Increased microbial biomass and turnover underpin efficient phosphorus acquisition by Brassica chinensis

Root-microbe interactions are critical in influencing crop productivity. Microbial phosphorus (P) mobilization in soil determines P availability to plants, but how plants modify root acquisition strategies in response to microbially mediated P availability remains unknown. We conducted a pot experiment with rice straw addition to investigate effects of microbial P on plant-available soil P (CaCl2-P and NaHCO3-P) as well as root morphological and exudation traits related to P acquisition in Brassica chinensis. Microbial P immobilization caused by straw addition decreased plant-available P in soil and induced growth of short and thick roots at day 21. In parallel with decline in microbial P in straw-amended soil from the 21st to 42nd day, B. chinensis stimulated root elongation to get close to length of roots without straw. Rhizosphere carboxylate exudation by B. chinensis was greater by seven-fold in the treatment with than without straw at day 42. Field experiments with five successive B. chinensis crops were used to test whether root-microbe interactions can be manipulated by straw amendment to increase crop P uptake and yield. Phosphorus uptake by the first three successive B. chinensis crops was similar with or without straw addition. However, further repeated straw addition increased soil microbial P pool by one-fold and quantity of phosphate-solubilizing bacterial phoD gene by fifty-eight percent, after the 4th compared with those after the 2nd crop. Phosphorus released from microbial biomass turnover underpinned growth of roots with large specific length and low tissue density that are highly efficient in P acquisition, resulting in increased shoot P accumulation and yield in the 4th and 5th B. chinensis crops. In summary, crops shift root strategies from the P-conservative towards P-acquisitive traits in response to microbial biomass turnover. Straw amendment can facilitate root-microbe interactions governing sustained P availability and effective root P-acquisition strategies, thus increasing P-use efficiency.