Soil organic matter (SOM) and associated litter represents the largest actively cycling pool of organic carbon (OC) and nitrogen (ON). Because soil acts as both a sink and a source for carbon, a detailed, mechanistic understanding of the controls on the conversion of litter OM to SOM, and its stability in soil is critical to accurately account for the changing balance between the atmospheric, terrestrial plant, and soil carbon reservoirs. In mid continent and northern North American forests there is an increasing awareness of the effect that detritivore macroinvertebrates, specifically earthworms (EW), have on litter decay dynamics and the associated nature of stabilized SOM. Well-documented effects of earthworm introduction into forests with few or no native EW include the depletion of organic horizons, forest floor litter, loss of soluble nutrients, and mixing of mineral and organic horizons. Surprisingly, however, earthworm activity, with its feedbacks to enzymatic activity, microbial community structure, and plant biopolymer alteration, is generally not one of the considerations applied to influences on SOM stabilization. This proposal seeks to document and quantify how these protective mechanisms interact in natural and experimental systems impacted by different degrees of EW activity. Our focus is primarily on identifying how differences in invasive EW activity and feeding habit interact with differences in litter chemical composition, mineralogy, and microbial enzyme activity among locations in the same temperate forest to alter the relative importance of physical, chemical, and biochemical protection mechanisms controlling SOM stabilization. This interdisciplinary, collaborative proposal presents a series of hypotheses and experiments to test fundamental components of the EW-Litter-SOM dynamics as summarized in the following four questions: 1). Do EW promote a different decay path for litter, reflected in its biopolymer, elemental, and isotopic composition, that might impact its biochemical recalcitrance and thus SOM stabilization? 2). How will the degree of stabilization and the biopolymer character of plant/microbial OM incorporated in EW casts be influenced by EW feeding habit, initial litter chemistry, and the microbial and mineral composition of soil? 3) Given that earthworms are known to impact soil structure and carbon allocation (i.e. litter translocation and incorporation of their casts with associated stable microstructures into soil) how will the known gradient in earthworm activity and litter chemistry across our field sites impact the specific source, chemistry, and amount of SOM that is biochemically protected (i.e. refractory biopolymers) and/or physically and chemically protected (i.e. aggregated and mineral associated)? 4) How will forest ecosystems at different levels of earthworm activity differ in the rate at which its soil organic carbon (SOC) moves through the terrestrial soil carbon reservoir? This work will employ detailed molecular, isotopic, mineralogical, ecological and microbiological methods to develop a mechanistic understanding of the processes that control soil organic matter storage in a system with a intense gradient in EW activity. To accomplish these tasks we have assembled an interdisciplinary team that includes a molecular and stable isotope biogeochemist, a soil molecular ecologist, a soil mineral-chemist, and an earthworm ecologist whose combined expertise is well suited to investigate the impacts of this ecological change on the stability of SOM. The intellectual merit of this proposal rests on the importance of these factors for soil carbon cycling, the lack of existing knowledge about these questions, the multidisciplinary perspective we apply, and the qualifications of the PIs. Additionally, this work has significant potential to benefit the SOM modeling community, which struggles for physically meaningful analyses that permit modeling of SOM dynamics. The EW-litter-soil system is particularly relevant today as most identified EW species in this region?s forests are non-native, and it is anticipated that over the next few decades they will expand farther into northern forests driven by rising surface temperatures, and local factors, e.g. soil transport, discarded fishing bait, and land use change. Knowledge gained from this study will contribute to the general understanding of major drivers in carbon cycling in Eastern deciduous forests of North America. The broader impacts of this work include the enhanced understanding of the role earthworms in driving change in north American forest soil processes and biogeochemical cycles that will be of great significance to both scientists and policy-makers. Additionally, the project will educate two Ph.D. students and numerous undergraduates, with great potential to attract underrepresented students through Purdue?s NSF-Funded AGEP and Native American programs, SERC?s REU program, and Johns Hopkins? Provost Undergraduate Research Award and will provide information for a high school teaching module.
Land use history significantly affects species composition and function of many ecosystems. This is clearly the case in the Mid-Atlantic region, where agricultural practices and later abandonment of agriculture had a detectable footprint on forest soils. Additionally, non-native earthworms have been influencing soil physical, chemical and biological characteristics. In this project we examined the combined effects of past land use, forest age and earthworm abundance and species composition on leaf litter decomposition, microbial activity and soil carbon dynamics. We used a combination of field observations and samplings, manipulative experiments and laboratory mesocosms to tease out the relative importance of these factors to forest succession. Our field site was the Smithsonian Environmental Research Center, Edgewater, MD, which encompasses one of the oldest farms in North America. Upland forests at this site consist of a mosaic of stands of different ages. To determine the chemical nature of soil and litter in these forest systems required a combination of approaches including soil physical fractionations of particles of different size and density, analysis of the stable isotopes of N and C of the soils, extraction of plant biopolymers in the soil and litter including lignin, cutin, suberin fatty acids, soil microbial enzyme and DNA extraction, structural mass spectrometry, Fourier transform infrared analysis, and thermogravimetric analysis. The combination of these tools with a partial least squares (PLS) method of treated the data allowed for the development of predictive models of soil C content for the sites. Overall, past agriculture practice has greatly influenced carbon and nitrogen content and physical distribution, but its extended legacy in the soil is most likely the result of the continuous invasive earthworm activity during afforestation. Our study suggests that the presence of abundant invasive earthworms at the Smithsonian Environmental Research Center may be dramatically altering the nature of forest floor and organic horizon recovery, transporting annual input of C and N from forest floor to deeper mineral soil, and altering soil aggregate structure. It is very likely that in the presence of earthworms, the recovery of forest soil organic matter from abandoned farmland will lead to a distinct chemical and physical recovery pathway, different from that of disturbed forests with no earthworm activity. The strong influence of earthworms on soil processes is likely to remain since the earthworm community is well established, and its biomass appears to be increasing in some forest stands. In our initial hypotheses we expected the activity of the native and invasive worm communities to be the primary control on the chemical and physical trajectory of the forest. The strong interaction between land use legacy, even after 140 year since agricultural abandonment, and earthworm occupation on the state of the soil was not anticipated. Moreover, new invasive species were detected in our study sites indication that this process is dynamic, and is likely to change over time especially a result of current land use change (suburban development). We also found that earthworm type, either surface litter or soil organic matter consuming, had an important control on the movement of leaf litter into the soil and its subsequent association with mineral particles. Our 5-year litter addition experiment demonstrated that young successional forests with high percentages of soil dwelling earthworms had the greater capacity to mix fresh leaf and wood litter into soil physical structures. This interdisciplinary project required close collaboration of a geochemist, a plant ecologist, a soil scientist and a soil ecologist. The grant supported three graduate students; two PhD Dissertations will be completed by June 2013. A total of fifteen undergraduates worked on several aspects of the project. Five papers were published, two are currently under review, and another six are in preparation. A supplement to this grant allowed 3 undergraduate Native American students from the Red Lake Nation to engage in a comparative study of the multifaceted scientific and policy issues related to invasive earthworm impacts on their tribal lands. They spent the summer of 2010 at Purdue University working on soils and litter from SERC and Red Lake forests. They presented their work at the 2010 Annual Meeting of the American Geophysical Union. Papers from this work. Crow, S. E., Filley, T.R., McCormick, M.M., Szlavecz, K., Stott, D.E. Gamblin, D. Conyers, C. (2009) Biogeochemistry, DOI: 10.1007/s10533-008-9260-1. Filley, T.R., McCormick, M.K., Crow, S.E., Szlavecz, K.E. Whigham, D.F., Johnston, C.T., van den Heuvel, R. (2008). Journal of Geophysical Research:Biogeosciences 113, G01027, DOI:10.1029/2007JG000542. Klotzbücher, T., Filley, T.R., Kaiser, K., Kalbitz, K. (2011) . Organic Geochemistry, 42, 1271-1278. DOI:10.1016/j.orggeochem.2011.07.007. Ma, Y, Filley, T.R., Crow, S.E., Johnston, C.T., McCormick, M., Szlavecz, K. (In review). Organic Geochemistry. McCormick, M.K., Parker, K.L., Szlavecz, K., Whigham, D.F. (In Review) AOB Plants. Szlavecz, K., M. McCormick, L. Xia, J. Saunders, T. Morcol, D. Whigham, T. Filley. (2011). Biological Invasions. 15:1165-1182. doi: 10.1007/s10530-011-9959-0. Zicsi A, Szlavecz K, Csuzdi Cs. 2011. Pedobiologia doi:10.1016/j.pedobi.2011.09.004
