Matric water potential as an ecological determinant in compost, a substrate dense system.

Water is a critical ecological factor in substrate dense matric ecosystems, of which composting systems are an example. Excessive moisture inhibits aerobic metabolism because of oxygen diffusion limitations. How a lack of water impedes composting activity has not been previously considered. An investigation of the relationship of matric water potential (water held by physical attractions) to gravimetric water content was carried out in a laboratory composting system using a sewage sludge and wood chip substrate. The gravimetric-matric water relationship was best fit by a 2∘ polynomial regression of y=64.049-0.142 x, andR= 0.95. It is proposed that the commonly observed failure of composting to occur under drier conditions is a physical limitation, that is bacteria progressively fail to physically colonize the substrate as matric potential decreases below approximately -20 kPa. In physically mixed systems, agitation and introduction of inoculum may promote colonization, permitting composting to be initiated and proceed at lower moisture contents. In nonmixed systems, promotion of thorough initial colonization may permit composting to continue in the presence of significant drying.