Most microbial decomposers and animal consumers feed on substrates that differ in elemental composition from their bodies. Both microbes and consumers, however, have a relatively homeostatic body composition, leading to nutrient excess or limitation depending on the carbon-to-nutrient ratios of their resources. When relatively more nutrients are available, carbon (C) is the most limiting element, and excess nutrients are released as net mineralization or excretion. When substrates are nutrient-poor, C is in excess and nutrient availability constrains growth. The widespread occurrence of these stoichiometric mismatches between food sources and organism elemental composition raises the question as to how organisms adapt to grow in such conditions. It has been proposed that in the presence of stoichiometric imbalances, growth may be reduced in relation to C uptake, resulting in lowered carbon-use efficiency (CUE). Here we propose an optimality framework to understand if downregulating CUE is an effective avenue for stoichiometric regulation when nutrients are scarce. Based on the assumption that CUE is optimally adjusted to maximize the organism growth rate, we derive an analytical equation linking CUE to substrate and organism biomass C-to-nutrient ratios, and the availability of inorganic nutrients. The theoretical predictions from this optimality framework are generally confirmed by a large collection of CUE estimates in both aquatic and terrestrial organisms, and for both microorganisms and animals. This framework offers concise equations that are amenable for inclusion in soil and ecosystem biogeochemical models.

Stefano Manzoni graduated in Environmental Engineering from Politecnico of Torino (Italy) in 2004. He moved to Duke University (NC, USA) to pursue a PhD in Hydrology in the Department of Civil and Environmental Engineering, with a focus on the linkages between hydrology and biogeochemical cycling in soils. He then worked as a Research Scientist in the Nicholas School of the Environment at Duke University where he investigated how plants respond to changing water availability, and later at the Swedish University for Agricultural Sciences. In 2014 he joined the Department of Physical Geography at Stockholm University, as a Senior Lecturer. There he continues to develop models in the broad areas of eco-hydrology and ecological stoichiometry to capture the hydro-climatic impacts on vegetation and soil processes from single-organism to global scales. In 2014, he received the “AGU Hydrologic Sciences Early Career Award” in recognition of his theories of soil water-biota interactions.