What determines the outcome of competition for organic nitrogen between plant roots and soil microbes?

Most nitrogen enters the soil as protein, requiring considerable modification by soil microbes before becoming inorganic nitrogen (ammonium; nitrate), which was historically considered to support all plant nitrogen requirements. It is now clear that plant roots and soil microbes compete fiercely for the carbon and nitrogen of early protein breakdown products (short peptides and amino acids) in the rhizosphere. However, despite being hotly debated and generating many high profile publications, the key factors determining the outcome of this competition remain a subject of speculation. Consequently, this remains a major knowledge gap in our understanding of carbon and nitrogen cycling in soils.

Many have suggested that the outcome of plant-microbe competition is controlled by the "preference" of individual plant species for particular forms of nitrogen. Others argue that the outcome is driven by the efficiency/activity of the soil microbial competitor or other factors such as the availability of inorganic nitrogen or the presence of fungal symbionts in plant roots. Some evidence suggests that in ecosystems where soil nitrogen mineralisation rates are low (e.g. those at high altitudes or latitudes), plants are more successful competitors for organic forms of nitrogen than in ecosystems where nitrogen mineralisation rates are high (e.g. agri-ecosystems). Similarly, an effect of root infection with a fungal symbiont on organic nitrogen acquisition is apparent in some investigations. However, which ecosystem characteristics drive the outcome of rhizosphere competition and which are coincidental has never been investigated in a coordinated manor.