Abstract:

The calcareous fluvo-aquic soil was collected and a microcosm study was carried out with root-mat and frozen-slicing method in laboratory. The pH in the root-soil interface with the control treatment was just slightly lower than in the bulk soil. However, the addition of NH₄⁺-N significantly decreased the pH value in the root-soil interface and the addition of NO₃^--N slightly increased the pH value in the root-soil interface. The magnitude of pH changes in the root-soil interface depended upon the concentrations of the nitrogen sources added. The contents of Ca₂-P, Fe-P and Al-P in the root-soil interface were much lower after treated with NH₄⁺-N and slightly higher after treated with NO₃^--N compared with control treatment. After treated with 100, 200 and 400 mg·kg^-1 NH₄⁺-N, the deficiency rates of Ca₂-P in the area 0-1 mm from the root plane were 37.1%, 45.9% and 57.7%, respectively, the deficiency rates of Fe-P were 23.4%, 29.1% and 38.2%, respectively, and the deficiency rates of Al-P were 25.1%, 28.0% and 33.2%, respectively. Compared with the control the deficiency rates of Ca₈-P in NH₄⁺-N and NO₃^--N treatments decreased and increased, respectively, but the differences were not obvious. The contents of Ca₁₀-P and O-P in the root-soil interface did not significantly change after treated with NH₄⁺-N or NO₃^--N, suggesting that Ca₁₀-P and O-P were remarkably difficult to be mobilized even at the presence of high concentration of NH₄⁺-N. The lowered pH in the root-soil interface induced by the addition of the NH₄⁺-N promoted the transformation of phosphates in the root-soil interface, enhanced the mobilization and bioavailability of phosphates, and thereby remarkably increased the absorption of phosphorus by roots.

Key words: ammonium, nitrate, root-soil interface, inorganic phosphorus, transformation, P absorption, Pinus thunbergii seedlings