Effect of agricultural practices on soil microbial communities

Abstract

Soil carbon sequestration is an important and immediate sink for removing atmospheric carbon dioxide and slowing global warming. Agricultural practices strongly impact CO2 emission from the soil and the loss of soil organic matter is lower when “no-till” agriculture is practiced. The present work was performed in order to investigate effects of different agricultural practices on soil microbial communities. Field experiments took place at three Italian locations strongly differing in pedological, chemical and climatic characteristics: Naples, Turin and Piacenza. Soils were subjected to four management practices such as traditional (TRA), minimum tillage (MIN) and amendments with compost (COM) known to be responsible in limiting CO2 release from agricultural soils, and synthetic metalporphyrins (POR) addition. Soil and rhizosphere samples from pots under wheat and maize cropping were sampled at different time and the composition of microbial groups directly implicated in OM mineralization, such as actinobacteria, fungi and cellulosolytic bacteria, as well as microbial groups involved in key bio-geochemical processes (e.g. aerobic free-living N2-fixing bacteria and ammonia-oxidizing bacteria) was estimated and culturable populations structure discussed. Moreover, the effect of synthetic metalporphyrins addition on the destiny of microbial populations responsible for important processes in soil biological activity was evaluated. As expected, different results were obtained depending on the field under study. A Principal Component Analysis (PCA) was applied to the overall set of results obtained from our field experiments to get a general outlook of the major factors influencing microbial distribution. The factors bulk – maize rhizosphere were the principal ones influencing distribution within the samples. In fact, a strong rhizosphere effect was exerted from the maize rhizosphere towards almost all microbial groups. Among the treatments only the compost addition (COM) showed a similar trend. Moreover, synthetic metalporphyrins amendment did not strongly influence the cultivable bacterial population. In fact, the strongest effect associated to the POR treatment in comparison with the control without porphyrin (NO POR) was related to the relatively higher counts of nitrogen - fixing bacteria found in the POR treatment and a decrease in ammonia-oxidizing bacteria. Our results emphasize the positive effect exerted by compost addition towards many microbial populations known to be important in the turn-over of OM, such as actinobacteria, fungi and cellulosolytic bacteria. This was particularly true in bulk soil, whereas in maize rhizosphere the different practices did not show such an obvious effect. This work was supported by FISR-MIUR Italy, (MESCOSAGR Project)