Soil organic matter in soil physical fractions in adjacent semi-natural and cultivated stands in temperate Atlantic forests

Changes from natural tree species to rapidly growing exotic species as well as intensification of forestry operations with heavy machinery can lead to changes in the quantity and quality of organic matter inputs to soil and to disruption of soil physical structure. These two ecosystem properties are tightly linked to organic matter dynamics. Five adjacent forest stands were selected to study soil organic matter dynamics in soil physical fractions. On one hand, two semi-natural broadleaved forests (Quercus robur, Fagus sylvatica) and an adult radiata pine plantation (40-year-old,) in order to study the effect of species change on these parameters, and on the other, a chronosequence of Pinus radiata plantations (40-year-old; 3-year-old; 16-year-old), to study the effect of mechanization during harvesting and intense site preparation. Samples of intact topsoil (0–5 cm) were collected and aggregate-size distribution, mean weight diameter (MWD), total C and N, particulate organic matter (POM)-C, POM-N and microbial biomass-C were determined in each aggregate size fraction. Microbial respiration and nitrogen mineralization were also assessed in each aggregate size fraction, during a 28 day incubation period. of POM-C and POM-N in the bulk soil due to mechanical site preparation were high relative to total soil C and N, which suggests that POM is a sensitive parameter to the effect of mechanization. The ratio C-POM:SOM was significantly related to MWD (R2 = 0.75, P < 0.001) reflecting that POM may play a key role in the topsoil aggregate formation in these stands. Semi-natural stands had a higher proportion of macroaggregates (0.25–2 mm) than the cultivated adult one. Megaaggregates (>2 mm) were the most abundant class in mature stands (82–92%), whereas macro- and microaggregates (<2 mm) were the most abundant ones in the intensely soil prepared P. radiata plantation (49%). tors for sustainable forest management related to soil organic matter should not only be assessed in terms of total C stocks but also with respect to sensitive organic matter and its degradability in different size classes.