Relationships between Aggregation and Hyphal Length and Microbial Biomass C in Soil Amended with Rice Straw or Azolla

抄録

The influence of the addition of rice straw (Oryza sativa) or azolla (Azolla pinnata) to soil on soil aggregation, hyphal length, and microbial biomass C was compared using four different systems. RS 1 and AZ 1 systems contained smaller amounts (2g of C kg^<-1> soil) of rice straw and azolla, respectively. RS 2 and AZ 2 systems contained larger amounts (6 g of C kg^<-1> soil) of rice straw and azolla, respectively. These plant materials were amended with soil, and the mixtures were incubated at 30℃ for 3 and 28 weeks under upland conditions. The amendment with plant material of rice straw or azolla increased the amounts of the >2.0 mm aggregates and microbial biomass C and the hyphal length to a larger extent compared to the control system (without plant materials). These degrees of increases varied depending on the kind and amount of the plant materials amended and the incubation periods, and tended to be much higher for 28 weeks than for 3 weeks incubation. For example, after 28 weeks incubation, the amount of the >2.0 mm aggregates was usually 13 times larger in the RS 2 and AZ 2 systems, compared to the control system. In the rice straw and azolla systems after 3 weeks incubation, the increasing patterns of the amount of the >2.0 mm aggregates were generally similar to those of the hyphal length and the amount of microbial biomass C. On the other hand, in the rice straw systems after 28 weeks incubation, the increasing patterns of the amount of the >2.0 mm aggregates were similar to those of the microbial biomass C. Whereas, in the azolla systems, the increasing patterns of the >2.0 mm aggregates were similar to those of the hyphal length. These findings seem to suggest that during long term incubation in the rice straw systems, microbial biomass played more important role than hyphae in the formation of the >2.0 mm aggregates. In contrast, in the azolla systems, the hyphae played more important role in the formation of the >2.0 mm aggregates. These may be due to the differences of the amounts of carbohydrate and C/N ratio between the rice straw and azolla. The amendments of rice straw or azolla tended to increase the amounts of the 0.1-0.25 mm and 0.25-0.5 mm aggregates. In contrast, these amendments decreased the amounts of the 0.5-1.0 mm and 1.0-2.0 mm aggregates to some extent. In the RS 2 and AZ 2 systems after 28 weeks incubation, the amount of the <0.1 mm aggregates decreased to a large extent.