Characteristics of nitrous oxide emissions from typical greenhouse vegetable fields in Beijing suburbs

Nitrous oxide( N2O) has been recognized as one of the most important trace gases in the atmosphere that causes global warming and stratospheric ozone depletion. Nitrogen( N) fertilizer is considered as the primary source of N2 O emissions from agricultural soils. As a large agricultural country,China consumes the greatest amount of synthetic N fertilizer which accounts for 30% of the world consumptions. Therefore,quantifying N2 O emissions from agricultural soils and seeking suitable mitigation measures have become a relatively hot issue in international global climate change studies.However,the task has proved to be uneasy because N2 O production and emission processes are very complex and are influenced by a number of soil and environmental variables,interacting soil water and N processes,crop uptake and management practices. Especially the N2 O emissions from the greenhouse vegetable systems are more complex because the system obtain relatively higher inputs of fertilizer( e. g.,N fertilizer application rate can be as high as 1,500 kg N /hm2),more water irrigation and cultivation disturbance. This paper reported a field experiment with intensive measurements of N2 O fluxes from a greenhouse vegetable system with varied management treatments in Fangshan District located in the western suburbs of Beijing,China. N2 O fluxes in conjunction with the main environmental drivers( i. e.,soil temperature,soil moisture,soil NO-3-N and soil NH+4-N) were observed from Feb. 2012 to Feb. 2013. Four treatments,i. e.,the control treatment( CK),the farmers' practice treatment( FP),the optimized fertilization treatment( OPT),and the OPT treatment with nitrification inhibitor amendment treatment( OPT+DCD),were implemented during the experimental period to test the impacts of fertilization on N2 O fluxes from the agroecosystem. The CK had no fertilizer applied; FP consisted of2,470 kg N /hm2 with 1,270 and 1,200 kg N /hm2 from synthetic fertilizer and manure,respectively; the OPT reduced synthetic fertilizer rate to 573 kg N /hm2. The field was planted with tomato,cabbage and lettuce rotation during the two experimental years. The results indicated that large amount of N2 O emissions were observed in the spring and summer periods when the soil had relatively high temperatures and moisture. N2 O emission peaks were measured following each event of fertilization or irrigation. The high peaks usually lasted for 3—5 days. During the experimental period, N2 O emission rates ranged from-0. 21—14. 26 mg N2 O m-2h-1with daily means ranging from 0. 03—0. 36 mg N2 O m-2h-1.The annual cumulative N2 O emissions ranged from 1. 69—20. 66 kg N hm-2,based on which the annual emission factor was calculated to be 0. 36% —0. 77% of the fertilizer N. Compared to the FP treatment,the OPT and OPT + DCD treatments both significantly reduced the annual N2 O emissions by 38. 09% and 61. 3%( P 0. 05),respectively. The N2 O emissions during the tomato,lettuce,cabbage and fallow periods accounted for 60. 65%,26. 32%,10% and 3. 3%of annual cumulative emissions,respectively. The fertilizer-induced N2 O emissions varied across the N application rates,the crop growing periods and the management treatments. N2 O fluxes were positively related to the soil water filled pore space( WFPS) when the WFPS values varied between 48. 88% —79. 88%( P0. 05). There was no consistent correlation between N2 O fluxes and the soil temperature at soil depth of 5 cm. Higher soil available nitrogen,especially nitrate,contributed higher N2 O emissions. In conclusion,alternative management practices such as reduced fertilizer application rate and amendment of DCD could effectively reduce N2 O emissions from the greenhouse vegetable field which usually emitted more N2 O than other croplands in China.