Missing links in the root-soil organic matter continuum.

The soil environment remains one of the most complex and poorly understood research frontiers in ecology. Soil organic matter (SOM), which spans a continuum from fresh detritus to highly processed, mineral-associated organic matter, is the foundation of sustainable terrestrial ecosystems. Heterogeneous SOM pools are fueled by inputs from living and dead plants, driven by the activity of micro- and mesofauna, and are shaped by a multitude of abiotic factors. The specialization required to measure unseen processes that occur on a wide range of spatial and temporal scales has led to the partitioning of soil ecology research across several disciplines. In the organized oral session 'Missing links in the root-soil organic matter continuum' at the annual Ecological Society of America meeting in Albuquerque, NM, USA, we joined the call for greater communication and collaboration among ecologists who work at the root-soil interface (e.g. Coleman, 2008). Our goal was to bridge the gap between scientific disciplines and to synthesize disconnected pieces of knowledge from root-centric and soil-centric studies into an integrated understanding of belowground ecosystem processes. We focused this report around three compelling themes that arose from the session: (1) the influence of the rhizosphere on SOM cycling, (2) the role ofmore » soil heterotrophs in driving the transformation of root detritus to SOM, and (3) the controlling influence of the soil environment on SOM dynamics. We conclude with a discussion of new approaches for gathering data to bridge gaps in the root-SOM continuum and to inform the next generation of ecosystem models. Although leaf litter has often been considered to be the main source of organic inputs to soil, Ann Russell synthesized a convincing body of work demonstrating that roots, rather than surface residues, control the accumulation of SOM in a variety of ecosystems. Living roots, which are chemically diverse and highly dynamic, also influence a wide range of soil processes, from the exudation of labile C compounds to the development of fungal associations. For example, Zoe Cardon demonstrated that the root-mediated redistribution of deep soil water to relatively dry shallower soil, increased soil CO{sub 2} efflux and nutrient cycling near the surface in an arid ecosystem. Andrew Kulmatiski also discussed the importance of rooting distribution throughout the soil profile for strategies of water uptake by different species in an African savanna. Later, Julie Jastrow demonstrated that living roots shape soil physical structure by promoting the formation of soil aggregates, which facilitated accrual of SOM in restored grasslands. Taken together, the evidence is compelling that living roots, and organic matter derived from root detritus, are important parts of the continuum of organic matter in the soil. Larger soil organisms (i.e. 50 {micro}m to many cm in body size) play an important role in the root-SOM continuum by grazing on roots and microbes, comminuting organic matter and aggregating soil in fecal pellets. However, litterbag and soil incubation studies necessarily exclude invertebrates, and research on faunal activity and trophic dynamics tends to be independent from research on the biogeochemistry of SOM cycling. Tim Filley used plant-derived biomarkers in invertebrate residues to bridge the gap between larger soil organisms, such as earthworms and beetle larvae, and SOM distribution. He found that larger soil organisms help to stabilize root-derived organic matter in soil aggregates. Similar coupling of biogeochemistry with food web studies could prove fruitful for describing mechanisms that underlie critical ecosystem processes. Despite considerable research efforts, the breadth of the microbial role in the root-SOM continuum remains unresolved. Using advanced pyrosequencing techniques, David Nelson demonstrated the importance of archea as nitrifiers in agricultural systems exposed to elevated [CO{sub 2}]. Rising atmospheric [CO{sub 2}] and other changing environmental factors add a layer of complexity to the quest to understand microbial process. For example, Claudia Boot demonstrated that microbially mediated C and N cycling in Mediterranean California grasslands is intricately linked with summer drought. Ongoing research across subdisciplines seeks to uncover the many complex links between soil organisms of all sizes and the root-SOM continuum. While the role that living organisms play in the transformation of root detritus to SOM is disproportionate to their body size, the nonliving soil environment also influences SOM cycling. However, destructive sampling can obscure feedbacks between abiotic and biotic processes, making it difficult to quantify the role of edaphic factors in the root-SOM continuum.« less