Gross mineralization, nitrification and N₂O emission under different tillage in the North China Plain

No-tillage cropping can increase soil carbon (C) stocks and aggregation, and subsequently impact the internal nitrogen (N) cycle and gas loss. The ¹⁵N 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 ± 0. 3-10. 6 ± 1. 5 mg N kg^-1 days^-1) were significantly higher than for no-tillage (1. 7 ± 0. 8-6. 8 ± 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 ¹⁵N 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 ‰. 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.