Variations of agricultural soil quality during the growth stages of sorghum and sunflower

Plant species and growth stage can differently affect the structure and functionality of soil microbial community. Besides, microorganism amount and functional diversity can dynamically respond to management practices, such as soil tillage and mineral fertilization that, in turn, can modify soil quality. In this study, we investigated the effects of two crop species, sorghum (Sorghum bicolor L.) and sunflower (Helianthus annuus L.), on soil microbial biomass, activity and diversity at a flat agricultural site. We evaluated the structure and the activity of microbial community in root-influenced soil (close to roots) and bulk soil (devoid of roots), during different plant growth stages, and we calculated a soil quality index, before sowing time and during the crops growth. The principal component analyses highlighted variations in soil properties at different plant growth stages, regardless of the crop species. At the first and the last stage of plant growth, microbial biomass, catabolic evenness, Shannon index and catabolic response to carbohydrates showed statistically lower values in root-influenced soil than in bulk soil. At the plant flowering, the lowest values of organic matter and N contents in root-influenced soil were related to an increase of catabolic responses to amino and carboxylic acids. Before plant harvesting, the highest soil metabolic quotient values were calculated, suggesting the presence of stress conditions for the soil microbial community and low energetic substrates use efficiency. Soil mineral fertilization induced decreases of soil pH and microbial diversity, favoring the fungal component rather than the bacterial one. Plant growth and crop management affected also soil N2O emission. Finally, a clear decrease of the soil quality index was measured during plant growth and statistically significant differences between close to roots and devoid of roots soils already occurred after two weeks since sowing.