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增温和氮沉降是驱动全球变化的2个重要因素。IPCC第六次评估报告中多个气候模型预测表明,未来气候仍将呈变暖趋势,相比于1850—1900年,2081—2100年全球表面平均温度将升高大约4.4 ℃[1]。人类活动导致氮沉降比例增加,由氮沉降引发的一系列环境问题已逐渐成为当今研究热点。研究指出,中国是全球氮沉降最严重的地区之一[2],其中,我国亚热带地区氮沉降量高达40 kg·hm−2·a−1[3]。土壤氮素是陆地生态系统的重要养分元素,同时也是植物生长所需的营养元素。自然界中的氮通常需要由微生物分解成简单无机氮如铵态氮和硝态氮,这个过程称为土壤氮矿化[4]。土壤氮矿化产生的铵态氮和硝态氮在土壤中进一步转化产生N2O。全球变化通过改变微生物分泌的胞外酶活性、微生物生物量以及微生物群落结构间接影响土壤氮矿化过程[5-8],从而对森林生态系统生产力产生深远影响。此外,森林土壤作为重要的温室气体N2O排放源,其巨大的排放潜能对调节全球气候十分重要[9]。因此,研究森林土壤氮矿化和N2O排放对增温和氮添加的响应具有重要意义。
温度是影响土壤氮矿化的重要环境因子,通常认为一定范围的增温会促进微生物对有机氮的矿化,因为温度升高提高土壤中微生物及其分泌的胞外酶活性 [5-6]。Butler等[10]在温带北方森林进行长达7 a的增温5 ℃实验发现,温度升高使土壤净氮矿化每年平均增加45%。此外,有研究发现,土壤氮矿化速率随温度增加呈先升高后降低的趋势,并在25 ℃时土壤氮矿化速率达到最大[11]。与此同时,土壤氮矿化过程受氮添加的直接作用,氮添加往往会促进土壤氮矿化[12]。然而,目前有关氮添加对亚热带土壤氮矿化的影响仍未达成一致观点。相比于温带北方地区,通常认为亚热带地区土壤氮有效性高,外源氮的持续输入可能导致亚热带地区土壤出现氮富集现象,导致土壤pH值降低,影响土壤微生物量、微生物多样性和群落结构等[8, 13-14],进而改变土壤氮矿化速率,引起土壤氮素的排放和淋溶[15]。此外,氮添加对土壤氮矿化作用的程度受氮添加量影响,如Gao等[16]在我国南部亚热带松林进行的氮添加实验发现,低氮添加的土壤氮矿化速率与对照没有显著差异,而高氮添加则降低了20%左右。N2O是一种重要的温室气体,人们越来越关注N2O浓度升高对全球气候变暖和氮循环的影响。同时,气候变暖和氮沉降可能反作用于N2O浓度的变化。如多数研究表明,增温提高了微生物丰度和活性以及增温增加土壤可溶性有机碳(DOC)的输入,促进了N2O排放[17-18];但也有研究指出,增温会减少N2O的排放[19]。增温对土壤N2O排放的影响仍未达成一致观点。有研究指出,氮添加会通过改变土壤pH环境进而作用于土壤N2O排放,如过酸的土壤环境会抑制N2O还原酶的活性[20],从而增加N2O排放。此外,氮添加为硝化反硝化过程提供充足底物,随着氮添加含量升高,N2O排放也明显增加[21]。
综上,这些单因子变化对森林土壤氮矿化及N2O排放的研究表明,森林土壤氮矿化及N2O排放对增温和氮添加的响应表现出复杂的特征,由于不同研究的气候区、养分背景等不同,增温和氮添加对森林土壤氮矿化及N2O排放的影响机制存在差异[13,19,22]。未来全球变化背景下增温和氮沉降将同时发生,然而有关增温和氮添加及二者交互对森林土壤氮矿化和N2O排放的影响研究较少,这不利于清晰认识土壤氮素转化过程中增温和氮添加及二者交互在其中发挥的作用和关系。因此,有必要对中亚热带森林土壤氮矿化和N2O排放进行研究。本研究以中亚热带杉木(Cunninghamia lanceolata (Lamb.)Hook.)人工幼林土壤为研究对象,采用室内培养的方法探究增温和氮添加对土壤氮矿化和N2O排放的影响,为研究全球变化背景下土壤氮循环过程提供基础数据。
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由图1可知:各处理的土壤铵态氮含量、硝态氮含量和矿质氮含量均表现为增温 + 高氮>高氮>低氮>增温 + 低氮>增温>对照,与对照处理相比,单独氮添加、增温和氮添加交互处理明显提高了土壤铵态氮含量、硝态氮含量和矿质氮含量,而增温处理增加不明显,说明氮添加能显著增加土壤矿质氮含量,且氮添加水平越高土壤矿质氮含量增加越明显,增温处理增加土壤矿质氮含量的效果比氮添加小。单独氮添加、增温和氮添加交互处理的土壤铵态氮含量在培养第1天和第3天达到最大值,土壤硝态氮含量在第14天达到最大值,说明单独氮添加、增温和氮添加交互处理增加的铵态氮可能一部分通过硝化作用转化为硝态氮。增温处理的土壤铵态氮含量、硝态氮含量和矿质氮含量随培养时间的增加呈上升趋势,并在培养结束时达到最大值,说明增温处理对土壤矿质氮的影响存在时间滞后性,增温时间越长,增加的矿质氮含量就越明显。单独氮添加、增温和氮添加交互处理的土壤铵态氮含量随培养时间延长呈不断减少的趋势,但仍比对照处理的高,土壤硝态氮含量在培养期间波动较大,变化趋势为先升高后降低再升高。表1表明:氮添加、培养时间及二者交互处理显著影响土壤铵态氮含量、硝态氮含量和矿质氮含量,培养时间也显著影响土壤净铵化速率、净硝化速率和净氮矿化速率,且氮添加与培养时间存在显著交互作用。此外,单独氮添加、增温和氮添加交互处理对土壤净氮矿化速率影响显著(P<0.05)。
图 1 增温和氮添加处理下土壤铵态氮、硝态氮和矿质氮含量随时间的变化
Figure 1. Changes of soil ammonium, nitrate and mineral nitrogen contents with time under warming and nitrogen addition treatments
由图2可知:与对照相比,单独增温处理对土壤净铵化速率、净硝化速率和净氮矿化速率影响不显著(P>0. 05),单独氮添加、增温和氮添加交互处理显著降低土壤净铵化速率,而低氮和增温 + 低氮处理显著提高了土壤净硝化速率(P<0.05)。此外,低氮处理显著提高土壤净氮矿化速率,而高氮和增温 + 高氮处理显著降低土壤净氮矿化速率(P<0. 05)。
图 2 增温和氮添加处理对培养28 d后的土壤净铵化速率、净硝化速率和净氮矿化速率的影响
Figure 2. Effects of warming and nitrogen addition on soil net ammoniation rate, net nitrification rate and net nitrogen mineralization rate after 28 days of incubation
由图3可知:主成分1(PC1) 和主成分2(PC2) 对土壤净铵态氮转化的解释率分别为64.4%和21.5%,2轴的累积贡献率为85.9%(图3a);PC1和PC2对土壤净硝态氮转化的解释率分别为78.0%和16.3%,2轴累积贡献率为94.3%(图3b);PC1和PC2对土壤净氮矿化的解释率分别为46.4%和40.4%,2轴累积贡献率为86.8%(图3c)。单独氮添加、增温和氮添加交互处理均与对照处理间隔距离较远,即对照处理能明显的与这几种处理区分开,说明单独氮添加、增温和氮添加交互处理使土壤净铵态氮转化(图3a)、土壤净硝态氮转化(图3b)和土壤净氮矿化(图3c)发生明显变化;其中,高氮和增温 + 高氮处理对土壤净铵态氮转化、土壤净硝态氮转化和土壤净氮矿化的影响较一致,而低氮和增温 + 低氮处理对土壤净硝态氮转化的影响较一致。
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增温、培养时间、增温 × 氮添加、增温 × 培养时间、氮添加 × 培养时间、增温 × 氮添加 × 培养时间均显著影响土壤N2O排放速率(P<0.05),而氮添加对土壤N2O排放速率无显著影响(表1)。图4表明:单独增温处理土壤N2O排放速率整体低于对照处理,在第5天单独增温处理土壤N2O排放速率达最高值0.36 μg·kg−1·h−1。0~5 d,与对照相比,氮添加处理降低了土壤N2O排放速率,且高氮处理降低的土壤N2O排放速率较低氮处理明显。低氮和高氮处理土壤N2O排放速率分别在第3天和第7天达到最高值,分别为0.32和0.33 μg·kg−1·h−1。整体上看,前14 天增温和氮添加交互处理土壤N2O排放速率低于对照处理,并在第14天达到最低值,随后土壤N2O排放速率逐渐升高,增温 + 低氮处理土壤N2O排放速率在第21天超过对照处理,并在第28天达到最高,增温 + 高氮处理的N2O排放速率最高值在第21天出现。
表 1 增温、氮添加和培养时间对土壤矿质氮、氮素转化和N2O排放速率影响的重复测量方差分析
Table 1. Repeated measures analysis of variance for the effects of warming, nitrogen addition and incubation time on soil mineralization nitrogen, nitrogen transform and N2O emission rates
处理
Treatment铵态氮含量
NH4+-N
content硝态氮含量
NO3−-N
content矿质氮含量
Mineralization
N content净铵化速率
Net
ammonification
rate净硝化速率
Net
nitrification
rate净氮矿化速率
Net nitrogen
mineralization
rateN2O排放速率
N2O
emission
rate增温
Warming0.885 0.605 0.744 0.659 0.801 0.718 < 0.001 氮添加
Nitrogen addition< 0.001 < 0.001 < 0.001 0.656 < 0.001 < 0.001 0.114 增温 × 氮添加
Warming × Nitrogen addition0.954 0.724 0.887 0.218 0.709 0.028 0.031 培养时间
Incubation time< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 增温 × 培养时间
Warming × Incubation time0.112 0.056 0.185 0.281 0.321 0.089 < 0.001 氮添加 × 培养时间
Nitrogen addition × Incubation time< 0.001 < 0.001 0.018 0.001 < 0.001 0.001 0.001 增温 × 氮添加 × 培养时间
Warming × Nitrogen addition ×
Incubation time0.463 0.935 0.946 0.007 0.894 0.622 0.027 图 4 增温和氮添加下土壤N2O排放速率随时间的变化
Figure 4. Changes of soil N2O emission rate with time under warming and nitrogen addition
图5表明:增温、低氮、高氮、增温 + 低氮和增温 + 高氮处理土壤N2O累积排放量显著低于对照处理,分别降低50%、21%、29%、62%和31%;单独增温处理土壤N2O累积排放量显著低于氮添加处理,增温 + 高氮处理土壤N2O累积排放量显著高于增温 + 低氮处理。
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图6表明:室内培养28 d后,增温和氮添加处理土壤pH值较对照处理显著降低(P<0.05),但增温和氮添加处理间土壤pH值无显著差异。
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由图7可知:土壤N2O排放速率与土壤净铵化速率无显著相关性(R2=0.03, P=0.30),与净硝化速率呈显著负相关(R2=0.11, P=0.03),而与土壤pH呈显著正相关(R2=0.72, P=0.03)。
增温和氮添加对中亚热带杉木人工林土壤氮矿化和N2O排放的影响
Effects of Warming and Nitrogen Addition on Soil Nitrogen Mineralization and N2O Emission in a Mid-subtropical Cunninghamia lanceolata Plantation
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摘要:
目的 研究增温和氮沉降对中亚热带森林土壤氮矿化和氧化亚氮(N2O)排放的影响,以期深入认识全球变化背景下中亚热带森林土壤氮循环过程。 方法 选取经过野外增温和氮添加处理的中亚热带杉木人工林土壤,将野外非增温处理和增温处理的土壤置于不同温度(20、25 ℃)培养箱中,同时对野外氮添加处理的土壤继续添加不同梯度的氮素(0.1、0.2 g·kg−1,以干土计),进行为期28 d的室内培养,研究增温和氮添加对土壤氮矿化和N2O排放的影响。 结果 与对照相比,增温和氮添加及二者交互处理增加了土壤铵态氮、硝态氮和矿质氮含量,且氮添加水平越高增加越明显,增温处理增加不显著。与对照相比,培养28 d后增温处理的土壤净铵化速率、净硝化速率和净氮矿化速率变化不显著,低氮、增温 + 低氮显著增加土壤净硝化速率,而高氮、增温 + 高氮显著降低土壤净氮矿化速率。与对照相比,增温和氮添加及二者交互处理总体降低土壤N2O排放速率,土壤N2O累积排放量也显著降低(P<0.05),其中,单独增温、低氮、高氮、增温 + 低氮和增温 + 高氮处理土壤N2O累积排放量显著低于对照50%、21%、29%、62%和31%。增温和氮添加及二者交互处理显著降低土壤pH值。相关性分析表明:土壤N2O排放速率与土壤pH值呈显著正相关,与硝化速率呈显著负相关,而与土壤铵化速率无显著相关。 结论 增温和氮添加降低土壤pH值,同时抑制土壤N2O排放,因此,全球变化背景下中亚热带森林土壤中存留的硝态氮可能以淋溶方式损失。 Abstract:Objective To study the effects of warming and nitrogen deposition on soil nitrogen mineralization and nitrous oxide (N2O) emission in mid-subtropical forests for further understanding soil nitrogen cycling process in subtropical forests under the background of global change. Method The soils taken from a mid-subtropical Cunninghamia lanceolata plantation treated with field warmed and nitrogen addition were selected, and the field un-warmed and warmed soils were placed in incubators at different temperatures (20, 25 ℃). The soil was continuously added with different gradients of nitrogen (0.1、0.2 g·kg−1) for 28 days of indoor cultivation to study the effects of warming and nitrogen addition at different concentrations on soil nitrogen mineralization and N2O emissions. Result Compared with the control, soil ammonium, nitrate and mineral nitrogen contents were increased by warming and nitrogen addition and their interaction treatment, and the increase was more obvious with the higher level of nitrogen addition, while the increase was not significant in warming treatment. Compared with the control, the net ammonification rate, net nitrification rate and net nitrogen mineralization rate of soil treated with warming after 28 days of culture had no significant changes. Low nitrogen, warming + low nitrogen treatments could significantly increase soil net nitrification rate, while high nitrogen, warming + high nitrogen could significantly decrease soil net nitrogen mineralization rate. Compared with the control, warming and nitrogen addition and their interaction treatment reduced the soil N2O emission rates, and the cumulative soil N2O emissions was also decreased significantly (P<0.05). The cumulative soil N2O emissions under the treatments of low nitrogen, high nitrogen, warming + low nitrogen and warming + high nitrogen were significantly lower than those of the control by 50%, 21%, 29%, 62% and 31%, respectively. At the same time, warming, nitrogen addition and their interaction treatments also significantly decreased the soil pH value. Correlation analysis showed that soil N2O emission rate was positively correlated with soil pH and was negatively correlated with the soil net nitrification rate, but not significantly correlated with soil ammonium rate. Conclusion Warming and nitrogen addition reduce soil pH and inhibit soil N2O emission. Therefore, nitrate nitrogen retained in mid-subtropical forests soils may be lost by leaching under the background of global change. -
Key words:
- mid-subtropical
- / nitrogen mineralization
- / N2O emission
- / warming
- / nitrogen addition
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表 1 增温、氮添加和培养时间对土壤矿质氮、氮素转化和N2O排放速率影响的重复测量方差分析
Table 1. Repeated measures analysis of variance for the effects of warming, nitrogen addition and incubation time on soil mineralization nitrogen, nitrogen transform and N2O emission rates
处理
Treatment铵态氮含量
NH4+-N
content硝态氮含量
NO3−-N
content矿质氮含量
Mineralization
N content净铵化速率
Net
ammonification
rate净硝化速率
Net
nitrification
rate净氮矿化速率
Net nitrogen
mineralization
rateN2O排放速率
N2O
emission
rate增温
Warming0.885 0.605 0.744 0.659 0.801 0.718 < 0.001 氮添加
Nitrogen addition< 0.001 < 0.001 < 0.001 0.656 < 0.001 < 0.001 0.114 增温 × 氮添加
Warming × Nitrogen addition0.954 0.724 0.887 0.218 0.709 0.028 0.031 培养时间
Incubation time< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 增温 × 培养时间
Warming × Incubation time0.112 0.056 0.185 0.281 0.321 0.089 < 0.001 氮添加 × 培养时间
Nitrogen addition × Incubation time< 0.001 < 0.001 0.018 0.001 < 0.001 0.001 0.001 增温 × 氮添加 × 培养时间
Warming × Nitrogen addition ×
Incubation time0.463 0.935 0.946 0.007 0.894 0.622 0.027 -
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