黒ボク土壌における土壌微生物バイオマスリンとインゲンマメのリン吸収量との関係解析と各種有機物のリン資材としての評価の試み

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  • Estimation of the Relationship between Soil Microbial Biomass Phosphorus and Phosphorus Uptake by Kidney Bean and the Evaluation of Organic Matter as a Phosphorus Source for Plant
  • クロ ボク ドジョウ ニ オケル ドジョウ ビセイブツ バイオマスリン ト インゲンマメ ノ リン キュウシュウリョウ ト ノ カンケイ カイセキ ト カクシュ ユウキブツ ノ リン シザイ ト シテ ノ ヒョウカ ノ ココロミ

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Andosols are widely distributed in upand fields in Japan and are characterized by a high phosphorus (P) retention capacity. Consequently, the efficiency of P fertilizer addition is low in Andosols and high rates of P fertilizer application are required. However, global P resources are becoming depleted, and excess accumulation of P in soil inhibits the uptake of trace element by plants, causes environmental pollution, and leads to outbreaks of soil-borne diseases. Available P in upland fields in Japan is usually estimated by using the Truog method, but the results do not correlate with plant P uptake in Andosols. Soil organic P is not usually regarded as available, but it is also an important P source for plants. P in microbes (biomass P) is particularly important, because P from dead microbes is readily released into the soil. Although biomass P is usually measured by the chloroform fumigation extraction method, this method is not suitable for analyzing biomass P in upland Andosol fields because the P released from dead microbes is adsorbed by reactive clay minerals, and therefore only a small proportion of the released P is extracted. We modified the chloroform fumigation extraction method by changing the ratio of soil to extract solution from 1 to 20 to 1 to 40. This modification increased P yield from the extraction and recovery rate of P added to the soil. We conclude that this method is suitable for measuring biomass P in Andosols. We conducted a field experiment on Andosol in Hokkaido to investigate the relationships of P uptake by kidney bean (Phaseolus vulgaris cv. Taisho-kintoki) with biomass P measured by our modified method and Truog P. The P content of shoots at harvest was significantly correlated (P<0.01) with biomass P but not with Truog P. Therefore, biomass P may serve as a reliable indicator of P availability in Andosols. However, studies have shown a positive correlation between biomass P and P uptake in only a few crop species, and field experiments are needed to test more species. The accuracy of biomass P as an indicator of P fertility may depend on various conditions such as soil inorganic P content, soil type, and growing season of the plants, and further investigation is required for each of these conditions. Applied organic matter can be a valuable source of P to plants. To estimate applied organic matter as a P source and identify the flow of P in soils, we analyzed P forms contained in a range of organic matter types and in soil to which the organic matter had been applied. Based on the results of this analysis, we classified the various types organic matter into four categories based on the P forms that were increased in the soil : (1) organic matter (including wheat straw, corn shoots and chaff, beet leaves, rice husks, chicken manure and bark compost) for which application increased organic P in the soil, (2) organic matter (including rice straw and both wheat straw and oats returned as green manure) for which application increased both organic P and inorganic P in the soil, (3) organic matter (only sheep manure compost) for which application increased inorganic P in the soil, and (4) organic matter (only sewage sludge compost) for which application increased available P in the soil. The main factors determining these outcomes by application of organic matter appeared to be P content, C : N ratio, and decomposition rate of organic matter. Biomass P was increased in soils to which a combination of wheat straw and oat green manure or to which sewage sludge compost was applied. Applied organic matter did not always increase biomass P in the soil. Differences in the ratios of C, N, and Pin the organic matter appeared to be the main reason for this occasional lack of response. Increasing biomass P may increase the efficiency of P fertilizer application, especially in soils with high P retention capacity such as Andosols. Therefore, future research should focus on how the ratios of C, N, and P in organic matter affect biomass P with a view to devising methods for organic matter application that will increase biomass P and hence the efficiency of fertilizer application.

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