Soil culturable microorganisms as affected by different soil managements in a two years wheat-faba bean rotation

Agricultural practices greatly influence soil chemical and microbiological parameters, which affect soil fertility. The aim of this study was to investigate the effects of different tillage (conventional (CT), minimum (MT) and no tillage (NT)), fertilization (fertilization versus no fertilization), soil depth (0-30, 30-60, and 60-90 cm) and crops (wheat versus faba bean) on the abundance of selected microorganisms. Total culturable bacteria (TCB) were predominant in the microbial soil community, while total culturable yeasts (TCY) represented the minority. The TCB and total culturable fungi (TCF) were positively correlated with the content of soil organic carbon, total nitrogen and available phosphorus. The minimum tillage increased significantly the size of microbial populations, representing a good compromise between soil aeration and available nutrients as well as the faba bean. In particular, the unfertilized legume resulted in a higher TCY number with respect to the fertilized one, suggesting that somehow these microorganisms lose their low ecological competition in the presence of inorganic phosphorus. Finally, all microbial populations studied reduced their size with soil depth because of the nutrients availability, even if the TCB decreased with a far lower percentage due to the major proliferation of bacteria at the expenses of the other microbial groups.