Juvenile tree growth correlates with photosynthesis and leaf phosphorus content in central Amazonia

Light and soil water availability may limit carbon uptake of trees in tropical rainforests. The objective of this work was to determine how photosynthetic traits of juvenile trees respond to variations in rainfall seasonality, leaf nutrient content, and opening of the forest canopy. The correlation between leaf nutrient content and annual growth rate of saplings was also assessed. In a terra firme rainforest of the central Amazon, leaf nutrient content and gas exchange parameters were measured in five sapling tree species in the dry and rainy season of 2008. Sapling growth was measured in 2008 and 2009. Rainfall seasonality led to variations in soil water content, but it did not affect leaf gas exchange parameters. Subtle changes in the canopy opening affected CO2 saturated photosynthesis (A pot, p = 0.04). Although A pot was affected by leaf nutrient content (as follows: P > Mg > Ca > N > K), the relative growth rate of saplings correlated solely with leaf P content (r = 0.52, p = 0.003). At present, reduction in soil water content during the dry season does not seem to be strong enough to cause any effect on photosynthesis of saplings in central Amazonia. This study shows that leaf P content is positively correlated with sapling growth in the central Amazon. Therefore, the positive effect of atmospheric CO2 fertilization on long-term tree growth will depend on the ability of trees to absorb additional amount of P

Gradual correction of phosphorus availability in the no-tillage system

ABSTRACT In areas cultivated under no-tillage system, the availability of phosphorus (P) can be raised by means of the gradual corrective fertilization, applying phosphorus into sowing furrows at doses higher than those required by the crops. The objective of this work was to establish the amount of P to be applied in soybean crop to increase content of P to pre-established values at the depth of 0.0 to 0.10 m. An experiment was carried out on a clayey Haplorthox soil with a randomized block experimental design distributed in split-split plot, with four replications. Two soybean crop systems (single or intercropped with Panicum maximum Jaca cv. Aruana) were evaluated in the plots. In addition, it was evaluated four P levels (0, 60, 120 and 180 kg ha-1 P2O5) applied in the first year in the split plots; and four P levels (0, 30, 60 and 90 kg ha-1 P2O5) applied in the two subsequent crops in the split-split plot. Contents of P were extracted by Mehlich-1 and Anion Exchange Resin methods from soil samples collected in the split-split plot. It was found that it is necessary to apply 19.4 or 11.1 kg ha-1 of P2O5, via triple superphosphate as source, to increase 1 mg dm-3 of P extracted by Mehlich-1 or Resin, respectively, in the 0.0 to 0.10 m layer of depth. The soil drain P character decreases as the amount of this nutrient supplied in the previous crops is increased.

Soil physical and Chemical properties of cultivated pasture on forest land, Roraima, Brazil .

Soil conditions under pasture were examined in a range of sites representing the sequence of conversion of forest to pasture at two locations in the vicinity of Ilha de Maracã, Roraima. Comparisons were made with adjacent savana. Soil bulk densities shown to increase after forest clearance and soil chemical data indicate that the initial beneficial effects on nutrient supply of burning forest debris are rather short-lived. Very low levels of available phosphorus prevail in areas of savanna and cultivated pasture of all ages. Variations in the status of older cultivated pastures are mainly attributable to different grazing levelt.

Activity of phosphorus of synthetic Fe-K-P compounds in superphosphate fertilizers as affected by pH and ionic strength

The concentration of orthophosphate ions released from Fe-K-P compounds (Fe3KH8(PO4)6 .6H2O and Fe3KH14(PO4)8 .4H2O) present in superphosphates increases with pH, which initially suggests that the agronomic effectiveness of P fertilizers containing high amounts of these compounds would also increase with soil pH but studies considering activity, instead of concentration, are necessary. With this purpose, both compounds were synthesized under laboratory conditions, characterized by elemental chemical analysis, optical microscopy, X ray diffractometry, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), and used in a solubility study. Solutions of 0.01, 0.05 and 0.1 mol L-1 NaCl with pH adjusted to 3.0, 4.0, 5.0, 5.5, 6.0, 6.5, 7.0 and 7.5 were prepared for the solubility study of H8-syn, H14-syn and a phosphate rock (PR) from Brazil. The orthophosphate activity as H2PO4- and HPO4(2-) was calculated in each situation as related to pH and ionic strength using software MINTEQ. The remaining precipitates after equilibrium were chemically analyzed and subjected to X ray, SEM and EDS. Results of chemical analysis and instrumental techniques confirmed the preparation method. The activity of orthophosphate ions of both compounds tended to decrease under increasing pH and/or ionic strength of the solution, which in turn suggests that an increase in the solution pH does not necessarily promote an increase in the P bioavailability for plant uptake. This can be important when evaluating agronomic data of P fertilizers with high contents of these two Fe-K-P compounds.

Biomass production and phosphorus use of forage grasses fertilized with two phosphorus sources

A major constraint to agricultural production in acid soils of tropical regions is the low soil P availability, due to the high adsorption capacity, low P level in the source material and low efficiency of P uptake and use by most of the modern varieties grown commercially. This study was carried out to evaluate the biomass production and P use by forage grasses on two soils fertilized with two P sources of different solubility. Two experiments were carried out, one for each soil (Cambisol and Latosol), using pots filled with 4 dm³ soil in a completely randomized design and a 4 x 2 factorial scheme. The treatments consisted of a combination of four forage plants (Brachiaria decumbens, Brachiaria brizantha, Pennisetum glaucum and Sorghum bicolor) with two P sources (Triple Superphosphate - TSP and Arad Reactive Phosphate - ARP), with four replications. The forage grasses were harvested at pre-flowering, when dry matter weight and P concentrations were measured. Based on the P concentration and dry matter production, the total P accumulation was calculated. With these data, the following indices were calculated: the P uptake efficiency of roots, P use efficiency, use efficiency of available P, use efficiency of applied P and agronomic efficiency. The use of the source with higher solubility (TSP) resulted, generally, in higher total dry matter and total P accumulation in the forage grasses, in both soils. For the less reactive source (ARP), the means found in the forage grasses, for use efficiency and efficient use of available P, were always higher when grown in Latosol, indicating favorable conditions for the solubility of ARP. The total dry matter of Brachiaria brizantha was generally higher, with low P uptake, accumulation and translocation, which indicated good P use efficiency for both P sources and soils. The forage plants differed in the P use potential, due to the sources of the applied P and of the soils used. Less than 10 % of the applied P was immobilized in the forage dry matter. Highest values were observed for TSP, but this was not reflected in a higher use efficiency of P from this source.

Phosphorus and root distribution and corn growth as related to long-term tillage systems and fertilizer placement

Soil and fertilizer management during cultivation can affect crop productivity and profitability. Long-term experiments are therefore necessary to determine the dynamics of nutrient and root distribution as related to soil profile, as well as the effects on nutrient uptake and crop growth. An 18-year experiment was conducted at the Federal University of Rio Grande do Sul State (UFRGS), in Eldorado do Sul, Brazil, on Rhodic Paleudult soil. Black oat and vetch were planted in the winter and corn in the summer. The soil management methods were conventional, involving no-tillage and strip tillage techniques and broadcast, row-and strip-applied fertilizer placement (triple superphosphate). Available P (Mehlich-1) and root distribution were determined in soil monoliths during the corn grain filling period. Corn shoot dry matter production and P accumulation during the 2006/2007 growing season were determined and the efficiency of P utilization calculated. Regardless of the degree of soil mobilization, P and roots were accumulated in the fertilized zone with time, mainly in the surface layer (0-10 cm). Root distribution followed P distribution for all tillage systems and fertilizer treatments. Under no-tillage, independent of the fertilizer placement, the corn plants developed more roots than in the other tillage systems. Although soil tillage systems and fertilizer treatments affected P and root distribution throughout the soil profile, as well as P absorption and corn growth, the efficiency of P utilization was not affected.

Leaf area of common bean genotypes during early pod filling as related to plant adaptation to limited phosphorus supply

Low phosphorus supply markedly limits leaf growth and genotypes able to maintain adequate leaf area at low P could adapt better to limited-P conditions. This work aimed to investigate the relationship between leaf area production of common bean (Phaseolus vulgaris) genotypes during early pod filling and plant adaptation to limited P supply. Twenty-four genotypes, comprised of the four growth habits in the species and two weedy accessions, were grown at two P level applied to the soil (20 and 80 mg kg-1) in 4 kg pots and harvested at two growth stages (pod setting and early pod filling). High P level markedly increased the leaf number and leaf size (leaf area per leaf), slightly increased specific leaf area but did not affect the net assimilation rate. At low P level most genotypic variation for plant dry mass was associated with leaf size, whereas at high P level this variation was associated primarily with the number of leaves and secondarily with leaf size, specific leaf area playing a minor role at both P level. Determinate bush genotypes presented a smaller leaf area, fewer but larger leaves with higher specific leaf area and lower net assimilation rate. Climbing genotypes showed numerous leaves, smaller and thicker leaves with a higher net assimilation rate. Indeterminate bush and indeterminate prostrate genotypes presented the highest leaf area, achieved through intermediate leaf number, leaf size and specific leaf area. The latter groups were better adapted to limited P. It is concluded that improved growth at low P during early pod filling was associated with common bean genotypes able to maintain leaf expansion through leaves with greater individual leaf area.

Sewage sludge as nitrogen and phosphorus source for cane-plant and first ratoon crops

The use of sewage sludge in Brazilian agriculture was regulated by the resolution no. 375 Conama, in 2006. However, there is a lack of research to adequate the mineral N and P fertilizer doses to be applied in agricultural fields treated with this residue. In a field experiment, the effects of application rates of sewage sludge and mineral N and P fertilizers on the productivity and technical characteristics of the cane-plant and first ratoon (residual effect) crops were evaluated. Four doses of sewage sludge (0, 3.6, 7.2 and 10.8 t ha-1, dry base), of N (0, 30, 60 and 90 kg ha-1) and of P2O5 (0, 60, 120 and 180 kg ha-1) were combined in a factorial and laid out on randomized block design, a with two replications. To evaluate the residual effect of the sludge, 120 kg ha-1 N and 140 kg ha-1 of K2O were applied in all plots. Sludge application at cane planting, with or without N and/or P fertilizer increased the stalk yield from 84 up to 118 t ha-1, with no alteration in the sugarcane quality, compared with the application of NPK fertilizer alone, resulting in a stalk yield of 91 t ha-1. The study of the response surface for stalk yield on lowfertility soil was the basis for a recommendation of mineral N and P fertilizer doses for sugarcane implantation as related to sewage sludge application rates. It was also concluded that a sludge application of 10.8 t ha-1, which is the sludge dose established based on the N criterion according to the resolution Conama nº 375, could a) reduce the use of mineral N by 100 % and of P2O5 by 30 %, with increments of 22 % in stalk yield, as a direct effect of sludge application to cane plant crop, and b) increase the stalk yield in the second harvest (first ratoon) by up to 12 % and sugar yield by up to 11 %, by the residual effect of sludge application to sugar cane.

Seeds enriched with phosphorus and molybdenum improve the contribution of biological nitrogen fixation to common bean as estimated by 15n isotope dilution

Seeds with a high concentration of P or Mo can improve the growth and N accumulation of the common bean (Phaseolus vulgaris L.), but the effect of enriched seeds on biological N2 fixation has not been established yet. This study aimed to evaluate the effect of seeds enriched with P and Mo on growth and biological N2 fixation of the common bean by the 15N isotope dilution technique. An experiment was carried out in pots in a 2 x 3 x 2 x 2 factorial design in randomized blocks with four replications, comprising two levels of soil applied P (0 and 80 mg kg-1), three N sources (without N, inoculated with rhizobia, and mineral N), two seed P concentrations (low and high), and two seed Mo concentrations (low and high). Non-nodulating bean and sorghum were used as non-fixing crops. The substrate was 5.0 kg of a Red Latosol (Oxisol) previously enriched with 15N and mixed with 5.0 kg of sand. Plants were harvested 41 days after emergence. Seeds with high P concentration increased the growth and N in shoots, particularly in inoculated plants at lower applied P levels. Inoculated plants raised from high P seeds showed improved nodulation at both soil P levels. Higher soil P levels increased the percentage of N derived from the atmosphere (%Ndfa) in bean leaves. Inoculation with the selected strains increased the %Ndfa. High seed P increased the %Ndfa in inoculated plants at lower soil P levels. High seed Mo increased the %Ndfa at lower soil P levels in plants that did not receive inoculation or mineral N. It is concluded that high seed P concentration increases the growth, N accumulation and the contribution of the biological N2 fixation in the common bean, particularly in inoculated plants grown at lower soil P availability.

Mobility of inorganic and organic phosphorus forms under different levels of phosphate and poultry litter fertilization in soils

The eutrophication of aquifers is strongly linked to the mobility of P in soils. Although P mobility was considered irrelevant in a more distant past, more recent studies have shown that P, both in organic (Po) and inorganic forms (Pi), can be lost by leaching and eluviation through the soil profile, particularly in less weathered and/or sandier soils with low P adsorption capacity. The purpose of this study was to determine losses of P forms by leaching and eluviation from soil columns. Each column consisted of five PVC rings (diameter 5 cm, height 10 cm), filled with two soil types: a clayey Red-Yellow Latosol and a sandy loam Red-Yellow Latosol, which were exposed to water percolation. The soils were previously treated with four P rates (as KH2PO4 ) to reach 0, 12.5, 25.0 and 50 % of the maximum P adsorption capacity (MPAC). The P source was homogenized with the whole soil volume and incubated for 60 days. After this period the soils were placed in the columns; the soil of the top ring was mixed with five poultry litter rates of 0, 20, 40, 80, and 160 t ha-1 (dry weight basis). Treatments consisted of a 4 x 5 x 2 factorial scheme corresponding to four MPAC levels, five poultry litter rates, two soils, with three replications, arranged in a completely randomized block design. Deionized water was percolated through the columns 10 times in 35 days to simulate about 1,200 mm rainfall. In the leachate of each column the inorganic P (reactive P, Pi) and organic P forms (unreactive P, Po) were determined. At the end of the experiment, the columns were disassembled and P was extracted with the extractants Mehlich-1 (HCl 0.05 mol L-1 and H2SO4 0.0125 mol L-1) and Olsen (NaHCO3 0.5 mol L-1; pH 8.5) from the soil of each ring. The Pi and Po fractions were measured by the Olsen extractant. It was found that under higher poultry litter rates the losses of unreactive P (Po) were 6.4 times higher than of reactive P (Pi). Both the previous P fertilization and increasing poultry litter rates caused a vertical movement of P down the soil columns, as verified by P concentrations extracted by Mehlich-1 and NaHCO3 (Olsen). The environmental critical level (ECL), i.e., the P soil concentration above which P leaching increases exponentially, was 100 and 150 mg dm-3 by Mehlich-1 and 40 and 60 mg dm-3 by Olsen, for the sandy loam and clay soils, respectively. In highly weathered soils, where residual P is accumulated by successive crops, P leaching through the profile can be significant, particularly when poultry litter is applied as fertilizer.

Rhizosphere properties of maize genotypes with contrasting phosphorus efficiency

An experiment was conducted in a growth chamber to evaluate characteristics of the rhizosphere of maize genotypes contrasting in P-use efficiency, by determining length and density of root hairs, the rhizosphere pH and the functional diversity of rhizosphere bacteria. A sample of a Red Oxisol was limed and fertilized with N, K and micronutrients. In the treatment with the highest P level, 174 mg kg-1 P was added. Each experimental unit corresponded to a PVC rhizobox filled with 2.2 dm-3 soil. The experiment was completely randomized with three replications in a 5 x 2 factorial design, corresponding to five genotypes (H1, H2 and H3 = P-efficient hybrids, H4 and H5 = P-inefficient hybrids) and two P levels (low = 3 mg dm-3, high = 29 mg dm-3). It was found that 18 days after transplanting, the nodal roots of the hybrids H3 and H2 had the longest root hairs. In general, the pH in the rhizosphere of the different genotypes was higher than in non-rhizosphere soil, irrespective of the P level. The pH was higher in the rhizosphere of lateral than of nodal roots. At low P levels, the pH variation of the hybrids H2, H4 and H5 was greater in rhizospheric than in non-rhizospheric soil. The functional microbial activity in the rhizosphere of the hybrids H3 and H5 was highest. At low soil P levels, the indices of microbial functional diversity were also higher. The microbial metabolic profile in the rhizosphere of hybrids H1, H2, H3, and H5 remained unaltered when the plants were grown at low P. The variations in the rhizosphere properties could not be related to patterns of P-use efficiency in the tested genotypes.

Remaining phosphorus estimated by pedotransfer function

Although the determination of remaining phosphorus (Prem) is simple, accurate values could also be estimated with a pedotransfer function (PTF) aiming at the additional use of soil analysis data and/or Prem replacement by an even simpler determination. The purpose of this paper was to develop a pedotransfer function to estimate Prem values of soils of the State of São Paulo based on properties with easier or routine laboratory determination. A pedotransfer function was developed by artificial neural networks (ANN) from a database of Prem values, pH values measured in 1 mol L-1 NaF solution (pH NaF) and soil chemical and physical properties of samples collected during soil classification activities carried out in the State of São Paulo by the Agronomic Institute of Campinas (IAC). Furthermore, a pedotransfer function was developed by regressing Prem values against the same predictor variables of the ANN-based PTF. Results showed that Prem values can be calculated more accurately with the ANN-based pedotransfer function with the input variables pH NaF values along with the sum of exchangeable bases (SB) and the exchangeable aluminum (Al3+) soil content. In addition, the accuracy of the Prem estimates by ANN-based PTF were more sensitive to increases in the experimental database size. Although the database used in this study was not comprehensive enough for the establishment of a definitive pedotrasnfer function for Prem estimation, results indicated the inclusion of Prem and pH NaF measurements among the soil testing evaluations as promising ind order to provide a greater database for the development of an ANN-based pedotransfer function for accurate Prem estimates from pH NaF, SB, and Al3+ values.

Phosphatase activity in sandy soil influenced by mycorrhizal and non-mycorrhizal cover crops

Cover crops may difffer in the way they affect rhizosphere microbiota nutrient dynamics. The purpose of this study was to evaluate the effect of mycorrhizal and non-mycorrhizal cover crops on soil phosphatase activity and its persistence in subsequent crops. A three-year experiment was carried out with a Typic Quartzipsamment. Treatments were winter species, either mycorrhizal black oat (Avena strigosa Schreb) or the non-mycorrhizal species oilseed radish (Raphanus sativus L. var. oleiferus Metzg) and corn spurry (Spergula arvensis L.). The control treatment consisted of resident vegetation (fallow in the winter season). In the summer, a mixture of pearl millet (Pennisetum americanum L.) with sunnhemp (Crotalaria juncea L.) or with soybean (Glycine max L.) was sown in all plots. Soil cores (0-10 cm) and root samples were collected in six growing seasons (winter and summer of each year). Microbial biomass P was determined by the fumigation-extraction method and phosphatase activity using p-nitrophenyl-phosphate as enzyme substrate. During the flowering stage of the winter cover crops, acid phosphatase activity was 30-35 % higher in soils with the non-mycorrhizal species oilseed radish, than in the control plots, regardless of the amount of P immobilized in microbial biomass. The values of enzyme activity were intermediate in the plots with corn spurry and black oat. Alkaline phosphatase activity was 10-fold lower and less sensitive to the treatments, despite the significant relationship between the two phosphatase activities. The effect of plant species on the soil enzyme profile continued in the subsequent periods, during the growth of mycorrhizal summer crops, after completion of the life cycle of the cover crops.

Kinetics of phosphorus sorption in soils in the state of Paraíba¹

The soil P sorption capacity has been studied for many years, but little attention has been paid to the rate of this process, which is relevant in the planning of phosphate fertilization. The purpose of this experiment was to assess kinetics of P sorption in 12 representative soil profiles of the State of Paraíba (Brazil), select the best data fitting among four equations and relate these coefficients to the soil properties. Samples of 12 soils with wide diversity of physical, chemical and mineralogical properties were agitated in a horizontal shaker, with 10 mmo L-1 CaCl2 solution containing 6 and 60 mg L-1 P, for periods of 5, 15, 30, 45, 60, 90, 120, 420, 720, 1,020, and 1,440 min. After each shaking period, the P concentration in the equilibrium solution was measured and three equations were fitted based on the Freundlich equation and one based on the Elovich equation, to determine which soil had the highest sorption rate (kinetics) and which soil properties correlated to this rate. The kinetics of P sorption in soils with high maximum P adsorption capacity (MPAC) was fast for 30 min at the lower initial P concentration (6 mg L-1). No difference was observed between soils at the higher initial P concentration (60 mg L-1). The P adsorption kinetics were positively correlated with clay content, MPAC and the amount of Al extracted with dithionite-citrate-bicarbonate. The data fitted well to Freundlich-based equations equation, whose coefficients can be used to predict P adsorption rates in soils.

Characterization and nutrient release from silicate rocks and influence on chemical changes in soil

The expansion of Brazilian agriculture has led to a heavy dependence on imported fertilizers to ensure the supply of the growing food demand. This fact has contributed to a growing interest in alternative nutrient sources, such as ground silicate rocks. It is necessary, however, to know the potential of nutrient release and changes these materials can cause in soils. The purpose of this study was to characterize six silicate rocks and evaluate their effects on the chemical properties of treated soil, assessed by chemical extractants after greenhouse incubation. The experimental design consisted of completely randomized plots, in a 3 x 6 factorial scheme, with four replications. The factors were potassium levels (0-control: without silicate rock application; 200; 400; 600 kg ha-1 of K2O), supplied as six silicate rock types (breccia, biotite schist, ultramafic rock, phlogopite schist and two types of mining waste). The chemical, physical and mineralogical properties of the alternative rock fertilizers were characterized. Treatments were applied to a dystrophic Red-Yellow Oxisol (Ferralsol), which was incubated for 100 days, at 70 % (w/w) moisture in 3.7 kg/pots. The soil was evaluated for pH; calcium and magnesium were extracted with KCl 1 mol L-1; potassium, phosphorus and sodium by Mehlich 1; nickel, copper and zinc with DTPA; and the saturation of the cation exchange capacity was calculated for aluminum, calcium, magnesium, potassium, and sodium, and overall base saturation. The alternative fertilizers affected soil chemical properties. Ultramafic rock and Chapada mining byproduct (CMB) were the silicate rocks that most influenced soil pH, while the mining byproduct (MB) led to high K levels. Zinc availability was highest in the treatments with mining byproduct and Cu in soil fertilized with Chapada and mining byproduct.