The (mineral) matrix reconsidered: Master control or product of carbon cycling?
Mineral control of soil carbon turnover has been an accepted paradigm for close on two decades, but fungi and plant roots are known for their ability to manipulate the mineral matrix such that it may release protected carbon. Simulation models have historically relied on the premise that carbon turnover should be a function of the molecular makeup of organic substrates, but over the last few years, arguments for an addition of decomposer functionalities to models have gained traction. Since all of the approaches mentioned above have both, shortcomings and undeniable merits, it seems that the time has come for a unifying "theory of everything" for soil carbon turnover. If traditions, contradictions and new insights are to be succesfully reconciled into a new unified theory of soil carbon turnover, the question to be resolved must be: How and where may mineral protection, substrate quality, microbial ecology and plant physiology converge to determine the residence time of soil carbon? In response to this question, I propose an evolutionary approach to the issue, beginning with an examination of the conditions under which plants colonized dry land some 400-500 million years ago.
I intend to show that plants have evolved to use belowground carbon inputs as a regulatory means to create, exploit and even condition the soil ecosystem towards optimum resource strength. I will further emphasize that, when algae began to evolve into plants, they did so in an environment that was already inhabited by microorganisms. Thus the evolution of root systems involved the development of sophisticated plant-microbial relationships covering the full scope of the mutualism-symbiosis-parasitismus continuum, effectively forming a plant-microbial metaorganism or holobiont that coevolves with the soil system it continues to create even today, when soil formation starts afresh on newly exposed or deposited mineral parent materials.
If coevolution of the plant holobiont with the surrounding mineral matrix is accepted as a fundamental process, the issue of soil carbon turnover morphs from a "what is the best predictor" – type question to the issue of "what controls soil bioreactor performance". I will then present arguments to show that analogies between soils and industrial bioreactor technology, informed by complex systems theory, offer a promising avenue towards a new, unified, mechanistic and quantitative theory of soil carbon turnover.