In deciduous forests, autumn leaf abscission causes a transfer of labile carbon (C) from trees to the forest floor. The initial decomposition of this labile C may represent a significant proportion of annual soil CO2 efflux, but the initial respiratory dynamics of freshly senesced litter have not been well characterized in the field. Laboratory studies of plant litter decay with controlled temperature and moisture have observed large peaks in respiration and microbial biomass during the initial stage of decay, but the extent to which this happens in the field is still largely unknown. Although laboratory studies have demonstrated that respiration and microbial biomass are dynamic and predictable, it is not clear that these dynamics occur under natural conditions. Here we explore what factors drive initial microbial respiration and biomass dynamics in litter decay in the field. We conducted a study at the Oak Openings Preserve in Northwest Ohio to examine the initial respiration dynamics of freshly senesced litter in five replicate plots. Each plot contained four soil collars in a 2x2 factorial design with added fresh Acer rubrum litter and added moisture, in which respiration was measured daily for 27 days. Upon collection, litter was immediately placed into the litter addition collars on the first day. Soil samples from additional replicate collars were collected on days 0, 3, 6, and 27 and analyzed for soil microbial biomass, dissolved organic carbon and nitrogen, and inorganic nitrogen.
Results/Conclusions
In contrast to the consistent patterns observed in laboratory incubations, we observed wide fluctuations in field respiration rates. Respiration rates in the field peaked three times during the experiment, coinciding with warmer air temperatures. The first respiration peaks occurred on days 2-3, the second respiration peak occurred on day 8, and the final respiration peak was on day 21. While litter respiration was positively correlated with temperature, added moisture generally had no effect, possibly due to frequent precipitation events during the experiment (total accumulation of 17.17 cm over the 27 days). These results differ from laboratory incubation studies of litter decay that concluded that microbial respiration is predictable, peaks in the first week, and increase with moisture addition. We conclude, the predictability of microbial respiration seen in laboratory experiments during the initial stage of leaf litter decomposition does not directly apply to field results without consideration of temperature as the major driving factor.