The status of soil organic matter decomposing microbiota in afforested and abandoned arable Arenosols

Organic matter decomposing soil microbiota status (abundance and diversity) have been investigated in former arable Arenosols (AR) that (i) were afforested 45 years ago with Scots pine (Pinus sylvestris L.) plantations or (ii) abandoned for the last 12 years (since 1993). The abundance of soil microbiota in soil organic layer (forest floor or dead grass fall) and in surface 0-2 and 2-10 cm mineral layers of former ploughed Ap horizon was estimated by the standard method of sowing organic or mineral soil suspensions on the agarised nutrient standard media and on media supplemented by mineral substances and different lignin monomers. The diversity of soil microbiota was determined only in surface mineral layers and was expressed according to the occurrence of the cytochrome P450 gene in mineral soil. The highest abundance of soil microbiota was found in organic layers of former arable Arenosols, especially, in forest floor of pine plantations, where mean abundance of microbiota was from 4-10 to even 20-30-fold higher than in surface mineral layers of former arable horizon. However, in the studied surface mineral soil layers mean microbial abundance, including lignin decomposing microbiota, was on average by 2-5 times higher in abandoned land than in pine plantations. In addition, it was determined that the microbiota diversity was two times higher in abandoned Arenosols where more actinobacteria and proteobacteria strains were, while in 45-year-old pine plantations organic matter decomposers were mainly proteobacteria strain related. It was concluded that the accumulation and the decomposition of forest floor, and lower abundance and diversity of microbiota in surface mineral layer preserve or even could promote higher sequestration rate of soil organic carbon in afforested than in abandoned arable Arenosols.