Organic Inputs Positively Alter the Bacteriome of Post-Agricultural Soils

Agriculture can degrade soils and reduce microbial diversity. The reduction in microbial diversity of degraded soils is due to their long-term agricultural use. In most cases, such areas are afforested but rarely succeed in converting them into first-generation pine forests without adequately revitalizing the soils and restoring the natural relationships characteristic of forest habitats. This is possible thanks to the positive changes in soil biodiversity. To facilitate and enhance this phenomenon in the present experiment, various forms of organic matter were added to the soil: pine bark compost scattered on the soil surface (BCS) or placed under the roots of the seedlings at planting (BCR), woody debris (WW) and sawdust (S), while the control plot was not treated. The studies started in 2001 on two experimental plots, a poorer one in Bielsk and a more fertile one in Czarne Czluchowskie (eastern and northern Poland, respectively). The following year, 2-year-old pine seedlings (Pinus sylvestris L.) were planted on the plots. After 20 years, the physicochemical properties of the soil and its microbial composition were determined and compared with the control. The results encourage the use of organic matter for established pine forest crops on post-agricultural land for revitalization: C, N, and P content increased both in the organic layer and in the topsoil (up to 40 cm), where most fine roots are located. The total content of exchangeable base cations (Ca, Mg, K) and the sorption capacity of the soils (a measure of the ion binding capacity of the soil) also improved. The genetic analyses carried out using the molecular method (NGS) showed positive changes in the composition of the soil microbiome. Compared to poorer soil conditions in richer habitats, the number of taxa increases when organic matter is added, leading to significant qualitative changes in the bacteriome. The addition of organic material from the forest had a positive effect on the bacterial communities, which in turn accelerated the changes in the diversity of bacteriomes characteristic of agricultural soils and brought them closer to the forest ecosystem. The organic horizon was restored, and at the same time, the biodiversity of the soil microbiome increased, which is important for the health and sustainability of pine stands on formerly agricultural land.