Organic matter and water-stable aggregates in soils

The water-stability of aggregates in many soils is shown to depend on organic materials. The organic binding agents have been classified into (a) transient, mainly polysaccharides, (b), temporary, roots and fungal hyphae, and (c) persistent, resistant aromatic components associated with polyvalent metal cations, and strongly sorbed polymers. The effectiveness of various binding agents at different stages in the structural organization of aggregates is described and forms the basis of a model which illustrates the architecture of an aggregate. Roots and hyphae stabilize macro-aggregates, defined as > 250 μm diameter; consequently, macroaggregation is controlled by soil management (i.e. crop rotations), as management influences the growth of plant roots, and the oxidation of organic carbon. The water-stability of micro-aggregates depends on the persistent organic binding agents and appears to be a characteristic of the soil, independent of management.     

黑土轻组分C与团聚体水稳性的关系

To evaluate the role of kaolinite and variable charge soils on the hydrolytic reaction of Al, the hydrolysis of Al ions in suspensions of a kaolinite and an Oxisol influenced by organic anions was investigated using changes of pH, Al adsorption, and desorption of pre-adsorbed Al. Kaolinite and the Oxisol promoted the hydrolytic reaction of Al above a certain initial Al concentration （0.1 mmol L^-1 for kaolinite and 0.3 mmol L^-1 for the Oxisol）. The Al hydrolysis accelerated by kaolinite and the Oxisol increased with an increase in initial concentration of Al and was observed in the range of pH from 3.7 to 4.7 for kaolinite and 3.9 to 4.9 for the Oxisol. The acceleration of Al hydrolysis also increased with the increase of solution pH, reached a maximum value at pH 4.5, and then decreased sharply. Al hydrolysis was promoted mainly through selective adsorption for hydroxy-Al. Soil free iron oxides compensated a portion of the soil negative charge or masked some soil surface negative sites leading to a decrease in Al adsorption, which retarded acceleration to some extent. For the Oxisol organic anions increased the proportion of adsorbed Al^3＋ in total adsorbed Al with the increase in soil negative surface charge and eliminated or reduced the acceleration of Al hydrolysis. Different organic anions inhibited the hydrolysis of Al in the order： citrate 〉 oxalate 〉 acetate （under initial pH of 4.5）. The formation of Al-organic complexes in solution also inhibited the hydrolysis of Al.     

Stimulation of nitrogen fixation in soddy-podzolic soils with fungi

Stimulation of nitrogen fixation in soddy-podzolic soils is related to the hydrolytic activity of fungi decomposing plant polymers. It was found that the rate of nitrogen fixation upon the simultaneous inoculation of the strains of nitrogen-fixing bacteria Bacillus cereus var. mycoides and the cellulolytic fungus Trichoderma asperellum into a sterile soil enriched with cellulose or Jerusalem artichoke residues is two to four times higher than upon the inoculation of the strains of Bacillus cereus var. mycoides L1 only. The increase in the nitrogen fixation depended on the resistance of the substrates added into the soil to fungal hydrolysis. The biomass of the fungi decomposing plant polymers increased by two–four times. The nitrogen-fixing activity of the soil decreased when the growth of the fungi was inhibited with cycloheximide, which attested to a close correlation between the intensity of the nitrogen fixation and the decomposition of the plant polymers by fungi. The introduction of an antifungal antibiotic, together with starch or with plant residues, significantly (by 60–90%) decreased the rate of nitrogen fixation in the soll.     

黑土开垦后水稳性团聚体与土壤养分的关系

Water-stable aggregates, which are an index for the evaluation of the structural properties of the soil, are affected by many factors. Zhaoguang Farm, Longzhen Farm, and Jiusan Farm were chosen as the representative study sites in the region of black soils, a typical soil resource in Northeast China. The variation in the content of 〉 0.25 mm water-stable aggregates and its relationship with the nutrients in black soil were investigated after different years of reclamation. The results showed that the 〉 0.25 mm water-stable aggregates were more in the surface than in the subsurface soil and they changed in the following order： Longzhen Farm 〉 Zhaoguang Farm 〉 Jiusan Farm. The water-stable aggregates decreased sharply at the initial stage of reclamation and then became stable gradually with time. They were significantly correlated with the contents of organic C, total N, total P, and CEC in black soil, with the correlation coefficients r being 0.76, 0.68, 0.61, and 0.81 （P 〈 0.01）, respectively; however, their relationships with available P, available K, and total K were unclear. These showed that organic matter was the cementation of soil water-stable aggregates. Increasing decompositions and decreasing inputs of organic matter after reclamation were responsible for the amount of reduction of the water-stable aggregates. Thus, to maintain good soil aggregate structure, attention should be paid to improvement of soil nutrient status, especially the supply of organic C and N.     

Bacterial diversity in water-stable aggregates of soils under conventional and zero tillage management

Reduced tillage of agricultural soils has been shown to result in greater macroaggregation, microbial biomass and microbial diversity. While it has been shown that macroaggregates contain more microbial biomass per unit soil mass than microaggregates, it is unclear how microbial diversity varies with soil aggregation. We investigated the functional diversity (catabolic potential) of bacteria, evaluated by calculating Shannon’s diversity index (H′), substrate richness (S) and substrate evenness (E) from potential substrate utilization patterns, in whole soil (i.e. not separated into different aggregate sizes) and aggregates of different sizes (2–4, 1–2, 0.5–1, 0.25–0.5, and 0.1–0.25 mm diameter) in loam and silt loam soils grown to barley and managed for 6 years under conventional tillage (CT) or zero tillage (ZT) systems in northern British Columbia. There were no significant tillage effects on bacterial diversity in whole soils. In soil aggregates, H′ and E were significantly higher under CT than under ZT on the loam at barley planting time, with no significant aggregate size effects. However, at barley-heading stage, all diversity indices in both soils were significantly higher under ZT than under CT, and they tended to increase with increasing aggregate size. Cluster analysis and principal component analysis of substrate utilization patterns also revealed differences in bacterial community structures between CT and ZT, but the substrates that were utilized differently between the two tillage systems were not the same between soil types or sampling times. The results during the cropping cycle imply that deterioration of soil structure is probably one factor that explains the adverse effects of soil tillage on soil microbial biomass and diversity.     

Texture and sesquioxide effects on water-stable aggregates and organic matter in some tropical soils

Many tropical soils include sesquioxides, which influence the stability of soil organic matter (OM) and aggregation to an extent that is not fully characterized. The present study was carried out on a range of 18 topsoil samples (0–10 cm) from low-activity clay (LAC) soils from sub-Saharan Africa and Brazil, and aimed: (i) at characterizing the size distributions of water-stable aggregates and organic constituents, (ii) at studying how these distributions were affected by texture and sesquioxides, and (iii) how they interacted.The distributions of stable aggregates were generally dominated by macroaggregates (> 200 μm), and those of organic constituents by fine OM (< 20 μm). Aggregation was not clearly affected by soil texture, while total soil carbon (Ct) and the amount of carbon (C) as fine OM increased with soil content in clay plus fine silts (< 20 μm). Stable macroaggregation correlated with Ct and with C amount as fine OM, but each of them correlated more closely with citrate-bicarbonate-dithionite-extractable aluminium (Al), which was not expected. Stable macroaggregation also correlated with C amounts as coarse- and medium-sized OM (> 200 and 20–200 μm, respectively), but each of them correlated more closely with oxalate-extracted Al. These results suggested that for the LAC tropical soils under study OM and aggregate stability depended closely on Al-containing sesquioxides, on Al-substituted crystalline hematite and goethite especially. These sesquioxides also seemed to play a dominant role in the relations between aggregation and OM. As far as soils rich in sesquioxides are concerned, this confirmed that OM is not the main aggregating agent, and suggested that physical protection within aggregates is not necessarily the main mechanism for OM stabilization. However, as soil sesquioxide content cannot be managed easily, the effect of land use on soil OM and aggregation was determinant at the local scale: indeed, for a given location, stable macroaggregation, Ct and C amount as fine OM generally decreased with land use intensification (i.e. cultivation, tillage, reduced surface cover).     

Enzymatic activity inside and outside of water-stable aggregates in soils under different land use

A method is presented for assessing the distribution of enzymatic activity inside and outside of water-stable aggregates. Two samples of water-stable aggregates >1 mm have been isolated from dry aggregates of 1–2 mm. To determine the enzymatic activity, a substrate has been added to one of the samples without disaggregation; the other sample has been preliminarily disaggregated. Enzymatic activity within waterstable aggregates has been assessed from the difference between the obtained results under the supposition that the penetration of substrate within the water-saturated aggregates is hampered, and enzymatic reactions occur only at the periphery. The levels and distributions of enzymatic (peroxidase, polyphenol oxidase, and catalase) activities in water-stable aggregates of soddy-podzolic soils under forest and plowland and typical chernozems of long-term field experiments have been studied. The peroxidase, polyphenol oxidase, and catalase activities of water-stable aggregates vary from 6 to 23, from 7 to 30, and from 5 to 7 mmol/(g h), respectively. The ratio between the enzymatic activities inside and outside of soil aggregates showed a higher dependence on soil type and land use, as well as on the input of organic matter and the structural state, than the general activity level in water-stable aggregates.     

PAM对黄土高原主要土壤类型水稳性团聚体的改良效果及机理研究

通过室内土柱培养,研究PAM对黑垆土、黄绵土、风沙土水稳性团聚体的改良效果,并探讨其作用机理和合理的施用浓度.结果表明,在浓度为0.05%～0.4%,PAM均可促进3种土壤＞0.25 mm水稳性团聚体的形成,并有效降低3种土壤团聚体分形维数,改善土壤结构;方差分析表明,PAM改良3种土壤结构的机理是一致的,将＜1mm的水稳性团聚体聚合为更大粒径的水稳性团聚体,使＞1mm的水稳性团聚体含量增加.PAM改良黑垆土、黄绵土和风沙土土壤水稳性团聚体达到显著水平时的浓度也不相同,其适宜浓度分别为0.2%～0.4%,0.05%,0.05%.3种土壤的黏粒含量和有机质含量的差异,可能是影响PAM对不同土壤的水稳性团聚体的改良效果差异的主要原因.     

Organic matter in density fractions of water-stable aggregates in silty soils: Effect of land use

The location of soil organic matter (SOM) within the soil matrix is considered a major factor determining its turnover, but quantitative information about the effects of land cover and land use on the distribution of SOM at the soil aggregate level is rare. We analyzed the effect of land cover/land use (spruce forest, grassland, wheat and maize) on the distribution of free particulate organic matter (POM) with a density <1.6 g cm⁻³ (free POM_<1.6), occluded particulate organic matter with densities <1.6 g cm⁻³ (occluded POM_<1.6) and 1.6-2.0 g cm⁻³ (occluded POM_1.6-2.0) and mineral-associated SOM (>2.0 g cm⁻³) in size classes of slaking-resistant aggregates (53-250, 250-1000, 1000-2000, >2000 μm) and in the sieve fraction <53 μm from silty soils by applying a combined aggregate size and density fractionation procedure. We also determined the turnover time of soil organic carbon (SOC) fractions at the aggregate level in the soil of the maize site using the ¹³C/¹²C isotope ratio. SOM contents were higher in the grassland soil aggregates than in those of the arable soils mainly because of greater contents of mineral-associated SOM. The contribution of occluded POM to total SOC in the A horizon aggregates was greater in the spruce soil (23-44%) than in the grassland (11%) and arable soils (19%). The mass and carbon content of both the free and occluded POM fractions were greater in the forest soil than in the grassland and arable soils. In all soils, the C/N ratios of soil fractions within each aggregate size class decreased in the following order: free POM_<1.6>occluded POM_<1.6-2.0>mineral-associated SOM. The mean age of SOC associated with the <53 μm mineral fraction of water-stable aggregates in the Ap horizon of the maize site varied between 63 and 69 yr in aggregates >250 μm, 76 yr in the 53-250 μm aggregate class, and 102 yr in the sieve fraction <53 μm. The mean age of SOC in the occluded POM increased with decreasing aggregate size from 20 to 30 yr in aggregates >1000 μm to 66 yr in aggregates <53 μm. Free POM had the most rapid rates of C-turnover, with residence times ranging from 10 yr in the fraction >2000 μm to 42 yr in the fraction 53-250 μm. Results indicated that SOM in slaking-resistant aggregates was not a homogeneous pool, but consisted of size/density fractions exhibiting different composition and stability. The properties of these fractions were influenced by the aggregate size. Land cover/land use were important factors controlling the amount and composition of SOM fractions at the aggregate level.     

Mycelium of arbuscular mycorrhizal fungi increases soil water repellency and is sufficient to maintain water-stable soil aggregates

Using an in vitro bioreactor system in which the arbuscular mycorrhizal (AM) fungus Glomus intraradices was grown in a soil devoid of detectable living microbes, we could show that the mycelium of this fungus contributed to the maintenance of water-stable soil aggregates and increased soil water repellency, as measured by water drop penetration time. This is to our knowledge the first demonstration of a causal link between AM fungal growth and water repellency of soil aggregates. Our results also place AM fungal contributions to soil aggregation on a firm mechanistic footing by showing that hyphae are sufficient to produce effects, in the absence of other soil biota, which have always been included in previous studies.     

The seasonal dynamics of yeast communities in the rhizosphere of soddy-podzolic soils

The annual dynamics of the number and taxonomic composition of yeast was studied in the rhizosphere of two plant species (Ajuga reptans L. and Taraxacum officinale Wigg.) in a forb-birch forest on soddy-podzolic soil. Eurybiont phyllobasidial cryptococci and red-pigmented phytobionts Rhodotorula glutinis were found to predominate in the phyllosphere of these plants, whereas the typical pedobionts Cryptococcus terricola and Cr. podzolicus occurred on the surface of roots and in the rhizosphere. The seasonal changes in the number and species composition of the yeast communities in the rhizosphere were more smooth as compared to those in the phyllosphere. In the period of active vegetation of the plants, the phytobiont yeasts develop over their whole surface, including the rhizoplane. Their number on the aboveground parts of the plants was significantly lower than that of the pedobiont forms. Thus, the above-and underground parts of the plants significantly differed in the composition of the dominant species of epiphytic yeasts.     

Distribution of nutrient elements within water-stable aggregates of two tropical agro-ecological soils under different land uses

This study evaluated carbon and nutrient distributions within water-stable aggregates (WSA) of soils of two contrasting ecosystems under different land uses. Surface soil samples were collected from uncultivated and cultivated land in rainforest and savannah agro-ecological areas and separated by wet-sieving technique into 4.76–2.0, 2.0–1.0, 1.0–0.50, 0.50–0.25 and <0.25 mm aggregate fractions. The results show that irrespective of the agro-ecological area, cultivation significantly (p < 0.05) reduced the macroaggregate fractions (>0.25 mm) to smaller diameters. Distribution of organic carbon (C), total nitrogen (N) and available phosphorus (P) within the WSA showed preferential enrichment of these elements in the large macroaggregate fraction (4.76–2.0 mm) for the uncultivated soils and microaggregate fraction (<0.25 mm) for the cultivated soils. The overall pattern indicates higher accumulation of C, N and P in the WSA of the uncultivated soils over the cultivated soils. Average distribution of total exchangeable bases (TEB), i.e., sum of Ca²⁺, Mg²⁺, K⁺ and Na⁺, within WSA of the uncultivated soils of the rainforest region were 7.35 and 7.39 cmol/kg for 4.76–2.0 and <0.25 mm fractions, respectively. The distributions of TEB for cultivated soils of the rainforest region were 2.76 cmol/kg (4.76–2.0 mm fraction) and 7.73 cmol/kg for <0.25 mm fraction. This showed that cultivation significantly (p < 0.05) led to 62% reduction in these nutrients in the 4.76–2.0 mm fraction and 5% increase in concentrations of these cations in <0.25 mm fraction. For savannah soils, distributions of TEB were 7.44 and 6.77 cmol/kg for 4.76–2.0 and <0.25 mm fractions, respectively, in uncultivated sites, whereas TEB were 2.19 cmol/kg (4.76–2.0 mm) and 6.35 cmol/kg (<0.25 mm) for cultivated savannah. This indicated that cultivation significantly (p < 0.05) led to 71% and 6% reductions in Ca²⁺, Mg²⁺, K⁺ and Na⁺ concentrations within the 4.76–2.0 and <0.25 mm aggregate fractions, respectively. However, there were 18% and 50% increase in these elements in the 2.0–1.0 and 1.0–0.50 mm fractions of the cultivated soils of the savannah region, respectively. The general trend showed that in uncultivated soils, the 4.76–2.0 and <0.25 mm fractions were preferentially enriched with Ca²⁺, Mg²⁺, K⁺ and Na ⁺; whereas, cultivation led to redistribution of these elements into the smaller aggregates. Since smaller aggregates are preferentially removed by erosion, this study underscores the need for sustainable soil management practices that would minimize nutrient loss when forest or fallow lands are converted to cropland.     

长期不同施肥下塿土有机碳和全氮在团聚体中的分布

【目的】研究小麦/玉米轮作体系不同施肥方式下土壤有机碳(SOC)和全氮(TN)在(?)土不同水稳性团聚体中的分布特征,以期深入理解不同施肥方式对土壤碳、氮固持的机制。【方法】采集(?)土21年长期肥料定位试验不同施肥处理0-10 cm和10-20 cm土层土样,分析其水稳性团聚体(2 mm、2~1 mm、1~0.5 mm、0.5~0.25 mm以及0.25 mm)有机碳和全氮的分布特征。试验设不施肥(CK),化肥氮磷钾配施(N、NP、NK、PK、NPK)和秸秆还田配施氮磷钾(SNPK)以及两个水平有机肥与氮磷钾配施(M1NPK、M2NPK)9个处理。【结果】长期施肥0-10 cm土层土壤团聚体SOC和TN含量明显高于10-20 cm,平均增幅20%以上。2~1 mm或1~0.5mm团聚体中SOC和TN的含量最高,0.25 mm团聚体最低。长期不施磷肥处理的土壤团聚体SOC和TN含量均与CK相似。NP、NPK以及SNPK处理,0-10 cm土层SOC较CK分别增加16%~43%、9%~40%和22%~47%;TN增幅分别为28%~48%、39%~61%和39%~91%。10 20 cm土层,NP、NPK以及SNPK处理2mm、2~1 mm、1~0.5 mm土壤团聚体SOC较CK增幅分别为35%~49%、17%~40%和45%~46%,TN增幅分别为44%~47%、39%~54%和54%~64%。长期有机肥与氮磷钾配施处理(M1NPK、M2NPK),0-10 cm土层的团聚体SOC平均较CK分别增加68%~122%和61%~163%,TN平均分别增加84%~133%和97%~175%;10-20 cm土层,SOC较CK平均增幅分别为20%~61%和39%~118%,TN增幅平均分别为43%~86%和107%~136%。SOC和TN主要储存于0.25 mm团聚体中(40%)2~1 mm团聚体储存最少(10%)。长期不施氮或不施磷对SOC和TN在团聚体中的储存比例基本没有影响。长期NP、NPK以及M1NPK、M2NPK均降低了各土层SOC和TN在2 mm或2~1 mm的储存比例增加了在1~0.25 mm团聚体储存比例。2 mm或2~1 mm团聚体的C/N比值高于微团聚体(0.25 mm),而与CK相比,长期施肥降低了土壤团聚体的C/N比值。【结论】关中地区(?)土长期偏施化肥对有机碳和全氮在团聚体的含量及分布没有显著影响而长期氮磷或氮磷钾化肥配合、氮磷钾与有机物配合均明显增加土壤团聚体的有机碳及全氮含量,特别是长期氮磷钾配合有机肥能显著增加土壤1~0.25 mm团聚体对土壤有机碳和全氮的固存比例,提高土壤有机碳和全氮储量减少温室气体的排放。     

Stable and labile components of humus in soddy-podzolic soils

A mathematical model was proposed to characterize the seasonal dynamics of the humus composition in loamy soddy-podzolic soils. The results of determining the composition of the organic matter during two successive seasons revealed the presence of labile and stable components in almost all the groups and fractions of the humic substances. The seasonal changes in the content of the humic substances, the exchangeable Ca, and the pH value at the equilibrium state of the soil were found to be identical during the successive years.     

Postagrogenic transformation of organic matter in soddy-podzolic soils

In the chronological sequence of postagrogenic soils, the restoration of the original differentiation of the soil profile and its horizons proceeded with different rates depending on the fallow age and the horizon depth. The layer sampling (at 5-cm intervals) showed that the plow horizon began to differentiate into a system of subhorizons in all the fallow soils. The zonal pedogenesis showed clear signs of manifestation already in the 15-year-old fallow. The upper part of the former plow horizon in the 15- and 60-year-old fallows under herbaceous plants was transformed into a soddy horizon, while a postagrogenic soil 90 years old already developed under a zonal type of vegetation and approached the control soil in its morphological features. The content and reserve of carbon in the soils showed a stable tendency of increasing (especially in the upper part of the plow horizon) during the entire postagrogenic period under study. The water permeability of the soils gradually increased and approached that of the virgin soil. However, the compacted subsurface horizon (the plow pan) disappeared only after 90 years. The assessment of the physicochemical properties of the soils and the structural and functional parameters of the humic acids indicated the expansion of the layer differentiation primarily within the homogeneous plow horizon. From the elemental analysis and 13C NMR spectroscopy data, the degree of aromaticity in the molecular structure of the humic acids gradually decreased, and the aliphatic part developed with the age of the fallow.     

红壤旱土和水稻土团聚体中磷素的分布特点

为深入探讨红壤磷素有效性的控制机理,研究了典型红壤旱土和水稻土团聚体中磷的组分与分布特点。结果表明:旱土和水稻土以粒径>0.2mm的团聚体为主,且在该级土壤团聚体中磷素含量较高;两种土壤>0.2mm团聚体中吸附态无机磷以Fe-P为主,而<0.02mm团聚体以O-P为主;水稻土>0.2mm和<0.002mm团聚体的各组分有机磷总量亦显著高于旱土对应的团聚体。说明土壤磷的形态分布与团聚体粒级有密切关系,旱土有效磷含量较低可能与其团聚体游离氧化铁含量较高、且有机磷含量较低有关。     

几株产荚膜细菌在土壤水稳性团聚体形成中的作用

试验研究几株产荚膜细菌在土壤水稳定性团聚体形成中的作用结果表明,菌株次生代谢产物即发酵物在土壤水稳定性团聚体形成中起作用,而菌体本身不起作用。细菌能够显著促进>0.25mm土壤水稳定性团聚体的形成,其效果随加入C源物质而异,蔗糖效果优于小麦秸秆和玉米秸秆,风化煤几乎不起作用。加入不同C源物质时,菌株之间表现出明显差异。     

Physical properties of soddy-podzolic soils in a long-term field experiment

The main physical properties of soddy-podzolic soils in a long-term field experiment have been studied. It is shown that their changes under the impact of a century-long application of lime, fertilizers, and manure have been relatively small. Reliable differences in the parameters of approximation of the dependence of penetration resistance on the soil water content in the variant with regular application of organic fertilizers have been revealed. This attests to stronger interparticle bonds (within the studied moisture range) in this variant. Interparticle bonds in the control variant and in the variant with lime application tend to increase with a decrease in the soil water content more significantly than those in other experimental variants. This may be due to the coarser texture of the soil in the control variant and to the aggregation of soil particles under the impact of lime in the variant with lime application.     

Transformation of humic substances in soddy-podzolic soils under agrogenic impacts

The transformation of humic substances in soddy-podzolic soils under agrogenic impacts of different intensities and durations was studied. Indices of the initial stages of degradation and regradation of soil humus were revealed, and the humus status of soils under different fertilizing conditions was estimated.     

Regularities of the distribution of eroded and aggraded soddy-podzolic soils on slopes

The distribution of differently eroded soddy-podzolic soils on slopes of different gradients, lengths, and aspects has been studied with the use of a quantitative parameter of slope erodibility K. The latter is calculated as a product of slope gradient in the power of 1.3 and the length of the flow line (m) in the power of 0.5. It is shown that moderately eroded soddy-podzolic soils appear on the slopes of northern and northeastern aspects with K equal to 0.4; on the slopes of southern aspect, this parameter is somewhat lower (0.24). Moderately eroded soils prevail on the slopes of northern and northeastern aspects with K equal to 0.7.