Journal of Conference Abstracts

Paradox Lost: Nitrogen Retention in Carbon Limited Soils

John D. Aber (john.aber@unh.edu)

Complex Systems Research Centre, University of New Hampshire,
Durham, New Hampshire 03824, U.S.A.

Forest ecosystems experiencing elevated N deposition generally show net retention of dissolved inorganic nitrogen (DIN). This long-term net N accumulation is a major factor reducing the negative impacts of N deposition on forests and surface waters. Studies on altered C:N ratios in forest floor organic matter under coniferous forests, long-term distribution of added ¹⁵N tracers, and ecosystem-level N budgets, all demonstrate that the majority of added N is retained in soils in non-extractable organic forms. The relative importance of the different mechanisms by which the net retention of DIN may occur has not been determined.

Short-term ¹⁵N pool dilution experiments have demonstrated that gross mineralisation and immobilization rates measured in soil are many times higher than net rates. The question posed by the ¹⁵N pool dilution results relative to sites receiving large amounts of added N is: what is the source of the labile carbon required to drive this process? Microbial activity under field conditions in soils is generally thought to be energy or carbon limited, not nitrogen limited. The measured absence of increased CO₂ efflux from soils following DIN additions supports the idea of C limitations in soils and emphasises the dilemma posed by DIN retention with high N inputs.

In contrast to experiments with intact systems, trench plot experiments and sites experiencing forest decline both tend to show rapid increases in nitrate mobility. This combination of experimental results suggests that the mechanisms leading to retention of N added through deposition or experimental amendments must result in little or no increase in CO₂ efflux from soils and require no large soil pools of labile carbon. Two mechanisms fit this requirement: (1) incorporation into root/mycorrhizal biomass or exudates using carbohydrates from photosynthesis directly, and (2) chemical or abiotic incorporation. Published results suggest that chemical incorporation occurs but cannot account for the large quantities of N incorporation reported in many studies. In contrast, incorporation of DIN into root/mycorrhizal biomass or exudates using carbohydrates from photosynthesis fits within the constraints set by the experimental observations, and by a set of calculations to be presented.

The Effect of Organic Substrate on Sulphate-Reduction Rates: Implications for Constructed Wetlands

D. Adam¹ (chedna@ecu-01.novell.leeds.ac.uk) & R. Raiswell² (raiswell@earth.leeds.ac.uk)

¹ Department of Chemical Engineering, Leeds University, Leeds, West Yorkshire, U.K.

² Department of Earth Sciences, Leeds University, Leeds, West Yorkshire, U.K.

The recent closure of virtually all of the United Kingdom's coal mines is likely to produce serious environmental problems with acid-mine drainage (AMD) and there is considerable interest in using constructed wetlands to treat AMD. Wetlands are thought to remove heavy metals from polluted drainage by a variety of methods, including the action of anaerobic sulphate-reducing bacteria (SRB). Most wetlands within the UK are based on a mushroom-compost substrate, although there is little scientific basis for the choice of this substrate, in terms of optimising rates of sulphate reduction

The purpose of this work was to investigate the sulphate-reduction process in the laboratory, using enrichment cultures of bacteria obtained from a wetland in South Wales. Specific studies were carried out to determine the suitability of mushroom compost as the selected source of organic material for the SRB as compared to other possible waste substrates. Reduction rate constants were obtained from anaerobic incubation experiments, some results of which are presented below. The index shown is a ratio of the rate constant for the substrate investigated, to the rate constant obtained for mushroom compost.

Substrate Index

Lactate 2.7

High level sewage sludge 2.1

Brewery waste 1.3

Low level sewage sludge 1.0

Mushroom compost 1.0

Horse manure 1.0

Newspaper 0.63

Grass 0.50

Sawdust 0.23

Paper towel 0.13

Straw 0.06

Toilet paper 0.06

Other studies were carried out to investigate the effect of temperature and pH on the laboratory reduction process. With lactate as substrate the apparent activation energy for the sulphate-reduction process was 39 kJ mol^-1 and the optimum pH was 7.3.