Studies on the plant debris in rice paddy soils

Abstract

Microscopic observations, chemical analyses, and measurements of the numbers of microbes in several fractions, separated according to the differences in size and specific gravity from the plough layer of paddy soils, were carried out. 1) Plant debris contained in paddy soil consisted of various particles different in size, shape, color, extent of decomposition, and content of adhering mineral particles. With the progress of decomposition, plant debris became smaller in size and darker in color, and the number of mineral particles adhering to it increased. The carbon content and C/N ratio in plant debris decreased with the decrease in particle size. 2) Numbers of Microbes (bacteria, actinomycetes and fungi) in various fractions of plant debris tended to decrease with the decrease in particle size. This corresponds well to the degree of humification of the plant debris fractions as judged by morphological characteristics and C/N ratios in various fractions of plant debris. Numbers of microbes in the clay fraction were much greater than in the silt fraction, and they were nearly equal to the microbial numbers in the fine plant debris of Napno soil and the coarse plant debris of Yamanashi soil. 3) Carbon content and numbers of microbes in various fractions of plant debris were found to have a high correlation. 4) The plant debris fractions (>0.037 mm) were 1.8–2.8% of the dry weight of the soil. 28–35% of total carbon, and 16–22% of total nitrogen in the soils. The number of microbes in the plant debris fractions amounted to 13–17% of total bacteria. 17–21% of total actinomycetes, and 21–25% of total fungi found in the soil.