固沙林恢复土壤酶化学计量特征与养分限制效应

[目的]探究土壤胞外酶及酶化学计量比对沙漠化土地植被恢复过程的响应特征及其养分限制效应，可深入揭示人工固沙林重建植被过程养分转化能力与机理。[方法]基于时空替代法，以榆林毛乌素沙区的半固定沙地为(0 a)对照、选择恢复25～56 a灌木与乔木固沙林时间序列样地，测定分析了0—10 cm和10—20 cm土层的胞外酶活性、酶生态化学计量的演变特征及其限制微生物养分需求效应。[结果]随着植被恢复年限增加，两种林地土壤β-葡萄糖苷酶(BG)、β-1,4-乙酰氨基葡萄糖苷酶(NAG)、亮氨酸氨肽酶(LAP)和碱性磷酸酶(AP)活性在不同土层均呈显著升高趋势。但酶活性的不均衡变化也显著改变了土壤酶化学计量特征，56 a植被恢复过程中，土壤酶C∶N,C∶P,N∶P呈增加趋势。土壤胞外酶化学计量比也表明植被恢复期间微生物生长受N限制显著增加，P限制并不明显，仅在乔木林恢复25 a时10—20 cm土层中出现P限制。[结论]固沙林植被恢复显著提升了土壤酶活性，增强了土壤碳氮代谢的能力，且在恢复过程中土壤微生物活动主要受到N养分的限制。     

葡萄糖添加对不同演替阶段森林土壤微生物及酶化学计量特征的影响

糖类作为一种重要的根系分泌物,如何影响土壤微生物及酶化学计量特征目前尚不清楚,制约着人们对上述过程的认识。为探究糖类对森林生态系统中土壤微生态环境和酶活性的影响,以黄土高原子午岭地区森林演替先锋(山杨林)、中期(油松林)和气候顶级群落(辽东栎林)为研究对象,通过野外土壤采样、添加3种不同浓度的葡萄糖(0.1,0.5,1 g/kg干土),和室内培养的方法,研究了葡萄糖添加对土壤微生物量和酶活性及化学计量的影响。结果表明:(1)随着森林演替,土壤中速效磷(SAP)、硝态氮(NO^-₃-N)等速效养分显著降低。在葡萄糖影响下,3种林地土壤全氮(TN)降低,山杨林土壤C/N值显著大于油松林和辽东栎林。(2)随着森林演替,山杨林和辽东栎林微生物碳(MBC)和微生物氮(MBN)的值显著大于油松林,均为先减小后增加。随葡萄糖浓度增加,3种林地MBC和MBN均增加,微生物(MBP)呈波动趋势。(3)山杨林和辽东栎林土壤酶活性均显著大于油松林。随葡萄糖浓度增加,油松林和辽东栎林4种酶活性均为先增加后减小。BG/(LAP+NAG)、BG/AP和(LAP+NAG)/AP辽东栎林的值最大,山杨林和辽东栎林的值显著大于油松林。(4)RDA分析表明,土壤酶、微生物量及其酶活性分别与有机质(SOC),MBC有着显著性正相关关系,土壤酶活性(除NAG)与pH值均呈显著负相关。本研究说明碳输入增加了3种林分土壤的激发效应,土壤微生物量增加,降低了土壤TN含量,改变了土壤养分化学计量,进而加剧了油松林地微生物N限制和辽东栎林地微生物P限制,影响根际土壤有机质分解和养分代谢等过程,从而影响了森林演替过程。     

毛乌素沙地三种植被下苔藓结皮的土壤理化效应

分析了沙蒿、沙柳、樟子松三种植被下有苔藓结皮、无苔藓结皮（对照）样地0—2,2—5,5—10 cm剖面土壤的理化指标,探讨毛乌素沙地苔藓结皮对土壤理化性状的影响及其与植被类型的关系。结果表明:（1）三种植被下苔藓结皮均能够明显提高土壤稳定性,增加细砂粒（0.02～0.2 mm）含量、降低粗砂粒（0.2～2 mm）含量。（2）沙柳及沙蒿植被下苔藓结皮均能够显著提高各层土壤的有机质、全氮、速效钾、全磷含量（P>0.05）,降低全钾、速效磷含量;樟子松下苔藓结皮0—2 cm剖面的理化效应同沙柳、沙蒿类似,但2—5,5—10 cm剖面内,有机质含量显著降低（P>0.05）,全氮、全钾、全磷、速效磷含量则无显著变化（P<0.05）。（3）荒漠生态恢复过程中,苔藓结皮对土壤理化性状的改善作用主要集中在表层土壤,改善程度同植被类型的关系密切。     

毛乌素沙地苔藓结皮对沙化土壤性质和细菌群落的影响

我国沙化土地占国土面积的17.93%,对北方地区经济和社会的发展、人们的生活以及生态平衡影响巨大。土壤生物结皮能有效防风固沙,微生物在其形成中起着重要作用。之前在对生物结皮微生物群落研究方面,缺乏对生物结皮层与同厚度裸沙表层之间以及它们各自下方土壤之间的比较与分析。采用高通量测序技术,对毛乌素沙地苔藓结皮层与裸沙表层,结皮下方土壤与裸沙下层土壤细菌群落结构进行了分析。结合土壤理化因子数据,分析了苔藓结皮对沙地细菌群落多样性的影响。结果表明：相比于裸沙表层苔藓结皮显著提高了土壤速效养分、全氮和有机质的水平,结皮下方土壤中速效养分、全磷和有机质含量均高于裸沙下层。结皮层及其下方土壤中粗粉砂和粉粒的含量显著高于裸沙表层和下层,表明苔藓结皮显著改善了沙地土壤的理化性质。速效磷、速效钾、速效氮、黏粒、粉粒、粗砂是影响沙地土壤细菌群落组成的重要环境因子。多样性分析显示,结皮下方土壤中细菌多样性最高。在苔藓结皮层中unclassifiled_f__Micromonosporaceae（小单孢菌科的未知属）,norank_c__Cyanobacteria（蓝藻纲的未知属）和Pseudonocardia（假诺卡氏菌属）的相对丰度都显著高于裸沙表层;在结皮下方土壤中norank_c__Acidobacteria（酸杆菌纲的未知属）和Rubrobacter（红色杆菌属）的相对丰度均显著高于裸沙下层,这些差异显著的物种对稳定沙地土壤结构具有重要作用。在研究生物结皮的过程中,对照沙地也要分层取样,微生物群落对比分析才更准确。因此,苔藓结皮的形成对沙地原有表层及下层土壤的细菌群落产生显著影响,利于生物固沙。研究结果为风沙治理和荒漠生态恢复提供了重要的微生物学理论依据。     

黄土高原刺槐林土壤酶化学计量沿着环境梯度变化

土壤酶在地下生态系统物质循环、能量流动以及信息交流起着重要作用,是当前生态学研究的热点问题之一,然而沿环境梯度土壤酶的变化机制尚不明确。因而选取由北到南黄土高原4个地区(神木、绥德、安塞、淳化)刺槐林为研究对象,通过测定土壤养分、微生物量和酶活性,分析了酶活性及其化学计量与气候、土壤养分与微生物量之间的关系。结果表明:(1)由北到南,β-1,4-葡萄糖苷酶(BG)、β-N-乙酰氨基葡萄糖苷酶(NAG)、亮氨酸氨肽酶(LAP)和碱性磷酸酶(AP)活性分别在28.72～110.66,1.60～3.32,6.84～10.25,14.17～32.60 [nmol/(g·h)]之间;LAP呈增加趋势;BG,NAG,AP先增加后减小而后又增加。(2)微生物量碳(MBC)、微生物量氮(MBN)和微生物量磷(MBP)分别为23.08～95.04,2.86～7.95,0.53～0.92 mg/kg,均呈增加趋势。(3) BG/(LAP+NAG)、BG/AP、(LAP+NAG)/AP分别为3.15～6.46,1.5～2.96,0.41～0.70;BG/(LAP+NAG)先减小后增大,淳化最高;BG/AP先减小后增大,在神木最高;(LAP+NAG)/AP呈持续减小趋势。(4)通过冗余分析,环境因子对土壤酶、微生物量总解释率达到57.36%,其中年均降雨量(MAP)、N/P影响最大且呈正相关关系,解释率分别为36.60%,14.80%;环境因子对土壤酶、微生物量化学计量比总解释率达到72.18%,其中土壤速效磷(A-P)与TN影响最大且呈正相关关系,解释率分别为30.5%,19.1%。微生物群落处于相对稳态,微生物量化学计量比与土壤养分化学计量比无显著性关系。土壤酶化学计量比沿环境梯度不处于稳态,依赖于土壤速效养分与土壤养分化学计量比。     

东祁连山高寒草甸土壤酶活性及其化学计量特征对海拔和坡向的响应

研究高寒草甸微尺度海拔和坡向下土壤酶活性及其化学计量特征的变化对于探讨草地生态系统养分循环过程具有重要的生态学意义。以东祁连山高寒草甸为研究对象,分析了7个海拔（2 800,3 000,3 200,3 400,3 600,3 800,4 000 m）和2个坡向（阴坡、阳坡）高寒草甸的土壤酶活性、化学计量特征变化规律及其与土壤理化因子之间的关系。结果表明：海拔和坡向的交互效应对土壤亮氨酸氨基肽酶（LAP）、β-葡萄糖苷酶（BG）、多酚氧化酶（PPO）和过氧化物酶（POD）酶活性、lnBG/ln （NAG+LAP）、lnBG/lnAP和ln （NAG+LAP）/lnAP有显著影响;LAP、AP、BG、POD、lnBG/ln （NAG+LAP）、lnBG/ln （NAG+LAP）随海拔的升高呈先升高后降低,β—1,4—乙酰基氨基葡萄糖苷酶（NAG）和PPO随海拔升高而降低;同一海拔,阳坡土壤AP、BG、POD、PPO酶活性低于阴坡,阳坡土壤NAG酶活性高于阴坡,海拔3 800 m以下阳坡LAP酶活性低于阴坡,除海拔3 000 m和4 000 m外的其他海拔阳坡的土壤lnBG/ln （NAG+LAP）高于阴坡,海拔3 000~3 400 m的土壤lnBG/lnAP和海拔3 000~3 600 m的土壤ln （NAG+LAP）/lnAP在阳坡低于阴坡。相关分析发现,土壤酶活性及其化学计量特征不同程度受土壤C、N、P资源及土壤水分条件等的调控,土壤含水量和有机碳是影响土壤酶活性的主要因子。综上,土壤酶活性及其化学计量特征在微尺度海拔和坡向上具有差异性,且受土壤C、N、P资源及土壤水分条件的综合影响,以土壤含水量和有机碳尤为突出。     

黄土丘陵区垂直陡壁表面不同类型生物土壤结皮养分与微生物养分限制特征

生物土壤结皮的发育类型对土壤养分和微生物代谢起着重要作用。为进一步明确在生物结皮发育过程中微生物的限制性养分与影响因素,研究选择黄土丘陵区垂直陡壁表面上的裸土(CK)、浅色藻结皮(LA)、深色藻结皮(DA)、藻藓混合结皮(AM)和藓结皮(M)为研究对象,分析了不同生物土壤结皮类型下碳(C)、氮(N)、磷(P)养分状况与胞外酶活性,并通过胞外酶化学计量来量化微生物的代谢限制。结果表明:LA,DA,AM和M这4种类型生物土壤结皮C,N,P养分含量和微生物生物量C,N,P均显著高于CK(p<0.05),并且SOC,TN,TP和微生物生物量C,N,P随CK,LA,DA,AM和M的顺序逐渐增大,藓结皮微生物量C,N,P分别是CK处理的18.3,27.6,14.1倍。生物土壤结皮的发育显著提高了C,N,P循环酶的活性,冗余分析结果表明土壤养分与酶活性密切相关。此外,通过酶计量的矢量模型结果来看,生物土壤结皮的发育造成微生物相对碳限制的增大与相对磷限制的减小,并受到速效养分含量的影响。偏最小二乘路径模型结果也表明生物土壤结皮的类型会间接影响微生物的代谢限制。总的来说,生物土壤结皮类型的变化会改善土壤养分状况与微生物量等性质,养分资源的供应状况会造成微生物养分代谢的变化。     

亚热带米槠天然林不同深度土壤胞外酶活性对隔离降雨的响应

随着全球变暖的加剧,降雨格局也在发生变化,土壤酶是森林生态系统化学过程的重要参与者,能够快速反映土壤环境的变化。因此研究亚热带土壤胞外酶活性对降水变化的响应,可有效评估气候变化对土壤有机质分解、转化的影响。以福建三明格氏栲自然保护区内的米槠天然林为研究对象,设置对照(CT)、隔离30%降雨(TE1)和隔离60%降雨(TE2) 3种处理,于2020年9月对不同土层(0—10,10—20,20—40,40—60 cm)土壤进行取样,研究9年的隔离降雨对不同土层土壤酶活性的影响。结果表明:不同程度隔离降雨后,4个土层的β—葡萄糖苷酶(βG)、β—N—乙酰氨基葡萄糖苷酶(NAG)和酸性磷酸酶(AP)活性均呈下降趋势。与CT相比,TE2处理0—40 cm土壤AP、βG和NAG酶活性均显著降低(P<0.05),40—60 cm土壤AP酶活性在TE1和TE2处理中降低均较为显著。随着土层加深,3种土壤胞外酶活性均逐渐降低。冗余分析(RDA)表明,在0—10 cm土壤中,土壤含水量(SWC)、总氮(TN)和可溶性有机碳(DOC)是调控土壤胞外酶活性的主要因素,10—20 cm土壤胞外酶活性主要受...     

科尔沁沙地结皮发育对土壤理化性质影响的研究

通过野外取样和室内分析,初步研究了科尔沁沙地植被恢复过程中结皮的理化性质及其结皮发育对下层土壤特性的影响。结果表明,半流动、半固定和固定沙丘分别发育了物理、地衣及苔藓结皮,结皮的厚度、紧实度、水分、粘粉粒含量及各种全效、速效养分随沙漠化逆转趋势依次增加。同时,结皮的存在增加了其下层土壤的容重值,提高了颗粒组成中的极细砂和粘粉粒含量,富集了结皮下土壤的有机质、全N、全P、速效N、速效P等养分,随着结皮的发育,结皮对下层土壤的上述影响作用从半流动沙丘到半固定、固定沙地呈逐渐增长的趋势。沙漠化逆转过程中,结皮对土壤养分的富集在0-5 cm范围内,由表及里呈递减趋势。同一沙丘结皮下0-2.5 cm层土壤的容重、粘粉粒含量、养分含量均高于2.5-5 cm层及对照无结皮层相对应的值。     

黄土丘陵区生物结皮对土壤磷素有效性及碱性磷酸酶活性的影响

黄土丘陵区生物结皮广泛发育,可影响土壤磷素有效性。目前鲜见生物结皮对土壤磷素有效性的研究报道。本文以该区不同年限退耕地的生物结皮为研究对象,通过野外调查和室内分析,研究了生物结皮对土壤全磷、有效磷及碱性磷酸酶活性的影响。结果表明,1）生物结皮的形成可显著提高结皮层土壤全磷含量,而下层(010 cm)全磷含量差异不显著; 2）生物结皮的形成可显著提高结皮层土壤有效磷含量,研究区生物结皮层土壤有效磷含量为3.27~5.87 mg/kg,占到同层土壤全磷含量的0.57%~0.95%,生物结皮层磷酸酶活性高于下层(010 cm) 381倍; 3）生物结皮对土壤磷素有效性及碱性磷酸酶活性的影响与生物结皮发育阶段有关; 4）生物结皮主要通过提高结皮层土壤碱性磷酸酶活性和有机质含量,降低土壤pH,进而提高了土壤磷素有效性。本文研究结果表明,生物结皮的形成有助于提高黄土丘陵区退耕地土壤磷素有效性。     

Stoichiometric responses of soil microflora to nutrient additions for two temperate forest soils

The ratios of soil carbon (C) to nitrogen (N) and C to phosphorus (P) are much higher in Chinese temperate forest soils than in other forest soils, implying that N and P might limit microbial growth and activities. The objective of this study was to assess stoichiometric responses of microbial biomass, enzyme activities, and respiration to N and P additions. We conducted a nutrient (N, P, and N + P) addition experiment in two temperate soils under Korean pine (Pinus koraiensis) plantation and natural broadleaf forest in Northeast China and measured the microbial biomass C, N, P; the activities of β-glucosidase (BG), N-acetyl-β-glucosaminidase (NAG), and acid and alkaline phosphomonoesterase (AP); and the microbial respiration in the two soils. Nitrogen addition increased microbial biomass N and decreased microbial biomass C-to-N ratio and microbial respiration in the two soils. Nitrogen addition decreased NAG activity to microbial biomass N ratio, P addition decreased AP activity to microbial biomass P ratio, and N, P, and N + P additions all increased BG activity to microbial biomass C ratio. These results suggest that microbial stoichiometry is not strictly homeostatic in response to nutrient additions, especially for N addition. The responses of enzyme activities to nutrient additions support the resource allocation theory. The N addition induced a decline in microbial respiration, implying that atmospheric N deposition may reduce microbial respiration, and consequently increase soil C sequestration in the temperate region.     

Ecoenzymatic stoichiometry reveals phosphorus addition alleviates microbial nutrient limitation and promotes soil carbon sequestration in agricultural ecosystems

Purpose

Variation in soil microbial metabolism remains highly uncertain in predicting soil carbon (C) sequestration, and is particularly and poorly understood in agroecosystem with high soil phosphorus (P) variability.

Materials and methods

This study quantified metabolic limitation of microbes and their association with carbon use efficiency (CUE) via extracellular enzymatic stoichiometry and biogeochemical equilibrium models in field experiment employing five inorganic P gradients (0, 75, 150, 225, and 300 kg P ha^?1) in farmland used to grow peas.

Results and discussion

Results showed P fertilization significantly increased soil Olsen-P and NO₃^?-N contents, and enzyme activities (β-1,4-glucosidase and β-D-cellobiosidase) were significantly affected by P fertilization. It indicated that P fertilization significantly decreased microbial P limitation due to the increase of soil available P. Interestingly, P application also significantly decreased microbial nitrogen (N) limitation, a phenomenon primarily attributable to increasing NO₃^?-N content via increasing biological N fixation within the pea field. Furthermore, P fertilization increased microbial CUE because the reduction in microbial N and P limitation leads to higher C allocation to microbial growth. Partial least squares path modeling (PLS-PM) further revealed that the reduction of microbial metabolic limitation is conducive to soil C sequestration.

Conclusions

Our study revealed that P application in agroecosystem can alleviate not only microbial P limitation but also N limitation, which further reduces soil C loss via increasing microbial CUE. This study provides important insight into better understanding the mechanisms whereby fertilization mediates soil C cycling driven by microbial metabolism in agricultural ecosystems.

     

Using metabolic tracer techniques to assess the impact of tillage and straw management on microbial carbon use efficiency in soil

Tillage practices and straw management can affect soil microbial activities with consequences for soil organic carbon (C) dynamics. Microorganisms metabolize soil organic C and in doing so gain energy and building blocks for biosynthesis, and release CO₂ to the atmosphere. Insight into the response of microbial metabolic processes and C use efficiency (CUE; microbial C produced per substrate C utilized) to management practices may therefore help to predict long term changes in soil C stocks. In this study, we assessed the effects of reduced (RT) and conventional tillage (CT) on the microbial central C metabolic network, using soil samples from a 12-year-old field experiment in an Irish winter wheat cropping system. Straw was removed from half of the RT and CT plots after harvest or incorporated into the soil in the other half, resulting in four treatment combinations. We added 1-¹³C and 2,3-¹³C pyruvate and 1-¹³C and U-¹³C glucose as metabolic tracer isotopomers to composite soil samples taken at two depths (0–15 cm and 15–30 cm) from each of the treatments and used the rate of position-specific respired ¹³CO₂ to parameterize a metabolic model. Model outcomes were then used to calculate CUE of the microbial community. Whereas the composite samples differed in CUE, the changes were small, with values ranging between 0.757 and 0.783 across treatments and soil depth. Increases in CUE were associated with a reduced tricarboxylic acid cycle and reductive pentose phosphate pathway activity and increased consumption of metabolic intermediates for biosynthesis. Our results suggest that RT and straw incorporation do not substantially affect CUE.     

Effects of nitrogen and phosphorus fertilization on the ratio of activities of carbon-acquiring to nitrogen-acquiring enzymes in a primary lowland tropical rainforest in Borneo,Malaysia

ABSTRACT

Previous meta-analyses revealed that the ratio of activities of carbon (C)-acquiring enzyme to nitrogen (N)-acquiring enzymes in tropical forest ecosystems was nearly identical to those in other ecosystems, despite of the N-rich condition in tropical forests. This could be explained by microbes in tropical forest soils, which require a large amount of N to produce N-rich acid phosphatase (AP) for catalyzation of the organic form of phosphorus (P) and compensation for poor P availability in soils. Based on this idea, we hypothesized that experimental P fertilization would reduce the allocation to N-acquiring enzymes compared with that of C-acquiring enzymes, i.e. that it would increase the ratios of activities of β-1,4-glucosidase (BG) to β-1,4-acetylglucosaminidase (NAG) and leucine aminopeptidase (LAP). We tested this hypothesis using an experimental fertilization site with factorial N (100 kg ha^?1 yr^?1) and P (50 kg ha^?1 yr^?1) addition in a primary tropical lowland forest in Bornean Malaysia, where our earlier work demonstrated that P fertilization reduced AP activity. Contrary to our hypothesis, the BG:NAG and BG:(NAG + LAP) ratios were not altered by either N or P fertilizations. This result indicated that AP production was not a reason for the maintenance of a relatively high investment in N-acquiring enzyme at our study site. Rather, NAG and LAP production was likely driven by C acquisition, rather than N acquisition, as the target substrates contained C as well as N. This idea was supported by the fact that neither the BG:NAG ratio nor the BG:(NAG + LAP) ratio was elevated by N addition. We propose that the ratios of activities of BG to NAG and LAP do not necessarily indicate the ratio of C:N acquisition, at least in our N-rich tropical forest ecosystem.     

Dynamics of soil microbial biomass C, N and P in disturbed and undisturbed stands of a tropical wet-evergreen forest

To understand the spatial and temporal dynamics of soil microbial biomass and its role in soil organic matter and nutrient flux in disturbed tropical wet-evergreen forests, we determined soil microbial biomass C, N and P at two soil depths (0–15 and 15–30 cm), along a disturbance gradient in Arunachal Pradesh, northeastern India. Disturbance resulted in considerable increase in air temperature and light intensity in the forest and decline in the soil nutrients concentration, which affected the growth of microbial populations and soil microbial biomass. There were significant correlations between bacterial and fungal populations and microbial biomass C, N and P. Soil microbial population was higher in the undisturbed (UD) forest stand than the disturbed forest stands during post-monsoon and less during rainy season due to heavy rainfall. Greater demand for nutrients by plants during rainy season limited the availability of nutrients to soil microbes and therefore, low microbial biomass C, N and P. Microbial biomass was negatively correlated with soil temperature and pH in all the forest stands. However, there were significant positive relationships among microbial biomass C, N and P. Percentage contribution of microbial C to soil organic C was higher in UD forest, whereas percentage contribution of microbial biomass N and P to total N and total P was higher in the moderately disturbed site than in the highly disturbed (HD) site. These results reveal that the nutrient retention by soil microbial biomass was greater in the selective logged stand and would help in the regeneration of the forest upon protection. On the other hand, the cultivated site (HD) that had the lowest labile fractions of soil organic matter may recover at a slower phase. Further, minimum and maximum microbial biomass C, N and P during rainy and winter seasons suggest the synchronization between nutrient demand for plant growth and nutrient retention in microbial biomass that would help in ecosystem recovery following disturbance.     

Responses of soil enzyme activity and microbial community compositions to nitrogen addition in bulk and microaggregate soil in the temperate steppe of Inner Mongolia

In order to explore the responses of soil enzyme activities and microbial community compositions to long-term nitrogen (N) addition in both bulk soil and microaggregate of chestnut soil, we conducted a 7-year urea addition experiment with N treatments at 6 levels (0, 56, 112, 224, 392 and 560 kg N ha^–1 yr^–1) in a temperate steppe of Inner Mongolia in China. Soil properties and the activities of four enzymes involved in carbon (C), nitrogen (N) and phosphorus (P) cycling were measured in both bulk soil and microaggregate, and phospholipid fatty acids (PLFAs) were measured in bulk soil. The results indicated that: 1) in bulk soil, N addition significantly decreased β-1,4-glucosidase (BG) and leucine aminopeptidase (LAP) activities at the treatment amounts of 224, 392 and 560 kg N ha^–1 yr^–1, and obviously suppressed β-1,4-N-acetylglucosaminidase (NAG) activity at the treatment amount of 560 kg N ha^–1 yr^–1. N addition enhanced total PLFAs (totPLFAs) and bacterial PLFAs (bacPLFAs) at the treatment amounts of 392 and 560 kg N ha^–1 yr^–1, respectively, but fungal PLFAs showed no response to N addition. The activities of BG, NAG and LAP were positively correlated with soil pH, but negatively correlated with the concentration of NH ₄⁺-N; 2) in microaggregate (53–250 μm), the activities of BG, NAG and AP showed no response to increased addition of N, but the significantly decreased LAP activity was observed at the treatment amount of 392 kg N ha^–1 yr^–1. These results suggested that enzyme activities were more sensitive to N addition than PLFA biomarkers in soil, and LAP activity in microaggregate may be a good indicator for evaluating N cycle response to long-term N addition.     

Responses of enzymatic activities within soil aggregates to 9-year nitrogen and water addition in a semi-arid grassland

Soil microorganisms secrete enzymes used to metabolize carbon (C), nitrogen (N), and phosphorus (P) from the organic materials typically found in soil. Because of the connection with the active microbial biomass, soil enzyme activities can be used to investigate microbial nutrient cycling including the microbial response to environmental changes, transformation rates and to address the location of the most active biomass. In a 9-year field study on global change scenarios related to increasing N inputs (ambient to 15 g N m⁻² yr⁻¹) and precipitation (ambient to 180 mm yr⁻¹), we tested the activities of soil β-glucosidase (BG), N-acetyl-glucosaminidase (NAG) and acid phosphomonoesterase (PME) for three soil aggregate classes: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm) and microaggregates (<250 μm). Results showed higher BG and PME activities in micro-vs. small macroaggregates whereas the highest NAG activity was found in the large macroaggregates. This distribution of enzyme activity suggests a higher contribution of fast-growing microorganisms in the micro-compared with the macroaggregates size fractions. The responses of BG and PME were different from NAG activity under N addition, as BG and PME decreased as much as 47.1% and 36.3%, respectively, while the NAG increased by as much as 80.8%, which could imply better adaption of fungi than bacteria to lower soil pH conditions developed under increased N. Significant increases in BG and PME activities by as much as 103.4 and 75.4%, respectively, were found under water addition. Lower ratio of BG:NAG and higher NAG:PME underlined enhanced microbial N limitation relative to both C and P, suggesting the repression of microbial activity and the accompanied decline in their ability to compete for N with plants and/or the accelerated proliferation of soil fungi under elevated N inputs. We conclude that changes in microbial activities under increased N input and greater water availability in arid- and semi-arid grassland ecosystems where NPP is co-limited by N and water may result in substantial redistribution of microbial activity in different-sized soil particles. This shift will influence the stability of SOM in the soil aggregates and the nutrient limitation of soil biota.     

造林恢复植被后土壤的化学计量学特征对其碳组分的影响

Afforestation is recognized as an important driving force for soil organic C(SOC) dynamics and soil element cycling.To evaluate the relationships between soil C:N:P stoichiometry and SOC fractions,soil C:N:P stoichiometry distributions at 0–200 cm soil depths were analyzed and the contents of SOC fractions were evaluated in 9 typical land-use systems on the Loess Plateau of China.The contents of light fraction organic C,particulate organic C(53,53–2 000,and2 000 μm),labile organic C,microbial biomass C,and dissolved organic C decreased with increasing soil depth and were higher in afforested soil than in slope cropland soil.Compared with the slope cropland,different vegetation types influenced soil C:N,C:P,and N:P ratios,especially when C:P and N:P ratios were significantly higher(P0.05).Moreover,SOC fractions at the 0–10 and 10–40 cm depths were particularly affected by soil C:P ratio,whereas those at the 40–100 and 100–200 cm soil depths were significantly affected(P0.05) by soil N:P ratio.These results indicate that changes in SOC fractions are largely driven by soil C:P and N:P ratios at different soil depths after afforestation.