东北黑土有机碳储量及其对大气CO2的贡献

开垦荒地和翻耕农田导致土壤结构破坏,加速土壤有机碳(SOC)损失.通常计算土体中SOC的损失时忽略了侵蚀和沉积作用产生的SOC在景观中的再分布,因而过高地估计了农业土壤对大气CO2的贡献.近年来,土壤科学研究表明,通过采用新的管理措施后,能使农田土壤由大气CO2碳源转变为碳汇.以东北黑土为例,计算其SOC库储量及耕种以来释放到大气中CO2的数量;评价侵蚀和沉积作用对SOC损失的影响;估算东北黑土采用新的管理方式后,该土类可固定大气CO2的潜力.根据第二次土壤普查资料和回归拟合方法,得出东北黑土1 m深度的SOC平均密度为12.54 kg C/m2,有机碳储量为646.2 TgC.应用修正的土壤流失方程(RUSLE)和有关该区土壤侵蚀资料,计算黑龙江和吉林两省每年土壤迁移的碳量为0.34～2.84 TgC/a,因沉积作用引起的SOC在景观中再分布的数量为0.27～2.27 TgC/a.由此计算自耕种以来,东北黑土净释放到大气中的CO2数量为34.6～434.6 TgC.如果采用新的管理措施后,东北黑土最大固碳潜力为244.3 TgC,在未来20年内土壤固碳潜力为30.9 TgC,平均每年1.55 TgC/a.     

利用燃煤飞灰作为时间标记物评价坡耕地黑土侵蚀物质和有机碳的再分配

燃煤飞灰(以下简称飞灰)作为时间标记物克服了放射性同位素137Cs示踪方法不能鉴定大气核爆炸之前的土壤再分布过程这一缺陷。本文利用土体中的飞灰研究坡耕地黑土有机碳的时空再分布特征。尝试建立飞灰在土壤中分层的方法,根据飞灰和土壤有机碳(SOC)随土壤深度的分布特征鉴定土壤堆积厚度,以及堆积土壤的相对年代。结果表明:用飞灰示踪技术鉴定的埋藏土壤表层与SOC含量随深度变化确定的埋藏表层吻合较好,景观中低洼部位在飞灰出现前就有一定的土壤堆积。各地貌部位坡肩侵蚀最为严重,有机碳含量最低;坡顶坡度较小,侵蚀微弱;坡脚和坡足发生沉积。土壤沉积速率在1.01~5.56mm a-1之间。研究结果还表明堆积部位埋藏层的SOC含量较高,说明有相当数量的有机碳被隐遁在目前的耕作层之下。因此,在评价农田土壤作为大气CO2“源”或“汇”时应该考虑景观中土壤物质迁移和埋藏作用的影响。     

耕作及水蚀影响下坡耕地土壤有机碳动态模拟

土壤侵蚀和沉积明显影响土壤有机碳(SOC)的积累与损耗,在以往土壤碳平衡模拟中却未得到应有的重视。本文以典型黑土漫岗坡耕地表层土壤为研究对象,利用CENTURY模型模拟特定质地下自然黑土有机碳的积累过程,估算研究区黑土有机碳及各组分的背景值;对比研究侵蚀泥沙对SOC富集的影响,将模型模拟值与实测值进行统计比较来验证模型;进而模拟侵蚀区开垦后SOC以及各组分随时间的变化,定量研究土壤侵蚀对SOC各组分损失的贡献。研究结果表明:黑土有机碳的累积大致可分为初期的快速积累和后期缓慢积累两个阶段,前期慢性有机碳库的累积对SOC库的增加贡献最大,后期SOC累积主要由惰性有机碳缓慢累积来完成。达到平衡状态时,研究区黑土有机碳含量为7 240 g m-2,以慢性和惰性有机碳为主,约占总SOC的97.4%。考虑泥沙对SOC的富集作用,模型模拟值与实测值更加吻合。自然黑土开垦后,微生物分解矿化作用是活性和慢性有机碳损失的主要途径,土壤侵蚀明显降低惰性有机碳含量,其贡献率随侵蚀速率的增加而增大。因研究区侵蚀不严重,土壤侵蚀对开垦以来的SOC库损耗贡献较小。     

坡耕地黑土活性有机碳空间分布及生物有效性

以一东北黑土区典型漫岗坡耕地为研究对象，测定不同侵蚀程度地形部位的活性碳组分，以分析土壤侵蚀对活性碳组分的影响以及沉积区侵蚀碳的归宿。研究结果表明：各地形部位表层黑土（0～20cm）水溶性有机碳（WSOC）含量介于14．6～20．0mg／kg之间，低于总SOC含量的0．15％WSOC在沿坡迁移的同时，向下淋溶也很显著。MBC含量为121．5～583．6mg／kg，占总SOC的1．0%～4．7％。Min—C变化范围为52．8～115．3mg／kg，土壤Min-C差异主要集中在表层。土壤侵蚀显著降低侵蚀部位表层土壤Min-C、WSOC和MBC含量，沉积区土壤MBC、Min—C含量及其商值较高，而WSOC却无显著累积。相关关系表明，表层土壤Min—C、WSOC和MBC均于总SOC含量呈显著正相关。初步的研究结果认为侵蚀物质的输入增加沉积区表层土壤微生物活性和土壤碳的矿化潜力，常年处于氧化环境中的侵蚀碳可能被矿化而难以累积。     

秸秆添加对石灰性土壤有机与无机碳释放的影响

在富含碳酸盐的石灰性土壤上,土壤本身CO2释放不仅来自土壤有机碳（SOC）的分解,也源于无机碳（SIC）的溶解。在秸秆还田下,石灰性土壤CO2释放来源达到三个（秸秆碳、SOC和SIC）,由于区分技术的限制,当前区分CO2释放三源的研究,尚少见报道。以华北石灰性农田土壤为研究对象,采用13C标记玉米秸秆添加土壤进行室内培养32周,设置4个处理,分别为无添加对照（CK）、低量秸秆添加（S1,相当于田间秸秆还田量9.6 t?hm-2）、中量秸秆添加（S2,秸秆还田量28.8 t?hm-2）和高量秸秆添加（S3,秸秆还田量48.0 t?hm-2）,利用秸秆碳、SOC与SIC之间的δ13C差异,借助稳定同位素溯源模型IsoSource,区分土壤CO2的释放来源,明确秸秆添加对石灰性土壤有机与无机碳释放的影响。结果表明,随着培养时间的进行,土壤释放CO2中源于秸秆的贡献呈下降趋势;秸秆分解对土壤CO2释放的贡献随着秸秆添加量增加而增加,对于S1、S2和S3处理,土壤释放CO2中源于秸秆、SOC和SIC的贡献比值约分别为3:3:4、5:2:3和6:2:2;与CK相比,S1处理降低SOC分解的激发效应（程度为9%）,S2和S3处理反而增加了SOC分解的激发效应（程度分别为22%和57%）;秸秆和SOC矿化增加SIC溶解的释放,随秸秆添加量增加而增加,S1、S2和S3处理提高SIC源CO2的释放程度分别为368%、561%和652%。因此,秸秆添加不仅影响SOC源CO2的释放,也增加了SIC源CO2的释放,若忽略SIC溶解对土壤CO2释放的贡献,可能导致SOC矿化量的高估,进而影响SOC激发效应评估的准确度。     

东北黑土有机碳的分布及其损失量研究

为了分析东北黑土土壤有机碳(SOC)的分布特征及其开垦以来黑土SOC的损失程度,我们于2004～2005年在黑龙江和吉林两省采集了32个自然黑土剖面样品,在每个自然黑土样品附近对应采集32个景观条件相似的耕作黑土样品。结果表明,自然黑土样品0～30cm土层SOC含量平均为32.20 g kg-1,最高可达63.46 g kg-1,黑龙江省自然黑土SOC含量(34.55 g kg-1)高于吉林省(23.80 g kg-1)。耕作土壤SOC平均含量为22.71 g kg-1,远低于自然土壤。受温度的影响,随着纬度的增加,自然黑土与耕作黑土SOC含量逐渐递增。由于土壤侵蚀以及耕垦和去除作物残留物等农业管理措施的综合作用,使得耕作黑土表层SOC含量小于自然黑土。与自然黑土相比,耕作黑土0～10cm土层SOC损失量在26.84%～46.57%之间,亚表层损失相对较少。黑土SOC含量下降也是土壤水土流失致使黑土层变薄的一个直接表现。耕作黑土表层流失厚度可以通过自然与耕作黑土剖面SOC含量的分异差值来估算。通过对土壤剖面上SOC的分布进行校正剔除土壤侵蚀的影响后得到的同等深度SOC含量的差值才可视为由耕作以及有机质输入量差异等因素造成的SOC损失量。未经校正而进行的自然黑土和耕作黑土同一深度SOC含量的比较可能过高估计了农业管理措施对土壤SOC损失量的影响。     

东北黑土区旱作农田土壤CO2排放规律

为研究农田土壤CO2排放规律,调控农田碳平衡,通过对东北黑土区旱作农田土壤CO2排放的定位连续观测,研究了玉米、大豆农田土壤CO2排放的季节变化规律;并估算了农田碳平衡。结果表明:1)农田土壤CO2排放通量随季节呈单峰曲线变化,7月份出现最大值;秸秆覆盖还田明显增加了农田土壤CO2排放;玉米或大豆生长发育对土壤CO2排放影响较小。2)地温的季节变化与土壤CO2排放通量季节变化规律一致,用指数方程和二次方程均可很好地模拟土壤CO2排放通量与地温之间的关系,但指数方程优于二次方程,以20cm土层地温的相关性最高,5cm土层地温的相关性最低。3)玉米、大豆农田在通常情况下为大气CO2的"汇",玉米-玉米-大豆轮作周期(3a)的碳汇年平均为4.53t/hm2,该碳汇可为固碳减排提供参考。     

坡耕地黑土有机碳和全氮的迁移与累积平衡

本文利用137CS示踪技术计算东北黑土坡耕地土壤再分布速率,结合表层土壤有机碳(SOC)和全氮(TN)含量动态,探索典型漫岗坡地SOC和TN流失量的空间分布特征,据此计算研究区近50年来SOC和TN迁移、累积平衡。研究结果表明:研究区土壤再分布速率介于-24.61~33.56 T/HM2/A,绝大部分地区处于中度和轻度侵蚀状态,约占研究区总面积的83.66%,沉积面积占总面积的15.62%。SOC和TN的流失量与土壤再分布速率相一致,坡肩部位SOC和TN流失量最大,侵蚀损失率分别为407.57 KG/HM2/A和39.94 KG/HM2/A;其次为坡背和坡顶,平均流失量分别为244.2 KG/HM2/A和17.93 KG/HM2/A;坡脚和坡趾表现为累积,平均累积量分别为-207.2 KG/HM2/A和-20.56 KG/HM2/A。整个研究区SOC和TN的相对流失量>50%的面积分别占10.45%和11.21%。整个研究区48年来土壤净迁移泥沙量为45.54 T/A,其中,SOC流失量为612.62 KG/A,TN流失量为47.20 KG/A。考虑迁移泥沙对土壤有机质的富集作用,迁移损失的SOC和TN量比原计算值高52%。     

黑土坡耕地侵蚀和沉积对物理性组分有机碳积累与损耗的影响

以侵蚀和沉积过程明显的黑土坡耕地为研究对象,通过测定不同地形部位表层和典型剖面土壤不同粒级的水稳性团聚体、颗粒态有机碳（POC）以及团聚体结合态有机碳含量,探讨土壤侵蚀和沉积对土壤有机碳（SOC）损失、迁移和累积过程的影响。研究结果表明：上坡三个侵蚀部位表层土壤大团聚体、矿质结合态有机碳（MOC）以及团聚体结合态有机碳含量随侵蚀速率增加而减小;沉积部位（尤其是坡脚）POC含量和POC/SOC较低,而MOC含量和MOC/SOC较高。始终处于沉积状态的坡脚部位,各粒级有机碳组分的深度分布均表现出土壤累积和埋藏特征,并随着粒级的减小累积现象趋于明显。上述结果反映了土壤侵蚀优先使与细颗粒和微团聚体结合的SOC迁移流失,并在低洼的沉积区累积;埋藏层中的侵蚀物质（如微团聚体、颗粒态有机质）通过深埋作用和重新团聚作用形成稳定的大团聚体,最终促进SOC的固定。     

北方旱区免耕对农田生态系统固碳与碳平衡的影响

农田系统对大气CO2库呈碳汇还是碳源效应取决于土壤有机碳的固定和温室气体释放之间的平衡,而耕作措施会改变土壤有机碳含量和储量,影响农田系统的碳循环与碳平衡。该研究以北方旱区山西临汾20 a长期保护性耕作定位试验为基础,田间原位测定土壤呼吸和土壤有机碳含量,确定各类农业投入碳排放参数,利用碳足迹方法综合分析不同耕作措施(传统耕作CT和免耕NT)下农田生态系统碳平衡。结果表明:在化肥、机械等农业投入产生的间接碳排放量方面,化肥投入碳排放量约占系统农业总投入碳排放量的73.5%~77.4%,是农业投入中主要的碳源。由于免耕减少了翻耕、旋耕和秸秆移除3道程序,NT比CT少排放约5.1%,NT产量显著提高28.9%,且碳生产力大于CT。0~60 cm土壤有机碳储量NT(50.86 Mg/hm2)比CT(46.00 Mg/hm2)高10.5%。与CT相比,在小麦休闲期和生育期NT土壤呼吸CO2释放总量高于CT。但根据农田系统碳平衡公式分析得出,NT更有利于农田生态系统固碳,呈碳汇效应,而CT表现为碳源。因此,长期免耕耕作能够提高农田土壤固碳量,减少大气温室气体排放,对于改善北方旱区土壤碳库和减排效果是一个良好的选择。     

基于碳足迹和碳承载力的新疆碳安全评价

[目的]探索新疆碳足迹和碳承载力的的变化,评价其碳安全程度,为新疆低碳经济的发展提供理论依据。[方法]利用碳足迹理论对新疆2000—2014年的碳足迹、植被碳承载力、净碳足迹进行计算分析,并利用碳压力指数(CTI)构建了碳安全评价模型。[结果]新疆碳足迹呈上升趋势,从2000年的1.08×108 t上升到2014年的5.04×108 t,其中化石能源碳足迹占总碳足迹的比重达到96%;碳承载力不断增加,草地固碳量所占的比重最大,其次是森林、农田、园地和城市绿地;人均净碳足迹、碳压力指数均呈现增长趋势,导致新疆碳安全程度不断下降,从2009年开始就处于极不安全的状态。[结论]化石能源消费的增加是导致新疆碳足迹升高和碳安全程度下降的主要原因,虽然其能源利用率不断提高,但在未来一段时间仍然面临严峻的生态环境问题。     

不同有机物料还田对华北农田土壤固碳的影响及原因分析

中国农业面临着废弃物数量大、污染严重,农田土壤生产力低的现实问题。该研究以增加农田土壤固碳为目标对砂质农田进行有机物料还田,将秸秆、猪粪、沼渣和生物炭4种物料用尿素调节等氮还田,对农田土壤有机碳、颗粒有机碳、可溶性有机碳和微生物量碳的含量进行测定,并探究不同有机物料还田对土壤有机碳的影响原因。研究结果表明:物料还田3a后,生物炭、猪粪和沼渣处理土壤有机碳(SOC)比秸秆处理分别高262.4%、26.8%和20.7%;2014—2015年生物炭处理的土壤微生物量碳(MBC)较秸秆处理降低2.9%~35.5%,猪粪处理和沼渣处理的土壤可溶性有机碳(DOC)分别提高17.1%~60.1%和7.2%~64.8%;2014—2015年生物炭、猪粪和沼渣处理土壤颗粒有机碳(POC)较秸秆处理提高10.8%~148.2%、9.5%~58.3%和11.3%~57.6%;物料还田后,土壤总有机碳(TOC)和POC呈极显著的回归关系(R2=0.67,P0.001),土壤DOC与MBC有极显著相关性(R2=0.52,P0.001)。与秸秆还田相比,生物炭还田有利于土壤POC的累积进而促进土壤有机碳的提升,猪粪和沼渣则通过提高土壤MBC、DOC和POC的含量,促进土壤有机碳的周转和固定。从农田土壤固碳角度而言,生物炭,猪粪和沼渣还田优于秸秆还田。     

栓皮栎林与油松林土壤有机碳及其组分的研究

土壤有机碳在可持续森林生产力的维持中起重要作用。本文以北京鹫峰地区栓皮栎林和油松林为研究对象,对土壤有机碳及其组分的含量与密度进行了研究。结果表明,在0~20 cm土层中,栓皮栎林土壤总有机碳、轻组有机碳和易氧化碳的含量分别达到14.05 g kg-1、2.97 g kg-1和0.38 g kg-1,显著高于油松林。栓皮栎林0~20 cm土层内土壤颗粒有机碳、轻组有机碳和易氧化碳密度分别较油松林高39.68%、77.77%、145.45%,两植被类型间的的差异均达到了显著水平。在土壤总有机碳中,颗粒有机碳、轻组有机碳和易氧化碳的分配比例分别为23.60%~41.40%、9.10%~33.33%和1.39%~2.80%。因此,栓皮栎林较油松林更有利于土壤总有机碳和活性有机碳的积累。     

陆地土壤碳循环研究进展

[目的]对近年来国内外碳循环方面的研究进展进行综述,为退化土地的生态恢复及环境治理和保护提供依据。[方法]总结了当前土壤有机碳循环研究中的几种主要方法(室内培养法、同位素示踪法、碳循环模型和计算机模拟法等),分析对比了这几种方法的特点和存在的问题,并对土壤有机碳循环机理和影响土壤有机碳循环的主要因素进行分析。[结果]目前,在土壤有机碳库的估算方法、数据依据、结果以及土壤有机碳循环模型上存在较大差异,给土壤有机碳变化和循环研究带来一定的困难。土地利用方式和土地覆盖变化是影响陆地土壤有机碳变化及循环最直接的人为因子。[结论]应注重土地利用及覆被变化在土壤碳循环中的作用及地位。并建立适用于中国国情的碳循环模型。未来的土壤碳循环研究应探索标准化、高精度的有机碳库储量估算方法。     

A framework for integrating the terrestrial carbon stock of estates in institutional carbon management plans

Many institutions have substantial landholdings, but few consider soil carbon preservation and augmentation in their carbon management plans. A methodical framework was developed to analyse terrestrial carbon stocks (soil and tree biomass) for credible carbon offsetting strategies in institutional land. This approach was demonstrated at two farms (805 hectares) managed by Newcastle University. Soil carbon for three depths (0–30 cm, 30–60 cm and 60–90 cm) and above-ground tree biomass were quantified. These data provided a terrestrial carbon baseline to evaluate future land management options and effects. Historical land-use records enabled the following comparisons: (1) agricultural land vs. woodland; (2) arable land vs. permanent grassland; (3) organic vs. conventional farming; (4) coniferous vs. broadleaved woodland; and (5) recent vs. long-established woodland. Carbon storage (kg/m²) varied with land usage and woodland type and age, but only agricultural land vs. woodland, and for agriculture, arable land vs. permanent grassland, significantly affected the 0–90 cm soil carbon. At the university-managed farms, current terrestrial carbon stocks were 103,620 tonnes in total (98,050 tonnes from the 0–90 cm soil and 5,569 tonnes from tree biomass). These terrestrial carbon stocks were equivalent to sixteen years of the current carbon emissions of Newcastle University (6,406 tonnes CO₂ equivalents-C per year). Using strategies for alternative land management, Newcastle University could over 40 years offset up to 3,221 tonnes of carbon per year, or 50% of its carbon emissions at the current rate. The methodological framework developed in this study will enable institutions having large landholdings to rationally consider their estates in future soil carbon management schemes.     

Organic matter availability for denitrification in soils of different textures and drainage classes

Abstract

Soils from the A, B, and C horizons representing three natural drainage classes and differing textures were chosen to study relationships between denitrification rate and estimates of available carbon. The highest correlation with denitrification rate was obtained with total organic C. Water‐extractable C, mineralizable C and 0.1 N Ba(OH)2‐extractable C produced less satisfactory correlations. When soils of the B and C horizons only were included in the regression analysis, 0.1 N Ba(OH)₂‐extractable C was found to be unsatisfactory as a predictor of available C for soil denitrifiers. None of the four methods for estimating available C were found adequate for B and C horizon soils which were relatively low in available C. Coarser‐textured soils with relatively low C levels had correspondingly low denitrification rates. Regressions of denitrification rate on mineralizable C or water‐extractable C were nonsignificant with poorly drained soils whereas they were highly significant with well or imperfectly drained soils.     

秸秆机械集中沟埋还田对农田净碳排放的影响

通过大田试验,采用秸秆机械集中沟埋和常规还田方式,将上季作物秸秆进行全量还田(秸秆沟埋量2.1 kg/m).设置沟埋深度为20 cm(D2),30 cm(D3),常规还田(CK)3个处理.利用West提出的净碳排放方程对CK、D2、D3农田各项投入造成的碳排放和土壤碳累积及农作物碳吸收进行比较.结果表明:CK、D2、D3稻麦轮作各项农田投入造成的碳排放量分别为9 018.19,6 459.9,7 162.86 kg/(hm2·a),表层0-28 cm土壤的碳储量分别为8 375.98,15 854.42,10 954.36 kg/(hm2·a),农作物年碳吸收量分别为10 912.42,12 863.95,12 585.51 kg/(hm2·a);农田净碳排放量分别为-10 270.2,-22 258.5,-16 377.0kg/(hm2·a),与CK相比,D2、D3的相对净碳排放量分别为-11 988.30,-6 106.81 kg/(hm2·a);D2、D3农业投入的碳减排量2 558.29,1 855.33 kg/(hm2·a)分别为碳增汇量28 718.4,23 539.9kg/(hm2·a)的8.91％,7.88％,秸秆集中沟埋还田对农田储碳减排能能力较常规还田强,其贡献优先排序是D2＞D3＞CK.     

Reduced tillage with residue retention improves soil labile carbon pools and carbon lability and management indices in a seven-year trial with wheat-mung bean-rice rotation

Soil total organic carbon (TOC) is a composite indicator of soil quality with implications for crop production and the regulation of soil ecosystem services. Research reports on the dynamics of TOC as a consequence of soil management practices in subtropical climatic conditions, where microbial carbon (C) loss is high, are very limited. The objective of our study was to evaluate the impact of seven years of continuous tillage and residue management on soil TOC dynamics (quantitative and qualitative) with respect to lability and stratification under an annual wheat-mung bean-rice cropping sequence. Composite soil samples were collected at 0-15 and 15-30 cm depths from a three-replicate split-plot experiment with tillage treatment as the main plots and crop residue levels as the sub-plots. The tillage treatments included conventional tillage (CT) and strip tillage (ST). Residue levels were high residue level (HR), 30% of the plant height, and low residue level (LR), 15%. In addition to TOC, soil samples were analyzed for particulate organic C (POC), permanganate oxidizable C (POXC), basal respiration (BR), specific maintenance respiration rate (qCO₂), microbial biomass C (MBC), potentially mineralizable C (PMC), and TOC lability and management indices. The ST treatment significantly increased the TOC and labile C pools at both depths compared with the CT treatment, with the effect being more pronounced in the surface layer. The HR treatment increased TOC and labile C pools compared with the LR treatment. The ST + HR treatment showed significant increases in MBC, metabolic quotient (qR), C pool index (CPI), C lability index (CL_I), and C management index (CMI), indicating improved and efficient soil biological activities in such systems compared with the CT treatment. Similarly, the stratification values, a measure of soil quality improvement, for POC and MBC were > 2, indicating improved soil quality in the ST + HR treatment compared with the CT treatment. The ST + HR treatment not only significantly increased the contents of TOC pools, but also their stocks. The CMI was correlated with qCO₂, BR, and MBC, suggesting that these are sensitive indicators of early changes in TOC. The qCO₂ was significantly higher in the CT + LR treatment and negatively correlated with MBC and CMI, indicating a biologically stressed soil condition in this treatment. Our findings highlight that medium-term reduced tillage with HR management has profound consequences on soil TOC quality and dynamics as mediated by alterations in labile C pools.     

长白山森林土壤有机碳库大小及周转研究

主要分析不同森林植被下有机碳的分解动态和土壤碳库各组分大小、周转时间。结果表明：土壤样品培养90天，CO2累计释放量表层大致为1723～5065mg／kg、下层大致为178～642mg／kg。分解速率总的趋势是前期快，后期慢，表层明显大于下层。大小顺序为：冷杉林〉针阔混交林和阔叶林〉针叶林。在不同植被下的表层和下层土壤中，活性碳占总有机碳的0．54％～1．67％，0．45％～5．48％．平均驻留时间为11～56天、60～88天；缓效性碳占总有机碳的23．0％～63．3％，33．2％～72．2％，平均驻留时间为4～70年、24～161年；惰效性碳占总有机碳的35．5％～75．5％．26．0％～65．%。表层土壤的总有机碳、活性碳、缓效性碳和惰效性碳含量都明显大于下层。凋落物的化学组成主要决定活性碳库、缓效性碳库含量，土壤的粘粒含量等性质主要决定惰效性碳库含量。     

Landuse impacts on soil organic carbon fractions in different rainfall areas of a subtropical dryland

Landuse can alter soil organic carbon (SOC) fractions by affecting carbon inflows and outflows. This study evaluated changes in SOC fractions in response to different landuses under variable rainfalls. We compared cropland, grassland and forest soils in high rainfall (Islamabad ~1142 mm) and low rainfall (Chakwal ~667 mm) areas of Pothwar dryland, Pakistan. Forest soils in both rainfall areas had highest SOC (11.32 g kg^?1), particulate organic carbon (POC, 1.70 g kg^?1), mineral-associated organic carbon (MOC, 7.17 g kg^?1) and aggregate-associated organic carbon (AOC, 7.86 g kg^?1). However, in rangeland and cropland soils, these varied with rainfall. Under high rainfall, SOC and MOC were 12% and 17% higher in rangeland than in cropland while POC and AOC were equal. Under low rainfall, SOC and MOC were higher in rangeland than in cropland by 7.21 and 1.79 g kg^?1 at 0–15 cm and equal at 15–30 cm depth. POC and AOC were higher in rangeland than in cropland, in both depths. Averagely, SOC, POC, MOC and AOC were 26%, 68%, 76% and 30% higher in high rainfall than in low rainfall soils. Sensitivity of SOC fractions to landuses observed under different rainfalls could provide useful information for soil management in subtropical drylands.