稻田养萍模式下不同施氮量对稻田氨挥发及红萍生物固氮作用的影响

Influence of Nitrogen Application Rates on Biological Nitrogen Fixation and Ammonia Volatilization Under Rice-azolla Cropping System

红萍对水体铵态氮浓度较为敏感,稻田放养红萍模式下,红萍的生物固氮作用及其抑制氨挥发的作用对不同施氮量的响应未知。红萍为水生蕨藻共生体,具有很强的生物固氮能力。红萍可作为优质绿肥放养于稻田,以替代部分化学氮肥,起到节能减排的效应。为明确稻田养萍模式下不同施氮量对红萍生物固氮作用和田间氨挥发的影响,采用盆栽试验设置了0、75、150、225、300 kg/hm^(2)共5个施氮(以纯N量计)水平,监测了稻田放养红萍和水稻单种各处理的氨挥发量、生物固氮速率和水稻产量。结果表明:①同一施氮水平下,稻田放养红萍可显著降低氨挥发日通量峰值及氨挥发总量。在施氮量为225 kg/hm^(2)时,稻田放养红萍对氨挥发总量的抑制作用最大,与水稻单种相比,抑制幅度可达83.2%。②红萍的生物固氮速率及固氮总量与施氮量呈线性负相关关系,随施氮量的增加,固氮速率和固氮量逐渐降低,施氮量300 kg/hm^(2)并放养红萍处理得到的固氮速率及总量同不施氮肥不养萍处理之间无显著差异。③与不养萍处理相比,放养红萍组各处理的水稻产量都明显增加,其中施氮量为225 kg/hm^(2)时水稻增产幅度最大,增幅达21.2%。综上,在施氮条件下,稻田养萍可显著抑制稻田氨挥发并提高水稻产量,随着施氮量的增加,红萍作为生物绿肥的固氮作用受抑制程度加重。在不牺牲水稻产量的前提下,稻田养萍可替代约75 kg/hm^(2)的化学氮肥,且能抑制83.2%的稻田氨挥发。

As an aquatic fern-cyanobacteria symbiont,azolla has strong biological nitrogen(N)fixation ability,thus it can be used as high-quality green manure in rice fields to partially replace chemical nitrogen fertilizer in order to save energy and reduce N emissions.However,azolla is sensitive to ammonium concentration in the water,it is still unclear that the response of azolla to different nitrogen application rates under rice-azolla cropping system.A pot experiment was conducted to clarify the effects of different urea-N application rates on ammonia(NH3)volatilization,biological N fixation(BNF)of azolla and rice yield,in which N application rate was setup as 0,75,150,225,300 kg/hm^(2).The results show that:1)Under the same N application level,azolla in rice field can significantly reduce the daily peak flux of NH3 volatilization and the total amount of NH3 volatilization.At N application rate of 225 kg/hm^(2),rice-azolla co-culture leads to the greatest inhibition of the seasonal NH3 volatilization,with a reduction of 83.2%compared with rice monoculture.2)BNF rate and the total amount of BNF of azolla are negatively correlated with N application rate.BNF rate and the total amount of BNF of azolla at 300 kg/hm^(2) is similar with that in no-N without azolla.3)Rice-azolla co-culture improves rice yield compared to rice monoculture,except no-N group,and the maximal improvement is achieved at 225 kg/hm^(2),with an increase of 21.2%compared with rice monoculture.In conclusion,rice-azolla cropping system can significantly inhibit NH3 volatilization in rice fields and increase rice yield under fertilized conditions,BNF of azolla is inhibited with the increase of N application rate.Without sacrificing rice yield,rice-azolla cropping system can replace about 75 kg/hm^(2) of chemical N fertilizer and can inhibit 83.2%of the NH3 volatilization.