Abstract:Peatlands play an important role in the global carbon cycle, and loss of dissolved organic carbon (DOC) has been shown to be important for peatland carbon budgets. The objective of this study was to determine how net production and export of DOC from a northern peatland may be affected by disturbance such as drainage and climate change.The study was conducted at a poor fen containing several pool-ridge complexes: (1) control site-no water table manipulation; (2) experimental site-monitored for one season in a natural state and then subjected to a water table drawdown for 3 years; (3) drained site-subjected to a water table drawdown 9 years prior to monitoring. The DOC concentration was measured in pore water along a microtopographic gradient at each site (hummock, lawn and hollow), in standing water in pools, and in discharge from the experimental and drained sites. The initial water table drawdown released ¾3 g of carbon per square metre in the form of DOC, providing a large pulse of DOC to downstream ecosystems. This value, however, represents only 1-9% of ecosystem respiration at this site. Seasonal losses of DOC following drainage were 8-11 g of carbon per square metre, representing ¾17% of the total carbon exchange at the experimental study site. Immediately following water table drawdown, DOC concentrations were elevated in pore water and open water pools. In subsequent seasons, DOC concentration in the pool declined, but remained higher than the control site even 11 years after water-table drawdown. This suggests continued elevated net DOC production under lower water table conditions likely related to an increase in vegetation biomass and larger water table fluctuations at the experimental and drained sites. However, the increase in concentration was limited to initially wet microforms (lawns and hollows) reflecting differences in vegetation community changes, water table and soil subsidence along the microtopographic gradient.

1. The factors affecting denitrification in soil have been studied by determining loss of nitrogen from soil under various conditions by total-N analysis.2. It was found that the rate of denitrification of nitrate in soil was dependent upon various factors such as the pH, temperature and water content of the soil and that, under conditions conducive to denitrification, 80–86% of nitrate-N added to Rothamsted soils was lost by denitrification in 5 days.3. The rate of denitrification was greatly affected by the pH of the soil. It was very slow at low pH (below 4·8), increased with rise in soil pH and was very rapid at pH 8·0–8·6.4. The rate of denitrification increased rapidly with rise in temperature from 2° to 25° C. The optimum temperature for denitrification was about 60° C.5. The degree of water saturation of the soil had a profound influence on the rate of denitrification. Below a certain moisture level practically no denitrification occurred; above this level denitrification increased rapidly with increase in moisture content. The critical moisture level was about 60% of the water-holding capacity of the soil.

1. Methods of investigating denitrification in soil are critically discussed with special reference to methods based on total-N analysis.2. A modified Kjeldahl method of determining nitrogen which includes nitrate and nitrite and is applicable to waterlogged soil is described and the use of this method in studies on denitrification in soil is illustrated and discussed.3. It is shown that rapid denitrification of nitrate in soil can be induced by incubating the soil under waterlogged conditions with organic materials such as glucose and that denitrification can be followed by total-N analyses if the organic material used to induce denitrification is not added in such excess that it also promotes significant fixation of atmospheric nitrogen.4. The percentage of added nitrate-N lost by denitrification on incubation of waterlogged soils with different amounts of nitrate and sufficient glucose for denitrification was found to be the same whatever the level of application of nitrate.5. Denitrification of nitrate in waterlogged soil containing glucose was found to be accompanied by a rapid but temporary accumulation of large quantities of nitrite and by the formation of smaller amounts of ammonia. Hydroxylamine could not be detected during denitrification, but it was found that this compound was rapidly decomposed in the soils examined by a process which appeared to be purely chemical.6. It is shown that denitrification of nitrate in soil is a microbiological process and that the viability of the micro-organisms responsible for denitrification is not affected by air-drying and storage of the soil.

Awet-oxidation technique for determining organic carbon in soil, plant material, and aqueous plant extracts is described. The oxidation is carried out by heating the soil or plant sample (containing 50-100 mg. carbon) with a mixture of potassium dichromate, sulphuric, and phosphoric acids for 10-15 min., and the carbon dioxide liberated is determined gravimetrically after absorption in soda-lime. With slight modifications, the method has proved useful for the determination of carbon in plant extracts and would also appear suitable for a soil extract. A method is described for the removal of carbonates without loss of organic carbon.It is shown that heating a wet soil at 105" C. causes considerable loss of organic carbon. Inert carbonaceous material, such as coal or charcoal, is oxidized by this wet-digestion method, thus producing erroneously high results for the carbon content of soil organic matter.The method is at least as accurate as the conventional dry-combustion technique, and is less time-consuming, only one hour beingrequired for a determination. The apparatus is of simple design and with the exception of the needle valve can be readily constructed from normal laboratory glassware. The method can deal with fairly large samples of materials, thus preserving an essential feature of established macro-methods.This technique of determining carbon is convenient for the fairly rapid analysis of soils and plant materials where an accurate total-carbon value is required.

1. Methods for the determination of ammonia and nitrate in soil are described. The ammonia and nitrate are extracted at pH 1·0–1·5 with a mixture of potassium sulphate and sulphuric acid, and the ammonia is determined by distillation with magnesium oxide at 25° C. in a modified Conway microdiffusion unit. Ammonia plus nitrate is determined on a separate sample of the same extract by reduction of the nitrate to ammonia with titanous hydroxide and subsequent distillation with magnesium oxide, both the reduction and distillation being carried out in a modified microdiffusion unit at 25° C.2. The methods are applicable to coloured extracts and are not affected by substances found to interfere with other methods of determining ammonia and nitrate.3. It is suggested that the methods may also prove useful for the determination of ammonia and nitrate in plant materials.