Soil available P was extracted with 0.5 mol L−1 NaHCO3 (pH 8.5) at a solution to soil ratio of 20:1
for 30 min, and determined by the ascorbic acid method at 880 nm absorbance (Lu, 1999). The available
K was extracted by NH4OAc (pH 7.0) and determined by atomic absorption spectrometry. The total
organic carbon was measured using the acid dichromate method (Lu, 1999). Available N was measured
according to the method of Lu (1999). Briefly, the air-dried soil samples with 1 mol L−1 NaOH were
incubated in a sealed glass container at 40 ◦C for 24 h, NH3 volatilized from the soil was trapped by
H3BO3 (pH 4.5), and the NH+4 in the H3BO3 was then determined by back-titration with standard
H2SO4 solution.