Leaf litter decomposition process in forest ecosystem: A comparison of methods

Background/Question/Methods

Above ground litter decomposition is the result of three interlinked processes: leaching, fragmentation and catabolism. However there is a discrepancy between this understanding and how decomposition is measured. Most studies measure decomposition as the progressive loss of mass from litter residues, confined in mesh bags. Thus, they provide a rough estimate of leaching and catabolism, while preventing fragmentation to occur. Alternatively, litter decomposition is studied as microbial respiration on litter residue. In this case, only catabolism is measured.   
This study aims to compare five different methods to determine mean residence time (MRT) of leaf litter, in forest ecosystems. Additionally, the methods performance at capturing the effects of water manipulation treatments on litter decomposition is evaluated. The methods compared are: 1) litter bag; 2) litter Input/standing litter Pool (IP); 3) ¹⁴C-bomb spike; 4) litter respiration; 5) Day-cent model. Decay rates were measured either in the field (1-3) or in the lab (4), using the same Arbutus unedo leaf litter, and the study site data were used to initialize the model (5). The site is a Mediterranean woodland, where a throughfall manipulation is in place since 2004. Litter decomposition was determined under control, wet (summer irrigation), and dry (exclusion of 20% of  throughfall) conditions.

Results/Conclusions

A part from the litter bag method, the other field-based, non intrusive, methods gave comparable results, with leaf litter C MRT being 1.68 a^-1, 1.70 a^-1 respectively, for the IP and the ¹⁴C bomb spike. Similarly, Daycent model estimate of litter C MRT was 1.78 a^-1. Additionally, these methods appreciated the effect of the water treatments, with higher MRT being measured in the dry, as compared to the control and wet treatments. The litter respiration, measuring only the microbially-mediated breakdown of litter, overestimated its MRT, when respiration data were extrapolated to the field. Single exponential model best fit mass loss data by the litter bag method, reaching an asymptotic value at about 50% of the original mass. Thus, half of the litter would remain undecomposed on the forest floor. We believe that this is due to mesh bags preventing physical and biological break-down of litter, and the subsequent incorporation of litter fragments into the soil organic matter (SOM). Litter fragments input belowground promotes aggregation and SOM formation, and they are likely to decompose at a different speed than aboveground. Current global synthesis work on leaf litter decomposition may need to be revisited in light of this finding.