Exchangeable cations in rock fractions and fine earth in soil profiles of different genesis

Data on accumulated exchangeable H, Al, Fe and Mn (M_a) cations in rock fractions in German soil profiles are scarce. The objective of this study was to describe the sum of accumulated M_a cations of fine earth and rock fragments in 11 deep soil profiles of varying genesis. Soil profiles were laid out at the sites Solling, Eifel, Harz mountains and the Erzgebirge and the parent materials included sandstones, siltstones, quartzite, slate, greywacke, diabase, gneiss and quartz porphyry. Exchangeable cations in the fine earth and rock fragments were measured in depths down to 6 m. Additionally, effective porosity and specific surface of rock fragments were determined. The effective porosity of the different rock fragments ranged from 4 to 28% (v/v), indicating that the rocks were accessible to solutions. For most samples, the cation exchange capacities (CEC) of the fine earth fractions were larger than those of the rock fragments, and the CEC (fine earth)/CEC (rock) ratios decreased with depth. All 11 profiles had small (<40%) amounts of exchangeable Na, K, Mg and Ca (M_b) cations in the fine earth fraction. Exchangeable M_a and M_b cations in the rock fragments changed similarly with depth as in the fine earth fractions for all profiles. Cumulative (rock + fine earth) M_a cations from 0—200 cm ranged from 474 to 1592 kmol_c ha⁻¹. The contribution of the rock fraction to the cumulative exchangeable M_a cations accounted for 13 to 85% of the total. The sum of exchangeable M_a cations was much higher than the cumulative acid deposition in western Germany since the beginning of industrialization, suggesting that carbonic acid and organic acids contributed largely to soil acidification. The rocks contribute significantly to buffering the acidity of the seepage water by silicate weathering and cation exchange. Therefore, acidification models which consider the fine earth fraction only, may lead to an overestimation of the rate of soil and groundwater acidification.     

The effect of five years acid treatment on leaching,soil chemistry and weathering of a humo-ferric podzol

This paper describes the effect of treating a nutrient-poor forest soil in monolith lysimeters with H₂SO ₄, pH 3.0, for 4.75 yr. The lysimeters were instrumented with porous cup probes to distinguish processes occurring in each soil horizon. In the A horizon base cation exchange and sulphate absorption were the principal proton- consuming processes whereas lower down the profile Al³⁺ dissolution from hydrous oxides dominated. Acid treatment thus reduced the amount of amorphous Al in the lower horizons, but exchangeable Al was unaffected. Sulphate absorbtion was positively correlated with the distribution of Al hydrous oxides. High rates of nitrification reduced the differences between acid and control monoliths, but acid treatment significantly reduced soil pH down to 75 cm and reduced the levels of exchangeable base cations in the litter and A horizons. Acid treatment increased the leaching rates of base cations and Al. Consideration of the total base cation content shows that acid treatment increased the rate of weathering by 0.7–1.4 k eq ha^?1 yr^?1. The results should be useful in modelling more realistic rates of acid input to similar soils.     

Chemical Characteristics and Acid Buffering Capacity of Surface Soils in Japanese Forests

To determine the geological distribution of acid buffering capacity and exchangeable Al of forest soils in Japan, surface soils under forest vegetation were collected nationwide from a total of 1,034 sites. Generally, surface soils in Japanese forests are mostly acidic and low in both exchangeable cation content and exchangeable Al. The median of soil pH(H₂O), total exchangeable cations, and exchangeable Al are 5.1, 76 mmol₍₊₎Kg^?1, and 19.6 mmol₍₊₎kg^?1, respectively. Acid buffering capacities of selected soils were determined using a soil column, which was comparable to the capacity that resulted from cation exchanges with protons. Soils with high buffering capacity and low exchangeable Al are widely distributed in Japan, and overlap with the areas of Andisol distribution. Volcanogenic materials seem to mask soil characteristics derived from underlying geology even though they are not Andisols. However, central to western Honshu Island, Shikoku Island, and northern Kyushu Island showed weak acid buffering capacities with high exchangeable Al potential in surface soils.     

Differential soil acidification caused by parent materials and land-use changes in the Pearl River Delta region

Soil acidification occurs widely across the world, which has been partly attributed to land-use change. However, measureable effect of land-use change as well as parent materials on soil acidification remains poorly understood. Here, a typical area with intensive land-use change in the Pearl River Delta of China was chosen for this study. Topsoil (0–20 cm) and subsoil (20–40 cm) samples (n = 169) under different land uses (paddy fields, vegetable lands and orchards) and parent materials (granite and alluvial sediment) were collected in 2020. Soil pH, exchangeable base cations, exchangeable acidity and pH buffering capacity were measured to evaluate the status of soil acidification. The change of soil pH over the last 15 years was evaluated via comparing with historical data (n = 329) in 2005. The results showed a higher exchangeable acidity and lower pH buffering capacity and exchangeable base cations of soils derived from granite compared with soils derived from alluvial sediment in 2020. In the last 15 years, significant soil acidification under different parent materials was observed under vegetable lands and orchards but not paddy fields. Faster pH decline was found under land-use change from paddy fields compared with the unchanged vegetable lands or orchards. Furthermore, stronger acidification under the same land-use change was observed for soils derived from granite compared with soils derived from alluvial sediment. These results indicate that land-use change induced soil acidification is dependent on parent materials. This study implies that cropping management such as suitable rotation operation may slow soil acidification, and measures including straw returning may ameliorate acidified soils.     

不同利用方式对潮棕壤交换性钾钠及盐基总量的影响

为深刻认识土地利用变化对土壤交换性离子的影响,本文对潮棕壤水稻田、玉米地、撂荒地和人工林地4种土地利用方式经过14年后,在0～150cm剖面中土壤交换性钾、钠、交换性盐基总量的剖面分布及交换性钾钠比值变化进行了比较研究。结果表明,林地和撂荒地各土层交换性K含量及土体中交换性K储量具有高于水稻田和玉米地的趋势;水稻田和撂荒地0～20cm各土层的交换性Na含量显著高于林地和玉米地;林地0～150cm深度土壤交换性Na储量显著高于其他三种利用方式(P<0.05);0～150cm深度内土壤交换性盐基总储量大小依次为林地、玉米地、撂荒地、水稻田;水稻田剖面土壤交换性K/Na随土层深度增加而增大,其他3种土地利用方式则是随土层深度增加而降低。土壤管理和植物自身的特性在土壤剖面中交换性离子的构成以及土壤交换性盐基库的重建等方面可能起到重要作用。     

不同灌溉方式对保护地土壤酸化特征的影响

自连续13a在同一地块以不同灌溉方式进行灌溉试验的保护地,分层采集沟灌、滴灌、渗灌3个处理0～60cm土层土壤样品,研究灌溉方式对土壤酸化特征的影响。结果表明,3种灌溉处理土壤活性酸度和交换性酸含量均随着土层加深而降低,各处理间土壤活性酸度在0～40cm土层差异明显,总体为沟灌>渗灌>滴灌;土壤交换性酸差异出现在0～30cm土层,为渗灌>沟灌>滴灌;土壤交换性Al3+随土层加深呈先增加后降低的变化趋势,且以滴灌含量最低。各处理土壤盐基饱和度(BS)随土层加深而增加,在0～30cm土层为滴灌>渗灌>沟灌。土壤pH与交换性酸、硝态氮含量呈极显著负相关,与盐基饱和度、特别是Ca2+饱和度呈极显著正相关;Al3+占交换性酸比例与有机质含量呈极显著负相关。总之,保护地土壤酸化与硝态氮含量、盐基饱和度、有机质含量关系密切;与沟灌和渗灌相比,滴灌更利于抑制土壤酸化。     

Zur Bestimmung austauschbarer Kationen in sauren Waldböden

On the determination of exchangeable cations in acid forest soils Different samples from acid forest soils were percolated with large amounts of H₂O. Significant amounts of anions, especially sulfate, were found in the percolates mainly accompanied by Na. K, Ca and Mg (M_b-cations). The dissolution of Al-Sulfates and subsequent exchange of M_b-cations by Al as dominant mechanism is proposed. Thus the common method for determination of the cation exchange capacity (CEC) of acid forest soils, the percolation with NH₄Cl may overestimate the CEC. The overestimation may be related to the sulfate content of the soil and also influences the calculation of relative CEC proportions of individual cations.     

Simulated acid rain effects on acidification of a Lexington silt loam

Intact soil cores were collected to a depth of 15 cm from a Lexington silt loam. Simulated precipitation with adjusted pH values of 3.7, 4.7 and 5.7 (control), was applied to the cores in increments of 500 mL day^?1 until totals which approximated 10, 20, 40, and 80 yr of effective rainfall in Louisiana were reached. The exchangeable acidity and Al and H₂O and KC1 pH were measured from 2.5 cm sections of the cores after treatment. Only the 3.7 treatment at the 80 yr volume significantly affected the soil pH and exchangeable acidity and Al.     

The sensitivity of Swiss forest soils to acidification and the risk of aluminum toxicity

In the context of pollution‐control strategies to minimize the detrimental effects of soil acidification, there is a need to know how and to what extent soils respond to acidifying substances. The purposes of this study were to assess the sensitivity of soil to acidification, in particular to a decrease in pH and in base saturation (BS), and the risk of Al toxicity for vulnerable plants using chemical indicators. These indicators were derived from soil data (pH, exchangeable cations, amount of fine earth) measured in the mineral horizons of 257 soil profiles throughout Switzerland. Based on the analysis of the distribution of pH and BS values in the soil collective, we assessed the sensitivity of soils to a decrease in pH and in BS. Soils that were considered sensitive to a decrease in pH had pH values between 4.8 and 7.0. The degree of sensitivity was estimated with the proportion of fine earth in the critical pH range to a depth of 100 cm. Soils that were considered sensitive to a decrease in BS had pH values between 3.6 and 5.5 and a BS between 10% and 95%. Since the effective cation‐exchange capacity (CEC_eff) of the fine earth might dampen the decrease in BS when acidity is added, the disposition for a decrease in BS was related to the relative amount of fine earth in the sensitive BS and to the mean CEC_eff of this fine‐earth fraction. The risk of Al toxicity for vulnerable plants was estimated using the ratio of base cations to Al at the cation‐exchange sites (BC : Al_exc). A BC : Al_exc of 0.2 was taken as a threshold value below which the risk for sensitive plants increases. The degree of risk was based on the proportion of fine earth in the critical BC : Al range (≤0.2) in the soil profile. These indicators taking into account the various aspects of soil acidification are derived from usually available data and represent therefore a cost‐effective tool to assess the sensitivity of soils to an input of acidity.     

Spatial variability of soil solution chemistry under Hinoki cypress (Chamaecyparis obtusa) in Tama Hills

A monitoring study on precipitation and soil solution was conducted to analyze soil acidification processes at the Rolling Land Laboratory (RLL), Hachioji, Tokyo based on the spatial variability of the soil solution chemistry around the Hinoki cypress (Chamaecyparis obtusa) trunk. Soil solution samples were taken at various distances from the tree trunks and at various depths. Soil solution pH at the depth of 10 cm decreased to 4.1–4.2 on the downslope side of large tree trunks, presumably due to the heterogeneity of throughfall input and extensive infiltration of acidic stemflow. Ammonium ions brought by throughfall and stemflow were nitrified and provided large amounts of H⁺. Protons were replaced with exchangeable cations. When base cations were depleted, aluminum ion became the dominant cation species. On the average, Ca²⁺ concentration in the soil solutions at the depth of 10 cm decreased from 0.28 mmol_c L^-1 at the reference site to 0.18 mmol_c L^-1 on the downslope side and Mg²⁺ concentration decreased from 0.30 mmol_c L^-1 to 0.15 mmol_c L^-1. Arithmetic mean aluminum concentration at the depth of 10 cm on the downslope side was 0.35 mmol_c L^-1. Here aluminum dissolution was the main acid sink. Based on the spatial variability of the soil solution chemistry, soil solution acidification processes were divided into four stages.     

Chemical effects of pH 3 sulphuric acid on a soil profile

Cores of podzolic soil (monolith lysimeters) were treated for 4.8 yr with 1500 mm yr^?1 of either 0.5 mM H₂SO₄ at pH 3, equivalent to 24 g S m^?2 yr^?1 (acid treated) or distilled water (controls). The acid treatment was about 37 times greater than the average annual input of H₃O⁺ from rain at the site from which the monoliths were taken. Acid treatment acidified the litter (from pH(CaCl₂)3.4 to pH(CaCl₂)2.6) and the mineral soil to a depth of 80 cm (mean pH(CaCl₂) decrease of 0.2 unit). In the litter and upper A horizon, ion-exchange reactions provided the main neutralizing mechanism, resulting in a decrease in the reserves of extractable (in 2.5 % acetic acid) Ca, Mg, and Mn of about 70 to 80 %. Dissolution of solid phase Al from hydrous oxides provided most neutralization below this depth. Al3⁺ was the principal soluble Al species throughout the profile. In the litter and upper A horizon, some of the mobilized Al³⁺ was retained on cation exchange sites resulting in an increase in exchangeable Al. Deeper in the profile, where the exchange sites were effectively saturated with Al³⁺, no increase in exchangeable Al occurred, and Al³⁺ was, therefore, available for leaching. Some reversible adsorption of SO₄ ^2?, associated with hydrous Al oxides, occurred in the Bs and C horizons. The results are discussed in relation to possible effects of acid deposition over regions of Europe and N. America.     

Soil Acidification and Solute Budgets for Forested Lysimeters in Nordrhein-Westfalen

Solute budgets and nitrogen use were quantified in two 400 m² forested lysimeters in St. Arnold, Nordrhein-Westfalen. The lysimeters are covered by a mixture of oak-beech and Weymouth pine, respectively. The average bulk deposition between May 1985 and May 1987 of NH, SO and NO₃ was 1.1, 1.7, and 0.4 kmol_c ha^?1 yr^?1 in the deciduous stand and 2.1, 2.1, and 0.8 kmol_c ha^?1 yr^?1 in the coniferous stand. The input of N is almost completely retained in the deciduous stand. In the coniferous stand about 30% of this N-input is leached as NO₃. Due to N-transformations, total proton turnover is 4.4 kmol_c ha^?1 yr^?1 in the coniferous stand and only 2.5 kmol_c ha^?1 yr^?1 in the deciduous stand. Ca-mobilization is the major acid buffering process in both lysimeters. Only the deciduous stand was limed in 1980 (90 kmol_c/ha). Mobilization of Al is only relevant down to a soil depth of 30 cm. Below a 30 cm depth, Al is immobilized. The amounts of exchangeable and silicate-bound Ca in the soil underlying the coniferous stand are very small, but no evidence was found for explanation of the observed high Ca-mobilization by artificial Ca-sources.     

Lysimeter study with a cambic arenosol exposed to artificial acid rain: II. Input-output budgets and soil chemical properties

The effects of artificial precipitation with different pH levels on soil chemical properties and element flux were studied in a lysimeter experiment. Cambic Arenosol (Typic Udipsamment) in monolith lysimeters was treated for 6 1/2 yr with 125 mm yr^?1 artificial rain in addition to natural precipitation. Artificial acid rain was produced from groundwater with H₂SO₄ added. pH levels of 6.1, 4 and 3 were used. ‘Rain’ acidity was buffered, mainly due to cation exchange with Ca²⁺ and Mg²⁺, which were increasingly leached due to the acid input. The H⁺ retention was not accompanied by a similar increase in the output of Al ions, but a slight increase in the leaching of Al ions was observed in the most acidic treatment. The net flux of SO₄ ^2? from the lysimeters increased with increasing input of H₂SO₄, but in the most acidified lysimeters significant sorption of SO₄ ^2? was observed. The sorption was, however, most likely a concentration effect. The ‘long-term’ acidification effects on soil were mainly seen in the upper O and Ah-horizons, where an impoverishment of exchangeable Ca²⁺ and Mg²⁺ was observed. An increased proportion of Al ions on exchange sites in the organic layer was observed in the pH 3-treated soil. By means of budget calculations the annual release of base cations due to weathering was estimated to be between 33 and 77 mmol_c m^?2.     

Spatial heterogeneity of soil solution chemistry in a mature Norway spruce (Picea abies (L.) Karst.) stand

The determination of the average soil solution concentrations in forest soils is hindered by the spatial heterogeneity of the soil conditions and the stand structure on all scales. The aim of this paper is to investigate the spatial heterogeneity of the soil solution chemistry within a mature stand of Norway spruce and to evaluate the implication of this heterogeneity for the sampling design for soil solutions. The site is a 140 years old Norway spruce stand of 2.5 ha located in the German Fichtelgebirge at 800 m elevation on granitic, deeply weathered bedrock. At 35 cm soil depth, 59 ceramic suction lysimeters (5 cm length, 2 cm diameter) were installed in a systematic grid of 25 · 25 m and soil solution was sampled at 3 dates in June and July 1994. The solutions were analysed for major cations and anions. Semi-variance of the concentrations at a given date revealed no systematic spatial patterns. The coefficients of variance of the element concentrations were between 36 and 298% with highest values for NH₄ ⁺-N. The implications of the observed heterogeneity for the appropriate number of replicates was investigated by Monte Carlo simulations. As an example, the probability that the measured average concentration of SO₄ ^2?-S is outside a ±10% range (related to the ‘true’ 59 lysimeter average) is about 68% if only 3 replicates would have been used, 41% with 10 replicates and 25% with 20 replicates. Due to the generally large spatial heterogeneity of the soil solution chemistry in forest soils the number of lysimeters used must be carefully adjusted to site conditions and the specific question.     

南方典型红壤区旱地与水田土壤酸度的剖面差异性

为探明红壤区不同耕地利用类型下土壤酸度的剖面变化特征及其主要影响因素,选取江西省余江县和湖南省祁阳县的典型水田、旱地两种耕地利用类型下、第四纪红色黏土母质发育的红壤,分5层(0～20、20～40、40～60、60～80和80～100 cm)测定土壤pH、交换性铝、交换态盐基阳离子及有机质含量等指标,分析剖面酸度特征及其相互关系。结果表明：土壤pH均随土层深度的增加呈增加趋势,不同耕地利用类型下以水田剖面p H较高,范围为5.80～6.43,旱地剖面p H较低,范围为4.68～5.41。土壤交换性铝含量以水田含量较低,范围为0.16～1.56 cmol/kg,旱地的含量较高,范围为4.22～7.02 cmol/kg,水田的交换性铝含量随土层深度的增加呈降低趋势,旱地则呈现相反的变化趋势。0～20 cm土层的交换性铝与有机质含量呈显著负相关,40～100 cm土层的交换性铝与交换态盐基阳离子含量呈显著负相关。耕地利用类型是影响土壤酸度的主要因素之一,旱地土壤酸度强于水田。增加耕层土壤有机质含量可能是减缓酸化、降低交换性铝含量的策略之一。     

Effects of fertilization practices on aluminum fractions and species in a wheat soil

Purpose

To better understand the effect of fertilizer practices on soil acidification and soil organic matter (SOM) stocks in a rice-wheat system, a field experiment was conducted to (i) investigate the influence of fertilizer practices on the Al forms in solid phases and the distribution of Al species in water extracts and (ii) explore the relationship between the Al forms, the quantity and composition of SOM, and soil acidity.

Materials and methods

Seven fertilizer treatments including CL (no fertilizer), NK, PK, NPK, N₂PK (PK and 125 % of N), NP₂K (NK and 125 % of P), and organic fertilizer (OF) were applied to induce various changes in pH and SOM composition (i.e., total C and N contents, C/N ratio, and SOM recalcitrant indices) in a rice-wheat system. After 6-year cultivation, different pools of Al forms (i.e., amorphous Al; organically bound Al of varying stability; exchangeable Al; water-soluble inorganic Al³⁺, Al-OH, Al-F, Al-SiO₃, and Al-SO₄; and organic Al monomers) were quantified and related with SOM composition and soil pH during the wheat phase.

Results and discussion

Fertilizer types significantly changed soil pH and SOM composition and which explained 84 % of the variance of Al forms using redundancy analysis. An interaction between soil pH and SOM quality on Al forms also existed but only accounted for a very small (6 %) portion of the variation. Compared to CL and chemical fertilizer, OF practice with relative low SOM stabilization is likely to favor the formation of amorphous Al in order to bind more SOM. The decrease in exchangeable acidity and water-extractable Al via hydroxyl-Al precipitation but not in the form of organo-aluminum complexes evidenced this phenomenon. In contrast, chemical fertilizer input increased exchangeable Al and water extract Al (especially Al³⁺), partly at the expense of organically bound Al. The destabilization of organic-aluminum complexes was a mechanism of pH buffering evidenced by the increased soluble Al-dissolved organic matter (DOM) as soil pH decreases. Further, the magnitude of this trend was much greater for elevated N input compared with P input.

Conclusions

Chemical fertilizer with relative high SOM stabilization favored the formation of exchangeable Al and soluble Al resulting in soil acidification, whereas OF with relative low SOM stabilization tended to transform exchangeable Al and soluble Al to amorphous Al, thereby alleviating soil acidification and enhancing C stocks in a rice-wheat system.

     

Simulation of the long-term soil response to acid deposition in various buffer ranges

A soil acidification model has been developed to estimate long-term chemical changes in soil and soil water in response to changes in atmospheric deposition. Its major outputs include base saturation, pH and the molar Al/BC ratio, where BC stands for divalent base cations. Apart from net uptake and net immobilization of N, the processes accounted for are restricted to geochemical interactions, including weathering of carbonates, silicates and Al oxides and hydroxides, cation exchange and CO₂ equilibriums. First, the model's behavior in the different buffer ranges between pH 7 and pH 3 is evaluated by analyzing the response of an initially calcareous soil of 50 cm depth to a constant high acid load (5000 mol_c ha^?1 yr^?1) over a period of 500 yr. In calcareous soils weathering is fast and the pH remains high (near 7) until the carbonates are exhausted. Results indicate a time lag of about 100 yr for each percent CaCO₃ before the pH starts to drop. In non-calcareous soils the response in the range between pH 7 and 4 mainly depends on the initial amount of exchangeable base cations. A decrease in base saturation by H/BC exchange and Al/BC exchange following dissolution of Al³⁺ leads to a strong increase in the Al/BC ratio near pH 4. A further decrease in pH to values near 3.0 does occur when the A1 oxides and/or hydroxides are exhausted. The analyses show that this could occur in acid soils within several decades. The buffer mechanisms in the various pH ranges are discussed in relation to Ulrich's concept of buffer ranges. Secondly, the impact of various deposition scenarios on non-calcareous soils is analyzed for a time period of 100 yr. The results indicate that the time lag between reductions in deposition and a decrease in the Al/BC ratio is short. However, substantial reductions up to a final deposition level of 1000 mol_c ha^?1 yr^?1 are needed to get Al/BC ratios below a critical value of 1.0.     

陕西(土娄)土中硝态氮运移特点及影响因素

利用不同深度的渗漏池研究了陕西(土娄)土中NO₃-N运移特点及影响因素。结果表明,NO₃-N淋失量与土壤深度呈指数曲线关系,与施N量呈线性相关;NO₃-N淋移深度随地而接水量(降水量加灌水量)的增加而增大;NO₃-N在土壤剖面中的分布大部分都集中在0～60cm,含量高峰一般出现在20～40cm;不同施N方法对NO₃-N的淋失和在土壤中的积累都有明显的影响。     

四环素类抗生素在土壤和堆肥中的吸附和降解

Two agricultural soils were collected from Dahu and Pinchen counties and swine manure compost (SMC) from Ping-tung County in Taiwan, China to investigate the sorption and dissipation of three tetracyclines (TCs), i.e., oxytetracycline (OTC), tetracycline (TC) and chlortetracycline (CTC), in compost, soils and soil/compost mixtures with different organic carbon (OC) contents. There were seven treatments in total. TCs were most strongly adsorbed to SMC in all treatments due to the high OC content. When SMC was present in the soils, the sorption of TCs was significantly enhanced, which might be attributed to the increased OC content and CEC. The adsorption of TCs showed non-linear adsorption isotherms and fitted well to the Freundlich model. After 49 d of incubation at 25 ℃ in soils and soil/compost mixtures in the dark, TCs elapsed in all substrates, with the time required for 50% degradation (DT₅₀) between 20 and 41 d, and the time for 90% degradation (DT₉₀) between 68 and 137 d. Soil amended with compost enhanced the stability of TCs and reduced their mobility. The dissipation of TCs in a soil environment was slow, indicating that these compounds might be persistent in soil.     

Effects of nitrogen fertilizer on the acidification of two typical acid soils in South China

Purpose

A laboratory incubation under constant temperature and humidity was conducted to estimate the impacts of nitrogen (N) fertilizers on the acidification of two acid soils (Plinthudult and Paleudalfs) in south China.

Materials and methods

The experiment had three treatments, i.e., control (CK), addition of urea (U), and addition of ammonium sulfate (AS). We measured soil pH, nitrate (NO₃ ^?), ammonium (NH₄ ⁺), exchangeable hydrogen ion (H⁺), and aluminum ion (Al³⁺) concentrations at various intervals during the 90 days of incubation. Soil buffering capacity (pHBC) was also measured at the end of the experiment.

Results and discussion

The application of N fertilizers resulted in soil acidification. The U treatment caused greater acidification of the Plinthudult soil than the AS treatment, while there were no differences between U and AS treatments on the acidification of the Paleudalfs. At the end of the trial, the pHBC of Plinthudult in AS treatment was greater than that in CK and U treatments, which may be due to the buffering system of NH₄ ⁺ and NH₄OH. However, the pHBC of Paleudalfs was unchanged between treatments. The dynamics of exchangeable H⁺ and Al³⁺ corresponded to that of soil pH. Correlation analysis showed that both soil exchangeable H⁺ and soil exchangeable Al³⁺ were significantly related to soil pH.

Conclusions

Application of urea and ammonium sulfate caused acidification in both soils and increased soil exchangeable Al³⁺ and H⁺ concentrations in the Paleudalfs. The application of urea increased exchangeable Al³⁺, and ammonium sulfate increased pHBC in the Plinthudult.