On Maintenance and Metabolisms in Soil Microbial Communities

Biochemistry is an essential yet often undervalued aspect of soil ecology, especially in soil C cycling. We assume based on tradition, intuition or hope that the complexity of biochemistry is confined to the microscopic world, and can be ignored when dealing with whole soil systems. This opinion paper draws attention to patterns caused by basic biochemical processes that permeate the world of ecosystem processes. From these patterns, we can estimate activities of the biochemical reactions of the central C metabolic network and gain insights into the ecophysiology of microbial biosynthesis and growth and maintenance energy requirements; important components of Carbon Use Efficiency (CUE).The biochemical pathways used to metabolize glucose vary from soil to soil, with mostly glycolysis in some soils, and pentose phosphate or Entner-Doudoroff pathways in others. However, notwithstanding this metabolic diversity, glucose use efficiency is high and thus substrate use for maintenance energy and overflow respiration is low in these three soils. These results contradict current dogma based on four decades of research in soil ecology. We identify three main shortcomings in our current understanding of substrate use efficiency: 1) in numeric and conceptual models, we lack appreciation of the strategies that microbes employ to quickly reduce energy needs in response to starvation; 2) production of exudates and microbial turnover affect whole-soil CUE more than variation in maintenance energy demand; and 3) whether tracer experiments can be used to measure the long-term substrate use efficiency of soil microbial communities depends critically on the ability of non-growing cells to take up tracer substrates, how biosynthesis responds to these substrates, as well as on how cellular activities scale to the community level.To move the field of soil ecology forward, future research must consider the details of microbial ecophysiology and develop new tools that enable direct measurement of microbial functioning in intact soils. We submit that 13C metabolic flux analysis is one of those new tools.