Our study showed that higher EC levels result in higher amount of
organic acids exudation in the rhizosphere of rice plants. Higher amount
of organic acids exudation in gypsumfertilizer treatmentsmay be one of
the carbon costs which reduce the rice growth and one of the carbon
pools for methanogenic bacteria to produce more CH4. Methane emission
was closely related with the organic carbon contribution from rhizosphere
exudates. At the same time, gypsum application was also a
source for SO4 2−. In the presence of SO4 2 −, sulfate reducing bacteria
might compete carbon substrate with methanogens but sulfate reducers
could not fully outcompete the carbon substrates which were
provided by rice plants such as rhizosphere exudation. Thus, the present
results of increased or decreased CH4 emission due to gypsum fertilizer
addition might more relate to the ratio of SO4 2− and carbon availability
of soils where rice plants are grown.