长江中下游稻区不同水旱轮作模式和氮肥水平对稻田CH4排放的影响

Effects of Various Paddy-Upland Crop Rotations and Nitrogen Fertilizer Levels on CH4 Emission in the Middle and Lower Reaches of the Yangtze River

【目的】明确长江中下游稻区不同水旱轮作模式与氮肥水平对稻田CH4排放的影响。【方法】以2003年至今的4种水(水稻)旱轮作长期定位试验为基础(分别为水稻-休闲(RF),水稻-紫云英(RC-G),水稻-小麦(RW)和水稻-稻草覆盖种植马铃薯(RP)),并设置3个氮肥水平,分别为N0(0)、N1(142.5 kg N·hm-2)和N2(202.5 kg N·hm-2)。于2016—2017利用静态箱-气象色谱法,在田间采集并测定水稻生长季CH4排放。【结果】(1)轮作模式与氮肥互作对稻田CH4排放的影响主要集中在移栽后7—30 d内,其CH4累积排放量约为整个生育期的51.9%—72.3%。(2)轮作模式与氮肥水平对稻田CH4排放存在显著的互作效应;N0水平下,冬季作物栽培(包括RP、RW和RC-G)显著提高稻季CH4累积排放量,与RF相比分别增加74.1%—145.1%、68.5%—109.9%和56.4%—108.6%。(3)增施氮肥(N1和N2)后,CH4排放对轮作模式的响应出现分化。其中,RF、RP和RW模式下稻季CH4排放量随氮肥施用量的增加而逐渐增加;N2水平下,RP、RW和RF的CH4累积排放量分别为51.2—55.8、45.3-51.5和25.0—30.5 g·m-2,分别比N0水平提高23.0%—38.4%、26.7%—33.7%和35.3%—43.5%;而与N1相比,则提高9.9%—19.7%、20.8%—23.1%和17.4%—18.8%。而RC-G模式下则表现为增施氮肥一定程度上降低了稻季CH4排放;与N0相比,N1和N2下稻季CH4累积排放量分别降低20.7%—42.4%和10.6%—16.6%。(4)进一步解析与土壤CH4排放相关微生物菌群产甲烷菌(mcrA)和甲烷氧化菌(pmoA)丰度变化,发现N0水平下秸秆及绿肥全量还田能够显著增加产甲烷菌和甲烷氧化菌丰度;相关微生物对氮肥的响应机制因轮作模式而有所差异,增施氮肥促进产甲烷菌的增殖,却抑制了甲烷氧化菌的生长,但其变化幅度因轮作处理而有所不同。随着氮肥增施,RP、RW和RF的mcrA丰度增加191.4%、160.6%和143.3%,而RC-G则仅有62.6%。(5)另外,随着氮肥施用量的增加,RF、RP和RW模式下mcrA/pmoA比值增加,其增加比例分别为71.4%—141.1%、197.1%—258.2%和84.6%—165.5%,而RC-G则相反,下降26.8%—42.3%。其变化规律与CH4排放基本一致。【结论】稻田系统中秸秆还田C/N的相对含量可能是干扰氮肥水平对稻田CH4排放作用的关键,当系统中碳冗余时,相关微生物活性受到土壤中有效氮制约,投入无机氮可以减轻氮的限制作用从而显著提高CH4排放;而碳不足时,继续投入无机氮,相关微生物繁殖由于受到土壤中有限碳源的限制其活性也会受到抑制,CH4排放相对减少。

【Objective】The study was carried out to evaluate the effects of various paddy-upland systems and nitrogen fertilizer levels on CH4 emissions from paddy fields in the middle and lower reaches of the Yangtze River.【Method】Field CH4 emissions were collected during the rice growing season based on the long-term paddy-upland crop rotation experiments(2003-by now),including rice-fallow(RF),rice-green manure(Chinese milk vetch;RC-G),rice-wheat(RW)and rice-potato with rice straw mulch(RP),with three nitrogen levels(N0),N1(142.5 kg N·hm-2)and N2(202.5 kg N·hm-2))from 2016 to 2017.【Result】(1)The results showed that the effect of crop rotation and nitrogen fertilizer on CH4 emission in paddy fields was significant mainly on the early stage of tillering(from 7 to 30 days after transplanting),which accounted for 51.9%-72.3%of the cumulative CH4 emission of the whole growth period.(2)In addition,both crop rotations and nitrogen levels affected the CH4 emission.Rotations with winter crops(including RP,RW and RC-G)significantly increased CH4 cumulative emissions in rice season at N0 level compared to the RF,being74.1%-145.1%,68.5%-109.9%and 56.4%-108.6%higher in RP,RW,and RC-G,respectively.(3)The response of CH4 emissions to rotations was different along with increasing nitrogen fertilizer(N0 to N1 and N2).CH4 emissions increased along with the increase of nitrogen fertilizer application under RF,RP and RW,at N2 level,CH4 cumulative emissions of RP,RW and RF were 51.2-55.8 g·m-2,45.3-51.5 g·m-2 and 25.0-30.5 g·m-2,respectively,with 23.0%-38.4%,26.7%-33.7%and 35.3%-43.5%higher than that of N0 level,and 9.9%-19.7%,20.8%-23.1%and 17.4%-18.8%higher than that of N1 level.While decreased or kept consistent in RC-G,CH4cumulative emissions under N1 and N2 decreased by 20.7%-42.4%and 10.6%-16.6%,respectively compare with N0.(4)Analyses of functional microbial in related to CH4 emission during early tillering stage showed that rotations with full return of straw and/or green manure could significantly increase the abundance of both methanogens and methane oxidizing bacteria under N0.The response mechanism of related microorganisms to nitrogen fertilizer varied with crop rotation pattern,and the application of nitrogen fertilizer promoted the proliferation of methanogens,but inhibited the proliferation of methane oxidizing bacteria,but the extent of the change varied with crop rotations.With the increase of nitrogen application,the mcrA gene abundance of RP,RW and RF increased by 191.4%,160.6%and 143.3%,respectively,while RC-G only increased by 62.6%.(5)In addition,the ratio of mcrA/pmoA in RF,RP and RW increased along with the increase of nitrogen application,which increased 71.4%-141.1%,197.1%-258.2%and 84.6%-165.5%,respectively.The RC-G showed a downward trend,which declined 26.8%-42.3%.The change rule was basically consistent with CH4 emission.【Conclusion】Combining with the properties of straw returning in winter,the amount of straw returning in RW and RP was significantly higher than that under RC-G in this study,while the ratio of C/N in RP and RC-G was significantly lower than that under RW.Therefore,the relative amount of C/N returned from straw might be the key to interfere with the effect of nitrogen fertilizer level on CH4 emission from paddy field systems.When carbon was abundant in the system,the relevant microbial activity was restricted by available nitrogen in the soil,and the input of inorganic nitrogen could reduce nitrogen limitation and significantly increase CH4 emission.When the carbon was insufficient and the inorganic nitrogen continues to be invested,the related microbial reproduction was inhibited by the limited carbon source in the soil,and the CH4emission was relatively reduced.