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.     

Influence of mineral and organic fertilizers on soil organic carbon pools

Abstract

The influence of farmyard manure (FYM) and equivalent mineral NPK application on organic matter content, hot water extractable carbon (HWC), microbial biomass C (C_mic), and grain yields in a long-term field experiment was assessed after 40 years in Hungary. The unfertilized plot, FYM fertilized plots and plots fertilized with equivalent NPK fertilizer contained 0.99%, 1.13% and 1.05% total organic carbon (TOC) respectively. Compared to the unfertilized plot, FYM application resulted in 8.2% higher TOC than equivalent NPK fertilization. The highest TOC was only 1.21%, much lower than expected for a soil containing 21.3% of clay. The quantity of HWC varied depending on the type of fertilization: Compared to control, FYM treatments lead to 29% more HWC than mineral fertilization (FYM: 328 mg kg^?1; NPK: 264 mg kg^?1). The impact of FYM and equivalent NPK fertilizer on C_mic was contrary. FYM and NPK resulted in 304 and 423 mg kg^?1 C_mic, respectively. The difference was 119 mg kg^?1; 42% as compared to the unfertilized plot. Despite the higher HWC content, FYM treatments lead to significantly less (35%) grain yields than equivalent NPK doses; C_mic content showed closer correlation to grain yields.     

Contribution of primary organic matter to the fatty acid pool in agricultural soils

Fatty acids as major compounds of soil lipids may affect many soil properties, but the input and turnover rates in soil are largely unknown. The objective of this study was to identify and quantify fatty acids in soils as a result of input from primary sources such as plant residues, farmyard manure and soil organisms, and to evaluate the corresponding turnover- and stabilization processes. The concentrations of n-C_10:0 to n-C_34:0 fatty acids were determined in the Ap horizon of a Phaeozem with long-term cropping of rye and maize and the treatments ‘Unfertilized’ (‘U’) and fertilized with ‘Farmyard manure’ (‘FYM’). The most important primary sources of fatty acids such as rye and maize stubble and roots, soil micro- and mesofauna, and the applied FYM were also investigated. The quantification of fatty acids by gas chromatography/mass spectrometry (GC/MS) showed that long-term FYM application led to larger concentrations of n-alkyl fatty acids in the plots grown with rye (‘U’: 48.1 μg g⁻¹, ‘FYM’: 57.7 μg g⁻¹, **P≤0.01, n=3) and maize (‘U’: 17.0 μg g⁻¹, ‘FYM’: 23.4 μg g⁻¹, ***P≤0.001, n=3). The observed bimodal fatty acid distribution in soils from n-C_10:0 to n-C_21:0 and from n-C_21:0 to n-C_34:0 with a predominance at n-C_16:0 and at n-C_28:0 was apparently due to input from crop residues, soil organisms and FYM. The short-chain lengths may have originated from the investigated primary sources. The major contributors to the long-chain lengths, with a maximum at n-C_28:0, were rye stubble and FYM. A change in mono-culture from rye to maize, 38 years prior to sampling, led to a decrease in fatty acid concentrations by factors of about 2.8 (‘U’) and 2.5 (‘FYM’). Therefore, rye-derived fatty acids and soil tillage had a larger impact on fatty acid pools than the input of primary organic matter. The changes in fatty acid distributions and pools under the consideration of the quantified input of primary organic matter led to the conclusion that the short-chained fatty acids were more rapidly decomposed than the long-chains.     

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.     

Effects and residual effects of straw,farmyard manuring,and mineral fertilization at Field F of the long‐term trial in Halle (Saale), Germany

The effects and residual effects of farmyard manure (FYM), straw fertilization and mineral N fertilization were investigated in Field F of the long‐term fertilization trial in Halle (Saale), Germany. With sufficient mineral N fertilization, FYM and straw did not directly affect yield. The application of FYM alone increased the yield of potato less than those of silage maize and sugar beet. With low mineral N fertilization, however, residual effects of FYM, applied to root crops, were observed in the following cereal crops. Application of more mineral N to root crops had the same residual effects. In case of omitted mineral N fertilization, the humus content of the soil decreased rapidly. This implies that almost no stable humic material had been accumulated by application of FYM and straw. The calculated N loss increased with enhanced organic fertilization. In case of mineral N fertilization the content of organic C (C_org) was slightly higher (1.4 to 1.5%) than without any N fertilization (1.3%). FYM and straw (with same amounts of dry matter) likewise enhanced the C_org‐content and, consequently, the content of decomposable C (C_dec). In general, organic fertilizers should not be applied in too large amounts to avoid N losses.     

Changes in the microbial community of an arable soil caused by long-term metal contamination

The effects of past applications of farmyard manure (FYM, applied from 1942 to 1967), metal‐contaminated sewage sludge (applied from 1942 to 1961) and mineral fertilizer (NPK, applied from 1942 until now) on the microbial biomass and community structure in a sandy loam, arable soil from the Woburn Market Garden Experiment, UK, were investigated in 1998. Concentrations of Cu, Ni and Zn in soils which previously received sewage sludge were less than current European Union (EU) limits, but the soil Cd concentration was more than twice the permitted limit. Organic‐C concentration in the FYM‐treated soil and contaminated soils was about twice that of NPK‐treated soil. The initial microbial biomass‐C and estimates of total bacterial numbers by acridine orange direct count were significantly (P < 0.05) greater in the FYM‐treated soil compared with the NPK‐treated and the most contaminated soils. Total phospholipid fatty acid (PLFA) concentration (another measure of biomass) was significantly greater in the FYM‐treated soil compared with either the low or high metal‐contaminated soils, both of which contained similar PLFA concentrations. In the metal‐contaminated soils, in contrast, fluorescent Pseudomonas counts, as a percentage of total plate counts, were at least 1.5 times greater than in the uncontaminated soils. The concentrations of these microbial parameters were significantly (P < 0.05) less in the NPK soil than in all the other treatments. Biomass‐C as a percentage of organic‐C was also significantly (P < 0.05) greater in the uncontaminated soils compared with the metal‐contaminated soils. Biomass specific respiration rates in the metal‐contaminated soils were c. 1.5 times those in the FYM‐treated soil. In the metal‐contaminated soils, the concentration of mono‐unsaturated and hydroxy‐fatty acids (derived from phospholipids), and lipopolysaccharide hydroxy‐fatty acids (all indicative of Gram‐negative bacteria) were significantly (P < 0.05) greater than branched fatty acids (indicative of Gram‐positive bacteria). Furthermore, Gram‐negative counts were 62–68% greater than Gram‐positive counts in the metal‐contaminated soils. Branched fatty acid concentration was significantly (P < 0.05) greater in the FYM‐treated soil than in the metal‐contaminated soils. Gram‐positive counts were also 63% greater than Gram‐negative counts in the FYM‐treated soil. We found that effects of the relatively small heavy metal concentration caused measurable decreases in soil microbial biomass‐C concentrations, acridine orange direct counts and Gram‐positive counts. There were also increases in biomass specific respiration rates, and the microbial community had changed substantially, nearly 40 years after the metal inputs ceased. We conclude that, at the very least, the current EU permitted limits for heavy metals in agricultural soils should not be relaxed.     

Long-term effect of straw and farmyard manure on soil organic matter in field experiment in the Czech Republic

The effect of long-term (45 years) mineral and organic fertilization on soil organic matter (SOM) quantity (organic C and N content) and quality (hot-water-soluble C content, microbial biomass C content, hydrophobic organic components of SOM, soil enzyme activities) was determined in a field experiment established in Trutnov (North Bohemia, sandy loam, Eutric Cambisol). Six treatments were chosen for investigation: unfertilized control, mineral fertilization (NPK), straw N, farmyard manure (FYM) and straw and FYM completed with mineral NPK. Soil samples were taken from the arable layer (0–20 cm) in spring over the period of 2004–2010. The positive effect of FYM on the total organic C and N content, hot-water-soluble C content and hydrophobic organic components of SOM was more than 50% higher than that of straw and mineral N fertilization. Application of straw N increased microbial biomass C content in soil and generated invertase activity above the level of FYM. Hot-water-soluble C content, hydrophobic organic components of SOM and urease activity were positively correlated with total organic C and N content (R = 0.58–0.98; p < 0.05). Addition of mineral NPK to both the straw and FYM emphasized the effect of organic fertilization in most of monitored characteristics.     

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.     

Influence of long-term fertilization with farmyard manure on soil organic matter: Characteristics of particle-size fractions

Summary The influence of more than 100 years of fertilization with farmyard manure on soil organic matter in comparison to unfertilized soil was studied in particle-size fractions using elemental (C and N) analyses and pyrolysis-field ionization mass spectrometry. Distinct differences in C and N concentrations and distribution and in the quality of organic matter between the size fractions and the fertilization treatments were observed. Clay-associated C and N were relatively higher in the unfertilized treatment, whereas the application of farmyard manure preferentially increased soil organic matter associated with the fine and medium silt fractions. Pyrolysis-field ionization mass spectrometry of soil fractions <20 m showed increasing values for lignin monomers and dimers and fatty acids with larger equivalent diameters, whereas the proportion of N compounds, mono- and polysaccharides and phenolics decreased in the larger size fractions. Sand fractions were particularly rich in lignin fragments, mono- and polysaccharides, and alkanes/alkenes. These relationships seemed to be independent of management practices. In the same size fractions of the different treatments, however, a higher relative abundance of N-compounds, mono- and polysaccharides, phenolics, lignin monomers, and alkanes/alkenes was observed in the unfertilized variant. Lignin dimers and fatty acids were more abundant in the farmyard manure treatment. Both trends together imply that soil enrichment in organic matter due to the application of farmyard manure largely reflects an increase in lignin building blocks and partly reflects an increase in lipids such as fatty acids in the silt fractions. Therefore these constituents are of particular importance in assessing the positive effects of farmyard manure on soil fertility.     

Micro-scale distribution of microorganisms and microbial enzyme activities in a soil with long-term organic amendment

Microbial ecology is the key to understanding the function of biodiversity for organic matter cycling in the soil. We have investigated the impacts of farmyard manure added over 120 years on organic matter content, enzyme activities, total microbial biomass and structure of microbial populations in several particle‐size fractions of a Luvic Phaeozem a few kilometres northeast of Halle, Germany. We compared two treatments: no fertilization (control) and 12 t farmyard manure (FYM) ha^?1 year^?1 since 1878. The fine fractions contained most C and N, microbial biomass, total amount of phospholipid fatty acids (PLFAs) and greatest invertase activity. Xylanase activity as well as fungal biomass increased only gradually with diminishing particle size, whereas the relative abundance of fungi decreased with diminishing particle size. The least diversity of the soil microbial community, indicated by the smallest Shannon index based on the abundance and amount of different PLFAs and small number of terminal restriction fragments (T‐RFs) of 16S rRNA genes, was in the sand fractions. The results supported the hypothesis that this microhabitat is colonized by a less complex bacterial community than the silt and clay fractions. Addition of FYM had enhanced the amount of organic matter, total microbial biomass, and xylanase and invertase activity, and induced a shift of the microbial community towards a more bacteria‐dominated community in the coarse sand fraction. Microbial communities in finer fractions were less affected by addition of FYM.     

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.     

Effects of humus content,farmyard manuring,and mineral‐N fertilization on yields and soil properties in a long‐term trial

Investigations carried out at Field F3 of the Halle long‐term fertilization trials using data from 1974 to 1983 showed that with adequate supply of mineral N‐fertilizer soil organic matter (SOM) had no significant effects of yield. Similarly enhanced SOM did not justify a reduction of mineral N (Stumpe et al., 2000). The studies presented here examine the effects of the SOM differences existing after the termination of those trials in 1986 up until 1997 (then mainly differences of hardly decomposable SOM) in comparison to farmyard manuring with enhanced mineral N application (3‐factor‐experiment). As with total SOM, hardly decomposable SOM did not directly affect yields. The effects of FYM treatment observed at lower mineral‐N levels were compensated for by enhanced mineral‐N supply. The direct effect of FYM (40 t ha^—1) corresponded to a mineral‐N supply of about 60 kg ha^—1 and the residual effect to about 20 kg ha^—1. The differences of the C‐content in the soil at the beginning of the present studies continued throughout the experimental period of 12 years. In addition, significant differentiation has been caused by FYM and N fertilization in comparison to unfertilized treatments. The major finding is that differences in SOM content do not lead to yield differences on physically good soils (chernozem‐like soils) if appropriate compensation by mineral‐N fertilization takes place.     

Influence of geogenic CO2 on mineral and organic soil constituents on a mofette site in the NW Czech Republic

Geogenic CO₂ emission on mofette sites may be a factor in soil formation. To demonstrate a CO₂ effect, we studied soils (0–60 cm depth) along a transect across a mofette in the NW Czech Republic. We determined CO₂ partial pressures (p(CO₂)), and the contents in the soil of carbon (C), nitrogen (N), sulphur and dithionite‐ and oxalate‐extractable iron and manganese. X‐ray diffractometry (XRD) and Fourier‐transform infrared (FTIR) spectroscopy methods were applied to the soils' particle‐size fractions. The CO₂ partial pressures varied considerably (0.001–1) along the transect and were positively correlated with both the C_org contents (5.5–432.9 g kg⁻¹) and the C:N ratio (9.3–32.2), indicating a decreased turnover of organic parent material with increasing CO₂. When the soil atmosphere was entirely composed of CO₂, pedogenic Fe oxide contents were small (minimum 0.5 g dithionite‐extractable Fe kg⁻¹) and poorly crystalline. XRD and FTIR spectroscopy revealed primary and secondary minerals such as quartz, feldspars, mica, illite, kaolinite and halloysite irrespective of CO₂ contents. A pronounced effect of CO₂ was found for soil organic matter (SOM), because the FTIR spectra did not reveal a normal accumulation of alkyl C and lipids of microbial origin in the clay fraction. This indicates that microbial synthesis and/or degradation of plant‐derived aliphatic species were reduced. We did not detect more organo–mineral associations, microbially formed polypeptides or pectin in clay fractions in comparison with the clay‐plus‐silt fractions at large p(CO₂). This indicates relatively unaltered particulate OM in the clay fraction. At large p(CO₂) values, the IR bands indicative of lignin became detectable and that of aryl ketones in lignin was positively correlated with p(CO₂). Thus, we suggest that microbial formation of SOM and degradation of lignin is restricted under an increased CO₂ atmosphere. We attribute less humification at increased CO₂ in the soil atmosphere to a decrease in oxidative transformations and decreased microbial activity.     

Phosphorus and molybdenum interaction effects during accumulation of molybdenum by burley tobacco 1

Greenhouse experiments were conducted to study the P and Mo interactions during uptake by burley tobacco (Nicotiana tabacum L., cv. ‘Ky 14') and the effect of their interactions on plant dry matter and Mo concentration. In Experiment 1, plants were grown in an intermittently irrigated gravel culture system. The nutrient variables were Mo (0–0.63 μeq/L) and P (0 to 1.8 meq/L). Experiment 2 was conducted in soil‐sand mixtures. The soil types used were Shelbyville silt loam (Mollic Hapludalf) and Baxter silt loam (Typic Paleudalf) each containing, respectively, 36 and 368 kg P/ha by the Bray 1 soil test. Treatments consisted of four rates of P (0, 84, 168, and 336 kg/ha) and five rates of Na₂MoO₄‐2H₂O (0, 0.56, 1.12, 2.24, and 4.48 kg Mo/ha).

In gravel culture a significant synergistic Mo × P interaction occurred for dry weight, but singly the effect of P was greater than Mo. Similarly, significant positive Mo × P interactions occurred for Mo concentrations in both gravel and soil culture. As contrasted to adding no P or Mo, adding the highest levels of P and Mo together increased Mo concentrations in leaves four, five, and 10 fold, respectively, in gravel culture, Shelbyville, and Baxter soils. The rate of accumulation of Mo was much greater at plant Mo concentrations above 1 μg Mo/g tissue than below, suggesting two mechanisms of Mo × P interaction were involved.     

Molecular Characteristics Influencing Retrogradation Kinetics of Rice Amylopectins

The enthalpy changes (ΔH) for melting of crystallites formed during retrogradation of 60% (w/w) amylopectins (AP) aged at 4°C were investigated using AP from 13 rice cultivars with well‐known structural properties. According to the Avrami equation, the resultant kinetic parameters for AP retrogradation were obtained in relation to structural factors. Generally, the AP systems studied showed two stages of retrogradation behavior during early (≤7 days) and late (≥7 days) storage. The Avrami exponent for early‐stage kinetics (n₁, 1.04–5.54) was greater than the corresponding value for late‐stage kinetics (n₂, 0.28–1.52). While the Avrami K constant of the early‐stage kinetics (K₁, 1.0×10^‐5 to 2.3×10^‐1 day^‐n) was lower than the corresponding value of late‐stage kinetics (K₂, 4.4×10^‐2 to 1.4 day^‐n). The ΔH values for late and infinite retrogradation stages showed a significantly positive correlation with the proportions of short chain (chain length [CL] ≤ 15 glucose units) and long chain (CL = 16–100 glucose units) fractions, respectively. Retrogradation of AP with a higher number‐average degree of polymerization, greater proportion of short chain fractions, and shorter average chain lengths revealed significantly greater n₁ values and smaller K₁ values. Values for n₂ and K₂ showed little influence from the molecular properties except for the proportion of extra long (CL>100 glucose units) and long chain fractions on K₂. The negatively linear relationships between log K and n suggest the importance of some nonstructural factors for AP retrogradation mechanisms in various starch systems.     

Effect of organic and inorganic sources of nutrients on soil microbial activity and soil organic carbon build-up under rice in west coast of India

The effect of medium-term (5 years) application of organic and inorganic sources of nutrients (as mineral or inorganic fertilizers) on soil organic carbon (SOC), SOC stock, carbon (C) build-up rate, microbial and enzyme activities in flooded rice soils was tested in west coast of India. Compared to the application of vermicompost, glyricidia (Glyricidia maculate) (fresh) and eupatorium (Chromolaena adenophorum) (fresh) and dhaincha (Sesbania rostrata) (fresh), the application of farmyard manure (FYM) and combined application of paddy straw (dry) and water hyacinth (PsWh) (fresh) improved the SOC content significantly (p < 0.05). The lowest (p < 0.05) SOC content (0.81%) was observed in untreated control. The highest (p < 0.05) SOC stock (23.7 Mg C ha^?1) was observed in FYM-treated plots followed by recommended dose of mineral fertilizer (RDF) (23.2 Mg C ha^?1) and it was lowest (16.5 Mg C ha^?1) in untreated control. Soil microbial biomass carbon (C_mb) (246 µg g^?1 soil) and C_mb/SOC (1.92%) were highest (p < 0.05) in FYM-treated plot. The highest (p < 0.05) value of metabolic quotient (qCO₂) was recorded under RDF (19.7 µg CO₂-C g^?1 C_mb h^?1) and untreated control (19.6 µg CO₂-C g^?1 C_mb h^?1). Application of organic and inorganic sources of nutrients impacted soil enzyme activities significantly (p < 0.05) with FYM causing highest dehydrogenase (20.5 µg TPF g^?1 day^?1), phosphatase (659 µg PNP g^?1 h^?1) and urease (0.29 µg urea g^?1 h^?1) activities. Application of organic source of nutrients especially FYM improved the microbial and enzyme activities in flooded and transplanted rice soils. Although the grain yield was higher with the application of RDF, but the use of FYM as an organic agricultural practice is more useful when efforts are intended to conserve more SOC and improved microbial activity.     

Long-term management impacts on soil C,N and physical fertility: Part II: Bad Lauchstadt static and extreme FYM experiments

Manure is a source of plant nutrients and can make a valuable contribution to soil organic matter (SOM). Two experimental sites were studied on a Halpic Phaeozem soil near Bad Lauchstadt in Germany. The first experiment, called the static experiment, commenced in 1902. The impact of fresh farmyard manure (FYM) (0, 20 and 30 t ha⁻¹ 2 year⁻¹) combined with P, K and N fertiliser application on total organic C (C_T), labile C (C_L), non-labile C (C_NL), total N (N_T), mean weight diameter (MWD) and unsaturated hydraulic conductivity (K_unsat) was investigated. The second experiment commenced in 1984 and investigated the effect of extreme rates of fresh FYM applications (0, 50, 100 and 200 t ha⁻¹ year⁻¹) and cropping, or a continuous tilled fallow on the same soil properties. At both sites a nearby grassland site served as a reference. On the static experiment, FYM application increased all C fractions, particularly C_L, where application of 30 t ha⁻¹ 2 year⁻¹ increased C_L by 70% compared with no FYM application. Fertiliser additions to the static experiment had a positive influence on C fractions while N_T increased from both FYM and fertiliser application. MWD increased as a result of FYM application, but did not reach that of the grassland site. Both fertiliser and FYM application increased K_unsat (10 mm tension) on the static experiment. In the second experiment application of 200 t ha⁻¹ year⁻¹ of FYM increased concentrations of C_L by 173% and of C_NL by 80%, compared with no FYM application to make them equivalent to, or greater than the grassland site. A continuously tilled fallow resulted in significant decreases in all C fractions, N_T and MWD compared with the cropped site, while K_unsat (10 mm tension) was increased on the 0 and 50 t ha⁻¹ year⁻¹ treatments as a result of a recent tillage. There was no difference in K_unsat between the cropped and the continuous tilled fallow at FYM applications of 100 and 200 t ha⁻¹ year⁻¹. There were similar significant positive correlations of all C fractions and N_T with MWD on both experimental sites but the relationships were much stronger on the extreme FYM experiment. Weaker relationships of C fractions and N_T with K_unsat (10 mm tension) occurred for the static experimental site but these were not significant for the extreme FYM experimental site. The strongest relationship between C fractions and K_unsat was with C_L. This research has shown that applications of FYM can increase SOM and improve soil physical fertility. However, the potential risk of very high rates of FYM on the environment need to be taken into consideration, especially since the application of organic materials to soils is likely to increase in the future.     

Effects of fertilizer type and rate on labile soil fractions of a sandy Cambisol—long‐term and short‐term dynamics

The application of density fractionation is an established technique, but studies on short‐term dynamics of labile soil fractions are scarce. Objectives were (1) to quantify the long‐term and short‐term dynamics of soil C and N in light fraction (LFOC, LFON, ρ ≤ 2.0 g cm^–3) and microbial biomass C (C_mic) in a sandy Cambisol as affected by 28 y of different fertilization and (2) to determine the incorporation of C₄‐C into these labile fractions during one growing season of amaranth. The treatments were: straw incorporation plus application of mineral fertilizer (MSI) and application of farmyard manure (FYM) each at high (MSI_H, FYM_H, 140–150 kg N ha^–1 y^–1) and low (MSI_L, FYM_L, 50–60 kg N ha^–1 y^–1) rates at four field replicates. For all three sampling dates in 2008 (March, May, and September), stocks of LFOC, LFON and C_mic decreased in the order FYM_H > FYM_L > MSI_H, MSI_L. However, statistical significance varied markedly among the sampling dates, e.g., with LFOC being significantly different (p ≤ 0.05) in the order given above (sampling date in March), significantly different depending on the fertilizer type (May), or nonsignificant (September). The high proportion of LFOC on the stocks of soil organic C (45% to 55%) indicated the low capacity of soil‐organic‐matter stabilization on mineral surfaces in the sandy Cambisol. The incorporation of C₄‐C in the LFOC during one growing season of amaranth was small in all four treatments with C₄‐LFOC ranging from 2.1% to 3.0% of total LFOC in March 2009, and apparent turnover times of C₃‐derived LFOC ranged from 21 to 32 y for the sandy soils studied. Overall, our study indicates that stocks of LFOC and LFON in a sandy arable soil are temporarily too variable to obtain robust significant treatment effects of fertilizer type and rate at common agricultural practices within a season, despite the use of bulked six individual cores per plot, a common number of field replicates of four, and a length of treatments (28 y) in the order of the turnover time (21–32 y) of C₃‐derived LFOC.     

Nitrogen distribution and 15N natural abundances in particle size fractions of a long‐term agricultural field experiment

The present study combined a physical fractionation procedure with the determination of the natural abundance of ¹⁵N to investigate the impact of organic manure and mineral fertilizer application, and fallow on changes of N associated with different soil particle size fractions. The long‐term field experiment was conducted since 1956 in Ultuna, Sweden, on an Eutric Cambisol. Nitrogen in bulk soil and in particle size fractions changed significantly since 1956. The N_t concentrations in bulk soil decreased in all treatments not receiving organic materials. Comparing the N contribution of particle‐size fractions to the total N amount revealed the following ranking: silt > clay > fine clay > fine sand > coarse sand. The relative contribution of N in silt sized particles significantly increased from low to high bulk soil N contents, whereas N in clay and fine clay fractions decreased. The C : N ratios of particle size fractions differed considerably more between treatments than C : N ratios in bulk soils. Generally, the C : N ratios decreased from coarse to fine fractions emphasizing the tendency of smaller fractions being more significant as N sink than as C_org sink. ¹⁵N abundances varied more between particle size fractions of single treatments than between bulk soil from differently treated plots. Within treatments we observed differences of up to 7.1 ‰ between particle size fractions. In most cases δ ¹⁵N values increased with decreasing particle sizes. This pattern on average was similar to changes in δ ¹³ C. Our results suggest that silt sized particles acted as medium‐term sink of introduced N and that ¹⁵N abundances in particle size fractions sensitively reflect changes in N status in response to soil management.     

Carbon content in soil particle size and consequence on cation exchange capacity of Alfisols

Abstract

Many of the cultivated soils of sub‐Saharan Africa typically have a surface horizon low in clay and with a low cation exchange capacity (CEC). In these soils, CEC is largely due to the soil organic matter (SOM). Measurements made on long‐term trials show that changes in CEC and SOM are positively correlated to one another, but not of same magnitude, suggesting that not all of the SOM plays an equal role as regards the soil CEC. To study the influence of the different SOM size fractions on the CEC, soils with or without application of manure or compost coming from trials in Chad and Côte d'Ivoire were separated without destruction of the SOM into five organo‐mineral fractions: “coarse sand”;, “fine sand”;, “coarse silt”;, “fine silt”;, and “clay”; made up of particles of sizes between 2,000 and 200, 200 and 50, 50 and 20, 20 and 2, and 2 and 0 μm, respectively. Fractionation was carried out by mechanical dispersion of the soil, wet sieving of the fractions larger than 20 μm, and decanting of the “clay”; and “fine silt”; fractions. The CEC of these fractions increases inversely with their size. The “clay”; fraction which contains half of the SOM contributes about 80% of the CEC of the soils. The CEC of the fractions is largely a function of their carbon (C) content, but the organic CEC per unit C of the “clay”; fraction appears to be four times greater than that of the other fractions (1,000 as against 270 cmol_c kg^‐1). Applications of manure or compost increase the CEC of the soils by increasing the soil C only when this C increase concerns the fine fractions of the SOM.