Methane formation and substrate utilisation in anaerobic rice soils as affected by fertilisation

Anaerobic incubation of air-dried rice soils was carried out to investigate the effects of long-term fertiliser application on CH4 formation and the utilisation of glucose and acetate. The soil samples were taken after the crop harvest in 1995 from the long-term experimental plots which had been fertilised with chemical fertiliser alone (CF), or chemical fertiliser in combination with rice straw compost (RC), cow manure (CM) or rice straw (RS) for 20 y. The soil from the CM plot showed the highest rate of CH4 formation, especially when supplemented with glucose or acetate. The RS plot soil showed significant CH4 formation only when glucose was added. CH4 formation in the CF and RC plots was low, even glucose or acetate was added. The pool sizes of acetate ranged from 360 nmol g−1 in the CF plot to 818 nmol g−1 in the RS plot. The rate of acetate metabolism was lowest in the CF plot (26.6 nmol g−1 h−1) and highest in the CM plot (47.6 nmol g−1 h−1), respectively. The results using [1-14C] and [2-14C]acetate and 14C-bicarbonate indicated that demethylation of acetate was the main pathway (>80%) of CH4 formation. CH4 formation from [1-14C]acetate or bicarbonate was less than 15% in all plots. Thus, the long-term application of different fertilisers did not change the pathway of methanogenesis. However, the portion of [2-14C]acetate converted to CH4 was different between plots; about 50% in the CF and RC plots and >90% and nearly 80% in the CM and RS plots. It was found that the efficiency of acetate utilisation and the metabolic rate of acetate were the important factors in determining the magnitude of methanogenesis from acetate in these soils. Addition of BES did not accumulate acetate, suggesting the presence of other acetate-utilising bacteria in these soils.