Properties of rice soils affecting methane production potentials: 1. Temporal patterns and diagnostic procedures

Thirty-one rice soils from different locations in the Philippines were incubated anaerobically for 100 d to determine methane (CH(4)) production potentials and to establish relationships between physico-chemical properties of soil and CH(4) production potential. These soils showed pronounced variations in pattern and magnitude of CH(4) production. Total CH(4) production over 100 d incubation ranged from 163 to 837 mug CH(4) g(-1) soil. Total N, soil texture (clay and sand fractions mainly) and cation exchange capacity (CEC) of the soils had significant effect on CH(4) production potential. Available K and active Fe content also affected the CH(4) production potentials of various soils. An assessment of CH(4) production with high accuracy could be obtained from soil redox potential (Eh) development during incubation; the difference between initial and equilibrium Eh allowed a computation of CH(4) production with more than 70% reliability. The CH(4) production potentials obtained over long incubation periods could be assessed, with reasonable accuracy, by a relatively short incubation experiments and fewer measurements of CH(4) production. Only three samplings of CH(4) production rate within a short incubation period of 37 d facilitated a prediction of total CH(4) production over 100 d incubation using the following algorithm: P(0-100) = 99.21 + 10.79X(4) + 11.69X(16) + 45.79X(37) (R(2) = 0.91; P< 0.01), where P(0-100) is the total CH(4) production during 100 d of incubation and X(n) is CH(4) production rate at n days of incubation. Longer incubation periods (86 d) were required to achieve a reliability of more than 95%.