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  土壤学报  2025, Vol. 62 Issue (3): 665-676      DOI: 10.11766/trxb202402070064       CSTR: 32215.14.trxb202402070064
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引用本文  

李晶, 陈伟明, 孙佳蕊, 等. 基于DNDC模型评估极端高温和长期变暖对水田土壤有机碳的影响——以福建省漳州市为例. 土壤学报, 2025, 62(3): 665-676.
LI Jing, CHEN Weiming, SUN Jiarui, et al. Assessing the Impacts of Extreme High Temperatures and Long-term Warming on Paddy Soil Organic Carbon Based on the DNDC Model-A Case Study in Zhangzhou of Fujian Province. Acta Pedologica Sinica, 2025, 62(3): 665-676.

基金项目

国家自然科学基金项目(42207271)、福建省自然科学基金项目(2022J05036)和气象灾害教育部重点实验室 & 气象灾害预报预警与评估协同创新中心联合开放课题(KLME202105)共同资助

通讯作者Corresponding author

张黎明, E-mail:fjaulmzhang@163.com

作者简介

李晶(1990—),女,福建龙岩人,博士,讲师,主要从事气候变化与陆地生态系统碳循环研究。E-mail:lijing@fafu.edu.cn
基于DNDC模型评估极端高温和长期变暖对水田土壤有机碳的影响——以福建省漳州市为例
李晶1,2,3, 陈伟明1,3, 孙佳蕊1,3, 谢希临3,4, 张华5, 沈金泉5, 廖文强5, 邢世和1,3, 张黎明1,3    
1. 福建农林大学资源与环境学院, 福州 350002;
2. 气象灾害教育部重点实验室、气象灾害预报预警与评估协同创新中心, 南京信息工程大学, 南京 210044;
3. 土壤生态系统健康与调控福建省高校重点实验室, 福州 350002;
4. 福建农林大学菌草与生态学院(碳中和学院), 福州 350002;
5. 福建省农田建设与土壤肥料技术总站, 福州 350003
摘要:全球正经历以变暖为主、极端事件趋强趋频的气候变化,但其对土壤有机碳(Soil Organic Carbon,SOC)的影响并不完全一致,区分并量化极端高温和长期变暖对SOC的影响是制定适应性策略的关键。利用1:5万大比例尺土壤数据库和DNDC(DeNitrification-DeComposition)模型,模拟了1980—2016年福建省漳州市水田土壤在气候平均态、极端高温、长期变暖及实测温度四种气候情景下SOC的动态变化。结果表明,长期变暖、极端高温及二者协同对漳州市水田SOC的净贡献量分别为13.81、–80.02和–66.14 Gg。因此,虽然在气候变暖背景下土壤仍具有较强的固碳能力,但未来频发的极端高温事件可能在一定程度上造成更大的碳损失。
关键词DNDC模型    长期变暖    极端高温    土壤有机碳    水稻土    
Assessing the Impacts of Extreme High Temperatures and Long-term Warming on Paddy Soil Organic Carbon Based on the DNDC Model-A Case Study in Zhangzhou of Fujian Province
LI Jing1,2,3, CHEN Weiming1,3, SUN Jiarui1,3, XIE Xilin3,4, ZHANG Hua5, SHEN Jinquan5, LIAO Wenqiang5, XING Shihe1,3, ZHANG Liming1,3    
1. College of Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou 350002, China;
2. Key Laboratory of Meteorological Disaster (KLME), Ministry of Education & Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science & Technology, Nanjing 210044, China;
3. University Key Laboratory of Soil Ecosystem Health and Regulation in Fujian Province, Fuzhou 350002, China;
4. College of Juncao Science and Ecology (College of Carbon Neutrality), Fujian Agriculture and Forestry University, Fuzhou 350002, China;
5. Fujian Cropland Construction and Soil and Fertilizer Station, Fuzhou 350003, China
Abstract: 【Objective】Soil organic carbon(SOC)is an essential indicator of soil health. It not only provides a carbon source for plant growth and maintains the physical structure of soil, but also releases carbon into the atmosphere in the form of greenhouse gases, such as carbon dioxide. Therefore, it plays a critical role in the global carbon balance. Currently, the world is experiencing climate change characterized predominantly by warming and increasing frequency and intensity of extreme weather events. However, the impacts of the changing climate, including long-term warming and extreme weather events on SOC are not entirely the same. Distinguishing and quantifying the effects of extremely high temperatures (EH) and long-term warming(LW)on SOC is the key to formulating adaptive strategies.【Method】In this study, we focused on paddy soils in Zhangzhou of Fujian Province, a typical subtropical region of China. Based on a 1: 50, 000 detailed soil database, we employed the biogeochemical process model (DeNitrification-DeComposition, DNDC) to simulate SOC dynamics under four climate scenarios: de-trended climate base state (CTRL), extreme high temperatures (EXP_EH), long-term warming (EXP_LW), and measured temperatures (EXP_obs).【Result】The results revealed that the total amount of carbon sequestered by paddy fields in Zhangzhou from 1980 to 2016 under the four different climate scenarios (CTRL, EXP_EH, EXP_LW, and EXP_obs) was 1, 032.17, 952.15, 1, 045.98 and 966.03 Gg, with the corresponding average annual sequestration rates of 93.98, 86.70, 95.24, and 87.96 kg·hm–2, respectively. The long-term warming led to a net increase of 13.81 Gg of SOC in paddy fields across Zhangzhou, while extremely high temperatures resulted in a net decrease of 80.02 Gg. The combined effect of these two factors was –66.14 Gg in SOC, indicating that long-term warming promoted the sequestration of organic carbon in paddy soils, while extremely high temperatures reduced the soil carbon sink capacity, with extremely high temperatures exerting a dominant negative effect. Also, the variations in annual carbon sequestration rates between different climate scenarios indicated that extremely high temperatures throughout the years from 1980 to 2016 had a negative effect on carbon sequestration in the paddy soils of Zhangzhou, but the long-term warming effect on SOC turned from positive to negative around the year of 2000. This may be related to the diminishing effect of warming on plant growth over time. At the county level, climate change had the greatest impact on the carbon sequestration of Nanjing County. Additionally, the extremely high temperatures and long-term warming caused –26.23% and 7.27% impacts on its carbon sequestration rate, respectively. Furthermore, among different terrain and topographical areas, the carbon sequestration rate of hilly and mountainous areas was significantly affected by extremely high temperatures and long-term warming, with –8.84% and 1.98% changes, respectively.【Conclusion】In conclusion, while the paddy soils in Zhangzhou still maintain a strong carbon sequestration capacity in the context of climate change, the increasing extreme high-temperature events in the future may potentially contribute to greater carbon losses to some extent.
Key words: DNDC model    Long-term warming    Extreme high temperatures    Soil organic carbon    Paddy soils    

全球变暖已成为当今世界面临最大的环境问题之一。根据政府间气候变化专门委员会第六次评估报告(IPCC AR6)显示,在经济快速增长情景下(SSP5-8.5),全球平均地表气温到21世纪中叶将升高1.5℃[1]。全球变暖背景下,趋强趋频的极端高温事件强烈地影响着陆地生态系统,对陆地碳循环的影响尤为突出[2-3]。土壤是陆地生态系统中最大的有机碳库,土壤有机碳(Soil Organic Carbon,SOC)不仅为植物生长提供碳源并维持土壤良好的物理结构,也以二氧化碳等温室气体形式向大气释放碳,对全球碳平衡起关键作用[4]。SOC不是一种单一化合物,其储量是进入土壤的有机物残体量与微生物分解损失量之间平衡的结果,因此受气候、植被、土壤理化特性及人类活动等诸多因素影响[5];且其周转速率相对缓慢,通常只有在较长时间尺度上(几十甚至百年)才能被发现或充分表现出来[6]。相较而言,气候变暖也是一个缓慢持续的过程,但极端高温事件的发生却具有极强的随机性和破坏力,且土壤在未来遭受极端高温的影响更为严重[7],使得陆地碳循环过程对极端高温事件的响应更加敏感。因此,全面准确理解气候长期变暖与极端高温影响下SOC的变化过程及其主控因子,不仅有助于正确评估SOC的变化方向和速率,对于预测气候变化及制定适应性策略均具有重要意义。

尽管众多研究结果表明SOC随温度升高而降低,但其对气候变化的响应仍不明确[8-14]。一方面,气候变暖有利于陆地生态系统中植物的生长,使得植物来源有机碳输入量增加,促进SOC积累[15];但新产生的碳可能通过激发效应加速或限制原始SOC分解,从而增加气候变暖与土壤碳库之间反馈预测的不确定性[16-17]。另一方面,微生物对温度较为敏感,变暖会使微生物更加活跃,刺激微生物对微生物残体碳的利用[18],土壤呼吸增加,导致土壤碳损失[19];但长期变暖会使植物及微生物产生适应性,一定程度上抑制了土壤碳损失[20]。相比长期变暖,极端高温对土壤有机碳影响的研究较为有限。研究表明,极端高温会在植物个体、器官和组织细胞不同水平上影响其功能[21],导致植物的光合作用减弱甚至植物生长停滞,且呼吸作用增强,进而加剧生态系统碳汇功能下降[222]。此外,极端高温会对土壤中微生物活性、酶活性和SOC稳定性及其分解速率产生复杂而重要的影响[23-25]。虽然极端高温会抑制微生物及酶的活性[26-28],降低土壤中有机质的分解速率;但同时也会降低土壤中SOC的稳定性[29-30],导致土壤中长期储存的有机碳得以释放,还可能直接促进SOC的分解速率,加速碳循环过程,从而影响土壤碳储存和生态系统的碳平衡。

农田生态系统是陆地生态系统碳循环过程中最为活跃的碳库,极易受到人类活动、气候变化等因素的强烈干扰,碳源/汇反馈机制复杂,引起学者的广泛关注[31-32]。目前,其研究方法主要包括地面观测调查、生态系统碳通量监测、大气反演和模型模拟等[33]。现场实测虽然可以获取SOC的准确数据,但很大程度上受限于监测成本和复杂的环境参数;且生态系统具有高度复杂和不确定性,仅根据点尺度、离散时间段的监测数据难以满足当前对生态系统碳循环动态变化研究的需求[34]。因此,通过建立不同界面元素与水热库,基于生理和生态过程表达不同库之间动态交换的生物地球化学过程模型被广泛应用于评估气候变化对SOC影响的研究中[911]。其中,DeNitrification-DeComposition(DNDC)模型是目前开发历史最长、应用广泛、扩展丰富、且在我国得到较好结果验证的一种生物地球化学模型[34]。例如,Jia等[11]利用DNDC模型估算了在不同气候情景下,我国北方干旱绿洲区焉耆盆地农田SOC的变化情况,结果表明在RCP 2.6(下降3%)、RCP 4.5(下降9.17%)及RCP 8.5(下降16%)情景下SOC储量均显示出快速下降趋势,且在高排放情景中下降得更快。车燕等[8]同样利用DNDC模型估算了在温度上升2、4、6℃情景下,福建省水田年均固碳速率分别为141、106和74 kg·hm–2,说明随着未来气温升高,福建省水田固碳速率虽将逐步下降,但整体仍呈“碳汇”状态。

目前,关于气候变化对SOC影响的研究主要集中于长期变暖的贡献[1135],但在长期变暖影响下,农田生态系统是碳源还是碳汇仍不明确,主要取决于影响有机碳分解造成的碳损失和净初级生产力提高带来的碳增益之间的竞争与平衡结果[36]。此外,据气象资料显示,我国极端高温事件呈多发重发态势[37]。考虑到极端高温事件发生的强随机性、复杂性及SOC响应的较长周转时间,短时间尺度的观测难以完全捕捉SOC对其响应的长期演变规律,因此通过科学合理的气候情景设置,利用生物地球化学模型准确模拟SOC的动态变化,进而量化及区分长期变暖与极端高温对农田SOC的影响,对于保障我国粮食安全和实现农业领域碳中和具有重要意义。本研究以我国东南沿海典型亚热带地区——福建省漳州市水田土壤为研究对象,基于1980年第二次全国土壤普查数据建立的1:5万大比例尺土壤数据库、1980—2016年农业管理资料及气象站点数据,采用DNDC模型对漳州市农田SOC进行长期动态模拟,旨在(1)评估气候变化背景下漳州市水稻土有机碳储量及密度的动态变化;(2)区分并量化漳州市农田SOC对极端高温和长期变暖的响应。

1 材料与方法 1.1 研究区域概况

漳州市(23°32′-25°13′N,116°53′-118°09′E)地处福建省闽南金三角腹地,其水稻种植区具有南亚热带季风气候特点,无霜期长(320~365d),气候温暖湿润(年均气温23.5℃、年均降雨量1 160 mm),热量、日照条件优越(年活动积温6500~7800℃、年均日照数2142.5 h),水稻土约占耕地总面积的74.81%,是福建省主要粮食产区之一。根据海拔和坡度,全市可划分为三种主要地貌类型区:丘陵山地区、谷地盆地区和平原台地区。漳州平原是福建省第一大平原,主要分布于东南沿海地带,其次为西北部的丘陵山地,谷地盆地零散分布[38]

1.2 模型介绍

DNDC模型[39-41]是由美国New Hampshire大学的李长生教授等基于生物地球化学概念发展起来,用于模拟预测陆地生态系统中碳氮循环的生物地球化学模型,由气候土壤、农作物生长、土壤有机质分解、硝化作用、反硝化和发酵作用等6个子模块构成。该模型自1992年首次发表以来,受到国内外学者广泛关注[42-44],并对其不断改进发展[45-46],在福建省农田生态系统研究中也得以应用和验证[47-49]。模型验证主要通过对模拟值与观测值二者的均方根误差(RMSE)、平均绝对误差(MAE)和皮尔逊相关系数(r)等统计指标进行评估。模型整体的不确定性主要来源于两个方面,一方面是由各项输入数据(如初始土壤属性、气象要素、农田管理措施等)的不确定性带来的随机误差(εr),另一方面是由模型内部结构或过程参数设定引起的系统误差(εs[50]

1.3 数据来源

本研究基于水稻土图斑进行模拟,所使用的漳州市1︰5万土壤数据库由空间数据库和属性数据库两部分组成。土壤空间数据是利用ArcGIS软件数字化1980年第二次全国土壤普查的漳州市1︰5万比例尺土壤图制作而成(含14 126个水稻土图斑)。漳州市土壤属性数据分别来源于1980年第二次全国土壤普查(119个)、2008年农业农村部测土配方施肥(3 832个)及2016年农业农村部耕地地力调查(3 802个)的水田土壤实测样点,其中包含DNDC模型运行所需的土壤有机质(Soil Organic Matter,SOM)、黏粒、pH和容重等数据。通过采用Shi等[51]提出的“PKB(pedological professional knowledge based method)”法将土壤空间数据和属性数据进行连接。对于2008年、2016年缺失的土壤容重,本研究采用罗玉叶等[49]根据2018年福建省水田土壤容重与SOM含量的回归模型y=2.033–0.234×ln(SOM)计算求得。

模型运行所需的农业管理资料来自1980—2016年漳州市农业统计年鉴,主要包括水稻的种植面积、播种/收获期、氮肥和有机肥数据、耕作制度及农业人口资料等。本研究的气象数据来源于漳州市两个国家基本站提取的日最高温、最低温和降雨量。极端高温阈值采用研究时段内多年同日百分位法,定义日最高温大于90%分位数即极端高温日(TX90p)。

1.4 气候情景设置

为定量化研究极端高温(Extreme Heat,EH)和长期变暖(Long-term Warming,LW)对福建省水田SOC的影响,本研究以1980—2016年气象数据为基础,设置了4种不同气候情景(1个控制组与3个试验对照组):控制组试验(CTRL)将1980—2016年每日最高气温设为当日去趋势气候态均值;试验组1(EXP_EH)在CTRL日最高温设置基础上,还原极端高温日的观测值;试验组2(EXP_LW)在CTRL日最高温设置基础上,还原非极端高温日的观测值;试验组3(EXP_obs)直接采用实际观测的日最高温数据。

1.5 数据统计分析

不同气候情景下1980—2016年漳州全市及其不同区县、地貌类型的水稻土有机碳总变化量(TSOC,Gg),面积加权年均有机碳变化速率(dSOC,kg·hm–2·a–1)计算公式如下:

$ \begin{array}{c}\text{ APS=}\sum\limits_{\text{i=1}}^{\text{n}}{\text{APS}}_{\text{i}}\end{array} $ (1)
$ \begin{array}{c}\text{ AMSC=}\sum\limits_{\text{f=1}}^{\text{h}}{\text{ASC}}_{\text{f}}\end{array} $ (2)
$ \begin{array}{c}\text{ TSOC=}\sum\limits_{\text{i=1}}^{\text{n}}{\text{(APS}}_{\text{i}}\times {\text{AMSC}}_{\text{i}})\end{array} $ (3)
$ \begin{array}{c}\text{ dSOC=TSOC/APS/37}\end{array} $ (4)

式中,APS为研究区水田土壤总面积,hm2;APSi为第i个水田土壤图斑面积,hm2;AMSC为某个图斑单位面积上1980—2016年期间SOC的总变化量,kg·hm–2;ASCf为某个图斑单位面积上第f年的SOC变化量,kg·hm–2h为模拟年份(h=1,2,3,…,37);AMSCi为第i个图斑单位面积上1980—2016年期间SOC的总变化量,kg·hm–2n为图斑编号。

1980—2016年各气候条件(极端高温、长期变暖及二者协同作用)对漳州市水稻土固碳速率变化的贡献量和贡献率计算公式如下:

$ \begin{array}{c}\Delta {\text{dSOC}}_{\text{EH}}{\text{=dSOC}}_{\text{EXP_EH}}-{\text{dSOC}}_{\text{CTRL}}\end{array} $ (5)
$ \begin{array}{c} \Delta {\text{dSOC}}_{\text{LW}}{\text{=dSOC}}_{\text{EXP_LW}}-{\text{dSOC}}_{\text{CTRL}}\end{array} $ (6)
$ \begin{array}{c} \Delta {\text{dSOC}}_{\text{LW+EH}}{\text{=dSOC}}_{\text{EXP_obs}}-{\text{dSOC}}_{\text{CTRL}}\end{array} $ (7)
$ \begin{array}{c} 贡献率=\frac{\Delta {\text{dSOC}}_{\text{t}}}{{\text{dSOC}}_{\text{CTRL}}}\times \text{100%[ }\end{array} $ (8)

式中,$\Delta {\text{dSO}}{{\text{C}}_{{\text{EH}}}}$为极端高温对固碳速率变化的贡献量,kg·hm–2·a–1$\Delta {\text{dSO}}{{\text{C}}_{{\text{LW}}}}$为长期变暖的贡献量,kg·hm–2·a–1$\Delta {\text{dSO}}{{\text{C}}_{{\text{LW + EH}}}}$代表极端高温和长期变暖的协同贡献量,kg·hm–2·a–1。贡献率则为各贡献量与控制组(CTRL)结果的比值,%。

2 结果 2.1 模型验证及不确定性

本研究分别采用漳州市2008、2016年水稻土SOC模拟值与经三倍标准差筛选的同年实测样点值(n2008=1725,n2016=1709)做模型验证(图 1)。2008、2016年水稻土SOC实测样点值分布范围分别为0.70~27.31 g·kg–1、0.70~27.31 g·kg–1,模拟的同期SOC结果分布范围为4.81~25.13 g·kg–1、4.63~25.00 g·kg–1,均落在实测样点值范围区间内。此外,模拟值与实测样点值呈显著相关(r2008=0.41,r2016=0.42,P < 0.01),平均绝对预测误差和均方根预测误差均较小(MAE2008=3.75,MAE2016=3.73;RMSE2008=4.65,RMSE2016=4.62),说明DNDC模型适用于漳州市水稻土有机碳的动态变化模拟监测。经计算,本研究中DNDC模型的系统误差εs为–0.063,随机误差εr为0.643,不确定性为–0.063±0.643 g·kg–1

图 1 漳州市2008年(a)和2016年(b)两期水田土壤有机碳模拟值与实测值线性回归 Fig. 1 Linear regressions between simulated and observed SOC values in paddy fields across Zhangzhou in 2008(a)and 2016(b), respectively
2.2 不同气候情景下漳州市水稻土有机碳的时空变化

本研究利用DNDC模型估算出1980—2016年漳州市水田土壤在去趋势气候态(CTRL)、仅受极端高温影响(EXP_EH)、仅受长期变暖影响(EXP_LW)及实际观测温度(EXP_obs)四种气候情景下固碳总量分别为1 032.17、952.15、1 045.98和966.03 Gg,年均固碳速率分别为93.98、86.70、95.24和87.96 kg·hm–2。漳州市水稻土受极端高温、长期变暖和二者协同影响量分别为–80.02、13.81和–66.14 Gg。与气候态(CTRL)相比,极端高温影响下水田土壤年均固碳速率下降7.75%,长期变暖影响下固碳速率则上升1.34%,二者协同影响下固碳速率下降6.41%。结果表明长期变暖总体上促进漳州市水田土壤碳固持,但极端高温不利于土壤固碳,且极端高温对漳州市水田土壤固碳的负面效应远大于长期变暖带来的促进作用。

从时间序列看(图 2),漳州市固碳总量和固碳速率整体上呈现明显的年际变化特征,1998年之前呈碳汇状态(即固碳总量和固碳速率均为正值);而1998年之后,许多年份表现为碳源状态,并与碳汇年份交替出现,二者存在1~4年的振荡周期,这可能与21世纪以来极端高温事件频发对SOC累积带来的负面效应有关。对比不同气候情景下,各气象条件对1980—2016年漳州市水田土壤逐年SOC累积的影响结果可知(图 3),极端高温常年对漳州市水稻土固碳起负作用(均为负值),而长期变暖的影响则在2000年左右由正转负,但总体呈现下降趋势,这可能与长期变暖对生态系统中植物源碳的促进作用随持续时间减弱有关[52]。长期变暖和极端高温二者协同作用的影响在1987年由正转负,较长期变暖单一影响下的转折年份提前约13年,进一步证明极端高温的负效应在漳州市水田土壤固碳过程中起主导作用。

图 2 1980—2016年不同气候情景下漳州市水田土壤固碳总量及其固碳速率年际变化 Fig. 2 Interannual variation of the total amount of soil carbon sequestration and carbon sequestration rate in paddy fields across Zhangzhou from 1980 to 2016 under different climate scenarios

图 3 1980—2016年极端高温(a)、长期变暖(b)及二者协同作用(c)对漳州市水田土壤固碳速率的影响 Fig. 3 Changes in the soil carbon sequestration rate in paddy fields across Zhangzhou from 1980 to 2016 caused by extreme high temperatures(a), long-term warming(b), and their synergistic effect(c), respectively
2.3 各县级行政区水稻土有机碳累积对不同气候情景的响应

模拟结果显示(图 4a),不同气候情景下漳州市各行政区中漳浦县和龙海市水稻土固碳总量较大,分别约占全市的17.8%(172.14~181.71 Gg)和19.5%(184.71~201.20 Gg);东山县固碳总量最小,仅占1.22%(11.73~12.47 Gg)。与控制组相比,极端高温与实测组固碳量均减少,长期变暖情景下则略微增长。就固碳速率而言(图 4b),华安县和龙文区水稻土年均固碳速率较高(分别为154.15~164.32 kg·hm–2和149.54~160.40 kg·hm–2),南靖县最低(23.02~33.47 kg·hm–2)。

注:LW代表长期变暖;EH代表极端高温;LW+EH代表长期变暖和极端高温协同作用。下同。  Note: LW represents the long-term warming; EH represents the extremely high temperature; LW+EH represents the synergistic effect of long-term warming and extremely high temperature. The same as below. 图 4 1980—2016年不同气候情景下漳州市各县级行政区水稻土固碳总量(a)和固碳速率(b),以及长期变暖、极端高温和二者协同作用对固碳速率影响的总贡献量(c)及贡献率(d) Fig. 4 Distribution of the total amount of carbon sequestration(a)and carbon sequestration rate(b)of paddy soils under different climate scenarios in each administrative district across Zhangzhou from 1980 to 2016, and the contribution amount(c)and contribution rate(d)caused by long-term warming, extreme high temperatures, and their synergistic effect, respectively

图 4c图 4d显示了气候条件对各地区固碳速率的影响情况,其中南靖县受气候条件影响最大,极端高温影响下固碳速率降低8.18 kg·hm–2·a–1,贡献率达26.23%,长期变暖影响下固碳速率提高了2.27 kg·hm–2·a–1,贡献率达7.27%;其余各地极端高温影响下固碳速率降幅介于5.33~9.00kg·hm–2·a–1,贡献率为4.93%~12.39%,长期变暖影响下升幅介于0.04~2.32 kg·hm–2·a–1,贡献率为0.04%~2.73%。由此可知,极端高温对SOC累积的贡献率(绝对值)远大于长期变暖结果,二者协同贡献率均为负值。

2.4 各地貌类型区水稻土有机碳累积对不同气候情景的响应

模拟结果显示(图 5a),不同气候情景下漳州市各地貌类型区中平原台地区固碳总量最大,约占全市的42.62%(406.91~441.43 Gg);其次是丘陵山地区,占漳州市固碳总量的32.08%(303.57~339.64 Gg);谷地盆地区最小,约占25.30%(241.66~265.25 Gg)。就年均固碳速率而言(图 5b),不同气候情景下平原台地区最高(96.82~105.04 kg·hm–2);谷地盆地和丘陵山地的固碳速率分别介于81.30~89.24 kg·hm–2和79.73~89.02 kg·hm–2。与控制组相比,各地貌类型区在极端高温和实测组情景中固碳量和固碳速率均下降,长期变暖情景下有所增加。

图 5 1980—2016年不同气候情景下漳州市各地貌类型区固碳总量(a)和固碳速率(b),以及长期变暖、极端高温和二者协同作用对固碳速率影响的总贡献量(c)及贡献率(d) Fig. 5 Distribution of the total amount of carbon sequestration(a)and carbon sequestration rate(b)of paddy soils under different climate scenarios in each landform type across Zhangzhou from 1980 to 2016, and the contribution amount(c)and contribution rate(d)caused by long-term warming, extreme high temperatures, and their synergistic effect, respectively

图 5c图 5d显示了气候条件对不同地貌类型区固碳速率的影响。其中,丘陵山地区受极端高温和长期变暖影响最大,相比于控制组,其在极端高温影响下固碳速率下降7.74 kg·hm–2,贡献率为8.84%,长期变暖影响使其固碳速率提高1.55 kg·hm–2,贡献率为1.98%;谷地盆地区受气候条件影响也较大,极端高温导致其固碳速率下降7 kg·hm–2,贡献率为7.93%,长期变暖效应则致其固碳速率提高了0.93 kg·hm–2,贡献率为1.06%;平原台地区最小,极端高温影响下固碳速率降幅为6.81%,长期变暖影响下升幅为1.09%。

3 讨论

1980—2016年漳州市水稻土固碳总量和年均固碳速率均呈现明显的年际变化特征,且1998年之后,漳州市水稻土许多年份表现为碳源状态,并与碳汇年份交替出现,二者呈1~4年的振荡周期,几乎与厄尔尼诺-南方涛动(El Niño-Southern Oscillation,ENSO)气候振荡同频。此外,有机肥施用量对水稻土固碳的影响也不容忽视。据统计,1980—2016年漳州市有机肥施用量前期持续增长,2006年起施用量显著减少。因此,2000年后漳州市水田土壤频繁呈碳源状态极可能由于此时段内极端高温频发、后期有机肥施用量减少共同导致。据Wang等[53]通过利用空间代替时间耦合Meta分析的最新研究结果表明,极端高温对全球平均SOC的变化表现出显著的消极影响,且随着极端热浪频率的增加,SOC的减少从2%增加至21%,本研究结果也与之相符。

前人研究表明[49],影响福建省水稻土固碳速率的主导因素为初始有机碳、黏粒含量和有机肥施用量。一般而言,初始有机碳水平较低的土壤固碳潜力较大[54-55];增施有机肥通过高碳投入直接影响SOC储量[56],改善土壤物理性质,促进土壤团聚体形成和稳定,保护SOC免受微生物和酶的分解[57];而黏粒是影响SOC积累和矿化的主导因素之一,通过提供物理和化学方面的保护,黏粒含量高的土壤可以有效减缓SOC分解并降低其矿化速率[58]。由于漳州市华安县、龙文区和龙海市的初始有机碳处于全市平均水平,黏粒含量较高,加上较高的有机肥投入量,故固碳速率居漳州市各县级行政区前列。

从不同气候情景看,气候条件变化对漳州市水稻土固碳速率的影响在空间分布上整体呈西北高-东南低的规律(图 4c图 4d),主要原因可能是地势和纬度差异导致其温度敏感性(Q10)不同。漳州市西北部主要分布海拔和坡度较高的丘陵山地,东南部主要分布地势较低的平原台地,统计结果也表明(图 5d),气候变化对丘陵山地区的影响最大,谷地盆地区次之,平原台地最小。相关研究表明,Q10随海拔升高而增大[50],较高海拔地区(如丘陵山地)对极端高温和长期变暖均有较高的温度敏感性,土壤可能以更高的速率释放CO2[59-60]。另一方面,在一定海拔高度范围内,有机碳的含量随海拔升高而增加,且海拔较高,温度较低,进一步降低了有机碳的矿化速率[61],因此导致更高的SOM分解率而不利于土壤碳固存。

4 结论

DNDC模型能够较好地模拟评估气候变化背景下漳州市水田土壤有机碳的动态变化。本研究通过四种不同气候情景设置,对SOC累积情况进行模拟监测,区分并量化其对极端高温和长期变暖的响应程度。结果显示,在气候平均态、长期变暖、极端高温和实测温度四种不同气候情景下,1980—2016年漳州市水田土壤仍表现出较好的固碳能力,固碳总量分别为1 032.17、1 045.98、952.15和966.03 Gg。其中,仅受长期变暖情景影响的水田土壤固碳量最大,而仅受极端高温影响的固碳量最小。此外,极端高温频发年份对应固碳能力低值年,且存在1~4年振荡周期,几乎与厄尔尼诺-南方涛动气候振荡同频,说明其对极端高温较为敏感。总体而言,长期变暖有利于土壤有机碳的固定,而极端高温则明显抑制农田生态系统土壤碳汇功能的发挥,且极端高温的负面影响远大于长期变暖的累积效益。虽然在气候变暖背景下土壤仍具有较强的固碳能力,但未来频发的极端高温事件可能在一定程度上造成更大的碳损失。

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