Soil aggregate-associated microbial community structure and nitrogen transformations in three different tillage systems

Quantifying soil nitrogen transformation processes associated with soil aggregates is noteworthy as microbial communities that are central to N cycle reside in the soil aggregate size fractions. The objectives of this research were: (1) to determine the population size of N mineralizers, nitrifiers, and denitrifiers; (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 aggregate fraction (0.5-1 mm) than in other aggregates. Correspondingly, the potential rates of N transformation processes (N mineralization, nitrification, and denitrification) and activities of enzymes involved in N mineralization process (N-acetyl-beta-glucosaminidase, L-glutaminase, L-asparaginase and arylamidase) also significantly varied (p<0.05) with aggregate size and tillage systems. However, NMS (Non-metric Multidimensional Scaling) analysis of the Phospho Lipid Fatty Acid (PLFA) profiles showed that microbial communities differed with tillage systems but not with aggregate size. Furthermore, gross N transformation rates were measured using 15N pool dilution techniques and FLUAZ program to better understand the productive and consumptive processes associated with microbial habitats. The mean gross N mineralization, nitrification, and immobilization rates were significantly greater (1.5-2 times) in no-till than in chisel and moldboard systems. Similarly the mean gross N mineralization and nitrification rates were 20-25% greater in intermediate aggregates (0.5-1mm), but the mean gross immobilization rates were greater in large aggregate size (2-4mm) than in other aggregates. Thus, this study demonstrated that higher organic C and N have resulted in more rapid N turnover in long-term no-till than in tilled systems. Yet, more detailed studies involving the measurement of NO3 leaching losses are required to formulate best N management practices in long-term no-till systems.