Effect of soil pH on cation and anion solubility

Abstract

The effect of soil pH on the exchangeability and solubility of soil cations (Ca, Mg, Na, K, and NH₄‐N) and anions (NO₃‐N, Cl, and P) was investigated for 80 soils, spanning a wide range in physical and chemical properties and taxonomic groups. This information is needed from environmental and agronomic standpoints to estimate the effect of changes in soil pH on leachability and plant availability of soil nutrients. Soils were incubated with varying amounts of acid (H₂SO₄) and base (CaCO₃) for up to 30 days. Although acid and base amendments had no consistent effect on cation exchangeability (as determined by neutral NH₄OAc), amounts of water‐soluble Ca, Mg, Na, K, NH₄‐N, and P decreased, while NO₃‐N and Cl increased with an increase in soil pH. The increase in cation solubility was attributed to an increase in the negative charge of the soil surface associated with the base addition. The change in surface electrostatic potential had the opposite effect on amounts of NO₃‐N and Cl in solution, with increases in N mineralization with increasing soil pH also contributing to the greater amount of NO₃‐N in solution. The decrease in P solubility was attributed to changes in the solubility of Fe‐, A1‐, and Ca‐P complexes. The logarithm of the amount of water‐soluble cation or anion was a linear function of soil pH. The slope of this relationship was closely related (R² = = 0.90 ‐ 0.96) to clay content, initial soil pH, and size of the cation or anion pool maintaining solution concentration. Although the degree in soil pH buffering increased with length of incubation, no effect of time on the relationship between cation or anion solubility and pH was observed except for NO₃‐N, due to N mineralization. A change in soil pH brought about by acid rain, fertilizer, and lime inputs, thus, affects cation and anion solubility. The impact of these changes on cation and anion leachability and plant availability may be assessed using the regression equations developed.     

Comparison of phosphate rock and triple superphosphate on a phosphorus‐deficient Kenyan soil

Abstract

Soil phosphorus (P) deficiency is a constraint to crop production in many regions of sub‐Saharan Africa, which could be overcome through use of either soluble P fertilizer or sufficiently reactive phosphate rock (PR). A field study was conducted with corn (Zea mays L.) for three growing seasons (18 months) on a P‐deficient, acid soil in Kenya to compare a soluble P source (triple superphosphate, TSP) and relatively reactive Minjingu PR from Tanzania. In the 18 months following application of 250 kg P ha^‐1, bicarbonate extractable inorganic soil P (P_i) was higher for application of TSP than PR, but Pi extracted with a mixed anion‐cation resin was comparable for TSP and PR. Inorganic P extracted by 0.1M NaOH, without prior extraction of resin and bicarbonate P_i, decreased during the 18 months following TSP application, but increased following PR application. After 18 months, about 7% of the added PR‐P remained as Ca‐bound P that was extracted with 1M HCl. The 1M HCl extractable P., however, underestimated residual PR‐P that gradually dissolved and supplied plant‐available P, as indicated by recovery of <40% of PR‐P added to soil in laboratory incubations even though PR solubility in HCl was >90%. Minjingu PR was an effective source of P for corn. Corn yields were comparable for TSP and PR, and the relative agronomic effectiveness of PR averaged 107% in Season 1 and 79% in Season 3. Anion resin and mixed anion‐cation resin appeared to be superior to bicarbonate and NaOH as a soil P test for use with both TSP‐ and PR‐treated soils.     

Addition of phosphorus to soils with low to medium phosphorus retention capacities. II. effect on soil phosphorus extractability

Abstract

Knowledge of the change in soil extractable phosphorus (P) as a consequence of soil P fertilization could be useful in discriminating soils with a potential for soil P release to runoff or movement of P along the soil profile. In this research, soils with low to medium P retention capacity were equilibrated for 90 days with soluble P (KH₂PO₄) at rate of 100 mg P kg^‐1 soil. After this period, soil samples both with and without the P addition were analyzed using six conventional methods: 1) Olsen, 2) Bray 1,3) Mehlich3,4) Egner, 5) Houba, dilute CaCl₂ solution, and 6) distilled water, and three “innovative”; P‐sink methodologies: 1) Fe oxide‐coated paper strip, 2) anion exchange resin membrane, and 3) cation‐anion exchange resin membrane. The soils without P addition had low levels of extracted P as determined by all nine procedures. Net increases in the amount of P extracted from the soils with added P ranged from 4.2 mg kg^‐1 (CaCl₂ extraction) to 57.6 mg kg^‐1 (cation‐anion resin membrane extraction). Relationships between change in extracted P and i) physical and chemical characteristics, and ii) soil P sorption properties are also presented and discussed.     

Participation of silicon in cation‐anion balance as a possible mechanism for aluminum and iron tolerance in some gramineae

Abstract

Silicon (Si) has been suggested as a factor in aluminum (Al) tolerance of some species of the gramineae when grown on acid soils. Silicon concentrations are generally much higher in monocot plants than in dicot plants, and the phenomenon is related to the fact that mineral cation:mineral anion uptake ratio is much higher in dicots than in monocots. When large amounts of anionic Si, supposedly as sulfate (SO₄ ^4‐), participate in cation‐anion balance to add to the excess of anion uptake, equivalent amounts of hydroxyl ions should be expelled from roots which can increase rhizosphere pH and decrease uptake of Al and iron (Fe). The magnitude of OH^? released by roots for a 5000 kg/ha crop with an excess uptake of 1% Si can be equivalent to 357 kg lime per hectare. This could be very significant in decreasing Al and Fe uptake from acid soils when localized in the rhizosphere. Success of agriculture on highly acid soils may be enhanced by use in a rotation of crops and cultivars that have the ability to accumulate Si.     

Factors Affecting Stability and Efficiency of Ion Exchange Resins in Studies of Soil Nitrogen Transformation

Abstract

Mixtures of cation and anion exchange resins are used as part of the resin core technique to determine nitrogen transformation in forest soils as they adsorb the NH₄‐N and NO₃‐N from soil solution percolating through the incubated soil cores. In the field, the exchange resins may be subjected to a variety of conditions, involving drying, rehydration, freezing, and thawing. This paper examines how these processes affect adsorption of NH₄‐N and NO₃‐N and the stability of the resins. Lab tests were performed on the anion resin Amberlite IRA‐93, the cation resin Amberlite IR‐120, a mixture of IRA‐93 and IR‐120, and the commercially‐mixed bed resin Amberlite MB1. The background content of NO₃‐N and NH₄‐N on the resins was large and highly variable between different batches of resins in spite of a 2 M NaCl pre‐rinse. The IR‐120 cation resin that was subjected to 48 hours air‐drying contained significantly less NH₄‐N than the moist resins, while the drying of the IRA‐93 anion resin caused a significant release of NO₃‐N from resins with no N addition. Although the variation was large, the mixed bed resin MB1 indicated a release of NH₄‐N, which supports results from long term in situ deployments. A reduced adsorption of NO₃‐N was found on the IRA‐93 anion resins and the MB1 mixed bed resins that were dried prior to N addition while the dry IR‐120 cation resins adsorbec significantly less NH₄‐N than the control resin. No effect of freezing and thawing efficiency was observed on resin stability or N adsorption efficiency. Sufficient blanks that have been subjected to similar moisture changes are necessary in N limited systems with low levels of available NH₄‐N and NO₃‐N.     

Relationship between chemical and mineralogical properties and the rapid response to acid load of soils in humid Asia: Japan,Thailand and Indonesia

Abstract

It is essential to determine the relationship between soil chemical and mineralogical properties and soil response to acid load to understand the acid-neutralizing capacity and cation behavior of different ecosystems. For 46 soil samples from a subsurface horizon in humid Asia, that is, Japan, Thailand and Indonesia, exchangeable cations, total bases and oxalate-extractable Al (Al_o) were determined, and acid titration was conducted to investigate the rapid soil response to acid load. The acid titration experiment indicated three types of soil response: (1) the release of base cations (particularly Ca and Mg) strongly correlated with exchangeable bases, which dominated the tropical soil samples, (2) the release of Al correlated with Al_o content, which dominated the Japanese soil samples, (3) acid and anion adsorption in soil samples with low acid-neutralizing capacity. To gain further information on the source of soil alkalinity, a column experiment with HCl was conducted using eight selected soil samples in which first-order kinetics were assumed to simulate the time-courses of cation release. In the column experiment, the amounts of Ca and Mg released were close to the exchangeable amounts, and Al_o dissolved more rapidly than Al in crystalline minerals. The rate constants of cation release were large for Ca and Mg, and small for Al, clearly indicating a difference between the exchange and dissolution reactions. Thus, rapid soil response to acid load differed among the soils. A cation exchange reaction was dominant in the tropical soils. In some tropical soils, Ca and Mg were present in exchangeable forms at a higher ratio in the total amounts and they were considered to be easily utilized by plants, but leached out from the soils. In the Japanese soils, including the Andisols, secondary mineral dissolution was conspicuous, resulting in a large acid-neutralizing capacity. In both the tropical and Japanese soils with low acid-neutralizing capacity, anion adsorption mainly contributed to acid neutralization.     

土壤加速酸化的主要农业驱动因素研究进展

土壤酸化是土壤质量退化的一个重要方面,农业活动对其有极其重要的驱动作用。本文从土壤酸化加速的农业主驱因素:化肥、作物及有机物料等方面阐述它们对土壤酸化的影响。认为化肥尤其是生理酸性肥料和含硫肥料的不合理施用加速土壤酸化,而氮肥的致酸除受氮素形态影响外,硝化作用及硝化产物的淋溶是重要的致酸原因,同时豆科作物的固氮致酸作用也不容忽视。作物通过选择性吸收盐基阳离子,通过秸秆和子粒转移出生产系统后,导致土壤盐基量减少,土壤表面交换性酸增加;作物根系呼吸、根系分泌物及土壤溶液中重碳酸盐的淋溶也引起土壤酸化;而秸秆和畜禽粪便对土壤酸化的影响除受土壤本身性质影响外,秸秆中的灰化碱含量、畜禽粪便中的碳及氮和盐分去向对酸化也有重要的影响。     

PHOSPHORUS DEFICIENCY IN PELARGONIUM: EFFECTS ON NITRATE AND AMMONIUM UPTAKE AND ACIDITY GENERATION

A sudden pH decline (SPD) of the substrate is an increasing problem in geranium growth systems, and the cause is unknown. In this study, we investigate whether a phosphorus (P) deficiency can cause SPD, and whether the effect is related to inhibition of ammonium (NH₄ ⁺) and nitrate (NO₃ ^?) uptake and a corresponding shift in the cation to anion uptake balance. Geraniums (Pelargonium x hortorum Bailey ‘Designer Dark Red’) were grown in hydroponic solutions with or without P, and the hydroponics systems were located in a growth chamber programmed for light/dark temperatures of 22/18 or 26/22°C. Acidification potential was measured by the amount of base required to maintain pH at 5.8. The results indicated that much greater amounts of base were required to maintain a stable pH with P-limited plants. Using periodic exposures to ¹⁵NH₄ ⁺ or ¹⁵NO₃ ^?, it was found that NO₃ ^? uptake was strongly inhibited as plants became P stressed. Tissue nutrient profiles showed that the NO₃ ^? uptake inhibition was accompanied by an increase in the cation to anion uptake ratio. Rhizosphere acidification was greater at higher temperature even though the cation and anion responses were unchanged in control plants, suggesting the involvement of carbon dioxide (CO₂) generated by root respiration. The results indicate that changes in cation and anion uptake and the associated increase in net H+ extrusion that occur under P-stress conditions can contribute to SPD in geranium culture systems.     

Comparative Effects of Two Forage Species on Rhizosphere Acidification and Solubilization of Phosphate Rocks of Different Reactivity

ABSTRACT

Dissolution of phosphate rocks (PR) in soils requires an adequate supply of acid (H⁺) and the removal of the dissolved products [calcium (Ca^{2 +}) and dihydrogen phosphate (H₂PO₄ ^?)]. Plant roots may excrete H⁺ or OH^? in quantities that are stoichiometrically equal to excess cation or anion uptake in order to maintain internal electroneutrality. Extrusion of H⁺ or OH^? may affect rhizosphere pH and PR dissolution. Differences in rhizosphere acidity and solubilization of three PRs were compared with triple superphosphate between a grass (Brachiaria decumbens) and a legume (Stylosanthes guianensis) forage species at two pH levels (4.9 and 5.8) in a phosphorus (P)-deficient Ultisol with low Ca content. The experiment was performed in a growth chamber with pots designed to isolate rhizosphere and non-rhizosphere soil. Assessment of P solubility with chemical extractants led to ranking the PRs investigated as either low (Monte Fresco) or high solubility (Riecito and North Carolina). Solubilization of the PRs was influenced by both forage species and mineral composition of the PR. The low solubility PR had a higher content of calcite than the high solubility PRs, which led to increased soil pH values (> 7.0) and exchangeable Ca, and relatively little change in bicarbonate-extractable soil P. Rhizosphere soil pH decreased under Stylosanthes but increased under Brachiaria. The greater ability of Stylosanthes to acidify rhizosphere soil and solubilize PR relative to Brachiaria is attributed to differences between species in net ion uptake. Stylosanthes had an excess cation uptake, defined by a large Ca uptake and its dependence on N₂ fixation, which induced a significant H⁺ extrusion from roots to maintain cell electroneutrality. Brachiaria had an excess of anion uptake, with nitrate (NO₃ ^?) comprising 92% of total anion uptake. Nitrate and sulfate (SO₄ ^{2 ?}) reduction in Brachiaria root cells may have generated a significant amount of cytoplasmic hydroxide (OH^?), which could have increased cytoplasmic pH and induced synthesis of organic acids and OH^? extrusion from roots.     

Soil properties affecting volatilization of ammonia from soils treated with urea

Abstract

The effects of various soil properties on ammonia (NH₃) volatilization from soils treated with urea were studied by measuring the NH₃ evolved when 20 soils selected to obtain a wide range in properties were incubated at ‐0.034 mPa soil moisture potential and 30°C for 10 days after treatment with urea. The nitrogen (N) volatilized as NH₃ from these soils represented from 0 to 65% of the urea‐N applied and averaged 14%. Simple correlation analyses showed that loss of NH₃ was negatively correlated (P<0.1%) with cation‐exchange capacity, silt content, and clay content and was positively correlated (P <0.1%) with sand content. Loss of NH₃ was also negatively correlated with total nitrogen content (P<1.0%), organic carbon content (P<1.0%), hydrogen ion buffering capacity (P<5.0%), and exchangeable acidity (P<5.0%), and was positively correlated with calcium carbonate equivalent (P <1.0%) and with soil pH after incubation with urea (P<1.0%), but was not significantly correlated with initial soil pH or soil urease activity. Multiple linear regression analyses indicated that the amount of urea N volatilized as NH₃ from the 20 soils studied increased with increase in sand content and decreased with increase in cation‐exchange capacity. They also indicated that soil texture and cation‐exchange capacity are better indicators of potential loss of urea N as NH₃ from soils fertilized with urea than are hydrogen ion buffering capacity or initial soil pH.     

Beziehung zwischen Kationen/Anionen-Aufnahme von Rotklee und Protonenabscheidung der Wurzeln

Relationship between the cation/anion uptake and the release of protons by roots of red clover Red Clover was cultivated in Mitscherlich pots on a brown podzolic soil. Besides a low N rate at the beginning of the experiment, the clover received only symbiontically fixed N. Soil pH dropped under clover from 7.2 to 4.5 during a period of 14 months. During this time seven cuts were obtained. In a parallel pot experiment with rye-grass grown on the same soil and under the same environmental conditions but supplied with NH₄NO₃ after each clipping, no drop in soil pH was observed. In the aerial plant parts of clover the cation excess was high and amounted to about 60 % of the H⁺ quantity required for the decrease in soil pH from 7.2 to 4.5. It is concluded that the cations taken up in excess were electrostatically balanced by an equivalent amount of protons secreted by the roots into the soil. The alcalinity assessed in the upper plant parts of clover was approximately equivalent to the cation excess. It is therefore supposed that the H⁺ release from roots resulted in an alkalinization in the plant cells which in return led to a synthesis of organic anions being equivalent to the amount of H⁺ released.     

Cation transport during unsaturated flow through two intact soils

In the quest for better understanding of cation movement through undisturbed soils, leaching experiments on 300-mm long undisturbed soil columns of two contrasting soils were carried out. One soil was a weakly-structured alluvial fine sandy loam, the other a well-structured aeolian silt loam. About 2000 mm of solutions of MgCl₂ and Ca(NO₃)₂ of 0·025 M were applied at unsaturated water flow rates of between 3 and 13 mm h^?1. Solute movement was monitored over several weeks by collecting effluent under suction at the base. In the sandy loam anion transport was influenced by exclusion from the double layer, whereas in the Ramiha soil anion adsorption occurred. Cation transport was described by coupling the convection-dispersion equation with cation exchange equations. Good simulations of the Mg²⁺ and Ca²⁺ concentrations in the effluent and on the exchange sites were obtained if 80% of the exchangeable cations, as measured using the 1 M ammonium acetate method, were assumed to be active. Local physical or chemical disequilibrium did not need to be explicitly taken into account. About 400 kg ha^?1 of native potassium was leached from the alluvial soil, but only about 10 kg ha^?1 was leached from the aeolian soil. The convection-dispersion equation coupled with exchange theory was found to describe cation transport under unsaturated flow through undisturbed soil satisfactorily.     

Nitrogen accumulation in nodulated and non‐nodulated pea plants,grown in a sandy soil at different acidities

Abstract

The growth of nitrate‐supplied and dinitrogen‐fixing pea plants was studied in a pot experiment with a sandy soil in a pH‐H_?O range from 3.4 to 5.6. Optimum growth in both treatments occurred at pH 5.0. At low pH, N₂‐plants yielded significantly less than NO₃‐plants. Planting of nodulated seedlings did not enhance yield in comparison with sowing in inoculated soil, indicating that nodulation was not the most sensitive process in restricting yield. Comparison of the nitrogen contents of shoots of planted and sown N₂‐plants allowed the suggestion that the synthesis of nitrogenous compounds was also not limiting yield. At low pH, root growth was severely reduced in dinitrogen‐fixing plants in comparison with nitrate‐supplied plants. This difference could be explained by the influence of the form of nitrogen nutrition on the cation‐anion uptake pattern of the plant and the resulting pH‐shift in the rhizosphere. It is to be expected that in an acid soil under field conditions the indirect effect of nitrate on root growth and nodulation via increase of the pH is more extensive than its direct negative effect on nodulation.     

Seasonal levels of water‐extractable cations and anions in soil under blight‐affected and healthy citrus trees

Abstract

Soil samples were collected under blight‐affected and healthy citrus trees at 30‐day intervals for 24 months, at 20‐to 50‐cm depth. Analyses of water extracts (1:1 soil: water) for K, Ca, Mg, Na, SO₄, and Cl showed few differences in cations, but lower anion levels under blighted trees. The cation/anion ratio was significantly higher under blight‐affected trees. Samples collected once at different locations in the same time period showed the same differences. In one location, K was lower and Na higher under healthy trees than under blight‐affected trees, in others K was higher too.     

Comparison of soil zinc extractants for detection of applied zinc and prediction of leaf zinc concentration

Abstract

Many soil extractants have been developed for determination of zinc (Zn) availability to plants. The optimum soil Zn extractant should be useful not only for prediction of plant Zn concentration but also for detection of applied Zn levels. The objectives of this study were: i) to compare soil Zn extradants for detecting applied Zn and for predicting peanut leaf Zn over a range of soil pH levels, and ii) to correlate other soil‐extractable Zn levels with Mehlich‐1. Soil and peanut leaf samples were taken from a field study testing pH levels as the main plots and Zn application rates in the sub‐plots. Extractable Zn was determined on soil samples using Mehlich‐1, Mehlich‐3, DTPA, MgNO₃, and many dilute salt extradants of varied strength and pH. Correlation of extractable soil Zn to cumulative applied Zn levels revealed Mehlich‐1, Mehlich‐3, DTPA, and AlCl₃ extradants to be among the best indicators of applied Zn. Leaf Zn concentration was best correlated with soil Zn extracted by dilute salts, such as KCl, CaCl₂, NH₄Cl, CaSO₄, and MgCl₂. Including soil pH as an independent variable in the regression to predict leaf Zn considerably improved R‐square values. The DTPA‐extractable soil Zn levels were very well correlated with Mehlich‐1‐extractable Zn. Mehlich‐3 extracted about 20% more soil Zn than Mehlich‐1, but Mehlich‐3 soil Zn was not as well correlated to Mehlich‐1 soil Zn as DTPA soil Zn. Lower pH solutions extracted more of the applied Zn, but more neutral solutions extracted Zn amounts which were better correlated with Zn uptake. On the other hand, Mehlich‐1, which had a lower pH, had better correlations with both applied Zn and leaf Zn than did Mehlich‐3. Shortening the DTPA extraction time to 30 minutes resulted in better correlations than the standard two hour extraction time. Chloride (Cl) was the best anion tested in relation to soil applied Zn recovery in combination with potassium (K), calcium (Ca), and aluminum (Al), and Cl optimized leaf Zn correlations for ammonium (NH₄), K, Ca, and magnesium (Mg). The larger the valence of the cation, the better the correlation with applied Zn and the poorer the correlation with leaf Zn.     

Increase in Cation Adsorption Induced by Surface Complexation of Sulfate on Andisols and Prediction by “Four-Plane Model”

Abstract

The “Four-plane model” is one of the surface complexation models developed for analyzing the electrostatic charge of synthetic oxides. This model which was applied to the B horizon of Andisols, was equilibrated with an electrolyte solution containing KCl, KNO₃, K₂SO₄ in the concentration range of 0.0033, 0.01, 0.033, 0.1 M, and pH range of 2 to 11. The amounts of NO₃ ^-, Cl^-, SO₄ ^2-, and K⁺ adsorbed were determined from the products remaining in the liquid phase. The increase in the pH value results in the increase of K⁺ adsorption, and decrease of anion adsorption. The increase in the equilibrium concentration increased both K⁺ and anion adsorption. The parameters of the model were determined by measurements and the iterative method. The model have a good agreement between calculated and measured values of cation and anion adsorption. Among the anion species, the amount of adsorbed anions was in the order of SO₄ ^2-?Cl^->NO₃ ^-. The differences in the degree of adsorption were mainly associated with the differences in the equilibrium constants of surface complexation. The surface complexation of the anions produced a negative charge and increased cation adsorption. The degree of surface complexation was most significant in the K₂SO₄ solution, and the phenomenon was considered to be important not only for anion adsorption but also for cation adsorption on Andisols.     

Effect on plant growth and nutrient uptake of a saturated gypsum solution given to a soil rich in CA

Abstract

Barley plants were grown in 201 pots containing a sandy soil rich in exchangeable and watersoluble Ca. Results from earlier experiments have indicated that the mode of action of supplementary Ca may differ according to, for example, the associate anion. In this experiment soil‐Ca was activated by placing NH₄NO₃ at three depths in the soil and by adding solutions of Ca salts. Yields were found to increase with successively deeper placements of NH₄NO₃ in treatments without Ca application, whereas only small differences between placement depths were observed when Ca was added as a saturated gypsum solution or equivalent amounts of CaCl₂. The very pronounced responses to Ca application were in good agreement with visual symptoms of Ca deficiencies later in the season and with the nutrient uptake rates and growth rate over the intire growth period.     

有机无机改良剂对滨海盐渍化土壤酶活性和土壤微生物量的影响

[目的]为解决滨海地区土壤盐分高和生态环境恶劣的问题,研究发酵园林废弃物与膨润土不同比例配合施用对滨海盐渍土的改良效果。[方法]通过滨海地区田间试验,采用单独施用68 kg/m~3发酵园林废弃物(T_1)、单独施用15 kg/m~3膨润土(T_2)和二者混合施用(T_3)的方法,分析不同处理组土壤酶活性、微生物量碳、氮的变化及其与土壤理化性质的相关关系。[结果]有机无机改良剂混施(T_3)在提高土壤酶活性和微生物量碳、氮方面具有更显著的效果。脲酶、蔗糖酶和脱氢酶分别为对照的10.1,9.0和4.5倍;土壤微生物量碳、氮分别比对照提高了24.8%和78.1%。此外,混施也可以显著改善土壤理化性质,使土壤盐分降低了62.7%,养分各项指标增幅为57.2%～101.4%。同有机改良剂处理相比,无机改良剂对土壤酶活和土壤微生物量的影响较小。速效钾与速效氮是影响土壤酶活性与微生物量的主要因子,而含盐量、容重则与土壤酶和微生物量呈负相关,具有抑制作用。[结论]发酵园林废弃物的加入对土壤酶活性和微生物量的增加起到了决定性的作用。最佳施用处理组为原土混合掺拌68 kg/m~3发酵园林废弃物和15 kg/m~3膨润土。     

Environmental Implication of Soils Amended with Anaerobically Digested Sewage Sludge in Hong Kong

Rainfall leaching soil column trials wereused to characterize the downward movement ofpotential contaminants through a sandy loam and sandysoil following the application of an anaerobicallydigested sewage sludge at the rates of 10 and 25%(v/v). Leachate pH did not vary significantly withsludge application except for sandy loam with 25%sludge, while initial electrical conductivity (EC) anddissolved organic carbon (DOC) increased linearly withsludge application and declined shortly to levelsfound in soils without sludge. A higher initialleaching loss of ammonium (NH₄ ⁺) was found in sandy soilthan loamy sand due to its lower cation exchangecapacity. Nitrate (NO₃ ^-) was the dominant anion inleachates with an average in excess of 10 mg L^-1 NO₃ ^- at all loading rates after 12 weeks. The highestconcentration of NO₃ ^- occurred with the highest sludgeapplication rate. Leachate zinc (Zn) content increasedin loamy sand columns at the high sludge loading rateat the end of the experiment owing to the reduced pHfollowing nitrification. No significant difference inleachate copper (Cu) and phosphate (PO₄ ^3-) contents werenoted for both soils receiving various sludgeapplication rates. Evaluation of the soluble nutrientspresent in the soil profiles at the end of theleaching experiment showed that EC, NH₄ ⁺ and PO₄ ^3- increased according to sludge application rate up toa depth of 20 cm. Significant accumulation of NO₃ ^- wasfound in sandy loam with sludge application to thedepth of 50 cm. Analyses of leachates and soils forthe selected contaminants revealed that NO₃ ^- leaching islikely to occur without plant growth at the currentapplication rate. Therefore, the application rate forsludge should not exceed 10% (v/v), and the provisionof vegetation on the amended soil would reduce theleaching loss of NO₃ ^-.     

Effects of biochar application in forest ecosystems on soil properties and greenhouse gas emissions: a review

Purpose

Forests play a critical role in terrestrial ecosystem carbon cycling and the mitigation of global climate change. Intensive forest management and global climate change have had negative impacts on the quality of forest soils via soil acidification, reduction of soil organic carbon content, deterioration of soil biological properties, and reduction of soil biodiversity. The role of biochar in improving soil properties and the mitigation of greenhouse gas (GHG) emissions has been extensively documented in agricultural soils, while the effect of biochar application on forest soils remains poorly understood. Here, we review and summarize the available literature on the effects of biochar on soil properties and GHG emissions in forest soils.

Materials and methods

This review focuses on (1) the effect of biochar application on soil physical, chemical, and microbial properties in forest ecosystems; (2) the effect of biochar application on soil GHG emissions in forest ecosystems; and (3) knowledge gaps concerning the effect of biochar application on biogeochemical and ecological processes in forest soils.

Results and discussion

Biochar application to forests generally increases soil porosity, soil moisture retention, and aggregate stability while reducing soil bulk density. In addition, it typically enhances soil chemical properties including pH, organic carbon stock, cation exchange capacity, and the concentration of available phosphorous and potassium. Further, biochar application alters microbial community structure in forest soils, while the increase of soil microbial biomass is only a short-term effect of biochar application. Biochar effects on GHG emissions have been shown to be variable as reflected in significantly decreasing soil N₂O emissions, increasing soil CH₄ uptake, and complex (negative, positive, or negligible) changes of soil CO₂ emissions. Moreover, all of the aforementioned effects are biochar-, soil-, and plant-specific.

Conclusions

The application of biochars to forest soils generally results in the improvement of soil physical, chemical, and microbial properties while also mitigating soil GHG emissions. Therefore, we propose that the application of biochar in forest soils has considerable advantages, and this is especially true for plantation soils with low fertility.