Organic-carbon and nitrogen stocks and organic-carbon fractions in soil under mixed pine and oak forest stands of different ages in NE Germany

The objective of this work was to evaluate the C and N stocks and organic-C fractions in soil under mixed forest stands of Scots pine (Pinus sylvestris L) and Sessile oak (Quercus petraea [Matt] Liebl) of different ages in NE Germany Treatments consisted of pure pine (age 102 y), and pine (age 90-102 y) mixed with 10-, 35-, 106-, and 124-y-old oak trees After sampling 0 layers, soils in the mineral layer were taken at two different depths (0-10 and 10-20 cm). Oak admixture did not affect total organic-C (TOC) and N stocks considering the different layers separately However, when the sum of TOC stocks in the organic and mineral layers was considered, TOC stocks decreased with increasing in oak age (r(2) = 058, p < 0.10) The microbial C (C-MB) was not directly correlated with increase of oak age, however, it was positively related with presence of oak species. There was an increase in the percentage of the C-MB-to-TOC ratio with increase of oak-tree ages On average, light-fraction C (C-LF) comprised 68% of the soil TOC in upper layer corresponding to the highest C pool in the upper layer. C-LF and heavy-fraction C (C-HF) were not directly affected by the admixture of oak trees in both layers. The C-HF accounted on average for 30% and 59% of the TOC at 0-10 and 10-20 cm depths, respectively Despite low clay contents in the studied soils, the differences in the DCB-extractable Fe and Al affected the concentrations of the C-HF and TOC in the 10-20 cm layers (p < 0 05) Admixture of oak in pine stands contributed to reduce topsoil C stocks, probably due to higher soil organic matter turnover promoted by higher quality of oak litter