Positive feedback between acidification and organic phosphate mineralization in the rhizosphere of maize ( Zea mays L.)

To test the hypothesis that rhizosphere acidification would enhance the hydrolyzation of organic phosphates by increasing phosphatase activity. A Petri dish experiment with sterile agar and a pot experiment with a low P soil were used. In the Petri dish experiment, roots of each plant were cultured in two compartments, each of which contained agar with one of three nitrogen combinations: NH 4 + /N0 (N0 = nitrogen free), NH 4 + /NO 3 - , and NO 3 - /N0. Phytin was supplied as the sole phosphorus (P) source to all compartments. In the pot experiment, the soil in each pot was treated with N0, KNO 3 , or (NH 4 ) 2 SO 4 ) together with 0 or 75 mg kg −1 phytin-P. Dry weight, P concentration, and P content of roots were highest in the NH 4 + compartments in the Petri dish experiment. In the pot experiment, dry weight, P concentration, and P content of both shoots and roots were higher with NH 4 + than with NO 3 - . NH 4 + treatments reduced rhizosphere pH, promoted the hydrolization of phytin, enhanced acid phosphatase activity in the rhizosphere, and increased phytin-P utilization relative to N0 and NO 3 - treatments. Phosphatase activity was negatively correlated with rhizosphere pH but was positively correlated with plant P content in both experiments. Rhizosphere acidification optimized the activity of acid phosphatase excreted by maize roots and promoted phytin mineralization. NH 4 + -induced acidification in the maize rhizosphere improved the growth of maize roots by improving P uptake from phytin; the improved growth, in turn, increased NH 4 + uptake and acidification.