Phosphorus saturation and pH differentially regulate the efficiency of organic acid anion-mediated P solubilization mechanisms in soil

Exudation of organic acid anions by plants as well as root-induced changes in rhizosphere pH can potentially improve phosphate (P_i) availability in the rhizosphere and are frequently found to occur simultaneously. In non-calcareous soils, a major proportion of P_i is strongly sorbed to metal oxi(hydr)oxides of mainly iron (Fe) and aluminium (Al) and organic anions are known to compete with P_i for the same sorption sites (ligand exchange) or solubilize P_i via ligand-promoted mineral dissolution. Root-induced co-acidification may also further promote P_i release from soil. The relative efficiency of these different solubilization mechanisms, however, is poorly understood. The aims of this study were to gain a better mechanistic understanding of the solubilizing mechanisms of four carboxylates (citrate, malate, oxalate, malonate) in five soils with high and low P surface site saturation. Results indicate that at a lower P saturation of solid phase sorption sites, ligand-promoted mineral dissolution was the main P_i solubilization mechanism, while ligand exchange became more important at higher soil P concentrations. Co-acidification generally increased P_i solubility in the presence of carboxylates; however the relative solubilizing effect of carboxylates compared to the background electrolyte (KCl) control decreased by 20–50%. In soils with high amounts of exchangeable calcium (Ca), the proton-induced Ca solubilization reduced soluble P_i, presumably due to ionic-strength-driven changes in the electric surface potential favoring a higher P_i retention. Across a wider soil pH range (pH 3–8), P_i solubility increased with increasing alkalinity, as a result of both, more negatively charged sorption sites, as well as DOC-driven changes in Fe and Al solubility, which were further enhanced by the presence of citrate. Overall, the relative efficiency of carboxylates in solubilizing P_i was greatest in soils with medium to high amounts of anionic binding sites (mainly Fe- and Al-oxy(hydr)oxides) and a medium P sorption site coverage, with citrate being most effective in solubilizing P_i.     

Gains and losses in soil nutrients associated with harvesting and burning eucalypt rainforest

In mixed eucalypt/rainforest in southern Tasmania, samples of surface soil 0 to 2 cm, 2 to 5 cm, and 5 to 10 cm were taken from a clear-felled coupe before and after burning in 1982, from a similar coupe after burning in 1979, and from an uncut area adjacent to each coupe. Factors compared were bulk density; total organic C, N, P, Ca, Mg, and K; pH; exchangeable Ca, Mg, and K; cation exchange capacity; extractable P; and N-mineralisation rates. The effect of burning was found to be restricted mainly to the upper 2 cm of soil. The combustion of organic matter caused losses of 7360 kg organic C and 211 kg N/ha; 348 kg Ca and 282 kg Mg and 151 kg K/ha were added to the soil in ash. Burning caused significant increases in pH, exchangeable Ca, Mg, and K, and in extractable P; cation exchange capacity was reduced. In the 6 months after burning only K was leached from the upper 2 cm of soil. Equilibrium levels of NH₄?N increased initially after the fire, but between 6 and 18 months, equilibrium levels and rate of production of NH₄?N during anaerobic incubation in soil of burned coupes differed little from that in adjacent uncut forest. Rates of production of NO₃?N during aerobic incubation were very low throughout the period of study. It is concluded that for soils developed on dolerite in mixed eucalypt/rainforest, a single regeneration burn probably improves the nutritional status of the soil. Nutrients lost from the area as particulate ash are in quantities that will probably be replaced in rainfall in 15 to 20 years.     

Phosphorus retention capacity of root bed media of sub-surface flow constructed wetlands

Phosphorus retention by sub-surface flow constructed wetlands is dependent upon the effluent quality, loading rate and type of root bed media. Three types of root bed media (Lockport dolomite, Queenston shale and Fonthill sand) at various stages of their use were sampled from a sub-surface flow wetland located in Sewage Waste Amendment Marsh Process Project (SWAMP), Niagara-on-the-Lake, Ontario, Canada, and their P sorption characteristics were investigated. Lower equilibrium P concentration (EPC_o) and higher P sorption maxima (S_max) and retention capacity (P_r) of untreated Fonthill sand compared to untreated Lockport dolomite and Queenston shale indicated that Fonthill sand could be better root bed media for sub-surface flow constructed wetland systems to remove P from wastewater. In general, EPC_o of the root bed media increased with the duration of their use. Untreated root bed media indicated that the greater the amount of poorly crystalline forms of aluminum (Al), and magnesium (Mg) contents, the greater their EPC₀. The P sorption maxima of root bed media increased with use by accumulation of amorphous and poorly crystalline forms of Al and iron (Fe). However, in alkaline root bed media, preferential P sorption occurred more on amorphous and poorly crystalline forms of Al than Fe. This study also indicated that increases in P sorption capacities of root bed media do not necessarily assure lower effluent P concentrations, since EPC_o increased by several fold with use.     

To recycle or steal? Nutrient resorption in Australian and Brazilian mistletoes from three low‐phosphorus sites

Resorption is the process by which nutrients are withdrawn from leaves prior to leaf fall. Mistletoes are generally thought not to rely on nutrient resorption; being xylem‐tapping parasites, they instead derive the nutrients required for new growth from their host plant, at little or no cost. We measured nutrient (N, P, K, Ca, Mg) resorption in 18 parasitic mistletoe–host species pairs distributed across three sites with notably low‐P soil, also quantifying relationships with leaf lifespan (LL) and specific leaf area (SLA). There was little or no evidence of N, Ca or Mg resorption. By contrast, on average ～30% of P and ～20% of K were resorbed prior to leaf fall. Longer LL in mistletoes was associated with lower N and P concentrations in mistletoes and in host leaves. We provide evidence that, even though mistletoes are relatively inefficient in terms of nutrient resorption compared to non‐parasite species, on low‐P soils their ecological and evolutionary strategies for conserving phosphorous involve modulation of both leaf lifespan and P concentration in senesced leaves.     

Effects of 60-year N,P, K and Ca fertilization on EUF-nutrient fractions in the soil and on yields of rye and potato crops

Summary In field experiments carried out uninterruptedly since 1923 on a sandy, grey-brown luvisol (8% clay) the EUF-nutrient fractions were determined after potato and rye crops in 1982 had been harvested.Annual applications of N fertilizer (90 kg N/ha) over a 60-year period raised both the organic and inorganic EUF-N fractions (Table 1). EUF-N_org values increased depending on crop rotation from 1.1–1.9 to 2.4–3.6 mg/100 g. This increase in EUF-N amounts was higher under rye monoculture than under potato monoculture. N leaching was more intensive under potato monoculture than under rye monoculture.Application of P, K and Ca fertilizers raised the EUF-P, EUF-K and EUF-Ca values at 20°C as well as at 80°C (Figs. 1, 2, 3). Analyses of soil samples from the 25–75 cm layer showed that the contents of EUF-Ca and EUF-K in this layer are as high as in the topsoil or even higher (Table 3).High yields of rye (>4 t/ha) and of potatoes (>30 t/ha) were obtained when in samples taken after harvest EUF-amounts of 3.5 mg N, 2 mg P, 3 mg K (in both topsoil and subsoil), 1.7 mg Mg and 20 mg Ca/100 g were released within 35 min. These levels were obtained with annual fertilizer application rates of 90 kg N, 60 kg P₂O₅ and 110 kg K₂O/ha.Applications of 1.6 t CaO/ha every 4 years were sufficient to maintain the soil pH at 6.0–6.5. Calculated on the basis of EUF-Ca contents, with the amounts of K-selective minerals being taken into account, this rate of liming corresponds with the needs of acid sandy soils.     

Young secondary vegetation and soil interactions in Izabal,Guatemala

Summary A study of the relationships of soils and vegetation was undertaken in the humid tropical region of Lake Izabal, Guatemala, Central America. Soils and associated 10-month-old secondary growth were analyzed for N, P, K, Ca, and Mg. In addition, organic matter, Al and pH were determined in the soil samples and plant biomass for the secondary growth were calculated. The secondary growth biomass averaged 9,710 kg/ha for the 10-month growth period. Total nutrient content of the vegetation increased linearly with the biomass, except for Mg. The antagonism of Mg on K and Ca nutrition was quite significant when Mg exceeded Ca in the soil under shifting cultivation. Pure stands ofHeliconia sp. andGynerium sp. appeared to be more efficient in accumulating P than stands of mixed vegetation. Chemical composition and dry matter production of native vegetation may provide additional information to evaluate soil fertility in the humid tropics.     

Depletion of Selenium in Soil Solution due to its Enhanced Sorption in the Rhizosphere of Soybean

The effect of plant roots on selenium (Se) mobility in soil was studied by a large-scale pot experiment in order to understand the environmental behavior of Se in agricultural soils under plant growth conditions. Soybean plants (Glycine max (L.) Merrill) were grown in a greenhouse for 84 d. The concentrations of Se and major elements (K, Ca, Mg, Na, and Al) in the soil solutions and in the plants were measured at different growth periods. Concentrations of Se and major cations in soil solution decreased as the soybean plants grew, while the concentrations of Al increased. It was assumed that the soybean roots released H⁺ with the uptake of cations; consequently, due to the acidification of the rhizosphere, Al³⁺ was released starting from the soil solid phase. The decreased Se concentration in the soil solution should be due to the enhancement of Se sorption onto the soil solid phase. The increase of Se sorption level in the rhizosphere was examined in a small-scale pot experiment. The soil–soil solution distribution coefficient of Se (K _d-Se) was observed as an index of Se sorption level. K _d-Se clearly increased in the rhizosphere soil after cultivation. The effects of pH and Al³⁺ in the rhizosphere on Se sorption were assessed by K _d-Se measurements at different levels of HCl and AlCl₃. In this third experiment, a decrease in pH increased K _d-Se values, but no specific effect was observed on Se sorption due to increased Al³⁺. These results show that the Se mobility in agricultural soil could be decreased by plant roots under plant growth conditions due to enhanced Se sorption in the rhizosphere.     

Effect of Sodium and Magnesium on Kinetics of Phosphorus Release in Some Calcareous Soils of Western Iran

Knowledge of the rate of release of phosphorus (P) from soils resulting from poor water quality application is essential for long-term planning of crop production while minimizing the impact on groundwater quality. In this study, we examined the effect of sodium adsorption ratio (SAR) and Ca:Mg ratio of water on P release of some calcareous soils from western Iran. Nine different solutions at a total electrolyte concentration of 100 mmol_c l^?1 and three levels of SAR (5, 15, 45) each with Ca:Mg ratios of 1:3, 1:1, or 3:1, prepared using solutions of NaCl, CaCl₂, and MgCl₂, were used to extract P from the soils. The geochemical Visual MINTEQ was used to calculate saturation indices and P species at the initial and end of P release. Significantly different quantities of P were extracted by the solutions. The maximum (average of five soils) (233.6 mg kg^?1) and the minimum (162.9 mg kg^?1) P were extracted by an SAR 45 solution with a Ca:Mg ratio of 1:3 and SAR 15 solution with Ca:Mg ratio of 3:1, respectively. Elovich model adequately described P release. The release rate for SAR 15 with Ca:Mg of 3:1 and SAR 45 with Ca:Mg 1:3 ranges from 16.3 to 31.3 mg kg^?1 h^?1 and from 20.0 to 32.8 mg kg^?1 h^?1, respectively. In the initial stage of P release the solution samples in most soils were saturated with respect to hydroxyapatite, octacalcium phosphate, ß-tricalcium phosphate, and undersaturated with respect to dicalcium phosphate dihydrate, dicalcium phosphate, and mangnesium phosphates. At the end of P release, all solutions were saturated with respect to hydroxyapatite and under saturated with respect to other phosphate minerals. The results imply that P release from soils could be increased during use of saline and sodic irrigation water containing high Mg concentration and that P fertilization management may need modification.     

Phosphorus deficiency does not enhance proton release by roots of soybean [Glycine max (L.) Murr.]

Little work has been done on root exudation in soybean under P deficiency. This study examined the effect of P supply on release of protons and carboxylates by roots of soybean (Glycine max Heinong 35), and to correlate the release with excess uptake of cations over anions. Plants were either reliant on N₂ fixation or supplied with nitrate and were grown in nutrient solution with 1–50 μM P for 7 weeks. Release of protons and carboxylates from roots, and concentrations of Ca, Mg, K, Na, P, S, Cl and N in plants were measured weekly from week 4. Unlike in many other species, P deficiency decreased proton release per unit root biomass in N₂-fixing plants and increased release of hydroxyl ions in nitrate-fed soybean. While P deficiency generally decreased uptake of K, Ca, Mg, S, Cl and P, it increased nitrate uptake per unit root biomass. Irrespective of P supply, amounts of protons released correlated well with excess uptake of cations over anions by the roots. Phosphorus deficiency increased release of carboxylates but the amounts released were small. The results suggest that soybean displays strategies of P acquisition through decreasing proton release which favors P mobilization in acid soils, and increasing root-to-shoot ratio and specific root length.     

Soil carbon increased by twice the amount of biochar carbon applied after 6 years: Field evidence of negative priming

Applying biochar to agricultural soils has been proposed as a means of sequestering carbon (C) while simultaneously enhancing soil health and agricultural sustainability. However, our understanding of the long‐term effects of biochar and annual versus perennial cropping systems and their interactions on soil properties under field conditions is limited. We quantified changes in soil C concentration and stocks, and other soil properties 6 years after biochar applications to corn (Zea mays L.) and dedicated bioenergy crops on a Midwestern US soil. Treatments were as follows: no‐till continuous corn, Liberty switchgrass (Panicum virgatum L.), and low‐diversity prairie grasses, 45% big bluestem (Andropogon gerardii), 45% Indiangrass (Sorghastrum nutans), and 10% sideoats grama (Bouteloua curtipendula), as main plots, and wood biochar (9.3 Mg/ha with 63% total C) and no biochar applications as subplots. Biochar‐amended plots accumulated more C (14.07 Mg soil C/ha vs. 2.25 Mg soil C/ha) than non‐biochar‐amended plots in the 0–30 cm soil depth but other soil properties were not significantly affected by the biochar amendments. The total increase in C stocks in the biochar‐amended plots was nearly twice (14.07 Mg soil C/ha) the amount of C added with biochar 6 years earlier (7.25 Mg biochar C/ha), suggesting a negative priming effect of biochar on formation and/or mineralization of native soil organic C. Dedicated bioenergy crops increased soil C concentration by 79% and improved both aggregation and plant available water in the 0–5 cm soil depth. Biochar did not interact with the cropping systems. Overall, biochar has the potential to increase soil C stocks both directly and through negative priming, but, in this study, it had limited effects on other soil properties after 6 years.     

Effect of varying Mg/Ca ratio and electrolyte concentration in the irrigation water on the soil properties and growth of wheat

This paper discusses the results of a pot experiment conducted to study the effect of irrigation waters having varying Mg/Ca ratio (2, 4, 8 and 16) and electrolyte concentration (20 and 80 meq/l) on the soil properties and growth of wheat crop in two different soils. The development of salinity in the soils generally increased at higher electrolyte concentration of the irrigation water, but it was of a greater magnitude in the heavy-textured black soil dominated by montmorillonite clay mineral than in the light-textured alluvial soil having illite type of clay mineral. The accumulation of soluble salts as a result of saline water irrigation was higher in the surface layer than in the subsurface layer in both soils. The adsorption of Na and Mg in the soils increased with an increase in the Mg/Ca ratio and electrolyte concentration of the irrigation water. These changes in soil properties were adequately reflected by the grain and dry matter yields of wheat crop, which showed a significant reduction with an increase in the Mg/Ca ratio and electrolyte concentration of the irrigation water. However, the effects of these treatments were more pronounced in the heavy black clay soil than in the alluvial soil. Thus, the role of Mg is different from that of Ca under the conditions used in the experiment.     

On the tropical soils; The influence of organic matter (OM) on phosphate bioavailability

Application of organic manure (OM) and crop residues in agricultural soils can potentially influence positively or negatively the availability of soil phosphorus (P) through soil mineralization, sorption, or desorption of soil-bound P. Traditionally, the addition of OM can reduce the capacity of the soil colloids to adsorb P, thus increasing the release of P in soil solution, but also added OM can increase the adsorption site and increase the fixation or sorption of P to soil colloids, thus reducing the availability of P in soil solution and loss to the environment. The highly weathered tropical soils (HWTS) are susceptible to P insufficiency because HWTS have high P adsorption and fixation; this is mainly due to high concentration of P adsorbent. The main P adsorbents in HWTS include Al, Fe, Ca, and clay minerals, which are principally the same binding or adsorbent for OM compounds, but in excess, are toxic (Al and Fe) to crops. Thus, the presence of OM in HWTS can compromise the adsorption and availability of P in agricultural soils following phosphatic fertilizer applications. In this study, the influence of OM on P adsorption and availability was characterized to have a clear understanding of how OM influences P availability in agricultural soils, especially in highly weathered tropical soil. It is clearly outlined that the application of OM and crop residues can positively or negatively influence the availability of P in agricultural soils for plant uptake and dictate the P that is available for loss to the environment. Thus, the addition of organic matter as a strategy to increase P bioavailability for plant uptake must be treated with care because their contribution is not strait forward to be positive in many agricultural soils.     

我国23个土壤磷素淋失风险评估Ⅱ.淋失临界值与土壤理化性质和磷吸附特性的关系

从13个省（市）采取23个耕地表层土壤,通过室内模拟试验测定其磷素淋失临界值和pH、有机质、＜0.01mm、＜0.002mm、交换性钙镁、活性铁铝、磷等温吸附特性等,以建立土壤磷素淋失临界值与土壤基本理化性质和磷吸附特性之间的关系.结果表明：土壤pH＜6.0时,随土壤pH提高临界值增加,土壤pH与临界值之间呈显著的指数关系;而当土壤pH＞6.0时,随土壤pH提高临界值减小,在pH6.5左右土壤磷素淋失临界值最高.土壤磷素淋失临界值与土壤有机质、活性铁（铝）、交换性钙之间存在显著的相关,而与交换性镁、CEC、＜0.01mm、＜0.002mm、K、Qm的相关性受土壤酸碱度影响.可以通过测定土壤有机质或活性铁的含量,来计算土壤磷素淋失临界值,评价土壤磷素淋失的风险.供试的23个土壤,除了采自湖北潜江的20号水稻土存在比较大的磷素淋失风险,其余土壤发生磷素淋失的风险很小.     

Effects of dissolved organic matter from sewage sludge on the atrazine sorption by soils

The effects of dissolved organic matter (DOM), water soluble organic matter derived from sewage sludge, on the sorption of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-trazine) by soils were studied using a batch equilibrium technique. Six paddy soils, chosen so as to have different organic carbon contents, were experimented in this investigation. Atrazine sorption isotherms on soils were described by the linear equation, and the distribution coefficients without DOM (K_d) or with DOM (K_d ^*) were obtained. Generally, the values of K_d ^*/K_d initially increased and decreased thereafter with increasing DOM concentrations of 0–60 mg DOC · L^?1 in soil-solution system form. Critical concentrations of DOM (DOM_np) were obtained where the value of K_d ^* was equal to K_d. The presence of DOM with concentrations lower than DOM_np promoted atrazine sorption on soils (K_d ^* > K_d), whereas the presence of DOM with concentrations higher than DOM_np tended to inhibit atrazine sorption (K_d ^* < K_d). Interestingly, DOM_np for tested soils was negatively correlated to the soil organic carbon content, and the maximum of K_d ^*/K_d (i.e.K _max) correlated positively with the maximum of DOM sorption on soil (X_max). Further investigations showed that the presence of hydrophobic fraction of DOM evidently promoted the atrazine sorption on soils, whereas the presence of hydrophilic DOM fraction obviously tended to inhibit the atrazine sorption. Interactions of soil surfaces with DOM and its fractions were suggested to be the major processes determining atrazine sorption on soils. The results of this work provide a reference to the agricultural use of organic amendment such as sewage sludge for improving the availability of atrazine in soils.     

Amelioration of acid soil infertility by phosphogypsum

Amelioration of subsoil acidity requires an increase in Ca status along with a decrease in Al status in subsoil. In this study, effects of phosphogypsum (PG) on the amelioration of subsoil acidity have been evaluated, using cultivated and woodland subsoils representing Cecil, Wedowee (both Typic Hapludult) and Bladen (Typic Albaquult) series. Subsoil (0.6–0.8 m) samples were collected and treated with either PG (approximately 2 Mg ha^-1 rate), Ca(NO₃)₂ or Mg(NO₃)₂ along with an unamended control treatment. A fertile topsoil amended with NH₄NO₃ was placed on top of all treated subsoil. Top and root growth of alfalfa [Medicago sativa (L.) cv. Hunter River] and soybean [Glycine max (L.) Merr. cv. Lee] were significantly greater in PG-amended than in unamended pots of the Cecil and Wedowee soils, although most growth was observed with the Ca(NO₃)₂-amended treatment. In the Bladen soil, however, none of the amendments evoked a significant growth response in either alfalfa or soybean. The concentration of Ca in the displaced soil solution (in soils with no plants) as well as tissue levels of Ca suggest that the growth response was partly due to an improved Ca availability in both PG or Ca(NO₃)₂-treated soils. Exchangeable Al decreased in PG-amended soils. The self-liming effect of PG, which is a release of OH^- due to ligand exchange between SO₄ ^2- and OH^-, as well as a decrease in exchangeable Al in PG-amended soil is greater in predominantly kaolinitic Cecil and Wedowee soils than in smectitic Bladen soil. As a result, significant growth response to PG amendment was observed in the Cecil and Wedowee soils, but not in the Bladen soil.     

Phosphorus retention in the filter materials shellsand and Filtralite P®—Batch and column experiment with synthetic P solution and secondary wastewater

Filtralite P^® and shellsand as ideal constructed wetland substrates have been tested for their P sorption capacity, both with batch and column experiments. Two columns were filled with Filtralite P^® and one column with shellsand. The shellsand (SSPS) and one of the Filtralite P^® columns (FLSP) were loaded with a synthetic P solution, while the second Filtralite P^® column (FLWW) was loaded with secondary wastewater. Ca, Mg, pH, and the P concentrations were measured in the inlet and the seven outlets along the height of the three vertical upflow columns for up to 303 days. An overall P removal rate of 92, 91, and 54% was measured in the columns SSPS, FLWW, and FLPS, respectively, for the entire experimental period. The comparison of FLWW and FLPS showed that FLWW kept its high P removal efficiency (91%) throughout the experimental period while the removal efficiency of FLPS decreased fast after reaching the 1 ppm effluent P concentration. The competition of other negative ions and the development of biofilm did not have a negative effect on P removal from wastewater. The batch experiments showed a better sorption capacity of Filtralite P^® at low initial concentrations, while for high initial concentrations the shellsand sorbed more. Shellsand had, however, a higher sorption capacity in batch experiments with used column material and high initial P concentrations. The results from both the batch and the column experiment suggest that the shellsand has a more durable P sorption capacity than the Filtralite P^® material, possibly due to the persistent high concentrations of Ca in the shellsand.     

Variation in Phosphorus Sorption with Soil Particle Size

Phosphorus (P) is considered a primary cause for surface water eutrophication that leads to anoxia. Understanding the relationships between soil particle size and P sorption helps devise effective best management practices (BMPs) to control P transport by erosion, leaching, and overland flow from agricultural land. Consequently, this study examined the effect of surface soil particle size on the sorption of P in five soil series (four Ultisols and one Entisol) from the Mid-Atlantic region. The sorption of P in each soil was assessed by equilibrating (after shaking for 24?h) 5?g soil containing varied amounts of KH₂PO₄ in 20?mL of 0.01?M KCl solution. Phosphorus in solution was determined by the molybdate blue method of Murphy and Riley. The P adsorption characteristics of these soils were described using the Langmuir isotherm. Results indicated that variability in P sorption was related to particle size and soil type. Soil organic matter content contributed a great deal to P sorption in the Entisol. However, soil clay had influence on the P sorption characteristics of each soil. The maximum P retentive capacities of soils (as determined by Sm from Langmuir equation) and P sorbed at 500?mg P kg^?1 addition showed a linear relationship (r2 = 0.94). Therefore, based on the results obtained, the single point method of Bache and Williams may be appropriate to describe the maximum P sorption capacity of non-sandy soils, as observed in this study.     

Intraspecific adoption and foster feeding of fledglings in the North Island robin

We sampled soils and vegetation within and outside two sheep and rabbit exclosures, fenced in 1979, on steep sunny and shady slopes at 770 m altitude on seasonally-dry pastoral steeplands. The vegetation of sunny aspects was characterised by higher floristic diversity, annual species, and low plant cover. Here the exotic grass Anthoxanthum odoratum dominated on grazed treatments, and the exotic forb Hieracium pilosella on ungrazed. Shady aspects supported fewer, and almost entirely perennial, species. Here Hieracium pilosella dominated grazed treatments, but co-dominated with the exotic forb H. praealtum and the native grass Festuca novae-zelandiae on ungrazed treatments. There was 43% more biomass in exclosures (P < 0.01). Most of the biomass difference (4285 kg/ha) was from greater root mass (2400 kg/ha). 1385 kg/ha of the difference was from herbage and the remainder (500 kg/ha) from litter. Exclosures had 50 to 100% more Ca, Mg, K and P in the biomass (P < 0.05), but the effect on soils was limited to significantly higher concentrations of total N (P < 0.05) and exchangeable Mg (P < 0.01) in 0-7.5 cm soils. We conclude that stopping grazing for 16 years on seasonally-dry steeplands results in greater plant cover, approximately double the biomass of standing vegetation, greater biomass in roots, and more biomass nutrients relative to grazed areas. However, it does not favour native species and has little effect on soil nutrients or soil carbon. Stopping grazing alone therefore cannot be regarded as a comprehensive short- or medium-term vegetation or soil rehabilitation option.     

Fly ash effect on improving soil properties and rice productivity in Korean paddy soils

Paddy soils in Korea generally require the addition of Si to enhance rice productivity. Coal combustion fly ash, which has a high available Si content and alkaline pH, was selected as a potential source of Si in this study. Two field experiments were carried out to evaluate rice (Oryza sativa) productivity in silt loam and loamy sand soils to which 0, 40, 80, and 120 Mg ha(-1) of fly ash were added with 2 Mg ha(-1) Si as a control. Fly ash increased the soil pH and available Si and P contents of both soils. The amount of available B increased to a maximum of 2.57 mg kg(-1), and the B content of the rice plants increased to a maximum of 52-53 mg kg(-1) following the addition of 120 Mg ha(-1) fly ash. The rice plants did not show toxicity effects. The highest rice yields were achieved following the addition of around 90 Mg ha(-1) fly ash. The application of fly ash increased Si, P and K uptake by the rice plants, but did not result in an excessive uptake of heavy metals in the submerged paddy soil. In conclusion, fly ash could be a good supplement to other inorganic soil amendments to improve the nutrient balance in paddy soils.     

三种苦苣苔对石灰土和红壤的适应性分析

为探讨苦苣苔科植物对其岩溶生境的适应性,该研究选取黄花牛耳朵(Primulina lutea)、紫花报春苣苔(Pri.purpurea)和桂林蛛毛苣苔(Paraboea guilinensis)三种苦苣苔科植物,将其栽种在石灰土及红壤两种不同类型的土壤中,观测记录其生长性状并对其叶片元素含量进行测定和比较。植株采集过程中,同时采集自然生境中三种苦苣苔科植物叶片及取样植物基部土壤,并对叶片及土壤元素的含量进行测定,作为今后苗圃试验的参照。结果表明:三种苦苣苔科植物在两种土壤上的生长状况及适应性具有差异,其在石灰土上生长良好,在红壤上生长较差;在两种不同土壤中,除N外,桂林蛛毛苣苔的叶片其他元素(P、K、Mn、Mg、Ca、Zn、Cu)差异极显著(P0.01);除P外,紫花报春苣苔的叶片其他元素(N、K、Mn、Mg、Ca、Zn、Cu)差异极显著(P0.01);除N、Cu、Ca外,黄花牛耳朵的叶片元素(P、K、Mn、Mg、Zn)差异极显著(P0.01);三种植物的叶片元素比值,除少数值没有差异外,大部分指标差异都极显著;对叶片元素与栽培土壤元素的相关性分析,发现植物叶片Mn元素与土壤中N、Ca、Mg、Zn、Mn、有机质含量等呈正相关,土壤P元素与叶片中N、P元素呈正相关,而与叶片中Zn元素呈负相关关系。在其他栽培条件一致的条件下,土壤因素及物种差别是造成黄花牛耳朵、紫花报春苣苔和桂林蛛毛苣苔适应性产生差异的主要原因。