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Mineral nitrogen dynamics in the casts of epigeic earthworms (Metaphire hilgendorfi: Megascolecidae)

Quantitative evaluation of earthworm activity and temporal dynamics of cast stability are necessary to understand the importance of nutrient cycling in soil ecosystem. To address how earthworm activity influences soil aggregation and mineral nitrogen (N), cast production rate of Metaphire hilgendorfi and N mineralization rate in the casts were monitored for 56 days under laboratory conditions. The casting rate was 0.79 g dry weight (DW) g−1 earthworm fresh weight (FW) day−1 and 92% of the casts were water-stable within 24 h of excretion. The casts within three days of excretion contained 333.8 mg ammonium-nitrogen ( -N) kg−1 and 11.1 mg nitrate-nitrogen ( -N) kg−1, which were 69.8 and 0.95 times higher than those of the soil, respectively. M. hilgendorfi excreted 270 µg N g−1 earthworm FW day−1 through their casting activity. Soil respiration was active in the first two weeks, whereas dissolved organic carbon in casts did not change during aging. Cast -N was nitrified rapidly with subsequent decrease in pH value from 6.5 to 5. Nitrification stopped at 28 days when half of the -N was nitrified. Gentle crushing of casts did not significantly influence respiration nor N mineralization. Therefore microbial access to neither carbon nor N was responsible for the nitrification cessation, but pH appeared to contribute. Dynamic changes in mineral N was observed during cast aging.