Paula Roberts

Acquisition and assimilation of nitrogen as peptide-bound and D-enantiomers of amino acids by wheat.

Nitrogen is a key regulator of primary productivity in many terrestrial ecosystems. Historically, only inorganic N (NH4 + and NO3 -) and L-amino acids have been considered to be important to the N nutrition of terrestrial plants. However, amino acids are also present in soil as small peptides and in D-enantiomeric form. We compared the uptake and assimilation of N as free amino acid and short homopeptide in both L- and D-enantiomeric forms. Sterile roots of wheat (Triticum aestivum L.) plants were exposed to solutions containing either 14C-labelled L-alanine, D-alanine, L-trialanine or D-trialanine at a concentration likely to be found in soil solution (10 µM). Over 5 h, plants took up L-alanine, D-alanine and L-trialanine at rates of 0.9±0.3, 0.3±0.06 and 0.3±0.04 µmol g−1 root DW h−1, respectively. The rate of N uptake as L-trialanine was the same as that as L-alanine. Plants lost ca.60% of amino acid C taken up in respiration, regardless of the enantiomeric form, but more (ca.80%) of the L-trialanine C than amino acid C was respired. When supplied in solutions of mixed N form, N uptake as D-alanine was ca.5-fold faster than as NO3 -, but slower than as L-alanine, L-trialanine and NH4 +. Plants showed a limited capacity to take up D-trialanine (0.04±0.03 µmol g−1 root DW h−1), but did not appear to be able to metabolise it. We conclude that wheat is able to utilise L-peptide and D-amino acid N at rates comparable to those of N forms of acknowledged importance, namely L-amino acids and inorganic N. This is true even when solutes are supplied at realistic soil concentrations and when other forms of N are available. We suggest that it may be necessary to reconsider which forms of soil N are important in the terrestrial N cycle.

Vegetation cover regulates the quantity, quality and temporal dynamics of dissolved organic carbon and nitrogen in Antarctic soils.

Populations of the two native Antarctic vascular plant species (Deschampsia antarctica and Colobanthus quitensis) have expanded rapidly in recent decades, yet little is known about the effects of these expansions on soil nutrient cycling. We measured the concentrations of dissolved organic carbon (DOC) and nitrogen (DON), amino acids and inorganic N in soils under these two vascular plant species, and under mosses and lichens, over a growing season at Signy Island in the maritime Antarctic. We recorded higher concentrations of nitrate, total dissolved nitrogen, DOC, DON and free amino acids in soil under D. antarctica and C. quitensis than in lichen or moss dominated soils. Each vegetation cover gave a unique profile of individual free amino acids in soil solution. Significant interactions between soil type and time were found for free amino acid concentrations and C/N ratios, indicating that vascular plants significantly change the temporal dynamics of N mineralization and immobilization. We conclude that D. antarctica and C. quitensis exert a significant influence over C and N cycling in the maritime Antarctic, and that their recent population expansion will have led to significant changes in the amount, type and rate of organic C and N cycling in soil.

Yield responses of wheat (Triticum aestivum) to vermicompost applications

Earthworm digested wastes (vermicompost) are being produced in increasing quantities and there is much interest in developing new markets for these products. In this paper, the responses of wheat (Triticum aestivum var. Einstein and Xi19) to vermicompost additions and to combinations of vermicompost and inorganic NPK fertilizer in field and glasshouse environments are considered. Plant response was determined by measuring a range of ontogenetic parameters (plant growth, chlorophyll content, fertile ears, tiller number and grain yield). The individual treatments involved the addition of 1, 10 and 30 t ha−1 of vermicompost to soil, or the coapplication of 1, 10 and 30 t ha−1 of vermicompost plus NPK fertilizer where the N addition rate was normalized to 150 kg N ha−1. Photosynthetic pigment development, plant growth, and yield were reduced in all treatments in the absence of inorganic fertilizer. However, all coapplication treatments resulted in similar yields to NPK fertilizer alone in both field and glasshouse experiments. It is our conclusion that vermicompost alone cannot provide a viable substitute for inorganic fertilizer without causing a significant loss of yield; however, it may enhance soil quality if integrated into conventional arable cropping practices which use inorganic fertilizers.

In-Vessel Cocomposting of Green Waste With Biosolids and Paper Waste

In comparison to traditional windrow composting, in-vessel composting techniques often represent more effective waste management options due to the reduced production of bioaerosols and leachate and the potential for better process control. Chemical processes occurring during the cocomposting of three common wastes (green waste, biosolids and paper processing waste) were studied using the forced aeration, static pile, in-vessel EcoPOD® composting system. Since no turning of the compost occurs within the static piles, spatial differences in the vessel were also monitored. These measurements revealed significant spatial gradients in temperature; however, this did not result in spatial differences in nutrients within the composting vessel. Significant differences in soluble N production were observed during the composting process following the series: green plus paper waste < green waste < green waste plus biosolids. After the active compost phase was over, and the compost was removed from the vessel and matured outside, we demonstrated that covering the compost was essential to preserve compost quality. Our study clearly shows that cocomposting of common waste feedstocks can be used to successfully manipulate the chemistry of the final compost making it suitable for multiple end uses. In addition, our study demonstrated that careful management of the compost maturing phase is also required to maximise quality and minimize pollution.

Responses of Common Pot Grown Flower Species To Commercial Plant Growth Media Substituted With Vermicomposts

Waste management is an increasingly important problem in many developed countries and much effort is currently being expended into identifying novel uses for waste products. We consider the effects of substituting proportions of earthworm-digested composted green waste (vermicompost) into commercial peat based growing media on the germination, growth, and productivity of Helianthus annuus, Cosmos bipinnatus and Eschscholzia californica. The percentage germination, time to flowering, seed production, overall heights and biomass were measured for all three species in treatments of commercial peat based growing media where the growth medium was substituted with vermicompost at rates of 0%, 20%, 40%, 60% and 100%. In addition, the total numbers of inflorescences was recorded in multiple flowering species. Results for Helianthus suggest that while vermicompost additions induced small species specific growth responses in germination rate, growth rate, overall height and total biomass, none were statistically significant. For Cosmos there was a significant increase in early plant growth rate and in the final flower numbers at substitution rates of 20% and 40% vermicompost. The total biomass and numbers of flowers produced by Eschscholzia was decreased significantly by substitution of 20% and 40% vermicompost. We conclude that the plant growth enhancement properties of this vermicompost seem more species specific than previously reported. Further, in order to preserve consumer confidence in waste-derived horticulture products, vermicompost marketing strategies should acknowledge these findings.

Dynamics of Nitrogen Speciation in Horticultural Soils in Suburbs of Shanghai, China

Dissolved organic nitrogen (DON) represents a significant pool of soluble nitrogen (N) in soil ecosystems. Soil samples under three different horticultural management practices were collected from the Xiaxiyang Organic Vegetable and Fruit Farm, Shanghai, China, to investigate the dynamics of N speciation during 2 months of aerobic incubation, to compare the effects of different soils on the mineralization of 14C-labeled amino acids and peptides, and to determine which of the pathways in the decomposition and subsequent ammonification and nitrification of organic N represented a significant blockage in soil N supply. The dynamics of N speciation was found to be significantly affected by mineralization and immobilization. DON, total free amino acids, and NH + -N were maintained at very low levels and did not accumulate, whereas NO − -N gradually accumulated in these soils. The conversion of insoluble organic N to low-molecular-weight (LMW) DON represented a main constraint to N supply, while conversions of LMW DON to NH + -N and NH + -N to NO − -N did not. Free amino acids and peptides were rapidly mineralized in the soils by the microbial community and consequently did not accumulate in soil. Turnover rates of the additional amino acids and peptides were soil-dependent and generally followed the order of organic soil > transitional soil > conventional soil. The turnover of high-molecular-weight DON was very slow and represented the major DON loss. Further studies are needed to investigate the pathways and bottlenecks of organic N degradation.

Influence of inorganic and organic nitrogen on enzymes of nitrogen assimilation and growth in tomato seedlings

Summary Recent evidence suggests that agricultural and horticultural crops may be able to take up significant quantities of dissolved organic nitrogen (DON). Our aims were to determine the effects of supplying different forms of N on growth and on the activities of N-assimilatory enzymes in tomato. Two genotypes of tomato were grown in sterile hydroponic culture without N (control), with NO3– or NH4+ (3 mM), or with organic-N in the form of glycine (1.5, 3.0, or 6.0 mM). The results showed that biomass production and N-contents were similar in both genotypes when supplied with NO3– or with glycine, and that this growth was much greater than in plants supplied with NH4+ alone, or without added N. In addition, the production of plant biomass was positively correlated with the concentration of glycine-N used; however, the magnitude of the response was genotype-dependent. The form of N supplied also significantly affected the activities of several key N-assimilatory enzymes in roots and shoots. For example, addition of glycine increased the activities of NADH-glutamate dehydrogenase (NADH-GDH), glutamate oxaloacetate transaminase (GOT), and glutamate pyruvate transaminase (GPT) in roots, compared with the NO3– or NH4+ treatments. Our results clearly demonstrate the intrinsic ability of tomato plants to use DON as a sole source of N. Further studies to investigate the functional significance of DON in horticultural systems under non-sterile conditions are therefore warranted.