No-tillage cropping can increase soil carbon (C) stocks and aggregation, and subsequently impact the internal nitrogen (N) cycle and gas loss. The N-15 pool dilution method was used to study gross N transformations, and relative proportions of nitrous oxide (N2O) emissions derived from denitrification versus nitrification-related processes under long-term tillage systems (no-tillage, rotary tillage and conventional tillage) in the North China Plain. In-field incubation experiments were repeated in successive growing seasons during April-November in 2007. Gross mineralization rates for rotary and mouldboard plough tillage (3.6 +/- A 0.3-10.6 +/- A 1.5 mg N kg(-1) days(-1)) were significantly higher than for no-tillage (1.7 +/- A 0.8-6.8 +/- A 1.1 mg N kg(-1) days(-1)). Gross mineralization was positively correlated with soil moisture and temperature, as well as with microbial biomass N and C. However, there was no consistent tillage effect on gross nitrification, and gross nitrification was positively correlated with soil moisture, but not with gross mineralization and microbial biomass. N2O emissions were higher in no-tillage (NT) than for conventional tillage (CT) during May-August. The N-15 labelling indicated that 26-92 % of the N2O was directly derived from the soil ammonium (NH4 (+)) pool. Emission rates of N2O from both nitrification and denitrification were positively correlated with NH4 (+) supply as expressed by gross mineralization, but not correlated with supply of nitrate as expressed by gross nitrification. The fraction of nitrified N emitted as N2O was positively correlated with changes in soil moisture and varied within 0.01-2.51 aEuro degrees. Our results showed that the tillage management impact on gross N transformation was not consistent with N2O emission, and more detailed information on the controls over N2O formation needs to be sought.
