Kinetics of Net Nitrification Associated with Soil Aggregates Under Conventional and No-Tillage In a Subtropical Rice Soil.

Tillage effects on soil nitrification kinetics at the aggregate scale were studied for a subtropical rice soil. Soil samples were separated into large aggregates (> 2.0 mm), macro-aggregates (2.0-0.25 mm), micro-aggregates (0.25-0.053 mm) and silt + clay fractions (< 0.053 mm) by wet-sieving. Net nitrification process was simulated by a zero- or first kinetics model.  Conventional tillage (CT) increased the proportion of the silt + clay fraction by 60% and decreased large-aggregates by 35% compared to RNT (ridge with no-till). Regression analysis showed that the time-dependent kinetics of net nitrification were best fitted by a zero-order model for the large-aggregate and silt + clay fractions but a first-order kinetic model for macro-, micro-aggregate and whole soil, regardless of tillage regime. Both potential nitrification rates (V_p) and net nitrification rates (V_a) were higher for macroaggregates than microaggregates. The N_p for whole soil under RNT was 38.7% higher than CT. The V_p and V_a for whole soil under RNT was 88.5% and 64.7% higher than under CT, respectively. Although nitrification was stimulated under RNT, the kinetics model of nitrification was not affected by tillage. This inferred that the interaction between substrate and enzymes in nitrification associated with aggregates was not altered by tillage. Nitrifying microorganisms are mainly associated with macro- and micro-aggregates rather than large-aggregates and silt + clay fractions.