• 中国中文核心期刊
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Volume 35 Issue 6
Nov.  2022
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Effects of Litter and Nitrogen Addition on Carbon and Nitrogen in Soil Leaching Solution of Subtropical Castanopsis fabric and Cunninghamia lanceolata Forest

  • Corresponding author: MA Hong-liang, mhl936@163.com
  • Received Date: 2022-02-09
    Accepted Date: 2022-05-18
  • Objective Litter is the main source of carbon and nitrogen in forest soils. The effect of litter decomposition on soil carbon and nitrogen by leaching was studied through analyzing the changes of dissolved carbon and nitrogen in litter or soil leaching solution, for exploring the relationship between litter decomposition and soil carbon and nitrogen. Method Soil and litter in broad-leaved and coniferous forests were collected from subtropical forests. Six treatments were set, including litter, soil, litter + soil, nitrogen + litter, nitrogen + soil, and litter + soil + nitrogen. Three replicates were set for each treatment. The nitrogen addition amount was 120mg NH4 + -N·kg−1 soil. The amount of litter added in coniferous and broad-leaved forests was 12.1 g·kg−1 and 19.7 g·kg−1, respectively. The litter was placed on the surface of soil or quartzite, and the soil moisture was controlled at 60% water-holding capacity. An incubation experiment was carried out with nitrogen addition by leaching to simulate nitrogen deposition in a dark incubator at 25 ℃ for 220 days. During the incubation period, the nitrogen solution leached in 5 times with different nitrogen amounts, each with 110 mL of solution (80, 10, 10, 10, 10 mg NH4 + -N in sequence according to the number of leaching times), and the leaching solution was collected and measured.. In addition, the dissolved organic carbon (DOC) and nitrogen (DON), and NH4 + -N and NO3-N in the leaching solution were also measured. Result The results showed that the litter leaching solution had lower inorganic nitrogen and DON, and higher DOC. Litter addition significantly reduced the NO3-N by 22.6% and 29.9% in the coniferous forest and broad-leaved forest soil leaching solution, respectively and increased the DOC by 181.4% in the coniferous forest soil leaching solution. However, litter addition significantly decreased the DON by 39.2% in the broad-leaved forest soil leaching solution and MBN by 53.2% in the broad-leaved forest soil. Under nitrogen addition, the interception of added nitrogen by litter was less, and the interception of litter in broad-leaved forest litter was higher than that in coniferous forest. Litter input to soil by leaching decreased DOC, while DON increased. Nitrogen addition increased soil leaching inorganic nitrogen and coniferous forest soil leaching solution DON, but the effect of litter on reducing soil nitrogen leaching was not weakened by nitrogen addition. Conclusion The litter slows down the negative impact of soil NO3-N output on water environment. Nitrogen addition can affect soil nitrogen changes by altering the output of DOC and DON in broad-leaved and coniferous forest litters.
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Effects of Litter and Nitrogen Addition on Carbon and Nitrogen in Soil Leaching Solution of Subtropical Castanopsis fabric and Cunninghamia lanceolata Forest

    Corresponding author: MA Hong-liang, mhl936@163.com
  • 1. State Key Laboratory for Subtropical Mountain Ecology of the Ministry of Science and Technology and Fujian Province, Fujian Normal University, Fuzhou 350007, Fujian, China
  • 2. School of Geographical Sciences, Fujian Normal University, Fuzhou 350007, Fujian, China
  • 3. Wanmulin Nature Reserve Management Station of Fujian Province, Jianou  353100, Fujian, China
  • 4. Jiangsu Mudu Senior High School, Suzhou 215000, Jiangsu, China

Abstract:  Objective Litter is the main source of carbon and nitrogen in forest soils. The effect of litter decomposition on soil carbon and nitrogen by leaching was studied through analyzing the changes of dissolved carbon and nitrogen in litter or soil leaching solution, for exploring the relationship between litter decomposition and soil carbon and nitrogen. Method Soil and litter in broad-leaved and coniferous forests were collected from subtropical forests. Six treatments were set, including litter, soil, litter + soil, nitrogen + litter, nitrogen + soil, and litter + soil + nitrogen. Three replicates were set for each treatment. The nitrogen addition amount was 120mg NH4 + -N·kg−1 soil. The amount of litter added in coniferous and broad-leaved forests was 12.1 g·kg−1 and 19.7 g·kg−1, respectively. The litter was placed on the surface of soil or quartzite, and the soil moisture was controlled at 60% water-holding capacity. An incubation experiment was carried out with nitrogen addition by leaching to simulate nitrogen deposition in a dark incubator at 25 ℃ for 220 days. During the incubation period, the nitrogen solution leached in 5 times with different nitrogen amounts, each with 110 mL of solution (80, 10, 10, 10, 10 mg NH4 + -N in sequence according to the number of leaching times), and the leaching solution was collected and measured.. In addition, the dissolved organic carbon (DOC) and nitrogen (DON), and NH4 + -N and NO3-N in the leaching solution were also measured. Result The results showed that the litter leaching solution had lower inorganic nitrogen and DON, and higher DOC. Litter addition significantly reduced the NO3-N by 22.6% and 29.9% in the coniferous forest and broad-leaved forest soil leaching solution, respectively and increased the DOC by 181.4% in the coniferous forest soil leaching solution. However, litter addition significantly decreased the DON by 39.2% in the broad-leaved forest soil leaching solution and MBN by 53.2% in the broad-leaved forest soil. Under nitrogen addition, the interception of added nitrogen by litter was less, and the interception of litter in broad-leaved forest litter was higher than that in coniferous forest. Litter input to soil by leaching decreased DOC, while DON increased. Nitrogen addition increased soil leaching inorganic nitrogen and coniferous forest soil leaching solution DON, but the effect of litter on reducing soil nitrogen leaching was not weakened by nitrogen addition. Conclusion The litter slows down the negative impact of soil NO3-N output on water environment. Nitrogen addition can affect soil nitrogen changes by altering the output of DOC and DON in broad-leaved and coniferous forest litters.

  • 森林生态系统中植物凋落物是土壤有机质(SOM)的主要来源[1-2],因此,凋落物的分解影响土壤氮素动态变化[3]。凋落物分解释放的溶解性有机碳(DOC),可以快速地增加土壤DOC,提高土壤养分可用性[4];也可通过激发效应,加快土壤有机质分解[1]。森林生态系统土壤溶解性有机氮(DON)一般认为主要来自凋落物和土壤腐殖质[5]。Huang等[6]发现,约80%的DON产生于新近凋落物层。也有研究发现,DON大部分产生自土壤原位(根系或微生物周转)而不是凋落物层[7]。由于DON很容易矿化产生无机氮,因此,DON能反映土壤有机氮矿化的难易程度[8]。在凋落物分解过程中,降雨淋溶可加速凋落物层内有机质降解和碳释放[9]、凋落物与土壤之间物质流动和相互影响[10]。可见,凋落物分解进入土壤的碳氮既可增加其在土壤中的保持[11],也可通过激发作用增加碳和养分的淋溶损失[2,12]。因此,探究这些过程与土壤碳氮矿化关系是深入理解凋落物的生态地位和环境效应必不可少的。

    氮沉降对凋落物分解的影响因凋落物化学性质的差异而变化[13],可促进凋落物分解[14]、或抑制其分解[15],甚至氮沉降可先促进后减缓其分解[16]。Fang等[17]的研究也表明,氮添加可显著提高有丰富凋落物的表层土壤微生物的代谢活动及其对碳基质的利用,促进土壤DOC形成的同时,增强微生物对低分子量氨基酸的吸收。由于不同形态沉降氮对微生物、酶及土壤有机质的影响存在差异,比如有机氮、无机氮,铵态氮(NH4+-N)、硝态氮(NO3-N)的影响各不相同[18-20]。可见,利用同一形态氮添加可直观揭示研究对象的响应差异,探究氮沉降对森林土壤碳氮的影响与凋落物或土壤独自响应差异的关系。

    凋落物分解输入与土壤淋出碳氮存在差异,凋落物分解和土壤氮转化受大气沉降氮的影响,因此,凋落物和土壤淋溶碳氮对沉降氮响应不同。本研究以亚热带天然阔叶林和人工杉木林土壤和凋落物为对象,采用氮淋溶模拟氮沉降的方法,探究凋落物分解与氮沉降对土壤溶解性碳氮的影响,揭示凋落物分解对土壤碳氮影响的重要性。

    • 万木林自然保护区位于福建省建瓯市(27°03′ N,118°09′ E),该研究选取天然阔叶罗浮栲(Castanopsis fabric, CAF)林和人工针叶杉木(Cunninghamia lanceolata, CUL)林[21]为对象,于2018年12月在罗浮栲和杉木林样地的上、中、下坡3个位置,分别按照对角线的方法选择8个点。每个点利用宽5 cm、长25 cm的铁铲采集深度0~20 cm土壤约2 kg,挑除石头、根系以及凋落物,充分混匀后,代表该位置样品;用自封袋将所需新鲜土壤样品密封于4 ℃冰箱保存,通过10目尼龙网收集新近凋落物(主要是叶和枝)。同时利用环刀采取原状土以测定土壤饱和持水量(WHC)。一部分土壤过2 mm筛,用于测定基本理化性质;另一部分土壤用于室内培养试验。土壤和凋落物基本性质见表1

      林分类型
      Forest type
      样品
      Samples
      全碳
      Total carbon/
      (g·kg−1)
      全氮
      Total nitrogen/
      (g·kg−1)
      碳氮比
      C/N
      铵态氮
      NH4 + -N/
      (mg·kg−1)
      硝态氮
      NO3-N/
      (mg·kg−1)
      溶解性有机氮
      DON/
      (mg·kg−1)
      针叶林
      Coniferous forest
      土壤 Soil 27.78 ± 0.13 a 2.12 ± 0.02 a 13.12 ± 0.06 a 15.05 ± 0.46 a 13.55 ± 0.31 a 26.80 ± 0.17 a
      凋落物 Litter 498.32 ± 1.62 A 4.77 ± 0.20 A 106.69 ± 0.62 A 2.16 ± 0.23 A 0.07 ± 0.02 A 20.12 ± 0.17 A
      阔叶林
      Broad-leaved forest
      土壤 Soil 35.44 ± 0.14 b 2.65 ± 0.03 b 13.38 ± 0.16 a 61.25 ± 1.88 b 45.90 ± 2.24 b 53.35 ± 5.77 b
      凋落物Litter 493.16 ± 0.60 B 9.74 ± 0.32 B 50.66 ± 1.69 B 4.28 ± 0.14 B 0.47 ± 0.02 B 16.19 ± 2.22 B
      注:不同小写字母代表不同林型土壤样品之间差异显著(p < 0.05),不同大写字母代表不同林型凋落物样品间差异显著(p<0.05)。 本文针叶林指杉木林,阔叶林指罗浮栲林(平均值±标准差, n = 3)
         Notes: Different lowercase letters represent significant differences between soils and different capital letters represent significant difference between litters (P < 0.05). In this study, coniferous forest is Cunninghamia lanceolata and broad-leaved forest is Castanopsis fabric (mean±SD, n = 3)

      Table 1.  Basic properties of soil and litter

    • 采用两因素(凋落物和氮)随机区组试验设计,共计6个处理,分别为:凋落物(L)、土壤(S)、凋落物 + 土壤(LS)、凋落物 + N(NL)、土壤 + N(NS)、凋落物 + 土壤 + N(NLS),每个处理3重复。

      土壤经去除碎石根系等杂物,过8 mm筛,于350 mL 注射器中(底面积为19.63 cm2,底部设置防土壤颗粒渗漏而允许液体渗漏装置,即采用60目尼龙网和玻璃棉铺垫底部),设置培养土壤于注射器10 cm高度,土壤体积为196.35 cm3,根据土壤密度(针叶林:1.2 g·cm−3;阔叶林:1.0 g·cm−3),计算该体积需土壤干质量为针叶林196.35 × 1.2=235.62 g,阔叶林为196.35 × 1.0=196.35 g。

      为了研究凋落物的作用,特意增加凋落物添加量,根据野外单位面积年凋落物现存量的3倍来确定凋落物添加量,凋落物现存量针叶林为4.82 t·hm−2·a−1,阔叶林为6.57 t·hm−2·a−1[21],计算3倍添加凋落物干质量针叶林为4.82 × 19.63 × 0.01 × 3=2.84 g,阔叶林为6.57 × 19.63 × 0.01 × 3=3.87 g。凋落物(枝叶)剪碎成约1 cm2 左右碎片后,将土壤水分条件调整到 60%饱和持水量(60% WHC),凋落物均匀平铺于土壤或石英石表面。在25 ℃培养箱中(25 ℃为微生物活动提供较适宜的土壤温度),开始为期220 d的避光培养实验,培养期间每3 d通过称质量法保持土壤水分恒定。

      根据样地年平均降雨量(1 673.3 mm)计算淋溶水量、培养时间,设计每次加水量和加水次数,每次淋溶之后,待土壤水分下降至60% WHC以下,再行调整水分。由于凋落物分解试验研究时间长短不同(有69 d培养试验[12]、10个月[3]和1 a[10]的野外试验),且考虑凋落物分解的阶段性和淋溶物的差异以及本研究的目的,本研究选择培养时间在200 d左右。

      按照5次淋溶氮添加总量为每千克土壤添加120 mg N(按照淋溶次数分配依次为80 + 10 + 10 + 10 + 10 mg),每次氮添加通过淋溶氮溶液(NH4Cl)110 mL实现。预培养20 d后,开始第1次淋溶(第0 天),根据降雨氮沉降量前多后少的原则,设计第1次氮添加量为主,基于试验土壤质量,分别为针叶林80 mg·kg−1 × 235.62 g=18.85 mg、阔叶林80 mg·kg−1 × 196.35 g=15.71 mg,以后4次(第60、120、180、220 d)淋溶的氮添加量均为针叶林10 mg·kg−1 × 235.62 g=2.36 mg、阔叶林10 mg·kg−1 × 196.35 g=1.96 mg。每次及时分析淋溶液样品中的铵态氮(NH4+-N)、硝态氮(NO3-N)、总溶解性氮 (TDN)、溶解性有机碳(DOC)。分析第60天和第220天土壤中的微生物生物量碳(MBC)或微生物生物量氮(MBN)。

    • 土壤质量含水量用烘干法测定,土壤饱和持水量用环刀法测定。土壤全碳、全氮用碳氮元素分析仪(Elemantar vario MAX CN,德国)测定。NH4+-N、NO3-N、TDN浓度使用连续流动分析仪(SKALAR SAN++ ,荷兰)测定。DOC使用岛津TOC-VCPH/TN分析仪测定。土壤MBC和MBN采用改进的氯仿熏蒸—0.5 mol·L−1 K2SO4溶液浸提法[22]

      淋溶液中DOC、TDN、NH4+-N、NO3-N均为5次淋溶液累积量/mg=(c0 + c60 + c120 + c180 + c220

      式中:c0、c60、c120、c180、c220分别为第0、60、120、180、220天各指标淋溶量/mg。

      式中:DON为溶解性有机氮累积量/mg,TDN为总溶解性氮累积量/mg,NH4+-N为铵态氮累积量/mg,NO3-N为硝态氮累积量/mg。

      土壤微生物量碳氮:  MBC=ΔEC/KCMBN=ΔEN/KN  式中:ΔEC为熏蒸与未熏蒸土壤DOC含量的差值,ΔEN为熏蒸与未熏蒸土壤TDN含量的差值;KC为0.45;KN为0.54。

    • 采用Excel 2010和Origin 9.0软件对数据进行处理和作图,运用SPSS 20.0中单因素方差分析(One way ANOVA)和S-N-K检验法分析各处理间DOC、DON、NH4+-N、NO3-N的差异显著性(α=0.05),运用双因素方差分析(Two way ANOVA)统计氮添加、凋落物及二者交互作用对土壤碳、氮的影响,运用皮尔逊(Pearson)相关系数分析各指标之间的相关性。数据符合正态分布(Q-Q图检验),所有数据均为平均值±标准差。

    2.   结果与分析
    • 方差分析(表2)显示:凋落物添加对淋溶液NH4+-N的影响不显著,但显著降低淋溶液NO3-N。LS处理的淋溶液NO3-N较S处理分别降低了22.6%(针叶林)和29.9%(阔叶林)(图1b)。氮添加后,NH4+-N淋出显著增加(表2图1a),且NL处理远高于NLS和NS处理;除NL处理,阔叶林淋溶液中NH4+-N显著高于针叶林43.9%~227.0%,且NS处理的淋溶液中的NH4+-N高于NLS处理。针叶林和阔叶林淋溶液中的NO3-N,在氮添加的NL处理较L处理分别升64 873.1%和1 920%;与LS和S处理比较,氮添加(NLS、NS、NL)使淋溶液中的NO3-N升高了30.0%~128.6%;但NLS处理较NS处理分别降低了11.4%和23.0%。未添加氮处理(LS、S、L)的阔叶林淋溶液中NO3-N显著高于针叶林44.7%~975.2%,但在NL处理显著低于针叶林66.6%。

      因素
      Factors
      针叶林 Coniferous forest阔叶林 Broad-leaved forest
      铵态氮 NH4+-N硝态氮 NO3-N铵态氮 NH4+-N硝态氮 NO3-N
      氮添加 N0.0220.0210.001ns
      凋落物 Lns0.015ns0.003
      氮添加 × 凋落物 N × Lnsnsnsns
      注:p < 0.05 表示因素对不同溶解性碳氮的影响显著:ns表示因素对不同溶解性碳氮的影响不显著(p>0.05),n =3
        Notes: p < 0.05 indicate the significant effects and ns indicate the non-significant effects of factors on soluble carbon or nitrogen (p>0.05), n = 3

      Table 2.  Variance analysis of the influence of nitrogen addition and litter on NH4+-N and NO3-N in the leaching of coniferous and broad-leaved forests (p value)

      Figure 1.  Changes of NH4+-N and NO3-N in leaching solution for the coniferous forest and broad-leaved forest

    • 方差分析(表3)显示:凋落物对淋溶液中的DON和DOC有显著影响。针叶林LS处理的DON较S处理的高,而阔叶林LS处理的DON较S处理的降低了39.2%(图2a)。L处理凋落物淋溶液中的DOC最高,且针叶林和阔叶林LS处理的DOC较S处理的分别升高了181.4%和74.6%(图2b)。

      因素
      Factors
      针叶林 Coniferous forest阔叶林 Broad-leaved forest
      溶解性有机氮 DON溶解性有机碳 DOC溶解性有机氮 DON溶解性有机碳 DOC
      氮添加 N0.000nsnsns
      凋落物 L0.0010.0140.0000.033
      氮添加 × 凋落物 N × L0.002nsnsns
      注:p < 0.05 表示因素对不同溶解性碳氮的影响显著;ns 表示因素对不同溶解性碳氮的影响不显著(p>0.05),n = 3
        Notes: p < 0.05 indicate the significant effects and ns indicate the non-significant effects of factors on soluble carbon or nitrogen (p>0.05), n = 3.

      Table 3.  Variance analysis of the influence of nitrogen addition and litter on DON and DOC in the leaching of coniferous and broad-leaved forests (p value)

      Figure 2.  Changes of DON and DOC in leaching solution for the coniferous forest and broad-leaved forest

      方差分析(表3)显示:氮添加、凋落物及二者交互作用对针叶林淋溶液中的DON有显著影响。氮添加后,NL处理针叶林淋溶液中的DON较L处理升高了972.6%;与LS和S处理比较,NLS和NS处理使淋溶液中的DON分别升高了80.9%和415.4%,但NLS处理较NS处理降低了53.7%。阔叶林仅NL处理的DON较L处理升高了147.4%;S和L处理阔叶林淋溶液中的DON分别显著高于针叶林157.0%和108.0%,但在氮添加处理显著低于针叶林34.5%~54.4%(图2a)。在针叶林和阔叶林淋溶液中,NL处理较L处理的DOC分别降低了12.8%和11.1%;在针叶林NLS处理较LS处理降低了32.5%,而在阔叶林NLS处理较NS处理升高64.3%。L、NS和NL处理阔叶林淋溶液中的DOC分别显著高于针叶林70.9%、78.3%和74.2%(图2b)。

    • 在第60天,LS处理针叶林土壤MBN较S处理降低了60.1%(图3a),而阔叶林土壤较S处理升高了53.2%(图3c);在针叶林和阔叶林,与LS和S处理比较,氮添加使土壤的MBN显著降低27.8%~93.0%;阔叶林NLS处理较NS处理显著升高了33.8%。与第60 d相比,220 d后,针叶林NLS、LS和NS处理土壤MBN分别显著增加1 850.0%、70.2%和690.0%,S处理显著降低56.6%;阔叶林LS处理显著降低38.9%,NS和S处理分别显著增加293.2%和226.8%。可见,随着培养针叶林土壤MBN有所降低,当凋落物存在情况下则有所增加,且氮添加使MBN增加更多。

      Figure 3.  Changes of soil MBN and MBC in the coniferous forest and the broad-leaved forest

      在第60天,土壤MBC在针叶林仅NLS处理较LS处理显著升高了74.4%(图3b),而在阔叶林土壤仅NLS处理较LS处理显著降低了45.0%(图3d)。与第60天相比,220 d后,NLS、LS、NS和S处理针叶林和阔叶林土壤MBC分别显著增加305.1%~595.1%和103.7%~194.2%。阔叶林中土壤MBN和MBC均显著高于针叶林土壤。

    • 针叶林淋溶液中NO3-N与DOC呈极显著的负相关,而与DON呈显著正相关(表4);阔叶林中淋溶的DOC与DON和NO3-N呈极显著负相关,NO3-N与DON呈极显著正相关。

      林分类型
      Forest type
      指标
      Indicators
      铵态氮
      NH4+-N
      硝态氮
      NO3-N
      溶解性有机氮
      DON
      针叶林
      Coniferous
      forest
      溶解性有机碳 DOC ns −0.861** ns
      铵态氮 NH4+-N ns ns
      硝态氮 NO3-N 0.574*
      阔叶林
      Broad-leaved
      forest
      溶解性有机碳 DOC ns −0.939** −0.665**
      铵态氮 NH4+-N ns ns
      硝态氮 NO3-N 0.715**
      注:ns,相关性不显著(p>0.05); *和**,相关性显著水平 p<0.05和p<0.01
        Notes: ns indicate the non-significant (p>0.05), * and ** indicate the significant correlation between indexes at p<0.05 and p<0.01 level, respectively

      Table 4.  Correlation analysis of each index in leaching for coniferous forest and broad-leaved forest

    3.   讨论
    • 凋落物(L)淋溶液的无机氮量远低于土壤(S)(图1),说明凋落物中溶解性氮较少,野外氮素的淋溶损失风险主要源自土壤。土壤淋溶液中存在大量的NO3-N和少量的DON,且淋溶液中NO3-N与DON正相关关系(表4),说明不但无机氮是氮淋溶损失的主要形态[23],水溶性的有机氮也存在淋溶风险,甚至有研究显示DON是主要的淋溶损失氮形态[24]。凋落物分解是土壤碳氮的主要来源[1-2],且凋落物与土壤一起培养时其分解速率高于其单独培养[25],但本研究发现凋落物的存在并没有增加土壤淋溶液中溶解性氮,与S处理比较,LS处理则降低土壤淋溶液中NO3-N,表明土壤表面凋落物的存在可以降低土壤氮淋溶损失,有利于氮的保持和环境友好,这对生态系统而言,意义远大于对凋落物或土壤的单独研究。

      凋落物中可淋溶的DOC高于土壤,因此,将提高土壤DOC和影响土壤氮转化,尤其是土壤淋溶液中的NO3-N,其与DOC是负相关关系(表4),可能DOC有利于微生物对氮的保持[26-27],这与Ma等[28]和Cheng等[29]的研究结论类似。Ma等[28]研究表明,溶解性碳输入对土壤氮降低的影响程度与输入碳的量有关,且降低程度在阔叶林大于针叶林。阔叶林凋落物质量(C/N比为50.7)显著高于针叶林(C/N比为106.7)(表1),有机碳氮易于分解、矿化,阔叶林凋落物比针叶林有较多的DOC淋溶(图2),因此,NO3-N降低的量也远高于针叶林土壤(图1)。在针叶林土壤,与S处理比较,LS处理增加第220天土壤MBN;而来自阔叶林凋落物的DOC进入土壤后较针叶林被更多地利用,阔叶林土壤淋溶液DOC降低(图2b),且释放更多的CO2 [30],碳损失加快[25],导致MBN并未增加。凋落物的存在提高针叶林土壤DOC和DON,而降低阔叶林土壤淋溶液DON和MBN,也可能因阔叶林凋落物和土壤有较高的单宁含量[31],阔叶林凋落物添加通过非微生物保持[32],在单宁的作用下[33]降低土壤无机氮。

      土壤是凋落物分解产物的汇,土壤和凋落物长期相互作用,因土壤微生物代谢活性的差异,针叶林土壤(DOC)和(DON)普遍低于阔叶林[34]。由于来自凋落物淋溶液中的溶解性有机物(DOC和DON)远高于无机物(NH4+-N和NO3-N),因此,在野外实际降雨条件下,森林地表可移动DOM的持续供给[35],凋落物层相互影响[9,36],进而影响土壤,使阔叶林土壤保持更多碳氮[37]

    • 氮添加可促进凋落物分解[38],加快物质淋溶损失。研究显示,欧洲赤松叶有很高的NH4+-N淋溶损失[39]。添加NH4Cl后,凋落物淋溶液回收大量的NH4+-N,说明凋落物只能截留少量的NH4+-N,而大部分NH4+-N进入土壤参与土壤氮转化。研究表明,氮沉降因是否凋落物存在而对土壤氮汇产生不同的影响[40]。Xiong等[41]研究发现,外施氮的9.8%~13.6%保留在森林凋落物中,森林凋落物在调控施氮对森林生态系统的影响方面作用显著。被截留的NH4+-N通过硝化作用提高了氮添加处理凋落物淋溶液的NO3-N,说明凋落物保持NO3-N的能力有限。氮添加增加了淋溶液中氮量,这与已有研究[39]一致,但与NS比较,NLS处理显著降低阔叶林土壤淋溶液中的无机氮(图1),且NO3-N降低量更多。究其原因,在土壤pH较低、碳含量较高的情况下,土壤以异养硝化为主[26],在酸性针叶林土壤异养硝化是NO3-N产生的主要机理,而阔叶林不是[42],阔叶林在保留凋落物情况下,土壤有较高的硝化潜势[43]。因此,添加NH4Cl后,一方面阔叶林土壤硝化作用高于针叶林,可能与阔叶林土壤自养硝化增加有关,第二,阔叶林凋落物促进土壤硝化作用,增加了土壤NO3-N的淋溶;同时,土壤淋溶液NO3-N在NLS处理低于NS处理,表明在氮沉降条件下,凋落物仍具有降低土壤NO3-N淋出的作用,从而减缓对水环境的负面影响。可见,有关降雨淋溶在土壤溶解性NO3-N产生机理、凋落物促进硝化与增加NO3--N固定方面的研究,还有待深入。

      在凋落物通过淋溶输入土壤DOC的同时,无机氮添加通过消耗DOC促进土壤或凋落物有机氮的矿化,增加土壤或凋落物淋溶液DON [11]。本研究显示,氮添加增加了针叶林淋溶液DON(图2),可能与针叶林土壤氨化作用受到抑制有一定关系[44],表明铵态氮沉降有促进针叶林DON淋溶损失的可能。可见,类似氮添加和凋落物特性对凋落物分解和土壤有机质影响不同的研究[20,38]、凋落物各分层生态化学计量特征差异和对氮沉降响应的不同[9,36]。在不同森林类型,土壤DON参与淋溶和氮转化的差异及驱动机理,亟待进一步探究。

    4.   结论
    • 凋落物的存在显著降低土壤NO3-N淋溶22.6%(针叶林)和29.9%(阔叶林);即使在氮添加情况下,凋落物也降低土壤NO3-N淋溶11.4%(针叶林)和23.0%(阔叶林),表明凋落物有利于土壤氮的保持。

      凋落物可淋溶的DOC是DON的73倍(针叶林)和60倍(阔叶林),表明DOC是凋落物影响土壤的主要调节物质。

      氮添加可改变针阔叶林凋落物DOC和DON输出的变化,进而影响土壤;降雨导致的溶解性碳氮自凋落物层向下进入土壤的淋溶过程,是建立起凋落物与土壤间碳氮关系必不可少的外部条件。

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