Soil aggregate-associated nitrogen dynamics and microbial community composition in three different tillage systems

Quantifying soil nitrogen transformation processes associated with soil aggregates is noteworthy as microbial communities central to N cycle reside in the soil aggregates of different sizes. The objective of this investigation was to determine both the rates of ammonium production and consumption processes associated with soil aggregate size fractions obtained from soils subjected to three different long-term (23 yrs old) tillage systems. Total C and N, microbial biomass C and N levels were significantly higher in the surface 0-10 cm of soils from no-till than in chisel and moldboard systems. N fluxes estimated from FLUAZ program demonstrated that gross N mineralization, nitrification and immobilization rates were significantly greater in no-till than in chisel and moldboard systems. Gross mineralization and nitrification rates were greater in intermediate (0.5-1mm) size fractions of all the three tillage systems. But, gross immobilization rates were enhanced (30%) in large aggregates (2-4mm) than in other size fractions. Nitrifying capacity (GNR/ GNMR) of the chisel (0.73) and moldboard (0.84) soils were greater than in the no-till (0.50) system. However, potential leaching capacity (GNR/GIR) of no-till soils was lower than for the tilled systems. Microbial community composition examined by PLFA (Phospho Lipid Fatty Acid) method after N addition illustrated that bacterial biomarkers predominated fungi in all the aggregate size fractions and tillage systems.  Though a significant correlation between gross nitrogen transformation rates and microbial community composition was observed, stepwise regression analysis (R² = 0.89, P<0.0001) showed that only gross immobilization rates were restrained by microbial communities. Thus, results of this study have illustrated that higher microbial biomass in long-term no-till soils and macroaggregate size fractions has resulted in more rapid N turnover via balanced ammonium production and consumption processes and therefore suitable crop and fertilizer management practices are required to prevent N losses from these systems.