黄土高原区不同植物凋落物搭配对土壤微生物量碳、氮的影响

黄土高原丘陵沟壑区进行的以退耕还林还草为主的生态建设,使得进入土壤生态系统有机物的种类和数量发生变化,不同种类凋落物混合对土壤微生物量碳、氮的影响是值得关注的问题。本文采用室内培养试验方法研究了采自黄土高原地区6种不同植物凋落物及等比例混合后对土壤微生物量碳、氮及矿质态氮含量的影响。结果表明,加入不同植物凋落物均显著提高了培养期间土壤微生物量碳、氮含量。总体平均,添加三种等量混合后植物凋落物的土壤微生物量碳、氮含量高于两种凋落物等量混合处理,而两种凋落物混合高于单种凋落物处理;土壤矿质态氮含量的变化则相反,即单种＞两种混合＞三种混合。单种和两种混合后土壤微生物量碳、氮含量与其碳氮显著相关,而三种凋落物混合后土壤微生物量碳、氮含量与其碳氮比无相关性,说明多种凋落物混合后土壤微生物量碳、氮含量受多种因素共同影响。因此,在黄土高原植被恢复重建中,有必要采用不同种类植物搭配,利用生物多样性促进生态系统健康持续发展。     

氮添加和凋落物处理对油松-辽东栎混交林土壤氮的影响

本研究采用随机区组设计,通过改变森林地表凋落物及添加不同水平的外源氮,探讨凋落物与氮添加及其交互作用对油松-辽东栎(Pinus tabuliformis-Quercus wutaishanica)混交林土壤氮的影响。凋落物处理包括:剔除凋落物(Littnil)、枝果凋落物加倍(Littwoody)、叶凋落物加倍(Littleaf)和混合凋落物加倍(Littmix);氮添加量分别为0(N_0)、5(N_5)和10g N·m~(-2)·a~(-1)(N_(10))。在处理5年后的生长季8月,采集地表0~5 cm土壤样品,测定与氮循环相关的土壤性状和微生物性状指标,包括土壤含水量、土壤p H、土壤有机碳、土壤全氮、铵态氮、硝态氮、微生物生物量氮、N-乙酰-β葡萄糖苷酶、蛋白酶和脲酶。结果表明:土壤铵态氮是土壤氮素的主要存在形式,占土壤无机氮的64.53%~87.02%;氮添加和凋落物处理对土壤pH无影响,而土壤含水量、土壤全氮、土壤有机碳、土壤铵态氮、土壤无机氮、微生物生物量氮、N-乙酰-β葡萄糖苷酶均表现为在叶凋落物加倍和混合凋落物加倍处理中高于枝果凋落物加倍和剔除凋落物处理;蛋白酶和脲酶不受凋落物处理的影响;另一方面,土壤碳氮比、土壤硝态氮和脲酶受氮添加的影响显著,土壤碳氮比随着施氮水平的增加而降低,硝态氮在剔除凋落物和枝果凋落物加倍的处理中随着施氮水平的增加先增加后降低,而在叶凋落物加倍和混合凋落物加倍处理中则随着施氮水平的增加而增加;脲酶活性随施氮水平的增加先上升后下降;氮添加和凋落物处理对所测定的指标未表现出交互作用。本研究也表明,有叶凋落物输入的处理显著改善了土壤质量。     

不同土壤水分条件下香樟凋落叶覆盖对土壤碳氮循环的影响

凋落物作为森林生态系统碳库的重要组成部分对森林土壤碳、氮循环具有重要作用.为探讨香樟凋落叶对土壤碳、氮循环的影响,室内模拟研究了10%、20%和30% 3种土壤含水量条件下香樟凋落叶覆盖森林土壤中碳、氮元素的变化.结果表明: 3种含水量条件下香樟凋落叶覆盖均显著增加了土壤CO₂排放速率和土壤溶解性有机碳(WSOC)含量,但显著降低了土壤中硝态氮含量,表明香樟凋落叶覆盖能够增强土壤呼吸强度和碳矿化,抑制土壤硝化作用;香樟凋落叶覆盖能够显著增加10%含水量土壤中铵态氮含量,但降低了20%和30%含水量土壤铵态氮含量,表明香樟凋落叶覆盖对土壤铵态氮含量的影响与土壤含水量有关.香樟凋落叶中部分单萜烯浓度在不同土壤含水量条件下分别与土壤CO₂排放速率和铵态氮含量呈显著正相关,而与土壤WSOC和硝态氮含量呈显著负相关,说明香樟凋落叶覆盖对土壤碳、氮循环的影响可能与凋落叶中的单萜烯有关.     

凋落物和林下植被对杉木林土壤碳氮水解酶活性的影响机制

凋落物和林下植被在森林生态系统土壤碳、氮循环过程中发挥重要作用,目前关于亚热带杉木人工林凋落物和林下植被对土壤碳氮水解酶活性影响机制还不清楚。在亚热带杉木人工林设立去除凋落物+去除林下植被(LR+UR)、去除凋落物+保留林下植被(LR+U)、凋落物加倍+保留林下植被(LD+U)、凋落物加倍+割倒林下植被归还(LD+UC)处理。通过研究各处理对土壤环境因子(土壤温度(ST)、土壤含水量(SWC)、土壤酸度(p H)、硝态氮(NO_3~--N)、氨态氮(NH_4~+-N)、溶解性有机碳(DOC))和土壤碳氮水解酶(β-1,4-葡萄糖苷酶(βG)和β-1,4-N-乙酰葡糖氨糖苷酶(NAG))活性的影响,揭示凋落物和林下植被对杉木林土壤碳氮水解酶活性影响机制。结果表明:(1)通过比较LR+U和LD+U处理发现,短期内添加凋落物有增加土壤p H、NH+4-N含量和提高βG、NAG活性的趋势,但未达到显著水平(P0.05);(2)LR+UR与LR+U处理间的比较发现,去除林下植被有降低SWC、p H、NH+4-N含量、NAG活性的趋势,并显著降低了土壤DOC含量和βG活性(P0.05);(3)将LR+UR分别与LD+U、LD+UC处理进行比较表明,添加凋落物同时保留林下植被显著增加了SWC、p H、DOC和NH+4-N含量,并增强了βG和NAG活性(P0.05),而且林下植被割倒归还比林下植被正常生长更有利于土壤βG、NAG活性的提高和SWC、DOC含量的增加,但是对ST和NO-3-N含量没有显著影响(P0.05);(4)土壤水解酶活性与SWC、DOC表现为极显著正相关关系(P0.01)。总之,凋落物和林下植被影响碳氮水解酶活性的机制,主要是通过调节SWC、增加DOC含量,影响βG、NAG活性;βG、NAG活性增强也会加快土壤有机质(SOM)分解,增加土壤DOC含量。     

秦岭不同海拔森林土壤-植物-凋落物化学计量特征对土壤氮组分的影响

为探究不同海拔森林土壤氮组分对土壤-植物-凋落物化学计量特征的响应规律,选取太白山1300～2600 m海拔范围内4种典型森林——锐齿栎林(Quercus aliena var.acuteserrata)、辽东栎林(Quercus liaotungensis)、红桦林(Betula albo-sinensis)、牛皮桦林(Betula albo-sinensis var. septen-trionalis)为研究对象,测定土壤、叶片、凋落物、根的碳(C)、氮(N)、磷(P)及土壤铵态氮、硝态氮、微生物生物量氮,分析不同森林土壤、植物、凋落物的化学计量比值的变化特征及其对氮组分的影响。结果表明:1) 4种森林土壤C、N、P含量的变化范围分别为36.77～59.80、2.91～4.76、0.13～0.80 g·kg-1。C、N含量在不同森林间变化趋势基本一致,均表现为牛皮桦林红桦林辽东栎林锐齿栎林; P含量的变化趋势表现为辽东栎林牛皮桦林红桦林锐齿栎林; 2)锐齿栎林叶片N∶P14,表明锐齿栎林生长较大程度受N限制;辽东栎林、红桦林、牛皮桦林叶片N∶P16,表明辽东栎林、红桦林、牛皮桦林生长较大程度受P限制; 3)不同森林间微生物量氮差异显著(P0.05),铵态氮含量无显著差异,硝态氮含量表现为锐齿栎林(0.33 mg·kg-1)牛皮桦林(0.28 mg·kg-1)辽东栎林(0.27 mg·kg-1)红桦林(0.17 mg·kg-1); 4)冗余分析结果表明,土壤-植物-凋落物N∶P值是影响土壤微生物量氮的重要因子,土壤C∶N是影响铵态氮、硝态氮含量的重要因子。本研究结果为太白山森林生态系统的保护和氮循环研究奠定基础。     

秦岭不同海拔森林土壤-植物-凋落物化学计量特征对土壤氮组分的影响

为探究不同海拔森林土壤氮组分对土壤-植物-凋落物化学计量特征的响应规律,选取太白山1300~2600 m海拔范围内4种典型森林--锐齿栎林(Quercus aliena var.acuteserrata)、辽东栎林(Quercus liaotungensis)、红桦林(Betula albo-sinensis)、牛皮桦林(Betula albo-sinensis var.septen-trionalis)为研究对象,测定土壤、叶片、凋落物、根的碳(C)、氮(N)、磷(P)及土壤铵态氮、硝态氮、微生物生物量氮,分析不同森林土壤、植物、凋落物的化学计量比值的变化特征及其对氮组分的影响。结果表明:1)4种森林土壤C、N、P含量的变化范围分别为36.77~59.80、2.91~4.76、0.13~0.80 g·kg^-1。C、N含量在不同森林间变化趋势基本一致,均表现为牛皮桦林>红桦林>辽东栎林>锐齿栎林;P含量的变化趋势表现为辽东栎林>牛皮桦林>红桦林>锐齿栎林;2)锐齿栎林叶片N∶P<14,表明锐齿栎林生长较大程度受N限制;辽东栎林、红桦林、牛皮桦林叶片N∶P>16,表明辽东栎林、红桦林、牛皮桦林生长较大程度受P限制;3)不同森林间微生物量氮差异显著(P<0.05),铵态氮含量无显著差异,硝态氮含量表现为锐齿栎林(0.33 mg·kg^-1)>牛皮桦林(0.28 mg·kg^-1)>辽东栎林(0.27 mg·kg^-1)>红桦林(0.17 mg·kg^-1);4)冗余分析结果表明,土壤-植物-凋落物N∶P值是影响土壤微生物量氮的重要因子,土壤C∶N是影响铵态氮、硝态氮含量的重要因子。本研究结果为太白山森林生态系统的保护和氮循环研究奠定基础。     

不同氮效率水稻生育后期根表和根际土壤硝化特征

通过田间试验研究了不同氮效率粳稻品种4007(氮高效)和Elio(氮低效)生育后期在N0(0 kgN hm-2)、N180(180 kgN hm-2)和N300(300 kgN hm-2)水平下根表、根际和土体土壤pH值、铵态氮(NH+4-N)和硝态氮(NO-3-N)含量、硝化强度和氨氧化细菌(AOB)数量.结果表明无论是齐穗期、灌浆期还是成熟期,根表土壤pH值均显著低于根际和土体土壤.土壤pH值范围在5.95至6.84之间变化.土壤NH+4-N含量随水稻生长显著下降,且随施氮量增加而显著增加.根表土壤NH+4-N有明显亏缺区,且随距水稻根表距离增加,NH+4-N含量逐渐升高.土壤NO-3-N含量随水稻生长显著增加,施氮处理均显著高于不施氮处理,但N180和N300处理差异不显著.NO-3-N含量表现为根际>土体>根表.水稻根表和根际土壤硝化强度随水稻生长显著下降,而土体土壤硝化强度随时间延长小幅增加.施氮显著提高4007水稻根表土壤在齐穗和收获期硝化强度以及Elio在齐穗期根际硝化强度,但在施氮处理N180和N300中无显著差异.在整个采样期间,土壤硝化强度均表现为根际>根表>土体.水稻根表和根际AOB数量随水稻生长而显著降低,而土体土壤AOB数量无显著变化.例如,根表土壤AOB数量在齐穗期、灌浆期和收获期分别为16.7×105、8.77×105个g-1 dry soil和8.01×105个g-1 dry soil.根表和根际土壤AOB数量无显著差异,但二者显著高于土体土壤AOB数量.就两个氮效率水稻品种而言,土壤pH值基本无差异.4007土壤NH+4-N含量均显著高于Elio.在齐穗期水稻根表、根际和土体土壤NO-3-N含量在N180水平下均表现为Elio显著高于4007.而在灌浆期和收获期,水稻根表、根际和土体土壤则表现为4007显著高于Elio.在所有采样期,两个水稻品种土体土壤硝化强度和AOB数量在3个施氮量下均无显著差异.Elio根表和根际土壤硝化强度和AOB数量在水稻灌浆期之前一直显著高于4007,而在灌浆期之后则显著低于4007,且最终产量和氮素利用率(NUE)显著低于4007,这可能是由于4007灌浆期后硝化作用强,根际产生的NO-3-N含量高,从而4007根吸收NO-3-N的量也高造成的.因此水稻灌浆期和收获期根表和根际硝化作用以及AOB与水稻高产及氮素高效利用密切相关.     

退化草地恢复过程中土壤氮素状况以及与植被地上绿色生物量形成关系的研究

 研究了在不同放牧率下形成的不同退化阶段的草地各形态氮素（全氮、硝态氮、铵态氮、无机氮和微生物氮）的变化情况,同时也研究了植被地上绿色生物量与各形态氮素季节变化的同步性关系。土壤全氮含量相对稳定,随草地植被状况和植物生长时期变化不大,说明土壤总氮库有相当的弹性。土壤硝态氮（NO－3-N）、铵态氮（NH+4-N）、无机氮（IN）和微生物氮（Micro-N）季节变化明显。土壤Micro-N和NO-3-N含量随植物生长逐渐降低,到植物枯黄期含量又回复到较高的水平;土壤NH+4-N含量随植物生长有逐渐升高的趋势;IN则随着植物的生长出现低-高-低-高的特点,且与植被地上绿色生物量呈显著负相关（R=-0.247, p<0.01）。在放牧条件下草原植物优先利用NO－3-N,NO－3-N与植被地上绿色生物量有显著的负相关性,是形成草原植被地上绿色生物量的有效性氮素。Micro-N能解释土壤IN 22.3%的变异（R2=0.223, p<0.01）,Micro-N是土壤无机氮的重要来源。土壤NH+4-N与Micro-N呈显著负相关（R=-0.222, p<0.01）,说明土壤微生物对土壤NH+4-N有偏好吸收。总体上,不同形态的氮素在各土壤层次间差异显著,随土壤层次的加深含量逐步降低。连续放牧11年恢复两年后,各氮素组分对放牧压力消除的响应并不一致。土壤全氮含量与停止放牧前相比变化差异不显著;而Micro-N对放牧压力消失的响应在不同处理下整个生长季的结果比较一致,即以前过度和中度放牧处理的Micro-N含量较高,无牧和轻牧含量较低;IN、NH+4-N和NO－3-N变化比较复杂,在不同放牧恢复处理上结果并不一致。总的来看,以前中度和过度放牧的IN、NH+4-N和NO－3-N含量较高,存在潜在损失的可能。经过两年的恢复,植被地上绿色生物量（8月）过牧处理与无牧处理差异不显著。     

凋落物与单宁酸对森林土壤无机氮的影响

采用室内培养试验,研究了不同凋落物和单宁酸对森林土壤硝态氮和铵态氮的影响.结果表明：凋落物和单宁酸加入均降低了土壤硝态氮和铵态氮含量.杉木凋落物使红壤硝态氮和铵态氮含量分别降低6.1%～25.9%和19.7%～68.6%.杉木凋落物中黄红壤无机氮含量的降幅大于毛竹,对铵态氮的影响极显著.与对照相比,单宁酸处理能显著降低黄红壤中铵态氮含量,单宁酸浓度越高,其降幅越大,至高浓度(HG)时,其降幅达31.9%～57.8%.随着培养时间的延长,低浓度单宁酸处理(HL)中硝态氮含量降幅逐渐增大,第84天达到4.5%;在HG处理下,第7～28天的硝态氮含量增加了10.3%～18.5%,而第56和85天分别降低 23.9%和42.3%.     

不同施氮处理玉米根茬在土壤中矿化分解特性

以黄土高原南部地区7a定位试验不同氮肥处理玉米根茬为研究对象,通过室内培养试验研究了施氮量分别为0、120和240 kg N/hm2处理玉米根茬(分别用R0、R120、R240表示)在15-20 cm和45-50 cm土层土壤中有机碳矿化及其对土壤微生物量碳、可溶性有机碳和矿质态氮含量的影响.结果表明,不同处理玉米根茬C/N为R0＞R240 ＞R120.培养条件下,R120和R240根茬的碳矿化速率高于R0根茬,R120与R240根茬之间差异不显著.不同处理根茬C/N与其培养过程中碳素累积表观矿化量呈极显著负相关关系.3种施氮量处理的玉米根茬在培养过程中有机碳矿化率、潜在碳矿化量、土壤微生物量碳、可溶性有机碳含量均为添加R120根茬的处理最高,R240次之,R0最低.添加R120和R240根茬显著提高了培养起始时土壤矿质态氮含量.R0、R120和R240根茬在15-20 cm土层土壤中的碳矿化率分别比其在45-50 cm土层土壤中高51.70％、26.41％和27.84％.在评价根茬还田对农田生态系统碳、氮等养分循环的作用时,应同时考虑施肥对根茬分解和转化的影响.     

Root-induced nitrogen mineralisation: A theoretical analysis

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of inorganic nitrogen (N) to plants by stimulating microbial activity. The release of soluble C compounds from roots is assumed to occur by one of two general processes: cortical cell death or exudation from intact cells. On the basis of several assumptions chosen to allow maximal amounts of N mineralisation to be calculated, greater amounts of net N mineralisation are theoretically possible at realistic soil C:N ratios of bacteria are grazed by predators such as protozoa, than if bacteria alone are active. More N is mineralised when the substrate released from the root has a high C:N ratio (as in cell death) than when it is relatively N-rich. The amounts of N that a root might realistically cause to be mineralised are unlikely to account entirely for high nitrate inflow rates that have been measured experimentally. However there are circumstances in which the loss of C from roots is essential if any N is to be mineralised and obtained by plants.     

Root-induced nitrogen mineralisation: A nitrogen balance model

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of nitrogen (N) to plants by stimulating microbial activity. Two models are described, both of which assume that C released from roots is used by bacteria to mineralise and immobilise soil organic N and that immobilised N released when bacteria are grazed by bacterial-feeding nematodes or protozoa is taken up by the plant. The first model simulates the individual transformations of C and N and indicates that root-induced N mineralisation could supply only up to 10% of the plant's requirement, even if unrealistically ideal conditions are assumed. The other model is based on evidence that about 40% of immobilised N is subsequently taken up by the plant. A small net gain of N by the plant is shown (i.e. the plant takes up more N than it loses through exudation), although with exudate of up to C:N 33:1 less than 6% of the plant's requirement is supplied by root-induced N mineralisation. It is argued, however, that rhizosphere bacteria do not use plant-derived C to mineralise soil organic N to any great extent and that in reality root-induced N mineralisation is even less important than these models indicate.     

The effect of cropping and fertiliser nitrogen rates on nitrogen balance in soil

Summary Fertilizer/soil N balance of cropped and fallow soil has been studied in a pot experiment carried out with grey forest soil (southern part of Moscow region) at increasing rates of¹⁵N labelled ammonium sulfate (0; 8; 16; 32 mg N/100 g of soil). The fertilizer¹⁵N balance has been shown to depend upon its application rate and the presence of growing plants. Fertilizer N uptake efficiency was maximum (72.5%) and gaseous losses-minimum (12.5%) at the application rate of 16 mg N/100 g of soil. Fertilizer N losses from the fallow soil were 130–220% versus those from the cropped soil. At the application of fertilizer N the plant uptake of soil N was 170–240% and the amount of soil N as N–NH₄ exchangeable + N–NO₃ in fallow was 350–440% as compared to the control treatment without nitrogen (PK).After cropping without or with N fertilizer application at the rates of 8 and 32 mg N/100 g of soil, a positive nitrogen balance has been found which is likely due to nonsymbiotic (associative) N-fixation. It has been shown that biologically fixed nitrogen contributes to plant nutrition.     

Experimental warming shifts coupling of carbon and nitrogen cycles in an alpine meadow

增温对高寒草甸生态系统碳氮循环耦合关系的影响 陆地生态系统碳吸收受土壤氮素可用性的调节。然而,全球变化背景下的不同生态系统组分的碳氮比及其所反映的碳氮循环耦合关系尚不十分清楚。本文运用数据同化的方法,将一个高寒草甸增温试验的14组数据同化到草地生态系统模型中,从而评估了增温如何影响陆地生态系统的碳氮循环耦合关系。研究结果表明,增温提高了土壤氮素的有效性,降低了土壤活性碳库的碳氮比,导致植物对土壤氮的吸收增加。但是由于植物叶片吸收的碳比吸收的氮增加更多,使得叶片中碳氮比增加,而根部的碳输入增加则低于氮的增加,导致根部的碳氮比减少。同时,增温降低了凋落物碳氮比,可能是在土壤高氮有效性的条件下,凋落物氮的固定得到增强;而且增温加速了凋落物的分解。同时增温还增加了慢速土壤有机质的碳氮比,使得该土壤碳库的碳固存潜力增大。由于大多数模型在不同的环境中通常使用相对固定的碳氮比,本研究所发现的气候变暖条件下碳氮比的差异变化可为模型参数化提供一个有效的参考,有利于模型对未来气候变化背景下生态系统碳氮耦合关系响应的预测。     

Global patterns of the dynamics of soil carbon and nitrogen stocks following afforestation: a meta-analysis

? Afforestation has been proposed as an effective method of carbon (C) sequestration; however, the magnitude and direction of soil carbon accumulation following afforestation and its regulation by soil nitrogen (N) dynamics are still not well understood. ? We synthesized the results from 292 sites and carried out a meta-analysis to evaluate the dynamics of soil C and N stocks following afforestation. ? Changes in soil C and N stocks were significantly correlated and had a similar temporal pattern. Significant C and N stock increases were found 30 and 50 yr after afforestation, respectively. Before these time points, C and N stocks were either depleted or unchanged. Carbon stock increased following afforestation on cropland and pasture, and in tropical, subtropical and boreal zones. The soil N stock increased in the subtropical zone. The soil C stock increased after afforestation with hardwoods such as Eucalyptus, but did not change after afforestation with softwoods such as pine. Soil N stocks increased and decreased, respectively, after afforestation with hardwoods (excluding Eucalyptus) and pine. ? These results indicate that soil C and N stocks both increase with time after afforestation, and that C sequestration through afforestation depends on prior land use, climate and the tree species planted.     

Controls on decomposition and soil nitrogen availability at high latitudes

At high latitudes, decomposition rates and soil nitrogen (N) availability are pivotal in determining ecosystem responses to climate change. The effects of temperature, soil moisture content, resource quality, and saprotrophic fungi as an example of soil organisms, on carbon (C) and N mineralisation are reviewed. The controls on N availability are less well characterised than those on decomposition, and C and net N mineralisation sometimes do not respond to these controls in a parallel manner. Increases in mean summer temperatures of 2–4°C predicted for high latitudes may not necessarily cause greater rates of decomposition and N mineralisation because of concomitant small rises in soil temperature together with interactions between the controls, including interactions of the temperature and moisture content of the substrata with the diversity and function of decomposer fungi. Research on decomposition and soil N availability has been carried out at several scales, at all of which future research remains necessary. It is not clear whether species diversity of decomposer fungi influences decomposition and C and N release above the microscale.     

Nonlinear root-derived carbon sequestration across a gradient of nitrogen and phosphorous deposition in experimental mesocosms

Enhanced sequestration of plant‐carbon (C) inputs to soil may mitigate rising atmospheric carbon dioxide (CO₂) concentrations and related climate change but how this sequestration will respond to anthropogenic nitrogen (N) and phosphorous (P) deposition is uncertain. We couple isotope, soil C fractionation and mesocosm techniques to assess the sequestration of plant‐C inputs, and their partitioning into C pools with different sink potentials, under an experimental gradient of N and P deposition (0, 10, 30, 60 and 100 kg N ha^?1 yr^?1; and 0, 2, 6, 12 and 20 kg P ha^?1 yr^?1). We hypothesized that N deposition would increase sequestration, with the majority of the C being sequestered in faster cycling soil pools because N deposition has been shown to accelerate the turnover of these pools while decelerating the turnover of slower cycling pools. In contrast to this hypothesis, sequestration into all soil C pools peaked at intermediate levels of N deposition. Given that P amendment has been shown to cause a net loss of soil C, we postulated that P deposition would decrease sequestration. This expectation was not supported by our data, with sequestration generally being greater under P deposition. When soils were amended simultaneously with N and P, neither the shape of the sequestration relationship across the deposition gradient, nor the observed sequestration at the majority of the deposition rates, was statistically predictable from the effects of N and P in isolation. The profound nonlinearities we observed, both for total sequestration responses and the partitioning of C into soil pools with different sink potentials, suggests that the rates of N and P deposition to ecosystems will be the critical determinant of whether they enhance or decrease the long‐term sequestration of fresh plant‐C inputs to soils.     

Pulse additions of soil carbon and nitrogen affect soil nitrogen dynamics in an arid Colorado Plateau shrubland

Biogeochemical cycles in arid and semi-arid ecosystems depend upon the ability of soil microbes to use pulses of resources. Brief periods of high activity generally occur after precipitation events that provide access to energy and nutrients (carbon and nitrogen) for soil organisms. To better understand pulse-driven dynamics of microbial soil nitrogen (N) cycling in an arid Colorado Plateau ecosystem, we simulated a pulsed addition of labile carbon (C) and N in the field under the canopies of the major plant species in plant interspaces. Soil microbial activity and N cycling responded positively to added C while NH₄⁺–N additions resulted in an accumulation of soil NO₃⁻. Increases in microbial activity were reflected in higher rates of respiration and N immobilization with C addition. When both C and N were added to soils, N losses via NH₃ volatilization decreased. There was no effect of soil C or N availability on microbial biomass N suggesting that the level of microbial activity (respiration) may be more important than population size (biomass) in controlling short-term dynamics of inorganic and labile organic N. The effects of C and N pulses on soil microbial function and pools of NH₄⁺–N and labile organic N were observed to last only for the duration of the moisture pulse created by treatment addition, while the effect on the NO₃⁻–N pool persisted after soils dried to pre-pulse moisture levels. We observed that increases in available C lead to greater ecosystem immobilization and retention of N in soil microbial biomass and also lowered rates of gaseous N loss. With the exception of trace gas N losses, the lack of interaction between available C and N on controlling N dynamics, and the subsequent reduction in plant available N with C addition has implications for the competitive relationships between plants species, plants and microbes, or both.     

氮添加对中国陆地生态系统植物-土壤碳动态的影响

近年来,中国大气氮沉降水平不断增加,过量的活性氮输入深刻影响了我国陆地生态系统碳循环。虽然已有大量的研究报道了模拟氮添加实验对我国陆地生态系统碳动态的影响,但是由于复杂的地理条件和不同的施氮措施,关于植物和土壤碳库对氮添加的一般响应特征和机制仍存在广泛争议。因此,采用整合分析方法,收集整理了172篇已发表的中国野外氮添加试验结果,在全国尺度上探究氮添加对我国陆地生态系统植物和土壤碳动态的影响及其潜在机制。结果表明,氮添加显著促进了植物的碳储存,地上和地下生物量均显著增加,且地上生物量比地下生物量增加得多。同时,氮添加显著增加了凋落物质量,但对细根生物量没有显著影响。氮添加显著降低了植物叶片、凋落物和细根的碳氮比。总体上,氮添加显著增加了土壤有机碳含量并降低了土壤pH值,但对可溶性有机碳、微生物生物量碳和土壤呼吸的影响并不显著。在不同的地理条件下,土壤有机碳含量对氮添加的响应呈现增加、减少或不变的不同趋势。回归分析表明,地上生物量与土壤有机碳含量之间,以及微生物生物量碳与土壤有机碳含量之间呈负相关关系。虽然氮添加通过增加凋落物质量显著促进了植物碳输入,但同时也会通过刺激微生物降解来增加土...     

长白山苔原带土壤碳、氮矿化对氮沉降的响应

高山苔原生态系统的土壤无机氮含量较低,对氮的缓冲性弱而易受外源氮输入的影响.本研究以长白山北坡苔原带土壤为研究对象,通过室内培养试验,以NH₄NO₃为外加氮源,设置3个施氮水平:对照(CK,0 kg·hm^-2),低氮(N₁,25 kg·hm^-2),高氮(N₂,50 kg·hm^-2),分析长白山苔原带土壤碳、氮矿化对氮沉降的响应.结果表明: 氮添加处理对长白山苔原带土壤碳矿化速率影响不显著,但对土壤碳矿化累积矿化量影响显著,N₂抑制了土壤的碳矿化作用.培养40 d后,氮添加处理提高了土壤无机氮含量;而培养80 d后,N₂与N₁的无机氮含量差异不显著,但都明显高于CK,氮输入促进了土壤氮的矿化.培养过程中,N₁处理下的微生物生物量碳、氮高于N₂和CK处理,说明低氮输入对土壤微生物活性的促进作用更明显.在未来氮沉降增加的背景下,长白山苔原土壤碳、氮周转可能加快,提高土壤无机氮含量.土壤中无机氮含量增加,虽然可以为植物生长提供更多生长所需的氮素,但也提高了土壤氮素的流失风险.