Water stability of earthworm casts in manure- and inorganic-fertilizer amended agroecosystems influenced by age and depth

Summary Earthworm casts can contribute significantly to surface soil structure in agroecosystems. We suggest that the water stability of such earthworm casts depends on the earthworm food source, and the depth at which casts are made. We studied the effects of fertilizer inputs to cropping systems as earthworm feedstocks, and depth of cast placement on cast water stability and texture. We collected air-dried earthworm casts from manure and inorganic fertilizer-amended agroecosystems. Cast water-stability index and the cast texture were measured at 1, 2, 4, 8, and 16 weeks. We found that fertilizer treatments did not account for all of the variation in the measured water stability of earthworm casts. The placement of casts at 10 cm depth resulted in small changes in the water-stability index with time, while casts placed near the soil surface were degraded rapidly in terms of both water stability, and textural attributes. This was less marked in the inorganic treatment than in organic treatments. The differences in casts from different earthworm species, especially in terms of ecological groupings, may have affected the stability of the casts collected. Our results suggest that the form of fertilizer inputs can influence cast stability to some extent. However, cast placement in terms of depth, which is largely a function of earthworm ecological group, may be more important overall.