环境科学  2021, Vol. 42 Issue (8): 3931-3942   PDF    
引用本文
王紫君, 王鸿浩, 李金秋, 伍延正, 符佩娇, 孟磊, 汤水荣. 椰糠生物炭对热区双季稻田N2O和CH4排放的影响[J]. 环境科学, 2021, 42(8): 3931-3942.
WANG Zi-jun, WANG Hong-hao, LI Jin-qiu, WU Yan-zheng, FU Pei-jiao, MENG Lei, TANG Shui-rong. Effects of Coconut Chaff Biochar Amendment on Methane and Nitrous Oxide Emissions from Paddy Fields in Hot Areas[J]. Environmental Science, 2021, 42(8): 3931-3942.
椰糠生物炭对热区双季稻田N2O和CH4排放的影响
王紫君, 王鸿浩, 李金秋, 伍延正, 符佩娇, 孟磊, 汤水荣     
海南大学热带作物学院, 海口 570228
收稿日期: 2020-11-29; 修订日期: 2021-01-26
基金项目: 海南省自然科学基金高层次人才项目（2019RC049）；国家自然科学基金项目（41661051，42067008）
作者简介: 王紫君(1997~), 女, 硕士研究生, 主要研究方向为农业环境与生态修复, E-mail: puss_wong97@163.com
通信作者: 伍延正, E-mail: wyz198712@163.com; 孟磊, E-mail: menglei@hainanu.edu.cn
摘要: 基于稻菜轮作模式，选择海南双季稻田为对象进行氧化亚氮（N2O）和甲烷（CH4）排放的原位监测，探究椰糠生物炭对该系统稻田温室气体排放的影响.试验设当地常规施肥对照（CON）、氮肥配施20 t·hm-2生物炭（B1）、氮肥配施40 t·hm-2生物炭（B2）及不施氮对照（CK）4个处理，采用静态箱-气相色谱法监测整个水稻种植季稻田N2O和CH4排放，并估算增温潜势（GWP）和温室气体排放强度（GHGI）.结果表明，早稻季N2O排放动态与土壤矿质氮含量密切相关，排放集中在水稻苗期与分蘖期施肥后，各处理早稻季N2O累积排放量为0.18~0.76 kg·hm-2，相较于CON处理，生物炭处理减排18%~43%，其中B2处理达显著水平；生物炭可能通过促进N2O的还原减少早稻苗期N2O排放；提高土壤硝态氮含量而增加了早稻分蘖期N2O排放.晚稻季N2O排放集中在抽穗期和成熟期，累积排放量为0.17~0.34 kg·hm-2，B1处理减排37%，B2增加3%，差异均不显著.稻田CH4排放高峰出现在早稻季后期与晚稻季前期.各处理早稻季CH4累积排放量为3.11~14.87 kg·hm-2，CK较CON处理增排39%，生物炭处理可能提高土壤通气性限制早稻季产CH4能力，B1和B2处理分别较CON减排28%和71%；晚稻季CH4累积排放量为53.1~146.3 kg·hm-2，排放动态与NH4+-N含量极显著正相关，CK和B1分别较CON处理增加52%和99%，B2处理显著增加176% CH4排放.早稻季B1和B2处理较CON分别增产12.0%和14.3%，晚稻季分别增产7.6%和0.4%.由于晚稻季甲烷排放的增加，施用生物炭增加了双季稻田总增温潜势（GWP），其中高量生物炭达显著水平；不同施用量生物炭对双季稻田温室气体排放强度（GHGI）无显著影响.椰糠生物炭在热区稻田温室气体减排方面的应用仍需进一步研究.
关键词: 椰糠      生物炭      热区      双季稻田      温室气体      增温潜势(GWP)     
Effects of Coconut Chaff Biochar Amendment on Methane and Nitrous Oxide Emissions from Paddy Fields in Hot Areas
WANG Zi-jun , WANG Hong-hao , LI Jin-qiu , WU Yan-zheng , FU Pei-jiao , MENG Lei , TANG Shui-rong     
College of Tropical Crops, Hainan University, Haikou 570228, China
Abstract: Based on the rice-vegetable crop rotation model, in-situ measurements of nitrous oxide (N2O) and methane (CH4) emissions were conducted in double-cropping rice fields in Hainan to determine the impact of coconut chaff biochar on greenhouse gas emissions. The experiment involved four treatments: conventional farming fertilization (CON), nitrogen fertilizer combined with 20 t ·hm-2 biochar (B1), nitrogen fertilizer combined with 40 t ·hm-2 biochar (B2), and no nitrogen fertilizer, as the control (CK). The N2O and CH4 emissions were measured using static chamber-gas chromatography during the two paddy seasons, and the global warming potential (GWP) and greenhouse gas intensity (GHGI) were also estimated. The results show that N2O emission dynamics during the early rice season are closely related to the mineral nitrogen content of the soil. The N2O is emitted at the rice seedling and tillering stages after fertilization. The cumulative N2O emission during the early rice season was 0.18-0.76 kg ·hm-2. Compared with the CON treatment, the biochar treatments reduced N2O by 18%-43%, and the B2 treatment resulted in a significant reduction. The addition of biochar may promote the reduction of N2O at the early rice seedling stage and increase N2O emissions by improving the soil NO3--N content at the early rice tillering stage. During the late rice season, N2O is emitted during the heading and maturity stages, and the cumulative N2O emission was 0.17-0.34 kg ·hm-2. The B1 treatment reduced emissions by 37%, and B2 increased emission by only 3%, which is not a significant difference. The peak of CH4 emissions from rice fields appeared in the late phase of the early rice season and prophase of the late rice season. The cumulative emission of CH4 in the early rice season was 3.11-14.87 kg ·hm-2. Compared with CON, the CK treatment increased emission by 39%. The biochar treatment may increase soil aeration and limit the ability of CH4 production in the early rice season, as B1 and B2 treatments reduced CH4 emissions by 28% and 71%. The cumulative CH4 emission in late rice season was 53.1-146.3 kg ·hm-2, and the emission dynamics were significantly positively correlated with NH4+-N content. CK and B1 treatments increased CH4 emissions by 52% and 99%, respectively compared with CON, and the B2 treatment significantly increased CH4 emissions by 176%. Compared with CON, the B1 and B2 treatments increased the yield by 12.0% and 14.3% when applied in the early rice season and by 7.6% and 0.4% when applied in the late rice season, respectively. Due to the increased methane emissions in the late rice season, biochar amendment increased the GWP of the double-cropping rice field, in which the high amount of biochar reached a significant level; different amounts of biochar had no significant effect on the GHGI of the double-cropping rice field. Thus, the application of coconut chaff biochar for the reduction of greenhouse gas emission, from rice fields in hot areas, requires further research.
Key words: coconut chaff      biochar      hot area      double-cropping rice fields      greenhouse gas      global warming potential (GWP)     

受人类活动影响, 近年来N2O与CH4等温室气体排放加剧, 引起全球气候变暖, 危害人类健康[1].水稻是世界主要粮食作物之一, 其生产过程是N2O和CH4的一大重要来源.稻田生态系统排放的CH4和N2O是土壤中有机碳和活性氮复杂转化过程的产物, 它的产生受气候类型、种植制度和施肥模式等多种因素影响[2].稻菜轮作是海南较为普遍的耕作模式, 水稻种植可有效降低土壤连作障碍, 减少病虫害发生, 提高瓜菜作物产量[3].稻菜轮作模式下, 瓜菜季需肥量大, 农户常过量施肥, 瓜菜收获后0~20 cm土层无机氮累积有的高达380 kg ·hm-2[4], 残留的大量氮肥会加剧稻田N2O排放, 同时热区高温高湿条件引发稻田干湿交替频繁, 有利于N2O的产生[5], 例如田伟等[6]的研究观测到琼北晚稻季N2O累积排放量达0.44~3.40 kg ·hm-2.此外, 海南全年暖热、雨量充沛, 也有利于土壤和作物碳代谢从而增加CH4排放[7].因此, 迫切需要寻找一种适合热区的温室气体减排措施.

由于N2O与CH4产生途径的复杂性和多样性, 仅仅通过改进肥料的投入量可能难以达到高效减排的目的[8].近年来, 生物炭在土壤中的应用被认为是最具潜力的减排措施之一, 它的减排潜势(以Ceq计)达到0.7×109 t ·a-1[9].然而至今它的减排效果仍存在诸多不确定性.大量研究结果表明, 施用生物炭可能通过土壤碳固存来减少温室气体排放, 并有改善土壤肥力、保持土壤水分及增加作物产量等作用[10~13].相反, 也有部分研究报道生物炭会显著增加温室气体排放[14, 15]或没有影响[16].这些研究之间往往因为地域、水热条件、作物类型及生物炭的性质(原料、热解条件)等的不同而显现出差异性结果.目前关于水旱轮作模式下稻田温室气体对施用生物炭的响应已有大量研究[17, 18], 然而这些研究大多基于稻麦轮作系统, 针对热区稻菜轮作模式下生物炭对稻田温室气体排放影响的报道较少[19, 20].椰糠是椰子在加工过程中从果皮中脱落的纤维性粉末, 其作为椰子的废弃物之一长期得不到合理的开发与利用[21, 22].而椰糠本身具有保湿、保温和疏松等特性, 将其制成生物炭用于作物生产, 可起到改良土壤pH、增加作物产量的效果[23].选择椰糠生物炭探究其对热区稻田温室气体排放的影响, 有利于实现热区废弃生物质资源的循环利用, 也为海南椰子产业的可持续发展创造条件.

基于此, 本试验基于稻菜轮作系统, 以海南双季稻田为研究对象, 在当地农户常规施肥基础上设置2个不同用量的椰糠生物炭处理, 原位监测水稻生长季的CH4和N2O排放, 并结合作物产量、增温潜势(GWP)和温室气体排放强度(GHGI), 系统地探究椰糠生物炭对热区双季稻田CH4和N2O排放的影响, 以期为生物炭的合理施用提供科学指导.

1 材料与方法 1.1 试验地概况

本试验地点位于海南省澄迈县桥头镇西岸村(110°04′E, 19°56′N).试验所在区域属于热带季风气候, 年平均气温23.80℃, 年降雨量1 786.10 mm, 年平均日照时数2 059 h.当地种植模式多为早稻-晚稻-冬季瓜菜.土壤为滨海沉积物母质发育的沙壤土.观测期间气温与降水量见图 1.

图 1 观测期间气温与降水量变化(2019年) Fig. 1 Dynamics of air temperature and precipitation during the sampling period(2019)

1.2 试验设计

田间试验共设4个处理, 即: 不施氮肥的空白对照(CK)、按当地农户习惯的常规施肥对照(CON)、添加20 t ·hm-2生物炭(B1)和添加40 t ·hm-2生物炭(B2).具体施肥方案见表 1.每个处理设3个重复, 共12个小区.试验小区随机区组排列, 每个小区面积21 m2(7 m×3 m), 各小区边界起垄, 并用地膜覆盖, 地膜两边埋入地下50 cm, 防止串水串肥.

表 1 试验施肥方案1)/kg ·hm-2 Table 1 Fertilization schemes/kg ·hm-2

早稻季于2019年4月19日进行整地, 次日混入生物炭与基肥并灌水插秧, 早稻品种为“特优3301”, 株距15 cm.于耕地前取土, 测定土壤基本理化性质(表 2). 5月13日施入分蘖肥, 5月31日晒田, 灌水后于6月13日下午施入穗肥, 7月20日采收测产.晚稻于7月30日插秧, 品种为“万金优366”. 10 d后施入蘖肥, 8月26日晒田, 9月9日施入穗肥, 10月27日采收.晒田期之前水稻田一直持续淹水, 晒田后按照灌水-复水后间歇性灌水-收获前落干调控水分.其他农田管理措施与当地一致.

表 2 水稻收割后土壤基本理化性质1) Table 2 Soil properties after rice harvest

供试生物炭由椰糠在600℃下厌氧热解制成, 其pH为9.7, 氮含量0.34%, 阳离子交换量(CEC)8.20 cmol ·kg-1, 比表面积为5.84 m2 ·g-1.

1.3 样品采集与测定

气体样品的收集用静态箱法, 装置包括PVC材料的箱体(50 cm×50 cm×90 cm)、外部包裹有泡沫塑料的保温层, 防止阳光照射温度升高过快.箱顶设有温度计和带三通阀的采气口.底座为四周带有凹槽的正方形框, 固定于小区内, 采集气体时将采样箱置于底座凹槽内, 向水槽中加入适量水, 形成密闭环境, 并用100 mL针管反复抽动几次, 使箱体内部及底座周边空气充分流动.于盖上采气箱的0、10、20和30 min抽取30 mL气体, 注入抽成真空的20 mL顶空瓶(Nichiden-rika Glass Co. Ltd.)中.为防止采样时的踩踏改变土壤结构, 设计了从田埂伸入小区的木桥.采气时间为上午08:00~11:00, 采集频率为一周一次, 施肥后第1、3和5 d加密采样.采集的气体带回实验室由气相色谱仪(岛津GC- 2014)分析, N2O检测器为ECD, 载气为氩甲烷; CH4检测器为FID, 载气为高纯氮, 检测器温度300℃.标准气体由中国计量科学研究院提供.

于采气后采集土壤样品, 频率为一周一次, 用靛酚蓝比色法(625 nm)和紫外双波长法(275 nm和220 nm) 分别测定NH4+-N和NO3--N含量.土壤基本理化测定方法如下：pH采用电位法(水土比2.5 ∶1), 有效N采用碱解扩散法; 有效P采用钼蓝比色法; 有效K采用1 mol ·L-1乙酸铵溶液(pH=7)浸提, 火焰光度计测定; 阳离子交换量采用乙酸铵交换-蒸馏法测定[24]; 有机质采用重铬酸钾-硫酸消化法[25].

1.4 数据处理与分析

N2O与CH4排放通量计算公式：

式中, F为排放通量, FN2O单位为μg ·(m2 ·h)-1, FCH4单位为mg ·(m2 ·h)-1; ρ为标准状态下N2O-N和CH4-C的密度(kg ·m-3); h为采样箱高度(m); Δc/Δt为采样过程中箱内气体摩尔分数变化速率; T为采样时箱内平均温度(℃).

累积排放量(f, kg ·hm-2)计算公式:

式中, n和i为采样次数, t为采样天数(d).

100 a尺度的农田土壤直接排放的N2O和CH4的增温潜势[26](GWPGHGS, 以CO2-eq计, kg ·hm-2) 计算公式：

式中, fN2O为N2O累积排放量, fCH4为CH4累积排放量, 单位为kg ·hm-2.

温室气体排放强度(greenhouse gas intensity, GHGI, 以CO2-eq计, kg ·kg-1) 的计算公式:

式中, Y为作物产量(kg ·hm-2).

使用SPSS 20.0和Origin 2018进行数据统计分析与制图.处理间差异采用Duncan多重比较法; 采用Pearson法对各变量间相关性进行分析, 显著水平为P < 0.05.

2 结果与分析 2.1 生物炭对稻田土壤理化性质的影响

由表 2可知, 与背景值相比, 水稻收割后CK和CON处理土壤的pH降低, 生物炭处理的pH升高; 早稻收割后, CK处理的土壤速效氮含量显著提升, 且均高于其它处理.相比于常规对照, 生物炭处理显著提高了早晚稻土壤pH, 降低早稻季土壤速效氮与晚稻季速效磷含量达显著水平; B2处理明显提高早稻季土壤有机质含量(P < 0.05); 晚稻季B1处理土壤速效钾含量显著高于CON处理(P < 0.05).与早稻季相比, 晚稻季B1处理有机质含量升高, 且显著高于对照处理(P < 0.05).

2.2 双季稻田N2O排放通量与累积排放量

结果表明, 早稻季N2O排放动态变化与晚稻季明显不同.早稻季的N2O排放高峰主要出现在水稻分蘖期, 通常于施肥后2~8 d达到峰值.仅CON处理在施入基肥后出现排放峰, 峰值达232.43 μg ·(m2 ·h)-1.除CK处理外, 其它处理在施入分蘖肥后第2 d均出现排放峰值, 其中B1处理峰值最高, 达350.54 μg ·(m2 ·h)-1, 其次为B2[149.85 μg ·(m2 ·h)-1]和CON处理[67.21μg ·(m2 ·h)-1], 见图 2(a).早稻季N2O的累积排放量高低顺序为CON>B1>B2>CK, CON与B1显著高于CK处理(P < 0.05).相比于CON对照, B1和B2分别减少了早稻季18%与43%的N2O排放量, 其中B2达到显著水平[(P < 0.05), 图 3(a)].晚稻季N2O的排放集中在水稻孕穗抽穗与成熟期(晒田后干湿交替时期), 各处理在施入穗肥后出现峰值, 其中B2处理峰值最高, 达到77.00μg ·(m2 ·h)-1.晚稻季N2O的累积排放量呈B2>CON>B1>CK的高低顺序, 各处理间无显著差异[(P>0.05), 图 3 (b)].此外, 不同处理下晚稻季N2O排放峰值及季节累积排放量均明显低于早稻季.

↓从左到右依次表示基肥、蘖肥、穗肥(早稻), 蘖肥、穗肥(晚稻), ↑表示晒田; CK表示不施氮处理, CON表示常规施氮处理, B1表示氮肥配施20 t ·hm-2生物炭, B2表示氮肥配施40 t ·hm-2生物炭 图 2 不同处理下双季稻田N2O与CH4排放变化(2019年) Fig. 2 Dynamics of N2O and CH4 emission fluxes from paddy fields with different treatments(2019)

同一图中不同小写字母表示处理间差异显著(P < 0.05); Δ表示处理与CON对照的比较：Δ=(f-fCON)/fCON×100%, 式中f为处理的N2O或CH4累积排放量; 虚线表示以CON对照为基准的参考线 图 3 双季稻田N2O与CH4累积排放量 Fig. 3 Cumulative emissions of N2O and CH4 from paddy fields with different treatments

2.3 双季稻田CH4排放通量与累积排放量

早稻季CH4排放动态和晚稻季也存在明显差异.早稻季CH4的排放高峰主要集中在孕穗抽穗与成熟期, 4个处理均出现多个峰值, 其中CK处理相比其它处理的峰值始终最高, 而B2处理一直处于最低排放水平[图 2(c)].早稻季CH4累积排放量范围在3.11~10.68kg ·hm-2, 各处理的排放高低顺序为CK>CON>B1>B2, 与CON对照相比, B2处理显著减少71% CH4排放量[(P < 0.05), 图 3(c)].除成熟期外, 晚稻季CH4的排放在各个时期均出现排放峰, 其中B2处理的4次排放峰均为最高, 其次是B1和CK, CON处理最低[图 2(d)].晚稻季CH4累积排放量为53.05~146.31kg ·hm-2, 显著高于早稻季, 各处理CH4的总排放量的顺序为B2>B1>CK>CON, 其中B2处理显著高于CON对照, 增加了176% CH4总排放[(P < 0.05), 图 3 (d)].其余处理与CON对照无显著差异(P>0.05).早稻季CH4的排放主要集中在水稻生长后期, 而晚稻季CH4主要出现在水稻生长前期, 与N2O排放存在明显“消长”动态.

2.4 双季稻田土壤矿质氮动态变化

双季稻田铵态氮的动态变化如图 4(a)所示.土壤中NH4+-N含量普遍在施肥后升高, 随着时间延续而下降.早稻季前期, B2处理的土壤NH4+-N含量始终最高, 在第29 d时迅速降低, 随后一直维持较低水平. B1和B2处理在施入分蘖肥后NO3--N含量迅速上升, 显著高于CK和CON对照, 随后各处理NO3--N水平基本一致.晚稻季各处理矿质氮含量变化较为平缓.施入穗肥后, 除CK外其余处理的NH4+-N含量升高, 然后缓慢下降.所有处理的NO3--N含量在零值上下波动[图 4(b)].

图 4 不同处理双季稻田土NH4+-N和NO3--N动态变化(2019年) Fig. 4 Dynamic changes in NH4+-N and NO3--N in paddy soils under different treatments(2019)

2.5 矿质氮、作物生物量、产量和环境因子与气体排放的关系

相关分析结果显示(表 3), 早稻季稻田N2O排放通量与土壤NH4+-N、NO3--N含量呈极显著正相关(P < 0.01), 与其它因子无显著相关性.早稻季CH4排放与土壤Eh有极显著负相关性, 与5 cm土温呈显著负相关(P < 0.05).晚稻季中, 稻田CH4排放通量与土壤NH4+-N含量有极显著正相关性(P < 0.01), 同时与土壤Eh、5 cm土温呈显著正相关(P < 0.05).

表 3 稻田N2O、CH4排放通量与土壤矿质氮和环境因子相关性1) Table 3 Correlations of N2O and CH4 emissions with mineral nitrogen content and environmental factors of paddy fields

双季稻田N2O、CH4排放总量与水稻生物量、产量及土壤理化相关性显示, N2O排放总量与各因子均无显著相关性(P>0.05).早稻季CH4排放总量与土壤有机质含量显著负相关(P < 0.05); 晚稻季CH4排放总量与土壤pH呈显著正相关(P < 0.05), 同时与土壤速效磷含量有极显著负相关性(P < 0.01, 表 4).

表 4 稻田N2O、CH4排放总量与水稻生物量、产量和土壤理化相关性1) Table 4 Correlations of N2O and CH4 cumulative emissions from paddy fields with rice biomass, yield, and soil properties

2.6 作物生物量、产量、增温潜势和气体排放强度

早稻季作物生物量在各处理间无显著性差异(P>0.05).晚稻季各处理作物的地上与总生物量的高低顺序为B1>B2>CON>CK.与CON对照相比, 其余处理无显著差异, 而B1处理的水稻地上生物量与总生物量显著高于CK对照(P < 0.05, 图 5).

同一图中不同小写字母表示处理间差异显著(P < 0.05) 图 5 水稻地下、地上部分生物量和总生物量 Fig. 5 Underground, aboveground biomass, and total biomass of rice

如表 5所示, 早稻季水稻产量B2>B1>CON>CK, 各处理间差异不显著.晚稻季产量B1>B2>CON>CK, 其中CK显著低于其它3个施氮处理.在100 a尺度下比较对两稻季总增温潜势的贡献率, 其中晚稻季贡献了75.1% ~95.1% (按季别分), CH4贡献了84.7% ~96.3%(按气体分).各处理间总GWP的高低顺序为B2>B1>CK>CON, B2处理的增温潜势显著高于CON对照(P < 0.05).GHGI呈B2>B1>CK>CON的顺序, 处理间无显著差异.

表 5 不同处理中水稻产量、N2O与CH4总增温潜势和温室气体排放强度1) Table 5 Yields, global warming potential, and greenhouse gas intensity of paddy fields under different treatments

3 讨论 3.1 生物炭对稻田N2O排放的影响

农田土壤的N2O排放主要来自硝化与反硝化过程[27, 28], 受施氮水平、水分管理和土壤类型等多种环境因素的限制[2].本试验中早稻季常规处理的N2O累积排放量为0.18~0.76 kg ·hm-2, 高于晚稻季0.17~0.34kg ·hm-2, 一方面因为早稻季土壤含有上茬作物残留的大量无机氮肥, 为N2O产生提供充足的底物; 另一方面晚稻季降水量高于早稻季, 强厌氧条件促进了反硝化过程的彻底进行, N2O更多地被还原为N2[29].早稻季N2O的季节排放动态与晚稻季明显不同, 主要受施肥与水分的影响.早稻季N2O的排放峰值出现在施入基肥与分蘖肥后, 此时土壤中氮含量丰富, 稻田由旱地转变为水田, 土壤孔隙中O2利用率高, 硝化与反硝化作用剧烈从而排放大量N2O[30, 31].前期N2O的高排放降低了后期N2O的矿质氮底物浓度; 同时后期的CH4排放与反硝化菌竞争碳源, 因此晒田覆水至早稻收割未观测到N2O排放峰.晚稻季稻田N2O排放通量与矿质氮含量无明显相关性.由于较高的降雨量, 前期未出现N2O排放峰, 而在施入穗肥时因水分落干观测到较小排放峰.

生物炭因其自身的化学性质与结构特点, 如较高的pH、巨大的比表面积及强吸附能力等, 直接或间接地影响硝化与反硝化过程, 从而影响土壤N2O气体排放[32].多数前人研究表明, 生物炭对稻田N2O具有减排能力[33, 34], Zhang等[16]的研究也表明, 生物炭显著降低了太湖平原稻田的N2O排放.生物炭的添加提升土壤pH, 利于反硝化过程中N2O还原酶的活性, 促进N2O还原[35]; 同时通过改善土壤曝气性抑制反硝化细菌及酶活性, 减少N2O生成[36]; 生物炭还可吸附土壤溶液中的无机氮, 降低N2O底物的可用性[37].但也有部分研究报道生物炭增加稻田N2O排放[38, 39].本研究中, 施用生物炭减少了早稻季18% ~43%的N2O排放总量, 但在水稻不同生长时期, N2O产生存在不同响应.Shen等[38]向双季稻田中添加秸秆与生物炭, 与常规相比, 生物炭降低了早稻季N2O的首个排放峰值, 但增加了第二个N2O排放峰, 与本研究结果相似.在水稻生长前期, 生物炭处理的土壤中碳源丰富, 反硝化微生物有更多的底物将N2O还原成N2[40].其次, 生物炭可能对土壤氮素具有缓释作用.土壤中的NH4+和NO3-被生物炭吸附, 减少矿质氮底物的利用而导致了前期N2O的产生减少[41, 42].在施入分蘖肥后, 生物炭处理的土壤氮素被逐渐释放, 促进了硝化与反硝化过程, 此时B1和B2处理土壤中的NO3-含量显著高于CON对照(图 4), 导致N2O排放增加, 而高量生物炭可能含有较多可利用性碳源, 促进了部分N2O向N2转化, 使得第二个N2O峰值低于B1处理[39].晚稻季施用生物炭对N2O排放无显著影响.祁乐等[43]的研究结果表明, 生物炭对紫色水稻土N2O的抑制作用不明显, 高量生物炭显著增加了水稻生育期N2O排放, 可能与其长期淹水条件有关, 生物炭只在土壤由干转湿的过程中对N2O排放产生差异影响[35].本研究中晚稻季降雨量较大, 稻田长期淹水条件可能影响生物炭减排效果[35].

3.2 生物炭对稻田CH4排放的影响

本试验中双季稻田全年CH4累积排放量在63.7~149.4kg ·hm-2, 与田伟等[6]的研究结果相近, 但常规施肥处理的CH4累积排放量(63.7 kg ·hm-2)显著低于石生伟等[44]研究中的湖南双季稻田CH4排放总量(191.7kg ·hm-2)和成臣等[45]研究的江西双季稻田累积排放量(233.5 kg ·hm-2).造成这种差异的原因可能是热区的干湿交替模式促进了甲烷氧化菌的生长[46].早稻季降雨量相对较少, 田间干湿交替过程较频繁, 不利于产甲烷菌的活性, 且有利于甲烷氧化, 导致早稻季CH4的排放相对较低, 仅贡献了双季稻田CH4排放总量的1.9% ~21.1%[46].其次, 早稻季较高的生物量可以增加根系泌氧, 促进甲烷氧化[47].水旱轮作模式下, 旱作期土壤Eh高, 再淹水后封闭于土壤孔隙中的氧气增加稻田对甲烷的氧化能力, 且旱季时间越久, 水稻季CH4排放越低, 但该作用仅能维持一季水稻生长季[48].早稻季CH4排放通量与Eh呈极显著负相关, 且排放主要集中在晒田后.稻田初期为弱还原状态, 随着时间延续土壤中氧气消耗, Eh下降逐渐形成强还原条件, 致使后期稻田CH4排放增加[49].晚稻季水稻移栽后气温较高, 大量早稻残茬为产甲烷菌提供了丰富的碳源, 淹水状态进一步促进产甲烷菌的生长, 因此CH4排放较高[50].晚稻季CH4排放动态与NH4+-N含量极显著正相关, 可能是高水分条件形成低氧环境, 加剧了NH4+-N和CH4对甲烷氧化酶的竞争, NH4+-N含量的升高会限制部分CH4的氧化[51].晚稻后期NH4+-N含量减少, 同时过深的水层对CH4排放还有一定的阻碍作用[44].双季稻田N2O与CH4排放存在明显的消长动态, 主要受稻田水分变化影响[52].

生物炭对稻田CH4排放的影响在不同地区存在差异.秦晓波等[53]的研究中, 生物炭可显著抑制华南地区双季稻田甲烷的排放; 李松等[54]的研究也发现太湖地区稻田CH4随着生物炭施用量的增加而减少.而部分研究中生物炭也可促进稻田CH4排放[55].生物炭对甲烷排放的影响主要取决于其对土壤通气性的增强与pH的提升[19, 38, 56].本试验早稻季中, 相比于常规对照, 20 t ·hm-2和40 t ·hm-2生物炭处理分别减少了28%和71%的CH4排放, 这与Qin等[57]和Wang等[58]的研究结果一致.生物炭通过提高稻田土壤孔隙度与通气性, 减少厌氧环境, 进而限制产甲烷菌的生命活动[46].其次, 添加生物炭提高了土壤有机质含量, 促进土壤碳循环, 而早稻季CH4总量与有机质含量呈负相关, 表明生物炭增强了早稻土壤的氧化能力, 增加甲烷氧化菌的活性, 减少了CH4的产生[57, 59], 这可能是本研究中生物炭减排的主要原因.晚稻季施用生物炭增加了99% ~176%的CH4排放, 其中高量生物炭达显著水平.晚稻前期雨水充沛, 对应着更高的甲烷排放, 说明高水分条件下生物炭增加土壤通气性的作用减弱, 减排能力下降, 其对甲烷排放的影响可能由其它因素主导[46].生物炭对土壤pH的提升可能增加了晚稻季部分产甲烷菌活性[60].本研究中晚稻季田间有大量早稻残茬, 碳源充足, CH4产生可能不受碳源限制; 同时生物炭输入可能造成晚稻季土壤C/N比过高, 加剧Ca-促磷酸根沉降反应(本研究用生物炭含钙23.7 g ·kg-1), 表现为有效磷含量的降低, 而较高的有效磷含量能促进甲烷氧化菌活性, 因此与CH4排放总量呈现显著负相关[61, 62].先前的研究表明, 生物炭对早晚稻田CH4排放的影响基本表现出一致的规律[53, 63].而本试验观测到早晚稻季CH4排放对生物炭的不同响应, 可能与稻菜轮作系统两个水稻季节的水热条件差异和生物炭的分解有关[64].因此, 生物炭对热区双季稻田温室气体排放的年际持续影响需要长期定位试验来探究.

3.3 生物炭对作物产量、GWP和GHGI的影响

受温度、降水与施肥等因素的影响, 双季稻的产量也存在差异.热区瓜菜季后土壤有大量的养分残余, 促使早稻季作物产量不受施氮的影响.而晚稻季时土壤中早稻季残存的养分已无法满足作物生长需求, 因此相比于空白对照, 施氮处理显著增加了作物产量(P < 0.05, 表 5).生物炭通过提高酸性土壤中Ca和Mg等基本养分的可用性来改善土壤肥力[65]; 提高氮素固存和氮素利用率[16, 66, 67]; 提高酶活性[68]以及改善土壤水分状况[69]来提高作物产量.与常规施肥对照相比, 经生物炭改良的土壤增加了早稻季12.0% ~14.3% 的水稻产量, 增产效应随施用量的增加而增强; 而在晚稻季增产效果却一般, 仅增加了0.4% ~7.6% 的水稻产量.Zhang等[70]的研究结果表明, 施氮下生物质炭的施用可在两年内保持水稻持续增产, 与本试验结果相反, 究其原因可能是热区较高的温度导致生物质炭加速矿化.有研究报道, 温度上升可能会加速生物炭的矿化过程, 而光照可以通过光化学氧化作用加速生物炭的降解, 随着时间延续对土壤肥力提升的作用减弱[71].晚稻季降雨量较大, 可能导致更多养分的流失, 降低了生物炭提高土壤肥力的效果, 而高量生物炭较高的pH促进土壤硝化, 加剧硝态氮随水流失, 减少作物对养分的吸收[72, 73].

通过对N2O和CH4总增温潜势(GWP)、作物季和气体贡献率以及温室气体排放强度(GHGI)的计算, 可以更直观地评估生物炭对热区温室气体排放与产量的综合效益.从作物季来看, 晚稻季气体排放贡献了主要的GWP.而两种稻田温室气体中, CH4占主导地位[74].与单施氮肥相比, 生物炭施用增加了双季稻田气体总GWP, 其中高量生物炭增量达到显著水平.前人研究中, 大多结果都显示生物炭减少了两稻季的增温潜势[56, 64], 与本研究情况不同, 可能是不同地区的水热条件与田间管理有所差异导致的.氮肥配施生物炭均在一定程度上增加了稻田总GHGI, 其减排和增产效果的微观机制和年际持续性仍然需要进一步地深入研究.

4 结论

(1) 热区稻菜轮作模式下, 稻田N2O的排放集中在早稻季前期和晚稻季后期; CH4排放集中在早稻季后期和晚稻季前期.晚稻季CH4累积排放量显著高于早稻季.

(2) 生物炭可减少早稻季N2O排放, 对晚稻季N2O无显著影响; 高量生物炭可降低早稻季CH4排放, 却显著增加晚稻季CH4排放.

(3) 从总体看, 氮肥配施低量和高量椰糠生物炭对热区双季稻田无明显固碳减排作用.

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