Predominant effects of litter chemistry on lignin degradation in the early stage of leaf litter decomposition

Abstract

Lignin has long been considered a recalcitrant component of plant materials. Its degradation is critical for litter decomposition and the formation of soil organic matter. However, little is known about lignin degradation dynamics and its controlling factors during leaf litter decomposition, especially in forest ecosystems. The objective of this study was to evaluate lignin degradation dynamics during litter decomposition and its controlling factors. We conducted a lab incubation experiment for 180 days with leaves of two tree species (Fagus lucida and Schima parviflora) from a mid-subtropical forest in China. The CuO oxidation method was applied for lignin determination. The mass loss of bulk litter and lignin (including V-, S- and C-type phenols) and the dynamics of litter microbial community composition, carbon, nitrogen (N) and phosphorus (P) were determined during the incubation. Lignin degradation appeared to be a two-phase process over the incubation period, including a fast decomposition phase (early stage) followed by a relatively slow decomposition phase (late stage). Additionally, 21.9% and 18.5% of the initial lignin contents were lost from F. lucida and S. parviflora litter, respectively. The acid to aldehyde ratios of V-type phenols [(Ac/Al)V] and S-type phenols [(Ac/Al)S] of both litters increased with incubation. Lignin degradation was significantly affected by the litter types but not by the soil microbial communities, given that the soil microbial communities were distinctly different for the two plant communities. In the early stage, the mass loss of lignin from the F. lucida litter was faster than the S. parviflora litter: the mass loss of S-type phenols from the F. lucida litter was faster than the S. parviflora litter, whereas the dynamics of the V-type phenols were comparable for the two species. For F. lucida litter, lignin was lost faster than the bulk litter in the early stage but no difference was observed after day 133. For S. parviflora litter, the degradation of lignin and bulk litter were generally comparable over the incubation period. The mass loss of lignin in the early stage was significantly correlated with lignin, N and P contents, and the carbon/N, carbon/P and N/P ratios. Among these factors, P played a prominent role in controlling lignin degradation. The important role of litter chemistry emphasized the importance of taking this into account in plant-soil models assessing carbon dynamics in forest ecosystems. These results improve our conceptual understanding of lignin degradation and its controlling factors.