Fig. 7: Conceptual figure illustrating the relative P incorporation rate into microbial phospholipids and DNA pools and the dominance of extra- and inter-cellular P recycling in soils with contrasting P availability (X-axis), and their response to changes in soil C availability (Y-axis).

Four modes and the specific mechanistic features underlie them have been proposed: 1. Low C availability in low P soil: rapid P recycling in microbial phospholipids is common because intensive P incorporation into phospholipids and rapid intracellular P recycling. The P from phospholipids is intensively reused within the cells through endogenous metabolism to support microbial maintenance without P release into soil solution, resulting in a more “conservative” ecosystem P cycling. 2. Low C availability in high P soil: soil microorganisms preferentially incorporate P into DNA for DNA reparation but without cell division. This facilitates the shifts of microorganisms from maintenance to the state of ready to grow, because preparation of the accurate and intact genetic information can help with the immediate start of microbial growth. 3. High C availability in low P soil, a disproportionately large P amount is incorporated into phospholipids mainly due to P “refill” of the depleted phospholipids pool. Phospholipids serves as an efficient P storage to resist to further P exhaustion following microbial growth. 4. High C availability in high P soil: P is invested for DNA and phospholipids duplication, but a disproportionately large amount of immobilized P is re-allocated to DNA to meet the boost of nucleic acids demand during rapid growth, leading to a faster DNA recycling. In this case, the extracellular P recycling is accelerated via taking up the P released from dying microorganisms by growing ones, which consequently results in an “open” P cycling in soil. Generally, a DNA P incorporation pattern coupled with intensified extracellular P recycling dominates in soils with high nutrient and high C availability, whereas a phospholipids P incorporation pattern with intensified intracellular P recycling dominates the soils depleted in nutrients and C. The concomitant shifts of fungal composition towards copiotrophs and microbial functions towards DNA synthesis after removal of C deficiency drive transformations from pattern 2 to pattern 4. However, bacterial compositional shifts towards G- and functional switch with strong organic C degradation and phospholipids metabolisms after P exhaustion can induce transformations from pattern 4 to pattern 3.