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

摘要

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 (p0.05) 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.