Effect of organic manure on organic phosphorus fractions in two paddy soils

We investigated the transformation of the organic P fractions from organic manure in two paddy soils (Ultisol, Entisol) and the influence of organic manure or cellulose on organic P under anaerobic conditions. The results obtained from the P fractionation experiment indicated that during the incubation labile and moderately labile organic P fractions increased in the Ultisol and decreased in the Entisol, which might be related to the difference in the organic matter content of both soils. Immediately after the application of organic manure, a large part of labile and moderately labile organic P supplied with the manure was transformed into moderately resistant organic P, possibly Ca- or Mg-inositol P were transformed into Fe-inositol P. During anaerobic incubation, the labile forms of organic P in the soils treated with organic manure were increased along with the incubation period in the first 4 weeks. The change in the moderately labile fraction was dramatic. It increased sharply in the first 2 weeks, then decreased, which was more pronounced in the soils treated with pig faeces. The moderately resistant fraction decreased during the whole incubation period. This indicated that under anaerobic conditions, the moderately resistant fraction can be transformed into labile and moderately labile organic P fractions, perhaps as Fe³⁺-inositol P is reduced to Fe⁺²-inositol P. Cellulose as an organic substrate had an increasing effect on organic P, especially when it was combined with inorganic P. Therefore, it is suggested that the application of inorganic P fertilizer combined with organic manure may be an effective way of protecting inorganic P against intensive sorption in soils.     

不同耕种年限红壤性水稻土理化性质的变化特征

以江西省余江县不同种植年限的红壤性水稻土为对象,研究了土壤有机质、pH、阳离子交换量(CEC)、游离氧化铁含量和氧化铁活化度随时间变化趋势;选择耕种7、22和80年的土壤剖面,研究了游离氧化铁和无定形氧化铁在剖面不同层次的分布。结果表明,耕层(0～20cm)土壤有机质含量随着水稻种植年限的增加而增加,淹水种稻导致土壤pH升高,但主要发生在种稻的前7年,7～80年间土壤pH变化不大。土壤CEC随着种植年限的增加而增加,在17年达到最大,然后随种稻时间的进一步增加而减小。土壤游离氧化铁含量随种稻时间呈下降趋势,但种稻后土壤氧化铁的活化度高于母土旱地红壤。对7年和80年水稻土,表层土壤游离氧化铁的含量低于底层土壤,说明铁在剖面中存在淋溶迁移。7年和22年水稻土表层土壤无定形氧化铁含量高于底层土壤,但对80年水稻土,土壤无定形氧化铁随采样深度的增加而增加。     

Effect of fertilizer application on methane emission/production in the paddy soils of Taiwan

Methane production in three types of rice paddy soil was investigated under greenhouse conditions. The amount of methane produced during the first crop season (March to July) was 2–6 times higher than that in the second crop season (August to December). Application of organic fertilizer hastened the drop in redox potential and increased methane production and emission. Methane production also increased with the depth of soil with high values in soil samples from 18 to 30cm depth. Methane production in the first crop season was 18.0, 54.3 and 49.4mgcm^–3 for 6tha^–1 straw application for Linkou, Tzawchyau and Jiaushi soils, respectively. The value was 33.4mgcm^–3 for the second crop season in Jiaushi soil. Methane emission was high during the flowering and maturity stages in the first crop season and the values were high during the tillering and flowering stages in the second crop season. Methane emission was high in Tzawchyau and Jiaushi soils in the first crop season. Methane emission rate reached a maximum from 12 noon to 3p.m. due to high temperature and a minimum at 3 to 6a.m. in both planted and unplanted soils. Received: 17 September 1996     

Mechanisms of fixation and release of ammonium in paddy soils after flooding III. Effect of the oxidation state of octahedral Fe on ammonium fixation

An experiment with two typical paddy soils from China and two clay minerals was conducted to study the effect of reduction of octahedral Fe^III on fixation of NH₄⁺ ions. Reduction of octahedral Fe^III was achieved by treating soils and clay minerals with dithionite‐citrate‐bicarbonate (DCB) followed by dialyzing the samples under oxygen free conditions. Reduction of Fe^III increased the negative charge of interlayers and resulted in a significantly higher ammonium fixation. Close positive correlations were found between the Fe²⁺ concentration or the ratio of Fe²⁺/Fe³⁺ and non‐exchangeable NH₄⁺‐N. Therefore, it is concluded that the reduction of octahedral Fe induced by flooding is one of the important prerequisites for the pronounced ammonium fixation in flooded soils. However, the relation between ΔFe²⁺ and Δfix‐N was not stoichiometric.     

Nitrogen cycling involving non-exchangeable ammonium in a gray luvisol

 There is considerable interest in determining the bio-availability of non-exchangeable NH₄ ⁺ (NEA) because it constitutes nearly 10% of the total N in the top 1 m of soil. NEA is NH₄ ⁺ present in the soil that is not extractable with neutral K salt solutions, e.g. 1M KCl. This study was conducted in 1994 and 1995 to quantify the amount of NEA released by a gray luvisolic clay loam soil in Alberta, Canada, during the growing season. Replicated plots under: (1) continuous bromegrass, (2) continuous barley, and (3) barley since 1991 but previously under a continuous forage legume, were sampled four times through the growing season to a depth of 80 cm. The first sampling was done before seeding of barley. Cropping systems and their interaction with time of sampling had no effect on NEA. The NEA-N pool in the soil decreased significantly during grain-filling and maturing of barley, and had returned to approximately its initial level by the following spring. The maximum amount of NEA-N released was 15 g m^–2 at barley grain-filling in 1994, and 10 g m^–2 3 weeks after barley had matured in 1995. Approximately 24% of the released NEA accumulated as 1M-KCl-extractable NH₄ ⁺ in 1994, 20–30% was absorbed by the barley crop, and most of the remainder was probably assimilated by soil micro-organisms and immobilized in soil organic matter. The release of significant amounts of NEA during crop growth has implications for N cycling and measurements of mineralization/immobilization turnover rates. Received: 21 July 1997     

Effect of climate and soil type on the immobilization of nitrogen by decomposing straw in northern and southern Europe

 Wheat straw enclosed in mesh bags was buried for periods up to 1 year over two seasons in Scottish, Danish and Portuguese soils treated with ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃. Scottish soils were: Terryvale, a poorly drained sandy loam; and Tipperty, an imperfectly drained brown forest soil with a higher clay content. The Danish soil (Foulum) was a freely drained sandy loam and the Portuguese soils were a sandy soil (Evora) and a clay soil (Beja). During the first month, ¹⁵N was being incorporated into the straw in the Tipperty, Terryvale and Foulum soils simultaneously as the total N content was decreasing. Subsequently, the straws began to show net immobilization and the total N content of the original straw was exceeded in Tipperty and Foulum soils after 4 months and 8 months, respectively. Net immobilization in Terryvale was detected only in the second season and did not occur in the first because of high soil moisture content. The rates of ¹⁵N incorporation were similar in the two Portuguese soils, and a loss of N was only detected after 8 months. After 1 month, in the two clay soils, Beja and Tipperty, ¹⁵NO₃ ^– was incorporated into straw to a greater extent than ¹⁵NH₄ ⁺ and this was attributed to ¹⁵NH₄ ⁺ fixation by clay minerals. In contrast, ¹⁵NH₄ ⁺ was more efficiently incorporated than ¹⁵NO₃ ^– under waterlogged conditions (Terryvale) and NO₃ ^– loss could be attributed to denitrification. The proportion of added ¹⁵N in the straw residue after 1 month varied between 6% and 18% for ¹⁵NH₄ ⁺ and 2% and 23% for ¹⁵NO₃ ^– and immobilization of N in the longer term tended to be greater in soils from northern Europe than from Portugal. Received: 19 January 1998     

Dynamics of soil microbial biomass and nitrogen availability in a flooded rice soil amended with different C and N sources

 A greenhouse experiment was conducted to compare effects of different C and N sources applied to a flooded soil on soil microbial biomass (SMB) C and N, extractable soil organic N (N_ORG), and NH₄ ⁺-N in relation to plant N accumulation of rice (Oryza sativa L.). In addition to a control without inputs (CON), four treatments were imposed receiving: prilled urea (PU), rice straw (RS), RS and PU (RS+PU), or Sesbania rostrata as green manure (SES). Treatments were arranged according to a completely randomized design with four replicates and further consisted of pots with and without transplanted rice. While plant effects on the SMB were relatively small, the application of organic N sources resulted in a rapid increase in SMB until 10 days after transplanting (DAT) followed by a gradual decline until 73 DAT. Plant N accumulation data in these treatments clearly indicated that the SMB underwent a transition from a sink to a source of plant-available soil N during the period of crop growth. Seasonal variation of the SMB was small in treatments without amendment of organic material (CON, PU) presumably due to a lack of available C as energy source. Extractable N_ORG was significantly affected by soil planting status and organic N source amendment, but represented only a small N pool with little temporal variation despite an assumed rapid turnover. Among the three treatments receiving the same amount of N from different sources, the recovery efficiency of applied N was 58% for PU and 28% for both RS+PU and SES treatments at 73 DAT. The N uptake of rice, however, was not driven by N availability alone, as most evident in the RS+PU treatment. We assume that root physiological functions were impeded after application of organic N sources. Received: 1 June 1999     

A critical assessment of methods for determining organic phosphorus in savanna soils

 Two newly introduced extraction techniques for determining total organic P (P₀) were compared with the standard high-temperature ignition method in selected savanna soils of Nigeria. The two extraction techniques were: (1) concentrated H₂SO₄ and dilute base sequential extraction (18 N H₂SO₄ and 0.5 N NaOH) and (2) basic EDTA method (0.25 M NaOH plus 0.05 M Na₂EDTA). The concentrated H₂SO₄ and dilute base method extracted significantly higher total P₀ than the high-temperature ignition method and the basic EDTA extraction. The high-temperature ignition and the basic EDTA extraction gave similar total P₀ values (mean=91 mg kg^–1 for ignition and 90 mg kg^–1 for basic EDTA). The precision of the methods, determined by coefficients of variation (CV, %) associated with each P₀ determination method in the soils, was better for the concentrated H₂SO₄ and dilute base extraction method (CV=13%) than the ignition method (CV=18%) and the basic EDTA method (CV=15%). The high C : P₀ ratios determined for the high-temperature ignition and basic EDTA extraction indicated that the two methods underestimated total P₀ in the soils. The concentrated H₂SO₄ and dilute base sequential extraction appears to be suitable for the rapid determination of P₀ in savanna soils because the method can be simplified to a single-step analysis. Received: 14 November 1997     

Studies on the mechanisms of fixation and release of ammonium in paddy soils after flooding. I. Effect of iron oxides on ammonium fixation

Experiments were conducted with two typical paddy soils from China and a vermiculite to study the influence of iron oxides on the fixation and release of ammonium. Removing iron oxides, especially amorphous iron oxides, from the soils favoured the release of non-exchangeable NH₄-N and stimulated the fixation of NH₄-N in the presence of added (NH₄)₂SO₄. Addition of artificial goethite and hematite to the original soils or to the soils free of iron oxides reduced the fixation of NH₄⁺-ions. This effect was also observed with vermiculite. We conclude that the coating of clay minerals with iron oxides has an impact on the diffusion of NH₄⁺-ions into and out of the interlayers of the clay minerals. The reduction and dissolution of iron oxides induced by low redox potential (E_h) after flooding of paddy soils is assumed to be an important mechanism controlling NH₄⁺-fixation in paddy fields.     

Lability of soil organic carbon in tropical soils with different clay minerals

Soil organic carbon (SOC) storage and turnover is influenced by interactions between organic matter and the mineral soil fraction. However, the influence of clay content and type on SOC turnover rates remains unclear, particularly in tropical soils under natural vegetation. We examined the lability of SOC in tropical soils with contrasting clay mineralogy (kaolinite, smectite, allophane and Al-rich chlorite). Soil was sampled from A horizons at six sites in humid tropical areas of Ghana, Malaysian Borneo and the Solomon Islands and separated into fractions above and below 250 μm by wet sieving. Basal soil respiration rates were determined from bulk soils and soil fractions. Substrate induced respiration rates were determined from soil fractions. SOC lability was significantly influenced by clay mineralogy, but not by clay content when compared across contrasting clay minerals. The lability of SOC was lowest in the allophanic and chloritic soil, higher in the kaolinitic soils and highest in the smectitic soil. Our results contrast with conventional concepts of the greater capacity of smectite than of kaolinite to stabilize SOC. Contents of dithionite-citrate-bicarbonate extractable Fe and Al were inversely related to SOC lability when compared across soil types. A stronger inverse correlation between content of ammonium-oxalate extractable Fe and SOC lability was found when considering the kaolinitic soils only and we conclude that the content of active Fe (hydr-) oxides controls SOC stabilization in the kaolinitic soils. Our results suggest that the validity of predictive models of SOC turnover in tropical soils would be improved by the inclusion of soil types and contents of Fe and Al (hydr-) oxides.     

Soil microbial biomass and nitrogen supply in an irrigated lowland rice soil as affected by crop rotation and residue management

 Processes that govern the soil nitrogen (N) supply in irrigated lowland rice systems are poorly understood. The objectives of this paper were to investigate the effects of crop rotation and management on soil N dynamics, microbial biomass C (C_BIO) and microbial biomass N (N_BIO) in relation to rice N uptake and yield. A maize-rice (M-R) rotation was compared with a rice-rice (R-R) double-cropping system over a 2-year period with four cropping seasons. In the M-R system, maize (Zea mays L.) was grown in aerated soil during the dry season (DS) followed by rice (Oryza sativa L.) grown in flooded soil during the wet season (WS). In the R-R system, rice was grown in flooded soil in both the DS and WS. Three fertilizer N rates (0, 50 or 100 kg urea-N ha^–1 in WS) were assigned to subplots within the cropping system main plots. Early versus late crop residue incorporation following DS maize or rice were established as additional treatments in sub-subplots in the second year. In the R-R system, the time of residue incorporation had a large effect on NO₃ ^–-N accumulation during the fallow period and also on extractable NH₄ ⁺-N, rice N uptake and yield in the subsequent cropping period. In contrast, time of residue incorporation had little influence on extractable N in both the fallow and rice-cropping periods of the M-R system, and no detectable effects on rice N uptake or yield. In both cropping systems, C_BIO and N_BIO were not sensitive to residue incorporation despite differences of 2- to 3-fold increase in the amount of incorporated residue C and N, and were relatively insensitive to N fertilizer application. Extractable organic N was consistently greater after mid-tillering in M-R compared to the R-R system across N rate and residue incorporation treatments, and much of this organic N was α-amino N. We conclude that N mineralization-immobilization dynamics in lowland rice systems are sensitive to soil aeration as influenced by residue management in the fallow period and crop rotation, and that these factors have agronomically significant effects on rice N uptake and yield. Microbial biomass measurements, however, were a poor indicator of these dynamics. Received: 31 October 1997     

Methane production potentials of twenty-eight rice soils in China

 Soil CH₄ production potentials were investigated by incubating air-dried soils under anaerobic conditions in the laboratory. Twenty-eight soils from different fields and locations were collected for this study. Soil CH₄ production during a 100-day incubation differed greatly and were significantly correlated with soil organic content (r=0.61, P<0.01). The statistical significance increased when soils were grouped according to soil reduction rates. A significant correlation was also found between CH₄ production and total N content (r=0.64, P<0.01) and between CH₄ production and soil particle sizes of 0.25–0.05 mm (r=0.48, P<0.05). A negative exponential correlation was found between CH₄ production and aerobic soil pH (r=–0.74, P<0.01). The 28 soils were stratified into four groups on the basis of variation in CH₄ production rates which were associated with the soil reduction rate and soil organic content. The faster the Eh of soil fell, the more CH₄ was formed. Adding rice straw to Hangzhou and Beijing soils increased CH₄ production. The increase in CH₄ production was more pronounced in the soil with the lowest organic matter content and slowest reduction rate than in the soil with highest organic matter and fastest reduction rate. Inorganic fertilizer had no significant influence on CH₄ production potentials of either type of soil. Received: 26 November 1997     

Methane production in unamended and rice-straw-amended soil at different moisture levels

 CH₄ production in an alluvial soil, unamended or amended with rice straw (1% w/w), was examined under nonflooded [–1.5 MPa, –0.01 MPa and 0 MPa (saturated) and flooded (1 : 1.25 soil to water ratio)] conditions during a 40-day incubation in closed Vacutainer tubes. CH₄ production was negligible at –1.5 MPa, but increased with an increase in the moisture level. Addition of rice straw distinctly increased CH₄ production in the soil at all moisture levels including –1.5 MPa. Evidence, in terms of the drop in redox potential and Fe²⁺ accumulated, suggested that the addition of rice straw hastened the reduction of the soil, even under nonflooded conditions; thus its addition stimulated even the nonflooded soil to produce CH₄ in substantial amounts. Our results indicate that many currently unidentified sources of CH₄, possibly including organic-amended nonflooded soils, may make a significant contribution to the global CH₄ budget. Received: 10 July 1997     

Soil organic nitrogen composition in Pinus forest acid soils: variability and bioavailability

 The total N content in the acid forest soils studied ranged between 0.41% and 1.43%, and in more than 98% was composed of organic N. Total hydrolysable organic N, hydrolysable unknown N (HUN) and α-aminoacidic N represented around 70%, 34% and 20% of the organic N, respectively, and varied in wide ranges. The percentages of amidic N and of the organic N compounds solubilised to NH₄ ⁺ were approximately 6% and 5%, respectively, and ranged in narrow intervals. Aminoglucidic N reached a maximum of 3.8% of the organic N and was undetectable in some of the samples analysed. Most of the hydrolysable N, HUN and α-aminoacidic N was solubilised with 1 N and 3 N HCl, while a high amount of the compounds recovered as NH₄ ⁺ (60%) was obtained with 6 N HCl. The distribution of aminoglucidic N in the four fractions of increasing hydrolytic intensity was very irregular. The organic N composition in the 0 to 5-cm and 5 to 10-cm layers was not significantly different. The variation among samples was determined mainly by the organic N compounds less resistant to acid hydrolysis (hydrolysable N and HUN less resistant to acid hydrolysis, amidic N and labile ammoniacal N) and by all α-aminoacidic N fractions. Aminoacidic N was positively correlated with electrical conductivity and negatively correlated with exchangeable Al. The net N mineralisation over 10 weeks of incubation was positive in all the soil samples analysed. The inorganic N content after the incubation and the microbial N content were positively correlated with other variables – mainly with amidic N and α-aminoacidic N, as well as with HUN and the hydrolysable N less resistant to hydrolysis. Received: 13 July 1999     

Comparison of two different methods to measure nitric oxide turnover in soils

 The NO turnover in soils was measured in two different experimental set-ups, a flow-through system, which is very time-consuming and needs rather sophisticated equipment, and a closed system using serum bottles. We compared the NO turnover parameters (NO consumption rate constant, NO production rate, NO compensation concentration) that were measured with both systems in different soils, under different conditions and in the presence of 10 Pa acetylene to inhibit nitrification. The values of the NO turnover parameters that were measured with the two systems under oxic conditions were usually comparable. The addition of acetylene did not affect the NO consumption rate constants of the soils with the exception of soil G1. However, the NO production rates and the NO compensation concentrations decreased significantly in the presence of acetylene, indicating that nitrification was the main source of NO in these soils. Only one soil (Bol) showed no nitrifying activity. Increasing soil moisture content resulted in decreasing NO consumption rate constants and NO production rates. Even at a high soil moisture content of 80% water holding capacity, nitrification was the main source of NO. The values of the NO turnover parameters that were measured with the two systems were not comparable under anoxic conditions. The NO consumption rate constants and the NO production rates were much lower in the closed than in the flow-through system, indicating that the NO consumption activity became saturated by the high NO concentrations accumulating in the closed system. Under oxic conditions, however, closed serum bottles were a cheap, easy and reliable tool with which to determine NO turnover parameters and to distinguish between nitrification and denitrification as sources of NO. Received: 21 April 1998     

不同利用年限红壤水稻土有机碳和养分含量的粒级分布变化

通过田间采样结合沉降法分级提取,研究了不同利用年限红壤水稻土有机碳和养分含量的粒级分布变化特征。结果表明,红壤水稻土有机碳和养分含量随土壤颗粒粒径的增大而下降,但在各粒级中的分布比例存在显著差异。<0.002mm、0.002～0.02mm、0.02～0.05mm、>0.05mm粒级的有机碳占全土有机碳的比例分别是29.2%、30.7%、11.9%、15.4%,氮的相应数值为36.7%、31.9%、10.2%、14.0%,磷为49.2%、26.5%、11.1%、12.4%,钾为36.9%、33.4%、12.9%、20.0%。总体来说,黏粒和粉粒中有机碳和养分的分布比例较高。红壤水稻土有机碳和养分含量及分布比例还随利用年限而有明显变化。开垦利用不到10a的水田土壤,有机碳和养分含量较低且主要集中在<0.002mm粒级中;而利用超过10a的水稻土,有机碳和养分在粉粒中(0.002～0.05mm)的比例大于50%。各利用年限的红壤水稻土多以0.02～0.05mm粒级的C/N为最高,并随利用年限延长而下降。红壤水稻土各粒级有机碳和养分含量及分布状况随利用年限的变化反映了土壤肥力熟化和养分有效性的提高过程。     

不同耕作措施下江苏省稻田土壤固碳潜力的模拟研究

以江苏省稻田为对象,整合DNDC和1:100万土壤数据库,以土壤图斑为基本模拟单元,定量估算少耕、免耕和综合措施（少耕 + 30% 秸秆还田）下江苏省稻田土壤的固碳潜力（0 ~ 30 cm）。模拟结果表明：相对于传统耕作,采用少耕、免耕和少耕 + 30% 秸秆还田均可明显地增加稻田SOC的积累,其在2009—2050年间的固碳潜力分别为24.5、47.7和43.8 Tg。免耕和少耕 + 30% 秸秆还田条件下稻田固碳速率大约是少耕的2倍。结合实际情况,少耕 + 30% 秸秆还田将是最可行的固碳措施之一。     

Exchangeable ammonium and nitrate from different nitrogen fertilizer preparations in polyacrylamide-treated and untreated agricultural soils

 High molecular weight, anionic polyacrylamide (PAM) is currently being used as an irrigation water additive to significantly reduce soil erosion associated with furrow irrigation. PAM contains amide-N, and PAM application to soils has been correlated with increased activity of soil enzymes, such as urease and amidase, involved in N cycling. Therefore we investigated potential impacts of PAM treatment on the rate at which fertilizer N is transformed into NH₄ ⁺ and NO₃ ^– in soil. PAM-treated and untreated soil microcosms were amended with a variety of fertilizers, ranging from common rapid-release forms, such as ammonium sulfate [(NH₄)₂SO₄] and urea, to a variety of slow-release formulations, including polymerized urea and polymer-encapsulated urea. Ammonium sulfate was also tested together with the nitrification inhibitor dicyandiamide (DCD). The fertilizers were applied at a concentration of 1.0 mg g^–1, which is comparable to 100 lb acre^–l, or 112 kg ha^–1. Potassium chloride-extractable NH₄ ⁺-N and NO₃ ^–-N were quantified periodically during 2–4 week incubations. PAM treatment had no significant effect on NH₄ ⁺ release rates for any of the fertilizers tested and did not alter the efficacy of DCD as a nitrification inhibitor. However, the nitrification rate of urea and encapsulated urea-derived NH₄ ⁺-N was slightly accelerated in the PAM-treated soil. Received: 16 January 1998     

Characterization of active nitrogen pools in soils under different cropping systems

 Studies were conducted to evaluate the relationships among different active N pools of organic matter in soils at two long-term cropping systems in Iowa. Results indicated that multi-cropping systems, particularly meadow-based systems, enhanced bioactivities of soils. Mono-cropping systems, particularly soybean, reduced soil microbial biomass and enzyme activities. The mineralizable N pool (potential N mineralization;N _o) was more sensitive to changes in the size of the microbial biomass N (N_mic) than to changes in organic N. One unit change in organic N did not lead to substantial changes in N _o, but 1 unit change in N_mic resulted in three or more units change in N _o. The active N pools and turnover rate were more sensitive to changes in organic C than to changes in microbial biomass C (C_mic). A unit change in organic C resulted in 10.6 units change in N _o, but a unit change in C_mic resulted in only 0.8 unit change in N _o. C_mic or N_mic are better indexes than organic C or N for the estimation of N _o or N availability, because biomass values are more highly correlated with cumulative N mineralized during 24 weeks of incubation, with r values ranging from 0.57 (P<0.001) to 0.88 (P<0.001). Received: 18 October 1999     

Neutral sugars in melanins synthesized by actinomycetes from Brazilian soils

 Nine actinomycete melanins synthesized under various culture conditions, eight of them by actinomycete samples isolated from Brazilian topsoils under savanna (cerrado) vegetation and one from an ATCC sample, were subjected to a two-step hydrolysis procedure and the sugars released qualitatively and quantitatively determined by capillary gas-liquid chromatography (GLC). Humic acids (HAs) extracted from these soils, analysed previously, were used for comparison. The neutral sugars glucose, galactose, mannose, xylose, arabinose, ribose, rhamnose and fucose and the alcohol sugar inositol were present in varying amounts in most of the melanins analysed. The same sugars were present in the HAs used for comparison, except for ribose. Some qualitative and quantitative differences observed in the two types of macromolecules would be expected, considering their origins. The results indicate that the actinomycete melanins have a qualitative sugar distribution pattern similar to that of the HAs from Brazilian tropical soils and of HAs reported for soils from other climatic regions. The possible participation of actinomycete melanins in the formation of soil humic substances is discussed. Received: 4 April 1997