Microbial functional responses to forest litter composition and biodiversity

Background/Question/Methods

The response of soil microbial decomposition to litter from different tree species, individually or over biodiversity gradients, is an important issue for understanding soil nutrient cycling and carbon storage. The objective of this research was to measure extra-cellular enzyme activities and nitrogen cycling rates in relation to a forest litter decomposition experiment. This experiment, part of the Biodiversity and Ecosystem Function-China project, was composed of two sub-experiments in subtropical China containing either ¹⁵N and Li labeled litter (labeled experiment) or unlabeled litter (unlabeled experiment). In the unlabeled experiment litter was added to soil mesocosms (plastic collars) in 1,2, and 4 species mixtures. In the labeled experiment litter was added to mesocosms in 1,2, or 3 species mixtures. Soil and litter samples were then taken at four time-points: 12d, 37d, 101, and 171d  (labeled experiment) and 21d, 49d, 105d, and 168d (unlabeled experiment). From each soil sample (0-5 cm) we measured the activity of 2 oxidative and 4 hydrolytic enzymes, as well as gross N mineralization and nitrification. We also measured total microbial biomass from the labeled experiment to determine microbial ¹⁵N and Li uptake. We then compared our results to litter decomposition rates from both experiments.

Results/Conclusions

We observed higher enzyme activities at the first two sample dates in spring, which we expected because nutrient cycling rates are generally seasonally higher then in our ecosystem. Between both experiments we found that b-glucosidase, phosphatase, and xylosidase were more responsive to different litter compositions than N-acetyl-glucosaminidase activities. We also found that while some species or species combinations altered enzyme activity and that there was a trend of higher activity in the 4 species mixture (unlabeled experiment) at the first two time-points. However in general there were no clear diversity effects. In contrast, monocultures had faster litter decomposition rates in the labeled experiment. We also observed between the experiments that microsite, elevation, slope, and other factors can alter decomposition rates and enzyme activities and these differences could alter treatment effects. In summary, in ecosystems where decomposition occurs very rapidly it could be difficult to determine the microbial role in decomposition other than in the first weeks after litter fall when enzyme substrates are readily available.