Atmospheric nitrogen deposition and soil fauna: The effect on litter decay process

Background/Question/Methods

During the next century, atmospheric nitrogen (N) deposition is projected to more than double, potentially slowing litter decomposition by altering the composition of soil biota and their functions in forest floor. In a previous study, long -term simulated N deposition (30 kg N^-1 ha^-1 yr^-1 plus ambient, since 1994) in a sugar maple-dominated forest has been found to significantly reduce the density of microarthropods in the forest floor.  In order to determine whether such decline in microarthropods has contributed to further inhibit litter decomposition, microcosm experiments were established with two types of litter (ambient and simulated N deposition litter) and two types of microarthropods community (ambient fauna extracted from ambient litter and fauna extracted from simulated N deposition litter) with 30 replicates for each treatment (120 microcosms in total). All the microcosms were incubated in mason jars, and headspace gas was analyzed for CO₂ concentration twice weekly during the first 40 days and once a week for the remaining of the study. We predicted that litter from the simulated N deposition treatment, together with fauna from that treatment, would show the lowest respiration rate. 

Results/Conclusions

After 96 days, CO₂ accumulation for each microcosm was fit into an exponential equation model A_t= A₀ (1- e^-kt) and the two parameters A₀ and k were estimated. A₀ indicates the total labile organic substrates potentially decayed by the biological community in the microcosms; and k is the first-order rate constant. Two-way ANOVA indicated that the respirations rates of the microcosms were significantly affected by the litter types, with a lower k value for litter from the simulated N deposition treatment (P = 0.003). This is consistent with previous finding that litter decomposition was inhibited under chronic N deposition. In contrast, there was a significant fauna by litter interaction on A₀ (P = 0.045), indicating that ambient fauna can potentially degrade more labile substrates when given with ambient litter, whereas fauna from the simulated N deposition treatment can potentially degrade more labile substrates when given litter from the simulated N deposition treatment.  These observations did not support our hypothesis.  Rather, they indicate that the effect of soil fauna on litter decomposition depends on litter origin, suggest that soil fauna have acclimated to changes in microbial community composition and function under simulated N deposition. Further analysis of the microbial and microarthropod community in the microcosms will help to elucidate such mechanism.