Annual N2O and CH4 emissions from intensively managed vegetable fields in Nanjing, China

Nitrous oxide (N2O) and methane (CH4) emissions were monitored from an intensive vegetable production region with four typical consecutive rotations and a bare fallow in Nanjing, China, from 2009 to 2010. The four consecutive rotations were established as follows: Celery (Apium graveolens dulce L.)-Tung choy (Ipomoea aquatica Forsk L.)-Baby bok choy (Brassica rapa Chinensis L.)-Amaranth (Amaranthus mangostanus L.) (C-T-Bb-A), Choy sum (Brassica rapa Chinensis L.)Celery-Tung choy-Bok choy (Brassica rapa Chinensis L.) (Cs-C-T-Bc), Garland chrysanthemum (Chrysanthemum coronarium L.)-Tung choy-Bok choy (G-T-Bc), and Celery-Choy sum-Lettuce (Lactuca sativa L.)-Bok choy (C-Cs-L-Bc). Annual cumulative N2O emissions were 237.7 +/- 18.3, 137.2 +/- 8.5, 100.9 +/- 7.5, 56.4 +/- 10.6 and 29.2 +/- 3.7 kg N ha(-1) from the G-T-Bc, C-T-Bb-A, Cs-C-T-Bc, C-Cs-L-Bc rotation fields and the bare fallow, respectively. Cumulative N2O emissions from the rotation fields, except the C-Cs-L-Bc rotation, were significantly higher than that from the bare fallow. Annual cumulative CH4 emissions were 9.0 +/- 3.5, 13.9 +/- 1.5, 18.8 +/- 1.2. 12.1 +/- 3.8 and 16.1 +/- 3.5 kg C ha(-1) from the bare fallow, C-T-Bb-A, Cs-C-T-Bc, G-T-Bc and C-Cs-L-Bc, respectively, and they did not show significant differences among systems. The annual N2O emission factor averaged 2.6%, ranging from 1.2% to 5.0%, and was higher than the current IPCC default value (1.0%). Total N fertilizer application rate explained 35.5% of annual N2O emissions. N2O emissions from intensively managed vegetable fields were characterized by high fluxes and emission factors that varied with cropping systems. Frequent year-round monitoring of N2O fluxes from intensively managed vegetable fields is essential for better constraint of the national N2O budget.