Soil Aggregate-Associated Microbial Community Structure and Nitrogen Transformations in Three Different Tillage Systems

Abstract

Soil management practices such as tillage largely influence soil aggregates and microbial communities that reside on different soil aggregate size fractions. Microbial communities that are central to the N cycle determine plant-available N and N losses via leaching and gaseous emissions. Although some research studies described the role of soil aggregates in C sequestration, very little information is available regarding the link between the soil aggregate size fractions and N cycling processes. The objectives of this research were: 1) to determine the population size of N mineralizers, nitrifiers, and denitrifiers by measuring the potential rates of N mineralization, nitrification, and denitrification, respectively; 2) to assess the activities of enzymes involved in the N mineralization process; 3) to quantify the gross N mineralization, nitrification, and immobilization rates; and 4) to relate the microbial community composition and rates of N processes associated with soil aggregate size fractions of no-till, chisel, and moldboard tillage systems.

Soil and microbial biomass C and N were 1.5 to 2 times greater in no-till than in moldboard systems and 15 to 20% greater in intermediate aggregates (0.5-1 mm) than other aggregates. Aggregate size had significant effects on potential N mineralization, nitrification, and denitrification rates for all three tillage systems. Potential activities of N-acetyl-β-glucosaminidase, L-glutaminase, and arylamidase were also significantly different (p<0.05) among aggregate size fractions. However, L-asparaginase activity did not vary significantly among aggregate sizes but did differ among tillage systems. This shows that different aggregate size fractions accommodate distinct microbial populations and communities associated with N cycling. Therefore, soil microbial community composition and gross N transformation rates associated with soil aggregate size fractions of the three tillage systems were quantified using the 15N pool dilution method. Nitrogen fluxes estimated from the FLUAZ program demonstrated that gross N mineralization, nitrification, and immobilization rates were significantly greater (1.5-2 times) in no-till than in chisel and moldboard systems. Gross N mineralization and nitrification rates were approximately 20 to 25% greater in intermediate aggregates (0.5-1mm) than in other aggregate size fractions. NMS analysis of the microbial community composition analyzed by Phospho Lipid Fatty Acid (PLFA) method both before (A = 0.3205, P <0.005) and after (A = 0.1951, P <0.005) 15N addition illustrated that microbial communities differed with tillage systems but not with aggregate size. Thus, this study demonstrated that higher microbial biomass in long-term no-till soils has resulted in more rapid N turnover via balanced N mineralization, nitrification, and immobilization processes than in tilled soils. However, more detailed studies involving the measurement of NO3 leaching losses are required to formulate best N management practices in different tillage systems.