農作物

It has been reported by several workers that a marked decrease in concentration of carbon dioxide was frequently observed in surrounding atmosphere of crops at later growing stage. This might cause a great decrease in the rate of photosynthesis and the production of dry matter in the crops. Especially, in the case of the crops which have higher rate of photorespiration such as rice and wheat, this effect will be much serious. In a practical farm in particular, where crops are fed on sufficient mineral nutrients to get high yield, it may be much important to keep the concentration of carbon dioxide in the surrounding atmosphere of the crops sufficiently high through decomposition of soil organic matter and then to keep the rate of photosynthesis of the crops much higher. It is well known that drying a soil and rewetting it results in a flush of decomposition of soil organic matter and a flush of ammonium and then of nitrate. According to the recent findings of T. Harada et at., besides the effect of previously drying a soil, flush of decomposition of soil organic matter occurs through the effects of the following pretreatments of soils, too: rising temperature, changing pH, treating physically, adding with neutral salts, agricultural chemicals and chelating agents, touching with plant roots, etc. Further, they found that with regard to the magnitude of the flush of decomposition of soil organic matter, the effects of the above-mentioned pretreatments of soils are parallel with the effect of previously drying a soil. It is sure that the fraction of the organic matter becoming decomposable through the effect of the above-mentioned pretreatments of soils including previously drying a soil are directly concerned with the supplement of carbon dioxide to atmosphere of fields and the naturally supplied nitrogen to crops. The purpose of this investigation is to make clear the relationship of the accumulation of this organic matter fraction in soils to the soil properties and the environmental conditions such as temperature, moisture, kind of organic matter added and form of nitrogen fed, etc., by means of the determination of the effect of drying a soil. The study presented here forms one part of this investigation. In this paper, the methods for the determinations of soil organic carbon employing coulombmeter, carbon dioxide evolved and the organic matter becoming decomposable through the effect of drying a soil were reported in the first place, and then correlations between mineralization of the straw carbon added and/or accumulation of the organic matter becoming decomposable through the effect of drying and soil properties were described. Design of the experiment to obtain information about the correlations mentioned above was as follows: Rice straw powder was added to soils different in kinds of main clay minerals, texture, cation exchange capacity (CEC) and the contents of carbon, free iron oxide, clay and exchangeable nitrogen. After adjusting the moisture contents to 60% of the maximum water holding capacities, the soils were incubated at 30℃ during the periods of 178 days. Mineralization amount of the straw carbon was determined periodically. After 63 and 164 day incubations, the accumulated amount of the organic matter becoming decomposable through the effect of drying each soil was determined. All of the experiments in this investigation were carried out in the laboratory, by addition with carboneceous materials such as straw to soils of which a large portion of the organic matter were destroyed with 10% hydrogen peroxide and removed previously. The soil properties deeply related with mineralization of the straw carbon and accumulation of organic matter becoming decomposable through the effect of drying a soil were determined statistically. The outline of the results obtained were as follows. Rate of the mineralization of the straw carbon was correlated with the contents of free iron oxide, exchangeable nitrogen, clay and CEC, and the chief properties affecting the mineralization were contents of nitrogen and free iron oxide. To obtain more detailed information on the effects of nitrogen and free iron oxide on the mineralization of the straw carbon, the effects of addition of ammonium nitrogen and iron hydrated oxide after Mackengie's method were determined. The results obtained from these experiments showed that at the beginning of the incubation addition of the iron hydrated oxide led to a decrease of the mineralization rate of the straw carbon, but at the stage that the mineralization rate was the highest this led to an increase of the mineralization rate. Addition of ammonium nitrogen resulted in an increase of the mineralization rate of the straw carbon, but the mineralization rate was not in linear proportion to the added amount of ammonium nitrogen, Accumulation of the organic matter becoming decomposable through the effect of drying was correlated with the contents of carbon and exchangeable nitrogen in the soils. To obtain more detailed information on the effect of available nitrogen to microbe on accumulation of the organic matter becoming decomposable through the effect of drying, the effect of addition of ammonium nitrogen was determined. The results showed that addition of ammonium nitrogen led to an increase of the amount of the organic matter becoming decomposable through the effect of drying and the amount was in linear proportion to the added amount of ammonium nitrogen. Furthermore, it was observed that larger amount of the organic matter becoming decomposable through the effect of drying was accumulated in the volcanic ash soils than in the other soils, and the accumulated amount of this organic matter in each soil was altered with the incubation periods.