Seasonal variation in casting activity and in the amounts and release to solution of phosphorus forms in earthworm casts

Abstract

Production of surface casts of earthworms (dominantly Allolobophora caliginosa) varied markedly during 1975. Casting activity was restricted to the April to September period and reached a maximum in early June. Seasonal variations in cast production were attributed to fluctuations in soil moisture and temperature, and in food supply. Although the total P content of casts was essentially constant throughout the year, the inorganic P content decreased from a maximum in May to a minimum in August. The progressive decrease in the amount of inorganic P between May and August is attributed to a reduction in the conversion of organic to inorganic P because of the lower microbial and phosphatase enzyme activity as a result of declining soil temperature. Seasonal variations in the release of inorganic P from casts to solution and in the amounts of rapidly-exchangeable P were more closely related to the seasonal pattern of cast production than to changes in the amount of inorganic P in the casts. In contrast, seasonal changes in the release of inorganic P to solution and in the amount of rapidly-exchangeable P in underlying soil were small. A greater amount of inorganic P was extracted by 0.5 m NaHCO 3 than by water from freshly-deposited casts collected at different times of the year, indicating that NaHCO 3 extracts some chemisorbed inorganic P from casts. Water-extractable inorganic P accounted for approximately 90% of the difference between inorganic P in casts and underlying soil. Consequently, essentially all of the additional inorganic P in casts is retained by a more-physical sorption type. The results obtained are discussed in terms of the likely effects of surface casting on the P cycle in a soil-plant system.