Paul F. Hendrix

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.

Detritus Food Webs in Conventional and No-tillage Agroecosystems

onservation tillage-crop planting systems that leave 30% or more of crop residues on the soil surface instead of plowing them under-is becoming widely adopted in US agriculture. The total area under conservation tillage is estimated to be between 24 and 36 million hectares, or about one-third of the nations cropland, which represents an increase of about 1,25% during the past decade (Christensen and Magleby 1983). The primary reasons for this increase are that conservation

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.

Invertebrates as webmasters in ecosystems

Webmaster functions in ecosystems webmasters in feedback interactions and food webs webmasters and ecosystems diversity webmasters in regional and global contexts. (Part contents).

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.

Abundance and distribution of earthworms in relation to landscape factors on the Georgia Piedmont, U.S.A.

Abundance and distribution of earthworms were studied on the Georgia Piedmont of the Southeastern U.S.A., at sites representing various ecosystem types, management practices, landscape positions, soil textures and soil erosion status. Earthworm abundance showed distinct seasonal patterns, with winter/spring maxima and summer minima. Numbers and biomass ranged from zero in plowed. mono-cropped soil at an upland site to over 1000 m-2 ( > 25g ash-free dry wt m-2) in no-tillage, double-cropped soil on bottomlands. Numbers and biomass in plowed, double-cropped soil, in a bottomland forest, and in grass meadows at both upland and bottomland sites were intermediate. Soil texture, as influenced by water erosion, strongly affected earthworm abundance. Moderately and severely eroded sandy clay loam supported significantly higher earthworm numbers and biomass than did slightly eroded soil with higher sand content. This effect may have resulted from low organic content and water holding capacity of the sandy soils. Of the soil texture variables, silt content was most highly correlated with earthworm biomass. Earthworm abundance was also related to quantity and quality of plant residue inputs in the agroecosystems, and to standing stocks of soil organic carbon across all sites studied. At most agroecosystem and forest sites, the predominant earthworm species were European lumbricids; native Diplocardia spp were most prominent in a meadow soil with high organic content.

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.

Soil respiration in conventional and no-tillage agroecosystems under different winter cover crop rotations

Carbon-dioxide efflux from conventional (CT) and no-tillage (NT) soils under winter ryegrain sorghum and crimson clover-grain sorghum rotations was measured for two consecutive cropping seasons using a static-absorption technique. Overall, CO2 output was significantly higher from NT than from CT soils, and from soils cropped to clover than from those cropped to rye. During the cool season, soil respiration was similar in NT and CT soils, and in an adjacent forest and old field. Carbon dioxide production ranged from 5 to 50 g CO2 m−2 day−1 over the 17 months of observation. Pulses of CO2 production were observed, following mowing of the winter crops, in both CT and NT. Plowing did not stimulate CO2 production in CT as was expected, but annual CO2 production in these systems may have been underestimated. Tillage appeared to affect the timing rather than the total amount of CO2 production. Linear regressions showed strong relationships between temperature and respiration in both CT (r = 0.88) and NT (r = 0.80) soils but not between soil moisture and respiration. However, soil moisture was significantly related to the contribution of surface residue to total respiration in NT (r = 0.80), suggesting that wetting and drying cycles may be more important to the decomposition of residues in NT than in CT.

Identification of uncultured bacteria tightly associated with the intestine of the earthworm Lumbricus rubellus (Lumbricidae; Oligochaeta)

Abstract The bacteria associated with the intestine and casts of the earthworm Lumbricus rubellus Hoffmeister, 1843, were examined by direct counts, culturability studies, 16S rRNA gene clone libraries, and fluorescent in situ hybridization (FISH). A significant fraction, 24–47%, of the total numbers of prokaryotes remaining in the intestine after casting were tightly associated with the intestinal wall. Bacterial 16S rRNA gene clone libraries constructed from washed earthworm intestinal tissue suggested that the bacterial community was dominated by a few phylotypes that were either absent from, or in low abundance, in the casts. The specific phylotypes present depended on the date of sampling and included representatives of the Acidobacteria, Firmicutes, β-Proteobacteria, and one phylogenetically deep, unclassified group. Juvenile earthworms subsequently collected contained three of the four phylotypes observed in the intestine clone libraries. The Firmicutes phylotype was examined by FISH and was found to be a short rod that represented only a small fraction of the total population of the juvenile samples. These results suggested that the microbial community tightly associated with the intestine was dominated by a small number of phylotypes and that this association was opportunistic rather than obligate.

Responses of trophic groups of soil nematodes to residue application under conventional tillage and no-till regimes

A laboratory and a field study were conducted to monitor the increase in numbers and 14C uptake of different trophic groups of soil nematodes in response to residue addition and to examine the relative importance of bacterivorous and fungivorous nematodes in conventional (CT) and no-till (NT) agroecosystems. In general, soil nematode numbers increased more rapidly in response to residue addition and became much more abundant (greater than five-fold) under laboratory conditions than in the field. Our results showed that bacterivorous nematodes responded to residue addition earlier than fungivorous nematodes under both CT and NT regimes in the laboratory and field studies. A depth effect was observed in NT, but not in the CT treatment; this reflected the vertical residue distribution in both tillage regimes. Soil nematodes were more abundant under NT than under CT in the field. The same pattern was observed at the beginning of the laboratory study but it reversed later. The ratios of fungivorous-to-bacterivorous nematodes (FN-to-BN) were not significantly different between CT and NT treatments at the beginning of the experiment. They were very low (less than 0.2) in both tillage regimes, indicating that bacterivorous nematodes were relatively more important than fungivorous nematodes in both tillage agroecosystems. However, the FN-to-BN ratios increased with time after residue decomposition started, particularly in the CT treatment. This suggested that the relative importance of fungivorous nematodes increased with the progress of residue decomposition. It was more pronounced in the CT treatment during the short period after residue application. In both the laboratory and field studies, the 14C specific activity of soil nematodes and the ratio of 14C bound in nematode biomass to total 14C decayed in the experiment (reported elsewhere) were significantly higher under CT than under NT, suggesting that soil nematodes use carbon more efficiently under CT than under NT. No significant difference of 14C specific activity of soil nematodes was found between the two depths under CT in both the studies; however, 14C specific activity was significantly higher in the 0–2.5 cm than in the 2.5–5.0 cm layer under NT in the laboratory study.