (60 μm phosphate in soil solution) and low (1 μm phosphate in soil solution) phosphorus availability. At high phosphorus availability, wildtype and mutant plants were equal in growth, phosphorus acquisition, fecundity and relative crowding coefficient (RCC). At low phosphorus availability, hairless plants accumulated less biomass and phosphorus, and produced less seed when planted with wildtype plants. Wildtype plants were unaffected by the presence of hairless plants in mixed genotype plantings. Wildtype plants had RCC values greater than one while hairless plants had RCC values less than one. We conclude that root hairs increase the competitiveness of plants under low phosphorus availability but do not reduce growth or competitiveness under high phosphorus availability.

Abstract: We report the isolation of nitrogen fixing, phytohormone producing bacteria from sugarcane and their beneficial effects on the growth of micropropagated sugarcane plantlets. Detection of the nitrogen fixing bacteria by ARA-based MPN (acetylene reduction assay-based most probable number) method indicated the presence of up to 106 bacteria per gram dry weight of stem and 107 bacteria per gram dry weight of root of field-grown sugarcane. Two nitrogen fixing bacterial isolates were obtained from stem (SC11, SC20) and two from the roots (SR12, SR13) of field-grown plants. These isolates were identified as Enterobacter sp. strains on the basis of their morphological characteristics and biochemical tests. The isolate SC20 was further characterized by 16S rRNA sequence analysis, which showed high sequence similarity to the sequence of Enterobacter cloacae and Klebsiella oxytoca. All the isolates produced the phytohormone indoleacetic acid (IAA) in pure culture and this IAA production was enhanced in growth medium containing tryptophan. The bacterial isolates were used to inoculate micro-propagated sugarcane in vitro where maximum increase in the root and shoot weight over control was observed in the plantlets inoculated with strain SC20. By using the15N isotope dilution technique, maximum nitrogen fixation contribution (28% of total plant nitrogen) was detected in plantlets inoculated with isolate SC20.

Abstract: Root gravitropism may be an important element of plant response to phosphorus availability because it determines root foraging in fertile topsoil horizons, and thereby phosphorus acquisition. In this study we seek to test this hypothesis in both two dimensional paper growth pouch and three-dimensional solid media of sand and soil cultures. Five common bean (Phaseolus vulgaris L.) genotypes with contrasting adaptation to low phosphorus availability were evaluated in growth pouches over 6 days of growth, and in sand culture and soil culture over 4 weeks of growth. In all three media, phosphorus availability regulated the gravitropic response of basal roots in a genotype-dependent manner. In pouches, sand, and soil, the phosphorus-inefficient genotype DOR 364 had deeper roots with phosphorus stress, whereas the phosphorus-efficient genotype G19833 responded to phosphorus stress by producing shallower roots. Genotypes were most responsive to phosphorus stress in sand culture, where relative root allocation to the 0–3- and 3–6-cm horizons increased 50% with phosphorus stress, and varied 300% (3–6 cm) to 500% (0–3 cm) among genotypes. Our results indicate that (1) phosphorus availability regulates root gravitropic growth in both paper and solid media, (2) responses observed in young seedlings continue throughout vegetative growth, (3) the response of root gravitropism to phosphorus availability varies among genotypes, and (4) genotypic adaptation to low phosphorus availability is correlated with the ability to allocate roots to shallow soil horizons under phosphorus stress.

Abstract: Changes in soil quality after 45 years of continuous production of corn (Zea mays L) by the conventional tillage method (C) compared with adjacent poplar forest (F) and native grassland (G) sites were examined The investigated parameters were: total and humified organic C, total N, light fraction content and composition, water-soluble organic C (WSOC), water-soluble carbohydrates (WSC), phenolic substances, biomass C, cumulative CO2-C (soil respiration) (Cm), enzyme activities (alkaline phosphatase, protease, β-glucosidase, urease, catalase and dehydrogenase) Empirical indexes of soil quality were also calculated: biomass C/organic C, specific respiration of biomass C (qCO2), death rate quotient (qD), metabolic potential (MP), biological index of fertility (BIF), enzyme activity number (EAN) and hydrolysing coefficient (HC) Results indicate that long-term corn production at an intensive level caused a marked decline in all examined parameters Between the undisturbed systems, native grassland showed higher values of soil quality parameters than forest site The indexes most responsive to management practices that may provide indications of the effects of soil cultivation, as well as of the differently undisturbed ecosystems were: organic C, WSC, Cm, protease, β-glucosidase, urease and HC Soil enzyme activities were well related with, and not more sensitive than organic carbon

Abstract: A greenhouse study was conducted in order to determine interactive effects of NaCl salinity and B on the growth, sodium (Na), chloride (Cl), boron (B), potassium (K) concentrations and membrane permeability of salt resistant Tomato (Lycopersicon esculentum L cv Lale F1) and salt sensitive cucumber (Cucumis sativus L cv Santana F1) plants Plants were grown in a factorial combination of NaCl (0 and 30 mM for cucumber and 0 and 40 mM for tomato) and B (0, 5, 10 and 20 mg kg−1 soil) Boron toxicity symptoms appeared at 5 mg kg−1 B treatments in both plants Salinity caused an increase in leaf injury due to B toxicity, but it was more severe in cucumber Dry weights of the plants decreased with the increasing levels of applied B in nonsaline conditions, but the decrease in dry weights due to B toxicity was more pronounced in saline conditions especially in cucumber Salinity × B interaction on the concentration of B in both plants was found significant However, increase in B concentrations of tomato decreased under saline conditions when compared to nonsaline conditions Contrary to this, B concentration of cucumber increased as a result of increasing levels of applied B and salinity Salinity increased Na and Cl concentrations of both plants Potassium concentration of tomato was not affected by salinity and B treatments, but K concentration of cucumber was decreased by salinity Membrane permeability of the plants was increased by salinity while toxic levels of B had no effect on membrane permeability in nonsaline conditions Membrane permeability was significantly increased in the presence of salinity by the increasing levels of applied B

Abstract: Root system dynamics, productivity and N use were studied in inter- and sole crops of field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) on a temperate sandy loam. A 32P tracer placed at a depth of 12.5, 37.5, 62.5 or 87.5 cm was employed to determine root system dynamics by sampling crop leaves at 0, 15, 30 and 45 cm lateral distance. 15N addition was used to estimate N2 fixation by pea, using sole cropped barley as reference crop. The Land Equivalent Ratio (LER), which is defined as the relative land area under sole crops that is required to produce the yields achieved in intercropping, were used to compare the crop growth in intercrops relative to the respective sole crops. The 32P appearance in leaves revealed that the barley root system grows faster than that of pea. P uptake by the barley root system during early growth stages was approximately 10 days ahead of that of the pea root system in root depth and lateral root distribution. More than 90% of the P uptake by the pea root system was confined to the top 12.5 cm of soil, whereas barley had about 25–30% of tracer P uptake in the 12.5 – 62.5 cm soil layer. Judging from this P uptake, intercropping caused the barley root system to grow deeper and faster lateral root development of both species was observed. Barley accumulated similar amounts of aboveground N when grown as inter- and sole crop, whereas the total aboveground N acquired by pea in the intercrop was only 16% of that acquired in the pea sole crop. The percentage of total aboveground N derived from N2 fixation in sole cropped pea increased from 40% to 80% during the growth period, whereas it was almost constant at 85% in intercropped pea. The total amounts of N2 fixed were 95 and 15 kg N ha−1 in sole cropped and intercropped pea, respectively. Barley was the dominant component of the pea-barley intercrop, obtaining 90% of its sole crop yield, while pea produced only 15% of the grains of a sole crop pea. Intercropping of pea and barley improved the utilization of plant growth resources (LER > 1) as compared to sole crops. Root system distribution in time and space can partly explain interspecific competition. The 32P methodology proved to be a valuable tool for determining root dynamics in intercropping systems.

Abstract: We investigated some mechanisms, which allow maize genotypes to adapt to soils which are low in available P. Dry matter production, root/shoot-ratio, root length and root exudation of organic acids and acid phosphatase were investigated in four maize genotypes grown under P-deficient and P-sufficient conditions in sterile hydroponic culture. A low-P tolerant, an acid-tolerant and a low-P susceptible genotype of maize were compared with a Swiss commercial cultivar. The study found increased root development and increased exudation of acid phosphatase under P-deficient conditions in all maize genotypes, except for the Swiss cultivar. Effects on root formation and acid phosphatase were greater for the low-P tolerant than for the low-P susceptible, and the acid soil tolerant genotypes. Organic acid contents in root tissues were increased under P deficiency and related to increased PEPC activity. However, the increase in contents was associated with an increase in exudation for the low-P tolerant genotype only. The low-P susceptible genotype was characterized by high organic acid content in roots and low organic acid exudation. The organic acids content in the phloem exudates of shoots was related to root exudation under different P supply, to the difference between lines in organic acids root content, but not to the low-P tolerance or susceptibility of maize genotypes.

Abstract: The water relations of arbuscular mycorrhizal plants have been compared often, but virtually nothing is known about the comparative water relations of mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis typically affects soil structure, and soil structure affects water retention properties; therefore, it seems likely that mycorrhizal symbiosis may affect soil water relations. We examined the water retention properties of a Sequatchie fine sandy loam subjected to three treatments: seven months of root growth by (1) nonmycorrhizal Vigna unguiculata given low phosphorus fertilization, (2) nonmycorrhizal Vigna unguiculata given high phosphorus fertilization, (3) Vigna unguiculata colonized by Glomus intraradices and given low phosphorus fertilization. Mycorrhization of soil had a slight but significant effect on the soil moisture characteristic curve. Once soil matric potential (Ψm) began to decline, changes in Ψm per unit change in soil water content were smaller in mycorrhizal than in the two nonmycorrhizal soils. Within the range of about −1 to −5 MPa, the mycorrhizal soil had to dry more than the nonmycorrhizal soils to reach the same Ψm. Soil characteristic curves of nonmycorrhizal soils were similar, whether they contained roots of plants fed high or low phosphorus. The mycorrhizal soil had significantly more water stable aggregates and substantially higher extraradical hyphal densities than the nonmycorrhizal soils. Importantly, we were able to factor out the possibly confounding influence of differential root growth among mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis affected the soil moisture characteristic and soil structure, even though root mass, root length, root surface area and root volume densities were similar in mycorrhizal and nonmycorrhizal soils.

Abstract: This study reports effects on soil solution chemistry and plant uptake of 55 elements (Ag, Al, As, B, Ba, Be, Bi, Br, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Gd, Ge, Hf, Hg, Ho, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, Nd, Ni, P, Pb, Pr, Rb, S, Sb, Sc, Se, Si, Sm, Sr, Tb, Th, Tl, U, V, W, Y, Yb, Zn, Zr) by raising the pH using addition of fine-grained precipitated calcium carbonate at 20 rates (yielding a soil solution pH range of 5.2 - 7.8) to A horizon samples of an acid Cambisol, cultivating a common grass (Agrostis capillaris L.) and determining the soil solution, root and shoot concentrations of these elements at the end of the experiment. For many of these elements, there is little or no previous information about concentrations in soil solutions, or in plant biomass, as related to soil pH/acidity or addition of calcium carbonate. Soil solutions were obtained by high speed centrifugation and ultrafiltration (0.2 mum) of samples at 60% water-holding capacity. Concentrations of elements were determined by ICP-ES or (in most elements) ICP-MS, using isotopes specified. Soil solution pH, HCO3 and organic C were also determined. Concentrations of elements in the biomass of A. capillaris were usually inversely related to soil solution pH. The most apparent (p <0.001) inverse, though often curvilinear, relationships between pH and concentrations in shoot biomass were measured for Ag, As, B, Ba, Eu, Ge, Li, Mn, Ni, P and Sr. Positive relationships (p <0.05) were only measured in Ca, Hg, Mg, Mo and S. For concentrations in root biomass, relationships were mostly, but not always, of the same sign and of a similar strength. Though soil solution pH and concentrations of elements were usually quite closely correlated, pH and/or HCO3- concentration more often accounted for a higher share of the variability in biomass concentration of elements than did soil solution concentration of the same element. (Less)

Abstract: The 6 billion people alive today consume about 25 million tonnes of protein nitrogen each year, a requirement that could well increase to 40–45 million tonnes by 2050. Most of them ultimately depend on the Haber-Bosch process to fix the atmospheric N2 needed to grow at least part of their protein and, over the earth as a whole, this dependency is likely to increase. Humans now fix some 160 million tonnes of nitrogen per year, of which 98 are fixed industrially by the Haber-Bosch process (83 for use as agricultural fertilizer, 15 for industry), 22 during combustion and the rest is fixed during the cultivation of leguminous crops and fodders. These 160 million tonnes have markedly increased the burden of combined nitrogen entering rivers, lakes and shallow seas, as well as increasing the input of NH3, N2O, NO and NO2 to the atmosphere. Nitrogen fertilizers give large economic gains in modern farming systems and under favourable conditions can be used very efficiently. Losses of nitrogen occur from all systems of agriculture, with organic manures being particularly difficult to use efficiently. Although nitrate leaching has received much attention as an economic loss, a cause of eutrophication and a health hazard, gaseous emissions may eventually prove to be the most serious environmentally. Scientists working on the use and fate of nitrogen fertilizers must be careful, clear headed and vigilant in looking for unexpected side effects.

Abstract: We quantified rates of soil respiration among sites within an agricultural landscape in central Iowa, USA. The study was conducted in riparian cool-season grass buffers, in re-established multispecies (switchgrass + poplar) riparian buffers and in adjacent crop (maize and soybean) fields. The objectives were to determine the variability in soil respiration among buffer types and crop fields within a riparian landscape, and to identify those factors correlating with the observed differences. Soil respiration was measured approximately monthly over a two-year period using the soda-lime technique. Mean daily soil respiration across all treatments ranged from 0.14 to 8.3 g C m−2 d−1. There were no significant differences between cool-season grass buffers and re-established forest buffers, but respiration rates beneath switchgrass were significantly lower than those beneath cool-season grass. Soil respiration was significantly greater in both buffer systems than in the cropped fields. Seasonal changes in soil respiration were strongly related to temperature changes. Over all sites, soil temperature and soil moisture together accounted for 69% of the seasonal variability in soil respiration. Annual soil respiration rates correlated strongly with soil organic carbon (R = 0.75, P < 0.001) and fine root (<2 mm) biomass (R = 0.85, P < 0.001). Annual soil respiration rates averaged 1140 g C m−2 for poplar, 1185 g C m−2 for cool-season grass, 1020 g C m−2 for switchgrass, 750 g C m−2 for soybean and 740 g C m−2 for corn. Overall, vegetated buffers had significantly higher soil respiration rates than did adjacent crop fields, indicating greater soil biological activity within the buffers.

Abstract: This paper reports a new barley mutant missing root hairs. The mutant was spontaneously discovered among the population of wild type (Pallas, a spring barley cultivar), producing normal, 0.8 mm long root hairs. We have called the mutant bald root barley (brb). Root anatomical studies confirmed the lack of root hairs on mutant roots. Amplified Fragment Length Polymorphism (AFLP) analyses of the genomes of the mutant and Pallas supported that the brb mutant has its genetic background in Pallas. The segregation ratio of selfed F2 plants, resulting from mutant and Pallas outcross, was 1:3 (−root hairs:+root hairs), suggesting a monogenic recessive mode of inheritance.

Abstract: On-farm and experimental measures of the proportion (%Ndfa) and amounts of N2 fixed were undertaken for 158 pastures either based on annual legume species (annual medics, clovers or vetch), or lucerne (alfalfa), and 170 winter pulse crops (chickpea, faba bean, field pea, lentil, lupin) over a 1200 km north-south transect of eastern Australia The average annual amounts of N2 fixed ranged from 30 to 160 kg shoot N fixed ha−1 yr−1 for annual pasture species, 37–128 kg N ha−1 yr−1 for lucerne, and 14 to 160 kg N ha−1 yr−1 by pulses These data have provided new insights into differences in factors controlling N2 fixation in the main agricultural systems Mean levels of %Ndfa were uniformly high (65–94%) for legumes growing at different locations under dryland (rainfed) conditions in the winter-dominant rainfall areas of the cereal-livestock belt of Victoria and southern New South Wales, and under irrigation in the main cotton-growing areas of northern New South Wales Consequently N2 fixation was primarily regulated by biomass production in these areas and both pasture and crop legumes fixed between 20 and 25 kg shoot N for every tonne of shoot dry matter (DM) produced Nitrogen fixation by legumes in the dryland systems of the summer-dominant rainfall regions of central and northern New South Wales on the other hand was greatly influenced by large variations in %Ndfa (0–81%) caused by yearly fluctuations in growing season (April–October) rainfall and common farmer practice which resulted in a build up of soil mineral-N prior to sowing The net result was a lower average reliance of legumes upon N2 fixation for growth (19–74%) and more variable relationships between N2 fixation and DM accumulation (9–16 kg shoot N fixed/t legume DM) Although pulses often fixed more N than pastures, legume-dominant pastures provided greater net inputs of fixed N, since a much larger fraction of the total plant N was removed when pulses were harvested for grain than was estimated to be removed or lost from grazed pastures Conclusions about the relative size of the contributions of fixed N to the N-economies of the different farming systems depended upon the inclusion or omission of an estimate of fixed N associated with the nodulated roots The net amounts of fixed N remaining after each year of either legume-based pasture or pulse crop were calculated to be sufficient to balance the N removed by at least one subsequent non-legume crop only when below-ground N components were included This has important implications for the interpretation of the results of previous N2 fixation studies undertaken in Australia and elsewhere in the world, which have either ignored or underestimated the N present in the nodulated root when evaluating the contributions of fixed N to rotations

Abstract: A pot experiment was carried out in a growth chamber to investigate P efficiencies and mycorrhizal responsiveness of modern (Krichauff and Excalibur) and old (Khapstein, Bobin, Comeback and Purple Straw) wheat cultivars (Triticum aestivum). The arbuscular mycorrhizal fungus (AMF) used in this study was Glomus intraradices. The growth medium was a soil/sand mixture with NaHCO3-extractable P of 9.4 mg P kg−1 and no extra P was added. Plant P efficiencies (uptake, utilisation and agronomic) were found to differ significantly between cultivars, but no general trends of changes with the year of release of the cultivar were found. AMF colonisation was found to decrease plant growth under our experimental conditions with low light intensity. Mycorrhizal responsiveness (MR) was measured in terms of the improvement in plant P nutrition (shoot P concentrations). MR was found to be generally lower in modern cultivars than in old cultivars, indicating that modern breeding programs may have reduced the responsiveness of modern wheat cultivars to arbuscular mycorrhizal fungi. MR was also found to decrease in general with increased plant P utilisation efficiency.

Abstract: 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 PH2O 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 (PH2O 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.

Abstract: The effect of water stress on proline accumulation was tested in two contrasted species of Mediterranean scrub: Halimium halimifolium (L.) Willk and Pistacia lentiscus L. Leaf water potential, stomatal resistance and proline content have been measured both in experimental and in natural water stress conditions. Both species accumulated proline in their leaves when leaf water potential dropped below a threshold value of −3.0 MPa, under natural as well as under experimental conditions. In the field, however, a time-lag between decrease of leaf water potential and proline accumulation could be observed. In Halimium halimifolium, proline accumulation appeared to be associated with severe stress conditions as most plants with high proline contents suffered irreversible wilting, especially in the greenhouse. P. lentiscus showed a different pattern, accumulating proline at two different times of the year, as a response to cold or to drought. The results of our study indicated that the role of proline in this species, rather than an osmotic agent, seems to be more related to a protective action in cases of severe stress conditions.

Abstract: Four quantitative trait loci (QTLs) for P uptake were previously identified in a rice population that had been developed from a cross between the indica landrace Kasalath (high P uptake) with the japonica cultivar Nipponbare (low P uptake). For further studies, near isogenic lines (NILs) were developed for a major QTL linked to marker C443 on chromosome 12 and for a minor QTL linked to C498 on chromosome 6. On a highly P-deficient upland soil (aerobic conditions), NIL-C443 had three to four times the P uptake of Nipponbare, whereas the advantage of NIL-C498 was in the range of 60-90%. The superiority of NILs over Nipponbare vanished when grown in the same soil under anaerobic paddy conditions. All genotypes had high P uptake when P was supplied at a rate of 60 kg P ha -1 , regardless of soil conditions. These results confirmed the presence of both QTLs and furthermore implied that QTLs affected absorption mechanisms that specifically increased P uptake in a P deficient upland soil. Additional experiments were conducted to investigate if the effect of QTLs is linked to an increase in root growth or due to more efficient P uptake per unit root size (higher root efficiency). Root size did not differ significantly between genotypes in the plus-P treatment. P deficiency, however, reduced the root surface area of Nipponbare by more than 80% whereas NIL-C443 maintained almost half of its non-stress root surface area. The low root growth of Nipponbare observed under P deficiency was probably the result of insufficient P uptake to sustain plant growth, including root growth. Genotypic differences in the ability to maintain root growth, therefore are likely caused by some mechanism that increases the efficiency of roots to access P forms not readily available. This however, only had an effect in aerobic soil. Potential mechanisms leading to higher P uptake of NILs are discussed.

Abstract: Separating the effects of soil temperature and moisture on soil CO2 efflux is critical to modeling and understanding the belowground carbon dynamics of forest ecosystems. We developed two analytical procedures to separate the effects of soil temperature and moisture, based on continuous measurements of the CO2 efflux, temperature and moisture of the soil at a ponderosa pine plantation in the Sierra Nevada Mountains in California, from May 1998 to August 1999. We found that the combined effects of temperature and moisture on the seasonal variation of soil CO2 efflux could be effectively separated and represented with the product of a temperature term and a moisture term. The relationship between soil CO2 efflux and temperature could be well described using a power function. This relationship was modified by soil moisture which affects only the coefficient, but not the exponent, of the power function. We also found that when soil moisture was held constant, the temperature effect explained 82% of the temporal variation in CO2 efflux of the soil. Similarly, when temperature was held constant, the moisture effects explained 84% of the variation. Temperature and moisture together explained 89% of the total temporal variations in soil CO2 efflux. A multiplicative formulation with power functions representing both temperature and moisture dependences was recommended for modeling soil CO2 efflux. This formulation can be used to model the seasonal trend of soil CO2 efflux of the forest based on temperature and moisture, two key variables influenced by climate change and management practices.

Abstract: Drought survival in perennial forage plants involves different adaptative responses such as delay of dehydration through water uptake, limitation of water loss and tolerance of tissues to dessication To compare the importance of these responses in contrasting cultivars of forage grasses at the whole plant level, we carried out two experiments under glasshouse conditions Plants of cocksfoot (Dactylis glomerata L) cultivars, cvs Currie, Medly (both of Mediterranean origin) and Lutetia (of continental origin), and of tall fescue (Festuca arundinacea L) cv Centurion (Mediterranean) were grown in 60 cm-deep cylinders to eliminate the effect of differences of root depth on water availability whilst allowing severe drought to be imposed at a realistic rate In both experiments, the cvs were ranked similarly for plant survival, with high mortality for Centurion, low for the Mediterranean cocksfoots Currie and Medly, and intermediate for Lutetia These differences could not be ascribed to water use during most of the drought period since water uptake and decrease in leaf extension were not significantly different between species and cultivars However, resistant cvs of cocksfoot were able to extract water for a longer period and at a lower soil water potential (Ψs) than other cvs The critical Ψs at plant death was −38 and −36 MPa for Medly and Currie and −30-,−26 MPa for Lutetia and Centurion Moreover, at a low soil water reserve (15–2%), membrane stability and water content were maintained for longer in enclosed immature leaf bases of cocksfoots cultivars, whereas the fescue Centurion exhibited accelerated lamina senescence and steady increase of membrane damage in surviving tissues Therefore, it is proposed that the drought resistance of tall fescue in the field can mainly be ascribed to its ability to develop a deep root system In cocksfoot, dehydration tolerance in surviving tissues and the ability of roots to extract water at low soil water potentials may, in addition to root depth, contribute significantly to plant survival under severe drought

Abstract: Effects of rate of nitrogen (N) fertilizer and stocking rate on production and N2 fixation by white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L.) were determined over 5 years in farmlets near Hamilton, New Zealand. Three farmlets carried 3.3 dairy cows ha−1 and received urea at 0, 200 or 400 kg N ha−1 yr−1 in 8–10 split applications. A fourth farmlet received 400 kg N ha−1 yr−1 and had 4.4 cows ha−1. There was large variation in annual clover production and total N2 fixation, which in the 0 N treatment ranged from 9 to 20% clover content in pasture and from 79 to 212 kg N fixed ha−1 yr−1. Despite this variation, total pasture production in the 0 N treatment remained at 75–85% of that in the 400 N treatments in all years, due in part to the moderating effect of carry-over of fixed N between years. Fertilizer N application decreased the average proportion of clover N derived from N2 fixation (PN; estimated by 15N dilution) from 77% in the 0 N treatment to 43–48% in the 400 N treatments. The corresponding average total N2 fixation decreased from 154 kg N ha−1 yr−1 to 39–53 kg N ha−1 yr−1. This includes N2 fixation in clover tissue below grazing height estimated at 70% of N2 fixation in above grazing height tissue, based on associated measurements, and confirmed by field N balance calculations. Effects of N fertilizer on clover growth and N2 fixation were greatest in spring and summer. In autumn, the 200 N treatment grew more clover than the 0 N treatment and N2 fixation was the same. This was attributed to more severe grazing during summer in the 0 N treatment, resulting in higher surface soil temperatures and a deleterious effect on clover stolons. In the 400 N treatments, a 33% increase in cow stocking rate tended to decrease PN from 48 to 43% due to more N cycling in excreta, but resulted in up to 2-fold more clover dry matter and N2 fixation because lower pasture mass reduced grass competition, particularly during spring.