Kyle Wickings

The origin of litter chemical complexity during decomposition

The chemical complexity of decomposing plant litter is a central feature shaping the terrestrial carbon (C) cycle, but explanations of the origin of this complexity remain contentious. Here, we ask: How does litter chemistry change during decomposition, and what roles do decomposers play in these changes? During a long-term (730 days) litter decomposition experiment, we tracked concurrent changes in decomposer community structure and function and litter chemistry using high-resolution molecular techniques. Contrary to the current paradigm, we found that the chemistry of different litter types diverged, rather than converged, during decomposition due to the activities of decomposers. Furthermore, the same litter type exposed to different decomposer communities exhibited striking differences in chemistry, even after > 90% mass loss. Our results show that during decomposition, decomposer community characteristics regulate changes in litter chemistry, which could influence the functionality of litter-derived soil organic matter (SOM) and the turnover and stabilisation of soil C.

The Effects of Soil Bacterial Community Structure on Decomposition in a Tropical Rain Forest

Soil microorganisms are key drivers of terrestrial biogeochemical cycles, yet it is still unclear how variations in soil microbial community composition influence many ecosystem processes. We investigated how shifts in bacterial community composition and diversity resulting from differences in carbon (C) availability affect organic matter decomposition by conducting an in situ litter manipulation experiment in a tropical rain forest in Costa Rica. We used bar-coded pyrosequencing to characterize soil bacterial community composition in litter manipulation plots and performed a series of laboratory incubations to test the potential functional significance of community shifts on organic matter decomposition. Despite clear effects of the litter manipulation on soil bacterial community composition, the treatments had mixed effects on microbial community function. Distinct communities varied in their ability to decompose a wide range of C compounds, and functional differences were related to both the relative abundance of the two most abundant bacterial sub-phyla (Acidobacteria and Alphaproteobacteria) and to variations in bacterial alpha-diversity. However, distinct communities did not differ in their ability to decompose native dissolved organic matter (DOM) substrates that varied in quality and quantity. Our results show that although resource-driven shifts in soil bacterial community composition have the potential to influence decomposition of specific C substrates, those differences may not translate to differences in DOM decomposition rates in situ. Taken together, our results suggest that soil bacterial communities may be either functionally dissimilar or equivalent during decomposition depending on the nature of the organic matter being decomposed.

A Source of Terrestrial Organic Carbon to Investigate the Browning of Aquatic Ecosystems

There is growing evidence that terrestrial ecosystems are exporting more dissolved organic carbon (DOC) to aquatic ecosystems than they did just a few decades ago. This “browning” phenomenon will alter the chemistry, physics, and biology of inland water bodies in complex and difficult-to-predict ways. Experiments provide an opportunity to elucidate how browning will affect the stability and functioning of aquatic ecosystems. However, it is challenging to obtain sources of DOC that can be used for manipulations at ecologically relevant scales. In this study, we evaluated a commercially available source of humic substances (“Super Hume”) as an analog for natural sources of terrestrial DOC. Based on chemical characterizations, comparative surveys, and whole-ecosystem manipulations, we found that the physical and chemical properties of Super Hume are similar to those of natural DOC in aquatic and terrestrial ecosystems. For example, Super Hume attenuated solar radiation in ways that will not only influence the physiology of aquatic taxa but also the metabolism of entire ecosystems. Based on its chemical properties (high lignin content, high quinone content, and low C:N and C:P ratios), Super Hume is a fairly recalcitrant, low-quality resource for aquatic consumers. Nevertheless, we demonstrate that Super Hume can subsidize aquatic food webs through 1) the uptake of dissolved organic constituents by microorganisms, and 2) the consumption of particulate fractions by larger organisms (i.e., Daphnia). After discussing some of the caveats of Super Hume, we conclude that commercial sources of humic substances can be used to help address pressing ecological questions concerning the increased export of terrestrial DOC to aquatic ecosystems.

Cover crop root contributions to soil carbon in a no-till corn bioenergy cropping system

Crop residues are potential biofuel feedstocks, but residue removal may reduce soil carbon (C). The inclusion of a cover crop in a corn bioenergy system could provide additional biomass, mitigating the negative effects of residue removal by adding to stable soil C pools. In a no‐till continuous corn bioenergy system in the northern US Corn Belt, we used 13CO2 pulse labeling to trace plant C from a winter rye (Secale cereale) cover crop into different soil C pools for 2 years following rye cover crop termination. Corn stover left as residue (30% of total stover) contributed 66, corn roots 57, rye shoots 61, rye roots 50, and rye rhizodeposits 25 g C m−2 to soil. Five months following cover crop termination, belowground cover crop inputs were three times more likely to remain in soil C pools than were aboveground inputs, and much of the root‐derived C was in mineral‐associated soil fractions. After 2 years, both above‐ and belowground inputs had declined substantially, indicating that the majority of both root and shoot inputs are eventually mineralized. Our results underscore the importance of cover crop roots vs. shoots and the importance of cover crop rhizodeposition (33% of total belowground cover crop C inputs) as a source of soil C. However, the eventual loss of most cover crop C from these soils indicates that cover crops will likely need to be included every year in rotations to accumulate soil C.

Temporal and Spatial Impact of Human Cadaver Decomposition on Soil Bacterial and Arthropod Community Structure and Function

As vertebrate carrion decomposes, there is a release of nutrient-rich fluids into the underlying soil, which can impact associated biological community structure and function. How these changes alter soil biogeochemical cycles is relatively unknown and may prove useful in the identification of carrion decomposition islands that have long lasting, focal ecological effects. This study investigated the spatial (0, 1, and 5 m) and temporal (3–732 days) dynamics of human cadaver decomposition on soil bacterial and arthropod community structure and microbial function. We observed strong evidence of a predictable response to cadaver decomposition that varies over space for soil bacterial and arthropod community structure, carbon (C) mineralization and microbial substrate utilization patterns. In the presence of a cadaver (i.e., 0 m samples), the relative abundance of Bacteroidetes and Firmicutes was greater, while the relative abundance of Acidobacteria, Chloroflexi, Gemmatimonadetes, and Verrucomicrobia was lower when compared to samples at 1 and 5 m. Micro-arthropods were more abundant (15 to 17-fold) in soils collected at 0 m compared to either 1 or 5 m, but overall, micro-arthropod community composition was unrelated to either bacterial community composition or function. Bacterial community structure and microbial function also exhibited temporal relationships, whereas arthropod community structure did not. Cumulative precipitation was more effective in predicting temporal variations in bacterial abundance and microbial activity than accumulated degree days. In the presence of the cadaver (i.e., 0 m samples), the relative abundance of Actinobacteria increased significantly with cumulative precipitation. Furthermore, soil bacterial communities and C mineralization were sensitive to the introduction of human cadavers as they diverged from baseline levels and did not recover completely in approximately 2 years. These data are valuable for understanding ecosystem function surrounding carrion decomposition islands and can be applicable to environmental bio-monitoring and forensic sciences.

Soil microarthropod communities reduce Heterorhabditis bacteriophora (Nematoda: Heterorhabditidae) host infection: Microarthropods and entomopathogenic nematodes

Biological control agents applied to soil, such as entomopathogenic nematodes (EPNs), are exposed to a wide variety of organisms other than the pest that they are utilized against. Interactions with these other organisms may positively or negatively affect EPN performance and may be important for managers to take into account when using EPNs to control pests. The present study assessed the effect of soil microarthropods on the EPN Heterorhabditis bacteriophora with respect to establishment within and infection of wax moth Galleria mellonella larvae in soil arenas. The presence of soil microarthropods at typical field densities significantly reduced the establishment of H. bacteriophora adults within G. mellonella larvae and also reduced the percentage of hosts infected in one of two replicated trials. Specifically, adult EPN establishment was negatively correlated with the abundance of mesostigmatid mites. The results of the present study indicate that soil microarthropods at natural abundances can reduce EPN establishment even within the structurally complex environment of soil, although this does not always lead to reductions in insect infection rates.

Impacts of alpine wetland degradation on the composition, diversity and trophic structure of soil nematodes on the Qinghai-Tibetan Plateau

Alpine wetlands on the Qinghai-Tibetan Plateau are undergoing degradation. However, little is known regarding the response of soil nematodes to this degradation. We conducted investigations in a wet meadow (WM), a grassland meadow (GM), a moderately degraded meadow (MDM) and a severely degraded meadow (SDM) from April to October 2011. The nematode community taxonomic composition was similar in the WM, GM and MDM and differed from that in the SDM. The abundance declined significantly from the WM to the SDM. The taxonomic richness and Shannon index were comparable between the WM and MDM but were significantly lower in the SDM, and the Pielou evenness showed the opposite pattern. The composition, abundance and diversity in the WM and SDM were relatively stable over time compared with other habitats. The abundances of all trophic groups, aside from predators, decreased with degradation. The relative abundances of herbivores, bacterivores, predators and fungivores were stable, while those of omnivores and algivores responded negatively to degradation. Changes in the nematode community were mainly driven by plant species richness and soil available N. Our results demonstrate that alpine wetland degradation significantly affects the soil nematode communities, suppressing but not shifting the main energy pathways through the soil nematode communities.

Author Correction: Impacts of alpine wetland degradation on the composition, diversity and trophic structure of soil nematodes on the Qinghai-Tibetan Plateau

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

Inoculative release of Heterorhabditis bacteriophora Poinar (Oswego) and Steinernema feltiae Filipjev (NY04) mixture can enhance biological control of soil-dwelling pests in turfgrass production systems

ABSTRACT In this study, the utility of inoculative releases of local strains of Heterorhabditis bacteriophora and Steinernema feltiae isolated from northern New York (NY) State for managing soil-dwelling pests in sod (turfgrass production systems) was evaluated. The objectives of the project were to (a) quantify changes in biocontrol potential during the sod production process and (b) evaluate the potential for the nematodes to suppress populations of soil-dwelling turf pests following harvest and installation at a new site. A field study was conducted on four sod fields in central NY State on soils with varying texture and organic matter content, and a post-harvest sod installation study was performed in the greenhouse on a sandy loam soil. Nematode-induced mortality of Galleria mellonella was increased significantly one month following inoculation but differed between fields with different soil types, likely due to variation in soil organic matter content. In the following year, nematode persistence in sod fields was low, likely driven by moderate to severe drought conditions. In contrast, upon harvest and installation, significant nematode-induced mortality was observed in soil receiving an installation of muck-produced sod. Enhanced nematode-induced mortality was detected in both the greater wax moth and third instar Japanese beetle larvae. This study indicates that inoculation of sod with entomopathogenic nematodes can improve biocontrol potential during sod production and after harvest, however, the utility of this practice is sensitive to soil conditions, and likely most efficacious on coarse-textured soils with high organic matter.