Effect of moisture, texture and aggregate size of paddy soil on production and consumption of CH4

Laboratory experiments were conducted to find out under which conditions the soil from Italian rice fields could change from a source into a sink of atmospheric CH4. Moist (30% H2O = 68% of the maximum water holding capacity (whc)) rice field soil oxidized CH4 with biphasic kinetics, exhibiting both a low (145 ppmv CH4) and a high (20.200 ppmv CH4) K-m value and V-max values of 16.8 and 839 nmol gdw(-1) h respectively. The activity with the low K-m allowed the oxidation of atmospheric CH4. Uptake rates of high CH4 concentrations (16.5% v/v) and of O-2 linearly decreased with aggregate size of soil between 2 and 10 mm. Atmospheric CH4 ( 1.8 ppmv) was consumed in soil aggregates <6 mm, but soil aggregates <6 mm released CH4 into the atmosphere. Similarly, net uptake of atmospheric CH4 turned into net release of CH4 when the soil moisture was decreased below a water content of about 20% whc. The uptake rate of atmospheric CH4 increased threefold when the soil was amended with sterile quartz sand. Flooded microcosms with non-amended and quartz-amended soil emitted CH4 into the atmosphere. The CH4 emission rate increased when the flux was measured under an atmosphere of N-2 instead of air, indicating that 30-99% of the produced CHI was oxidized in the oxic soil surface layer. Removal of the hood water resulted in increase of CH4 emission rates until a water content of about 75-82% whc was reached, and subsequently in a rapid decrease. However, the soil microcosms never showed net uptake of atmospheric CH4. Our results show that the microorganisms consuming atmospheric CH4 were inactivated at an earlier stage of drainage than the microorganisms producing CH4 irrespective of the soil rosity which was adjusted by addition of quartz sand. Hence. it is unlikely that the Italian rice fields can act as a net sink for atmospheric CH4 even when drained. (C) 2001 Elsevier Science Ltd. All rights reserved.