Comparison of different extractants for the determination of inorganic sulphate in gypsum‐free agricultural soils

Four frequently used extractants (H₂O, 0.1 M NaCl, 0.016 M KH₂PO₄, and 0.5 M NaHCO₃) as well as different extraction conditions have been tested for sulphate extraction from gypsum‐free agricultural soils. Water is the preferable extractant for soils with pH > 6. Two extraction steps have to be carried out for complete extraction (> 95%). A 0.016 M KH₂PO₄ solution was found to be the most efficient extractant for soils with a pH < 6 within a single extraction step. A shaking frequency of 170 min^‐1 and a duration of extraction of 4 hours are the optimized conditions for the sulphate extraction with H₂O and KH₂PO₄ solution.     

Comparison of different laboratory methods with lysimetry for soil solution composition — experimental and model results

Compositions of soil solution obtained by the following methods were compared with those obtained by lysimetry: centrifugation; 2:1 extracts of air dried (2:1_dried) and field moist (2:1_moist) samples; saturation extracts; the ‘equilibrium soil pore solution’︁ method using columns with undisturbed (ESPS) and composited soil (ESPS_comp); and a method using pressure. Two soil depths of a Spodic Dystric Cambisol at Solling, Germany, were sampled with 10 to 12 replications. A coupled equilibrium model was used to describe the effect of soil to solution ratio on the solution composition. The model included multiple cation exchange and inorganic complexation, and for the subsoil solubility products of AlOHSO₄ and Al(OH)₃. Saturation extracts gave similar results as lysimetry and thus may be useful for calculating output fluxes. However, biological transformations (N mineralisation, solubilisation of organic matter) occurred during the preparation of saturation extracts. Composition of soil solutions obtained by either 2:1_dried extracts or centrifugation differed greatly from the results of other methods, indicating that these two methods may not be the best means to investigate equilibrium soil solutions. The values of molar ion ratios depended largely on the method used to obtain soil solutions: Ca²⁺/Al³⁺ ratios for each depth ranged from less than 0.3 (which suggests that liming is required urgently) to greater than 1 (liming not necessary). Modelling described the effect of soil to solution ratio on element concentrations for the methods pressure, saturation extracts, ESPS_comp and 2:1_moist extracts qualitatively with a few exceptions. The model suggested that differences in element concentrations using these methods may be mainly due to dilution, cation exchange and solubilisation of sparingly soluble salts, depending on the soil to solution ratio used.     

Conversion of cropland into grassland: Implications for soil organic‐carbon stocks in two soils with different texture

Soil organic‐carbon (SOC) stocks are expected to increase after conversion of cropland into grassland. Two adjacent cropland and grassland sites—one with a Vertisol with 23 y after conversion and one with an Arenosol 29 y after conversion—were sampled down to 60 cm depth. Concentrations of SOC and total nitrogen (N_tot) were measured before and after density fractionation in two light fractions and a mineral‐associated fraction with C adsorbed on mineral surfaces. For the soil profiles, SOC stocks and radiocarbon (¹⁴C) concentrations of mineral associated C were determined. Carbon stocks and mineral‐associated SOC concentrations were increased in the upper 10 cm of the grassland soil compared to the cropland. This corresponded to the root‐biomass distribution, with 59% and 86% of the total root biomass at 0–5 cm soil depth of the grasslands. However, at the Arenosol site, at 10–20 cm depth, C in the mineral‐associated fraction was lost 29 y after the conversion into grassland. Over all, SOC stocks were not significantly different between grassland and cropland at both sites when the whole profile was taken into account. At the Arenosol site, the impact of land‐use conversion on SOC accumulation was limited by low total clay surface area available for C stabilization. Subsoil C (30–50 cm) at cropland of the Vertisol site comprised 32% of the total SOC stocks with high ¹⁴C concentrations below the plowing horizon. We concluded that fresh C was effectively translocated into the subsoil. Thus, subsoil C has to be taken into account when land‐use change effects on SOC are assessed.     

Determination of organic and inorganic carbon, δ13C,and nitrogen in soils containing carbonates after acid fumigation with HCl

In carbonate‐containing soils a reliable determination of organic C requires a method that effectively separates organic and inorganic C without altering the organic matter. This study was conducted to determine whether HCl vapor completely removes carbonates even in dolomite‐rich soils and to what extent a widely used acid‐fumigation method has to be modified for humus‐rich soils. Furthermore, it was tested whether HCl fumigation alters organic‐C content. Since C and N parameters are often analyzed simultaneously we also tested the influence of acid‐vapor treatment on N content and on δ¹³C of soil organic matter. We applied fumigation with 37% HCl for 8 and 32 h using 9 carbonate‐containing soil samples. Inorganic C ranged from 7 to 124 and organic C from 9 to 267 g kg^–1. The maximum contents of dolomite and calcite were 940 and 640 g kg^–1, respectively. A time of 8 h was enough to completely remove all carbonates. Neither the content nor the δ¹³C of organic C were significantly affected by fumigation. In contrast, N contents were altered by acid treatment. Based on these results and on our experience in analyzing more than 1000 soil samples, a recommended procedure for acid fumigation of carbonate‐containing soils with a wide range of organic‐ and inorganic‐C contents was derived. Samples pretreated in this way can be analyzed reliably for their organic‐C content and δ¹³C. Furthermore, N and inorganic‐C contents can be determined with a quality sufficient for many purposes.     

Consequences of sodium exclusion for the osmotic potential in the rhizosphere – Comparison of two maize cultivars differing in Na+ uptake

According to the biphasic model of growth response to salinity, growth is first reduced by a decrease in the soil osmotic potential (Ψ_o), i.e., growth reduction is an effect of salt outside rather than inside the plant, and genotypes differing in salt resistance respond identically in this first phase. However, if genotypes differ in Na⁺ uptake as it has been described for the two maize cultivars Pioneer 3906 and Across 8023, this should result in differences in Na⁺ concentrations in the rhizosphere soil solution and thus in the concentration of salt outside the plant. It was the aim of the present investigation to test this hypothesis and to investigate the effect of such potential differences in soil Ψ_o caused by Na⁺ exclusion on plant water relations. Sodium exclusion at the root surface of intact plants growing in soil was investigated by sampling soil solution from the rhizosphere of two maize cultivars (Across 8023, Pioneer 3906). Plants were grown in a model system, consisting of a root compartment separated from the bulk soil compartment by a nylon net (30 μm mesh size), which enabled independent measurements of the change of soil solution composition and soil water content with increasing distance from the root surface (nylon net). Across 8023 accumulated higher amounts of sodium in the shoot compared to the excluder (Pioneer 3906). The lower Na⁺ uptake in the excluder was partly compensated by higher K⁺ uptake. Pioneer 3906 not only excluded sodium from the shoot but also restricted sodium uptake more efficiently from roots relative to Across 8023. This was reflected by higher Na⁺ concentrations in the rhizosphere soil solution of the excluder 34 days after planting (DAP). The difference in Na⁺ concentration in rhizosphere soil solution between cultivars was neither due to differences in transpiration and thus in mass flow, nor due to differences in actual soil water content. As the lower Na⁺ uptake of the excluder (Pioneer 3906) was only partly compensated by increased uptake of K⁺, soil Ψ_o in the rhizosphere of the excluder was more negative compared to Across 8023. However, no significant negative effect of decreased soil Ψ_o on plant water relations (transpiration rate, leaf Ψ_o, leaf water potential, leaf area) could be detected. This may be explained by the fact that significant differences in soil Ψ_o between the two cultivars occurred only towards the end of the experiment (27 DAP, 34 DAP).