Michael H. Beare

MICROBIAL AND FAUNAL INTERACTIONS AND EFFECTS ON LITTER NITROGEN AND DECOMPOSITION IN AGROECOSYSTEMS

We conducted field experiments to test the general hypothesis that the com- position of decomposer communities and their trophic interactions can influence patterns of plant litter decomposition and nitrogen dynamics in ecosystems. Conventional (CT) and no-tillage (NT) agroecosystems were used to test this idea because of their structural sim- plicity and known differences in their functional properties. Biocides were applied to ex- perimentally exclude bacteria, saprophytic fungi, and microarthropods in field exclosures. Abundances of decomposer organisms (bacteria, fungi, protozoa, nematodes, microar- thropods), decomposition rates, and nitrogen fluxes were quantified in surface and buried litterbags (Secale cereale litter) placed in both NT and CT systems. Measurements of in situ soil respiration rates were made concurrently. The abundance and biomass of all microbial and faunal groups were greater on buried than surface litter. The mesofauna contributed more to the total heterotrophic C in buried litter from CT (6-22%) than in surface litter from NT (0.4-1/1%). Buried litter decay rates (1.4-1.7%/d) were -2.5 times faster than rates for surface litter (0.5-O.7%/d). Ratios of fungal to bacterial biomass and fungivore to bacterivore biomass on NT surface litter generally increased over the study period resulting in ratios that were 2.7 and 2.2 times greater, respectively, than those of CT buried litter by the end of the summer. The exclusion experiments showed that fungi had a somewhat greater influence on the decomposition of surface litter from NT while bacteria were more important in the de- composition of buried litter from CT. The fungicide and bactericide reduced decomposition rates of NT surface litter by 36 and 25% of controls, respectively, while in CT buried litter they were reduced by 21 and 35% of controls, respectively. Microarthropods were more important in mobilizing surface litter nitrogen by grazing on fungi than in contributing to litter mass loss. Where fungivorous microarthropods were experimentally excluded, there was less than a 5% reduction in mass loss from litter of both NT and CT, but fungi- fungivore interactions were important in regulating litter N dynamics in NT surface litter. As fungal densities increased following the exclusion of microarthropods on NT surface litter, there was 25% greater N retention as compared to the control after 56 d of decay. Saprophytic fungi were responsible for as much as 86% of the net N immobilized (1.81 g /m2) in surface litter by the end of the study when densities of fungivorous microarthropods were low. Although bacteria were important in regulating buried litter decomposition rates and the population dynamics of bacterivorous fauna, their influence on buried litter N dynamics remains less clear. The larger microbial biomass and greater contribution of a bacterivorous fauna on buried litter is consistent with the greater carbon losses and lower carbon assimilation in CT than NT agroecosystems. In summary, our results suggest that litter placement can strongly influence the com- position of decomposer communities and that the resulting trophic relationships are im- portant to determining the rates and timing of plant litter decomposition and N dynamics. Furthermore, cross placement studies suggest that the decomposer communities within each tillage system, while not discrete, are adapted to the native litter placements in each.

A Hierarchical approach to evaluating the significance of soil biodiversity to biogeochemical cycling

The significance of biodiversity to biogeochemical cycling is viewed most directly through the specific biogeochemical transformations that organisms perform. Although functional diversity in soils can be great, it is exceeded to a high degree by the richness of soil species. It is generally inferred from this richness that soil systems have a high level of functional redundancy. As such, indices of species richness probably contribute little to understanding the functioning of soil ecosystems. Another approach stresses the value of identifying “keystone” organisms, that is those that play an exceptionally important role in determining the structure and function of ecosystems. Both views tend to ignore the importance of biodiversity in maintaining the numerous and complex interactions among organisms in soils and their contributions to biogeochemical cycling. We describe some of those interactions and their importance to ecosystem function.

Decay Rates, Nitrogen Fluxes, and Decomposer Communiies of Single‐ and Mixed‐Species Foliar Litter

Decomposition rates, N fluxes, and abundances of decomposer organisms were quantified in mixed-species litterbags (containing leaves of two or three of the following tree species: Acer rubrum, Cornus florida, and Quercus prinus) and in litterbags containing leaves of a single species. Data from single-species litterbags were used to generate predicted decay rates, N fluxes, and abundances of decomposer organisms for mixed-species litter- bags, against which observed values could be compared to determine if significant inter- action effects occurred when litter of different species, and different resource quality, was mixed. Decay rates of-mixed-species litterbags during the 1-yr study were not significantly different than predicted from decay rates of individual component species. However, there were significant interaction effects on N fluxes and abundances of decomposer organisms. In the C. florida-A. rubrum and C. florida-A. rubrum-Q. prinus litter combinations there were significantly greater initial releases of N and lower subsequent N immobilization than predicted. In the A. rubrum-Q. prinus and C. florida-A. rubrum-Q. prinus litter combi- nations, lengths of fungal hyphae were significantly less than predicted on at least half the collection dates. Bacterial numbers in the mixed-litter combinations were also generally less than predicted. Nematode abundances, especially fungivores, were generally greater than predicted in mixed-species litterbags until the last sample date. Observed mean abun- dances of nematodes over all dates were 20-30% greater than predicted. Microarthropod abundances were more variable, but tended to be lower than predicted. Our results indicate that measurement of N flux in single-species litterbags may not reflect actual N flux in the field, where leaves of several tree species are mixed together. The differences in N flux between single- and mixed-species litterbags can affect ecosystem-level estimates of N release or accumulation in decomposing litter. For example, estimates of ecosystem-level N fluxes at our field site, based on data from single-species litterbags, resulted in a 64% underestimate of N released by day 75 and a 183% overestimate of N accumulated in the litter by day 375, relative to estimates based on data from mixed-species litterbags. We suggest that the deviation of observed N fluxes in mixed-species litterbags from those predicted using single-species litterbags are the result of differences in the decomposer community, such as lower microbial and microarthropod densities and higher nematode densities, resulting when litter of varied resource quality is mixed together. Longer term studies will be needed to determine if the differences between observed and predicted decomposer communities in mixed-species litter combinations influence the latter stages of decomposition where invertebrate-microbial interactions may have a greater effect on decay rates and nutrient release.

A substrate-induced respiration (SIR) method for measurement of fungal and bacterial biomass on plant residues

The substrate-induced respiration (SIR) method was modified and adapted to measure fungal, bacterial and total microbial contributions to glucose-induced respiration and the potentially active microbial biomass on decaying plant residues of differing composition. Decomposing residues from natural and agricultural ecosystems were chopped and sieved to include the > 1 mm fraction for routine SIR analyses on a continuous flow-through respiration system. SIR procedures were optimized for sample size (0.5–1.0g dry wt), glucose concentration (80mg g−1), antibiotic concentrations (16mg streptomycin g−1; 80 mg cycloheximide g−1), total solution volume (5 ml), antibiotic preincubation conditions (12 h at 4 C), and total assay time following glucose addition (2–3 h). Analyses of antibiotic selectivities for target populations were made from agar plate culture experiments with mixed residue-microbial populations under in vitro and in situ exposure to antibiotics. The results support those concentrations optimized by SIR and emphasize the importance of independent analysis of antibiotic selectivity. Measures of fungal, bacterial and total SIR (μg CO2-C g−1 dry residue h−1) were linearly correlated (P<0.001) with biovolume-derived estimates of total (r2 = 0.91) and FDA-active (r2 = 0.93) fungal biomass, bacterial biomass (r2 = 0.87) and total microbial biomass (r2 = 0.91), respectively. Fungal to bacterial SIR ratios (1.6:1 to 2.5:1) emphasize the dominance of fungi in the early stages of plant residue decay (⩽ 3 weeks). Plant residue SIR rates were 38–600 times greater than those of soils. Biomass specific SIR rates from plant residues (72 ng CO2 -Ch−1 μg−1 biomass-C) were 5– 6 times higher than those reported from soil SIR. These findings suggest both a larger microbial biomass as well as a much higher proportion of physiologically active microorganisms on plant residues as compared to soils.

Influences of mycelial fungi on soil aggregation and organic matter storage in conventional and no-tillage soils

Abstract Soil microbial community composition, aggregation and organic matter (SOM) content can be markedly influenced by tillage and crop management practices. This study was undertaken to determine to what extent differences in populations of mycelial fungi in conventional (CT) and no-tillage (NT) soils contribute to soil aggregation and soil organic matter storage. Fungicide (Captan) and control treatments were established in long-term CT and NT plots on a well-drained Hiwassee sandy clay loam soil (clayey kaolinitc thermic Rhodic Kanhapludult). The effects of these treatments on total and FDA-active fungal hyphal lengths, total bacteria (0–15 cm) and in situ soil respiration rates were measured at approximately monthly intervals. Soil carbohydrates and water-stable aggregate (WSA) distributions were quantified on the final sample date. Surface soil (0–5 cm) of NT had more macroaggregates (> 250 μm diam) and 1.30 to 1.46 times higher densities of fungal mycelia as compared to CT soils. The higher populations of fungal mycelia corresponded to a nearly two-fold higher concentration of acid-hydrolysable carbohydrates, which were composed of proportionally more microbial- than plant-derived sugars. Treatment with the fungicide resulted in a 40% reduction in > 2000 μm WSA and lower concentrations of carbohydrates in NT surface soils, but had no significant effects in CT soils. The contributions of fungi to aggregate stability may represent an important biotically-regulated mechanism for the protection of soil organic matter and may help to explain the greater retention of soil organic carbon in NT than in CT soils.

Relationships between fungal and bacterial substrate-induced respiration, biomass and plant residue decomposition

Abstract Residues of six plant species were incubated in the field and analyzed for decomposition rates, fungal, bacterial and total substrate-induced respiration (SIR), total fungal and bacterial biomass and changes in residue composition during 161 days. Plant residues included crimson clover ( Trifolium incarnatum L.), hairy vetch ( Vicia villosa Roth), crabgrass [ Digitaria sanguinalis , (L.) Scop.], winter rye ( Secale cereale L.), grain sorghum ( Sorghum bicolor L. Moench) and chestnut oak ( Quercus prinus L.) leaves. Plant residues were incubated in litterbags placed on the soil surface in no-tillage ( T. incarnatum, V. villosa, S. bicolor, S. cereale ), old-field ( D. sanguinalis ) or hardwood forest ( Q. prinus ) plots at the Horseshoe Bend Experimental Area in Athens, Ga and collected periodically for analyses. Decomposition rate constants ( k ) were greatest for V. villosa followed by T. incarnatum, D. sanguinalis, S. cereale, S. bicolor and Q. prinus . Net N loss generally followed the pattern of dry matter loss. Net N gain was observed after 100 days of decay for those residues with high initial C:N ratios. Initial N concentration was exponentially related with the annual decay rale constant ( r 2 = 0.93) for all species; however, on an individual species basis, lignin content was best correlated to dry matter weight loss. Total SIR was greatest on T. incarnatum and V. villosa followed by D. sanguinalis, S. bicolor, S. cereale and Q. prinus . Across all sample dates, residue carbon-to-nitrogen ratio was the best predictor of total SIR. Measurements of potential fungal and bacterial activity by SIR as well as biomass-C estimates by direct counts indicated that fungi were the dominant decomposers of these surface residues. For most residues, lignin content through time exerted the greatest influence on fungal SIR and fungal biomass-C ( r = −0.56 to −0.93). Total SIR, fungal SIR and total fungal biomass tended to decrease through time as residues decomposed. Total SIR, on any given sample date, was significantly correlated with residue dry weight remaining and annual decay rate constants were exponentially related to overall mean values of total SIR for all residues excluding S. cereale ( r 2 = 0.99). Residue SIR rates as a measure of the potentially active microbial biomass reflected the resource qualities of the plant residues investigated here and were positively correlated to their decomposition rates.

A comparison of methods for measuring water-stable aggregates: implications for determining environmental effects on soil structure

Abstract This paper describes the effects of different pretreatment conditions and wet-sieving procedures on water-stable aggregate distributions of sandy and clayey textured soils (Cecil, Pacolet, and Hiwassee Series) from the Piedmont of the southern Appalachian mountains of Georgia, USA. Four soil pretreatment procedures were compared: (1) air-dried, capillary wetted (AD-CW), (2) air-dried, tension wetted (AD-TW), (3) air-dried, slaked (AD-SL), and (4) field-moist, capillary wetted (FM-CW). Air-drying soils resulted in a greater quantity of aggregates in the coarser fractions (> 250 μm), as compared to field-moist soils, with a consequent reduction in the finer fractions (> 250 μm). Differences between methods of wetting air-dried soils were more pronounced for the clayey soils where both AD-CW and AD-SL resulted in a greater proportion (22–24%) of the soil mass in the finer fractions (> 250 μm), as compared with AD-TW (4.5%). The FM-CW procedure had the lowest coefficients of variation (2–8%) for repeated measurements. The AD-CW and FM-CW procedures were also used to compare the effects of cropping systems [conventional tillage (CT) (soybean/fallow), CT (sorghum/fallow), and no-tillage (NT) (sorghum/clover)], erosion classes (slight, moderate, and severe) and irrigation (drip-irrigated or nonirrigated) on water-stable aggregates (> 250 μm). In general, water-stable aggregates increased with decreasing intensity of cultivation, increasing severity of erosion and irrigation. Air-drying soils resulted in less differences in water-stable aggregates between treatments, but provided more detailed information on the interactive effects of cropping system and irrigation as compared with the field-moist condition. Similar differences were observed in aggregate disruption rates that were calculated from the aggregates recovered after wet-sieving for intervals of 1–32 min. Although water-stable aggregates were lower by FM-CW, this procedure showed greater separation of treatment means (e.g. erosion classes) than the AD-CW procedure. We also compared single versus multiple-sieve techniques for describing the effects of pretreatment conditions on aggregate distributions. For the clayey Pacolet soil, a fine fractionation into eleven aggregate size classes revealed the greatest differences (P > 0.05) between the FM-CW and AD-CW procedures, while a coarse fractionation into macro- (> 250 μm) and micro- (> 250 μm) aggregates showed no differences. However, for the sandy Hiwassee soil differences in aggregate distributions between the FM-CW and AD-CW procedures were found at most levels of fractionation, but were not detected by comparing the calculated mean weighted diameters. In general, our findings emphasize the value of comparing soil specific responses to different pretreatment conditions, particularly those that compare the distributions of aggregates among size classes, as a means for describing environmental influences on soil structure.

Earthworms and enchytraeids in conventional and no-tillage agroecosystems: A biocide approach to assess their role in organic matter breakdown

SummaryEarthworm and enchytraeid densities and biomass were sampled over an 18-month period in conventional and no-tillage agroecosystems. Overall, earthworm densities and biomass in the no-till system were 70% greater than under conventional tilling, and enchytraeid densities and biomass in the no-till system were 50%–60% greater. To assess the role of annelids in the breakdown of soil organic matter, carbofuran was applied to field enclosures and target (earthworm and enchytraeid biomass, standing stocks of organic matter) and non-target effects (bacteria, fungi, protozoa, nematode and microarthropod densities, litter decay rates, plant biomass) were determined in two 10-month studies. In the winter-fall study, carbofuran reduced the annelid biomass, and total soil organic matter standing stocks were 47% greater under no-till with carbofuran compared to control enclosures. Twelve percent of the difference could have been due to non-target effects of carbofuran, as determined from litterbag decay rates. In the summer-spring study, carbofuran again significantly reduced the annelid biomass, and treated pens in the no-till area had significantly greater standing stocks of fine organic matter (43%–45%). Although the densities of bacteria and nematodes were reduced in carbofuran-treated litterbags under a no-till system, the rates of decay were not reduced and estimates of the amount of organic matter processed could not be adjusted for non-target effects. A 76% difference in the standing stock of coarse organic matter between control and carbofuran-treated pens in the conventional-till system indicated further non-target effects. We concluded that our estimates of the amount of organic matter processed by annelids in no-till and conventionally tilled agroecosystems represented a maximum potential because of the confounding non-target effects of carbofuran.

Labile soil carbon pools in subtropical forest and agricultural ecosystems as influenced by management practices and vegetation types

Abstract Carbon storage in agricultural and forest soils has attracted attention recently due to its potential as a substantial carbon sink. Labile soil C pools are especially important because they are more vulnerable to climatic change and disturbance and play vital roles in nutrient cycling. Southern Appalachian forest soils and those from conventional tillage (CT), no-tillage (NT) and fescue sods at three sites in the Georgia piedmont were analyzed for total C, total N, carbohydrates, and microbial biomass C. The sizes of soil labile C pools (carbohydrates and microbial biomass) and their contributions to the total soil C pool differed significantly among ecosystems. The highest carbohydrate contents and microbial biomass C were found in forest soils, but agricultural soils had a significantly higher proportion of the soil organic matter present as carbohydrates and as microbial biomass. This difference probably reflects the quality of soil organic matter. Soil microbial biomass C was more sensitive to changes in management regimes than soil carbohydrates. Management practices signfiicantly affected organic C, carbohydrate contents, microbial biomass C and organic C turnover rates in agricultural soils, whereas differences in the quality of organic input due to different vegetation types substantially influenced soil labile C pools in forest soils. High mannose-to-xylose ratios in highly sandy agricultural soils indicate that plant-derived materials are rapidly metabolized by microorganisms and that organic C protection in sandy soils is largely dependent on reducing microbial access through effective residue management such as surface placement.

Characterization of a substrate-induced respiration method for measuring fungal, bacterial and total microbial biomass on plant residues

Abstract A substrate-induced respiration (SIR) method is described to measure the contributions of fungi and bacteria to total glucose-induced microbial respiration on plant residues of differing composition. Relationships between fungal, bacterial and total SIR and biomass were used to develop regression equations for predicting microbial biomass C from measures of SIR. Total SIR rates (100–2000 μg CO2-C g−1 h−1) and biomass-specific SIR rates (64–72 ng CO2-C h−1 μg−1 biomass C) from plant residues were considerably greater than those calculated from the literature for soils. Results of longer term decomposition studies indicate that the C:N ratios of plant residues through time account for the greatest amount of the variation in total SIR. Annual decomposition rate constants (k) for plant residues were positively correlated (r2=0.99) to overall mean estimates of total SIR. The plant residue SIR method has advantages over conventional direct count methods because it distinguishes a physiologically active component of the microbial biomass. Furthermore, it allows separation of fungal and bacterial components that may aid in understanding microbial controls on plant residue decomposition.