Microbial activity during summer in humus layers under Pinus silvestris and Alnus incana

In this work we investigated changes in the originally similar humus layers under coniferous and deciduous trees after a 40-year growth period of trees. We studied whether the contrasting humus layers differed in microbial community structure, biomass and enzyme activity patterns. Microbial structure was analyzed as phospholipid fatty acid (PLFA) profile and activity as enzyme activity patterns including arylsulphatase, α-glucosidase, β-glucosidase, β-xylosidase, cellobiosidase, chitinase, phosphomonoesterase (PME, acid), phoshodiesterase (PDE), esterase, lipase and alanine- and leucine aminopeptidase (AP) activities. Samples from the humus layer were collected separately under Scots pine (Pinus silvestris) and alder (Alnus incana) at 2-week intervals. CO2 efflux was monitored continuously. The alder humus layer contained higher microbial biomass, estimated as the sum of PLFAs, and higher enzyme activities than the Scots pine humus layer. There were also differences in enzyme activity patterns between the two sites. In the humus layer under pines, all the enzyme activity potentials tended to increase when the humus layer temperature and CO2 efflux increased from the end of May to the end of July. The potential activities remained high in the autumn when the humus layer temperature decreased. Some of the enzyme activities under alder had minima but others maxima in the middle of summer when the humus layer was dry. The different patterns of enzyme activity maxima and CO2 efflux measured at the proximity of alder and pine trees are discussed, as well as the impact of root exudates and degradable litter.