Sample collection and analysis
Plant samples were collected every 10 days after rice transplanted. N, P, and K contents after drying were weighed up and measured . The soil samples were also collected at the beginning of the trial (1998) and soil organic matter content was measured 5 years later (2002). Total N, total P, and total K were determined by digestion of the plant samples with H2SO4 and H2O2. For all samples, the concentrations of N and P were determined colorimetrically by the molybdenum-blue and indophenol-blue methods, while total K was determined by the flame atomic absorption spectrometry