Nitrogen Increases Early-Stage and Slows Late-Stage Decomposition Across Diverse Grasslands

To evaluate how increased anthropogenic nutrient inputs alter carbon cycling in grasslands, we conducted a litter decomposition study across 20 temperate grasslands on three continents within the Nutrient Network, a globally distributed nutrient enrichment experiment We determined the effects of addition of experimental nitrogen (N), phosphorus (P) and potassium plus micronutrient (Kμ) on decomposition of a common tree leaf litter in a long-term study (maximum of 7 years; exact deployment period varied across sites). The use of higher order decomposition models allowed us to distinguish between the effects of nutrients on early- versus late-stage decomposition. Across continents, the addition of N (but not other nutrients) accelerated early-stage decomposition and slowed late-stage decomposition, increasing the slowly decomposing fraction by 28% and the overall litter mean residence time by 58%. Synthesis. Using a novel, long-term cross-site experiment, we found widespread evidence that N enhances the early stages of above-ground plant litter decomposition across diverse and widespread temperate grassland sites but slows late-stage decomposition. These findings were corroborated by fitting the data to multiple decomposition models and have implications for N effects on soil organic matter formation. For example, following N enrichment, increased microbial processing of litter substrates early in decomposition could promote the production and transfer of low molecular weight compounds to soils and potentially enhance the stabilization of mineral-associated organic matter. By contrast, by slowing late-stage decomposition, N enrichment could promote particulate organic matter (POM) accumulation. Such hypotheses deserve further testing

Assessment of Ferric-Reducing/Antioxidant Power in the Foliage of Several Tree Species

Recently considerable attention has been given to the presence of non-enzymatic antioxidant compounds present in plant foliage. Carotenoids (xanthophylls + carotenes) have been identified as a class of compounds that have antioxidant abilities. New evidence suggests that phenylpropanoids (hydroxycinnamic acids + flavonoids) may also play a role in oxygen freeradical scavenging. The Ferric-Reducing/Antioxidant Power (FRAP) assay was used to examine these oxygen-reducing compounds in the foliage of ten tree species. The FRAP assay is a rapid and direct test that has been used primarily to quantify the capability of non-enzymatic antioxidants in animal systems and has rarely been used for plants. Foliage from five coniferous (Juniperus virginiana, Pinus sylvestris, Picea pungens, Thuja occidentalis and Ginko biloba) and five deciduous (Rhamnus catharticus, Acer saccharum, Fraxinus pennsylvanica, Robinia pseudoacacia and Quercus rubra) species was examined. Soluble carotenoids and phenylpropanoids were extracted from foliage in a methanol solution. Concentrations of these compounds were measured spectrophotometrically and FRAP assays performed on these extracts. There was a significant positive correlation (P\u3c0.05) observed between concentrations of soluble phenylpropanoids (assessed at 300 nm) and FRAP values in T. occidentalis, A. saccharum and F pennsylvanica. In addition there was a weak, but significant (P\u3c0.10), positive relationship observed between these two variables across all species and was strongest for the deciduous lifeform (P\u3c0.05). These results suggest that phenylpropanoid concentrations may play an important role in the scavenging of oxygen free-radicals in the foliage of some tree species

Decomposition of a Native Grass Species in the Antartic Tundra

We examined the decomposition of a native grass species (Deschampsia antarctica) using sizeselective litter bags in the Antarctic tundra. Litter bags were constructed of either 0.12 cm (small) or 1.2 cm (large) nylon mesh and placed on Bonaparte Point (64o46\u27S; 64o03\u27W) on Anvers Island along the western Antarctic Peninsula for approximately one year. Small litter bags excluded native herbivore invertebrates while large bags allowed invertebrates to access the plant material. We measured lignin, alpha-cellulose, holo-cellulose and total soluble carbohydrate and lipid concentrations of this material as well as changes in total organic carbon and nitrogen. Cellulose and lignin concentrations were high averaging 60 and 14.5% respectively, of the total dry weight of the plant material. Concentrations of soluble carbohydrates and fats averaged less than 1 and 5% respectively, suggesting rapid loss of these materials due to leaching. Decomposition rates averaged 58% for both small and large litter bags and did not significantly differ between mesh sizes. Our results suggest that invertebrate herbivores do not play a significant role in decomposition of the native grass species. In addition, decomposition of plant material was significantly greater in this wet maritime climate relative to drier inland areas of the Antarctic continent

Solar Ultraviolet-B Radiation Increases Phenolic Content and Ferric Reducing Antioxidant Power in Avena sativa

We examined the influence of solar ultraviolet-B radiation (UV-B; 280-320 nm) on the maximum photochemical efficiency of photosystem II (Fv/Fm), bulk-soluble phenolic concentrations, ferric-reducing antioxidant power (FRAP) and growth of Avena sativa. Treatments involved placing filters on frames over potted plants that reduced levels of biologically effective UV-B by either 71% (reduced UV-B) or by 19% (near-ambient UV-B) over the 52 day experiment (04 July - 25 August 2002). Plants growing under near-ambient UV-B had 38% less total biomass than those under reduced UV-B. The reduction in biomass was mainly the result of a 24% lower leaf elongation rate, resulting in shorter leaves and less total leaf area than plants under reduced UV-B. In addition, plants growing under near-ambient UV-B had up to 17% lower Fv/Fm values early in the experiment, and this effect declined with plant age. Concentrations of bulk-soluble phenolics and FRAP values were 17 and 24% higher under near-ambient UV-B than under reduced UV-B, respectively. There was a positive relationship between bulk-soluble phenolic concentrations and FRAP values. There were no UV-B effects on concentrations of carotenoids (carotenes + xanthophylls)

Appendix C. Figure showing proportion initial mass remaining versus proportion initial N for green leaves versus litter.

Figure showing proportion initial mass remaining versus proportion initial N for green leaves versus litter

Appendix E. Results of NMDS ordinations of PLFA groups (Gram positive bacteria, Gram negative bacteria, actinomycetes, fungi).

Results of NMDS ordinations of PLFA groups (Gram positive bacteria, Gram negative bacteria, actinomycetes, fungi)

Appendix B. Parameters obtained by fitting decomposition data to asymptotic and single exponential decay models to data showing treatments, sites, and substrates.

Parameters obtained by fitting decomposition data to asymptotic and single exponential decay models to data showing treatments, sites, and substrates

Appendix D. Extracellular enzyme activities on substrates harvested during the first three years of the experiment by substrate (averaged over sites and treatments).

Extracellular enzyme activities on substrates harvested during the first three years of the experiment by substrate (averaged over sites and treatments)