Different dynamics and controls of enzyme activities of leaf and root litter during decomposition

Litter enzyme dynamics are strongly shaped by litter, soil, and microbial attributes during decomposition, however, enzyme dynamics of leaf and root litter remains unresolved due to contrasting differences in rates and controls on leaf and root litter decomposition.

Herein, we conducted a 784-day field experiment to evaluate the relative importance of litter, alkaline soil, and microbial attributes to enzyme activities and their C:N:P stoichiometry of leaf and root litter during decomposition under subtropical land use change of China.

We found that only the C- and N-acquiring enzyme activities of shrub leaves were greater than those of wood and crop, and there was no significant difference in P-acquiring enzyme activity among the three species of leaves. Both the C- and P-acquiring enzyme activities of crop roots were significantly lower than those of afforested lands (i.e., woodland and shrubland). The N-acquiring activities of wood roots were significantly lower than those of shrub and crop. At the temporal dynamics, the C-, N-, and P-acquiring enzyme activities of the leaves decreased with mass loss, which was affected by the shift in litter nutrients (e.g., N and P) and soil moisture during decomposition. In contrast, the three enzyme activities of roots increased with mass loss, largely due to the increase in microbial biomass of bacteria regulated by litter stoichiometry. The enzymatic C:nutrient (N and P) ratios declined with mass loss, but the enzymatic P:N ratios remained relatively constant with mass loss during the leaf litter decomposition. Whereas, both of the enzymatic C:nutrient ratios and enzymatic P:N ratios decreased with mass during the root litter decomposition. Our results showed that the enzymatic C:N:P stoichiometry of decaying leaves and roots was predominantly predicted by microbial biomass and bacterial biomass, respectively.

Overall, we outlined the pattern of contrasting contributions of litter, soil, and microbial attributes to enzyme dynamics during decomposition, which provided a framework for better understanding litter C, N, and P dynamics in relation to microbial resource allocation strategy during decomposition.

Methods
We selected three sites with different land use types (i.e., woodland, shrubland, and cropland). At each study site, three independent 10 m × 10 m plots were selected. Each plot was located 100 m apart from one another. We conducted a 784-day in situ litter decomposition experiment, where litter from each species and site was incubated at their own site (e.g., crop litter was placed back into the cropland). In each woodland and shrubland, freshly fallen senesced leaves were collected from litter traps (1 m × 1 m). Roots (diameter ≤2 mm) were excavated using a root auger at a depth of 20 cm under each tree in the two plantations. For cropland, the leaves and roots (diameter ≤2 mm) of maize were collected after harvesting. Living roots were selected as the experimental materials based on color, luster, and elasticity. The sampled roots were placed in an incubator and transported to the laboratory. Adherent soil particles and extraneous organic material were gently removed from the root samples. Thereafter, all collected leaf and root litter samples were oven-dried (55 °C) in the laboratory. Subsequently, the leaf and root litters were sterilized at 121 °C for 20 min to ensure that the most likely import of the microbial community was through the soil rather than litter. Finally, all collected leaf and root litter samples were oven-dried (55 °C) in the laboratory for further experiments and analysis.

On October, 2016, a total of 5.0 g of oven-dried leaves and roots were placed in nylon bags (15 cm × 20 cm, with a 1-mm mesh size). A total of 450 litterbags prepared for this experiment (3 plots × 3 species × 2 litter types ×5 decomposition times × 5 replicates). After removing the floor litter (if any), leaf litterbags were fixed to the mineral soil surface. The root litterbags were placed in the soil at a depth of 10 cm at an angle of approximately 30° to the vertical. An additional set of samples from each litter was prepared for initial morphological traits and chemical analyses. After 77, 168, 265, 419, and 784 days of decomposition, we measured soil temperature and moisture in each plot with a portable instrument (SIN-TH8, SinoMeasure, China). At each decomposition time, five leaf and root litterbags were retrieved separately from each plot to ensure sufficient sample testing for all variables. Litterbags were transported to the laboratory where the exterior of the bags was brushed free of adhering soil. Two subsamples of litter were immediately frozen and stored at -20 °C until the analysis of enzyme activity and phospholipid fatty acid (PLFA); the remaining litters were carefully cleaned of mineral soil particles, living soil animals, and debris adhering to the litter materials, and then dried (55 °C for 48 h) in an oven before weighing. See Table S1 for a detailed description of replication statement.