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氧化亚氮(N2O)是三大温室气体之一，其百年增温潜势是二氧化碳的296倍[1]，大气寿命约为116年[2]，对平流层的臭氧消耗有着重要影响。农田尤其旱地是N2O排放的主要源头，占全球排放的25%[3]。不同气候类型和农田管理措施都对N2O的排放产生影响。土壤N2O的排放来源一般是由微生物硝化和反硝化作用产生。硝化作用是指土壤微生物将氨通过土壤氨单加氧酶和羟胺还原酶氧化为亚硝态氮或硝态氮，这些过程中会释放出N2O[4]，反硝化作用是指微生物将亚硝态氮或硝态氮通过土壤硝酸还原酶(NR)、亚硝酸还原酶(NIR)、一氧化氮还原酶和氧化亚氮还原酶作用下，最终还原为N2，其过程中也会释放出N2O[5]。中国南方红壤区高温多雨气候以及长期不合理的氮肥施用加剧了土壤酸化[6]，同时氧化亚氮还原酶受到土壤低pH影响导致还原能力降低，使得N2O排放增加，因此，在酸化红壤旱地进行N2O排放特征的研究具有重要意义。
气候、土壤酸碱度、施肥等因素影响着土壤N2O排放过程和排放量。王良等[7]在夏玉米连续两年不同施氮量下研究发现，温度、降雨量及施氮量通过影响硝化反硝化过程相关微生物活性显著影响N2O排放。黄晶等[8]通过对小麦–玉米季N2O的排放研究发现，在酸化土壤中，施用氮磷钾处理的土壤累积N2O排放量显著高于氮钾处理，因为均衡施肥提高了土壤微生物碳和微生物氮含量。Zhang等[9]连续两年监测发现，酸性土壤夏玉米季硝化作用进行较慢，夏玉米对硝态氮的吸收利用也促进了反硝化过程，因而N2O排放通量显著高于中性和碱性土壤。Cheng等[10]研究发现，当土壤pH<4.8时，旱地土壤中由反硝化作用主导N2O排放。Cui等[11]对全球作物类型分析发现，玉米季N2O排放与土壤有机碳含量呈正相关，有机碳含量高有利于硝化过程排放N2O，而土壤水分增加会产生厌氧条件，促进反硝化作用。Antonio等[12]研究表明，施用氮肥通过增加土壤铵态氮和硝态氮含量加强硝化反硝化作用，进而产生更多的N2O。Blum等[13]通过对反硝化过程酶研究发现，土壤中硝酸还原酶的最适pH为7左右，亚硝酸还原酶为5.8～6.7，过酸的土壤环境会抑制这两种酶活性，可见探究这两种酶活性在酸化红壤中的变化特征对调控氮转化具有重要意义。林先贵等[14]研究发现磷肥的添加通过促进土壤中微生物数量及活性，促进土壤中的硝化和反硝化等氮循环过程，因此探究磷肥投入对酸化红壤N2O排放的影响，对实现N2O的减排具有重要意义。
当前田间N2O排放研究大多关注周年排放量，较少针对关键生育期，研究N2O排放特征及其与土壤化学、酶活性等因素之间的关系，尤其是在长期施肥基础上的相关研究较少。参与反硝化过程的相关酶(DE、NR和NIR)在酸化红壤中的活性变化特征及其与N2O排放之间的关系尚不清楚。因此，本研究依托中国农业科学院祁阳红壤实验站的长期定位试验，运用静态箱法−气相色谱法监测长期施肥下酸化红壤玉米季N2O排放特征，同时测定了关键生育期土壤的化学性质和酶活性，探讨气候因素、土壤化学性质和反硝化相关酶活性对N2O排放特征的影响，阐明N2O排放与土壤反硝化相关酶活性的关系，揭示长期不同施肥下玉米关键生育期N2O排放特征及驱动因素，以期为酸化红壤旱地减少N2O排放提供理论依据。
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试验位于中国农业科学院祁阳红壤实验站(26°45′12″N，111°52′32″E)。该区域为典型亚热带湿润季风气候，年均气温18.0℃，≥10℃的积温5600℃，年均降水量1255 mm，蒸发量1470 mm，无霜期约300 天，日照时数约1610 h。供试土壤为旱地红壤，成土母质为第四纪红土，种植制度为小麦–玉米轮作。1990年试验开始时，0—20 cm土壤理化性质分别为：pH为5.7，有机碳含量为6.67 g/kg，全氮含量为1.07 g/kg，全磷含量为1.03 g/kg，全钾含量为13.3 g/kg，碱解氮含量为79.0 mg/kg，有效磷含量为10.8 mg/kg，速效钾含量为122.0 mg/kg。
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本研究选取长期定位施肥试验3个处理：不施肥(CK)、施氮钾化肥(NK)和氮磷钾化肥(NPK)。试验小区面积为200 m2，中间用60 cm深水泥埂隔开。肥料年施用量为N 300 kg/hm2、P2O5 120 kg/hm2和K2O 120 kg/hm2。供试化肥为尿素(N 46%)、过磷酸钙(P2O5 12%)、氯化钾(K2O 60%)。试验种植制度为冬小麦–夏玉米轮作。玉米季化肥施用量占总施用量的70%，所有肥料在作物种植前一次性施入，其他管理措施则与当地农民常用措施保持一致。
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采用静态箱法采集气体样品，静态箱和底框均为不锈钢制作，采气孔位于箱顶中间部位。底框于玉米基肥施用前放置，深度约为15 cm，底框内有玉米植株。田间采样从施肥后(2021年3月26日)开始至作物收获(2021年8月5日)结束，采样期间的日均温和降雨量变化如图1所示。采样频率一般1周采集1次，施肥和降雨后增加采样频率。每次采样的具体时间为当天上午9：30—11：30。采样前用水密封采气箱和底框之间的缝隙，同时用50 mL注射器使箱内气体混合均匀，之后在第0、20、40和60 min分别采样1次，每个处理3次重复，同时使用数字温度计监测箱内温度。
图 1 玉米季降雨量和气温
Figure 1. Precipitation and temperature during maize season
通过标准气体和待测气体的峰面积来计算待测气体浓度。通量的计算公式为[15]：
$ F=\rho \times V/A\times \Delta c/\Delta t\times 273/(273+T) $ 式中：F为N2O的排放通量[μg/(m2·h)]，ρ为标准状态下气体密度(1.964 kg/m3)；V是采气箱内有效空间体积(m3)；A为采集箱覆盖的土壤面积(m2)；
$\Delta c $ 为气体浓度差；$\Delta t $ 为时间间隔(h)；$\Delta c/\Delta t $ 为4个采样时间点的斜率；T为采样时箱内温度(℃)。N2O累积排放量计算公式为：
$ E={\sum }\left[\right({F}_{i+1}+{F}_{i})/2]\times ({D}_{i+1}-{D}_{i})\times 24/1000\times 10 $ 式中：E为N2O累积排放量(g/hm2)；Fi和Fi+1分别为第i和i+1次采样时N2O平均排放通量[μg/(m2·h)]；Di和Di+1分别为第i和i+1次采样时间(d)；N2O排放总量是将3次重复的各次观测值按时间间隔加权平均后再进行平均化处理。
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分别于施肥后(2021/03/26)、苗期(2021/04/03—2021/05/02)、喇叭口期(2021/05/17—2021/05/30)、灌浆期(2021/06/16—2021/07/01)和成熟期(2021/07/02—2021/08/09)，在各小区玉米行间随机选取3点，采集0—20 cm表土并混匀，每个小区重复3次，土样经风干后过0.85 mm筛。土壤化学性质参照《土壤农化分析》[16]进行相关指标的测定。土壤pH采用电位法测定(水∶土=2.5∶1)，土壤有机碳采用重铬酸钾氧化—容量法测定，全氮采用凯氏定氮法测定，碱解氮采用碱解扩散法测定。铵态氮和硝态氮采用2 mol/L的氯化钾浸提，用流动分析仪测定。土壤反硝化酶(DE)用硝酸钾培养比色法测定[17]，硝酸还原酶(NR)用酚二磺酸比色法测定[18]，亚硝酸还原酶(NIR)用α-萘胺比色法测定[19]。
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采用Excel 2016进行数据整理。采用Origin 2022进行图的绘制。用SPSS 19.0进行差异显著性分析(Ducan法)和相关性分析(Pearson法)。用R语言(4.1.2)中RandomForest进行随机森林模型分析。玉米各生育期的平均温度和累计降雨量分别为：苗期17.32℃和190.25 mm，喇叭口期21.18℃和186.44 mm，灌浆期27.43℃和37.80 mm，成熟期30.34℃和66.55 mm。
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N2O排放通量随时间延长呈脉冲式变化，各处理在施肥后均出现排放峰值(图2)。N2O排放通量随生育期推进整体呈先升高后降低的趋势，峰值出现在喇叭口期，CK、NK和NPK处理在喇叭口期的平均日累积排放量为(1.60±0.33)、(31.03±4.90)和(18.98±1.97) g/hm2，NK和NPK处理的排放通量无显著差异，均显著高于CK。
图 2 玉米季N2O排放通量变化
Figure 2. Dynamics of N2O emission fluxes during maize season
从表1可以看出，NK和NPK处理各生育期N2O累积排放量均显著高于CK，增幅分别达4.32和3.70倍。NPK处理的N2O累积排放量在苗期显著高于NK处理，增幅为84.7%，成熟期与NK处理无显著差异，在生育期中间的喇叭口期和灌浆期较NK处理分别显著降低了38.9%和30.7% (P<0.05)。然而，总排放量NK与NPK处理之间无显著差异。
表 1 玉米关键生育期N2O累积排放量(g/hm2)
Table 1. Cumulative N2O emission flux at the key growth stages of maize
处理
Treatment苗期
Seedling喇叭口期
Big trumpet灌浆期
Filling成熟期
Harvest累积排放量
Cumulative emissionsCK 112.3±17.4 C 22.5±8.0 C 27.4±3.5 C 127.4±3.9 B 377.4±31.0 B NK 221.3±40.1 B 434.5±68.7 A 238.0±43.1 A 565.5±22.1 A 2009.3±213.9 A NPK 400.3±34.7 A 265.7±27.5 B 164.9±73.6 B 486.7±134.6 A 1774.5±110.5 A 注：表中数据为平均值±标准误 (n=3)，同列数据后不同大写字母表示同一生育期处理间差异显著 (P<0.05)。
Note: The data in the table are mean ± standard error (n=3). Different capital letters after data in a column indicate significant difference among treatments at the same growth dtage (P<0.05). -
随着玉米季生育期推进，NK处理土壤pH整体呈下降趋势，从苗期到成熟期降低了17.8%，其他各处理无显著差异(表2)。与CK相比，NK和NPK处理土壤pH显著降低。施肥后，整个玉米季各处理土壤有机碳含量整体呈上升趋势，从苗期到灌浆期CK、NK和NPK处理土壤有机碳分别升高15.2%、16.4%和16.2%。此外，NPK处理的土壤有机碳均显著大于NK。土壤NO3−-N和NH4+-N作为硝化反硝化的底物显著影响土壤中N2O的排放。在整个玉米季生育期内，CK、NK和NPK处理土壤NH4+-N呈下降趋势，NK和NPK处理均在苗期达到最高，之后NH4+-N含量明显下降，在成熟期降至最低。从结果看出，施加氮肥，土壤表层NH4+-N显著增高。在整个玉米季生育期，CK和NK处理土壤NO3−-N含量整体呈增加趋势，施肥显著增加NO3−-N含量。NPK处理的NO3−-N整体呈下降趋势，从苗期到灌浆期下降59.9%。除苗期NPK处理NO3−-N含量高于NK处理外，其它时期NK处理NO3−-N含量显著高于NPK处理。
表 2 玉米关键生育期各处理土壤化学性质
Table 2. Soil chemical properties in each treatment at the key growth stages of maize
处理
Treatment生育期
Growth stagepH 有机碳 (g/kg)
Organic matter铵态氮 (mg/kg)
NH4+-N硝态氮 (mg/kg)
NO3−-NCK 苗期 Seedling 6.04±0.35 aA 6.69±0.07 bC 5.76±0.35 aC 4.57±0.80 bC 喇叭口期 Big trumpet 6.09±0.26 aA 8.09±0.25 aC 7.37±0.50 aC 3.24±0.30 bcC 灌浆期 Filling 5.75±0.08 aA 7.89±0.45 aC 5.08±0.11 abC 10.03±0.52 aB 成熟期 Harvest 5.82±0.17 aA 7.02±0.41 abB 2.94±0.23 bC 10.46±1.34 aC NK 苗期 Seedling 4.78±0.29 aB 8.41±0.45 bB 231.73±1.21 aA 13.51±2.22 bB 喇叭口期 Big trumpet 4.11±0.25 bB 9.16±0.29 abB 28.25±0.40 bA 34.99±4.59 aA 灌浆期 Filling 4.15±0.29 bB 10.06±0.56 aB 8.83±0.34 cA 41.05±4.62 aA 成熟期 Harvest 3.93±0.16 bC 8.61±0.44 abB 5.57±0.20 dA 40.31±1.68 aA NPK 苗期 Seedling 4.37±0.15 aB 9.84±0.50 bA 80.36±9.20 aB 30.95±2.81 aA 喇叭口期 Big trumpet 4.35±0.13 aB 10.97±0.34 abA 14.07±1.56 bB 22.88±0.36 bB 灌浆期 Filling 4.38±0.13 aB 11.74±0.28 aA 6.51±0.38 cB 12.40±1.00 cB 成熟期 Harvest 4.33±0.08 aB 10.85±0.69 abA 4.12±0.38 cB 22.63±2.92 bB 注：表中数据为平均值±标准误 (n=3)，同列数据后不同大写字母表示同一生育期处理间差异显著，不同小写字母表示相同处理不同生育期间差异显著 (P<0.05)。
Note: The data in the table are mean ± standard error (n=3). Different capital letters after data in a column indicate significant difference among treatments, and lowercase letters indicate significant difference among growth periods under the same treatment (P<0.05). -
不同处理玉米季生育期反硝化酶活性动态变化不同(图3)。CK、NK和NPK处理的反硝化酶活性从苗期到成熟期呈下降趋势，分别降低59.1%、66.9%和29.1%；NPK处理显著高于其他处理，且一直处于较高水平；在苗期和喇叭口期CK处理显著高于NK，而在灌浆期和成熟期时两者无显著差异。在整个玉米季各处理土壤的NR活性呈先降低再升高的趋势，以喇叭口期活性最低；CK、NK和NPK的喇叭口期较苗期分别降低了51.2%、50.9%和38.4%；玉米全生育期的NR活性NPK>NK>CK。CK和NK处理NIR活性整体呈先降低后升高的趋势，且不同处理的最小值对应的生育期不同，CK处理在喇叭口期NIR活性最低，NK处理在喇叭口期和灌浆期NIR活性最低。喇叭口期CK和NK处理的NIR活性较苗期分别下降33.4%和76.2%，而喇叭口期NPK处理NIR活性较最大值降低23.5%。全生育期内NIR活性以NPK处理最高，其次为CK，NK处理最低。
图 3 玉米关键生育期土壤酶活性
Figure 3. Soil enzyme activities at key growth stages of maize
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Pearson分析结果表明，N2O累积排放量与生育期平均气温以及土壤NO3−-N含量、有机碳含量、NR活性呈显著或极显著正相关，与pH、生育期降雨量和反硝化酶活性呈显著负相关(图4)。土壤NH4+-N含量与生育期平均气温呈显著负相关，与生育期降雨量呈显著正相关。土壤NO3−-N含量与pH呈显著负相关。土壤有机碳与pH呈显著负相关，与NO3−-N含量呈显著正相关。反硝化酶活性与生育期平均气温呈显著负相关，与生育期降雨量和土壤有机碳呈显著正相关。NR与生育期平均气温和土壤有机碳呈显著正相关，与生育期降雨量和pH呈显著负相关。NIR与土壤有机碳、反硝化酶活性和NR呈显著正相关。
图 4 玉米关键生育期N2O累积排放量影响因素相关性分析
Figure 4. Correlation between influencing factors and cumulative N2O emission at key growth stages of maize
随机森林模型结果(图5)表明，苗期pH、NO3−-N、土壤有机碳和生育期降雨量是最重要的解释变量，其中累计重要性大小依次为土壤化学性质(pH，NH4+-N，NO3−-N和土壤有机碳，50.69%)、反硝化相关酶活性(反硝化酶，NR和NIR，24.53%)、施肥因素(氮肥、磷肥、钾肥，15.30%)、气候因素(生育期降雨量和生育期平均气温，16.86%)。而在喇叭口期，生育期降雨量、土壤有机碳、钾肥和NO3−-N是最重要的解释变量，累计重要性由高到低为土壤化学性质(39.51%)、施肥因素(23.08%)、反硝化相关酶活性(20.74%)、气候因素(20.05%)。在灌浆期，NR是最重要的解释变量，其次为反硝化酶活性、土壤有机碳和生育期降雨量，累计重要性为反硝化相关酶活性(34.41%)>土壤化学性质(30.91%)>施肥因素(26.25%)>气候因素(19.26%)。土壤有机碳是成熟期最重要的解释变量，其次为NO3−-N、NR和生育期降雨量，累计重要性为土壤化学性质(40.24%)>反硝化相关酶活性(27.45%)>施肥因素(20.28%)>气候因素(16.71%)。
图 5 玉米关键生育期各因素对N2O累积排放量的重要性
Figure 5. Importance of various factors for the cumulative N2O emission at the key growth stages of maize
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不同施肥下N2O排放通量不同。本研究的排放通量特征与前人研究[20]结果基本一致。尿素水解产生的铵态氮作为硝化作用底物直接影响了N2O的排放[7]。尿素进入土壤后在相同的施氮量下NPK处理NH4+-N在苗期达到最高(80.36±9.20) mg/kg，而NK处理高达(231.73±1.21) mg/kg，说明NPK处理的NH4+-N在苗期的消耗高于NK处理，因此在苗期NPK处理的N2O排放通量增加速率高于NK。土壤中NH4+-N含量减少是导致土壤N2O排放的原因之一[21]。NPK处理(苗期至喇叭口期)的土壤NH4+-N减少了66.29 mg/kg，而NK处理的土壤NH4+-N减少了203.23 mg/kg，导致喇叭口期NK处理的土壤N2O排放通量迅速增加。玉米季灌浆期NK和NPK处理的N2O排放通量降低，可能是由于土壤NH4+-N含量减少且没有形成适宜进行反硝化的环境。成熟期各处理N2O排放通量快速增加，原因可能是此时土壤中NH4+-N含量较低而NO3−-N含量高，且成熟期较高的温度和降水为反硝化提供了适宜的条件，N2O排放通量随之升高[22]。
影响不同生育期N2O累积排放量的重要因素各不相同。本研究中玉米季施肥处理的N2O累积排放量显著高于不施肥处理，但NK和NPK处理的玉米季累积排放量无显著差异，与前人研究结果[20]不同。这可能由于在各关键生育期pH的相对重要性均高于磷肥，然而受作物养分吸收的影响，导致虽然在各关键生育期NK和NPK处理N2O累积排放量不同，但整个生长季累积排放量无显著差异。影响苗期N2O累积排放量最重要的几个因素是pH、NO3−-N和土壤有机碳。其中NPK处理的pH和NK处理无显著差异，而NPK处理的NO3−-N和土壤有机碳显著高于NK，同时NO3−-N和土壤有机碳与N2O累积排放量显著正相关，因此苗期NPK处理的N2O累积排放量显著高于NK处理，可能是NPK处理均衡施肥促进土壤中微生物活性导致苗期硝化过程进行较多[23]。生育期降雨量、土壤有机碳、钾肥和NO3−-N是影响喇叭口期N2O累积排放量的重要因素。NPK处理的土壤有机碳显著高于NK处理，而NO3−-N含量显著低于NK处理，由于NK处理NH4+-N从苗期至喇叭口期大量减少，因此NPK处理的喇叭口期N2O累积排放量显著低于NK处理的原因为NPK处理该时期硝化反应较弱[24]，虽然此时NK (4.11±0.25)和NPK (4.35±0.13)处理的pH均低于4.8，但pH并不是影响喇叭口期N2O累积排放量的最重要因素，所以此时反硝化反应并不能主导N2O排放。在灌浆期NR、反硝化酶活性、土壤有机碳和生育期降雨量是影响N2O累积排放量最重要的因素。NPK处理的NR和反硝化酶活性均显著高于NK处理，同时NR与N2O累积排放量显著正相关，反硝化酶活性与N2O累积排放量显著负相关，但NK处理的反硝化底物NO3−-N含量显著高于NPK处理，这可能导致NK处理灌浆期的N2O累积排放量显著高于NPK处理[25]。土壤有机碳、NO3−-N、NR是影响成熟期N2O累积排放量最重要的因素。NPK处理的土壤有机碳和NR显著高于NK处理，但NO3−-N显著低于NK处理，这可能是两施肥处理间成熟期N2O累积排放量无显著差异的原因。NR和土壤有机碳均为灌浆期和成熟期影响NK和NPK处理的N2O累积排放量的重要因素，可能是由于这两个生育期的N2O来源主要为反硝化作用。
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气候因素通过影响土壤环境来影响硝化反硝化过程产生N2O[26−27]。夏玉米季较高的大气温度促进土壤温度升高，在喇叭口期土壤中NR活性达到最高，因此N2O排放通量达到一个峰值[27]。7月、8月均有连续降雨，干湿交替的环境也抑制N2O还原为N2[28]，在7月15日和8月5日监测的N2O排放通量均显著上升。
N2O排放也受到土壤化学性质的影响，低pH抑制硝化、反硝化酶活性，减少N2O排放[29−30]。本试验选取的NK和NPK处理pH均低于4.5，土壤pH与N2O排放呈极显著负相关关系，pH降低可能通过促进NR来促进反硝化过程，且土壤pH对氧化亚氮还原酶活性的抑制更强，因此N2O排放量增加。有机碳和无机氮的增加促进土壤N2O排放[31−33]。本研究玉米季施肥促使土壤有机碳增加了13.0%～54.6%，长期施肥通过提高土壤养分并促进作物生长，导致更多的根际残留物和分泌物归还土壤，从而促进了土壤有机碳增加[31]。土壤有机碳通过为土壤中的微生物提供碳源，促进硝化反硝化作用进而增强N2O排放[32]。NH4+-N作为硝化过程的底物也对N2O排放有重要影响[33]，氮肥的投入可以增加土壤中的NH4+-N，NK和NPK处理较不施肥处理分别增加了89.5%～40.1%，同时NH4+-N含量从苗期到灌浆期逐渐减少，对N2O关键生育期累积排放量影响的重要性也随之下降。NO3−-N与N2O累积排放量呈显著正相关，在土壤中硝化作用产生NO3−-N，同时该过程也会产生N2O，当氮肥投入后，硝化作用增强，硝化作用产生的N2O越多，NO3−-N含量也随之增加，整个玉米季施肥促使NO3−-N增加了1.16～2.85倍。同时NO3−-N作为反硝化过程的底物，其含量的增加也会促进反硝化作用产生N2O[33]。
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在反硝化过程中产生N2O的关键酶有NR和NIR[34]。反硝化酶活性变化可作为区分旱地N2O产生途径的指标[35]。反硝化酶活性与N2O关键生育期累积排放量呈显著负相关，表明本研究的N2O多来源于硝化反应。从苗期开始，反硝化酶活性的相对重要性在灌浆期升至最高，说明灌浆期时反硝化过程可能进行的最多。NR作为反硝化作用第一步反应的酶，其活性受到底物NO3−-N含量影响[36]。且NR与N2O关键生育期累积排放量显著正相关，硝化作用产生N2O的同时也产生了大量的NO3−-N，促进了NR活性升高，因此本研究的N2O更多的来源于硝化作用[37]。NR与反硝化酶相似，均在灌浆期达到相对重要性最大，也说明此生育期反硝化进行的最多。同时灌浆期的N2O累积排放量在各关键生育期的排放较低，因此本研究N2O的产生主要来源于硝化作用，与Cheng等[10]不同。NIR是反硝化过程中由亚硝态氮转化为气态一氧化氮的酶，但NIR与N2O排放无显著相关性，表明反硝化过程产生的N2O较少。同时NIR与反硝化酶和NR均呈显著正相关，原因可能是NIR的底物亚硝态氮来源于NR产物，同时反硝化酶活性高低代表了反硝化的进行。
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玉米不同生育期提高土壤N2O累积排放的主要影响因素不同，苗期为土壤pH，大喇叭口期为累积降雨量，灌浆期为土壤硝酸还原酶活性，成熟期为有机碳含量。酸性红壤上，施用氮肥可降低土壤pH、提高土壤硝态氮和有机碳含量，进而显著增加N2O排放量。长期氮钾化肥配施与氮磷钾化肥配施条件下，玉米生育期内土壤N2O累积排放量没有显著差异，氮磷钾配施显著增加了苗期N2O累积排放量，而氮钾配施增加了大喇叭口期和灌浆期N2O累积排放量。
长期施肥下红壤玉米关键生育期氧化亚氮排放差异及其影响因素
Red soil N2O emission difference caused by fertilizers and other factors at the key growth stages of maize
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摘要:
【目的】 探究红壤区玉米关键生育期氧化亚氮(N2O)的排放量及影响因素，为红壤区玉米季温室气体减排提供理论支撑。 【方法】 典型红壤长期施肥定位试验始于1990年，选取不施肥(CK)、氮钾化肥配施(NK)、氮磷钾化肥配施(NPK) 3个处理，监测玉米苗期、喇叭口期、灌浆期和成熟期N2O排放、温度和降雨量，测定了表层土壤理化性状和硝酸还原相关酶的活性。 【结果】 与CK相比，NK和NPK处理均显著提高了N2O累积排放量，NPK处理显著增加苗期N2O累积排放量，而NK处理显著提高了喇叭口期和灌浆期N2O累积排放量，但两个处理的玉米生育期N2O累积排放总量没有显著差异。从玉米苗期到成熟期，NK处理土壤pH整体呈下降趋势，降低了17.8%，而CK和NPK处理无显著变化；CK、NK和NPK处理土壤有机碳(SOC)整体呈上升趋势，分别较灌浆期升高了15.2%、16.4%和16.2%，NH4+-N含量呈逐渐降低趋势，而NO3−-N含量呈逐渐上升趋势。NPK处理苗期土壤NO3−-N含量显著高于NK处理，喇叭口期和灌浆期均低于NK处理，NH4+-N含量全生育期均显著低于NK处理。从苗期到成熟期，各处理反硝化酶(DE)活性呈下降趋势，CK、NK处理的降幅(59.1%、66.9%)高于NPK处理(29.1%)；各处理硝酸还原酶(NR)活性均在喇叭口期最低，NK和NPK处理各生育期NR活性高于CK；亚硝酸还原酶各处理也在喇叭口期最低，CK和NK处理分别下降33.4%和76.2%，NPK处理较最大值降低23.5%。Pearson分析结果表明，各生育期N2O累积排放量与生育期平均气温(TEM)、NO3−-N、SOC和NR呈显著正相关关系，与生育期累积降雨量(PCP)、pH和反硝化酶活性呈显著负相关关系。随机森林模型分析结果表明，苗期、喇叭口期、灌浆期、成熟期与N2O累积排放量相关最显著的因素分别为pH、PCP、NR、SOC。 【结论】 在玉米不同生育期，影响土壤N2O累积排放的主要因素不同，苗期为土壤pH，大喇叭口期为累积降雨量，灌浆期为土壤硝酸还原酶活性，成熟期为土壤有机碳含量。酸性红壤上，施用化肥氮可降低土壤pH，提高土壤硝态氮和有机碳含量，进而显著增加N2O排放量。虽然长期施用氮钾肥与施用氮磷钾肥玉米生育期的N2O累积排放量没有显著差异，但施用氮磷钾肥显著提升苗期N2O排放量，而施用氮钾肥增加大喇叭口期和灌浆期N2O排放量。 Abstract:【Objectives】 We studied the nitrous oxide (N2O) emission at the key growth stages of maize in red soils under different fertilizer applications, and the main factors relative to the emission, so as to provide theoretical base for reduction of N2O emission in the red soil area. 【Methods】 The research based on a long-term fertilization experiment in the typical red soil region, started since 1990. Three of the treatments were selected for the research: no fertilization (CK), N and K fertilizer combined application (NK), and N, P and K fertilizer combined application (NPK). At seedling, big trumpet, grain filling and maturing stage of maize, the temperature, rainfall and N2O emission were monitored, the top soil chemical indexes (pH, SOC, NH4+-N and NO3−-N content), and the activities of relative enzymes were analyzed. 【Results】 The N2O emission was pulsed throughout maize season, and the NK and NPK treatment led to significantly higher soil cumulative N2O emission than CK. NPK treatment significantly increased the N2O emission at seedling stage, while CK treatment increased that at big trumpet and grain filling stage, the two treatments had similar total N2O emission. From seedling to maturing stage, soil pH in NK treatment kept decreasing, with decrease range of 17.8%, while the pH in CK and NPK treatment did not changed significantly. Soil organic carbon kept an overall upward trend, with an increase range of 15.2%, 16.4% and 16.2% in CK, NK, and NPK treatment, respectively. The NH4+-N in all treatment soils decreased while NO3−-N increased gradually. The NO3−-N in NPK treatment was higher than NK treatment at seedling stage but lower at big trumpet and grain filling stage, and the NH4+-N in NPK treatment was significantly lower than in NK treatment at all the stages. The activity of soil denitrification enzyme (DE) decreased by 59.1%, 66.9%, and 29.1% from seedling to maturing stage in CK, NK and NPK treatment, respectively. The nitrate reductase (NR) activities in all the three treatment soils decreased to the lowest at the big trumpet stage and then increased, the activities in NK and NPK treatment soils were still higher than in CK soil at each growth stage. Nitrite reductase activities decreased to the lowest at grain filling stage, with the lower range 33.4% and 76.2% in CK and NK treatment, and 23.5% NPK treatment than the maximum. The Pearson correlation analysis showed the cumulative N2O emission was positively correlated with mean temperature (TEM), NO3−-N, SOC and NR, negatively correlated with cumulative precipitation (PCP), soil pH and DE. Random forest model analysis showed the most important factors affecting cumulative N2O emissions were soil pH at seedling stage, PCP at big trumpet stage, NR at filling stage, and SOC at harvest stage. 【Conclusions】 In the red soil area, the important factors related to N2O emission are soil pH at seedling stage, cumulative rainfall at big trumpet stage, NR at filling stage, and SOC at harvest stage. Nitrogen fertilizer application decreases soil pH, increases nitrate and organic carbon content, thereby increases N2O emission. NK and NPK combined application would not influence the total N2O emission, but NPK application would increase N2O emission at maize seedling stage, while NK application would increase that at big trumpet and grain filling stage. -
图 4 玉米关键生育期N2O累积排放量影响因素相关性分析
Figure 4. Correlation between influencing factors and cumulative N2O emission at key growth stages of maize
图 5 玉米关键生育期各因素对N2O累积排放量的重要性
Figure 5. Importance of various factors for the cumulative N2O emission at the key growth stages of maize
表 1 玉米关键生育期N2O累积排放量(g/hm2)
Table 1. Cumulative N2O emission flux at the key growth stages of maize
处理
Treatment苗期
Seedling喇叭口期
Big trumpet灌浆期
Filling成熟期
Harvest累积排放量
Cumulative emissionsCK 112.3±17.4 C 22.5±8.0 C 27.4±3.5 C 127.4±3.9 B 377.4±31.0 B NK 221.3±40.1 B 434.5±68.7 A 238.0±43.1 A 565.5±22.1 A 2009.3±213.9 A NPK 400.3±34.7 A 265.7±27.5 B 164.9±73.6 B 486.7±134.6 A 1774.5±110.5 A 注：表中数据为平均值±标准误 (n=3)，同列数据后不同大写字母表示同一生育期处理间差异显著 (P<0.05)。
Note: The data in the table are mean ± standard error (n=3). Different capital letters after data in a column indicate significant difference among treatments at the same growth dtage (P<0.05).
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表 2 玉米关键生育期各处理土壤化学性质
Table 2. Soil chemical properties in each treatment at the key growth stages of maize
处理
Treatment生育期
Growth stagepH 有机碳 (g/kg)
Organic matter铵态氮 (mg/kg)
NH4+-N硝态氮 (mg/kg)
NO3−-NCK 苗期 Seedling 6.04±0.35 aA 6.69±0.07 bC 5.76±0.35 aC 4.57±0.80 bC 喇叭口期 Big trumpet 6.09±0.26 aA 8.09±0.25 aC 7.37±0.50 aC 3.24±0.30 bcC 灌浆期 Filling 5.75±0.08 aA 7.89±0.45 aC 5.08±0.11 abC 10.03±0.52 aB 成熟期 Harvest 5.82±0.17 aA 7.02±0.41 abB 2.94±0.23 bC 10.46±1.34 aC NK 苗期 Seedling 4.78±0.29 aB 8.41±0.45 bB 231.73±1.21 aA 13.51±2.22 bB 喇叭口期 Big trumpet 4.11±0.25 bB 9.16±0.29 abB 28.25±0.40 bA 34.99±4.59 aA 灌浆期 Filling 4.15±0.29 bB 10.06±0.56 aB 8.83±0.34 cA 41.05±4.62 aA 成熟期 Harvest 3.93±0.16 bC 8.61±0.44 abB 5.57±0.20 dA 40.31±1.68 aA NPK 苗期 Seedling 4.37±0.15 aB 9.84±0.50 bA 80.36±9.20 aB 30.95±2.81 aA 喇叭口期 Big trumpet 4.35±0.13 aB 10.97±0.34 abA 14.07±1.56 bB 22.88±0.36 bB 灌浆期 Filling 4.38±0.13 aB 11.74±0.28 aA 6.51±0.38 cB 12.40±1.00 cB 成熟期 Harvest 4.33±0.08 aB 10.85±0.69 abA 4.12±0.38 cB 22.63±2.92 bB 注：表中数据为平均值±标准误 (n=3)，同列数据后不同大写字母表示同一生育期处理间差异显著，不同小写字母表示相同处理不同生育期间差异显著 (P<0.05)。
Note: The data in the table are mean ± standard error (n=3). Different capital letters after data in a column indicate significant difference among treatments, and lowercase letters indicate significant difference among growth periods under the same treatment (P<0.05).
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