荣慧(1995—),女,山东曹县人,硕士研究生,主要从事土壤结构与有机碳周转关系的研究。E-mail:
土壤有机碳(SOC)矿化一般通过培养松散土样来测定,但是松散土样与原状土的结构存在很大差异,二者之间SOC矿化的关系尚不明确;通过填装土柱可以获得接近田间状态的土壤样品,但填装的紧实程度会改变土壤孔隙结构,因此可能影响SOC矿化。本研究首先以施用不同量有机肥的红壤为研究对象,设置松散土样和填装土柱两个处理,采用室内培养法比较二者之间SOC矿化的差异;然后选择其中一种土壤填装土柱,设置BD1.1、BD1.3、BD1.5、BD1.7四个紧实程度处理,容重分别为1.1、1.3、1.5和1.7 g·cm–3,利用X射线显微CT(Computed Tomography,CT)成像技术分析土壤孔隙结构,分析紧实程度对土壤孔隙结构及SOC矿化的影响。结果表明,松散土样与填装土柱的SOC矿化量有显著差异,培养结束时(第57天),松散土样的有机碳累积矿化量约是填装土柱的4倍。紧实程度增加较大程度地降低了土壤的总孔隙度和大孔隙度,降低比例分别为12.9%~17.4%和18.7%~88.5%;并且使充气孔隙度从63.6%降至8.2%,而充水孔隙度从36.4%增至91.8%。填装土柱的SOC矿化量随紧实程度增加呈先增加后降低的趋势,培养结束时(第28天),BD1.5的SOC矿化量最高。回归分析的结果表明,SOC矿化量与总孔隙度、大孔隙度(> 16 μm)、充水孔隙度(Water-filled pore space,WFPS)或充气孔隙度(Air-filled pore space,AFPS)之间存在显著的非线性关系。当总孔隙度或大孔隙度低于46%或3.7%时,SOC矿化量随孔隙度增加而增加;反之,SOC矿化量随孔隙度增加而降低。SOC矿化量与WFPS或APFS之间的关系呈现出类似的规律,当WFPS为66%或AFPS为34%时,SOC矿化量最高。以上结果说明,通过培养松散土样测定SOC矿化将会高估田间SOC的矿化潜力;紧实程度的变化会改变土壤的孔隙结构进而影响填装土柱的SOC矿化;SOC矿化量与孔隙度之间存在显著的非线性关系。
Soil organic carbon (SOC) mineralization is generally measured by laboratory incubation of loose soil samples. However, the structure of loose soil samples is of great difference from that of bulk soil samples. The relationship between SOC mineralization of loose soil samples and bulk soil samples is not clear. Soil samples close to field conditions can be obtained by repacking soil columns. Nevertheless, compactness can affect soil pore structure and may influence SOC mineralization. Therefore, this study aimed to evaluate whether it is accurate to represent SOC mineralization in the field by incubating loose soil samples and how compactness influences soil pore structure or SOC mineralization in repacked soil columns.
Soil samples were collected from a long-term field experimental site with treatments receiving different amount of pig manure. In our first incubation experiment, all of these soils were selected and two treatments were set up in each soil: loose soil samples and repacked soil columns. In the second incubation experiment, only one soil was used, and the soil was repacked into columns with four bulk densities, which were 1.1(BD1.1), 1.3(BD1.3), 1.5(BD1.5)and 1.7(BD1.7)g·cm–3. The samples of these two experiments were incubated for 57 d and 28 d, respectively. SOC mineralization was measured during incubation, and soil pore structure was quantified using X-Ray micro-computed tomography (μ CT)imaging.
At the end of incubation(57 d), the cumulative amount of SOC mineralization was significantly different between loose soil samples and repacked soil columns. The cumulative amount of SOC mineralization in the loose soil samples was about 4 times that of the repacked soil columns. In the second experiment, the total porosity decreased by 12.9%, 14.8% and 17.4%, respectively under BD1.3, BD1.5 and BD1.7 compared with BD1.1. In relative to BD1.1, the increase of compactness decreased macro-porosity by 19.0%, 65.5% and 88.5%, respectively under BD1.3, BD1.5 and BD1.7. In addition, the water-filled pore space (WFPS) increased from 36.4% to 91.8% and air-filled pore space (AFPS) decreased from 63.6% to 8.2%. At the end of incubation(28 d), the cumulative amount of SOC mineralization generally increased as bulk density increased up to 1.5 gžcm–3, after which there was a decrease. The regression analysis showed that there was a significant nonlinear relationship between the cumulative amount of SOC mineralization and total porosity, macro-porosity, WFPS and AFPS. The cumulative amount of SOC mineralization increased with increasing total porosity and macro-porosity until a level of 46% and 3.7% was respectively reached, afterwards it began to decline. Also, the relationship between the cumulative amount of SOC mineralization and WFPS and AFPS showed the same trend. The cumulative amount of SOC mineralization was the highest when WFPS was 66% or AFPS was 34%.
Laboratory incubation using loose soil samples will overestimate the potential of SOC mineralization in the field, while a change of compactness will modify soil pore structure and subsequently affect SOC mineralization. There is a significant nonlinear relationship between the cumulative amount of SOC mineralization and porosity.
土壤有机碳(SOC)矿化指SOC分解产生CO2的过程,认识SOC的矿化规律对于阐明土壤碳库的周转过程并对其进行有效调节具有十分重要的作用[
通过室内培养法测定SOC矿化时,一般采用过筛后的松散土样进行培养[
填装土柱时,填装的紧实程度会改变土壤的孔隙结构,因此可能会影响SOC的矿化过程。一些学者发现,SOC矿化量随紧实程度增加逐渐降低,且SOC矿化量与容重之间存在显著的负相关关系,并认为这可能是紧实程度增加使土壤的通气性变差,从而抑制了微生物对SOC的分解导致的[
土壤样品采集于江西省鹰潭市余江县中国科学院红壤生态实验站(28°15′20″N,116°55′30″E)的长期定位试验田。该区属于中亚热带湿润季风气候,年均温度17.6℃,多年平均降雨量1 795 mm。土壤为第四纪红黏土母质发育的红壤(黏化湿润富铁土),黏粒、粉粒和砂粒的含量分别为36.3%、42.5%和21.2%。长期定位试验开始于2002年,本研究选择3个施肥处理,分别为:不施肥(Control);施低量有机肥(LM,N 150 kg·hm–2·a–1)和施高量有机肥(HM,N 600 kg·hm–2·a–1)。有机肥为猪粪,来自试验站附近的养殖场,平均pH为7.72,全氮32.9 g·kg–1(干基,下同),全钾14.0 g·kg–1,全磷20.2 g·kg–1,全碳306.5 g·kg–1。每个施肥处理设置3次重复,以顺序区组排列,小区大小为2 m × 2 m。种植作物为玉米,品种为苏玉24。每年4月中旬播种,7月下旬玉米收获,之后至次年4月土地休闲。种植密度为每小区20株,相当于50 000株·hm–2。于2019年7月玉米收获后采集0~20 cm的表层土样,每个小区随机采集3个点混合为一个样品,室温下风干,挑去根系、石块等。将同一施肥处理土样均匀混合,过2 mm筛,分别称为Control、LM和HM土壤,供试土壤的基本性质见
供试土壤的基本理化性质
Soil physicochemical properties under three fertilization treatments
土壤 |
pH | 土壤有机碳 |
全氮 |
阳离子交换量 |
田间持水量 |
注:Control、LM和HM分别代表采集自长期定位实验的不施肥、施低量有机肥和施高量有机肥处理的土壤样品。Note:Control,LM and HM in the table represent the soil samples collecting from a long-term fertilization experimental site receiving no manure,low manure and high manure,respectively. | |||||
Control | 4.64 | 3.67 | 0.49 | 14.54 | 0.23 |
LM | 4.88 | 6.93 | 0.76 | 14.98 | 0.26 |
HM | 5.77 | 9.38 | 0.98 | 16.94 | 0.25 |
培养试验一选用Control、LM和HM三种施肥处理的土壤,设置松散土样和填装土柱两个处理,每个处理设置3个重复,比较松散土样和填装土柱SOC矿化的差异。松散土样处理:取20.0 g过2 mm筛的土样加入500 mL培养瓶中,松散铺于瓶底(
松散土样(a)与填装土柱处理(b)示意图
Schematic diagram of loose soil samples(a)and repacked soil columns(b)
试验一采用土壤呼吸法测定SOC矿化量。将土样含水量调节为田间持水量的75%,然后置于22℃的培养箱中避光培养57 d,填装土柱培养前需在4℃的培养箱中水平衡3 d。分别在培养的第1、2、4、8、11、15、22、29、37、43、50和57天时采集气体。采气前,先将培养瓶置于22℃的室温下通风20 min,然后用硅橡胶塞密封瓶口。向培养瓶中注入20 mL新鲜空气,混合均匀后,从中抽取20 mL气体注入真空集气瓶;培养6 h后,再次采气。采气结束后,去除硅橡胶塞,用保鲜膜裹住瓶口并扎孔。采集的气体利用气相色谱(Gas chromatography,GC. Agilent 7890A,Agilent Technologies,Santa Clara,CA,USA)测定CO2浓度,进而计算CO2的产生速率(
式中,
培养试验二选取HM土壤,设置BD1.1、BD1.3、BD1.5、BD1.7四个紧实程度处理,容重分别为1.1、1.3、1.5和1.7 g·cm–3,每个处理设置3个重复,分析不同紧实程度对SOC矿化的影响。土柱填装方式同培养试验一。
试验二采用碱液吸收法测定SOC矿化量。同样地,将土柱含水量调节为田间持水量的75%(0.19 g·g–1),培养前放入4℃的培养箱中水平衡3 d。水平衡结束后,将土柱放入500 mL培养瓶中,然后将盛有5 mL 0.5 mol·L–1 NaOH溶液的特制吸收瓶小心地置于培养瓶内,加盖密封,放置在22℃的恒温培养箱中,黑暗条件下培养28 d。在培养的第1、3、5、7、9、14、21和28天取出吸收瓶,除第28天外,其余时间均换上新的吸收液继续培养。将取出的吸收瓶中的溶液完全洗入三角瓶中,加入1 mol·L–1的BaCl2溶液2 mL和2滴酚酞指示剂,用标准酸(约0.03 mol·L–1 HCl)滴定直至红色消失。通过HCl消耗量计算CO2释放量,并进一步计算出SOC矿化速率及累积矿化量。CO2释放量(mg·kg–1)的计算公式为:
式中,
培养试验二结束后,利用X射线显微CT(Phoenix Nanotom X-ray μCT,GE,Sensing and Inspection Technologies,GmbH,Wunstorf,Germany)扫描不同容重的填装土柱。扫描电压为90 kV,电流为90 μA,曝光时间为1.25 s。样品在样品台水平匀速旋转360°,在此过程中共采集1 201幅图像,空间分辨率为16 μm。利用Datos|x2 Rec软件进行图像重建,然后利用VG Studio Max 2.2软件生成2 302张8位灰度图像,存储为tiff格式。利用ImageJ软件进行图像处理和分析,首先进行高斯滤波降低图像的噪声。为减少边际效应和光束硬化引起的伪影,选择图像中心区域作为感兴趣区域(Region of interest,ROI)。ROI的大小为1 600 × 1 600 × 1 500体元,实际大小为25.6 mm × 25.6 mm × 24 mm。利用目视法确定阈值,将图像分割为土壤基质和孔隙两部分。由于分辨率的限制,从图像中获取的孔隙均为大于分辨率(16 μm)的孔隙,本文中称为大孔隙(Macropore)。ROI中大孔隙体积占ROI体积的比例称为大孔隙度(Macro-porosity)[
填装土柱总孔隙度(Total porosity,TP)根据式(3)计算:
式中,
充水孔隙度(Water-filled pore space,WFPS)根据式(4)计算:
式中,
充气孔隙度(Air-filled pore space,AFPS)根据式(5)计算:
利用SPSS 21.0(SPSS Inc.,Chicago,IL,USA)进行数据分析。采用双因素方差分析(Two-way ANOVA)考察施肥处理和样品状态以及它们的交互作用对SOC累积矿化量的影响;利用T检验(T-test)比较松散土样和填装土柱之间的差异性;利用单因素方差分析(One-way ANOVA)检验不同施肥处理以及不同紧实程度之间的差异性,采用最小差异显著法(Least Significant Difference,LSD)进行多重比较,显著性水平为0.05。利用回归分析建立SOC矿化量和总孔隙度、大孔隙度、充水孔隙度和充气孔隙度之间的关系[
由SOC矿化速率变化曲线(
不同施肥处理下松散土样和填装土柱的有机碳矿化速率和累积矿化量
SOC mineralization rate and cumulative SOC mineralization amount of loose soil samples and repacked soil columns under different fertilization treatments
在同一施肥处理的土壤中,除第29天外,其他采样点均表现为松散土样的SOC矿化速率显著高于填装土柱(
SOC矿化速率在培养第1天最高,约为39.3~63.7 mg·kg–1·d–1;之后,矿化速率逐渐降低,并在培养14 d后基本稳定(
不同紧实程度填装土柱的有机碳矿化速率(a)和累积矿化量(b)
SOC mineralization rate(a)and cumulative SOC mineralization amount(b)of the repacked soil columns with different bulk densities
填装土柱的总孔隙度随紧实程度增加而降低,相较于BD1.1而言,BD1.3、BD1.5和BD1.7的总孔隙度分别降低了12.9%、14.8%和17.4%(
不同紧实程度填装土柱的总孔隙度、大孔隙度、充水孔隙度及充气孔隙度
Total porosity, macro-porosity, water-filled pore space and air-filled pore space of the repacked soil columns with different bulk densities
处理 |
总孔隙度 |
大孔隙度 |
充水孔隙度 |
充气孔隙度 |
BD1.1 | 58.5 | 7.0 | 36.4 | 63.6 |
BD1.3 | 50.9 | 5.7 | 49.4 | 50.6 |
BD1.5 | 43.4 | 2.4 | 66.9 | 33.1 |
BD1.7 | 35.8 | 0.8 | 91.8 | 8.2 |
不同紧实程度填装土柱的二维结构
2-D structure of the repacked soil columns with different bulk densities
SOC矿化量与总孔隙度、大孔隙度、充水孔隙度以及充气孔隙度之间存在显著的非线性关系(
土壤有机碳累积矿化量与总孔隙度、大孔隙度、充水孔隙度及充气孔隙度之间的关系
Relationships between cumulative SOC mineralization amount and total porosity, macro-porosity, water-filled pore space and air-filled pore space
对于松散土样而言,其矿化速率在第8天时达到峰值(Control松散土样在第15天),之后持续下降,这是因为在有机质分解过程中,微生物优先利用易分解的活性组分,如糖类、蛋白质等,随着活性组分减少,其分解速率降低[
统计结果显示,在3种施肥处理的土壤中,除培养第29天外,其他时间均表现为松散土样的SOC矿化速率显著高于填装土柱(
培养结束时,无论在松散土样或填装土柱中,LM和HM土壤的有机碳累积矿化量均显著高于Control土壤(
对于填装土柱而言,紧实程度是影响其SOC矿化的一个重要因素。根据培养试验一的结果,对于不同施肥处理的土壤,培养期内松散土样的SOC累积矿化量相对于填装土样的增幅分别为278%、345%和319%,其差异较小。因此在培养实验二中,仅选用HM土壤为试验对象,通过填装土柱,分析紧实程度对SOC矿化的影响。培养28 d后,不同紧实程度处理之间SOC累积矿化量没有显著差异(
紧实程度对土壤通气性的影响源于其对孔隙结构的影响。当容重从1.1 g·cm–3增至1.7g·cm–3时,土壤总孔隙度降低了15%左右,而大孔隙(> 16 μm)度降低了18.7%~88.5%(
土样培养时的结构状态对SOC矿化有极显著的影响,松散土样的SOC矿化量约是填装土柱的4倍,因此土样过筛后分散状态下培养可能会高估田间SOC矿化的潜力。填装土柱的紧实程度影响SOC矿化,SOC矿化量随紧实程度增加呈先增加后降低的趋势,容重为1.5 g·cm–3时SOC矿化量最高。SOC矿化量随紧实程度的增加与土壤孔隙的改变有关,其与孔隙度之间存在显著的非线性关系。当孔隙度较低时(总孔隙度和大孔隙度分别为 < 46%和 < 3.7%时),SOC矿化量随孔隙度增加而增加;反之,SOC矿化量随二者增加而降低。此外,SOC矿化量随WFPS增加先增加后降低,当WFPS约为66%时,SOC矿化量最高。
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