黄土丘陵区不同演替阶段生物结皮对土壤CO2通量的影响

生物结皮是土壤表面具有光合活性的致密复合层,是土 气界面CO₂通量的影响因子之一.本文采用改进的Li-8100土壤碳通量测量系统,研究了黄土丘陵区退耕地上不同演替阶段生物结皮对土壤CO₂通量的影响.结果表明: 光照条件下,生物结皮土壤CO₂通量较除去生物结皮显著下降,其中藻结皮和藓结皮分别下降了92%和305%;生物结皮对土壤CO₂通量的降低程度与其生物组成和生物量有关,深色藻结皮和藓结皮土壤CO₂通量较裸地分别降低了141%和484%.生物结皮土壤CO₂通量的日变化呈降低-升高-降低的趋势,而裸地CO₂通量日变化趋势为单峰曲线,藻结皮、藓结皮的碳吸收峰值分别出现在8:00和9:00前后,其CO₂通量分别为0.13和-1.02 μmol CO₂·m^-2·s^-1;藻结皮24 h CO₂通量排放总量较裸地增加7.7%,而藓结皮减少了29.6%.生物结皮对土壤CO₂通量的影响显著,在评价退耕地土壤碳循环时,应考虑生物结皮的影响.     

黄土丘陵区草本植物覆盖下生物结皮对坡面径流流速的削减作用

生物结皮是干旱半干旱地区的常见地被物,与植物共同影响坡面径流及流速。迄今鲜有研究关注植物和生物结皮共同覆盖对流速的影响,是干旱半干旱地区坡面侵蚀驱动因素研究的薄弱环节。本研究以黄土丘陵区退耕草地为对象,通过野外模拟降雨试验,研究草本植物覆盖下有无生物结皮及不同组成生物结皮(多藻少藓、多藓少藻和藓)对径流流速的影响。结果表明: 植物和植物+生物结皮覆盖显著降低了流速,植物覆盖较裸土降低70.7%,植物+生物结皮覆盖较裸土降低83.1%;植物和生物结皮共同覆盖下,植物和生物结皮对径流流速的削减效益分别为70.7%和12.4%。植物覆盖下生物结皮对流速的影响程度与其组成有关,多藻少藓结皮、多藓少藻结皮和藓结皮对流速的削减效益分别为11.5%、12.4%和19.4%。流速与藓盖度呈显著负相关,与藻盖度呈显著正相关,藓结皮盖度(x)与流速(y)的关系式为:y=-2.081x+0.03(R²=0.469)。当植物盖度一定时(40%±10%),生物结皮组成中藓盖度是影响共同覆盖坡面流速的关键因子。综上,草本植物覆盖下生物结皮有显著减缓流速的作用,且作用程度与其组成有关。表明在研究退耕草地坡面侵蚀动力机制时,生物结皮的作用应予以考虑。     

藓结皮对陕北黄土高原两种质地土壤呼吸的影响

使用LI-840便携式土壤碳通量分析仪,测定了黄土高原典型小流域砂土(风沙土)和砂质壤土(黄土)上典型藓结皮与无结皮的呼吸速率,分析两种质地土壤上藓结皮对土壤呼吸的影响。结果显示,在小时尺度上,砂土和砂质壤土上有无藓结皮的呼吸速率均呈单峰型曲线,峰值出现在14:00时,大小分别为2.49和2.66μmol·m-2·s-1;相较于无结皮,有藓结皮的两种质地土壤的呼吸速率峰值分别降低了8.6%和5.8%。在日尺度上,有藓结皮的砂土和砂质壤土的日均呼吸速率分别较无结皮降低了9.1%和5.3%,且有藓结皮的砂土日均呼吸速率比有藓结皮的砂质壤土低7.1%。此外,两种质地土壤的呼吸速率与表层2 cm土壤温度均呈显著线性相关,表明表层土壤温度是影响生物结皮土壤呼吸的主要因素;同时,藓结皮使表层土壤温度降低0.3~0.5℃,因此推测藓结皮可能会通过降低土壤温度进而影响土壤呼吸速率。研究表明,黄土高原两种质地土壤上广泛发育的藓结皮均能显著降低土壤呼吸速率,其中砂土上的藓结皮更为突出,因此在区域碳通量估算和碳循环研究中应予以充分重视。     

混合生物结皮对土壤养分的影响与群落结构之关联——以黄土丘陵区的生物结皮为例

自然条件下生物结皮是藻、藓及地衣等结皮类型以不同比例组成的混合群落，显著影响土壤养分含量，目前混合生物结皮对土壤养分的影响与其群落结构的关系尚不清楚，限制了混合生物结皮土壤养分的评估。为此，研究通过测定单一组成的藻结皮、藓结皮以及80%藻+20%藓、60%藻+40%藓、40%藻+60%藓和20%藻+80%藓4个不同藻藓比例的混合生物结皮土壤有机碳、全氮、全磷、速效磷、铵态氮和硝态氮含量，研究了混合生物结皮土壤养分与其群落结构之间的关联。结果显示：(1)藓结皮层土壤有机碳、全氮、速效磷、铵态氮和硝态氮含量显著高于藻结皮，分别高出166.4%、77.2%、55.1%、56.2%和42.2%。(2)藻藓混合生物结皮土壤有机碳、全氮、速效磷和铵态氮含量与组成和盖度等结构特征有关，可以通过单一类型生物结皮土壤养分含量与盖度加权预测混合生物结皮土壤养分储量。(3)混合生物结皮土壤有机碳、全氮、速效磷和铵态氮储量实测值(x)与预测值(y)拟合的线性函数分别为y=0.97x、y=0.96x、y=1.18x和y=0.92x。(4)混合生物结皮对全磷和硝态氮含量的影响与群落结构无关。生物结皮对下层0—5 ...     

生物土壤结皮对雨滴动能的响应及削减作用

在野外调查采样的基础上,采用单滴雨滴法研究了黄土高原水蚀风蚀交错区不同类型生物结皮对雨滴动能的响应及削减作用.结果表明:生物结皮对雨滴动能的响应与其生物组成密切相关,击穿1 cm厚浅色藻结皮和80％盖度藓结皮的累积雨滴动能分别为0.99 J和75.56 J;相同组成的生物结皮对雨滴动能的响应与其生物量有关,生物量越大,击穿生物结皮所需累积雨滴动能越大;当藻结皮叶绿素a含量(表征藻结皮生物量)从3.32 μg·g-1增至3.73 μg·g-1时,对应的雨滴动能则由0.99 J增至2.17 J;当藓结皮的生物量从2.03 g·dm-2升至4.73 g·dm-2时,其对应的雨滴动能由6.08 J增至75.56 J;生物结皮演替过程中对雨滴动能的响应呈“S”形曲线变化.不同生物量的藻结皮对雨滴动能耐受能力的差异不显著;随单位面积苔藓生物量的增加,藓结皮对雨滴动能的耐受能力显著增加.藓结皮对雨滴动能的耐受能力在生物量2.03～4.73 g·dm-2之间呈线性增加;当藓结皮平均生物量达3.70 g·dm-2时,可以抗击62.03 J的雨滴动能.生物结皮对雨滴动能有显著的削弱作用,且这种削弱作用随着生物量的增加而增加.     

模拟降水对古尔班通古特沙漠生物结皮表观土壤碳通量的影响

水分是控制干旱区生态过程的重要环境因素,在水分受限制的生态系统中,降水通过改变土壤的干湿状态直接控制地下生物过程。生物结皮作为干旱区主要的地表覆盖物,能利用空气中有限的水分进行光合作用,其自身的碳交换是干旱区土壤碳通量的重要组成部分。通过模拟0(对照)、2、5 mm和15 mm 4个降水梯度,利用红外气体分析仪,对古尔班通古特沙漠中部生物结皮以及裸地表观土壤碳通量进行测量,探讨不同强度降水条件下生物结皮对表观土壤碳通量的影响,结果表明:(1)降水增加了生物结皮表观土壤碳释放量,2、5 mm和15 mm 3种降水处理累积碳释放量分别是对照的151.48%、274.97%、306.44%,并且随着降水后时间的延长,表观土壤碳通量逐渐减小直至达到降水前的水平;(2)生物结皮与裸地的表观土壤碳通量对降水的响应不同,对照和最大降水量下,生物结皮表观土壤碳通量大于裸地,但是2 mm和5 mm降水后,生物结皮表观土壤碳通量小于裸地,并且二者在2 mm降水时差异显著(P<0.05),而在其它降水处理下无显著差异;(3)连续两次降水事件,活性碳在初级降水后的大量释放使得二次降水后释放量下降,其中裸地碳释放量下降速率与降水强度正相关。本研究说明,在探求荒漠地区土壤碳交换对降水的响应规律时,应该考虑生物结皮的影响以及连续降水事件的差异。     

黄土丘陵区生物土壤结皮表面糙度特征及影响因素

地表糙度是影响地表径流和侵蚀过程的重要属性.生物结皮在干旱半干旱区广泛分布,是地表糙度的影响因子之一.本文采用链条法测定了黄土丘陵区不同发育阶段生物结皮表面糙度特征,分析了不同发育阶段生物结皮表面糙度对坡向、土壤含水量和冻融作用的响应及其与各理化性质的相关性,初步探索了生物结皮对地表糙度的影响及其相关因素.结果表明: 生物结皮显著改变地表糙度,随着生物结皮从藻结皮向藓结皮演替,其糙度先降低后增加,生物结皮发育形成10年以后,其表面糙度基本趋于稳定;研究区早期形成的藻结皮表面糙度较裸土降低47.0%,深色藻结皮(藓类盖度<20%)较裸土降低20.4%,混生结皮(藓类盖度为20%～60%)和苔藓结皮(藓类盖度>70%)表面糙度与深色藻结皮基本一致;坡向对发育10年以上的生物结皮表面糙度的影响不显著;土壤含水量影响地表糙度特征.研究区浅色藻结皮表面糙度随水分变化较为剧烈;随着生物结皮发育,深色藻结皮、混生结皮和苔藓结皮表面糙度随水分的变化趋于平缓.冻融增加了生物结皮表面糙度.浅色藻结皮经两次冻融后表面糙度增加29.7%;深色藻结皮、混生结皮和藓结皮表面糙度的影响需经过反复冻融才有所体现.生物结皮表面糙度与藓结皮盖度呈显著正相关(P＜0.1).     

沙地不同发育阶段的人工生物结皮对重金属的富集作用

生物结皮广泛存在于沙地生态系统,具有重要的生态功能.通过对位于库布齐沙漠腹地达拉特旗火力发电厂附近沙地中不同年代人工生物结皮及物理结皮中重金属含量的测定,旨在分析不同类型生物结皮及物理结皮在不同的发育阶段对重金属富集的影响和重金属污染的指示程度.通过分析和比较,得出以下结论:不同年代生物结皮和物理结皮各种重金属含量均表现为随着发育时间的增加而增加的趋势,生物结皮重金属含量多数类型表现为:Zn＞Cr＞Ni＞Pb＞Cu＞As＞Co＞Cd＞Hg的顺序关系,少数类型表现为:Cr＞Zn＞Ni＞Pb＞Cu＞As＞Co＞Cd＞Hg的顺序关系.通过单因素方差分析(ANOVA),不同年代藓结皮中各种元素差异显著性(P＜0.05)明显低于藻结皮和物理结皮.相同年代生物结皮和物理结皮重金属含量都表现为同样地规律:藓结皮＞藻结皮＞物理结皮,表明相同背景条件下,藓结皮对各种重金属的富集能力明显比藻结皮和物理结皮的富集能力强.通过单因素方差分析(ANOVA),Hg和Ni元素含量在所有相同年代样地的生物结皮和物理结皮均无差异,Cr、Zn、Cu、Co元素含量均存在差异(P＜0.05),但差异有的表现在藓结皮和藻结皮之间,而有的表现为藻结皮和物理结皮之间,而Pb、As、Cd元素含量则表现为有的年代有差异,有的年代无差异.通过污染因子CF值分析,藓结皮对污染的指示作用要明显比藻结皮和物理结皮敏感.同时生物结皮对于重金属的富集具有一定的选择性,像Hg、Ni、Zn、Cu、Pb、Co等元素生物结皮相对富集较少,而像Cr、Cd、As等元素相对富集较多.     

黄土丘陵区生物结皮对土壤可蚀性的影响

在采样分析生物结皮对土壤理化属性影响的基础上,采用EPIC模型估算与模拟降雨试验相结合的方法,研究了黄土丘陵区不同生物量、不同土壤质地和不同季节的生物结皮对土壤可蚀性(K值)的影响.结果表明： 生物结皮显著降低了土壤可蚀性,生物结皮层土壤可蚀性较下层土壤降低17%;土壤可蚀性随生物结皮生物量的增加呈降低趋势,藓结皮土壤可蚀性K值较藻结皮土壤降低21%;生物结皮土壤可蚀性在不同季节因其生物活性不同而存在差异,雨季中显著高于雨季前和雨季末;不同质地土壤上生物结皮对可蚀性的影响不同,可蚀性K值为砂壤>粉壤>砂土;模拟降雨条件下测定表明,生物结皮的发育使土壤可蚀性较对照（下层5~10 cm土壤）降低约90%.
     

黄土丘陵区生物结皮对土壤可蚀性的影响

在采样分析生物结皮对土壤理化属性影响的基础上,采用EPIC模型估算与模拟降雨试验相结合的方法,研究了黄土丘陵区不同生物量、不同土壤质地和不同季节的生物结皮对土壤可蚀性(K值)的影响.结果表明： 生物结皮显著降低了土壤可蚀性,生物结皮层土壤可蚀性较下层土壤降低17%;土壤可蚀性随生物结皮生物量的增加呈降低趋势,藓结皮土壤可蚀性K值较藻结皮土壤降低21%;生物结皮土壤可蚀性在不同季节因其生物活性不同而存在差异,雨季中显著高于雨季前和雨季末;不同质地土壤上生物结皮对可蚀性的影响不同,可蚀性K值为砂壤>粉壤>砂土;模拟降雨条件下测定表明,生物结皮的发育使土壤可蚀性较对照（下层5~10 cm土壤）降低约90%.
     

植物、土壤及土壤管理对土壤微生物群落结构的影响

土壤微生物是土壤生态系统的重要组成部分,对土壤微生物群落结构多样性的研究是近年来土壤生态学研究的热点。本文综述了有关植物、土壤类型以及土壤管理措施对土壤微生物群落结构影响的最新研究结果,指出植物的作用因植物群落结构多样性、植物种类、同种植物不同的基因型,甚至同一植物不同根的区域而异;而土壤的作用与土壤质地和有机质含量等因素有关;植物和土壤类型在对土壤微生物群落结构影响上的作用存在互作关系。不同的土壤管理措施对土壤微生物群落结构影响较大,长期连作、大量的外援化学物质的应用降低了土壤微生物的多样性;而施用有机肥、免耕可以增加土壤微生物群落结构多样性,有利于维持土壤生态系统的功能。     

Effect of soil moisture on bioremediation of chlorophenol-contaminated soil

A chlorophenol-contaminated soil was tested for the biodegradability in a semi-pilot scale microcosm using indigenous microorganisms. More than 90% of 4-chlorophenol and 2,4,6-trichlorophenol, initially at 30 mg kg^–1, were removed within 60 days and 30 mg pentachlorophenol kg^–1 was completely degraded within 140 days. The chlorophenols were degraded more effectively under aerobic condition than under anaerobic condition. Soil moisture had a significant effect with the slowest degradation rate of chlorophenols at 25% in the range of 10–40% moisture content. At 25–40%, the rate of chlorophenol degradation was directly related to the soil moisture content, whereas at 10–25%, it was inversely related. Limited oxygen availability through soil agglomeration at 25% moisture content might decrease the degradation rate of chlorophenols.     

耕作方式对潮土土壤团聚体微生物群落结构的影响

为探究不同耕作方式对潮土土壤团聚体微生物群落结构和多样性的影响,采用磷脂脂肪酸(PLFA)法测定了土壤团聚体中微生物群落。试验设置4个耕作处理,分别为旋耕+秸秆还田(RT)、深耕+秸秆还田(DP)、深松+秸秆还田(SS)和免耕+秸秆还田(NT)。结果表明:与RT相比,DP处理显著提高了原状土壤和>5 mm粒级土壤团聚体中真菌PLFAs量和真菌/细菌,为真菌的繁殖提供了有利条件,有助于土壤有机质的贮存,提高了土壤生态系统的缓冲能力;提高了5～2 mm粒级土壤团聚体中细菌PLFAs量,降低了土壤革兰氏阳性菌/革兰氏阴性菌,改善了土壤营养状况;提高了<0.25 mm粒级土壤团聚体中微生物丰富度指数。总的来说,深耕+秸秆还田(DP)对土壤团聚体细菌和真菌生物量有一定的提高作用,并且在一定程度上改善了土壤团聚体微生物群落结构,有利于增加土壤固碳能力和保持土壤微生物多样性。冗余分析结果表明,土壤团聚体总PLFAs量、细菌、革兰氏阴性菌和放线菌PLFAs量与土壤有机碳相关性较强,革兰氏阳性菌PLFAs量与总氮相关性较强。各处理较大粒级土壤团聚体微生物群落主要受碳氮比、含水量、pH值和团聚体质量分数的影响,较小粒级土壤团聚体微生物群落则主要受土壤有机碳和总氮的影响。     

Estimating respiration of roots in soil: Interactions with soil CO2, soil temperature and soil water content

Little information is available on the variability of the dynamics of the actual and observed root respiration rate in relation to abiotic factors. In this study, we describe I) interactions between soil CO₂ concentration, temperature, soil water content and root respiration, and II) the effect of short-term fluctuations of these three environmental factors on the relation between actual and observed root respiration rates. We designed an automated, open, gas-exchange system that allows continuous measurements on 12 chambers with intact roots in soil. By using three distinct chamber designs with each a different path for the air flow, we were able to measure root respiration over a 50-fold range of soil CO₂ concentrations (400 to 25000 ppm) and to separate the effect of irrigation on observed vs. actual root respiration rate. All respiration measurements were made on one-year-old citrus seedlings in sterilized sandy soil with minimal organic material.Root respiration was strongly affected by diurnal fluctuations in temperature (Q₁₀ = 2), which agrees well with the literature. In contrast to earlier findings for Douglas-fir (Qi et al., 1994), root respiration rates of citrus were not affected by soil CO₂ concentrations (400 to 25000 ppm CO₂; pH around 6). Soil CO₂ was strongly affected by soil water content but not by respiration measurements, unless the air flow for root respiration measurements was directed through the soil. The latter method of measuring root respiration reduced soil CO₂ concentration to that of incoming air. Irrigation caused a temporary reduction in CO₂ diffusion, decreasing the observed respiration rates obtained by techniques that depended on diffusion. This apparent drop in respiration rate did not occur if the air flow was directed through the soil. Our dynamic data are used to indicate the optimal method of measuring root respiration in soil, in relation to the objectives and limitations of the experimental conditions.     

生物质炭对水稻土团聚体微生物多样性的影响

生物质炭施用对土壤微生物群落结构的影响已有报道,但土壤团聚体粒组中微生物群落对生物质炭施用的响应的研究还相对不足。以施用玉米秸秆生物质炭两年后的水稻土为对象,采用团聚体湿筛法,通过高通量测序对土壤团聚体的微生物群落结构与多样性进行分析,结果表明:(1)与对照相比,生物质炭施用显著促进了大团聚体(2000—250μm)的形成,并提高了团聚体的稳定性。(2)不同粒径团聚体间微生物相对丰度存在显著差异。在未施生物质炭的处理(C0)中,随着团聚体粒径增大,变形菌门、子囊菌门、β-变形杆菌目、格孢腔菌目的相对丰度逐渐降低,而酸杆菌门、担子菌门、粘球菌目、类球囊霉目的相对丰度逐渐升高。(3)生物质炭施用显著改变了团聚体间的微生物群落结构。与C0处理相比,生物质炭施用处理的大团聚体中变形菌门、鞭毛菌门和β-变形杆菌目的相对丰度分别显著提高了14.37%、33.28%和33.82%;微团聚体(250—53μm)中酸杆菌门、子囊菌门和粘球菌目的相对丰度分别显著降低了20.15%、19.93%和17.66%;粉、黏粒组分(53μm)中担子菌门的相对丰度升高90.25%,而子囊菌门和鞭毛菌门的相对丰度分别降低12.15%和12.58%。由此可见,生物质炭不仅改变土壤团聚体组成和分布,同时伴随着土壤微生物群落结构的改变。     

Available soil nitrogen in relation to fractions of soil nitrogen and other soil properties

The available N of 27 soils from England and Wales was assessed from the amounts of N taken up over a 6-month period by perennial ryegrass grown in pots under uniform environmental conditions. Relationships between availability and the distribution of soil N amongst various fractions were then examined using multiple regression. The relationship: available soil N (mg kg^–1 dry soil)=(N_min×0.672)+(N_inc×0.840)+(N_mom×0.227)–5.12 was found to account for 91% of the variance in available soil N, where N_min=mineral N, N_inc=N mineralized on incubation and N_mom=N in macro-organic matter. The N mineralized on incubation appeared to be derived largely from sources other than the macro-organic matter because these two fractions were poorly correlated. When availability was expressed in terms of available organic N as % of soil organic N (N_ao) the closest relationship with other soil characteristics was: N_ao=[N_inc×(1.395–0.0347×CN_mom]+[N_mom×0.1416], where CN_mom=CN ratio of the macro-organic matter. This relationship accounted for 81% of the variance in the availability of the soil organic N.The conclusion that the macro-organic matter may contribute substantially to the available N was confirmed by a subsidiary experiment in which the macro-organic fraction was separated from about 20 kg of a grassland soil. The uptake of N by ryegrass was then assessed on two subsamples of this soil, one without the macro-organic matter and the other with this fraction returned: uptake was appreciably increased by the macro-organic matter.     

The porosity of soil aggregates from bulk soil and from soil adhering to roots

Summary Total porosity and pore-size distribution (p.s.d.) were determined in soil aggregates taken in plots planted with maize and treated with farmyard manure and three rates of compost. Soil aggregates were collected from the soil adherent to the maize roots (root soil aggregates) and from bulk soil (bulk soil aggregates). Mercury intrusion porosimetry was used to evaluate the total porosity and the p.s.d. Treatments did not affect the total porosity of the bulk soil aggregates. The same was observed for the root soil aggregates. However the total porosity of the root soil aggregates was always lower than that of the bulk soil aggregates. The loss of total porosity was found to be due to a decrease in the percentage of larger pores with respect to the total.     

Temporal dynamics of soil bacterial network regulate soil resistomes

Soil bacteria are diverse and form complicated ecological networks through various microbial interactions, which play important roles in soil multi-functionality. However, the seasonal effects on the bacterial network, especially the relationship between bacterial network topological features and soil resistomes remains underexplored, which impedes our ability to unveil the mechanisms of the temporal-dynamics of antibiotic resistance genes (ARGs). Here, a field investigation was conducted across four seasons at the watershed scale. We observed significant seasonal variation in bacterial networks, with lower complexity and stability in autumn, and a wider bacterial community niche in summer. Similar to bacterial communities, the co-occurrence networks among ARGs also shift with seasonal change, particularly with respect to the topological features of the node degree, which on average was higher in summer than in the other seasons. Furthermore, the nodes with higher betweenness, stress, degree, and closeness centrality in the bacterial network showed strong relationships with the 10 major classes of ARGs. These findings highlighted the changes in the topological properties of bacterial networks that could further alter antibiotic resistance in soil. Together, our results reveal the temporal dynamics of bacterial ecological networks at the watershed scale, and provide new insights into antibiotic resistance management under environmental changes.     

The breakdown of malathion in soil and soil components

The disappearance of the organophosphorus insecticide, malathion, from a silt loam soil and from its organic and inorganic components was examined. Half-lives and the time taken for 90% decomposition in nonsterile, sodium azide-treated, and 2.5 Mrad-irradiated soils were similar (3/4–1 1/2 days and 4–6 days, respectively) but breakdown in autoclaved soils was negligible. Decay in nonsterile sand, silt, and clay minus organic matter fractions was 3–6 times slower than that recorded in the original soil. Breakdown of malathion in the clay plus organic matter fraction (organo-mineral complex) was rapid (half-life, 1 day), as was the case in the separated organic matter (half-life, 1 3/4 days). Filter-sterilized organic matter was not as effective in catalyzing the breakdown of malathion (half-life, 4 days), and no loss occurred from any of the autoclaved components. Irradiation doses of 2.5 and 5.0 Mrad had little influence on the ability of soil to degrade malathion. Thereafter, increases up to 20 Mrad had a more drastic, though far from totally inhibitory, effect. Our results suggest that either the colloidal organic matter itself, or a fraction associated with it, is the most important single factor concerned with the rapid breakdown of malathion in the soil studied. Direct microbial metabolism is a slower process and may have a significant role in malathion disappearance in coarsetextured soils low in colloidal organic matter. The catalytic component of the organic matter is suggested to be a stable exoenzyme and is supportive of reports by other workers. The quantitative effect of organo-mineral complex (containing the active degradative ingredient) additions to sand and silt fractions on the rate of subsequent malathion decay is also described.