Changes in microbial community structure and physiological profile in a kaolinitic tropical soil under different conservation agricultural practices

Conservation management practices are essential for developing a sustainable agriculture. However, changes in agricultural practices impact soil microbial communities, which in turn influence soil ecosystem ecology and functioning. Investigations about the effect of agricultural practices on microbial community structure and functional profile in tropical soils are limited to a few types of land use and soil management. In this study, we used a 9-year old field experiment with a kaolinitic Kandiudalfs located in Northeastern Brazil to assess the impact of six agricultural practices (three tillage methods combined with two cropping systems) and fallow plots dominated by Brachiaria sp. on soil phospholipid fatty acid (PLFA)-based microbial community structure and stress status, community-level physiological profile (CLPP), microbial biomass-C (MB-C), basal respiration and pH. The three tillage methods assessed were conventional tillage (CT; plowing + disking), chisel plowing (CP; no disking) and no-till (NT; no plowing nor disking), while the two cropping systems were monoculture (MM; sole maize) and intercropping (MP; maize with pigeonpea). Statistical analyses were performed to assess the differences in the following contrasts: agriculture (all agricultural practices) vs fallow; conventional tillage vs conservation tillage (CP + NT); chisel plowing vs no-till; and maize monoculture vs intercropping. Microbial community structure was more affected by the different agricultural practices and fallow than CLPP, but changes did not correspond to increasing in soil physical quality and crop system complexity. On the other hand, CLPP showed lower differences between fallow vs agricultural soils and was unaffected by tillage methods, but was changed according to cropping systems - properly reflecting the increases in crop system complexity. Fallow soils decreased amino acid degradation capability, microbial stress status, gram-positive bacteria and actinomycetes biomarkers, and increased arbuscular mycorrhizal fungi (AMF) and gram-negative bacteria biomarkers, MB-C and basal respiration. Conventional tillage unexpectedly led to increases in AMF and decreases in stress biomarkers compared to conservation tillage. Intercropping decreased soil pH and increased MB-C compared to monoculture. Our results showed that microbial community structure and physiological profile are distinct regarding their responses to the agricultural practices analyzed, as well as on their correlations with the soil and ecosystem benefits promoted by conservation practices. Therefore, taxonomic and functional approaches of microbial community profiling are potentially useful, but need to be combined with soil physicochemical data in order to help understanding the complex effects of using sustainable agriculture practices on the soil environment.