Phosphate Speciation and Citrate-Induced Mobilization of P in an Acric Oxisol

Billions of dollars are spent annually in Brazil on imported phosphorus (P) fertilizers for agricultural crops produced on Oxisols. Phosphate fixation by Oxisols is a main limitation to crop productivity because these highly weathered soils have a high phosphate adsorption capacity. The objective was to determine whether reaction of an Acric Oxisol with citrate increases P availability. Columns of P-amended subsoil material were leached with 1 mM citrate solution, and effluent was monitored. Solid-phase speciation was measured for different stages of leaching using X-ray absorption near edge structure (XANES) spectroscopy at the P K-edge. When soil columns were leached with up to 56 column pore volumes of 1 mM citrate solutions, no P was detected in column effluent samples, whereas aluminum (Al) and iron (Fe) increased up to 30 and 1.4 µmol L^?1, respectively. The XANES analysis suggested that with increased leaching, a minor proportion of phosphate adsorbed on Fe-oxides increased as phosphate adsorbed on Al-oxides decreased. The results indicated that citrate tends to induce minor species redistribution of phosphate between Al- and Fe-oxide bound forms, but P mobilization was trivial compared with complexometric mobilization of Fe and Al.     

Dynamics of phosphorus fractions in a savanna Alfisol under continuous cultivation

Abstract. We examined the dynamics of inorganic P (P_i) and organic P (P_o) pools of a savanna Alfisol under continuous cultivation complemented with nitrogen (N) and phosphorus (P) fertilizers with or without cow manure (D), using a modified Hedly fractionation. Continuous cultivation without P fertilizer decreased the concentration of P_i and P_o, pools including the residual P fraction compared with an unfertilized treatment with natural vegetation. Adding P fertilizers alone or in combination with D (P, NPK, D+P, D+NP and D+NPK) increased the concentration of labile P_i pools, but decreased the concentration of P_o, pools and the residual P fraction. The tendency of the residual P fraction to decrease along with P_o pools suggested that the residual P fraction was largely P_o. This is consistent with the correlation between the residual P fraction and total P_o (r=0.74^**) and the residual P fraction and organic carbon content (r=0.47^**). Analysis of the relationships between plant available P, estimated by resin-extractable P, and the more stable P_i and P_o, pools indicated that 85% of the variation in resin-P was atiributable to the hydroxide extractable P_i, (OH-P_i) and HCI-P_i which acted as sinks for fertilizer-P. The contribution of the residual P fraction to resin-P was indirect via bicarbonate-extractable P_o (HCO₃-P_o) and OH-P_o. These results also clarify why attempts to find P extraction methods which correlate well with response to P and to P uptake in the savanna have given inconsistent results.     

Environmental and demographic correlates of tree recruitment and mortality in north Australian savannas

Tropical savannas cover approximately 20% of the earth’s land area, and therefore represent an important carbon store. Under scenarios of future climate change it is thus important to understand the demographic processes determining tree cover, namely tree recruitment, growth and mortality. This study measured tree recruitment and mortality in 123 (0.08 h) plots in Kakadu, Nitmiluk and Litchfield National Parks, in the Australian monsoonal tropics, over two consecutive 5-year intervals. Plots were located in two important habitats, both dominated by eucalyptus—lowland savanna and savanna growing on sandstone plateaux. All trees with diameter at breast height (DBH) ≥5 cm were tagged and identified. Recruitment was calculated as the proportion of tagged trees present at the end of an interval that were not present at the beginning. There were a total of 6666 and 6571 tree-intervals for mortality and recruitment, respectively. We used Akaike Information Criterion (AIC)-based model selection and multi-model inference to relate tree mortality and recruitment to fire frequency, mean annual rainfall (MAR), stand basal area, tree density and eco-taxonomic group. Recruitment decreased with tree density in both savanna types, and in lowland savanna, with the frequency of fires. In sandstone savanna, recruitment increased with MAR. Effects of fire on recruitment were better explained by season than severity of fire, while fire severity had a stronger influence on mortality. Mortality decreased with tree size up to about 25 cm DBH, but increased sharply when DBH exceeded 50 cm. Mortality increased with stand basal area, and increased with the frequency of late dry season fires in lowland savanna only. There was little evidence that mortality was affected by the frequency of early dry season fires or MAR. Both recruitment and mortality rates were higher for Acacia and Proteaceae species than for pantropical or Myrtaceae (including Eucalyptus) species. We identified several negative feedbacks, mediated by changes in tree density and stand basal area that help confer long-term stability to savanna tree cover. Nonetheless, changes such as a long-term increase in MAR or an increase in frequency or severity of fires are likely to result in changes in tree density, stand basal area and therefore carbon storage potential of savannas.     

The nature and spatial variability of lowland savanna soils: Improving the resolution of soil properties to support land management policy

A fresh approach is presented to address the increasingly urgent need for alternative land management strategies in savannas. We illustrate how fine‐scale information of soil characteristics can be used to enable a more precise delimitation of sites suitable for different forms of land stewardship, including agro‐pastoral activities, forestry, biodiversity and tourism. By collating data from previous soil surveys, and augmenting this with targeted new surveys, we produce the first national data set of soil properties for the lowland savannas of Belize. Most of these soils are typical of savanna soils worldwide, that is acidic (mean surface pH = 5.7), nutrient‐poor (mean surface total exchangeable bases [TEB] = 3.4 cmol kg^?1) and coarse‐textured (mean surface clay = 13.0%). Nevertheless, there is a marked spatial variability across the country in these soil properties. Some soils exhibit unusually high subsurface clay fractions (max = 73%), whilst other sites have exceptionally high pH in lower horizons (max = 8.4). Cluster analysis is used to group sites with similar soil properties. Across 79 sites, there is a clear division between soils with high clay percentages and those with coarser textures. These are sub‐divided into five groups based on further differences in parent material, chemical properties and site characteristics. Mapping the locations of these groups enables more specific land use recommendations to be made. This ability to make targeted land use recommendations from fine‐scale soil information represents an advance over the previous national land use policy, where all savanna lands were considered unsuitable for any form of agriculture. This approach could be applied to marginal savannas worldwide.     

金沙江干热河谷稀树灌草丛植被恢复方式研究

论述了金沙江干热河谷稀树灌草丛植被区自然概况,植物群落演替基本理论及封山育林(自然演替)与人工造林(人工促进恢复)2种恢复方式区基本情况。通过对比研究,结论是：封山育林自然演替3年后的稀树灌草丛,植物群落比人工促进恢复方式的复杂,生物多样性体现较好,灌木类逐渐转向耐旱、耐热、耐火、更新力强的抗逆类型发展,每获得1kg生物量的投入是人工播种方式的1／3。     

Changing perceptions of fire management in savanna parks

Parks are managed to preserve their pristine state. Fire has had a varied role in this, depending on shifting paradigms of savanna functioning. Formerly, an equilibrium theory of functioning prevailed where fire was at first an unavoidable evil, and then an evil to be avoided as far as possible. Subsequently, fire was regarded as natural and was applied with fixed return period and seasonal timing to compartments with fixed boundaries. Recently, non‐equilibrium theory has supplemented equilibrium models in explaining a savanna functioning that is partly event‐driven. In these terms savannas are understood to have high patch diversity arising from catenal variation, species individuality and the spatio‐temporal variability of events. Policy and practice in fire management is accordingly shifting towards regarding fire parameters (e.g. return period, seasonal timing, intensity and extent) not as scalars but as vectors (i.e. with frequency distributions), and to burning under diverse conditions.     

Reforming communal rangeland policy in southern Africa: challenges,dilemmas and opportunities

In the savanna rangelands of southern Africa, the debate about land reform tends to be about the redistribution of formerly freehold ranches and fencing-off the rangeland commons into ranches for better-off African farmers. The position of those who favour privatisation has been strengthened by the belief that the only environmentally sound way to manage the range is to subdivide it into private ranches because traditional open-range pastoralism is environmentally destructive. This point of view is at variance with an ever-increasing body of research. Insufficient attention has been given to the sustainable nature of communal rangeland in savanna areas and to the needs of the poor who depend upon them. Following decades of policies that further reduced and fragmented the commonage, governments have the challenging task of developing and implementing policies for those whose livelihoods depend on more secure and sustainable access to the remaining communal rangelands.     

Effect of Phosphorus Fertilizer on Soya Bean Residue Turnover in the Tropical Moist Savanna

The contributions of soya bean (Glycine max) to the maintenance of soil N, organic matter and physical properties in any cropping system is dependent on the amount of the crop residue returned after grain harvest. This amount of residue is a function of the dry matter accumulated during growth. In the topical moist savanna (MS) of West Africa where soya bean production has increased especially due to the cultivation of more hectarage of land, increase in soya bean dry matter with the resulting residue is limited by P deficiencies. In this study, the effect of P application on residue turnover by soya bean varieties of different maturity classes was evaluated across the MS. The amount of root residue in the late varieties was double that of the early and medium varieties. The effect of P application on root residue was also greater in the late varieties. Although root residue was 0.35–0.72 Mg ha^?1, this was about 17–21 % of total dry matter at harvest. Among the varieties, litter residue averaged less than 1 Mg ha^?1 in the early and medium varieties, and was 32 % higher in the late varieties. Litter residue increased by 42–46 % with P application. The total amount of soya bean residue that is a potential source of organic material in a cropping system after the export of grain is small and averaged 2.88 Mg ha^?1 . Of this, root residue constituted 18 %, litter residue 41 % and stover residue 40 %. In this study C/N ratio averaged 17.1, 34.8 and 32.2 for root, litter and stover, respectively. The amount of total residue obtained in this study shows that the benefit of the effect of soya beans on soil organic matter and physical properties derivable from a single soya bean crop is small.     

Influence of bushdumps on the herbaceous understorey in a mesic Eastern Cape savanna

Abstract

Studies of the effects of woody vegetation on herbaceous vegetation in southern Africa have focused almost exclusively on savannas with isolated trees with no attention given to multi‐species bushclump savannas. The influence of multi‐species bushdumps on herbaceous vegetation was investigated in a mesic savanna in the Eastern Cape. Development of bushdumps appeared to reduce herbaceous production underneath the clumps by up to 90%. The herbaceous layer underneath the bushdumps was distinctly different to that of the open grassland and it also had slightly higher quality compared to that of grassland. Effective bush control measures are warranted if these savanna areas are to sustain beef and mutton production as is currently the case.     

Autoclaving kills soil microbes yet soil enzymes remain active

Biocidal treatment of soil is used to remove or inhibit soil microbial activity, and thus provides insight into the relationship between soil biology and soil processes. Chemical (soil pH, phosphodiesterase, protease) and biological (substrate induced respiration) characteristics of three contrasting soils from tropical savanna ecosystems in north Queensland, Australia were measured in field fresh samples and following autoclaving (121 °C/103 kPa for 30 min on two consecutive days). Autoclaving treatment killed the active soil microbial biomass and significantly decreased protease activity (90%) in all three soils. Phosphodiesterase activity in kaolinitic soils also significantly decreased by 78% and 92%. However, autoclave treatment of smectitic soil only decreased phosphodiesterase activity by 4% only. This study demonstrates phosphodiesterase can remain stable in extreme conditions. This might be a characteristic vital to the cycling of phosphorus in shrink–swell clays in Australian tropical savanna ecosystems.