Influence of temperature and moisture on the relative contributions of heterotrophic and autotrophic nitrification to gross nitrification in an acid cropping soil

Nitrate can be produced through autotrophic and heterotrophic nitrification. Soil temperature and moisture are key factors affecting nitrification; however, how they influence the relative importance of autotrophic and heterotrophic nitrification is still unknown. The aim of this study was to determine the effects of soil temperature and moisture on autotrophic and heterotrophic nitrification in an acid cropping soil. An acid cropping soil (pH(H2O) 4.5) with high organic matter content (6.2 %) was collected from a wheat cropping system and used in a laboratory incubation experiment. Nitrogen as (NH4Cl)-N-15 and (KNO3)-N-15 was applied to soil samples which were then incubated for 7 days under 15 A degrees C, 25 A degrees C and 35 A degrees C and 50 % and 70 % water-filled pore space (WFPS). N-15 dilution and acetylene (C2H2) inhibition techniques were used to distinguish autotrophic and heterotrophic nitrification. We measured nitrate (NO3 (-)) and ammonium (NH4 (+)) concentration and (NO3)-N-15 (-) and (NH4)-N-15 (+) enrichment during the incubation. The result showed that more heterotrophic nitrification was observed under lower temperature (15 A degrees C) conditions. Around 69 % of the NO3 (-) produced was a result of heterotrophic nitrification at 15 A degrees C and 50 % WFPS and 50 % at 15 A degrees C and 70 % WFPS. However, at 25 and 35 A degrees C, nitrification was largely autotrophic regardless of moisture. Our results demonstrate that heterotrophic nitrification can be an important N transformation pathway in the studied soil. The contribution of heterotrophic nitrification decreased with increasing temperature and moisture.