Changes in soil organic compound composition associated with heat‐induced increases in soil water repellency

Soil heating, as for example experienced during vegetation fires, often increases soil water repellency; however, no detailed analysis of the soil chemical changes associated with this increase has been conducted to date. Here we characterize the changes in organic compound composition associated with heat‐induced increases in water repellency for three Australian eucalypt‐forest soils (one sandy loam, two sands). Laboratory heating (300°C) strongly increased water drop penetration times (WDPTs) in all soils. Soils were extracted by accelerated solvent extraction (ASE) with an iso‐propanol/ammonia mixture (IPA/NH₃ 95:5) and pure iso‐propanol (IPA). Extracts were fractionated into less and more polar fractions and analysed by GC‐MS. Water repellency was eliminated in unheated and heated soils by IPA/NH₃, but not by pure IPA. Before heating, total solvent extracts were dominated by n‐alkanols, terpenoids, C₁₆ acid, C₂₉ alkane, β‐sitosterol and polar compounds. After heating, dominant compounds were aromatic acids, aldehydes, levoglucosan, simple sugars and glycosides. Heating resulted in a sharp absolute decrease of homologous aliphatic series of alkanols and alkanes, a shift of fatty acid signature to members <C₂₀ and an increase in total content of aromatic compounds. Heating also caused the formation of complex high‐molecular‐weight compounds detected in the more polar fractionated extracts and low‐molecular‐weight oxo‐ and hydroxyacids and aromatics in the IPA/NH₃ solvent. We speculate that these compounds in conjunction with fatty acids of <C₁₂ interact with organic and mineral soil surfaces and cause the observed strong increases in soil water repellency following heating.     

Do surface active substances from water repellent soils aid wetting?

Soil water repellency is usually unstable, as exemplified by the common method of quantifying repellency degree – the water drop penetration time (WDPT) test. Dynamic penetration and infiltration of water into repellent soils is generally attributed to either reduction of the solid‐liquid interfacial energy (γ_SL) or reduction of the liquid‐vapour interfacial energy (γ_LV), or both. The reduction of γ_SL can result from conformation changes, hydration, or rearrangement of organic molecules coating soil particle surfaces as a result of contact with water, while the reduction of γ_LV can result from dissolution of soil‐borne surface active organic compounds into the water drop. The purpose of this study was to explicitly test the role of the second mechanism in dynamic wetting processes in unstably repellent soils, by examining the drop penetration time (DPT) of water extracts from repellent soils obtained after varying extraction times and at different soil : water ratios. It was indeed found that soil extracts had lower surface tensions (γ_LV approx. 51–54 mN m⁻¹) than distilled water. However, DPT of the soil extracts in water repellent soils was generally the same or greater than that of water. Salt solutions with the same electrical conductivity and monovalent/divalent cation ratio as the soil extracts, but lacking surface active organic substances, had the same DPT as did the extracts. In contrast, DPT of ethanol solutions prepared with the same γ_LV, electrical conductivity, and monovalent/divalent cation ratio as the soil extracts, was much faster. Ethanol solutions are usually used as an agent to reduce γ_LV and as such, to reduce DPT. It is concluded that the surface‐active, soil‐derived organic substances in aqueous soil extracts do not contribute to wetting dynamics, and as such, this mechanism for explaining kinetics of water penetration into water repellent soils is rejected. It is also concluded that the rapid penetration of ethanol solutions must be due not only to changes in γ_LV, but to also to changes in either or both γ_SL and the solid‐vapour interfacial energy (γ_SV). These results stand in sharp contrast to well‐accepted logical paradigms.     

Comparison of two alkaline treatments in the extraction of organic compounds associated with water repellency in soil under Pinus taeda

Isopropanol:NH₃ and Methanol:KOH (saponification) alkaline treatments are usually applied in the study of soil organic matter. The first is used in studies of soil water repellency, and the latter in the extraction of ester-bound lipids from soil. In this study, isopropanol:NH₃ and methanol:KOH treatments were applied separately in a solvent extracted repellent soil, in order to compare their efficiency in the extraction of water repellent compounds. The soil sample was taken from a site under a 16 year old Pinus taeda stand. The amount and class of organic compounds released by each treatment were compared using gas chromatography-mass spectrometry (GC-MS). Both treatments resulted in wettable soil after alkaline extraction. In general, alkaline treatments yielded extracts with the same class of organic compounds. Alkanoic acids, α,ω-alkanedioic acids, hydroxyalkanoic acids, aromatic compounds, and alkanols were identified, indicating the preservation of suberin and cutin biopolyester in the soil. Large differences were observed in the amounts of ω-hydroxyalkanoic acids, as well as in the quantity and distribution of dihydroxyalkanoic and trihydroxyalkanoic acids. In contrast to methanol:KOH, isopropanol:NH₃ was not efficient in the extraction of whole aliphatic biopolyesters, mainly pine cutin-related products. Methanol:KOH was more effective in hydrolysis. The presence of biopolyesters in water repellent soil under the P. taeda stand seems to play an important role in water repellency.     

Water repellency of volcanic ash soils from Ecuadorian páramo: effect of water content and characteristics of hydrophobic organic matter

Water repellency of volcanic ash soils from the Ecuadorian páramo was studied by a combination of extraction and analysis of water‐repellent products, Molarity Ethanol Droplet values, water contact‐angle measurements by capillary rise, and N₂ adsorption isotherms. The undried samples studied are hydrophilic, but exhibit water repellency after moderate drying (48 hours at 30°C). The advancing water contact‐angle measured by capillary rise varies from 78° to 89°. These water contact‐angles decrease strongly after extraction of organic materials by an isopropanol–water mixture. Elemental analysis, infrared spectra and gas chromatography‐mass spectrometry analyses were used to characterize the extracts. The results show that long‐chain fatty acids and more complex non‐polar alkyl components (waxes) are the main water‐repellent materials. The deposition of such extracted materials onto hydrophilic sand leads to the rapid increase of water contact‐angle until values close to those measured on the soil samples are achieved. Assuming a coating of the mineral surface by organic hydrophobic products and using Cassie's law, the water contact‐angle of extracted materials was computed. The values ranged from 100° to 157°. Nitrogen specific surface areas of the soils studied were very small, indicating a low adsorbent–adsorbate interaction on hydrophobic surfaces. These results partially validate the hypothesis of water‐repellent materials that occur as coatings at least after a drying process.     

Extraction and characterization of water–repellent materials from Australian soils

Organic materials responsible for water–repellency in some Australian soils were extracted with an amphiphilic mixture of iso -propanol/15.7 m ammonia (7:3, v:v) in a Soxhlet apparatus, after which the water–repellent soils were rendered wettable. The successful extraction by an organic solvent system indicates that the bulk of hydrophobicity in these soils is not covalently linked to the surface of the sand. The extracted materials restored hydrophobicity on acid washed sands or ignited sands at levels comparable to the original soils.
Spectroscopic and chromatographic examination of the extracted materials indicated that both free and esterified long–chain, 16–32 carbon atom, fatty acids were present with a bimodal distribution showing maxima at C₁₆ and C₂₂. The ¹³C–NMR and infrared spectra of the most hydrophobic extract suggest that hydrophobicity is caused by molecules with extensive polymethylene chains. Calculations with model compounds indicate that at least a close packed monolayer is required before measurable hydrophobicity can be detected with the molarity of ethanol droplet penetration test.     

Total lipid extracts from characteristic soil horizons in a podzol profile

The podzolization process is studied through lipids in nine characteristic podzol horizons. Organic matter accumulates particularly with aluminium in the Bh horizon, while the hard, cemented Bs horizon below this is formed mainly by iron oxides. The low soil pH seems to have no great influence on the preservation of lipids as reflected by the absolute amounts present and the presence of bacterial lipid markers throughout the profile. Independent of soil pH, lipids accumulate in organically enriched horizons. Albeit, high molecular weight organic compounds accumulate to a relatively greater extent than lipids in these horizons. A lipid signal related to the aerial parts, i.e. leaves and flowers, of Calluna is observed only in the O horizon. This ‘n‐alkane, steroid and triterpenoids’ signal is quickly lost in the underlying Ah horizon due to (bacterial) oxidation. The other total lipid extracts obtained are dominated by root‐derived compounds. In subsoil horizons rich in organic matter, i.e. the Ahb and Bh horizons, root‐derived friedooleanan and steroid compounds dominate the total lipid signal. Degraded horizons, poor in organic matter, i.e. the E2, Bhs, Bs and B/C horizons, are dominated by C₂₂ and C₂₄ω‐hydroxy acids, long‐chain (> C₂₀) n‐alkanoic acids with a strong even‐over‐odd predominance and C₂₂ and C₂₄n‐alkanols. Steroid and root‐derived triterpenoids with a friedooleanan structure have been removed from these horizons through degradation. Based on total organic carbon content and lipid composition, the formation of an E1 horizon has started, but is not yet complete. In the Ahb horizon, a contribution from buried vegetation to the total lipid signal is still present, although degradation and an input from roots have significantly altered the original signal. Overall, lipid data indicate that degradation (microbial oxidation) is an important process that should be taken into account, in addition to leaching, when describing podzolization processes in soils.     

Water repellency of fly ash‐enriched forest soils from eastern Germany

Fly ash‐enriched soils occur widely throughout the industrial regions of eastern Germany and in other heavily industrialized areas. A limited amount of research has suggested that fly ash enrichment alters the water repellency (WR) characteristics of soil. This study concentrates on the influence of fly ash enrichment on WR of forest soils with a focus on forest floor horizons (FFHs). The soils were a Technosol developed from pure lignite fly ash, FFHs with lignite fly ash, and FFHs without lignite fly ash enrichment. Three different methods (water drop penetration time, WDPT, test; water and ethanol sorptivity measurement and the derived contact angle, θ_R; and the Wilhelmy‐plate method contact angle, θ_wpm) were used to characterize soil WR. Additionally, carbon composition was determined using ¹³C‐NMR spectra to interpret the influence of the organic matter. This study showed that the actual WR characteristics of undisturbed, fly ash‐enriched soils can be explained in terms of the composition of soil organic matter, with the fly ash content playing only a minimal role. Regardless of the huge amounts of mainly mineral fly ash enrichment, all undisturbed FFHs were comparable in their WR characteristics and their carbon compositions, which were dominated by recently‐formed organic substances. The pure fly ash deposit was strongly influenced by lignite remains, with the topsoil having a greater content of recent plant residues. Thus, the undisturbed topsoil was more repellent than the subsoil. When homogenized samples were used, we found a distinct effect of fly ash enrichment and structure on WR. Water repellency of the pure fly ash horizons did not differ distinctly, while the fly ash enrichment in the FFHs caused a significant reduction in WR. The methods used (WDPT, θ_R and θ_wpm) identified these differences similarly. These results led to the assumption that water‐repellent structures of the topsoils were probably the result of hydrophobic coatings of recently formed organic substances, whereby the initially high wettability of the mainly mineral, hydrophilic fly ash particles was reduced.     

Effects of compaction on soil surface water repellency

Water repellency can reduce the infiltration capacity of soils over timescales similar to those of precipitation events. Compaction can also reduce infiltration capacity by decreasing soil hydraulic conductivity, but the effect of compaction on soil water repellency is unknown. This study explores the effect of compaction on the wettability of water repellent soil. Three air‐dry (water content ～4 g 100 g^?1) silt loam samples of contrasting wettability (non‐repellent, strongly and severely water repellent) were homogenized and subjected to various pressures in the range 0–1570 kPa in an odeometer for 24 h. Following removal, sample surface water repellency was reassessed using the water drop penetration time method and surface roughness using white light interferometry. An increase in compaction pressure caused a significant reduction in soil surface water repellency, which in turn increases the soil's initial infiltration capacity. The difference in surface roughness of soils compacted at the lowest and highest pressures was significant (at P > 0.2) suggesting an increase in the contact area between sessile water drops and soil surfaces was providing increased opportunities for surface wetting mechanisms to proceed. This suggests that compaction of a water repellent soil may lead to an increased rate of surface wetting, which is a precursor to successful infiltration of water into bulk soil. Although there may be a reduction in soil conductivity upon compaction, the more rapid initiation of infiltration may, in some circumstances, lead to an overall increase in the proportion of rain or irrigation water infiltrating water repellent soil, rather than contributing to surface run‐off or evaporation.     

Nature and Origin of Lipids in Clay Fractions from a Fluvisol in a Sewage Sludge Deposition Field

Few studies on free lipids in total solvent extracts from soil clay fractions directly measured by gas chromatography–mass spectrometry (GC/MS) have been reported so far. In this study, we aimed to examine the free lipids in the clay fraction separated from a Fluvisol profile on which sewage sludge was deposited 6 years ago and provide information on the sources, diagenetic processes and organic pollutants derived from the sludge. Clay fractions were separated from the four horizons of a Fluvisol and analysed for the biochemically stable lipid pool. The GC/MS analysis of the lipid fraction showed that lipid signatures were dominated by alkanes (C₁₇–C₃₃), alkanoic acids (C₁₂–C₁₈), alkanols (C₁₄–C₃₀), aromatic acids and phthalate esters. Sources of lipids show predominant bacterial contribution as shown by the alkane, fatty acids and n-alkanol distributions. The preservation of lipids of microbial origin in the clay fraction was revealed by the presence of even number, branched alkanes and short-chain and branched fatty acids. These results imply that similar pedogenic processes took place in this soil profile affected by hydromorphic conditions with some quantitative changes in the clay lipid compositions between different soil horizons. Some aromatic acids and xenobiotics such as phthalates were detected in the lipid extracts of the clay fractions in depth (0–85 cm) of the soil profile, which has implications for pollution of soils and ground waters in situations of sewage sludge deposition.     

Determination of oxalic acid (ethanedioic acid) in soil extracts using high performance liquid chromatograph

Abstract

This paper describes a method for quantifying oxalate in soil HC1 extracts using reversed‐phase ion‐pairing high performance liquid chromatography with UV detection at 220 nm. The method was adapted from a procedure for determining urinary oxalate (6). The mobile phase was 10 percent KH₂PO₄ and 5 mM TBA adjusted to pH 2.0 with H₃PO₄. The analytical column was a totally porous, reversed‐phase silica based C‐8 column (Hibar Li‐Chrosorb?). An important step in this method was the pre‐ treatment of each soil extract with a reversed‐phase C‐18 column (SPICE? C‐18). Sample pre‐treatment removed complex, non‐polar and low polarity compounds often present in soil extracts.

The method was applied to calcareous, agricultural and organic soils materials. An oxalate accumulating fungus, Endothia Parasitica, was used as a verification of method applicability to plant‐fungal materials. Oxalate extraction was accomplished by placing 1:2 suspensions (soil: 0.1 M HCl) on a reciprocal shaker for two hours and subsequently collecting the solution by filtering through a Whatman No. 42 filter paper.

Coefficients of variation for replicate oxalate determinations were less than 10 percent. Recovery of oxalate from spiked extracts of soil and plant material were consistently between 82 and 104 percent. Detection limits were approximately 1 μM which was greater than the concentration of oxalate in saturation paste extracts. Oxalate was detected in some of these samples but could not be quantified.     

The determination of total phosphorus improves the accuracy of the bicarbonate extraction as an availability index

The efficient use of phosphorus (P) in agriculture should rely on accurate soil P tests (SPT). Organic P contributes to P supply to plants; however, it is not usually taken into account in assessing P fertilizer requirements. We hypothesized that there would be an increased accuracy of bicarbonate extraction as SPT in predicting P uptake by plants if total P (TP) in this soil extract is taken into account. We conducted a soil P depletion experiment with 36 soils involving four consecutive crops in pots. Molybdate‐reactive P (MRP) and total P were determined in extracts centrifuged at 19,000 g (Bic‐MRP_C and Bic‐TP_C) or not (Bic‐MRP and Bic‐TP). MRP in extracts explained <47% of the variance in the cumulative P uptake, while total P (centrifuged at 19,000 g or not) provided the most accurate estimation of P uptake (59% with Bic‐TP) and threshold values for fertilizer response (R² = 0.58 with Bic‐TPc). When soils were separated in two groups according to their Ca carbonate equivalent to clay ratio, the variance in the cumulative P uptake explained by Bic‐MRP was above 63%, and that explained by Bic‐TP was above 73%. This separation also enabled more realistic estimation of the threshold values for fertilizer response. It can be concluded that the use of total P instead of MRP in bicarbonate extraction was promising in terms of improving its accuracy in assessing P fertilizer requirements.     

Phosphorus in sequentially extracted fen peat soils: A K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy study

The speciation of phosphorus (P) in native and degraded peat soils is an analytical challenge, and synchrotron‐based P K‐edge X‐ray absorption near‐edge structure (XANES) is a suitable method to gain information on P species in soils and organic materials. The objective of the present study was to test if P K‐edge XANES reflected differences in P fractions in fen peat due to sequential extraction and peat degradation. We investigated each one top‐ and subsoil sample of a Fibric Histosol, which differed in the degree of humification (H8 vs. H5) and concentration of total P (P_t) (1944 mg kg^–1 vs. 436 mg kg^–1). In the topsoil, residual P, H₂SO₄‐P, and NaOH‐P accounted for roughly the same proportions of P_t (≈30%). In the subsoil, residual P (64% of P_t) was more abundant than NaOH‐P (21% of P_t) and H₂SO₄‐P (10% of P_t). Among many different P reference standards, the P XANES spectra reflected differences in mineral P more distinctive than in organic P compounds. Phosphorus XANES spectra of the residues after each sequential extraction step all showed a prominent white‐line peak at around 2152 eV. Stepwise removal of resin‐P, NaHCO₃‐P, and NaOH‐P were reflected mainly by the peak intensity but scarcely by distinct spectral features. Extraction with H₂SO₄ led to the disappearance of spectral features of Ca and Mg phosphates which is a first direct hint to these compounds in the peat. In conclusion, a combined sequential fractionation and spectroscopic (³¹P NMR, P K‐ and L‐edge XANES with linear‐combination fits) approach is proposed to overcome limitations of the present study and gain more insight into the P species in peat soils.     

Limitations when quantifying microbial carbon and nitrogen by fumigation‐extraction in rooted soils

Soil microbial C and N (C_mic, N_mic) estimation by the chloroform fumigation‐extraction method is erroneous in densely rooted soils due to CHCl₃‐labile C and N compounds. The effect of a pre‐extraction with 50 mM K₂SO₄ and a pre‐incubation (conditioning at 25 °C for 7 days) on the flush in extractable, CHCl₃‐labile C (C‐flush) and N (N‐flush) was tested with reference to rooting density (0.3—75 mg root dry matter g^—1) in one arable and 3 grassland soils. In the arable soil and in the second horizon (10—20 cm) of a grassland soil, C‐flush values were not affected by the pre‐extraction. However, the pre‐extraction considerably reduced C‐flush values in the top soils of the grassland (above 10 cm). Only about 42 % was found in the pre‐extracted roots and the rest was lost during the pre‐extraction. The estimated concentrations of N_mic decreased due to pre‐extraction of soil samples with low root biomass. Clearly, the concentrations of N_mic were underestimated by introducing the pre‐extraction. Soil pre‐incubation reduced C‐flush values only slightly, whereas N‐flush values were not affected. It can be concluded that (1) CHCl₃‐labile root C and N is partly extracted with K₂SO₄ after pre‐incubation and (2) CHCl₃‐labile C and N removed with the roots during pre‐extraction is partly derived from microbial biomass. Soils with low rooting density (arable soils, grassland soils below approximately 10 cm depth) should therefore be fumigated and extracted without pre‐extraction. In densely rooted soils, fumigation extraction with and without pre‐extraction probably gives estimates for the minimum and maximum of C_mic and N_mic.     

Quantification of long‐chain fatty acids in dissolved organic matter and soils

The objective was to develop and adapt a versatile analytical method for the quantification of solvent extractable, saturated long‐chain fatty acids in aquatic and terrestrial environments. Fulvic (FA) and humic (HA) acids, dissolved organic matter (DOM) in water, as well as organic matter in whole soils (SOM) of different horizons were investigated. The proposed methodology comprised extraction by dichloromethane/acetone and derivatization with tetramethylammonium hydroxide (TMAH) followed by gas chromatography/mass spectrometry (GC/MS) and library searches. The C_10:0 to C_34:0 methyl esters of n‐alkyl fatty acids were used as external standards for calibration. The total concentrations of C_14:0 to C_28:0 n‐alkyl fatty acids were determined in DOM obtained by reverse‐osmosis of Suwannee river water (309.3 μg g^—1), in freeze‐dried brown lake water (180.6 μg g^—1), its DOM concentrate (93.0 μg g^—1), humic acid (43.1 μg g^—1), and fulvic acid (42.5 μg g^—1). The concentrations of the methylated fatty acids (n‐C_16:0 to n‐C_28:0) were significantly (r² = 0.9999) correlated with the proportions of marker signals (% total ion intensity (TII), m/z 256 to m/z 508) in the corresponding pyrolysis‐field ionization (FI) mass spectra. The concentrations of terrestrial C_10:0 to C_34:0 n‐alkyl fatty acids from four soil samples ranged from 0.02 μg g^—1 to 11 μg g^—1. The total concentrations of the extractable fatty acids were quantified from a Podzol Bh horizon (26.2 μg g^—1), Phaeozem Ap unfertilized (48.1 μg g^—1), Phaeozem Ap fertilized (57.7 μg g^—1), and Gleysol Ap (66.7 μg g^—1). Our results demonstrate that the method is well suited to investigate the role of long‐chain fatty acids in humic fractions, whole soils and their particle‐size fractions and can be serve for the differentiation of plant growth and soil management.     

Investigation of Australian olive mill waste for recovery of biophenols

Olive mill waste is a potential source for the recovery of phytochemicals with a wide array of biological activities. Phytochemical screening of hexane, methanol, and water extracts revealed a diversity of compounds, perhaps overlooked in previous studies through intensive cleanup procedures. Methanol and water extracts contained large amounts of biophenols, and further testing of polar extraction solvents, including ethyl acetate, ethanol, propanol, acetone, acetonitrile, and water/methanol mixtures, highlighted the latter as the solvent of choice for extraction of the widest array of phenolic compounds. Stabilization of the resulting extract was best achieved by addition of 2% (w/w) sodium metabisulfite. Quantitative data are reported for nine biophenols extracted using 60% (v/v) methanol in water with 2% (w/w) sodium metabisulfite. Six compounds had recoveries of greater than 1 g/kg of freeze-dried waste: hydroxytyrosol glucoside, hydroxytyrosol, tyrosol, verbascoside, and a derivative of oleuropein.     

Approaching a Standardized Method for the Hot-Water Extraction of Peat Material to Determine Labile SOM in Organic Soils

Peatland soils are the most effective and important long-term terrestrial carbon (C) storages. To estimate potential C loss, a valid characterization of soil decomposability, in particular the labile fraction, is of great interest. One of the most labile fractions is hot-water-extractable organic matter (HWOM), often measured as hot-water-extractable carbon (C_hwe) and nitrogen (N_hwe). Various studies describe different extraction procedures for mineral soils. Because of methodical differences, it is difficult to compare extracted HWOM amounts directly to each other. For peatland soils, few studies exist. The aim of the present study is the development of a standardized method for the hot-water extraction of peat materials. Therefore, we extracted HWOM in various replicates from different peats on the basis of a standardized extraction method for mineral soils (1 h extraction at 100 °C). We tested how differences in soil/water ratios, extract treatment (filtering vs. not filtering), and sample pretreatment (freeze drying vs. air drying) influence HWOM amounts. The results clearly illustrated the influence of changing soil/water ratios on HWOM amounts. Mean C_hwe concentrations ranged between 8 and 34 g kg^?1 whereas N_hwe ranged between 0.2 and 2.6 g kg^?1. We recommend the extraction under soil/water ratios of 1/800 to provide sufficient volume of solvent for C_hwe. If relative differences for N_hwe amounts are greater than 15 percent, samples should be extracted again under soil/water ratios greater than 1/300 to avoid analytical errors due to unintended dilution effects. Filtering of centrifuged and decanted extracts before analysis is not necessary. Peat material should be either air dried or freeze dried before extraction.     

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.     

Occurrence, prediction and hydrological effects of water repellency amongst major soil and land-use types in a humid temperate climate

Knowledge of soil water repellency distribution, of factors affecting its occurrence and of its hydrological effects stems primarily from regions with a distinct dry season, whereas comparatively little is known about its occurrence in humid temperate regions such as typified by the UK. To address this research gap, we have examined: (i) water repellency persistence (determined by the water drop penetration time method, WDPT) and degree (determined by the critical surface tension method, CST) for soil samples (0–5, 10–15 and 20–25 cm depth) taken from 41 common soil and land‐use types in the humid temperate climate of the UK; (ii) the supposed relationship of soil moisture, textural composition and organic matter content with sample repellency; and (iii) the bulk wetting behaviour of undisturbed surface core samples (0–5 cm depth) over a period of up to 1 week. Repellency was found in surface samples of all major soil textural types amongst most permanently vegetated sites, whereas tilled sites were virtually unaffected. Repellency levels reached those of the most severely affected areas elsewhere in the world, decreased in persistence and degree with depth and showed no consistent relationship with soil textural characteristics, organic matter or soil moisture contents, except that above a water content of c. 28% by volume, repellency was absent. Wetting rate assessments of 100 cm³ intact soil cores using continuous water contact (–20 mm pressure head) over a period of up to 7 days showed that across the whole sample range and irrespective of texture, severe to extreme repellency persistence consistently reduced the maximum water content at any given time to well below that of wettable soils. For slightly to moderately repellent soils the results were more variable and thus hydrological effects of such repellency levels are more difficult to predict. The results imply that: (i) repellency is common for many land‐use types with permanent vegetation cover in humid temperate climates irrespective of soil texture; (ii) supposedly influential parameters (texture, organic matter, specific water content) are poor general predictors of water repellency, whereas land use and the moisture content below which repellency can occur seem more reliable; and (iii) infiltration and water storage capacity of very repellent soils are considerably less than for comparable wettable soils.     

Nonhydrolyzable forms of soil organic nitrogen: Extractability and composition

The chemical identity of organic nitrogen (N) containing compounds in soils is only partially known, because 20—35% of soil N can not be hydrolyzed and identified by wet‐chemical methods. Therefore a new methodology for investigations of the extractability and composition of nonhydrolyzed N was developed using a combination of selective extraction, wet‐chemical analyses and pyrolysis‐mass spectrometry. Residues of organic matter hydrolysis with 6 M HCl from particle‐size separates and whole soils of five sites in Thyrow, 2 × Halle, Lauterbach and Bad Lauchstädt (Germany) were treated with dithionite/citrate/bicarbonate (DCB) to remove pedogenic oxides and bound N‐containing compounds. Between 13 and 61% (mean 34 ± 13%) of nonhydrolyzed N (N_nhydr) was extracted with DCB. For all particle‐size separates, there was a close positive correlation between the contents of nonhydrolyzed N and DCB extractable Al and Fe, respectively. Univariate analysis of variance and Pearson correlation coefficients showed that the specific surface areas of samples were the major factor determining the contents of N_nhydr. About 30—50% of the variation in N_nhydr could be explained by the variation in the contents of pedogenic oxides. In the DCB extraction residues an additional portion of 12 to 66% of N was hydrolyzed by 6 M HCl. About 75% of total N in the DCB extracts was hydrolyzed, and 29% was identified as α‐amino‐N. Amino acid analysis showed that the DCB extracts from clay and fine silt contained all amino acids characteristic of soils. Pyrolysis‐field ionization mass spectrometry (Py‐FIMS) of a freeze‐dried DCB extract indicated the presence of peptides (about 5% of total ion intensity) and heterocyclic N‐containing compounds (about 3% of total ion intensity). In summary, these results provide evidence that organic‐mineral bonds at reactive surfaces (silicates, pedogenic oxides, alkali‐extractable organic substances) are the main factors for the nonhydrolyzability of significant amounts of organic N, including peptides. It is concluded that the processes of trapping and binding of proteinaceous compounds proceed as postulated for recent models of soil organic matter (SOM) and soil particles. Molecular mechanics calculations show large gaps between SOM and the mineral matrix and as well as fine pores and voids in SOM which have a strong potential to occlude and bind peptides (hydrogen bonds).     

Changes in soil organic matter composition are associated with forest encroachment into grassland with long-term fire history

This study investigates if Araucaria forest (C₃ metabolism) expansion on frequently burnt grassland (C₄ metabolism) in the southern Brazilian highland is linked to the chemical composition of soil organic matter (SOM) in non‐allophanic Andosols. We used the ¹³C/¹²C isotopic signature to group heavy organo‐mineral fractions according to source vegetation and ¹³C NMR spectroscopy, lignin analyses (CuO oxidation) and measurement of soil colour lightness to characterize their chemical compositions. Large proportions of aromatic carbon (C) combined with small contents of lignin‐derived phenols in the heavy fractions of grassland soils and grass‐derived lower horizons of Araucaria forest soils indicate the presence of charred grass residues in SOM. The contribution of this material may have led to the unusual increase in C/N ratios with depth in burnt grassland soils and to the differentiation of C₃‐ and C₄‐derived SOM, because heavy fractions from unburnt Araucaria forest and shrubland soils have smaller proportions of aromatic C, smaller C/N ratios and are paler compared with those with C₄ signatures. We found that lignins are not applicable as biomarkers for plant origin in these soils with small contents of strongly degraded and modified lignins as the plant‐specific lignin patterns are absent in heavy fractions. In contrast, the characteristic contents of alkyl C and O/N‐alkyl C of C₃ trees or shrubs and C₄ grasses are reflected in the heavy fractions. They show consistent changes of the (alkyl C)/(O/N‐alkyl C) ratio and the ¹³C/¹²C isotopic signature with soil depth, indicating their association with C₄ and C₃ vegetation origin. This study demonstrates that soils may preserve organic matter components from earlier vegetation and land‐use, indicating that the knowledge of past vegetation covers is necessary to interpret SOM composition.