Characterization of Soil Microbial Communities in Agricultural and Forest Ecosystems

Abstract

Soil microbial community plays a critical role in nutrient cycling. The overall objective of this work was to determine responses of soil microbial communities to agricultural and forest management practices. Field experiments were conducted in one forestry site and two field crop sites. Soil microbial community structure was determined using phospholipid fatty acid (PLFA) analysis and automated ribosomal intergenic spacer analysis (ARISA). The objective of the first field experiment was to determine effects of fertigation on soil microbial biomass, activity and microbial community structure in a loblolly pine plantation. This experiment was established in 2000 on a Blanton sandy soil near Aiken, South Carolina. The experiment was arranged in a randomized complete block design of three blocks with fertilization and irrigation as factors. Soil samples were collected in November of 2006 and May and September of 2007. The soil organic carbon was significantly higher for fertilized and fertigated treatments. Soil pH was higher in the irrigated treatment compared to the control and fertilized treatments. PLFA profiles showed that fertigation as well as sampling time influenced changes in the soil microbial community. Bacterial and fungal ARISA profiles revealed changes in soil microbial community associated with fertigation. The PLFA and fungal ARISA profiles showed fertigation treatment and sampling time influenced soil microbial community structure. Soil organic carbon for fertigation treatment was significantly higher than in the control and positively correlated with fungal biomarker. These results indicate that fertigation can influence soil microbial community structure and activity along with soil chemical and biochemical properties in soil. The objective of the second field experiment was to examine effects of nitrogen sources and soil pH on soil microbial communities in a long-term crop rotation system. The long-term soil fertility experiment, Cullars Rotation, consisting of a three-year rotation of cotton, corn, wheat, soybean and clover, was established in 1911 on Marvyn sandy loam soil. Soil samples were collected in June and October of 2008 and February of 2009 at two depths (0-5 and 5-15 cm). Soil pH values for no input and no lime plots were lower than other treatment plots. Fungal biomarker (18:2ω6,9) concentration was lower in the surface soil and subsurface soil for no lime treatment. The arbuscular myccorrhizal biomarker (16:1ω5) concentration was lower in the surface soil and subsurface soil for no input treatment. Bacterial biomarkers, fungal biomarker (18:2ω6,9) and arbuscular myccorrhizal biomarker (16:1ω5) were positively correlated to soil organic carbon content. Multivariate analyses of PLFA and ARISA profiles showed that changes in soil microbial communities were associated with soil pH and nitrogen source (inorganic fertilizer vs legumes). The objective of the third field experiment was to examine effects of tillage practices on soil microbial communities. This experiment, located in Belle Mina, AL, was arranged in a randomized complete block factorial design with four replications on a Decatur silt loam soil. Tillage treatments included conventional tillage and no tillage in a continuous corn production system. Soil samples were taken at depths of 0-5 and 5-15 cm in April of 2008. The long-term no-tillage treatment resulted in higher soil carbon and nitrogen contents, viable microbial biomass, and phosphatase activities at the 0-5 cm depth than the conventional tillage treatment. Soil microbial community structure, assessed using phospholipid fatty acid (PLFA) analysis and automated ribosomal intergenic spacer analysis (ARISA), varied by tillage practice and soil depth. The abundance of PLFAs indicative of fungi, bacteria, arbuscular mycorrhizal fungi, and actinobacteria was consistently higher in the no-till surface soil. Results of principal components analysis based on soil physicochemical and enzyme variables were in agreement with those based on PLFA and ARISA profiles. Soil organic carbon was positively correlated with most of the PLFA biomarkers. These results indicate that tillage practice and soil depth were two important factors affecting soil microbial community structure and activity, and conservation tillage practices improve both physicochemical and microbiological properties of soil. Overall, these results indicate that changes in soil microbial community structure were influenced by changes in soil properties due to management practices, such as fertigation, lime application, crop rotation with winter legumes and conservation tillage.