Spatiotemporal characteristics of enzymatic hotspots in subtropical forests: In situ evidence from 2D zymography images

Abstract

Soil enzymes play a central role in organic matter decomposition and nutrient cycling in forest ecosystems. However, the spatiotemporal characteristics of in situ enzyme activities in heterogeneous soil and their drivers are still poorly understood. We applied soil zymography in the field to visualize the seasonal dynamics of enzyme activity along soil profiles (28 cm depth) in Quercus variabilis (oak) and Pinus massoniana (pine) pure and mixed forests in southern China. Temporally, the enzymatic hotspot areas were largest in summer (21.9% ± 7.1%), smallest in autumn (6.1% ± 4.9%), and medium-sized in spring (13.4% ± 5.9%) and winter (12.5% ± 7.7%) in all forest soils. The seasonal characteristics were attributed to changes in soil temperature and moisture. Spatially, the enzymatic hotspot areas were the largest at 0–10 cm (6.7% ± 3.8%), followed by 10–20 cm (5.1% ± 3.9%) and 20–28 cm (2.6% ± 2.3%) in all forest soils. In addition, the enzymatic hotspot areas of mixed forest (6.5–33.3%) were larger than those of pure oak (1.3–28.5%) and pine (0.2–22.8%) forests. This positive effect of mixed forest on enzymatic hotspot areas was mainly attributed to the higher microbial biomass and total soil faunal abundance than those in pure forests, especially under dry and cold climate conditions. These results suggest that the negative effects of drought and cold on enzymatic hotspot areas could be mitigated in the mixed forest. This study provided the first in situ evidence of the seasonal dynamics of enzymatic hotspots in forest soil profiles, which are of paramount importance for understanding nutrient cycling and ecosystem functioning.