Enzymes in Forest Soils

Contrasting distribution of enzyme activities in the rhizosphere of European beech and Norway spruce

Recent policies and silvicultural management call for forest regeneration that involve the selection of tree species able to cope with low soil nutrient availability in forest ecosystems. Understanding the impact of different tree species on the rhizosphere processes (e.g., enzyme activities) involved in nutrient mobilisation is critical in selecting suitable species to adapt forests to environmental change. Here, we visualised and investigated the rhizosphere distribution of enzyme activities (cellobiohydrolase, leucine-aminopeptidase, and acid phosphomonoesterase) using zymography. We related the distribution of enzyme activities to the seedling root morphological traits of European beech (Fagus sylvatica) and Norway spruce (Picea abies), the two most cultivated temperate tree species that employ contrasting strategies in soil nutrient acquisition. We found that spruce showed a higher morphological heterogeneity along the roots than beech, resulting in a more robust relationship between rhizoplane-associated enzyme activities and the longitudinal distance from the root apex. The rhizoplane enzyme activities decreased in spruce and increased in beech with the distance from the root apex over a power-law equation. Spruce revealed broader rhizosphere extents of all three enzymes, but only acid phosphomonoesterase activity was higher compared with beech. This latter result was determined by a larger root system found in beech compared with spruce that enhanced cellobiohydrolase and leucine-aminopeptidase activities. The root hair zone and hair lengths were significant variables determining the distribution of enzyme activities in the rhizosphere. Our findings indicate that spruce has a more substantial influence on rhizosphere enzyme production and diffusion than beech, enabling spruce to better mobilise nutrients from organic sources in heterogeneous forest soils.

Distribution of Soil Extracellular Enzymatic, Microbial, and Biological Functions in the C and N-Cycle Pathways Along a Forest Altitudinal Gradient

The diverse chemical, biological, and microbial properties of litter and organic matter (OM) in forest soil along an altitudinal gradient are potentially important for nutrient cycling. In the present study, we sought to evaluate soil chemical, biological, microbial, and enzymatic characteristics at four altitude levels (0, 500, 1,000, and 1,500 m) in northern Iran to characterize nutrient cycling in forest soils. The results showed that carbon (C) and nitrogen (N) turnover changed with altitude along with microbial properties and enzyme activity. At the lowest altitude with mixed forest and no beech trees, the higher content of N in litter and soil, higher pH and microbial biomass nitrogen (MBN), and the greater activities of aminopeptidases affected soil N cycling. At elevations above 1,000 m, where beech is the dominant tree species, the higher activities of cellobiohydrolase, arylsulfatase, β-xylosidase, β-galactosidase, endoglucanase, endoxylanase, and manganese peroxidase (MnP) coincided with higher basal respiration (BR), substrate-induced respiration (SIR), and microbial biomass carbon (MBC) and thus favored conditions for microbial entropy and C turnover. The low N content and high C/N ratio at 500-m altitude were associated with the lowest microbial and enzyme activities. Our results support the view that the plain forest with mixed trees (without beech) had higher litter quality and soil fertility, while forest dominated by beech trees had the potential to store higher C and can potentially better mitigate global warming.

Labile organic carbon pools and enzyme activities of Pinus massoniana plantation soil as affected by understory vegetation removal and thinning

The effects of forest management on carbon (C) sequestration are poorly understood, particularly in the Three Gorges Reservoir area. We aimed to identify the effects of forest management on C sequestration in Pinus massoniana plantations. An intact control forest (CK), a site undergoing regular shrub cutting with the simultaneous removal of residues (SC), a site under low-intensity thinning (LIT), and a site under high-intensity thinning (HIT) were compared for soil labile organic carbon (LOC), related enzyme activities, and soil characteristics. Soil organic carbon (SOC) significantly decreased in the HIT treatment as compared with that in the CK treatment. Soil EOC, DOC, MBC contents in treated plots were higher than those in the CK treatment; particularly, the HIT treatment significantly increased those values in 0–10 cm layer. Thinning resulted in a decrease in cellulase and amylase activities, but an increase in invertase activity. In addition, the SOC content was significantly correlated with four enzymes activities and LOC components, which suggested that the soil LOC components and enzymes activities were sensitive to the changes of SOC. Our results suggest that high-intensity thinning treatment in Pinus massoniana plantation could significantly decrease the SOC content and lead to an increase of LOC components.

Functional profiling and distribution of the forest soil bacterial communities along the soil mycorrhizosphere continuum

An ectomycorrhiza is a multitrophic association between a tree root, an ectomycorrhizal fungus, free-living fungi and the associated bacterial communities. Enzymatic activities of ectomycorrhizal root tips are therefore result of the contribution from different partners of the symbiotic organ. However, the functional potential of the fungus-associated bacterial communities remains unknown. In this study, a collection of 80 bacterial strains randomly selected and isolated from a soil-ectomycorrhiza continuum (oak-Scleroderma citrinum ectomycorrhizas, the ectomycorrhizosphere and the surrounding bulk soil) were characterized. All the bacterial isolates were identified by partial 16S rRNA gene sequences as members of the genera Burkholderia, Collimonas, Dyella, Mesorhizobium, Pseudomonas, Rhizobium and Sphingomonas. The bacterial strains were then assayed for β-xylosidase, β-glucosidase, N-acetyl-hexosaminidase, β-glucuronidase, cellobiohydrolase, phosphomonoesterase, leucine-aminopeptidase and laccase activities, chitin solubilization and auxin production. Using these bioassays, we demonstrated significant differences in the functional distribution of the bacterial communities living in the different compartments of the soil-ectomycorrhiza continuum. The surrounding bulk soil was significantly enriched in bacterial isolates capable of hydrolysing cellobiose and N-acetylglucosamine. In contrast, the ectomycorrhizosphere appeared significantly enriched in bacterial isolates capable of hydrolysing glucopyranoside and chitin. Notably, chitinase and laccase activities were found only in bacterial isolates belonging to the Collimonas and Pseudomonas genera. Overall, the results suggest that the ectomycorrhizal fungi favour specific bacterial communities with contrasting functional characteristics from the surrounding soil.