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Dynamics of nitrogen associated to acid insoluble substances derived from plant litter

A study was conducted on three 15N-labeled litter types (i.e., straw, stripe maple, and American beech) to determine the effects of three years incubation and NH4 +-N amendments [10% of total nitrogen (N)] on acid insoluble substances (AIS), and on the N content of AIS (i.e., AIS-N). Based on C:N ratios, all litter types lost considerable mass after three years incubation, however, AIS concentrations increased in only two of the litter types. This suggests that AIS of different litter types decomposed at different rates. The NH4 +-N amendments followed by a 21-day conditioning period did not affect AIS concentrations. The AIS-N concentration of fresh litters were much higher than expected suggesting much more diverse substrates than lignin only. Three years incubation significantly increased AIS-N concentrations of all litter types, probably the result of recalcitrant microbial by-products that accumulated during decomposition. The NH4 +-N amendments significantly increased AIS-N concentrations of both incubated and non-incubated litters after 21 days conditioning, and these increases corresponded to 20%–69% of added NH4 +-N. Analyses of changes in AIS-15N pools revealed, however, that only a small portion of the added NH4 +-N was actually transferred to the AIS-N pool. Therefore, the increase in the AIS-N pool due to the addition of NH4 +-N solution was the result of pool substitution or displacement reactions between the added NH4 +-N and other non-AIS N pools. We conclude that AIS-N pools can be subdivided into classes of compounds according to origin and stability.