『Abstract
The role of soil phosphorus (P) in the eutrophication of fresh
water systems is well established. It is crucial therefore to
assess the potential loss of P from soil in the various scenarios
where soil can come into contact with water. To date, such assessment
has often been based on soil P tests that are used for agronomic
purposes (e.g. fertilizer recommendations). The purpose of this
work was to examine the usefulness of one such test (viz. the
Olsen test, which is based on extraction with bicarbonate) for
predicting not only the amount of soil P available to plants,
but also that which can be desorbed to water in a group of 32
Portuguese soils, of which 29 were acid and 3 calcareous. To this
end, we (i) assessed the total amount of phytoavailable P in soil
by successively pot-cropping Chinese cabbage, buckwheat and rye;
and (ii) measured the amount of phosphate-P desorbed to a dilute
electrolyte mimicking fresh water over periods of up to 218 days
at soil:solution ratios of 1:100, 1;1000 and 1:10000. Total phytoavailable
P and Olsen P were found to bear a quadratic relationship, with
Olsen's extractant underestimating the content in phytoavailable
P of soils with high Olsen P contents relatively to soils with
low contents. The “change point” at which phytoavailable P began
to increase rapidly per unit change in Olsen P was 53 mg Olsen
P kg-1 soil. For the acid soils, a significant quadratic
relationship was found between the amount of P desorbed to water
and Olsen P at the three soil:solution ratios studied. However,
these relationships became less significant when only the soils
with an Olsen P value of less than 50 mg kg-1 were
considered. For the acid soils, the change point at which P input
to water began to increase rapidly per unit change in Olsen P
was 20, 61 and 57 mg kg-1 at the 1:100, 1:1000 and
1:10000 ratio, respectively. At comparable Olsen P values, the
calcareous soils released more phosphate to water than the acid
soils. On the basis of our results, we suggest the following environmental
threshold values for Olsen P in acid soils: 20 mg kg-1
for P desorption scenarios where the soil:solution ratio is high
(e.g. drainage water) and 50 mg kg-1 for desorption
scenarios where the soil:solution ratio is low (e.g., runoff,
water in reservoirs). Both values are higher than the agronomic
threshold above which plants are well supplied with P.
Keywords: Acid soils; Eutrophication; Olsen P; Phosphate; Phosphorus;
Soil P test』
Introduction
Materials and methods
Soil sampling and analysis
Pot cropping experiment
Experiments of desorption of P to a dilute electrolyte
Statistical analyses
Results and discussion
Soil properties
Plant P uptake in relation to Olsen P
Relationship between the amount of P released to water and Olsen
P
The Olsen P method as an agronomic and environmental soil P test
Acknowledgements
References
※リンの分析は、Olsen and Sommers(1982)などによる。