Sampling and analysis of plant materials
At harvest, all rice plants within the microplots were collected
and the fresh weights were recorded for roots, straw, and grain
individually. The roots were sampled to 15-cm depth within the
rice rows by a steel tube (5-cm diameter) and washed with distilled
water to remove soil. Plant material was oven-dried at 70 C to
constant weight, powdered (mesh size of 0.1 cm) by a hammer mill,
and stored dry in sterilized plastic bags for later analysis of total N
and 15N content using the same process as for soil (see above). Plant
agronomic indices (Guindo et al., 1994) were calculated as follows: