川西亚高山粗枝云杉人工林地上凋落物对土壤呼吸的贡献

采用Li-8100土壤碳通量分析仪对川西亚高山典型的粗枝云杉(Picea asperata)人工林土壤呼吸(凋落物去除和对照)及其环境因子进行为期1年的连续观测。结果表明:凋落物去除处理和对照土壤呼吸速率均具有显著的季节动态变化,并呈现一致的动态特征,变动范围分别为0.35—4.39μmol m-2s-1和0.40—5.15μmol m-2s-1。整个观测期间,凋落物去除对土壤温度、水分以及土壤呼吸速率产生的差异均不显著。与对照相比,凋落物去除分别使土壤呼吸速率和土壤水分平均下降了14.21%和4.95%。两种处理的土壤呼吸速率和土壤温度均呈显著指数相关,与土壤水分呈显著线性相关。凋落物去除和对照的土壤温度敏感性(Q10)分别为3.84和4.09。凋落物对土壤呼吸速率的年均贡献率为14.93%,且存在明显季节动态。可见,地表凋落物是亚高山森林土壤呼吸的重要组成部分。     

去除和添加凋落物对枫香（Liquidambar formosana）和樟树（Cinnamomum camphora）林土壤呼吸的影响

2007年1月至12月,在长沙天际岭国家森林公园,使用LI-COR-6400-09连接到LI-6400便携式CO2/H2O分析系统,测定亚热带枫香(Liquidambar formosana)和樟树(Cinnamomum camphora)林去除和添加凋落物(931.5 g · m-2a-1和1003.4 g · m-2a-1)的土壤呼吸速率以及5 cm土壤温、湿度,研究凋落物对2种森林生态系统中土壤呼吸速率的影响.结果表明:枫香和樟树林去除和添加凋落物的土壤呼吸速率季节变化显著,在季节动态上的趋势与5 cm土壤温度相似,均呈单峰曲线格局,全年去除凋落物土壤呼吸速率平均值分别为1.132 μmol CO2 · m-2s-1和1.933 μmol CO2 · m-2s-1,分别比对照处理1.397 μmol CO2 · m-2s-1和2.581 μmol CO2 · m-2s-1低18.62%和26.49%;添加凋落物土壤呼吸速率平均值分别为2.363 μmol CO2 · m-2s-1和3.267 μmol CO2 · m-2s-1,分别比对照处理高71.31%和39.18%.两种群落去除和添加凋落物土壤呼吸的季节变化均与5 cm土壤温度呈显著指数相关(P﹤0.001),与5 cm土壤湿度相关性不显著(P>0.05);土壤温度和湿度可以共同解释去除和添加凋落物后土壤呼吸变化的95.2%、93.7%和90.0%、92.8%.枫香和樟树群落去除和添加凋落物土壤呼吸温度敏感性Q10值分别为3.01、3.29和3.02、4.37,均比对照处理Q10值2.98和2.94高.这证明凋落物是影响森林CO2通量的一个重要因子.     

云南香格里拉地区亚高山草甸不同放牧管理方式下的碳排放

青藏高原由于高寒低温限制了有机碳的分解,大量的碳积累在土壤碳库中,对全球升温的反应很敏感.放牧可能会对该区草甸的碳排放产生显著影响.采用闭合箱动态法测定了云南香格里拉地区不同放牧管理方式下的亚高山草甸生态系统呼吸与土壤呼吸.常年放牧草甸与季节性放牧草甸的生态系统呼吸和土壤呼吸均呈现相似且明显的单峰季节变化特征,7月份达最大值,生态系统呼吸分别为9.77、8.03 μmol CO2 m-2 s-1,土壤呼吸分别为8.05、7.74 μmol CO2 m-2 s-1;1月份达最低值,生态系统呼吸分别为0.21、0.48 μmol CO2 m-2 s-1,土壤呼吸分别为0.16、0.49 μmol CO2 m-2 s-1.受一天中气温和土温的影响,常年放牧草甸的生态系统呼吸与土壤呼吸的日变化在夏季与冬季均呈现明显的单峰曲线变化,最高值都出现在14:00左右,最低值出现在凌晨.在夏季6～10月份,常年放牧草甸的呼吸显著大于季节性放牧草甸,表明较高的放牧强度增加了亚高山草甸的碳排放.土壤温度的指数模型F = aebT比土壤水分能更好地解释呼吸的变异性(R2 = 0.50～0.78,P < 0.0001).二元回归模型F=aebTWc比单因子模型的效果更好(R2=0.56～0.89,P < 0.0001).土壤呼吸在整个亚高山草甸生态系统呼吸中占主导地位,在常年放牧草甸与季节性放牧草甸分别为63.0%～92.7%和47.5%～96.4%,地上植物呼吸随生长季的变化而变化,在生长旺季占有较大的比例.生态系统呼吸和土壤呼吸的长期Q10(1a)是短期Q10(1d)的2倍左右.季节性放牧草甸的长期Q10小于常年放牧草甸,表明在温度上升的背景下,放牧压力较小的草甸碳库较为稳定,具有较好的碳截存能力.     

黄土高原小麦田土壤呼吸季节和年际变化

本研究采用自行开发的全自动多通道通量箱法对黄土高原小麦田土壤呼吸变化特征进行了连年原位监测,在此基础上,采用斜率同质性模型（homogeneity-of-slop model, HOS模型）对农田土壤呼吸季节和年际变化的成因进行了解析。结果表明,在日尺度上,土壤呼吸主要受土壤温度的影响,呈现出明显的单峰变化趋势,在12：00左右达最大值。季节尺度上,土壤呼吸冬季较低,春季上升,7月下旬达最大值,之后下降,土壤呼吸季节变化主要受土壤温度和湿度的影响。土壤呼吸年际间变化较大,其变化幅度为815.72 g C?m-2? yr-1～980.12 g C?m-2?yr-1。土壤温度是引起土壤呼吸年际变化最重要的因素,逐步回归分析表明,土壤温度和土壤含水量均与土壤呼吸呈显著的正相关,可以用Rsoil = 1.761 0.119Tsoil - 1.30SWC,（R2=0.48, p<0.0001）来描述土壤呼吸与土壤温度和土壤水分的关系。降水量与土壤呼吸没有显著的相关关系。用HOS模型对土壤呼吸年际变化的成因进行分析的结果表明,功能变化（function change, FC）、环境因子季节变化、环境因子年际变化以及随机误差分别可以解释年际变化的10.6%、 58.4%、4.5%、和 26.5%,功能变化对土壤呼吸年际变化的影响要大于土壤温度年际变化的影响。因此,在土壤呼吸的预测中,不能仅仅依靠环境因子的变化来预测土壤呼吸的变化,还必须考虑到功能变化对土壤呼吸的影响。     

垄沟覆膜栽培冬小麦田的土壤呼吸

通过大田试验研究了垄沟覆膜栽培条件下冬小麦生长过程中土壤呼吸规律。结果表明,垄沟覆膜栽培条件下垄脊土壤呼吸速率高于平作栽培,而垄沟部土壤呼吸速率小于平作。冬小麦生育期内垄脊平均呼吸速率为(2.06±0.44)μmol CO2·m-2·s-1,垄沟为(0.75±0.11)μmol CO2·m-2·s-1,而平作栽培为(1.14±0.20)μmol CO2·m-2·s-1。土壤呼吸季节变化显著,越冬期低,夏季高。不同生育期土壤呼吸日变化规律不同,越冬前和返青期土壤呼吸与土壤温度成正相关,随着土壤温度的升高而增加,呈单峰曲线;拔节期后垄脊部的土壤呼吸日变化明显,呈现双峰曲线;而平作和垄沟的土壤呼吸速率平稳,没有明显峰值。5 cm土壤温度与土壤呼吸之间的相关性最好。在一定范围内(<24—31℃),土壤呼吸随着温度的增加而增加,温度过高反而会抑制土壤呼吸速率。土壤呼吸f(R)与5 cm土壤温度之间的关系可以用二次函数表示;5 cm土壤温度T和土壤含水量W的交互效应可用函数:f(R)=a(bT2+cT)(1+dln(2W)/T)+e表示。垄沟覆膜栽培显著改变了冬小麦田的土壤呼吸作用。     

西天目山毛竹林土壤呼吸特征及其影响因子

毛竹(Phyllostachys edulis)是中国南方重要的森林资源,在区域碳平衡中扮演重要的作用。研究毛竹林土壤呼吸特征及影响因子有助于了解其土壤CO2释放过程的关键驱动因子,为进一步揭示毛竹林土壤碳循环特点提供科学依据。以浙江省西天目山自然保护区毛竹林为研究对象,利用LI-Cor8100开路式土壤碳通量测量系统测定(2007年5、8、11月,2008年1、3月)土壤呼吸速率及环境因子,同时取0—20 cm土层土样测定土壤酶活性,结果表明:(1)毛竹林土壤呼吸具有典型的日动态和季节变化模式,日动态变化较为平缓,土壤呼吸的季节变化较为显著(P0.05),最大值(5.99μmol.m-2.s-1)出现在2007年8月,最小值(1.08μmol.m-2.s-1)出现在2008年1月。(2)回归方程表明,土壤呼吸与土壤5 cm温度呈极显著的指数相关关系(P0.001),与土壤体积含水量相关性较弱(P0.05),与近地面大气温度和CO2浓度分别呈极显著的指数相关关系(P0.001)和显著的线性相关关系(P0.05)。(3)相关分析表明,土壤尿酶、蔗糖酶、纤维素酶活性与土壤呼吸均呈正相关,其中纤维素酶活性达到显著水平。综合分析表明毛竹林土壤温度是调控土壤呼吸季节变化的主要驱动因子,近地面大气环境及土壤酶活性的变化也对其产生不容忽视的影响。     

亚热带沟叶结缕草草坪土壤呼吸

随城市化进程加速,城市草坪生态系统释放CO2将对区域碳循环产生重要影响。采用LI-8100开路式土壤碳通量测量系统对亚热带沟叶结缕草草坪（Zoysia matrella）土壤呼吸进行为期1a的定位研究,结果表明：草坪土壤呼吸季节动态呈现为单峰曲线,全年土壤呼吸速率的变化范围在38.99—368.50 mg C?m-2?h-1之间,年通量为1684 g C?m-2?a-1。土壤温度、总生物量、以及二者的交互作用对土壤呼吸季节变化的解释程度接近,分别为89%、88%和90%,但仅二者的交互作用进入土壤呼吸的逐步回归方程,表明草坪土壤呼吸的季节变化主要受土壤温度与总生物量共同驱动。春末修剪草坪对土壤呼吸速率没有显著影响。在秋末无雨时期,浇水后1—2d土壤湿度对土壤呼吸的促进作用可掩盖同期降温的影响,使土壤呼吸速率显著升高。     

呼伦贝尔地区贝加尔针茅草甸草原土壤CO_2排放特征

采用Li-8150多通道土壤碳通量自动观测系统,于2009年6月—2010年6月对呼伦贝尔地区贝加尔针茅草甸草原土壤呼吸进行连续野外观测,分析该地区土壤CO2通量排放特征.结果表明:生长季内贝加尔针茅草甸草原土壤呼吸日动态呈单峰曲线,最高值出现在13:00—15:00,最低值出现在5:00—6:00,土壤呼吸呈明显的季节变化,与土壤温度和土壤含水量季节动态相吻合.土壤呼吸与各层土壤温度和土壤含水量关系可以用线性模型和指数-乘幂模型来表示;土壤呼吸与各层土壤温度呈显著的指数回归关系,Q10变化范围分别为1.68~2.14和3.03~3.60,非生长季土壤温度对土壤呼吸的影响更为显著;生长季内土壤呼吸与10 cm土壤含水量呈显著正相关.2009和2010年土壤CO2年排放量分别为488.47和507.20 g C·m-2·a-1,生长季排放量约占年排放量的90%.     

万木林保护区毛竹林土壤呼吸特征及影响因素

2009年1-12月,利用Li-Cor 8100开路式土壤碳通量系统测定福建省万木林自然保护区毛竹林土壤呼吸速率,分析毛竹林土壤呼吸动态变化及其与凋落物量的关系.结果表明:毛竹林土壤呼吸月变化呈明显的双峰型曲线,峰值分别出现在6月(6.83 μmol·m-2·s-1)和9月(5.59μmol·m-2·s-1).土壤呼吸速率的季节变化较明显,最大值出现在夏季,最小值出现在冬季;土壤呼吸速率与土壤5 cm温度呈显著正相关关系(P＜0.05),与土壤含水量无显著相关性(P＞0.05);毛竹林凋落物量月变化呈单峰型曲线.毛竹林土壤呼吸速率与当月凋落物归还量呈显著正相关(P＜0.05).土壤温度和凋落物量的双因素模型可以解释土壤呼吸速率变化的93.2%.     

模拟氮沉降对华西雨屏区慈竹林土壤呼吸的影响

2007年12月至2008年11月,在华西雨屏区采用0(对照)、50、150、300kg.hm-2.a-1施氮处理和红外CO2分析法,研究了模拟N沉降对慈竹林土壤呼吸特征的影响.结果表明:慈竹林土壤呼吸速率年内季节变化呈明显的单峰型曲线,7月末最高,为(3.36±0.20)μmol.m-2.s-1,2月末最低,为(0.33±0.07)μmol.m-2.s-1.土壤呼吸速率与土壤温度之间呈极显著指数相关(P0.001),10cm深的土壤温度解释了土壤呼吸速率季节变化的91.6%;而土壤含水量与土壤呼吸之间相关性不显著(R2=0.0758).2008年6—11月根呼吸对土壤总呼吸的贡献率在46%～59%.50、150和300kg.hm-2.a-1施氮处理的年CO2释放量分别比对照低23.6%、46.7%和50.5%.0、50、150和300kg.hm-2.a-1施氮处理的土壤呼吸速率Q10值分别为3.72、3.51、2.95和2.71.     

Effect of Tylenchorhynchus nudus on Growth of Kentucky Bluegrass

The effect of Tylenchorhynchus nudus on growth of Kentucky bluegrass was investigated under controlled environmental conditions in both a phytotron and a greenhouse. The nematode significantly reduced weights of clippings, crowns and roots. Pathogenicity was greater in sandy loam soil than in loam and was enhanced by submitting plants to nutrient and/or moisture stresses; soil nutrient level was most critical. The results suggest that T. nudus contributes significantly to summer decline of bluegrass lawns in South Dakota.     

向家坝水电站工程扰动区不同类型边坡土壤酸碱度与肥力分析

对向家坝水电站工程扰动区7种不同类型边坡土壤的酸碱度（pH)进行检测,并运用标准综合级别法对其肥力状况进行分析.结果表明：在向家坝水电站工程扰动区,植被混凝土边坡土壤呈微碱性,其他边坡土壤均呈不同程度的酸性,土壤pH值依次为:植被混凝土(7.68±0.04)>弃渣地(6.96±0.02)>框格梁覆土(6.93±0.16)>客土喷播(6.75±0.35)>厚层基材(6.67±0.36)>天然次生林(5.68±1.25)>天然林(4.71±0.23);不同类型边坡土壤肥力的综合级别值大小为: 植被混凝土(3.06±0.02）<厚层基材(4.06±0.78)<天然林(4.20±0.10)<天然次生林(4.32±0.16)<框格梁覆土(4.62±0.06)< 客土喷播(4.90±0.01)<弃渣地(5.00±0.10),植被混凝土肥力状况最佳,弃渣地肥力状况最差.采用人工修复技术添加的水泥、复合肥及腐殖质在调节边坡土壤pH值和维持其肥力方面起到一定的作用,可为植被生长提供良好的土壤条件.     

Plant-soil feedbacks provide an additional explanation for diversity-productivity relationships

Plant-soil feedbacks (PSFs) have gained attention for their role in plant community dynamics, but their role in productivity has been overlooked. We developed and tested a biomass-specific, multi-species model to examine the role of PSFs in diversity-productivity relationships. The model predicts a negative relationship between PSFs and overyielding: plants with negative PSFs grow more in communities than in monoculture (i.e. overyield), and plants with positive PSFs grow less in communities than in monoculture (i.e. underyield). This effect is predicted to increase with diversity and saturate at low species richness because the proportion of 'self-cultivated' soils rapidly decreases as species are added to a community. Results in a set of glasshouse experiments supported model predictions. We found that PSFs measured in one experiment were negatively correlated with overyielding in three-species plant communities measured in a separate experiment. Furthermore, when parametrized with our experimental PSF data, our model successfully predicted species-level overyielding and underyielding. The model was less effective at predicting community-level overyielding and underyielding, although this appeared to reflect large differences between communities with or without nitrogen-fixing plants. Results provide conceptual and experimental support for the role of PSFs in diversity-productivity relationships.     

Localized application of soil organic matter shifts distribution of cluster roots of white lupin in the soil profile due to localized release of phosphorus

Background and Aims

Phosphorus (P) is a major factor controlling cluster-root formation. Cluster-root proliferation tends to concentrate in organic matter (OM)-rich surface-soil layers, but the nature of this response of cluster-root formation to OM is not clear. Cluster-root proliferation in response to localized application of OM was characterized in Lupinus albus (white lupin) grown in stratified soil columns to test if the stimulating effect of OM on cluster-root formation was due to (a) P release from breakdown of OM; (b) a decrease in soil density; or (c) effects of micro-organisms other than releasing P from OM.

Methods

Lupin plants were grown in three-layer stratified soil columns where P was applied at 0 or 330 mg P kg⁻¹ to create a P-deficient or P-sufficient background, and OM, phytate mixed with OM, or perlite was applied to the top or middle layers with or without sterilization.

Key Results

Non-sterile OM stimulated cluster-root proliferation and root length, and this effect became greater when phytate was supplied in the presence of OM. Both sterile OM and perlite significantly decreased cluster-root formation in the localized layers. The OM position did not change the proportion of total cluster roots to total roots in dry biomass among no-P treatments, but more cluster roots were concentrated in the OM layers with a decreased proportion in other places.

Conclusions

Localized application of non-sterile OM or phytate plus OM stimulated cluster-root proliferation of L. albus in the localized layers. This effect is predominantly accounted for by P release from breakdown of OM or phytate, but not due to a change in soil density associated with OM. No evidence was found for effects of micro-organisms in OM other than those responsible for P release.     

Lignin characteristics in soil profiles in different plant communities in a subtropical mixed forest

Aims Lignin is generally considered as an important indicator of soil organic carbon (SOC) storage and dynamics. To evaluate the effects of plant communities and soil depth on soil lignin is critical to better understand forest carbon cycling.Methods We compared lignin content and chemical signature in three soil depths of four major plant communities in a subtropical forest, which located in the north part of Wuling Mountains, China. Lignin was measured using CuO oxidation method.Important findings Both lignin content and its biochemical signature in plant litter varied among communities. However, these differences were mostly no longer exist in the upper soil layers. Lignin chemistry in soils inherited some of the biochemical signature of lignin in litter, but in a diminished magnitude. These results suggest that different plant communities had similar decomposition process with varying rates, caused diminished differences in lignin content and its biochemical signature. Lignin content decreased with soil depth, but the biochemical signature of lignin was not significantly different among soil layers for all communities, which suggests that vertical movement of lignin within the soil profile is very likely a key process causing this similar biochemical signature. These results emphasized the important roles of lignin inputs and soil eluviation in shaping lignin characteristics and distribution in forest soils, which pinpoint the urgent need to consider hydrological processes in studying forest soil carbon cycling.     

珍贵固氮树种降香黄檀与二代巨尾桉混交种植对土壤微生物群落结构和功能的影响

该研究以二代巨尾桉/降香黄檀混交林(MP)作为研究对象,并以邻接的二代巨尾桉纯林(PP)作为对照(CK),采用磷脂脂肪酸(PLFAs)和土壤酶活性分别表征土壤微生物群落结构和功能,重点探究南亚热带PP引入降香黄檀混交种植对土壤微生物群落结构和功能的影响。结果表明:(1)与PP相比,MP的土壤有机碳(SOC)、总氮(TN)、铵态氮(NH_4~+-N)、硝态氮(NO_3~--N)和p H值分别显著或极显著增加了61.92%(P0.05)、60.12%(P0.05)、72.87%(P0.01)、488.49%(P0.01)和15.97%(P0.05);(2) MP的真菌/细菌(F/B)显著降低,但总微生物生物量、革兰氏阴性菌/阳性菌(G~-/G~+)并无显著变化;(3) MP的土壤微生物群落组成发生了显著变化,而p H值、NH_4~+-N和凋落物质量(C/N_(litter))是驱动其微生物群落组成发生变异的最显著性因子;(4) MP的β-葡萄糖苷酶(BG)和N-乙酰-葡萄糖苷酶(NAG)活性显著提高,而过氧化物酶(PER)活性显著降低,酚氧化酶(PO)和酸性磷酸酶(ACP)活性没有显著变化。该研究说明在连续短周期经营的桉树林中套种固氮树种降香黄檀,将可能是提高桉树林土壤质量的一种有效的经营管理措施。     

Persistence of Heterorhabditis Infective Juveniles in Soil: Comparison of Extraction and Infectivity Measurements

The persistence of Heterorhabditis megidis in soil was studied over a 4-week period. On days 0, 2, 14, and 28, infective juveniles (IJ) were extracted by centrifugal flotation, Baermann funnel, and baiting of soil with Tenebrio molitor larvae, which were then dissected. Extraction efficiencies on day 0 were 82% by centrifugal flotation, 56% by Baermann funnel, and 19.8% by bait insect. The relative efficiency of the three methods changed over time. The relationship between the density of nematodes in the soil and the proportion recovered by dissection was non-linear. Up to a dose of approximately 60 IJ/insect, less than 12% became established, while at higher doses (up to 200 IJ/insect) the invasion efficiency was 23%. Mortality of bait insects increased from day 0 to day 2, but decreased to day 28. A novel method of assessing soil pathogenicity by preparing a soil density series and calculating the dose of soil or IJ that kills 50% of the bait insects gave a similar pattern. This method is recommended as a means of tracking changes in pathogenicity over time when bait insect mortality in undiluted soil is at or near 100%. Two methods of preparing a series of Heterorhabditis IJ densities in soil, either by diluting the soil itself with IJ-free soil or by adding diluted suspensions of IJ to the soil, resulted in the same bait insect mortalities.     

Impact of grazing intensity on seasonal variations in soil organic carbon and soil CO2 efflux in two semiarid grasslands in southern Botswana

Biological soil crusts (BSCs) are an important source of organic carbon, and affect a range of ecosystem functions in arid and semiarid environments. Yet the impact of grazing disturbance on crust properties and soil CO₂ efflux remain poorly studied, particularly in African ecosystems. The effects of burial under wind-blown sand, disaggregation and removal of BSCs on seasonal variations in soil CO₂ efflux, soil organic carbon, chlorophyll a and scytonemin were investigated at two sites in the Kalahari of southern Botswana. Field experiments were employed to isolate CO₂ efflux originating from BSCs in order to estimate the C exchange within the crust. Organic carbon was not evenly distributed through the soil profile but concentrated in the BSC. Soil CO₂ efflux was higher in Kalahari Sand than in calcrete soils, but rates varied significantly with seasonal changes in moisture and temperature. BSCs at both sites were a small net sink of C to the soil. Soil CO₂ efflux was significantly higher in sand soils where the BSC was removed, and on calcrete where the BSC was buried under sand. The BSC removal and burial under sand also significantly reduced chlorophyll a, organic carbon and scytonemin. Disaggregation of the soil crust, however, led to increases in chlorophyll a and organic carbon. The data confirm the importance of BSCs for C cycling in drylands and indicate intensive grazing, which destroys BSCs through trampling and burial, will adversely affect C sequestration and storage. Managed grazing, where soil surfaces are only lightly disturbed, would help maintain a positive carbon balance in African drylands.     

Effect of soil microorganisms and labile C availability on soil respiration in response to litter inputs in forest ecosystems: A meta‐analysis

Litter inputs can influence soil respiration directly through labile C availability and, indirectly, through the activity of soil microorganisms and modifications in soil microclimate; however, their relative contributions and the magnitude of any effect remain poorly understood. We synthesized 66 recently published papers on forest ecosystems using a meta‐analysis approach to investigate the effect of litter inputs on soil respiration and the underlying mechanisms involved. Our results showed that litter inputs had a strong positive impact on soil respiration, labile C availability, and the abundance of soil microorganisms, with less of an impact related to soil moisture and temperature. Overall, soil respiration was increased by 36% and 55%, respectively, in response to natural and doubled litter inputs. The increase in soil respiration induced by litter inputs showed a tendency for coniferous forests (50.7%)> broad‐leaved forests (41.3%)> mixed forests (31.9%). This stimulation effect also depended on stand age with 30‐ to 100‐year‐old forests (53.3%) and ≥100‐year‐old forests (50.2%) both 1.5 times larger than ≤30‐year‐old forests (34.5%). Soil microbial biomass carbon and soil dissolved organic carbon increased by 21.0%‐33.6% and 60.3%‐87.7%, respectively, in response to natural and doubled litter inputs, while soil respiration increased linearly with corresponding increases in soil microbial biomass carbon and soil dissolved organic carbon. Natural and doubled litter inputs increased the total phospholipid fatty acid (PLFA) content by 6.6% and 19.7%, respectively, but decreased the fungal/bacterial PLFA ratio by 26.9% and 18.7%, respectively. Soil respiration also increased linearly with increases in total PLFA and decreased linearly with decreases in the fungal/bacterial PLFA ratio. The contribution of litter inputs to an increase in soil respiration showed a trend of total PLFA > fungal/bacterial PLFA ratio > soil dissolved organic carbon > soil microbial biomass carbon. Therefore, in addition to forest type and stand age, labile C availability and soil microorganisms are also important factors that influence soil respiration in response to litter inputs, with soil microorganisms being more important than labile C availability.     

The potential for nitrification and nitrate uptake in the rhizosphere of wetland plants: a modelling study

BACKGROUND AND AIMS: It has recently found that lowland rice grown hydroponically is exceptionally efficient in absorbing NO3-, raising the possibility that rice and other wetland plants growing in flooded soil may absorb significant amounts of NO3- formed by nitrification of NH4+ in the rhizosphere. This is important because (a) this NO3- is otherwise lost through denitrification in the soil bulk; and (b) plant growth and yield are generally improved when plants absorb their nitrogen as a mixture of NO3- and NH4+ compared with growth on either N source on its own. A mathematical model is developed here with which to assess the extent of NO3- absorption from the rhizosphere by wetland plants growing in flooded soil, considering the important plant and soil processes operating. METHODS: The model considers rates of O2 transport away from an individual root and simultaneous O2 consumption in microbial and non-microbial processes; transport of NH4+ towards the root and its consumption in nitrification and uptake at the root surface; and transport of NO3- formed from NH4+ towards the root and its consumption in denitrification and uptake by the root. The sensitivity of the model's predictions to its input parameters is tested over the range of conditions in which wetland plants grow. KEY RESULTS: The model calculations show that substantial quantities of NO3- can be produced in the rhizosphere of wetland plants through nitrification and taken up by the roots under field conditions. The rates of NO3- uptake can be comparable with those of NH4+. The model also shows that rates of denitrification and subsequent loss of N from the soil remain small even where NO3- production and uptake are considerable. CONCLUSIONS: Nitrate uptake by wetland plants may be far more important than thought hitherto. This has implications for managing wetland soils and water, as discussed in this paper.