环境科学  2022, Vol. 43 Issue (11): 5149-5158   PDF    
引用本文
邵晓辉, 汤水荣, 孟磊, 伍延正, 李金秋, 缑广林. 不同施肥措施对热带地区稻菜轮作体系土壤CH4和N2O排放的影响[J]. 环境科学, 2022, 43(11): 5149-5158.
SHAO Xiao-hui, TANG Shui-rong, MENG Lei, WU Yan-zheng, LI Jin-qiu, GOU Guang-lin. Effect of Different Fertilization Treatments on Methane and Nitrous Oxide Emissions from Rice-Vegetable Rotation in a Tropical Region, China[J]. Environmental Science, 2022, 43(11): 5149-5158.
不同施肥措施对热带地区稻菜轮作体系土壤CH4和N2O排放的影响
邵晓辉, 汤水荣, 孟磊, 伍延正, 李金秋, 缑广林     
海南大学热带作物学院, 海口 570228
收稿日期: 2021-12-21; 修订日期: 2022-03-12
基金项目: 海南省高层次人才基金项目(2019RC064);国家自然科学基金青年科学基金项目(41807022);海南大学高层次人才科研启动基金项目(KYQD(ZR)1858)
作者简介: 邵晓辉(1995~), 男, 硕士研究生, 主要研究方向为土壤碳氮循环与环境效应, E-mail：shaoxiaohuishi@163.com
通信作者: 汤水荣, E-mail: tangshuirong@163.com; 孟磊, E-mail: menglei94@sohu.com
摘要: 研究稻菜轮作模式下土壤甲烷(CH4)和氧化亚氮(N2O)排放对不同施肥措施的响应, 对补充我国热带地区CH4和N2O排放研究的不足具有重要的指导意义.在辣椒季设置4种施肥处理: 磷钾肥(PK)、氮磷钾肥(NPK)、等氮条件下50%有机肥替代化肥(NPK+M)和100%有机肥替代(M), 水稻种植季未设置施肥处理, 研究辣椒季不同施肥条件下CH4和N2O的排放规律以及对早稻生长季水稻产量、CH4和N2O排放的后续影响.采用密闭静态箱-气相色谱法测定稻菜轮作土壤CH4和N2O, 同时测定作物产量, 并估算全球增温潜势(GWP)和温室气体排放强度(GHGI).结果表明: 1辣椒季和早稻季4种施肥处理下土壤CH4的累积排放量分别为0.9~2.7 kg ·hm-2和5.5~8.4 kg ·hm-2, 与NPK处理相比, 辣椒季NPK+M和M处理CH4累积排放量分别减少35.3%和7.6%; 而早稻季NPK+M和M处理CH4累积排放量均增加37.5%和55.1%, 其中早稻季M处理达到显著水平.2辣椒季和早稻季4种施肥处理下N2O的累积排放量分别为0.5~3.0 kg ·hm-2和0.3~0.5 kg ·hm-2, 相对NPK处理, 辣椒季NPK+M和M处理降低33.7%和16.0%的N2O累积排放量, 其中NPK+M处理达到显著差异, 早稻季NPK+M处理N2O累积排放量降低23.5%, M处理却增加9.1%, 但均未达到显著水平.3 4种施肥处理下辣椒和早稻的产量分别为3055.6~37722.5 kg ·hm-2和5850.9~6994.4 kg ·hm-2, 与NPK处理相比, NPK+M和M处理显著增加辣椒产量.各施肥处理GWP为508.0~1864.4 kg ·hm-2, NPK+M和M处理相对NPK处理分别下降25.7%和5.7%, 其中NPK+M处理达到显著差异.辣椒季各处理的GWP对总GWP的贡献率为69.2% ~78.1%, N2O对总GWP的贡献率为77.3% ~85.3%.辣椒季和早稻季GHGI分别为0.03~0.09 kg ·kg-1和0.04~0.24 kg ·kg-1, 与NPK处理相比, 辣椒季M和NPK+M处理使GHGI显著下降71.5%和54.7%, 早稻季NPK+M和M处理GHGI值分别下降44.0%和20.8%, 其中NPK+M处理达到显著差异.综合作物产量及温室气体减排效果考虑, 化肥和有机肥配施(NPK+M)可推荐为海南稻菜轮作模式下一种最优的减排稳产的施肥措施.
关键词: 固碳减排      全球增温潜势      有机肥替代化肥      稻菜轮作      热带地区     
Effect of Different Fertilization Treatments on Methane and Nitrous Oxide Emissions from Rice-Vegetable Rotation in a Tropical Region, China
SHAO Xiao-hui , TANG Shui-rong , MENG Lei , WU Yan-zheng , LI Jin-qiu , GOU Guang-lin     
College of Tropical Crops, Hainan University, Haikou 570228, China
Abstract: The study of the effects of different fertilization treatments on soil methane (CH4) and nitrous oxide (N2O) emissions in rice-vegetable rotation systems is of great significance to supplement the research gap on greenhouse gas emissions in tropical regions of China. In this study, four fertilization treatments were set up during the pepper season: phosphorus and potassium fertilizer application (PK); nitrogen, phosphorus, and potassium (NPK) application; half application of nitrogen, phosphorus, and potassium plus half application of organic fertilizer (NPK+M); and application of organic fertilizer (M). There was no fertilizer application during the following early rice season. The objective of our study was to investigate the rules of CH4 and N2O emissions under different fertilization treatments in the pepper growth season, and the effects of different fertilization treatments in the pepper growth season on rice yield, and CH4 and N2O emissions in the following early rice growth season. The close static chamber-gas chromatography method was applied to determine soil CH4 and N2O emissions. We measured crop yield, estimated global warming potential (GWP), and calculated greenhouse gas emission intensity (GHGI). Our results showed that: ① the cumulative CH4 emission under the four fertilization treatments ranged between 0.9 kg·hm-2 to 2.7 kg·hm-2 during the pepper growth season and between 5.5 kg·hm-2 to 8.4 kg·hm-2 during the early rice growth season. Compared with NPK, NPK+M and M reduced the cumulative CH4 emission in the pepper growth season by 35.3% and 7.6%, respectively; however, NPK+M and M increased the cumulative CH4 emission in the early rice season by 37.5% and 55.1%, respectively. There was a significant difference in cumulative CH4 emission between M and NPK in the early rice growth season. ② The cumulative N2O emission under the four fertilization treatments varied from 0.5 kg·hm-2 to 3.0 kg·hm-2 in the pepper growth season and from 0.3 kg·hm-2 to 0.5 kg·hm-2 in the early rice growth season. The cumulative N2O emission was significantly decreased by 33.7% in NPK+M and by 16.0% in M, compared with that in NPK. In the early rice growth season, the cumulative N2O emission was decreased by 23.5% by NPK+M but was increased by 9.1% by M. There was no significant difference in the cumulative N2O emission among the four fertilization treatments. ③ The yields of pepper and early rice under the four fertilization treatments were 3 055.6-37 722.5 kg·hm-2 and 5 850.9-6 994.4 kg·hm-2, respectively. Compared with that in NPK, NPK+M and M significantly increased pepper yield. The GWP under the four fertilization treatments in the pepper-early rice rotation system varied from 508.0 kg·hm-2 to 1 864.4 kg·hm-2. Compared with NPK, NPK+M significantly decreased GWP by 25.7% and M insignificantly decreased GWP by 5.7%. The pepper growth season with the four fertilization treatments contributed to 69.2%-78.1% of the total GWP, and N2O contributed to 77.3%-85.3% of the total GWP. The GHGI ranged between 0.03 kg·kg-1 and 0.09 kg·kg-1 in the pepper growth season and between 0.04 kg·kg-1 and 0.24 kg·kg-1 in the early rice growth season. Compared with that in NPK, both M and NPK+M significantly reduced the GHGI by 71.5% and 54.7%, respectively, in the pepper growth season. In the early rice season, NPK+M significantly decreased the GHGI by 44.0%, but M non-significantly decreased the GHGI by 20.8%. The peak in N2O emission in the tropical pepper-early rice rotation system appeared after fertilization, and N2O emissions primarily occurred in the pepper growth season. However, CH4 emission was mainly concentrated in the early rice season. Considering the overall enhancing effects on crop yield and mitigation of greenhouse gas emissions, the co-application of chemical and organic fertilizers (NPK+M) can be recommended as an optimal fertilization practice to mitigate greenhouse gas emissions and maintain crop yield in pepper-rice rotation systems of Hainan, China.
Key words: C sequestration and mitigation      global warming potential      substitution of chemical fertilizer with manure      rice-vegetable rotation      tropical region     

近年来由于二氧化碳(CO2)等温室气体大量排放导致全球气候变暖已日益引起人们的普遍关注.据IPCC(2018年)统计, 预计2030~2052年间全球平均气温将上升约1.5℃, 甲烷(CH4)和氧化亚氮(N2O)作为两种重要的温室气体, 其增温效应均次于CO2, 而大气中CH4和N2O主要来自于农田系统, 其贡献率分别约占全球CH4和N2O总量的33%和69%[1~4].我国作为农业大国, 农业高度集约化, 大量肥料投入容易造成一系列严重的生态环境问题.因此, 研究稻田生态系统中CH4和N2O排放规律和减缓对策, 对于固碳减排和保障粮食安全等方面具有十分重要的意义.

作为农业生产的重要保障和常见的农田管理措施之一, 施肥在提升土壤肥力和作物产量的同时, 也影响着温室气体的排放, 而在农田生态系统中温室气体的排放和作物产量也因肥料种类、施用量和施用方式存在较大的差异[5, 6].有研究表明, 合理的施肥管理不仅可以提高作物产量, 而且有利于培肥土壤, 同时也是一种实现化肥零增长的长期有效措施[7, 8].但关于不同施肥管理对农田土壤CH4和N2O排放的影响存在一定差异[6, 9].有研究指出, 肥料的施入会导致稻田CH4大量排放, 而与化肥相比, 施用有机肥未促进CH4排放[10~12].由此可见优化施肥对减少稻田CH4排放潜力巨大.一般认为在水稻种植前期, 由于长期淹水会促进稻田土壤的反硝化作用, 势必会引起N2O的排放.稻田土壤N2O的排放量约占农业源N2O排放的10%, 过量使用氮肥将导致大气中N2O浓度持续增加[13, 14], 而施用有机肥对N2O排放的影响受多重因素的影响且具有较大的不确定性[15, 16].一方面, 有机肥中的氮是农田产生N2O的重要来源; 另一方面, 有机肥的施入会增加土壤有机碳的含量, 有机碳可以固持土壤矿质氮减少N2O的排放.因此, 深入研究不同气候带、土壤类型和施肥模式下稻田N2O和CH4的排放规律及机制, 对准确估算我国农田生态系统N2O和CH4排放量和科学制定碳中和的施肥措施具有重要的指导意义.

海南省地处我国热带, 常年高温多雨, 雨热同期, 具有鲜明的热带季风气候特征.特定的气候条件使得稻-菜轮作模式成为当地最常见的种植模式之一[17].作为保障当地农民增收和农业增效的传统特色产业, 冬季瓜菜则是施肥的重点对象.与亚热带和温带地区的轮作模式相比, 海南稻菜轮作系统具有气温高、降雨量大和瓜菜季施肥量大等特点, 瓜菜季施氮肥量可高达750 kg·hm-2, 大量氮肥的投入势必会影响土壤CH4和N2O排放[18, 19].目前, 关于辣椒季大量施用氮肥是否会引起来年早稻土壤中CH4和N2O的大量排放多停留于假设, 缺乏田间数据的有力支撑.此外, 关于热带地区整个稻菜轮作模式下CH4和N2O所引起的增温潜势与排放强度对不同施肥措施的响应研究还鲜见报道.因此, 本研究利用田间原位监测试验平台, 观测了热带地区典型稻菜轮作模式下农田CH4和N2O排放特征对辣椒季施肥的响应规律, 目的在于通过确定热带地区稻菜轮作系统CH4和N2O排放量和主要影响因子, 以期为准确编制稻田生态系统N2O和CH4排放清单和通过优化施肥以促进增产减排提供科学参考.

1 材料与方法 1.1 试验地概况

本试验于2020年12月至2021年8月在海南省澄迈县桥头镇西岸村稻菜轮作试验田(110°04′E, 19°56′N)进行, 该地区属于典型的热带季风气候, 全年温度高且降雨量大(年平均气温24.8℃, 年均降雨量1 819.2 mm).试验田土壤为滨海沉积物母质发育的沙壤土, 辣椒种植前土壤的ω(有机质)和ω(全氮)分别为29.6 g·kg-1和1.8 g·kg-1, 土壤pH为5.7(表 1).本试验观测期间气温和降水量变化见图 1.

表 1 土壤基本性质 Table 1 Basic properties of soil

图 1 采样期间气温与降水量变化 Fig. 1 Dynamics of air temperature and precipitation during the sampling period

1.2 试验设计

本试验在辣椒种植季设置4种施肥处理：只施磷钾肥(PK)、施氮磷钾肥(NPK)、等氮条件下50%有机肥替代化肥(NPK+M)和100%有机肥替代(M), 氮肥施用量为当地常规施肥量.采用完全随机区组试验, 每个处理3个重复, 小区面积为21 m2 (3 m×7m).每个小区边界用防水膜覆盖, 防止窜水窜肥.辣椒季施用N、P和K肥分别为400、282和500 kg·hm-2.有机肥为豆粕商品有机肥(含N 3.8%), N肥为复合肥(N∶P∶K=22∶8∶12).辣椒季施肥共分5次施入, 5次氮肥配比为2∶2∶3∶2∶1, NPK+M处理和M处理中有机肥在施基肥时全部施用(P和K含量不足时以常规磷肥和钾肥补充), 复合肥水溶后施入, 早稻季均不施肥, 整个辣椒季4种施肥处理中P和K的含量均分别处于同一水平.

辣椒季于2020年12月初开始育辣椒苗, 品种为泡椒(薄冠008), 于12月15日在各小区内翻耕起垄, 12月28日施入基肥并开始移栽辣椒苗.12月29日开始采气, 每隔20 d左右追肥1次, 共追肥4次, 辣椒为多次采收计产, 4月26日收获.早稻季于2021年5月8日耕地, 5月10日按底座和插秧, 5月11日开始采气, 8月15日收割水稻.每隔5~7 d采集1次气样, 采气时间为08:00~11:00.

1.3 样品采集与测定 1.3.1 气体样品

采用静态暗箱法采集温室气体, 采气箱用一层隔热锡箔纸包裹, 防止太阳直射引起箱温升高, 立体矩形箱体(长宽高分别为50、50和90 cm)顶部有抽气孔和温度计孔.采集气体前, 将采气箱垂直安放在带有凹槽的底座上(3 cm高)并灌注少许水, 以防止漏气.分别在0、10、20和30 min用塑料注射器抽取大约30 mL气体, 注入事先抽成真空的玻璃瓶(日电理化玻璃, 日本)中, 随后用气相色谱(岛津GC-2014, 日本)对气样CH4和N2O浓度进行分析.CH4和N2O的标准气体由中国计量科学研究院提供.

1.3.2 土壤样品

在2020年辣椒移栽前, 采集0~20 cm土壤, 自然风干后进行过筛处理, 土壤养分测定参考土壤农化分析[20].新鲜土样中的铵态氮(NH4+-N)和硝态氮(NO3--N)采用2 mol·L-1 KCl-连续流动分析仪(Proxima1022/1/1, 法国)测定.

1.4 数据处理与分析方法

CH4和N2O排放通量计算均如公式(1)所示[21]：

(1)

式中, f为排放通量, fCH4单位为mg·(m2·h)-1, fN2O单位为μg·(m2·h)-1; ρ为标准状态下CH4-C和N2O-N的密度(kg·m-3); h为采气箱的高度(m); Δc/Δt为采集气体过程中箱中气体摩尔分数变化速率; T为采集气体时箱内平均温度(℃).

累积排放量计算方法如公式(2)所示[21]：

(2)

式中, E为累积排放量, 单位为kg·hm-2; n和i为采样次数; t为采样天数(d).

100 a尺度上以等量CO2增温为基准, 稻田排放CH4和N2O增温潜势(GWPGHGS, kg·hm-2)计算方法如公式(3)所示[22]：

(3)

温室气体排放强度(GHGI, 以CO2-eq计, kg·kg-1)为GWP与作物产量(kg·hm-2)的比值, 反映单位产量条件下的增温潜势[22].

使用Excel 2013和Origin 2020进行数据分析和作图, 采用R软件(V.4.1.2)进行统计分析, 处理间的差异采用Duncan多重比较法.运用Pearson法对各变量进行相关性分析, 显著水平为P < 0.05.本研究中所有数据均以3次重复的平均值±标准差表示.

2 结果与分析 2.1 辣椒-早稻季不同施肥处理土壤CH4排放通量及累积排放量

在整个辣椒生育期, 由于4种施肥处理的水分条件及管理均保持一致, 且未造成长期的厌氧条件, 故CH4排放通量极低, 基本上在零值上下波动(图 2).在整个观测期间, 虽出现几个小的CH4排放峰值, 但土壤CH4的氧化与排放处于一个相对平衡的状态, 这可能是由于辣椒地在灌溉施肥过程中, 造成短期的厌氧条件从而刺激了CH4排放.就施肥管理而言, 各处理下土壤CH4排放趋势一致.分蘖期和孕穗抽穗期是早稻季CH4排放的主要时期.PK处理CH4排放通量始终处于平稳水平, 而其它3种施肥处理均出现多个CH4排放峰值, 前期各处理CH4排放通量的大小表现为：M＞NPK+M＞NPK＞PK.

实线箭头表示施肥 图 2 不同施肥处理下辣椒季和早稻季CH4的排放通量变化 Fig. 2 Dynamics of CH4 emission fluxes in pepper season and early rice season under different fertilization treatments

在整个辣椒季, 各处理CH4累积排放量的变化范围为0.9~2.7 kg·hm-2[图 3(a)].与PK处理相比, NPK和M处理均显著增加CH4的累积排放量, 增幅分别为200.2%和177.5%.尽管NPK+M和M处理相对NPK处理降低CH4累积排放量, 但均未达到显著水平.早稻季各处理CH4累积排放量为5.5~8.4 kg·hm-2[除PK外，图 3(b)], NPK、M和NPK+M处理相比PK处理分别增加19.7%、64.5% 和85.6%, M和NPK+M处理与PK处理差异显著.M和NPK+M处理的CH4累积排放量较NPK处理分别上升55.1%和37.5%, 其中M处理与NPK处理之间的CH4累积排放量差异显著.

不同小写字母表示不同处理间差异显著(P < 0.05) 图 3 不同施肥处理下辣椒季和早稻季CH4的累积排放量 Fig. 3 Cumulative CH4 emissions in pepper season and early rice season under different fertilization treatments

2.2 辣椒-早稻季不同施肥处理土壤N2O排放通量及累积排放量

施肥后辣椒季各处理土壤N2O排放增加, 通常于施肥后第2~8 d出现排放峰值, 整个排放峰约持续1周(图 4).辣椒季PK、NPK、NPK+M和M处理土壤N2O排放通量分别为-2.6~41.9、23.0~482.1、18.0~165.9和21~279.9 μg·(m2·h)-1, 平均排放通量分别为19.2、124.7、79.4和95.2 μg·(m2·h)-1, NPK和M处理排放峰值显著高于PK和NPK+M处理(P < 0.05).前期M处理N2O排放速率较高, 而后期NPK处理N2O排放速率高.早稻季各处理N2O排放通量峰值高低顺序为：NPK>M>NPK+M>PK.早稻的分蘖期后仅有M处理出现小的N2O排放峰值, 而其他3个处理的土壤N2O排放通量均处于较低水平.

实线箭头表示施肥 图 4 不同施肥处理下辣椒季和早稻季N2O排放通量变化 Fig. 4 Changes in N2O emission fluxes in pepper season and early rice season under different fertilization treatments

各处理辣椒季N2O累积排放量大小顺序为：NPK＞M＞NPK+M＞PK[图 5(a)], NPK、NPK+M和M处理较PK处理均显著增加.NPK和NPK+M处理与M处理间排放结果无显著性差异, 而NPK与NPK+M处理间排放结果差异显著.早稻季各处理N2O累积排放量为0.3~0.5 kg·hm-2[图 5(b)], NPK、M和NPK+M处理相比PK处理均增加, NPK和M处理结果显著高于PK处理.M处理相比于NPK处理N2O的累积排放量增加9.1%, 而NPK+M处理相比NPK处理N2O的累积排放量降低23.5%.

不同小写字母表示不同处理间差异显著(P < 0.05) 图 5 不同施肥处理下辣椒季和早稻季N2O的累积排放量 Fig. 5 Cumulative N2O emissions in pepper season and early rice season under different fertilization treatments

2.3 辣椒-早稻季不同处理土壤NH4+-N和NO3--N含量的动态变化

在辣椒生育期, 肥料施入土壤后, 引起土壤NH4+-N含量迅速增加.NPK、NPK+M和M处理的ω(NH4+-N)含量在施基肥第1 d后达到峰值分别为147.8、168.8和156.4 mg·kg-1[图 6(a)], 在第1次追肥后第3 d达到峰值分别为68.0、32.7和142.5 mg·kg-1, 在第2次追肥后第6 d达到峰值分别为108.8、73.9和93.9 mg·kg-1, 在第3次追肥和第4次追肥后第2 d和第1 d分别达到峰值.在PK处理中, 土壤NH4+-N含量在前期出现峰值, 但后期一直呈现平稳状态.由于早稻季未施用肥料, 导致各处理土壤NH4+-N动态变化规律基本一致, 且始终未出现明显的峰值.与NH4+-N类似, NPK、NPK+M和M处理在辣椒季均出现ω(NO3--N)峰值[图 6(b)], 第1次峰值分别为177.5、133.2和16.8mg·kg-1, 最后1次峰值分别为61.7、63.9和38.6 mg·kg-1.早稻季各处理NO3--N含量均处于较低水平, 未出现明显的峰值.

实线箭头表示施肥 图 6 不同施肥处理下辣椒季和早稻季NH4+-N和NO3--N含量动态变化 Fig. 6 Changes in NH4+-N and NO3--N contents in pepper season and early rice season under different fertilization treatments

2.4 作物产量、全球增温潜势和温室气体排放强度

各处理辣椒季产量在3 055.6~37 722.5 kg·hm-2之间, NPK、NPK+M和M处理相比PK处理显著增加494.1%、1 134.6%和892.2%(表 2). NPK+M和M处理较NPK处理显著增加109.2%和68.1%.早稻季各处理间产量大小顺序为：NPK+M＞M＞NPK＞PK, 其中NPK+M处理相比PK处理差异显著, NPK+M和M处理较NPK处理产量均有增加, 且达到显著性差异.

表 2 不同施肥处理中作物产量、CH4和N2O总增温潜势和温室气体排放强度1) Table 2 Crop yields, global warming potential of CH4 and N2O, and greenhouse gas emission intensity in different fertilization treatments

整个辣椒-早稻轮作系统中各处理GWP值为508.0~1 864.4 kg·hm-2(表 2).NPK、M和NPK+M处理相较PK处理显著增加267.0%、246.2%和172.8%, NPK+M处理相对NPK处理显著降低.辣椒季贡献69.2%~78.1%的GWP(按作物季分), N2O贡献77.3%~85.3%的GWP(按气体分).

辣椒季各处理GHGI大小顺序为：NPK＞PK＞M＞NPK+M.相比PK处理, M和NPK+M处理GHGI显著降低49.1%和68.0%, M和NPK+M处理相较NPK处理GHGI值显著下降71.5%和54.7%.早稻季各处理GHGI大小顺序为：NPK＞M＞NPK+M＞PK. NPK、M和NPK+M处理相比PK处理GHGI分别增加484.2%、351.1% 和227.0%, 各处理GHGI显著高于PK处理.NPK+M处理相比NPK处理GHGI值显著下降44.0%(表 2).

2.5 环境因子与热区辣椒-早稻CH4和N2O排放的关系

相关分析结果表明, 土壤CH4排放与N2O排放显著负相关(P < 0.05).土壤NH4+-N与CH4排放呈极显著负相关(P < 0.01), 和N2O排放极显著正相关.土壤NO3--N含量与N2O排放和CH4排放呈显著正相关(表 3).土壤pH与N2O排放和CH4排放呈极显著负相关, 5 cm土温与CH4排放呈显著正相关, 但与N2O排放的相关性不显著.

表 3 CH4和N2O排放通量与NH4+-N、NO3--N、5 cm土温和pH的相关性1) Table 3 Correlations of CH4 and N2O fluxes with NH4+-N, NO3--N, soil temperature below 5 cm soil surface, and pH

3 讨论 3.1 不同施肥及环境因子对热带地区稻菜轮作土壤CH4排放的影响

稻田往往被认为是CH4和N2O重要的排放源, 在关注施肥对作物产量产生积极影响的同时, 也要兼顾肥料的施用所产生的环境效应. CH4排放受温度、pH和施肥措施等影响[22, 23].本研究中CH4排放与5 cm土温显著正相关, 与土壤pH极显著负相关.这与Zhong等[24]和郑聚峰等[25]的研究结果一致.本试验中辣椒季CH4累积排放量为0.9~2.7 kg·hm-2, 相对PK处理, 各处理CH4累积排放量均有所增加.相对与早稻季, 辣椒季各处理CH4排放贡献极低.施肥通过改变产CH4菌和CH4氧化菌等微生物活性, 进而影响土壤CH4排放[26, 27], 辣椒季土壤多数时间为好氧环境, 好气土壤中生物氧化作用强, 使得旱地通常被认为是大气中CH4的吸收汇[28, 29].这与胡玉麟等[19]在海南三亚地区所报道的瓜菜季结果一致.值得关注的是, 有机肥的施入会使土壤进一步形成厌氧环境, 一方面为产CH4菌提供了充足的营养底物, 另一方面为产CH4菌创造极为有利的生长环境, 从而导致土壤CH4的大量排放[30, 31], 然而本研究在辣椒季中有机肥处理CH4排放均低于单施化肥处理, NPK+M处理CH4累积排放量最低, 这与姜姗姗等[23]和郑聚峰等[25]的结果一致, 有机肥与化肥配施会显著提升CH4氧化菌的多样性和丰富度, 导致大量的CH4在排放到大气之前已被氧化成CO2[26, 32].

与辣椒季相比, CH4的排放主要集中在早稻季, M处理CH4累积排放量相对NPK处理显著升高.大量研究结果表明, 有机肥肥效慢但肥效长, 可为作物生长持续提供所需的各种养分[33, 34].在本研究中, 早稻季除了有机肥的后效作用外, 根系凋落物和秸秆残留在土壤中, 增加了有机碳含量和生物有效性, 为产CH4菌过程提供了充足的底物供应, 从而促进土壤CH4的排放[27, 35].

3.2 不同施肥及环境因子对热带地区稻菜轮作土壤N2O排放的影响

农田土壤N2O排放主要来自硝化与反硝化过程, 易受肥料类型与用量、水分管理方式和气候条件等多种因素的共同制约[36].本研究中N2O排放主要集中在施肥后, 土壤中外源氮素以及本地有机氮的矿化能为硝化作用和反硝化作用提供充足的底物, 从而促进N2O的大量排放, 这与多数研究结果一致[23, 26].本试验中辣椒季N2O累积排放量为0.5~3.0 kg·hm-2, 这与胡玉麟等[19]所报道的瓜菜季的研究结果一致, 但高于钟川等[37]报道的N2O累积排放量(0.7~0.8 kg·hm-2).这可能是由于好氧条件下土壤硝化作用较强[38, 39].肥料类型作为影响N2O排放的主要因素[40], 而目前施用有机肥对稻田N2O排放研究结果不尽一致.有研究表明, 与化学肥料相比, 施用有机肥可以提高土壤C/N, 促使微生物对无机氮素产生强烈的竞争, 进而降低N2O排放[41], 本研究中施入有机肥处理N2O排放均低于单施氮肥处理, 这与Xia等[42]和邹建文等[43]的研究结果不同.这可能是由于辣椒季施入的氮肥量较大, 导致土壤NH4+-N的含量始终处于较高水平, 为土壤硝化细菌提供了充足的底物, 从而促进土壤N2O的大量排放; 而添加有机肥同时会增加土壤中碳源与氮源, 改变土壤C/N, 促进发生彻底的反硝化过程, 从而减少N2O的排放[44].

从本研究结果看, 稻菜轮作体系中辣椒季不同施肥对早稻季N2O排放无显著性影响.这可能是因为：一方面, 在辣椒生长发育期, 不管施入何种肥料, 随着辣椒生长吸收和氮的挥发, 土壤无机氮最终会保持较低水平, 不足以促进早稻季土壤N2O排放[45, 46]; 另一方面, 水稻种植期间土壤大部分时间处于淹水状态, 易形成强烈的土壤还原环境, 使得氮素进行彻底的反硝化作用, 所产生的N2O可被彻底还原为N2[47].

3.3 不同施肥对作物产量、温室气体增温潜势和排放强度的影响

合理施肥是稻田增产的重要途径与保障.有研究表明, 施用有机肥既能提升作物产量, 又能有效地提高土壤肥力[48].本研究中NPK+M和M处理较NPK处理均显著增加辣椒产量, 其中NPK+M处理提升效果最好.这可能是由于施入有机肥改善土壤物理结构, 提高土壤保肥能力, 使整个辣椒生育期养分供应充足[49].在早稻季中, 由于均未施肥, 除PK处理外, 各处理间产量无显著性差异, 其中NPK+M处理水稻产量最高, M处理次之.这也间接证明有机肥肥效慢, 具有长效性, 但要准确量化有机肥施用量, 在保证当季作物正常生长的同时又能最大限度的满足下茬作物的营养需求, 还有待更进一步研究.

本研究中, 从作物季来看, 辣椒季气体排放贡献了主要的GWP.相较于PK处理, 各处理由于肥料的施入促进了土壤N2O和CH4排放总量, 从而导致GWP均有所上升.与NPK处理相比, 由于辣椒季NPK+M和M处理施入有机肥抑制了土壤N2O和CH4排放总量, 降低了GWP, 其中NPK+M处理达到显著性差异.杨丹等[35]的研究表明有机肥能导致GWP增加, 但有明显增产效果.田伟等[17]的研究表明, 有机肥替代化肥能降低N2O和CH4排放, 结论不同可能是水热条件有所差异导致.同时, 在研究GWP中土壤有机碳含量变化也是一个重要的影响因子, 特别是有机肥替代化肥后, 土壤有机碳变化必将引起GWP的显著变化, 在今后研究中, 关注有机肥添加后土壤碳的动态变化对衡量温室气体排放意义重大.本试验中与当地种植管理模式NPK处理相比, 辣椒季和早稻季NPK+M和M处理GHGI均显著下降.这与Zhang等[49]的研究结果一致.化肥配施有机肥促进土壤微生物对无机氮的调控, 提高氮素利用率, 进而提高作物产量, 达到增产效果[50].

4 结论

(1) 热带地区辣椒-早稻轮作系统中施肥会引起N2O排放出现峰值, 且主要集中在辣椒种植季, 而CH4的排放则主要集中在早稻季, 且存在明显的滞后效应.轮作系统由于气体排放产生的GWP主要是辣椒季的N2O排放.

(2) 与NPK处理相比, 有机肥配施化肥处理减少辣椒种植季N2O和CH4排放, 却增加后茬早稻季CH4排放, 但并未引起N2O的显著性排放.

(3) 综合作物产量、GWP和GHGI考虑, 有机肥配施化肥既能增加作物产量, 又能降低GWP和GHGI, 可推荐为当地最优的减排稳产的施肥管理模式.

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