Decomposition is a critical source of plant nutrients, and drives the largest flux of terrestrial C to the atmosphere. Decomposing soil organic matter typically contains litter from multiple plant species, yet we lack a mechanistic understanding of how species diversity influences decomposition processes. Here, we show that soil C and N cycling during decomposition are controlled by the composition and diversity of chemical compounds within plant litter mixtures, rather than by simple metrics of plant species diversity. We amended native soils with litter mixtures containing up to 4 alpine plant species, and we used 9 litter chemical traits to evaluate the chemical composition (i.e., the identity and quantity of compounds) and chemical diversity of the litter mixtures. The chemical composition of the litter mixtures was the strongest predictor of soil respiration, net N mineralization, and microbial biomass N. Soil respiration and net N mineralization rates were also significantly correlated with the chemical diversity of the litter mixtures. In contrast, soil C and N cycling rates were poorly correlated with plant species richness, and there was no relationship between species richness and the chemical diversity of the litter mixtures. These results indicate that the composition and diversity of chemical compounds in litter are potentially important functional traits affecting decomposition, and simple metrics like plant species richness may fail to capture variation in these traits. Litter chemical traits therefore provide a mechanistic link between organisms, species diversity, and key components of belowground ecosystem function.decomposition ͉ species richness ͉ chemical diversity ͉ litter mixtures

Abstract. Rapid changes in climate and land use and the resulting shifts in species distributions and ecosystem functions have motivated the development of the National Ecological Observatory Network (NEON). Integrating across spatial scales from ground sampling to remote sensing, NEON will provide data for users to address ecological responses to changes in climate, land use, and species invasion across the United States for at least 30 years. Although NEON remote sensing and tower sensor elements are relatively well known, the biological measurements are not. This manuscript describes NEON terrestrial sampling, which targets organisms across a range of generation and turnover times, and a hierarchy of measurable biological states. Measurements encompass species diversity, abundance, phenology, demography, infectious disease, ecohydrology, and biogeochemistry. The continental-scale sampling requires collection of comparable and calibrated data using transparent methods. Data will be publicly available in a variety of formats and suitable for integration with other long-term efforts. NEON will provide users with the data necessary to address large-scale questions, challenge current ecological paradigms, and forecast ecological change.

Litter effects of two co-occurring alpine species on plant growth, microbial activity and immobilization of nitrogen. -Oikos 104: 336-344.We measured the litter chemistry of two co-dominant alpine species, Acomastylis rossii, a forb characterized by a low growth rate and N uptake capacity, and Deschampsia caespitosa, a grass characterized by a high growth rate and N uptake capacity, and examined the effect litter of these two species had on the growth of Deschampsia phytometers in a greenhouse. We also examined the influence of litter from the two species on microbial respiration and immobilization of N, in two separate laboratory microcosm experiments and in the field. We hypothesized that Acomastylis litter would reduce plant growth more than Deschampsia litter, corresponding with either 1) suppression of microbial activity and thus a decrease in N mineralization, or 2) by stimulation of microbial biomass and increasing microbial immobilization of N. Relative to Deschampsia litter, Acomastylis litter had higher total water soluble organic carbon (DOC), and higher total phenolic concentration. Deschampsia litter had 30 times higher carbohydrate (primarily glucose and fructose) concentrations than Acomastylis litter. Soil respiration, microbial biomass N, and consumption of DOC and N were higher with the Acomastylis litter treatment than the Deschampsia litter treatment in experimental microcosms, and both respiration and microbial biomass N were higher in field soils under canopies dominated by Acomastylis relative to those dominated by Deschampsia. These results indicate that phenolics in Acomastylis are a C source for soil microorganisms, rather than an inhibitor of microbial activity. Deschampsia phytometers grew significantly less, had higher root: shoot biomass ratios, and acquired less nitrogen in the Acomastylis litter treatment relative to the control and Deschampsia litter treatments. The results of these experiments indicate that Acomastylis litter influences soil N cycling by increasing microbial activity and N immobilization, which may influence N supply to neighboring plants. This mechanism has the potential to influence competitive interactions between Acomastylis and its neighbors.

Abstract. Global change drivers influence ecological processes at multiple scales and manifest across most of Earth as changes in biodiversity, biogeochemical cycles, infectious disease incidence, and ecohydrology. Small-scale investigations provide compelling evidence of specific effects of global change on local systems, but are of limited use in modeling complex ecological processes at continental-to-global scales. Long-term observations distributed across a diversity of habitat types are needed to improve the ability to forecast ecological change at large spatial and temporal scales. This special issue introduces the Terrestrial Observation System (TOS) of the National Ecological Observatory Network (NEON), a long-term, continental-scale ecological research platform designed to deliver these large-scale datasets. The TOS measures biodiversity of key biota (soil microbes, insects, plants, small mammals), ecosystem productivity and biogeochemistry, infectious disease dynamics, phenology, and population dynamics. The articles in this special issue describe the scientific rationale for the sampling designs of the TOS, including an overview of protocols, locations, and frequencies of measurements. The science designs are a culmination of design requirements scoped by NEON and the National Science Foundation, best available practices put forth by the scientific community, input from technical working groups, and consideration of logistical and financial constraints by NEON staff. Within each site, measurements have been collocated to the extent possible to optimize linkages among different sampling elements. Integrated analyses of terrestrial observations with sensor-based, imagery, and remote-sensing data collected by other NEON subsystems can facilitate scaling of measured parameters to larger spatial and temporal scales. NEON is designed to collect data for 30 years, and make these data freely available on a public data portal (data.neonscience.org). Samples and specimens will be archived and available to the scientific community upon request. The open access approach to the Observatory will provide users with the resources necessary to map, understand, and predict the effects of global change drivers on ecological processes at a continental scale.