In situ long‐term redox potential measurements in a dyked marsh soil

The long‐term measurement of soil redox potential (E_H) by permanently installed Pt electrodes may be restricted by electrode breakdown (electrode rupture and resin leakage) and contamination, especially under wet and strongly reducing soil conditions. The E_H of a slightly alkaline (pH 7.1 to 7.3) Calcaric Gleysol developed from marine sediment in the dyked marsh of Schleswig‐Holstein, Northern Germany, was monitored weekly during a 4‐year period using permanently installed Pt electrodes. Measurements were performed in fivefold at 10, 30, 60, 100, and 150 cm. Furthermore, water table level was recorded. Sulfide occurred in 150 cm as a heritage of the previous marine environment. Mean water table level was 84 cm below the soil surface but was characterized by both short‐term and seasonally strong fluctuations. Levels of water table ranged from 33 to >200 cm below soil surface. In consistence with water table level, the E_H continually decreased with soil depth. Mean redox conditions were oxidizing at 10 (550 mV) and 30 cm (430 mV), weakly reducing at 60 cm (230 mV), and moderately reducing at 100 (120 mV) and 150 cm depth (–80 mV). Soil hydrology differed markedly during the study as expressed by periods of water saturation for each depth. This was reflected by Pt electrodes response, since period of water saturation and E_H were significantly negatively correlated as calculated for each year and depth (r_s = –0.971; n = 20; P < 0.01). The 60‐cm depth was most frequently influenced by water table fluctuations, showed the largest E_H range (920 mV) and the most distinct seasonal pattern in E_H. Good function of the electrodes in this depth can be concluded even after long time of installation in soil. Although established in a sulfide‐bearing environment, three of five electrodes at 150 cm showed a substantial increase (+500 mV) in E_H during summer of the third and fourth years of investigation, which had low water tables. It is not clear whether the non‐response of two electrodes was due to electrode contamination or spatial variation in E_H. When operating in reducing systems, this problem can be circumvented by installing a large number of electrodes or by a regular replacement of electrodes. Using properly constructed and permanently installed Pt electrodes, soil E_H can be monitored for extended periods under wet and reducing soil conditions.     

Comparison of redox potential dynamics in a diked marsh soil: 1990 to 1993 versus 2011 to 2014

As revealed by an earlier study, young diked marsh soils on the west coast of Schleswig‐Holstein (Germany) are characterized by pronounced redox potential (E_H) dynamics. Since soil forming processes occur over a short period of time in these man‐made environments, the impact of pedogenesis on E_H was examined by comparing the E_H dynamics measured from November 1989 to October 1993 (weekly measurements) with those measured from November 2010 to October 2014 (hourly measurements) at the same study site in Polder Speicherkoog, Northern Germany. In addition, the necessity for high resolution E_H measurements was assessed as well as the impact of climate change on E_H. Redox potentials were determined in both monitoring campaigns with permanently installed platinum electrodes at 10, 30, 60, 100, and 150 cm soil depths. Soil properties were determined in November 1989 and in August 2013. In 24 years of soil formation, bulk density was demonstrated to increase by 28.5% and 33.3% in 10 and 20 cm depths, respectively, and the sulfide‐bearing Protothionic horizon lowered from 105 to 135 cm below surface level. Overall, E_H dynamics were similar at all soil depths during both study periods with topsoil compaction not affecting E_H. Annual alterations of E_H were primarily driven by the variable climatic water balance (CWB) and by the corresponding water table (WT) fluctuations. These fluctuations resulted in occasional aeration of the subsoil and subsequent oxidation of sulfides. A forecast of CWB to 2100 predicts an intensified WT drawdown by elevated evapotranspiration rates that should amplify sulfide oxidation. To deduce the soil redox status on a seasonal or annual scale, readings taken at daily intervals are sufficient. To identify biogeochemical processes, it is necessary to monitor E_H on an hourly basis because increases in E_H values of up to 540 mV have been observed within a 24 hour period in temporarily waterlogged horizons.     

Iron-cyanide complexes in soil under varying redox conditions: speciation, solubility and modelling

The distribution of iron‐cyanide complexes between ferrocyanide, [Fe^II(CN)₆]^4–, and ferricyanide, [Fe^III(CN)₆]^3–, in soils on contaminated sites depends on the redox potential, E_H. We carried out microcosm experiments in which ferrocyanide (20 mg l^?1) was added to an uncontaminated moderately acidic subsoil (pH 5.2), and varied the E_H of the soil suspension between 200 and 700 mV over up to 109 days. Ferrocyanide and ferricyanide were analysed by capillary isotachophoresis. At redox potentials ranging from 400 to 700 mV, small amounts of iron‐cyanide complexes were adsorbed, and ferrocyanide was almost completely oxidized to ferricyanide. Decreasing E_H to 200 mV led to nearly complete removal of iron‐cyanide complexes from solution, and the complexes were not mobilized after subsequent aeration (E_H > 350 mV). Under weakly to moderately reducing conditions (E_H ≈ 200 mV), iron‐cyanide complexes were removed from solution by precipitation, which occurred, presumably in the form of e.g. Fe₂[Fe^II(CN)₆], Fe₄[Fe^II(CN)₆]₃ or Mn₂[Fe^II(CN)₆], after reductive dissolution of Mn and Fe oxides. Four different sets of geochemical model calculations were carried out. The species distribution between ferrocyanide and ferricyanide in solution was predicted reliably under varying pH and redox conditions when iron‐cyanide complex concentrations and Fe concentrations, excluding Fe bound in iron‐cyanide complexes, were used in model calculations. In model calculations on the fate of iron‐cyanide complexes in soil, adsorption reactions must be considered, especially under oxidizing conditions. Otherwise, the calculated iron‐cyanide complex concentrations are larger than those actually measured.     

Capacity,selectivity, and reversibility for nitrate exchange of a layered double‐hydroxide (LDH) mineral in simulated soil solutions and in soil

The leaching of nitrate is an important way of N losses from agricultural soils in humid regions. Nitrate leaching is difficult to control as most soils under crop production do not have anion‐exchange properties, and nitrate remains mobile in the solution. The present work evaluated the potential use of a synthetic layered double‐hydroxide (LDH) mineral as a nitrate exchanger in soil. The LDH used was a chloride form of a magnesium‐aluminum layered double hydroxide with the formula: [Mg²⁺_0.82Al³⁺_0.18(OH)₂]^0.18+[(Cl^–)_0.18 0.5(H₂O)]^0.18–. Experiments were carried out in aqueous solutions as well as in soil with the following objectives: (1) to characterize the nitrate adsorption capacity on the LDH, (2) to study its selectivity for nitrate adsorption in solution, (3) to evaluate the reversibility for nitrate exchange, and (4) to study the nitrate adsorption capacity and nitrate diffusion towards the LDH in soil.     

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.     

Changes in the chemical composition of water percolating through the soil profile in a moderately polluted catchment

Throughfall (TF), stemflow (SF), soil solution below the organic layer (SS_org) and at 50 cm depth (SS₅₀), and output with stream water (SW) were measured and analyzed for four years in a moderately polluted forest catchment in southern Poland. The input of water with stemflow was ca. 6% of input with TF. However, due to higher concentrations of most ions in SF, the input of most elements with SF was from 8% to 9%. Sulphate (SO₄ ^2–), chloride (Cl^–) and magnesium (Mg²⁺) were the only ions steadily increasing in concentrations in water percolating through the soil profile. Nitrogen reached the forest floor mainly as ammonium (NH₄ ⁺). In the soil organic layer the NH₄ ⁺ concentration decreased, while concentrations of nitrate (NO₃ ^–) and hydrogen (H⁺) increased, probably due to nitrification. For NO₃ ^–, sodium (Na⁺) and calcium (Ca²⁺), the highest concentrations were found in SS_org and SW. This indicates both efficient cycling in the biotic pool of the ecosystem and intensive weathering processes in the mineral soil below the plant rooting zone. The latter was especially pronounced for Mg and Ca. Concentrations of zinc (Zn), lead (Pb) and cadmium (Cd) were the highest in SS_org and SS₅₀. As this was accompanied by a low pH and constant input of H⁺, NH₄ ⁺ and heavy metal ions to the catchment area, it may pose a serious threat to forest health.     

Impacts of land‐use intensity on soil organic carbon content,soil structure and water‐holding capacity

The impact of land‐use intensity is evaluated through changes in the soil properties in different areas of the traditional central Spanish landscape. Soil organic carbon (SOC) content, bulk density, aggregate stability and water‐holding capacity (WHC) in the topsoil of active and abandoned vineyards, livestock routes (LR) and young Quercus afforested areas were analysed. These different types of land use can be interpreted as having a gradient of progressively less impact on soil functions or conservation. As soil use intensity declines, there is an increase in SOC content (from 0.2 to 0.6%), WHC (from 0.2 to 0.3 g H₂O per g soil) and aggregate stability (from 4 to 33 drop impacts). Soils beneath vines have lost their upper horizon (15 cm depth) because of centuries‐old tillage management of vineyards. Except for an increase in bulk density (from 1.2 to 1.4 g/cm³), there were no differences in soil characteristics 4 yr after the abandonment of vine management. LR can be considered sustainable uses of land, which preserve or improve soil characteristics, as there were no significant differences between topsoil from LR and that from a 40‐yr‐old Quercus afforested area. SOC content, one of the main indicators for soil conservation, is considered very low in every case analysed, even in the more conservative uses of land. These data can be useful in understanding the slow rate of recovery of soils, even after long‐term cessation of agricultural land use.     

Prediction of long‐term sustainability of constructed urban soil: impact of high amounts of organic matter on soil physical properties and water transfer

Sustainability of urban soils lies in their ability to facilitate water and air permeabilities. Exogenous organic matter has been shown to have a positive impact on these properties. Under urban conditions, a large one‐time input of an organic amendment was made to the reconstituted soil. Two organic materials, green‐waste compost (gw) or cocompost from sewage sludge and wood chips (sw), were mixed with sandy loam soil (40% v/v) and placed in 600‐L containers. Containers received a 29‐cm thick layer of sandy loam soil–organic matter mix over a 28‐cm thick layer without organic amendment. Volumetric water content, dry bulk density, hydraulic conductivity at saturation and water retention were measured over 5 yrs in the soils and values for the mixes and a control compared. After this time, dry bulk density was greater (1.54 g/cm³) in control than in gw or sw soils (1.31 and 1.11 g/cm³, respectively), whereas hydraulic conductivity at saturation was smaller (4 × 10^?7 m/s) than in gw (3.4 × 10^?6) or sw (3.7 × 10^?6 m/s). HYDRUS 1D water balance model indicated that below 27 cm depth in the control after 5 yrs, there was a high degree of anoxia, lasting >200 days per year, compared with <40 days in gw and sw. Amplification of the risk of anoxia below 27 cm depth after 10 yrs was 323, 151 and 100 days in the control, gw and sw, respectively. Organic matter amendment could support sustainable urban soils for ten years after soil reconstitution.     

Effects of consecutive applications of gypsum in equal,increasing, and decreasing quantities on soil hydraulic conductivity of a saline‐sodic soil

The objective of this study was to determine the effects of consecutive application of gypsum dissolved in leaching water on hydraulic conductivity of a saline‐sodic soil. Drainage type plastic columns with a 10 cm diameter were used in this laboratory experiment. Soil depth within columns was 30 cm with an average bulk density of 1.38 g cm^–3. Leaching water was applied in six equal portions. Total gypsum was applied at 1, 3, and 5 portions after dissolving in leaching water. In dissolution, equal (1.273 + 1.273 + 1.273 Mg ha^–1), increasing (0.637 + 1.273 + 1.910 Mg ha^–1) and decreasing (1.910 + 1.273 + 0.637 Mg ha^–1) quantities of gypsum were used. Results were compared with the control treatment, in which total amount of gypsum were mixed with surface layer of soil column before leaching. Hydraulic conductivity of soil increased in all treatments. The maximum hydraulic conductivity value was obtained at consecutive application of gypsum at decreasing quantities.     

Effects of compaction on soil hydraulic properties,penetration resistance and water flow patterns at the soil profile scale

The aim of this study was to quantify the effects of compaction on water flow patterns at the soil profile scale. Control and trafficked plots were established in field trials at two sites. The trafficked treatment was created by four passes track‐by‐track with a three‐axle dumper with a maximum wheel load of 5.8 Mg. One year later, dye‐tracing experiments were performed and several soil mechanical, physical and hydraulic properties were measured to help explain the dye patterns. Penetration resistance was measured to 50 cm depth, with saturated hydraulic conductivity (K_s), bulk density, and macroporosity and mesoporosity being measured on undisturbed soil cores sampled from three depths (10, 30 and 50 cm). Significant effects of the traffic treatment on the structural pore space were found at 30 cm depth for large mesopores (0.3–0.06 mm diameter), but not small mesopores (0.06–0.03 mm) or macroporosity (pores > 0.3 mm). At one of the sites, ponding was observed during the dye‐tracing experiments, especially in the trafficked plots, because of the presence of a compacted layer at plough depth characterized by a larger bulk density and smaller structural porosity and K_s values. Ponding did not induce any preferential transport of the dye solution into the subsoil at this site. In contrast, despite the presence of a compacted layer at 25–30 cm depth, a better developed structural porosity in the subsoil was noted at the other site which allowed preferential flow to reach to at least 1 m depth in both treatments.     

The impact of reduced atmospheric depositions on soil microbial parameters in a strongly acidified Norway spruce forest at Solling,Germany

To simulate a future ion input reduction scenario in forests, a large scale field experiment was set up in a (1999) 66‒year‒old Norway spruce plantation at Solling, central Germany. Throughfall input of H⁺, SO₄^2—, and N‒compounds is artificially reduced by means of a permanent roof construction below the canopy and a de‒ionizing equipment since 1991. Here we present long term soil solution records for SO₄^2—, NO₃^—, Al³⁺ and the pH of the 10 cm mineral soil sampling depth. A significant decrease in ion concentrations since the start of the treatment is observed, but no change of the soil solution pH. Even in the fourth year pH values remained well within the aluminium buffer range (pH < 4.2). Three years after the start of the experiment (July 1994) it was examined whether microbial biomass (C_mic), specific activity (heat production per unit biomass), and the percentage of C_mic in organic C material indicated any changes. Furthermore chemical standard parameters (CEC, base saturation, pH) were analyzed for all soil samples. Results indicate that despite of drastic decreases of soil solution ion concentrations in the upper soil horizons microbial parameters were not affected and that the soil solid phase is not deacidified by the treatment until now.     

Oxygen and redox potential gradients in the rhizosphere of alfalfa grown on a loamy soil

Oxygen (O₂) supply and the related redox potential (E_H) are important parameters for interactions between roots and microorganisms in the rhizosphere. Rhizosphere extension in terms of the spatial distribution of O₂ concentration and E_H is poorly documented under aerobic soil conditions. We investigated how far O₂ consumption of roots and microorganisms in the rhizosphere is replenished by O₂ diffusion as a function of water/air‐filled porosity. Oxygen concentration and E_H in the rhizosphere were monitored at a mm‐scale by means of electroreductive Clark‐type sensors and miniaturized E_H electrodes under various matric potential ranges. Respiratory activity of roots and microorganisms was calculated from O₂ profiles and diffusion coefficients. pH profiles were determined in thin soil layers sliced near the root surface. Gradients of O₂ concentration and the extent of anoxic zones depended on the respiratory activity near the root surface. Matric potential, reflecting air‐filled porosity, was found to be the most important factor affecting O₂ transport in the rhizosphere. Under water‐saturated conditions and near field capacity up to –200 hPa, O₂ transport was limited, causing a decline in oxygen partial pressures (pO₂) to values between 0 and 3 kPa at the root surface. Aerobic respiration increased by a factor of 100 when comparing the saturated with the driest status. At an air‐filled porosity of 9% to 12%, diffusion of O₂ increased considerably. This was confirmed by E_H around 300 mV under aerated conditions, while E_H decreased to 100 mV on the root surface under near water‐saturated conditions. Gradients of pO₂ and pH from the root surface indicated an extent of the rhizosphere effect of 10–20 mm. In contrast, E_H gradients were observed from 0 to 2 mm from the root surface. We conclude that the rhizosphere extent differs for various parameters (pH, Eh, pO₂) and is strongly dependent on soil moisture.     

Building and testing conceptual and empirical models for predicting soil bulk density

The development of pedotransfer functions offers a potential means of alleviating cost and labour burdens associated with bulk‐density determinations. As a means of incorporating a priori knowledge into the model‐building process, we propose a conceptual model for predicting soil bulk density from other more regularly measured properties. The model considers soil bulk density to be a function of soil mineral packing structures (ρ_m) and soil structure (Δρ). Bulk‐density maxima were found for soils with approximately 80% sand. Bulk densities were also observed to increase with depth, suggesting the influence of over‐burden pressure. Residuals from the ρ_m model, hereby known as Δρ, correlated with organic carbon. All models were trained using Australian soil data, with limits set at bulk densities between 0.7 and 1.8 g cm^?3 and containing organic carbon levels below 12%. Performance of the conceptual model (r² = 0.49) was found to be comparable with a multiple linear regression model (r² = 0.49) and outperformed models developed using an artificial neural network (r² = 0.47) and a regression tree (r² = 0.43). Further development of the conceptual model should allow the inclusion of soil morphological data to improve bulk‐density predictions.     

Vertical distribution of soluble organic nitrogen,nitrogen mineralization,nitrification, and amidohydrolase activities in a manure‐treated soil

Recent studies indicate that soil soluble organic nitrogen (SON) plays an important regulatory role in the soil–plant N cycle. The aims of this study were to identify the vertical distribution of SON and its correlation with N mineralization, nitrification, and amidohydrolase activities, in a soil repeatedly amended with cow manure or chemical fertilizer. For this purpose, soil samples were collected from 0–20, 20–40, 40–60, 60–80, and 80–100 cm depths of a calcareous soil, which has been annually amended for 5 y with cow manure (CM) at two rates of 50 (CM₅₀) and 100 (CM₁₀₀) Mg CM ha^–1 y^–1. Treatments with chemical fertilizer (CF) and a control (CT) were also included. Soluble organic N, N mineralization, nitrification rates, as well as L‐glutaminase and L‐asparaginase activities were determined. Both CM₅₀ and CM₁₀₀ enhanced SON content throughout the soil profile. Nitrogen‐mineralization rate (N_m) was increased at the 0–20 cm depth of the CM₁₀₀ treatment and remained unaffected at the deeper depths. Nitrification rate (N_n) was significantly higher at the 0–60 cm depth of CM₁₀₀ compared to CF and CT. L‐glutaminase and L‐asparaginase activities were significantly increased at the 0–40 cm depth in both CM₅₀ and CM₁₀₀ compared to CF and CT. The amidohydrolase activities could not be detected below 40 cm, regardless of the fertilizer treatments. Our results suggest that SON makes a minor contribution to N mineralization in deep soil layers. It was also concluded that changes in the SON throughout the soil profile were not associated with changes in the N‐transformation rates (N_m and N_n) and amidohydrolase activities. While we conclude that SON is a major N pool in the whole profile of the manure applied soil further investigation is required to characterize SON and to investigate the bioavailability of SON for microbial activity in different soil depths.     

Deep Tillage,Soil Moisture Regime,and Optimizing N Nutrition for Sustaining Soil Health and Sugarcane Yield in Subtropical India

A field experiment was conducted at ICAR-Indian Institute of Sugarcane Research, Lucknow, with three tillage practices (T₁: Control- two times ploughing with harrow and cultivator, each followed by planking before sugarcane planting; T₂: Deep tillage with disc plough (depth 25–30 cm) before planting followed by harrowing, cultivator, and planking; and T₃: Subsoiling at 45–50 cm and deep tillage with disc plough/moldboard plough (depth 25–30 cm) followed by harrowing, cultivator, and planking before planting, two soil moisture regimes (M₁: 0.5 irrigation water (IW)/cumulative pan evaporation (?CPE) ratio and M₂: 0.75 IW/CPE ratio) at 7.5 cm depth of IW, and four N levels (N₁- 0, N₂- 75, N₃- 150, and N₄-225 kg N ha^?1) in sugarcane plant crop. Deep tillage and subsoiling increased porosity and reduced bulk density in surface/subsurface soil. Further, these physical changes also improved soil biological and chemical properties responsible for higher crop growth and yield. Deep tillage and subsoiling reduced the compaction by 6.12% in 0–15 cm depth in sugarcane plant crop at maximum tillering stage. The highest N uptake (158.5 kg ha^?1) was analyzed with deep tillage and subsoiling compared to all other tillage practices. Maintaining suboptimal moisture regime with deep tillage and subsoiling showed the highest IW use efficiency (157.16 kg cane kg^?1 N applied). Mean soil microbial biomass carbon (SMBC) in ratoon crop was higher compared to plant crop. During initial tillering stage, ratoon crop showed higher SMBC with application of deep tillage and subsoiling (1209 mg CO₂-C g^?1 soil day^?1) at 0–15 cm depth and 1082.9 mg CO₂-C g^?1 soil day^?1 at 15–30 cm depth. Thus, it could be concluded that besides improving sugarcane yield, soil health could be sustained by adopting subsoiling (45–50 cm depth) and deep tillage (20–25 cm depth), with soil moisture regime of 0.75 IW/CPE and application of 150 kg N ha^?1 in sugarcane (plant crop).     

Depth and Width of the Wetted Zone in a Sandy Soil after Leaching Drip‐Irrigation Events and Implications for Nitrate‐Load Calculation

Abstract

The quantitative assessment of nitrate‐nitrogen (NO₃‐N) leaching below the root zone of vegetable crops grown with plasticulture (called load) may be done using 150‐cm‐deep soil samples divided into five 30‐cm‐long subsamples. The load is then calculated by multiplying the NO₃‐N concentration in each subsample by the volume of soil (width×length×depth, W×L×D) wetted by the drip tape. Length (total L of mulched bed per unit surface) and D (length of the soil subsample) are well known, but W is not. To determine W at different depths, two dye tests were conducted on a 7‐m‐deep Lakeland fine sand using standard 71‐cm‐wide plasticulture beds. Dye tests consisted of irrigation lengths of up to 38 and 60 h, digging transverse sections of the raised beds at set times, and taking measurements of D and W in 30‐cm‐deep increments. Most dye patterns were elliptically elongated. Maximum average depths were similar (118 and 119 cm) for both tests despite differences in irrigation duration and physical proximity of both tests (100 m apart in the same field). Overall, D response (cm, both tests combined) to irrigation volume (V, L/100 m) was quadratic (D_comb.avg=?2×10^?7 V²+0.008 V+34), and W responses (using maximum and mean values at each 30‐cm increment depth, W_max and W_mean, respectively) to D (cm) were linear (W_max=?0.65D+114 and W_mean=?0.42D+79). Predicted W_max were 104, 84, 64, 44, and 25 cm in 30‐cm depth increments. Load calculations using NO₃‐N concentrations of 7.2, 5.0, 3.9, 3.0, and 2.9 µg/kg for the 15, 46, 77, 107, and 137 cm depths, respectively, were 21.2, 37.6, 28.2, and 39.1 kg/ha for W values of 40 cm, bed width (71 cm), W_mean, and W_max, respectively. These load calculations ranged from simple to double based on the choice of W estimate used, which illustrates the importance of knowing W accurately when load is calculated from field measurements. These W_max and W_mean values may be used for load calculations on sandy soils but are likely to overestimate load because they were determined without transpiring plants and may need to be adjusted for different soil types.     

Predicting soil solution chemical attributes from more easily measured soil and soil solution parameters

Abstract

An understanding of how soil solution ionic strength (I_s) and major cation activities influence crop growth is often limited by the extensive measurements required to characterize ionic composition and subsequent speciation exercises. Easily measured solution and soil attributes need to be identified that can predict these important solution parameters. Soil and soil solution chemical properties of four Ultisols in the Coastal Plain and Piedmont of North Carolina were used to develop models to predict ionic strength and solution cation attributes. GEOCHEM‐PC‐predicted I_s was linearly related to electrical conductivity (EC) across soils (r²=0.92), confirming that I_s for soil solutions with complex composition can be estimated from their electrical conductivity. Models of the form lnM_s=a+blnEC+clnM_E, or modifications thereof, were developed for predicting solution aluminum (Al), calcium (Ca), magnesium (Mg), and postassium (K) levels (M_s) from a knowledge of EC and either soil exchangeable cation #OPM_E) or cation saturation (MSAT_E) attributes. For each cation, total and free solution concentration and activity in absolute and saturation terms were investigated. The best models explained, at most, 68% of the variability associated with total solution Al concentration (Al_s‐T) or 74% when Al_s ‐T was expressed as a percent of major solution cations. Greater than 85% of the variability associated with solution Ca and Mg could also be accounted for, but only 67% of the variability associated with solution K could be explained. Including soil pH and interaction terms (M_ExEC, M_ExpH, and ECxpH) in models improved the relationship for total Al concentration (R²=0.87) and solution Ca parameters (R² ≥0.93), but not for solution Mg and K indices. None of the models could account for >30% of the variability associated with free concentration and activity of Al³⁺, suggesting that the prediction of these parameters for a particular Al species could not be made from a knowledge of soil pH, solution EC, and M_E or MSAT_E data.     

Populations of methanogenic bacteria in paddy field soil under double cropping conditions (rice-wheat)

The methanogenic populations able to use H₂–CO₂, methanol, and acetate were investigated in paddy field soil in situ under double cropping conditions [rice (Oryza sativa L.) as a summer crop under flooded conditions and wheat (Triticum aestivum L.) as an upland winter crop] over 2 years approximately bimonthly by the most probable number method. Three fields, one without fertilizer, one treated with inorganic fertilizer (mixed fertilizer including urea, ammonium phosphate, and potassium sulfate), and one treated with wheat straw plus inorganic fertilizer, were examined. The population of H₂–CO₂, methanol, and acetate utilizers in the paddy field soil at a depth of 1–6 cm was 10³–10⁴, 10⁴–10⁵, and 10⁴–10⁵ g^-1 dry soil, respectively. These values were almost constant during the 2 years irrespective of moisture regime (flooded or nonflooded), crop (rice or wheat), fertilizer treatment, and soil depth (0–1, 1–10, and 10–20 cm).     

Soil pH and nitrogen changes following cattle and sheep urine deposition

Abstract

The relationship between animal urine deposition and variability in soil chemical composition and crop growth is not well established in the semi‐arid region of West Africa. This study was conducted to examine the changes over time in soil pH and mineral nitrogen (N) concentrations at the micro sites of cattle and sheep urine patches in comparison to those occurring in fertilizer urea placement zones. The urine and fertilizer solution containing each 400 mg N (800 kg N ha^‐1) were spread onto individual plots covering a surface area of 4‐cm radius. The treatments included a control, which consisted of distillate water. Soil samples from three replicate plots were taken in 4‐cm increments to a depth of 16 cm and distance of 16 cm on a grid pattern at days 1, 7, 21, 49, 90, 120, and 150 after application. Significant pH and mineral N gradients develop in the vicinity of the fertilizer and urine placement zones declining towards the periphery and the deeper soil layers. The pH at the center of the urine zone remained above 7.5 throughout the 150 days of the study period. After the initial increase, the soil pH below the fertilizer placement sites declined to the control level by day 90. Concentrations of ammonium (NH₄) + nitrate (NO₃) also increased markedly in the immediate soil layers of the urine and urea placement zones, and then decreased over time probably due to N losses by volatilization and leaching. Concentrations of mineral N at the periphery of the placement site were similar for all treatments throughout the study period, indicating very little lateral N diffusion. These results provided evidence that animal urine causes significant variabilities in soil chemical composition, even in short distance from the deposition zones. The high soil solution pH in the vicinity of the urine patches alleviate the potential of aluminum (Al) toxicity while increasing the phosphorus (P) availability to crop plants.     

An evaluation of anion exchange resin used to measure nitrate movement through soil

Abstract

The attribute that ion‐exchange resins remove ions from solutions moving through them can be used to measure nitrate transport through soils. The characteristics of nitrate adsorption by resins must be known to interpret nitrate accumulation on ion‐exchange resins embedded in soil. The extent to which anion exchange resins retain NO₃‐ from soil leachate was measured in 15.9 cm diam.by 60 cm long intact cores of Nolin (fine silty mixed mesic Dystric Fluventic Eutrochroept) soil. A NC₃ ^‐‐selective resin and a non‐selective resin were tested. Columns were fertilized at a rate of 300 kg N/ha and 150 kg Br/ha and leached with 50 cm of water. Under these conditions, both resins retained approximately 80% of the NO₃‐ and Br^‐ leached through the soil. This compared with greater than 95% retention in laboratory columns containing only resin. The difference in retention was attributed to different flow through the resin associated with the method of resin emplacement.