Phosphorus uptake from green manures and phosphate fertilizers applied in an acid tropical soil

Quantifying the relative contribution of different phosphorus (P) sources to P uptake can lead to greater understanding of the mechanisms that increase available P in integrated P management systems. The P-32-P-33 double isotope labeling technique was used to determine the relative contribution of green manures (GMs) and P fertilizers to P uptake by Setaria grass (Setaria sphacelata) grown in an amended tropical acid soil (Bungor series) in a glasshouse study. The amendments were factorial combinations of GMs (Calopogonium caeruleum , Gliricidia sepium and Imperata cylindrica) and P fertilizers [phosphate rocks (PRs) from North Carolina (NCPR), China (CPR) and Algeria (APR), and triple superphosphate (TSP)]. Dry matter yield, P uptake, and P utilization from the amendments were monitored at 4, 8, and 15 weeks after establishment (WAE). The GMs alone or in combination with P fertilizers contributed less than 5% to total P uptake in this soil, but total P uptake into Setaria plants in the GM treatments was three to four times that of the P fertilizers because the GMs mobilized more soil P. Also, the GMs markedly increased fertilizer P utilization in the combined treatments, from 3% to 39% with CPR, from 6-9% to 19-48% with reactive PRs, and from 6% to 37% with TSP in this soil. Both P GM and the other decomposition products were probably involved in reducing soil P-retention capacity. Mobilization of soil P was most likely the result of the action of the other decomposition products. These results demonstrate the high potential of integrating GMs and PRs for managing P in tropical soils and the importance of the soil P mobilization capacity of the organic components. Even the low-quality Imperata GM enhanced the effectiveness of the reactive APR more than fourfold.

Management of a previously eroded tropical Alfisol with herbaceous legumes: Soil loss and physical properties under mound tillage

A study was carried out on a previously eroded Oxic Paleustalf in Ibadan, southwestern Nigeria to determine the extent of soil degradation under mound tillage with some herbaceous legumes and residue management methods. A series of factorial experiments was carried out on 12 existing runoff plots. The study commenced in 1996 after a 5-year natural fallow. Mound tillage was introduced in 1997 till 1999. The legumes - Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides - were intercropped with maize in 1996 and 1998 while yam was planted alone in 1997 and 1999. This paper covers 1997-1999. At the end of each year, residues were either burned or mulched on respective plots. Soil loss, runoff, variations in mound height, bulk density, soil water retention and sorptivity were measured. Cumulative runoff was similar among interactions of legume and residue management in 1997 (57-151 mm) and 1999 (206-397 mm). However, in 1998, cumulative runoff of 95 mm observed for Mucuna-burned residue was significantly greater than the 46 mm observed for cowpea-burned residue and the 39-51 mm observed for mulched residues of cowpea, Mucuna and Pueraria. Cumulative soil loss of 7.6 Mg ha(-1) observed for Mucuna-burned residue in 1997 was significantly greater than those for Pueraria-mulched (0.9 Mg ha(-1)) and Mucuna-mulched (1.4 Mg ha(-1)) residues whereas in 1999 it was similar to soil loss from cowpea treatments and Pueraria-burned residue (2.3-5.3 Mg ha(-1)). There were no significant differences in soil loss in 1998 (1-3.2 Mg ha(-1)) whereas Mucuna-burned residue had a greater soil loss (28.6 Mg ha(-1)) than mulched cowpea (6.9 Mg ha(-1)) and Pueraria (5.4 Ms ha(-1)). Mound heights (23 cm average) decreased non-linearly with cumulative rainfall. A cumulative rainfall of 500 mm removed 0.3-2.3 cm of soil from mounds in 1997, 3.5-6.9 cm in 1998 and 2.3-4.6 cm in 1999, indicating that (detached but less transported) soil from mounds was far higher than observed soil loss in each year. Soil water retention was improved at potentials ranging from -1 to -1500 kPa by Mucuna-mulched residue compared to the various burned-residue treatments. Also, mound sorptivity at -1 cm water head (14.3 cm h(-1/2)) was higher than furrow sorptivity (8.5 cm h(-1/2)), indicating differences in hydraulic characteristics between mound and furrow. Pueraria-mulched residues for mounds had the highest sorptivity of 17.24 cm h(-1/2), whereas the least value of 6.96 cm h(-1/2) was observed in furrow of Mucuna-burned residue. Pueraria phas eoloides was considered the best option for soil conservation on the previously eroded soil, cultivated with mound tillage. (c) 2005 Elsevier B.V. All rights reserved.

Stem and crown dimensions as predictors of thinning responses in a crowded tropical rainforest plantation of Flindersia brayleyana F. Muell.

A 67-year-old plantation of Flindersia brayleyana F. Muell. in the wet tropics of north-cast Queensland had developed with minimal management. Before thinning, the stand had a canopy stem density of 770 stems ha(-1) of which 564 were F brayleyana, a stand basal area of 78 m(2) ha(-1), a mean stem diameter at breast height (dbh) of 36 cm, and a mean dbh increment of 5.2 mm year(-1) over the life of the plantation and 0.5 mm year I at the time of thinning. Sixty-three percent of the trees had crown ratios (crown diameter determined from foliage projected area: dbh) of less than 12. Thinning treatments removed 57% of the canopy stems and 45% of the stand basal area. Mean dbh increment over 2.5 years after thinning on basaltic soil was 5.8 +/- 0.3 mm year(-1), but for trees with crown ratio

Prevalence, intensity and risk factors for soil-transmitted helminth infection in a South Indian fishing village

A study of the prevalence, intensity and risk factors for soil-transmitted helminth infection was undertaken among school children aged 5-9 years attending a primary school in the fishing village in Peda Jalaripet, Visakhapatnam, South India. One hundred and eighty nine (92.6%) of 204 children were infected with one or more soil transmitted helminth parasites. The predominant parasite was Ascaris lumbricoides (prevalence of 91%), followed by Trichuris trichiura (72%) and hookworm (54%). Study of age-specific prevalence and intensity of infection revealed that the prevalence and intensity of A. lumbricoides infection was higher among younger children than older children. While aggregation of parasite infection was observed, hookworm infection was more highly aggregated than either A. lumbricoides or T. trichiura. Multivariate analysis identified parental occupation, child's age and mother's education as the potential risk factors contributing to the high intensity of A. lumbricoides infection. Children from fishing families with low levels of education of the mother had the highest intensity of A. lumbricoides infection. As the outcome of chemotherapy programs to control soil transmitted helminth infection is dependant on the dynamics of their transmission, there is a need for further studies to better define the role of specific factors that determine their prevalence, intensity and aggregation in different epidemiological settings. (C) 2004 Elsevier B.V. All rights reserved.

Chloride as a signature indicator of soil textural and hydrologic stratigraphies in variable charge deep profiles

Soil properties that influence water movement through profiles are important for determining flow paths, reactions between soil and solute, and the ultimate destination of solutes. This is particularly important in high rainfall environments. For highly weathered deep profiles, we hypothesize that abrupt changes in the distribution of the quotient [QT = (silt + sand)/clay] reflect the boundaries between textural units or textural (TS) and hydrologic (HS) stratigraphies. As a result, QT can be used as a parameter to characterize TS and as a surrogate for HS. Secondly, we propose that if chloride distributions were correlated with QT, under non-limiting anion exchange, then chloride distributions can be used as a signature indicator of TS and HS. Soil cores to a depth of 12.5 in were taken from 16 locations in the wet tropical Johnstone River catchment of northeast Queensland, Australia. The cores belong to nine variable charge soil types and were under sugarcane (Saccharun officinarum-S) production, which included the use of potassium chloride, for several decades. The cores were segmented at I m depth increments and subsamples were analysed for chloride, pH, soil water content (theta), clay, silt and sand contents. Selected bores were capped to serve as piezometers to monitor groundwater dynamics. Depth incremented QT, theta and chloride correlated, each individually, significantly with the corresponding profile depth increments, indicating the presence of textural, hydrologic and chloride gradients in profiles. However, rapid increases in QT down the profile indicated abrupt changes in TS, suggesting that QT can be used as a parameter to characterize TS and as a surrogate for HS. Abrupt changes in chloride distributions were similar to QT, suggesting that chloride distributions can be used as a signature indicator of QT (TS) and HS. Groundwater data indicated that chloride distributions depended, at least partially, on groundwater dynamics, providing further support to our hypothesis that chloride distribution can be used as a signature indicator of HS. Copyright (c) 2005 John Wiley & Sons, Ltd.

Competitive sorption reactions between phosphorus and organic matter in soil: a review

The incorporation of organic matter ( OM) in soils that are able to rapidly sorb applied phosphorus ( P) fertiliser reportedly increases P availability to plants. This effect has commonly been ascribed to competition between the decomposition products of OM and P for soil sorption sites resulting in increased soil solution P concentrations. The evidence for competitive inhibition of P sorption by dissolved organic carbon compounds, derived from the breakdown of OM, includes studies on the competition between P and (i) low molecular weight organic acids (LOAs), (ii) humic and fulvic acids, and (iii) OM leachates in soils with a high P sorption capacity. These studies, however, have often used LOAs at 1 - 100 mM, concentrations much higher than those in soils ( generally < 0.05 mM). The transience of LOAs in biologically active soils further suggests that neither their concentration nor their persistence would have a practical benefit in increasing P phytoavailability. Higher molecular weight compounds such as humic and fulvic acids also competitively inhibit P sorption; however, little consideration has been given to the potential of these compounds to increase the amount of P sorbed through metal - chelate linkages. We suggest that the magnitude of the inhibition of P sorption by the decomposition products of OM leachate is negligible at rates equivalent to those of OM applied in the field. Incubation of OM in soil has also commonly been reported as reducing P sorption in soil. However, we consider that the reported decreases in P sorption ( as measured by P in the soil solution) are not related to competition from the decomposition products of OM breakdown, but are the result of P release from the OM that was not accounted for when calculating the reduction in P sorption.

Burial depth and cultivation influence emergence and persistence of Phalaris paradoxa seed in an Australian sub-tropical environment

Emergence and persistence characteristics of Phalaris paradoxa seeds in no- and minimum-till situations and at different burial depths were studied in a sub-tropical environment. Three experiments were carried out using naturally shed seeds. In the first experiment, seedlings emerged from May through to September each year, although the majority of seedlings emerged in July. In the second experiment with greater seed density, cultivation in March of each year stimulated seedling emergence, altered the periodicity of emergence and accelerated the decline of seeds in the seedbank compared with plots that received no cultivation. The majority of seedlings in the cultivated plots emerged in May whereas the majority of seedlings in the undisturbed plots emerged in July. Emergence accounted for only 4-19% of the seedbank in both experiments over 2 years. Seed persistence was short in both field experiments, with less than 1% remaining 2 years after seed shed. In the third experiment, burial depth and soil disturbance significantly influenced seedling emergence and persistence of seed. Seedlings emerged most from seed mixed in the top 10 cm when subjected to annual soil disturbance, and from seed buried at 2.5 and 5.0 cm depths in undisturbed soil. Emergence was least from seed on the soil surface, and buried at 10 and 15 cm depths in undisturbed soil. Seeds persisted longest when shed onto the soil surface and persisted least when the soil was tilled. These results suggest that strategic cultivation may be a useful management tool, as it will alter the periodicity of emergence allowing use of more effective control options and will deplete the soil seedbank more rapidly.

Modeling the age of tropical moist forest fragments in heavily-cleared lowland landscapes of Colombia

Increasingly, large areas of native tropical forests are being transformed into a mosaic of human dominated land uses with scattered mature remnants and secondary forests. In general, at the end of the land clearing process, the landscape will have two forest components: a stable component of surviving mature forests, and a dynamic component of secondary forests of different ages. As the proportion of mature forests continues to decline, secondary forests play an increasing role in the conservation and restoration of biodiversity. This paper aims to predict and explain spatial and temporal patterns in the age of remnant mature and secondary forests in lowland Colombian landscapes. We analyse the age distributions of forest fragments, using detailed temporal land cover data derived from aerial photographs. Ordinal logistic regression analysis was applied to model the spatial dynamics of mature and secondary forest patches. In particular, the effect of soil fertility, accessibility and auto-correlated neighbourhood terms on forest age and time of isolation of remnant patches was assessed. In heavily transformed landscapes, forests account for approximately 8% of the total landscape area, of which three quarters are comprised of secondary forests. Secondary forest growth adjacent to mature forest patches increases mean patch size and core area, and therefore plays an important ecological role in maintaining landscape structure. The regression models show that forest age is positively associated with the amount of neighbouring forest, and negatively associated with the amount of neighbouring secondary vegetation, so the older the forest is the less secondary vegetation there is adjacent to it. Accessibility and soil fertility also have a negative but variable influence on the age of forest remnants. The probability of future clearing if current conditions hold is higher for regenerated than mature forests. The challenge of biodiversity conservation and restoration in dynamic and spatially heterogeneous landscape mosaics composed of mature and secondary forests is discussed. (c) 2004 Elsevier B.V. All rights reserved.