Effect of cropping systems on phosphatases in soils

Phosphatases are widely distributed in nature and play a major role in phosphorus nutrition of plants. The effects of crop rotations and nitrogen fertilization on the activities of phosphatases (acid phosphatase, alkaline phosphatase, and phosphodiesterase) were studied in soils from two long‐term cropping systems at the Northeast Research Center (NERC) in Nashua and the Clarion Webster Research Center (CWRC) in Kanawha, Iowa, USA. Surface soils (0—15 cm) were taken in 1996 and 1997 from replicated field plots in corn, soybeans, oats, or meadow (alfalfa) that received 0 or 180 kg N ha^—1 before corn. Because of differences in organic C contents among soils of the two sites, the soils from the CWRC sites contained greater enzyme activity values than those from the NERC site. Plots under oats or meadow showed the greatest activity values, whereas those under continuous corn at the CWRC site and soybean at the NERC site showed the least activities. Analysis of variance indicated that the activities of the phosphatases were significantly affected by crop rotation (P < 0.001) in both years at the NERC site but not at the CWRC site. Nitrogen fertilization affected the activity of acid phosphatase in soils from the CWRC site in both years and alkaline phosphatase only in 1997; but it did not affect the activities of the phosphatases in the soils from the NERC site. With the exception of alkaline phosphatase (CWRC) and phosphodiesterase (NERC) in soils sampled in 1997, activities of alkaline phosphatase and phosphodiesterase were significantly correlated with microbial biomass C (C _mic) in soils from both sites and years, with r values ranging from 0.366* to 0.599***. Cropping systems and N fertilization affected the specific activities of phosphomonoesterases, especially acid phosphatase, but not of phosphodiesterase. Regression analysis showed that activities of phosphatases were significantly correlated with organic C contents of soils from the NERC site but not from the CWRC site.     

Protease extraction from soil by sodium pyrophosphate and chemical characterization of the extracts

Two arable soils and one pasture soil had previously been air-dried for 6 d and stored at room temperature. The enzyme activities remaining after this treatment were constant. The soils were then extracted with 140 mM sodium pyrophosphate at pH 7.1. Amino acid N and total organic C content of soils and soil extracts, together with humic and fulvic acids content of soil extracts were determined. Total organic C was determined in soil residues obtained after extraction. Chemical characterization of the organic matter of soils, soil extracts and soil residues was carried out by pyrolysis–gas chromatography (Py–GC). Protease activity was determined in soil extracts and soil residues by using three different substrates: N-benzoyl- -argininamide (BAA), specific for trypsin; N-benzyloxycarbonyl- -phenylalanyl -leucine (ZPL), specific for carboxypeptidases, and casein, essentially non-specific. Comparative studies between specific activities referred to organic C in soils, soil extracts and soil residues and their corresponding pyrogram composition, and also between total extracted or residual activity and the humine or unhumified organic matter content of the corresponding soil, allowed us to establish hypotheses about the type of organic matter the enzymes are associated with. From 12% to 21% of the soil organic C (33% to 39% of which were humic acids) and from 3% and 18% of amino acid N were extracted from soil using pyrophosphate. Py–GC analyses showed that pyrophosphate was effective in extracting condensed humic substances and glycoproteins and that the organic matter present in soil extracts was especially rich in intact or partially-decomposed fresh residues of carbohydrate origin and also in certain humus-associated proteins. Extracted BAA-hydrolysing activity accounted for 11% to 36% of the soil activity, depending on soil type. Extracted ZPL- and casein-hydrolysing activities were, with one exception, remarkably high, accounting for about 100% or even more of the soil activity, depending on soil type. According to the results BAA-hydrolysing proteases are probably mostly associated with highly condensed humus, ZPL-hydrolysing proteases with less condensed humic substances and casein-hydrolysing proteases with fresh organic matter.     

The distribution of nitrogen in some highly organic tropical volcanic soils

The qualitative and quantitative distribution of N-compounds in 10 tropical soils, and in a number of humic materials extracted from representative samples thereof, was determined after 6 N HCl hydrolysis.Eighty to 98% of the total N in the soils and humic materials was hydrolysable by 6n HCl. Slightly less than one half the hydrolysable N in the soils and humic fractions consisted of amino acids. Well-drained soils and fulvic acids extracted from them contained unusually high concentrations of the acidic amino acids, aspartic and glutamic acids. Between 80 and 95% of the amino acids in the soils was accounted for in the humic materials + NaOH-insoluble organic residues. NH⁺₄-N released by acid hydrolysis was generally higher for the soil samples than for the humic materials. Amino sugar-N constituted relatively small proportions of the total N in the soils and humic fractions.Our data suggest that large quantities of amorphous allophanic materials coupled with relatively high enzymic activity are responsible for the observed accumulation of acidic amino acids in the well-drained tropical volcanic soils.     

Aspartase Activity of Soils Under Different Management Systems

Abstract

Recent interest in soil tillage, cropping systems, and residue management has focused on low‐input sustainable agriculture. This study was carried out to evaluate the effects of various management systems on aspartase activity in soils. This enzyme [L‐aspartate ammonia‐lyase, EC 4.3.1.1] catalyzes the hydrolysis of L‐aspartate to fumarate and NH₃. It may play a significant role in the mineralization of organic N in soils. The management systems consisted of three cropping systems [continuous corn (Zea mays L.) (CCCC); corn‐soybean [Glycine max (L.) Merr.]‐corn‐soybean (CSCS); and corn‐oat (Avena sativa L.)‐meadow‐meadow (COMM) {meadow was a mixture of alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.)] at three long‐term field experiments initiated in 1954, 1957, and 1978 in Iowa and sampled in June 1987. The plots received 0 or 180 (or 200) kg ha^?1 before corn and an annual application of 20 kg P and 56 kg K ha^?1. The tillage systems (no‐tillage, chisel plow, and moldboard plow) were initiated in 1981 in Wisconsin and sampled in May 1991. The crop residue treatments were: bare, normal, mulch, and double (2×) mulch. The residue in the study was corn stalks. Results showed that, in general, crop rotation in combination with N fertilizer treatments affected aspartase activity in the following order: COMM>CSCS>CCCC. Because of nitrification of the NH₄ ⁺ or NH₄ ⁺‐forming fertilizers, which resulted in decreasing the pH values, N fertilizer application, in general, decreased the aspartase activity in soils in the order: CCCC>CSCS>COMM. The effect of tillage and residue management practices on aspartase activity in soils showed a very wide variation. The trend was as follows: no‐till/2× mulch>chisel plow/mulch>moldboard plow/mulch>no‐till normal>chisel plow/normal>no‐till bare>moldboard plow/normal. Aspartase activity decreased with increasing depth in the plow layer (0–15 cm) of the no‐till/2× mulch. The decreased activity was accompanied by decreasing organic C and pH with depth. Statistical analyses using pooled data (28 samples) showed that aspartase activity was significantly, linearly correlated with organic C (r=0.78^***) and exponentially with soil pH (r=0.53**). The variation in the patterns and magnitudes of activity distribution among the profiles of the four replicated plots was probably due to the spatial variability in soils.     

Amino acids and amino sugars extracted by EUF from a sandy soil incubated with green manure,bacterial biomass or cellulose

The extraction of soils by the electro-ultrafiltration (EUF) method yields organic N which has been used as an index for mineralisable N in soils. This EUF extractable organic fraction contains a mixture of various N compounds not yet completely identified. It has been proposed that the amino N compounds are more indicative for the potentially mineralisable N in soils than the total organic N extracted (Mengel et al., 1999). An amendment of soils with easily mineralisable organic matter may, therefore, alter the amino N concentrations of the organic N extracted. Our determination of the amino N compounds aimed to prove this hypothesis. The principle of our experiment was to mix soil with green manure, bacterial biomass and cellulose, respectively, and to incubate the treated soil aerobically for 80 days at 20°C in the laboratory. Control treatments without organic amendment were also incubated. Soil samples were taken several times during the incubation period and analysed for the inorganic N (NO₃⁻-N and NH₄⁺-N) and for the EUF extractable organic N. Amino acids and amino sugars were determined in the hydrolysed EUF extracts. The concentrations of amino acids and amino sugars in the organic N extracted varied with time and differed between the treatments. Glutamic acid has been found to be the most relevant amino acid in the EUF extracts and was particularly indicative for the existence of mineralisable green manure in the soil. Glucosamine was the most relevant amino sugar in the EUF extracts and this amino sugar appears to be indicative for the easily mineralisable relics of microbial cells in the soil.     

Organic nitrogen compounds extracted from arable and forest soils by electro-ultrafiltration and recovery rates of amino acids

Summary Extraction of synthetic amino acids dissolved in water by means of electro-ultrafiltration (EUF) showed average recovery rates of about 75%. Higher losses were obtained, particularly with cysteine, methionine and NH₄ ⁴; the latter, probably being deprotonated at the cathode, may be lost in form of NH₃. The EUF extracts of three arable and two forest soils were investigated for their N compounds. In the arable soils only about 3% of the total organic N extracted by EUF was free amino acids; about 23%–55% consisted of amino N (hydrolysable N) and the rest was non-hydrolysable N. The two forest soils contained higher amounts of EUF-extractable organic N compared with the arable soils. In the two forest soils the content of free amino-acid N amounted to 8% and 11% of the EUF organic N, and the proportion of hydrolysable N from total EUF-organic N was 41% and 46%. It is suggested that the amino-acid N and the hydrolysable N can be easily mineralized.     

Characterization of humus-protease complexes extracted from soil

Pyrophosphate (140 mM, pH 7.1) extracts of two arable soils and one pasture soil were ultrafiltrated separating the extracted material into three fractions: A_I with nominal molecular weight (nmw) > 100 kD, A_II with nmw between 10 kD and 100 kD and R with nmw < 10 kD. Protease activity was determined in the fractions by using three different substrates: N-benzoyl-l-argininamide (BAA), specific for trypsin; N-benzyloxy-carbonyl-l-phenylalanyl l-leucine (ZPL), specific for carboxypeptidases; and casein, essentially a non-specific substrate. The derivative fractions were also analysed for their amino acid N and humic (HA) and fulvic (FA) acid contents. The organic matter of extracts and derivative fractions obtained from the pasture soil was analysed by isoelectric focusing (IEF) and that of fractions analysed by pyrolysis gas chromatography (Py-GC). Activities of the extract were monitored for their thermal stability and those of the extract and derivative fractions for their optimal pH.Due to the mechanical disintegrating action of sodium pyrophosphate over the humic substances during the fractionation process the amount of total organic C and FA in the fractions was ranked as R > A_II > A_I. The lowest amino acid N/organic C was found in the R fraction, whereas A_II fraction was rich in humic acids, carbohydrates and amino acid N and A_I fraction showed the lowest carbohydrate content. At least 70% of the total BAA- and ZPL-hydrolysing activity was associated to particles with nmw higher than 10 kD and at least 30% of these activities were present in particles with nmw higher 100 kD. Casein-hydrolysing activity was quite evenly distributed among the three fractions (A_I, A_II and R). The extracted protease-organic complexes were resistant to thermal denaturation and some of them showed optimal activity at pH values higher than 10 as a result of the polyanionic characteristics of the humic material surrounding enzyme molecules and of the presence of alkaline protease. Comparison of data obtained in Py-GC analyses and in protease activity suggests that BAA-hydrolysing activity was associated to a highly condensed humic matter and ZPL-hydrolysing activity to less resistant humic substances, while at least some of the extracted casein-hydrolysing activity was present as glyco-proteins not associated to humus. BAA-hydrolysing activity was probably inhibited by fresh organic matter of carbohydrate origin whereas lignin derived organic matter probably inhibited ZPL- and casein-hydrolysing activity.     

Components of soil organic matter under grass and arable cropping

Components of the organic matter have been studied in three soils from adjacent sites with different long-term treatments: soil I, prolonged arable cultivation; soil II, 17 years under grass after prolonged arable cultivation; and soil III, old pasture. Contents of total organic C in the top 15cm were 0.9% in soil I. 1.7% in soil II and 4.8% in soil III. The light fraction, comprising partially decomposed materials with a specific gravity < 2.06, represented greater proportions of the organic C in soils II and III (20–23 per cent) than in soil I (8.5 per cent). The light fraction of soil III had a relatively high N content.The proportions of the soil organic C released, by hydrolysis, as neutral sugars, uronic acids, amino sugars, amino acids and phenolic acids were generally similar in the three soils, although uronic acids and phenolic acids constituted somewhat greater proportions in soils II and III than in I.The light fractions contained greater proportions of neutral sugars and phenolic acids, and smaller proportions of amino sugars and amino acids than the whole soils.     

Centrifugal extraction and determination of free amino acids in soil solutions by TLC using tritiated 1-fluoro-2,4-dinitrobenzene

A method is described to separate and quantitate free amino acids from soil solutions extracted rapidly by centrifugation at 10,000 g for 5 min. Centrifugation of soil samples with forces up to 20,000 g did not affect the content of ammonium and water soluble organic carbon in solution, therefore it was concluded that the integrity of microbial cells was unaffected.The method of amino acid analysis involved treatment of the soil solution with tritiated 1-Fluoro-2,4-dinitrobenzene under alkaline conditions after addition of a known concentration of a standard mixture of amino acids. After extraction, the dinitrophenyl (DNP)-amino acids were separated by two-dimensional thin layer chromatography. Quantification was effected by counting the ³H-activity of each spot and comparing it to that of spots of a standard amino acid mixture of known concentration. The method has been tested and found satisfactory on a variety of cultivated soils, except for an Andept soil with a high content of metallic ions in solution.The total free amino acid content in cultivated and virgin soil samples ranged between 0.32 and 4.72 μg g^?1 soil.Alanine, phenylalanine and threonine were the most abundant followed by valine and the aspartic-glutamic acid complex. Basic amino acids, those containing S (methionine and cystine), and the isoleucine-leucine pair were the least abundant and frequently not detected. With the Chernozemic order the total amino acid content decreased in the following order: Brown > Dark Brown > Black. Virgin soil samples contained more but not always a wider range of free amino acids than did cultivated soils. A Luvisolic soil cropped for 50 yr to a 5-yr rotation contained fewer and a narrower range of free amino acids than did the same soil cropped to a wheat-fallow rotation.     

Amino acid composition of soil peptides chromatographed by high performance liquid chromatography on C18 and C8 columns

Summary Low molecular weight fractions (LMW; <5000 daltons) of organic matter were isolated from three soils by a mild extraction procedure and gel-permeation chromatography. The peptides present in the LMW soil extracts were separated on a Whatman Partisphere C18 and a Beckman Ultrapore C8 column by reverse-phase high performance liquid chromatography (HPLC). The peptide fractions were collected, acid hydrolyzed, and analyzed for amino acid composition. The C8 bonded-phase column gave better separation of the LMW soil peptide material than the C18 column. The total quantities of amino acids released from LMW peptides by acid hydrolysis were greater than the quantities released by immobilized protease hydrolysis (Warman and Isnor 1990). Total soil N present in the form of LMW peptides in these three soils ranged from 4 to 15%. The total recovery of peptide amino acid-N showed little difference between the C18 and C8 columns for two of the soils tested.     

Nitrogen compounds extracted by electroultrafiltration (EUF) or CaCl2 solution and their relationships to nitrogen mineralization in soils

The objective of the investigation was to identify the most important organic N-containing fractions extracted from soils by electroultrafiltration (EUF) or a CaCl₂ solution, respectively, and their importance for nitrogen mineralization. The investigation comprised 19 agricultural and one forest top soil. Net N mineralization was tested in Mitscherlich pot experiments with three treatments: (1) fallow soil without N fertilizer, (2) soil cultivated with rye grass without N fertilizer, (3) soil cultivated with rye grass with N fertilizer. The highest proportion of N in the extracts was the amino N fraction (amino acids + peptides) amounting to approximately 60% of the total N extracted by CaCl₂ and to about 40% of the total N extracted by EUF. The proportion of amino sugars from total N extracted was in average 10% for the CaCl₂ and 5.2% for the EUF extracts. The proportion of heterocyclic N bases derived from nucleic acids amounted in average to 4.8% and 3.6% for the CaCl₂ and EUF extract, respectively. Amino N (amino acids + peptides) were correlated best with net N mineralization (EUF, r = 0.81***, CaCl₂, r = 0.86***). The correlation between amino sugars and net N mineralization was r = 0.55* for the EUF extract and r = 0.49* for the CaCl₂ extract. The heterocyclic N bases did not correlate with net N mineralization. Correlations between Norg extracted by CaCl₂ versus net N mineralization were higher than those obtained by the EUF extract. Net N mineralization was about four times higher in the fallow soils than in the treatment with grass and no N fertilizer. In the treatment with grass + N fertilizer on average no net N mineralization occurred, moreover there was a tendency of N immobilization. It is assumend that in the treatments with grass cultivation, organic C released by roots stimulated the assimilation of mineral N and amino acids by soil microorganisms resulting in a low net N mineralization. Net N mineralization led to a highly significant depletion in the Norg pools and particularly in the amino N and amino sugar pools in the treatment with grass and without N fertilizer. This depletion was particularly evident in the CaCl₂ extracts. The results justify the conclusion that the Norg obtained with both extraction methods originates from a dynamic N pool into which N flows in and out. The amino N extractable with EUF or CaCl₂ is a reliable indicator for the net N mineralization potential of soils.     

Carbon and nitrogen isotope composition of bulk soils, particle-size fractions and organic material after treatment with hydrofluoric acid

Soils and sediments contain only small amounts of organic matter, and large concentrations of paramagnetic metals can give poor solid‐state nuclear magnetic resonance (NMR) spectra of organic matter. Pretreatment of samples with hydrofluoric acid (HF) dissolves significant proportions of the mineral matrix and extracts paramagnetic elements. We investigated the effects of 10% HF treatment on the stable isotope content of carbon (C) and nitrogen (N) of organic matter from soils, composts and shales. Additionally we inferred molecular and isotopic characteristics of lost materials from calculations of isotope mass balances. Treatment with HF enriched C and N in mineral samples substantially (factors 2.5–42.4), except for Podzol B horizons (1.1–1.7) and organic material (1.0–1.3). After treatment most of the C (59.7–91.7%) and N (53.7–86.6%) was recovered, although changing C/N ratios often indicated a preferential loss of N‐rich material. Isotope ratios of C and N in the remaining material became more negative when net alterations exceeded 0.3‰. The isotope ratios of the lost material contained more ¹³C (1–2‰) and ¹⁵N (1–4‰) than the initial organic matter. Acid hydrolysis typically removes proteins, amino acids and polysaccharides, all of which are enriched in ¹³C, and in the case of proteins and amino acids, enriched in ¹⁵N as well. We conclude that HF treatment released fresh, soluble, probably microbial, biomass in addition to carbohydrates. Net changes of the bulk chemical composition of organic matter were small for most soils, size fractions and plant material, but not for samples containing little organic matter, or those rich in easily soluble organic matter associated with iron oxides, such as Podzol B horizons.     

Hydrolysis of low-molecular-weight soil peptides by immobilized proteases in column and batch reaction systems

Summary Organic matter was extracted from three soils, a cultivated Berwick sandy loam, a cultivated Franklin loamy sand, and an uncultivated Cumberland silty loam. Gel-permeation chromatography was used to separate organic matter extracts into high- (HMW) and low-molecular-weight (LMW) fractions. Reversed-phase high performance liquid chromatography was used to separate and collect the LMW peptide fractions. Peptide samples were hydrolyzed with immobilized proteases attached to beaded agarose and carboxymethyl cellulose in column and batch reaction systems. The chromatograms suggested that peptides are bound to common soil components. The amino acids released in the greatest percentages were relatively non-polar. Large percentages of serine, glycine, alanine, threonine, and valine were observed in the LMW soil peptides. Little aspartic acid, asparagine, glutamic acid, glutamine, arginine, and no histidine was detected in the LMW soil peptides. The soil peptides released different amino acid percentages and quantities when hydrolyzed by immobilized proteases attached to different supports. The quanitities of amino acids released by batch hydrolysis differed from those obtained with column hydrolysis. Greater quantities of amino acids were released (by both types of immobilized protease) from the LMW peptide hydrolysates of the two cultivated soils than from the uncultivated soil.     

Changes in chemical properties of organic matter with intensified rice cropping in tropical lowland soil

Rice systems in Asia have intensified rapidly in the past 30 years, and significant areas of irrigated lowland rice are now supporting two or three rice crops per year. Our objective was to compare the chemical composition of soil organic matter (SOM) from four fields with different histories of rice cropping intensity and soil submergence: (i) a single-crop rainfed, dryland rice system without soil submergence, (ii) an irrigated rice and soybean rotation, and irrigated (iii) double- or (iv) triple-crop rice systems in which soil remains submerged during much of the year. In all four soils, extracted mobile humic acid (MHA) and calcium humate (CaHA) fractions were of modern age by ¹⁴C-dating, and represented about 20% of total N and organic C. The MHA was enriched in N and hydrolysable amino acids (AA) compared with CaHA in all soils. With increased frequency of irrigated rice cropping, however, there was a large increase in phenolic content of SOM. We speculate that slower lignin decomposition caused by deficiency of O₂ in submerged soil leads to incorporation of phenolic moieties into young SOM fractions. The increased phenolic character of these fractions may influence N cycling and the N supplying capacity of lowland soils supporting two or three annual crops of irrigated rice.     

THE EFFECT OF SOIL FACTORS ON THE EXTRACTION OF SOIL ORGANIC MATTER BY ANHYDROUS FORMIC ACID

Twenty-five soils, having a wide range of organic matter contents, were extracted with anhydrous formic acid containing 10 per cent acetylacetone, and the extracted material precipitated in two fractions with diisopropyl ether. Precipitates comprised from 5.1 to 51.1 per cent of the original soil organic matter, the proportion extracted tending to be greatest from acid soils of fairly high organic matter content and least from neutral or slightly alkaline soils of low organic matter content. Soil clay content appeared to have no effect on the efficiency of organic matter extraction, but was the most important soil factor governing the proportion of the total soil-N extracted. Amounts of N extracted ranged from 10.2 to 57.8 per cent of the original soil N content, extraction efficiency being greatest with soils of low clay content and low pH. There was evidence to suggest that soil clay afforded some protection to N compounds against extraction. The results indicate that formic acid/acetylacetone is most effective with soils in which much of the organic matter is only partly humified.     

Land-use effects on amino sugars in particle size fractions of an Argiudoll

Identifying amino sugar pools from different land-use systems may advance our knowledge of land-use effects on the fate of microbially-derived substances. Surface soils (0–10 cm) from (1) native pasture, (2) a >80-years-arable site, and (3) a >80-years-afforested site were fractionated into clay, silt, fine-, and coarse-sand fractions. Then, soil organic carbon, N, glucosamine, galactosamine, mannosamine, and muramic acid were analyzed.Afforestation did not influence the amino sugar content in bulk soil, whereas cultivation reduced the content by 54%. The concentrations of amino sugars in g kg⁻¹ SOM declined after both long-term cropping and afforestation by 6% and 13%, respectively, relative to that in the grassland. The amino sugar depletion at the forest site occurred mainly from the silt fraction (by 25%), while that in the cultivated site was mainly due to preferential loss of amino sugars from clay (by 19% compared with the grassland). Both ratios of glucosamine to galactosamine and glucosamine to muramic acid increased when the prairie was converted to forest or cultivated land, suggesting that bacterial N especially is better preserved than fungal N under prairie conditions.     

Characteristics of dissolved organic matter and phenolic compounds in forest soils under silver birch (Betula pendula), Norway spruce (Picea abies) and Scots pine (Pinus sylvestris)

The aim was to characterize dissolved organic matter in soils under different tree species. Molecular size distribution and chemical composition of dissolved organic carbon and nitrogen were determined in water extracts from humus layers and mineral soils taken from silver birch ( Betula pendula Roth.), Norway spruce ( Picea abies (L.) Karst.) and Scots pine ( Pinus sylvestris L.) stands. Concentrations of tannins and 15 phenolic acids in the humus layers were measured. Per unit of organic matter, the concentrations of dissolved organic C and N were larger in birch and spruce humus layers than in the pine humus layer. In the underlying mineral soil, the concentrations of dissolved organic C were similar at all sites, but the concentration of dissolved organic N was greater in spruce and pine soils than in birch soil. In all soils, the 10–100 kDa fraction was the most abundant molecular size group and hydrophobic acids the most abundant chemical group of dissolved organic C. In all humus layers, hydrophobic acids and hydrophilic bases were the major components of dissolved organic N. There were only minor differences in the concentrations of total tannins in the humus layers under different tree species. Small-molecule tannins (about < 0.5 kDa) were most abundant in the birch humus, and large-molecule tannins in the pine humus. Coniferous humus contained more ferulic and p -coumaric acids than did the birch humus. The concentrations of 3,4 and 3,5-dihydroxybenzoic acid, vanillic acid and 4-hydroxybenzoic acid were similar in all soils.     

Urease activity of microbial biomass in soils as affected by cropping systems

 The impacts of crop rotations and N fertilization on different pools of urease activity were studied in soils of two long-term field experiments in Iowa; at the Northeast Research Center (NERC) and the Clarion-Webster Research Center (CWRC). Surface soil samples (0–15 cm) were taken in 1996 and 1997 in corn, soybeans, oats, or meadow (alfalfa) plots that received 0 or 180 kg N ha^–1, applied as urea before corn and an annual application of 20 kg P and 56 kg K ha^–1. The urease activity in the soils was assayed at optimal pH (THAM buffer, pH 9.0), with and without toluene treatment, in a chloroform-fumigated sample and its nonfumigated counterpart. The microbial biomass C (C_mic) and N (N_mic) were determined by chloroform fumigation methods. The total, intracellular, extracellular and specific urease activities in the soils of the NERC site were significantly affected by crop rotation, but not by N fertilization. Generally, the highest total urease activities were obtained in soils under 4-year oats–meadow rotations and the lowest under continuous corn. The higher total activities under multicropping systems were caused by a higher activity of both the intracellular and extracellular urease fractions. In contrast, the highest values for the specific urease activity, i.e. of urease activity of the microbial biomass, were found in soils under continuous soybean and the least under the 4-year rotations. Total and extracellular urease activities were significantly correlated with C_mic (r>0.30* and >0.40**) and N_mic (r>0.39** and >0.44**) in soils of the NERC and CWRC sites, respectively. Total urease activity was significantly correlated with the intracellular activity (r>0.73***). About 46% of the total urease activity of the soils was associated with the microbial biomass, and 54% was extracellular in nature. Received: 25 May 1999     

有机质对红壤烤烟氮素累积分配特征的影响

利用15N示踪技术,研究了有机质含量对红壤烤烟氮累积分配特征的影响。结果表明,随着土壤有机质含量增加,烤烟氮素累积时期延长,且累积量增加。烤烟后期吸收的氮素,在低有机质含量红壤上来自土壤供氮,中有机质含量来自肥料供氮,高有机质含量来自肥料供氮与土壤供氮。烤烟吸收总氮量中29.07%~40.26%来自肥料供氮,59.74% ~70.93%来自土壤供氮,表明烤烟吸收氮素大部分来自土壤供氮。氮素在烟株不同部位分配量表现为:烟叶烟茎烟根;烟叶各部位中的分配量为:在低有机质含量的红壤,下、中、上3个部位分配量相等,中有机质含量和高有机质含量上则为上部叶中部叶下部叶。有机质含量对下部叶氮素分配量影响不大,其它部位均表现为有机质含量越高,氮素分配量越大。烤烟不同部位中肥料氮比例表现为下部叶中部叶烟根烟茎上部叶,土壤氮比例表现为上部叶烟茎烟根中部叶下部叶;并且土壤有机含量越高,各部位中土壤氮的比例越高,肥料氮的比例越低,上部叶受土壤供氮影响最大。红壤上烤烟氮肥利用率在25.42%~30.61%之间,低有机质含量土壤氮肥利用率较低,中、高有机质含量利用率相对较高。在施肥过程中,低有机质红壤上应在N 90 kg/hm2基础上适当增加氮肥施用量,中等有机质含量上保持不变,高有机质含量上应适当降低氮肥用量。     

Characteristics of the organic matter in a slightly and in a severely crusted soil

Chemical and spectroscopic characteristics were determined on the organic matter in two Luvisolic soils from northwestern Alberta, Canada. One of these (the Debolt soil) is known to crust only slightly while the other (the Demmitt soil) exhibits severe crusting after heavy rainfalls. The Debolt top soil is richer in total C, total N, carbohydrates and proteinaceous materials than the Demmitt top soil. Humic acid extracted from the Debolt soil is more aliphatic, and contains fewer CO₂H groups than does humic acid extracted from the Demmitt soil. The quality of the organic matter appears to play a role in soil crusting.