A new sampling technique to monitor concentrations of CH4, N2O and CO2 in air at well‐defined depths in soils with varied water potential

A new sampling technique for measuring the concentrations of trace gases (CH₄, CO₂ and N₂O) in the soil atmosphere from well‐defined depths is described. Probes are constructed from silicone tubing closed with silicone septa on both ends, thereby dividing an inner air space from the outer soil atmosphere without a direct contact. The gas exchanges between the inner and outer atmosphere only by diffusion through the walls of the silicone tube. Tests revealed that the gases N₂O, CO₂ and CH₄ in the enclosed space reached 95% equilibrium with the surrounding atmosphere at 20°C within 7 h or faster. The probe measurements are reproducible: the standard deviation of samples taken from 26 probes stored in the laboratory atmosphere equalled that of a standard gas. The probes can easily be constructed and installed at specified depths in the soil. The method has the following advantages compared with other methods that use spaces with holes in them for gas exchange: (i) the silicone probe enables trace gases to be sampled in wet soils, including ones that are waterlogged or temporarily saturated; (ii) the sampling itself does not create low pressure and hence does not create mass flow in the soil matrix from undefined depths; and (iii) the probe can be made to take samples of gas of any required size. The silicone probes did not show ageing effects during 18 months of use in the field in a mineral soil under grass. The probes yielded comparable results: three probes inserted at 5 cm depth in a uniformly treated 100‐m² plot provided nearly identical average trace gas concentrations within the measurement period.     

Evidence for effects of CO2 on soil solution chemistry in spodosols by a simple in-field extractor

Degassing of CO₂ during collection of soil solution may alter the chemical composition of the solution, especially in well-buffered soils. We used a simple syringe extractor for field sampling of soil solution along with ambient soil air in order to test the influence of degassing of CO₂ on solution pH in acid soils (Spodosol B and C horizons collected in Central Maine, USA). Soil air concentrations of CO₂ varied from 0.36 to 1.35 ml l-¹ during sampling immediately after snow melt. Degassing increased solution pH by 0.3 to 0.5 pH units. Both in-situ and degassed pH were predicted by the Reuss and Johnson soil chemical equilibrium model. The results suggest, (i) that the simple method is useful for determination of solution from wet soil under ambient soil air conditions and (ii) that degassing plays a significant role for soil solution chemistry even in Spodosol B and C horizons.     

Feldmethode zur Bestimmung der substrat‐induzierten Bodenatmung

A field method for the measurement of substrate‐induced soil respiration A novel method for in situ measurements of microbial soil activity using the CO₂ efflux combined with kinetic analysis is proposed. The results are compared with two conventional, laboratory methods, (1) substrate‐induced respiration using a ’︁Sapromat’ and (2) dehydrogenase activity. Soil respiration was measured in situ after addition of aqueous solutions containing 0 to 6 g glucose kg^—1 soil. The respiration data were analysed using kinetic models to describe the nutritional status of the soil bacteria employing few representative parameters. The two‐phase soil respiration response gave best fit results with the Hanes' or non‐parametric kinetic model with Michaelis‐Menten constants (K_m) of 0.05—0.1 g glucose kg^—1 soil. The maximum respiration rates (V_max) were obtained above 1 g glucose. Substrate‐induced respiration rates of the novel in situ method were significantly correlated to results of the ’︁Sapromat’ measurements (r² = 0.81***). The in situ method combined with kinetic analysis was suitable for the characterisation of microbial activity in soil; it showed respiration rates lower by 59% than measured in the laboratory with disturbed samples.     

Phosphorus availability and sorption under alternating waterlogged and drying conditions

Abstract

The effect of alternating waterlogged and drying conditions on phosphorus (P) availability and sorption was studied in three soils of contrasting chemical and physical properties. Soils were treated with two levels of P (0 and 50 mg kg^‐1; P0 and P50), waterlogged for 21 days, then allowed to dry at room temperature for 14 days. The availability of P, iron (Fe), and manganese (Mn) over the waterlogged and drying periods was determined by shaking samples of each soil with 1M NaOAc (pH 3). Increasing concentrations of 1M NaOAc (pH 3) extractable P (P_ac) over the waterlogged period was attributed to solubilization of Fe(OH)₃ materials under reducing conditions with the release of sorbed and occluded P. The released P appeared to be resorbed by freshly precipitated amorphous Fe(OH)₂ material since earlier studies showed that water soluble P concentrations in these soils were reduced to negligible levels under waterlogged conditions. The Fe(OH)₂ material remained readily extractable with 1M NaOAc (pH 3) since Fe_ac increased dramatically with waterlogging. Drying the waterlogged soils caused a rapid decrease in P_ac, Fe_ac and Mn_ac suggesting the Fe(OH)₂ may have been transformed into more stable forms [e.g., Fe(OH)₃]. Much of the changes in P_ac appeared to be due to changes in Fe_ac, with limited influence from Mn_ac. and mineralization of organic P. Phosphate sorption isotherms were determined using the standard batch technique for air‐dry, waterlogged (with and without ponded water), and waterlogged/dried conditions. Sorption isotherms were not affected by waterlogging and subsequent drying. Most soils sorbed all of the added phosphate irrespective of moisture treatment.     

Influence of air drying and soil gas‐phase carbon dioxide on DTPA‐extractable iron in calcareous soils

Abstract

The extraction of a field‐moist soil with DTPA will result in a level of extractable iron (Fe) lower than that of the air‐dried soil. Soil gas‐phase carbon dioxide (CO₂) levels may be considerably higher than ambient atmospheric levels, especially in wet soils in the field. This study was undertaken to determine whether gas‐phase CO₂ level influences the quantity of Fe extracted by DTPA. Three moist calcareous soils were incubated for 21 days, each at three different partial pressures of CO₂, after which the moist soils were extracted with DTPA. A sample of each soil was also air dried, and was subsequently extracted with DTPA. In each case, DTPA‐extractable Fe from the moist sample was lower than that from the air‐dried sample; however, DTPA‐extractable Fe increased with increasing CO₂ partial pressure of in the moist soils. DTPA‐extractable Fe concentration for a given soil following air drying was not significantly influenced by the CO₂ partial pressure during incubation of the originally field‐moist soil. DTPA‐extract pH of the moist soils followed the same trend as soil‐solution pH (i.e., as CO₂ concentration of the soil gas‐phase increased, soil solution pH and DTPA extract pH both decreased); however, the slope of the pH versus log P_CO2 curve was less pronounced in the DTPA extract due to the buffering capacity of the triethanolamine. From this study, it is concluded that elevated soil gas‐phase CO₂ partial pressure does not contribute to the lower level of DTPA‐extractable Fe observed when the extraction is performed on a field‐moist versus an air‐dried soil; increased CO₂ partial pressure actually resulted in a slight increase in concentration of DTPA‐extractable Fe obtained from a field‐moist soil.     

Time‐resolved in‐situ pH measurement in differently treated,saturated and unsaturated soils

The pH‐value is of utmost relevance for soil properties and functioning. Hence, a time‐resolved in‐situ measurement is mandatory but lacking. As an alternative, a two‐probe pH electrode with gel‐covered reference electrode was newly constructed and tested for a continuous, in‐situ pH recording in saturated and unsaturated soil. This was done using samples from a set of 14 soils with different composition and pH$ _{\rm CaCl_2} $ ranging from 3.5 to 7.5 in batch and repacked soil column experiments. In the latter, changes in pH and redox potential were monitored upon transport of citrate‐phosphate buffer and pig slurry through the soil columns. The pH measurements were largely stable even upon substantial shifts in soil moisture content down to air‐dry conditions. The results of the pH measurements agreed with standard methods using settled soil suspensions in electrolyte solutions and the conventional combination (single‐probe) pH electrode. Testing the suspension effect, it was found that measuring pH directly in the soil is recommended. The pH measured in‐situ was closest to pH values determined in 0.01 M CaCl₂ suspensions according to DIN ISO 10390 (DIN, 2005 ). The transport of citrate buffer and pig slurry as pH active substances through soil induced strong effects on the pH and in part on the redox potential; the reversible effects lasted over days, which may affect the mobility and speciation of nutrients and pollutants as well as microbial processes.     

Methane and nitrous oxide fluxes, and carbon dioxide production in boreal forest soil fertilized with wood ash and nitrogen

Wood ash has been used to alleviate nutrient deficiencies and acidification in boreal forest soils. However, ash and nitrogen (N) fertilization may affect microbial processes producing or consuming greenhouse gases: methane (CH₄), nitrous oxide (N₂O) and carbon dioxide (CO₂). Ash and N fertilization can stimulate nitrification and denitrification and, therefore, increase N₂O emission and suppress CH₄ uptake rate. Ash may also stimulate microbial respiration thereby enhancing CO₂ emission. The fluxes of CH₄, N₂O and CO₂ were measured in a boreal spruce forest soil treated with wood ash and/or N (ammonium nitrate) during three growing seasons. In addition to in situ measurements, CH₄ oxidation potential, CO₂ production, net nitrification and N₂O production were studied in laboratory incubations. The mean in situ N₂O emissions and in situ CO₂ production from the untreated, N, ash and ash + N treatments were not significantly different, ranging from 11 to 17 μg N₂O m^?2 h^?1 and from 533 to 611 mg CO₂ m^?2 h^?1. However, ash increased the CH₄ oxidation in a forest soil profile which could be seen both in the laboratory experiments and in the CH₄ uptake rates in situ. The mean in situ CH₄ uptake rate in the untreated, N, ash and ash + N plots were 153 ± 5, 123 ± 8, 188 ± 10 and 178 ± 18 μg m^?2 h^?1, respectively.     

干湿交替对新疆绿洲农田土壤CO2排放的影响

[目的]分析不同土壤水分变化及干湿交替对土壤CO_2排放的影响,为绿洲农田土壤碳循环提供科学依据。[方法]选取新疆绿洲棉田土壤,通过室内控制模拟试验,以及用气相色谱仪分析CO_2浓度。[结果](1)与60%WFPS(土壤充水孔隙度)相比,40%WFPS对土壤CO_2排放起到了显著的抑制作用(p0.05),而80%WFPS对土壤CO_2排放无显著性影响(p0.05)。培养结束时,与60%WFPS的土壤CO_2累积排放量相比,40%WFPS的土壤CO_2累积排放量降低26%(p0.05),而80%WFPS的土壤CO_2累积排放量仅增加0.04%(p0.05)。(2)多次干湿交替循环后,干湿交替处理下的土壤CO_2累积排放量显著低于恒湿处理。在不同干旱强度处理中,重度干旱(SD)处理对土壤CO_2排放速率响应程度大于适度干旱(MD)处理,但多次干湿交替循环后,SD处理下的土壤CO_2累积排放量却显著小于MD处理。随干湿交替循环次数的增加,干湿交替对土壤CO_2排放速率的影响显著降低,特别是对土壤CO_2排放速率最高值的影响最大。[结论]在新疆绿洲棉田土壤中,干湿交替能降低土壤CO_2排放量,降低量随干旱强度的增大而增大。     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

Comparing concurrent in situ and batch‐extracted trace element concentrations

Different procedures to investigate dissolved trace element concentration at the transition from unsaturated to saturated zone in soils were compared by concurrent sampling of soil solution and solid soil material in this zone. The in situ sampled soil solution from the percolated water was used to measure in situ concentrations, while solid soil material was used to measure concentrations at two liquid–solid ratios using batch experiments on 250 sample pairs. The liquid–solid ratios were 2 L kg^–1 and 5 L kg^–1. At 5 L kg^–1, the ionic strength was adjusted with Ca(NO₃)₂ to a sample‐specific value similar to in situ, while at 2 L kg^–1, the ionic strength was not adjusted. The extracted concentrations of most trace elements exhibited a statistically significant but weak correlation (p value < 0.01) to the corresponding in situ concentrations. In the liquid–solid ratio of 2 L kg^–1 extracts, Pb and Cr showed very poor comparability with the in situ equivalent. A likely cause was the enhanced dissolved‐organic‐C release in the extract due to the lower ionic strength compared to in situ conditions in combination with effects from drying and moistening soil samples. For the other elements, correlation increased in the order As < Cu, Zn, Sb, Mo, V < Cd, Ni, Co where adjustment of the ionic strength led to slightly better results. In addition to the element‐specific shortcomings, it appeared that low concentration levels of in situ concentrations were generally underestimated by batch extraction methods. The liquid–solid ratio of 2 L kg^–1 extracts could only be used as a method to predict exceedance of thresholds if a safety margin of approximately one order of magnitude higher than the thresholds was adopted. The ability of the batch‐extraction methods to estimate in situ concentrations was equally limited.     

Simulating the effect of potassium fertilization on carbon sequestration in soil

The impact of horticultural management on carbon sequestration in soils has been limited so far to tillage and nitrogen fertilization. Our objective was to evaluate by mathematical modeling the effect of potassium fertilization on CO₂ binding in cropland soils. The developed model integrates three subunits: (1) A published simulator of crop dry‐matter (DM) production in response to N, P, K fertilization, but not DM partitioning; (2) a published soil–crop–atmosphere model predicting crop yield and DM partitioning as a function of N but not K fertilization; (3) an original model computing the organic‐inorganic carbon transformations, inorganic‐carbon reactions and transport in soil, CO₂ diffusion, and soil carbon sequestration. The model described the K‐fertilization effect on C binding in soil as a function of the soil pH, the Ca²⁺ concentration in the soil solution, hydraulic properties, air temperature, and crop DM production, and partitioning characteristics. In scenarios of corn (Zea mays L.) growth in clayey soil and wheat (Triticum aestivum L.) in loam soil, the computed K‐induced CO₂ sequestration amounted to ≈ 14.5 and 24 kg CO₂ (kg K)^–1, respectively (0 vs. 100 kg ha^–1 K application). The soil CO₂ sequestration declined by 8% when corn grew in sandy instead of clayey soil and by 20% when the temperature was 10°C higher than the temperature prevailing in mild semiarid zones. All predicted CO₂‐sequestration results were approximately 30‐fold higher than the 0.6 kg CO₂ emitted per kg of K manufactured in industry.     

Partitioning of metals (Cd, Co, Cu, Ni, Pb, Zn) in soils: concepts, methodologies, prediction and applications – a review

Prediction of the fate of metals in soil requires knowledge of their solid–liquid partitioning. This paper reviews analytical methods and models for measuring or predicting the solid–liquid partitioning of metals in aerobic soils, and collates experimental data. The partitioning is often expressed with an empirical distribution coefficient or K_d, which gives the ratio of the concentration in the solid phase to that in the solution phase. The K_d value of a metal reflects the net effect of various reactions in the solid and liquid phases and varies by orders of magnitude among soils. The K_d value can be derived from the solid–liquid distribution of added metal or that of the soil‐borne metal. Only part of the solid‐phase metal is rapidly exchangeable with the solution phase. Various methods have been developed to quantify this ‘labile’ phase, and K_d values based on this phase often correlate better with soil properties than K_d values based on total concentration, and are more appropriate to express metal ion buffering in solute transport models. The in situ soil solution is the preferred solution phase for K_d determinations. Alternatively, water or dilute‐salt extracts can be used, but these may underestimate in situ concentrations of dissolved metals because of dilution of metal‐complexing ligands such as dissolved organic matter. Multi‐surface models and empirical models have been proposed to predict metal partitioning from soil properties. Though soil pH is the most important soil property determining the retention of the free metal ion, K_d values based on total dissolved metal in solution may show little pH dependence for metal ions that have strong affinity for dissolved organic matter. The K_d coefficient is used as an equilibrium constant in risk assessment models. However, slow dissociation of metal complexes in solution and slow exchange of metals between labile and non‐labile pools in the solid phase may invalidate this equilibrium assumption.     

Method for estimation of CO2(aq) plus H2CO3°, HCO3− and pH in soil solutions collected under field conditions

A method for the collection of soil solution and the determination of pH, H₂CO₃* (= CO₂(aq) plus H₂CO₃°), HCO₃^? and CO₃^2?, was developed which excluded atmospheric gases during the entire procedure. The soil solution was collected by tension lysimeters without exposure to the atmosphere. Using a closed system, the sample was transferred to a titration beaker for the analysis of pH, H₂CO₃* and HCO₃^?. The analysis of CO₂-acidity was done by titration with 0.0454 N Na₂CO₃ to the end point pH of 8.3. It was immediately followed by an acidimetric titration for the determination of alkalinity using 0.005 N H₂SO₄ under gentle N₂ flow; the equivalence point was determined graphically from the titration curve. In standard solutions, this method gave nearly 100% recovery of H₂CO₃* and HCO₃^?. In soil solutions, the pH markedly increased and H₂CO₃* decreased upon exposure to the atmosphere. The values of the sum of CO₂-acidity and alkalinity in soil solutions at a depth > 5 cm agreed well with the values of total inorganic carbon obtained by CO₂ infrared detection following CO₂ degassing. For solutions obtained from 100 cm and 300 cm depth (limestone) the measured distribution of H₂CO₃* and HCO₃^? was in agreement with the calculated values based on pH-measurement and total inorganic carbon. This comparison was unsatisfactory for the concentration of H₂CO₃* in solutions of the surface (0–15 cm) soil, possibly because the mathematical model as well as the interpretation of the titration curves did not consider any organic compounds in the solution.     

Shaker speeds for aerobic soil slurry incubations

Abstract

Rates of soil microbial activity are frequently assayed using shaken slurries of soil. Soil slurries are typically shaken at speeds of 120 to 200 revolutions min^‐1 in an effort to maintain uniform concentrations of oxygen (O₂) and other substrates. Few studies have examined whether or not the shaking speeds were sufficient to achieve adequate aeration and mixing. In this study, concentrations of O₂ and rates of nitrification and denitrification were measured in soil slurries (100 mL solution + 10 g soil) contained in 250‐mL Erlenmeyer flasks. The flasks were shaken at speeds ranging from 112 to 200 rev. min^‐1 (corresponding to relative centrifugal forces of 0.14 to 0.45 x gravity). While concentrations of O₂ in the bulk solution remained high throughout the incubations, significant rates of denitrification occurred at shaking speeds ≤ 155 rev. min^‐1. Nitrification rates generally increased with shaking speeds up to 180 rev. min^‐1. Nitrification rates at 180 and 200 rev. min^‐1 were not significantly different. These results suggest that shaking speeds ≤ 180 rev. min^‐1 are necessary to ensure adequate aeration in soil slurries.     

Eine Methode zur permanenten pH-Wert-Registrierung mittels keramischer Saugkerzen im Mineralbodeninput eines sauren Waldbodens

A new method for permanent registration of pH value in mineral soil input of an acid forest soil The organic top layers of acid forest soils in low mountain ranges of Germany which receive acid precipitation are of special ecological interest to the observation for their saisonal chemical behaviour. Root systems of young spruce forests tend to accumulate in these layers because of Al-toxicity in mineral soil. Conventional procedures for a weekly sampling of soil solution do not account for short term events. Thus, a measuring device for permanent pH-logging of soil solution in mineral soil input was developed. A microprocessor controls the vacuum in the ceramic cups containing pH-glass electrodes in order to maintain steady water flux into the cups. The accuracy of the measured pH was tested with a theoretical equilibrium modell concerning CO₂-degassing and with a laboratory experiment. Below pH 4.7 no influence of CO₂-losses on the pH inside the cups occurs. Permanent measurements in mineral soil of a spodosol in The Fichtelgebirge showed fluctuations of higher pH values immediately after precipitation and decreasing values later on. In late spring this behaviour is not yet pronounced.     

Long‐term carbon loss and CO2‐C release of drained peatland soils in northeast Germany

Peatlands are common in many parts of the world. Draining and other changes in the use of peatlands increase atmospheric CO₂ concentration. If we are to make reliable quantitative predictions of that effect, we need good information on the CO₂ emission rates from peatlands. The present study uses two different methods for predicting CO₂‐C release of peatland soils: (i) a 40‐year field investigation of balancing organic carbon stocks and (ii) short‐term CO₂‐C release rates from laboratory experiments. To estimate long‐term losses of peat, and its resulting C input to the atmosphere, we combined highly detailed maps of surface topography and its changes, and the organic C contents and bulk densities of a drained peatland from different years. Short‐term CO₂‐C release rates were measured in the laboratory by incubating soil samples from several soil horizons at various temperatures and soil moistures. We then derived nonlinear CO₂‐C production functions, which we incorporated into a numerical simulation model (HYDRUS). Using HYDRUS, we calculated daily soil water components and CO₂‐release for (i) real‐climate data from 1950 to 2003 and (ii) a climate scenario extending to 2050, including an increase in temperature of 2°C and 20% less rainfall during the summer half year, i.e. from April to September inclusive. From our field measurements, we found a mean annual decrease of 0.7 cm in the thickness of the peat. Large losses (> 1.5 cm year^?1) occurred only during periods when groundwater levels were low (i.e. a deep water‐table). The annual CO₂‐C release results in a mean loss from the peat of about 700 g CO₂‐C m^?2, mostly as a direct contribution to the atmosphere. Both methods produced very similar results. The model scenarios demonstrated that CO₂‐C loss is mainly controlled by the groundwater (i.e. water‐table) depth, which controls subsurface aeration. A local climate scenario estimated a c. 5% increase of CO₂‐C losses within the next 50 years.     

Monitoring nitrate in soils with coated wire ion specific electrodes

Abstract

The high sensitivity and low per‐unit cost of nitrate (NO₃‐) ‐ sensitive coated wire electrodes (CWEs) makes them attractive alternatives to chemical extraction for NO₃‐ assessment in soils and solutions. We used CWEs in the field to quantify soil NO₃‐ in fresh soil extracts, in a laboratory incubation comparing CWEs with anion exchange membranes and soil extracts, and in the field in fertilized and unfertilized soils. Freshly calibrated electrodes performed well in the field when used to measure NO₃‐ in soil extracts and when installed directly in soils. The output of electrodes placed in situ in soils responded linearly, decreasing as the NO₃‐ contents of soils were increased. Soil‐installed electrodes performed better in the laboratory than in the field. When the electrodes were left in soils for long periods (days‐to‐weeks), we had problems with both their durability and the stability of their response to NO₃‐. Nitrate‐sensitive CWEs will be used to the greatest advantage in controlled settings where their output can be calibrated frequently and their contact with solution ensured.     

Nitrogen fertilizer–induced mineralization of soil organic C and N in six contrasting soils of Bangladesh

A 90‐day laboratory incubation study was carried out using six contrasting subtropical soils (calcareous, peat, saline, noncalcareous, terrace, and acid sulfate) from Bangladesh. A control treatment without nitrogen (N) application was compared with treatments where urea, ammonium sulfate (AS), and ammonium nitrate (AN) were applied at a rate of 100 mg N (kg soil)^–1. To study the effect of N fertilizers on soil carbon (C) turnover, the CO₂‐C flux was determined at nine sampling dates during the incubation, and the total loss of soil carbon (TC) was calculated. Nitrogen turnover was characterized by measuring net nitrogen mineralization (NNM) and net nitrification (NN). Simple and stepwise multiple regressions were calculated between CO₂‐C flux, TC, NNM, and NN on the one hand and selected soil properties (organic C, total N, C : N ratio, CEC, pH, clay and sand content) on the other hand. In general, CO₂‐C fluxes were clearly higher during the first 2 weeks of the incubation compared to the later phases. Soils with high pH and/or indigenous C displayed the highest CO₂‐C flux. However, soils having low C levels (i.e., calcareous and terrace soils) displayed a large relative TC loss (up to 22.3%) and the added N–induced TC loss from these soils reached a maximum of 10.6%. Loss of TC differed depending on the N treatments (urea > AS > AN >> control). Significantly higher NNM was found in the acidic soils (terrace and acid sulfate). On average, NNM after urea application was higher than for AS and AN (80.3 vs. 71.9 and 70.9 N (kg soil)^–1, respectively). However, specific interactions between N‐fertilizer form and soil type have to be taken into consideration. High pH soils displayed larger NN (75.9–98.1 mg N (kg soil)^–1) than low pH soils. Averaged over the six soils, NN after application of urea and AS (83.3 and 82.2 mg N (kg soil)^–1, respectively) was significantly higher than after application of AN (60.6 mg N (kg soil)^–1). Significant relationships were found between total CO₂ flux and certain soil properties (organic C, total N, CEC, clay and sand content). The most important soil property for NNM as well as NN was soil pH, showing a correlation coefficient of –0.33** and 0.45***, respectively. The results indicate that application of urea to acidic soils and AS to high‐pH soils could be an effective measure to improve the availability of added N for crop uptake.     

Residue incorporation mitigates tillage‐induced loss of soil carbon in laboratory microcosms

Annual horticultural systems rely on frequent and intensive tillage to prepare beds, manage weeds and control insects. But this practice reduces soil organic carbon (SOC) through accelerated CO₂ emission. Crop residue incorporation could counteract this loss. We investigated whether vegetable systems could be made more resilient by including a high‐residue grain crop such as sweet corn (Zea mays L. var. rugosa), in the rotation through the use of conventional (no residue, no soil sieving) and organic (residue incorporated and soil sieved) soil management scenarios. We evaluated short‐term emission of CO₂‐C and soil C content in incubated Chromosol and Vertosol soils (Australian Classification) with and without sieving (simulated tillage) or the incorporation of ground sweet corn residue. Residue treatment emitted 2.3 times more CO₂‐C compared to the no‐residue treatment, and furthermore, sieved soil emitted 1.5 times more CO₂‐C than the unsieved across the two soil types. The residue incorporation had a greater effect on CO₂‐C flux than simulated tillage, suggesting that C availability and form can be more important than physical disturbance in cropping soils. The organic scenario (with residue and sieved) emitted more CO₂‐C, but had 13% more SOC compared with the conventional scenario (without residue and unsieved), indicating that organic systems may retain more SOC than a conventional system. The SOC lost by soil disturbance was more than offset by the incorporation of residue in the laboratory conditions. Therefore, the possible SOC loss by tillage for weed control under organic management may be offset by organic residue input.     

Labile lead in polluted soils measured by stable isotope dilution

It is well known that lead (Pb) is strongly immobilized in soil by adsorption or precipitation. However, the reversibility of these reactions is poorly documented. In this study, the isotopically exchangeable Pb concentration in soils (E‐value) was measured using a stable isotope (²⁰⁸Pb). Soils were collected at three industrialized sites where historical Pb emissions have resulted in elevated Pb concentrations in the surrounding soil. Lead concentrations ranged from background values, in the control soils collected far from the emission source, to highly elevated concentrations (5460–14440 mg Pb kg^?1). The control soil of each site was amended in the laboratory with Pb(NO₃)₂ to the same total Pb concentrations as the field‐contaminated soils. The %E values (E‐value relative to total Pb content) were greater than 84% in the laboratory‐amended soils, and ranged from 45% to 78% (mean 58%) in the field‐contaminated soils. The relatively large labile fractions of Pb in the field‐contaminated soils show that the majority of Pb is reversibly bound despite the fact that the binding strength is large. The Pb concentrations in soil solution were up to 3500‐fold larger for the laboratory‐amended soils than for field‐contaminated soils at corresponding total Pb concentrations. These differences cannot be explained by differences in labile fractions of Pb but are attributed to the decrease in soil solution pH upon addition of Pb²⁺‐salt.