Long-term effects of organic amendments on bacterial and fungal communities in a degraded Mediterranean soil

Addition of organic amendments is a common practice to restore fertility and to increase productivity of degraded soils. Long-term effects of this practice on Mediterranean soils are controversial, with previous works showing contrasting results about the durability of the organic material added and its effects on the structure of microbial communities. This article presents results from a long-term soil remediation experiment, where a range of soil chemical and biochemical indicators, as well as indices of microbial diversity and community structure, were analysed 13 years after the first application of two organic amendments (leonardite and biosolid compost) at different doses, in an area contaminated by trace elements. In general, differences in chemical and biochemical properties and trace element availability between control and treated soils were still very evident, mainly in those soils treated with the highest amendment dose. The structure and composition of the soil microbial community was significantly affected by the type of management. The addition of both amendments favoured the increase of the fungal/bacterial ratio in the soil community, although a correlation with the C/N ratio of amendments was not found. The abiotic factors that acted as main drivers of the belowground communities differed between bacteria (more sensitive to Zn and Cd contamination) and fungi (soil pH and nitrogen content). Organic amendments had a direct positive effect on these abiotic factors, especially on the soil pH, a key factor in achieving long-term remediation. The results revealed that the effect of both amendments on the soil is maintained years after their application, although it is necessary to repeat their application to maintain soil pH within appropriate ranges and achieve a long-lasting recovery of soil functions.