Our research subjects in FY 1996 were, (1)^*carbon balance in paddy fields, (2) soil cellulase activity, (3) soil resource evaluation for environmental load capacity, (4)^*clay mineralogical classification of lowland soils, (5) soil information system, (6) dynamic behavior of heavy metals and herbicides in soil, (7)^*plant hormonal substances contained in manure, (8) estimation of deep percolation rate, (9)^*application of the TDR method for volcanic ash soils, (10)^*roles of root exudates and cell wall functions in the uptake of Fe and Al bound soil P, (11) plant uptake ability of organic N, (12)^*soil erosion estimation method on fields using ¹³⁷Cs, (13)^*influences by rising sea level on agricultural production, and (14) acid buffering capacity of soils. Research subjects with ^* were nominated as NIAES's major research activities in FY 1996, and fourteen original papers were published from the Division.

Topic1

Carbon Cycle in Paddy Field Ecosystem

The agricultural sector is considered to be one of the main anthropogenic sources of greenhouse gas (GHG) emission. A quantitative study on the carbon cycle was carried out for three years at a paddy field in Tsukuba where rice was planted yearly from May to September.

In a paddy field ecosystem, carbon contents of the solid phase (soil and rice plant), aqueous phase (flooded water, irrigation water and percolated water) and gaseous phase (carbon dioxide and methane) were analyzed. In measuring gas flux by the closed chamber method, CO₂ flux was measured under light and shaded conditions. Under light conditions, and 30kg as photosynthesis and respiration progressed simultaneously, while under shaded condition, only respiration progressed. By subtracting the flux in dark from the flux in light condition, input from photo-synthesis and output from respiration were calculated separately, and the following results were obtained. The carbon contents of the plow layer soil and the rice plant produced were estimated to be 4600 and 650 kg/10a,respectively.In the rice plant, carbon was distributed throughout paddy (233kg), straw (263kg), stubble (136kg), and tiller (16kg) per 10a after harvest. Yearly input of C to the paddy field ecosystem was estimated to be 1300 kg/10a as gaseous C (CO₂ fixation) and 10 kg/10a as aqueous C (irrigation and precipitation), respectively. Yearly output of C per 10a was estimated to be 600kg as plant respiration, 100kg as soil respiration, 10kg as methane emission and 30kg as percolation of flooded water. Consequently, net CO₂ fixation was calculated to be 700 kg/10a/year. In 700kg, 650kg was fixed in rice plants and 30kg was leached to underground water. Soil organic matter is known to be a major pool of carbon within the biosphere, about 1400 × 10¹⁵ g globally, roughly twice that in atmospheric CO₂. Because even a small change in C content in soil could affect atmospheric CO₂ concentration, further monitoring of C content change is important to predict global climate change in the future. (Fig.1)

Topic2

Conservation of Rice Terraces

Rice terraces are distributed on sloping lands in many east and southeast Asian countries. In Japan,the area of rice terraces on slopes steeper than 1/20 was 419,000 ha in 1983 and it decreased to 363,000 ha in 1994 due to abandonment. Most of the rice terraces on steep slopes are becoming marginal lands, because of the small and irregular lots and poor access roads obliging intensive labor for cultivation. (Fig.2)

Failure of the terrace walls after the abandonment, and the diversion to other crops, were strongly influenced by soil type and water management as shown in Table 1. These aspects, as well as socioeconomic conditions, must be considered to keep rice terraces for food security and environmental conservation of the country.

Topic3

Cell Wall Solubilize Soil-P: New Aspect in Nutrient Uptake by Plant Root

Groundnut showed a superior ability to take up P from a soil with low P fertility compared with sorghum and soybean. This ability was not related to its better root development or production of root exudates capable of solubilizing iron- and aluminum-bound P.

Therefore, P absorption by a direct contact reaction between the root surface and soil particles was presumed. The cell wall prepared from fresh roots of groundnut grown on sand showed higher P solubilizing activity than those of sorghum and soybean (Table 2). This finding corresponds well with observations in fields and soil pot experiments using a soil with low P fertility. The reaction site of P solubilizing activity was regarded as stable against heating and enzyme digestions. These findings are probably the first evidence to demonstrate that cell walls of plant root are involved in P solubilizing activity.

Fig.1 Carbon balance of paddy field ecosystem (kg C/10a/yaer)

Fig.2 Rice Terrace

Table.1 Rice terrace management and soil type

Table.2 Effects of root cell wall on solubility of sparingly soluble phosphorus