Macroaggregate Size Research Articles

Do nitrogen isotope patterns reflect microbial colonization of soil organic matter fractions?

Different theories have been brought forward to explain the commonly observed δ15N enrichment with depth in soil profiles, including the discrimination against 15N during N decomposition and the buildup of 15N-enriched microbial residues. A combination of soil organic matter (SOM) size and density fractionations, 15N determinations, and phospholipid fatty acid (PLFA) analyses was conducted on soils from a pristine N-limited Nothofagus forest in southern Chile. The purpose of this study was to investigate which SOM fractions mostly reflect the 15N-enrichment pattern and to link 15N SOM enrichment with microbial community composition. Nitrogen-15 enrichments were greater for the microaggregate ( 150 μm) size fraction, with Rayleigh isotope enrichment factors averaging −8.5‰ and −3.7‰, respectively. The macro-organic matter density fractions (>150 μm) showed intermediate enrichment factors of −5.1‰ and −7.3‰ for the light ( 1.37 g cm−3) fraction, respectively. The abundance of fungal and bacterial PLFAs was significantly higher in the microaggregate compared to the macroaggregate size fraction, but their relative abundance did not change between aggregate size fractions. Our data link differential 15N enrichment of SOM fractions to “total” microbial abundance and, as such, corroborates existing theories of microbial-induced 15N enrichment. Isotopic fractionation during microbial N decomposition processes alone could not explain the large 15N enrichment in the microaggregate size fraction (−8.5‰) and the heavy density fraction (−7.3‰). We therefore suggest that microbial turnover and accretion of 15N-enriched microbial (especially fungal) compounds was an additional driver for 15N enrichment of this soil profile.

Eubacterial communities in different soil macroaggregate environments and cropping systems

Different positions within soil macroaggregates, and macroaggregates of different sizes, have different chemical and physical properties which could affect microbial growth and interactions among taxa. The hypothesis that these soil aggregate fractions contain different eubacterial communities was tested using terminal restriction fragment length polymorphism (T-RFLP) of the 16S ribosomal gene. Communities were characterized from two field experiments, located at the Kellogg Biological Station (KBS), MI, USA and the Ohio Agricultural Research and Development Center (OARDC), Wooster, OH, USA. Three soil management regimes at each site were sampled and management was found to significantly affect T-RFLP profiles. The soil aggregate erosion (SAE) method was used to isolate aggregate regions (external and internal regions). Differences between eubacterial T-RFLP profiles of aggregate exteriors and interiors were marginally significant at KBS (accounting for 12.5% of total profile variance), and not significant at OARDC. There were no significant differences among macroaggregate size classes at either site. These results are in general agreement with previous studies using molecular methods to examine microbial communities among different soil macroaggregate size fractions, although further study of communities within different aggregate regions is warranted. Analysis of individual macroaggregates revealed large inter-aggregate variability in community structure. Hence the tertiary components of soil structure, e.g. arrangement of aggregates in relation to shoot residue, roots, macropores, etc., may be more important than aggregate size or intra-aggregate regions in the determination of the types of microbial communities present in aggregates. Direct microscopic counts were also used to examine the bacterial population size in aggregate regions at KBS. The proportion of bacterial cells with biovolumes >0.18 μm 3 was higher in aggregate interiors than in exteriors, indicating potentially higher activity in that environment. This proportion was significantly related to percent C of the samples, while total bacterial cell counts were not.

Production of macroaggregates from dissolved exopolymeric substances (EPS) of bacterial and diatom origin

Exopolymeric substances (EPS) isolated from a pure culture of the marine bacterium Marinobacter sp. and the marine diatom Skeletonema costatum (axenic) were partially purified, chemically characterized and used as dissolved organic matter (DOM) for the production of macroaggregates. The role of organic particles such as transparent exopolymeric particles (TEP) and Coomassie stained particles (CSP) in the production of macroaggregates was experimentally assessed. Three experimental rolling tanks containing sterile medium with: (1) EPS, (2) EPS + live diatom cells and (3) EPS + killed bacteria, and three control tanks without any added EPS were used for macroaggregate production. Changes in abundance and average size of macroaggregates were monitored using image analysis, whereas TEP and CSP were enumerated microscopically. In the presence of microbial EPS, macroaggregates of a size of 23–35 mm 2 were produced. Aggregate size and abundance considerably varied with both time and source of EPS. No correlation was observed for macroaggregate size and abundance with either TEP or CSP. One-way ANOVA demonstrated significant differences in the variance of particle abundance and size in tanks having only EPS or EPS in combination with live diatom cells. Our data suggest that production of macroaggregates was influenced by polymer chemistry and surface properties of colliding particles, whereas TEP and CSP concentrations were influenced by molecular weight of EPS and the presence of growing cells. Interestingly, macroaggregates were formed in the near absence of TEP and CSP, highlighting the role of other unknown processes in the transformation of DOM to particulate organic matter (POM) in aquatic environments.

Dissipation of acetochlor and its distribution in surface and sub-surface soil fractions during laboratory incubations

Pesticides in soil are subject to a number of processes that result in transformation and biodegradation, sorption to and desorption from soil components, and diffusion and leaching. Pesticides leaching through a soil profile will be exposed to changing environmental conditions as different horizons with distinct physical, chemical and biological properties are encountered. The many ways in which soil properties influence pesticide retention and degradation need to be addressed to allow accurate predictions of environmental fate and the potential for groundwater pollution. Degradation and sorption processes were investigated in a long-term (100 days) study of the chloroacetanilide herbicide, acetochlor. Soil cores were collected from a clay soil profile and samples taken from 0-30 cm (surface), 1.0-1.3 m (mid) and 2.7-3.0 m (deep) and treated with acetochlor (2.5, 1.25, 0.67 microg acetochlor g(-1) dry wt soil, respectively). In sterile and non-sterile conditions, acetochlor concentration in the aqueous phase declined rapidly from the surface and subsoil layers, predominantly through nonextractable residue (NER) formation on soil surfaces, but also through biodegradation and biotic transformation. Abiotic transformation was also evident in the sterile soils. Several metabolites were produced, including acetochlor-ethane sulphonic acid and acetochlor-oxanilic acid. Transformation was principally microbial in origin, as shown by the differences between non-sterile and sterile soils. NER formation increased rapidly over the first 21 days in all soils and was mainly associated with the macroaggregate (>2000 microm diameter) size fractions. It is likely that acetochlor is incorporated into the macroaggregates through oxidative coupling, as humification of particulate organic matter progresses. The dissipation (ie total loss of acetochlor) half-life values were 9.3 (surface), 12.3 (mid) and 12.6 days (deep) in the non-sterile soils, compared with 20.9 [surface], 23.5 [mid], and 24 days [deep] in the sterile soils, demonstrating the importance of microbially driven processes in the rapid dissipation of acetochlor in soil.

Changes in aggregation do not correspond with changes in labile organic C fractions in soil amended with 14C-glucose

Dry-sieved aggregates (> 2 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm and < 0.25 mm) were amended with 14C glucose at 615 ng C g −1 soil (low glucose, LG) or 2457 ng C g −1 soil (high glucose, HG). We followed the changes in water- and acid-extractable carbohydrate C, 14C microbial biomass and 14C microbial products and water-stable aggregation (MWD) in each aggregate size class over 56 days. CO 2-efflux was measured to determine mineralisation of native and glucose-derived C in each aggregate size class. Glucose amendments did not affect the MWD of the > 2 mm aggregates. Both rates of amendment increased the MWD of the < 0.25 mm aggregate size class, but only the HG treatments generally increased the MWD of the 0.5–2 mm aggregate size classes. Increases in acid- and water-extractable carbohydrate C in the amended soils, relative to non-amended soils, were generally evident only up to 7 days after amendment. After this time water-extractable carbohydrate C in the amended aggregates was generally less than that in the non-amended aggregates. The proportions of 14C accumulated in the microbial biomass increased with declining macro-aggregate size, whereas the accumulations of 14C as microbial products were generally greater in the larger macro-aggregate size classes. These differences in C turnover were poorly correlated with clay content and pore size distribution in each aggregate size class. Glucose amendment initially resulted in inconsistent effects on the mineralisation of native C, however, after 21 days the HG treatments consistently increased the mineralisation of all but the 0.25–0.5 mm aggregate size class. This delayed priming effect was attributed to increased exposure of native organic C by enzyme activity in the amended soils and indicated that the amount of semi-protected mineralisable organic C increased with increasing macro-aggregate size. Despite the differences in water-extractable carbohydrate C, 14C microbial biomass and 14C microbial products between amendment treatments, generally all were poorly correlated with MWD in each treatment. This indicated that gross changes in the amounts of freshly deposited organic C did not reflect the labile organic C fractions involved in the stabilisation of aggregates in this sandy soil.

Aggregate‐Protected and Unprotected Organic Matter Pools in Conventional‐ and No‐Tillage Soils

AbstractNo‐tillage (NT) practices can result in greater soil aggregation and higher soil organic matter (SOM) levels than conventional‐tillage (CT) practices, but the mechanisms for these effects are poorly known. Our objectives were to describe the size and quality of biologically active pools of aggregate‐associated SOM in long‐term CT and NT soils of the southeastern USA. Samples were collected from replicated CT and NT plots on a Hiwassee sandy clay loam (clayey, kaolinitic, thermic Rhodic Kanhapludult) and separated into four aggregate size classes (&gt;2000, 250–2000, 106–250, 53–106 µm) by wet sieving. Potentially mineralizable C and N and N2O emissions were measured from 20‐d laboratory incuhations of intact and crushed macroaggregates (&gt;250 µm) and intact microaggregates (&lt;250 µm). Three primary pools of aggregate‐associated SOM were quantified: unprotected, protected, and resistant C and N. Aggregate‐unprotected pools of SOM were 21 to 65% higher in surface soils of NT than of CT, with greater differences in the macroaggregate size classes. Disruption of macroaggregates increased the mineralization of SOM in NT but had little effect in CT. Rates of mineralization from protected and unprotected pools of C were higher in surface soils of CT than of NT. Macroaggregate‐protected SOM accounted for 18.8 and 19.1% of the total mineralizable C and N (0–15 cm), respectively, in NT but only 10.2 and 5.4% of the total mineralizable C and N in CT. Our results indicate that macroaggregates in NT soils provide an important mechanism for the protection of SOM that may otherwise be mineralized under CT practices.

Carbon and Nitrogen Distribution in Aggregates from Cultivated and Native Grassland Soils

AbstractLong‐term cultivation of grassland soils reduces soil organic C and N content and has been associated with a deterioration in the aggregate structure of the soil. This study examined the effects of bare fallow (moldboard plow), stubble mulch fallow (subtill), and no‐till fallow management on aggregate size distribution and aggregate organic C and N contents compared with a native (virgin) grassland soil. Aggregate size fractions were separated by wet sieving and the proportion of soil was quantified for each aggregate size class. Mineral‐associated (silt and clay) organic matter was isolated by dispersing aggregates in sodium hexametaphosphate and removing tbe sand and particulate organic matter (POM) by passing the dispersed aggregates through a 53‐µm sieve. The POM fraction is composed primarily of partially decomposed root fragments and has an average C/N ratio of about 16. A large proportion of the total soil dry weight (50–60%) was isolated in the small macroaggregate (250–2000 µm) size class. The native grassland soil was more stable than the cultivated soils when slaked, and the no‐till soil was more stable than the stubble mulch and bare fallow soil when slaked. Reduced tillage management is effective at increasing the proportion of macroaggregates and results in the accumulation of wheat (Triticum aestivum L.) derived POM within the aggregate structure compared with bare fallow soil. It has previously been shown that the POM fraction accounts for the majority of the soil organic matter (SOM) initially lost as a result of cultivation of grassland soils. The data reported in this study relates the loss of structural stability from cultivation to losses of organic C and N from the POM fraction.

EFFECTS OF CROPPING ON MACRO-AGGREGATION OF A MARINE CLAY SOIL

Field and growth chamber experiments were conducted to evaluate the short-term effects of selected crops on macro-aggregation of a Kamouraska clay soil. Under field conditions, the growth of barley (Hordeum vulgare L.) and alfalfa (Medicago sativa L.) for up to 2 yr resulted in increased macro-aggregate size and stability compared to a fallow control and to initial conditions. Under these two crops, the proportion of water-stable aggregates of the 2- to 6-mm fraction increased from 25% in May 1986 to 40% in September 1987 at the expense of the 0.25- to 1.0-mm fraction which decreased from 37% to 19% over the same period. Macro-aggregation after 2 yr was not different in corn (Zea mays L.) and fallow control. Seasonal variations in aggregate stability were significant but small and less important than the effects of cropping treatments. Results of the growth chamber experiment agreed with those obtained under field conditions suggesting that controlled environment experiments can be used to model cropping effects on the aggregation of this soil. Key words: Aggregation, soil structure, clay soil, corn, barley, alfalfa

Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation

Cultivation is known to influence the organic matter status and structural stability of soil. We investigated the effects of 69 yr of cultivation on the nature, distribution and activity of microbial biomass (MB) in different aggregate size classes of an Orthic Brown Chernozemic soil. Cultivation decreased MB content, its activity and enzyme activity in soil. Microaggregate (<0.25mm) size classes in both native and cultivated soils contained lower organic-C, MB-C, fungal biomass, arylsulfatase, acid phosphatase and respiratory activities as compared to macroaggregates. However, the negative effects of cultivation were more pronounced on macroaggregate size classes. Nutrient ratios of both whole aggregates and microbial biomass were narrower in aggregates from cultivated soil as compared to native soil. In both native and cultivated soils, mineralization of C. N and S was greater in macroaggregates as compared to that in microaggregates. The greatest effect of cultivation on nutrient and microbial characteristics was observed in the 0.25 to 1.00 mm dia size classes. These results suggest that microbial biomass, especially fungal biomass, plays an important role in the formation of macroaggregates and is the labile organic matter that serves as the primary source of C and nutrients released following cultivation.

Aggregate Structure and Carbon, Nitrogen, and Phosphorus in Native and Cultivated Soils

AbstractThis work corroborates the hierarchical conceptual model for soil aggregate structure presented by Tisdall and Oades (1982), extends it to North American grassland soils, and elaborates on the aspects relating to the influence of cultivation upon losses of soil organic matter. Aggregate size distributions observed for our soils are very similar to those of Australian soils, indicating that the microaggregate‐macroaggregate model may hold for a wide array of grassland soils from around the world. The use of two wetting treatments prior to sieving demonstrated that the native sod soil had the same general structural characteristics as cultivated soil but the macroaggregates were more stable. When slaked, native and cultivated soil present in the microaggregate size classes had less organic C, N, and P than the soil remaining as macroaggregates, even when expressed on a sand‐free basis. Moreover, the C/N, C/P, and N/P ratios of microaggregates were narrower than those of macroaggregate size classes. Much more C and N was lost than P under the conditions of this study. Element‐specific differences in microbial catabolic processes vs. extracellular enzyme activity and its attendant feedback controls are postulated to account for this difference. When the macroaggregates were crushed to the size of microaggregates, mineralizable C as a percent of the total organic C was generally greater for macro‐ than microaggregates early in the incubation for the cultivated soil and throughout most of the incubation for the native sod soil. Mineralizable N as a percent of the total organic N was greatest in macroaggregates even when the macroaggregates were not crushed. The macroaggregate‐micraggregate conceptual model is applied to help explain accumulation of soil organic matter under native conditions and its loss upon cultivation.

Influence of Soybean and Corn Cropping on Soil Aggregate Size and Stability

AbstractHigher rates of soil loss have been observed for soybean [Glycine max (L.) Merr.] cropping than for corn (Zea mays L.) cropping. The objective of this study was to determine whether 4 yr of continuous soybean and continuous corn cropping had altered the size and stability of soil aggregates within the tillage zone, which could affect soil seal formation and erodibility. Samples for analyses were obtained in June and October of 1980 from the Monona (fine‐silty, mixed, mesic, Typic Hapludolls) and Clarion (fine‐loamy, mixed, mesic, Typic Hapludolls) soils in Iowa. The mean‐weight diameter of dry‐sieved aggregates was significantly (p &lt;0.05) lower for soybeans than corn in October, but the values were similar in June. The mean‐weight diameter of wet‐sieved aggregates was lower for soybeans than corn in both June and October, but the differences were not statistically significant. The mass of clay released from the bulk soil and two macroaggregate size fractions with laboratory shaking was slightly, but significantly (p &lt;0.10), higher for corn than soybeans.