施用控释氮肥对稻田土壤微生物生物量碳、氮的影响

借助农业部望城红壤水稻土生态环境野外观测试验站的控释氮肥试验,研究了施用控释氮肥对水稻不同生育期间稻田土壤微生物生物量碳、氮动态变化的影响。试验共设5个处理:①CK,(不施氮肥);②Urea(施用尿素);③CRNF(施用与处理②等氮量的控释氮肥);④70%CRNF(施用控释氮肥,用氮量为处理②的70%);⑤50%CRNF+M(施用控释氮肥和猪粪,总氮量为处理②的70%,其中控释氮肥用量为处理②的50%,猪粪含氮量为处理②的20%)。结果表明,施肥后10 d,施氮处理土壤微生物生物量碳和氮均达最高,随生育进程推进逐渐下降,成熟期有一定的回升;施肥初期,施用等氮量的控释氮肥处理(CRNF)土壤微生物量碳、氮含量较尿素处理(Urea)分别增加5.4%和22.5%,而水稻生育中后期,控释氮肥处理(CRNF)土壤微生物量碳、氮含量较尿素处理(Urea)下降幅度大,该处理向地上部提供氮素营养较尿素处理高;施氮量较高的CRNF处理,土壤微生物生物量碳低于控释氮肥节氮处理(70%CRNF),但在大多数取样时期,土壤微生物量氮高于控释氮肥节氮处理(70%CRNF);控释氮肥配施有机肥的节氮处理较其他单施化肥处理显著增加土壤微生物生物量碳、氮含量。控释氮肥与有机肥配施,不仅能节约氮肥用量,而且能明显地提高土壤微生物生物量碳、氮的含量。     

生物质炭配施有机物料对贫瘠红壤酶活性和微生物碳源代谢功能的影响

为进一步促进红壤固碳培肥,于2017和2018年通过田间试验研究了两种有机物料(玉米秸秆和羊粪)单施以及与生物质炭配施对贫瘠红壤养分含量、碳转化相关酶活性和微生物底物利用速率的影响。试验设置6个处理,即不施有机物料(对照)、玉米秸秆、羊粪、单施生物质炭、玉米秸秆与生物质炭配施、羊粪与生物质炭配施。结果表明:与对照相比,有机物料施用显著增加了土壤pH值、有机碳、全氮、有效磷和速效钾含量;与单施秸秆和羊粪相比,生物质炭与秸秆或羊粪配施显著增加了土壤有机碳、速效钾和碱解氮含量,但两者无交互效应。与对照相比,有机物料施用显著提高了β-葡萄糖苷酶(BG)、纤维二糖水解酶(CB)、β-木聚糖苷酶(XYL)和过氧化物酶(PERO)活性;与单施秸秆相比,生物质炭与秸秆配施处理酚氧化酶、过氧化物酶活性分别显著降低了28.6%、22.2%;与单施羊粪相比,生物质炭与羊粪配施处理α-葡萄糖苷酶(AG)、BG、XYL和PERO活性分别显著降低了46.1%、50.9%、41.6%和31.3%。与对照相比,有机物料施用显著提高了土壤基础呼吸和微生物对碳水化合物的利用速率,而生物质炭配施处理对碳水化合物、羧酸类底物的利用速率存在显著抑制作用。微生物碳源利用速率与BG和PERO活性呈显著正相关。因此,有机物料与生物质炭配施更有利于提高土壤养分含量,降低有机碳分解酶和微生物碳源代谢活性,从而促进红壤固碳培肥,有利于贫瘠红壤的地力提升。     

紫色土坡耕地C、P与微生物生物量C、P对不同施肥的响应

分析了长期不同施肥处理下紫色土坡耕地土壤碳(C)、磷(P)与微生物生物量C(MBC)、P(MBP)的变化及其耦合特征.结果表明:农家肥或秸秆配施无机肥(包括N、NP和NPK)处理下紫色土坡耕地土壤总有机碳(TOC)变化范围为90.8~100.8 g·kg-1,高于单施氮肥处理的62.2 g·kg-1;农家肥与无机肥配施处理下土壤总磷(TP)变化范围为0.65~0.84g·kg-1,而秸秆与无机肥配施处理下土壤TP仅为农家肥与无机肥配施的23%~38%;单施氮肥处理下MBP显著小于农家肥和秸秆与无机肥配施处理;农家肥和秸秆配施无机肥处理下MBC/MBP在5~26,TOC/TP分别在92~137和296~653,而单一氮肥处理下MBC/MBP和TOC/TP分别高达59和2000.表明农家肥和无机肥配施更有利于增加土壤P的有效性,提高土壤供P潜力.     

长期施用有机物料下黑土氮素有效性及其与作物产量的关系

利用长期定位施肥试验,于2011年采样研究长期施用不同有机物料(秸秆、有机肥)对东北黑土氮素有效性的影响,并分析氮供应与作物生物量及产量的关系.结果表明:化肥配施有机肥、化肥配施低量秸秆均可显著提高黑土农田土壤无机氮和硝态氮含量,进而提高作物产量;尽管化肥配施高量秸秆提高了土壤氮矿化速率,但可能由于硝态氮淋溶或者其他形式的氮素损失或转化,与化肥配施低量秸秆处理相比,化肥配施高量秸秆下土壤无机氮量并没有增加.可见,适量施用有机物料对提高土壤氮素有效性、提高作物产量及保护环境具有重要意义.     

长期施肥对红壤微生物生物量碳氮和微生物碳源利用的影响

采集湖南省祁阳县红壤长期定位施肥19年的土壤样品,分析长期不同施肥红壤的微生物生物量碳、氮和微生物碳源利用率,以揭示长期施肥对红壤微生物学性状的影响.结果表明：施肥19年后,有机肥单施或与化肥配合施用均显著提高土壤微生物生物量碳、氮和微生物碳源利用率.单施有机肥的土壤微生物生物量碳、氮含量分别为231和81 mg·kg^-1,化肥有机肥配施分别为148和73 mg·kg^-1,均显著高于化肥配施秸秆、不施肥和单施化肥;施用有机肥和化肥配施秸秆的土壤微生物生物量氮占全氮的比例平均为6.0%,显著高于单施化肥和不施肥.Biolog-ECO分析中,平均吸光值(AWCD)的大小为：化肥有机肥配施、单施有机肥>对照>单施化肥、化肥配施秸秆.单施有机肥或与化肥有机肥配施增加了红壤微生物对碳水化合物、羧酸、氨基酸、聚合物、酚类和胺类的碳源利用率;化肥配施有机肥的红壤微生物对聚合物类碳源利用率最高,化肥配施秸秆的红壤微生物对碳水化合物类碳源的利用率最高.表明施用有机肥能显著提高红壤的微生物生物量碳、氮和微生物碳源利用率,提高红壤肥力,保持作物高产.     

不同秸秆还田和耕作方式对夏玉米农田土壤呼吸及微生物活性的影响

采用基质诱导呼吸法和CO2释放量法,研究了冬小麦季长期不同耕作方式(常规翻耕、免耕和深松)和秸秆处理(秸秆还田和无秸秆还田)对夏玉米田土壤呼吸及微生物活性的影响.结果表明:秸秆还田和保护性耕作主要在O～ 10 cm土层起作用.秸秆还田能明显提高土壤微生物生物量碳和微生物活性,降低呼吸熵,在苗期和开花期提高土壤呼吸,而在灌浆期、腊熟期和收获期降低土壤呼吸;在相同秸秆处理条件下,深松和免耕比常规翻耕能显著降低土壤呼吸和呼吸熵,提高微生物生物量碳和微生物活性.整个生育期,秸秆还田结合保护性耕作能显著提高微生物生物量碳和微生物活性,降低呼吸熵,与常规翻耕无秸秆还田相比,深松秸秆还田和免耕秸秆还田0～10 cm土层微生物生物量碳平均提高了95.8％和74.3％,微生物活性提高了97.1％和74.2％.     

氮肥对玉米生长季土壤呼吸的影响

在玉米生长季,采用温室盆栽试验,利用分根箱法和根去除法,研究了氮肥对土壤呼吸、土壤基础呼吸、根系呼吸和根际微生物呼吸的影响.试验设4个处理:不种植玉米不施氮肥(CKO)、不种植玉米施氮肥(CKN)、种植玉米不施氮肥(MO)和种植玉米施氮肥(MN).结果表明:不种植玉米处理(CKN和CKO)土壤呼吸速率(土壤基础呼吸)为13.41～77.27 mg C·m-2·h-1,施用氮肥对土壤基础呼吸没有显著影响;种植玉米条件下,施氮处理(MN)的平均土壤呼吸速率为138.54 mg C·m-2·h-1,显著高于不施氮处理(MO),增幅达17.7%,尤其在玉米的抽穗期和开花期增幅明显.施氮肥处理土壤基础呼吸、根系呼吸和根际微生物呼吸对土壤呼吸的贡献率分别为36.2%、45.9%和17.9%,而不施氮肥处理分别为35.5%、36.9%和37.6%.     

滴灌条件下秸秆还田配施氮肥对宁夏扬黄灌区春玉米产量和土壤理化性质的影响

围绕宁夏扬黄灌区土壤质地黏重、养分匮乏等导致作物产量低的问题,于2017、2018年在秸秆全量粉碎还田(12000 kg·hm^-2)的同时配施4个氮肥施用水平(0、150、300、450 kg N·hm^-2),以秸秆不还田常规施氮(225 kg N·hm^-2)为对照,研究秸秆还田配施不同量氮肥对滴灌玉米田土壤理化性状和产量的影响.结果表明: 与秸秆不还田处理相比,秸秆还田配施氮肥300和450 kg·hm^-2处理0~20 cm土层平均土壤容重分别降低3.3%和5.4%,土壤孔隙度分别增加3.7%和7.1%;秸秆还田配施氮肥对土壤养分和玉米产量的提升效果显著,其中以秸秆还田配施氮肥300和450 kg·hm^-2表现最佳;秸秆还田配施氮肥可显著增加土壤贮水量和产量,秸秆还田配施氮肥300 kg·hm^-2处理在2017和2018年土壤贮水量最高可分别增加13.6%和22.1%,产量分别增加31.1%和46.0%.分析产量构成因素发现,秸秆还田配施氮肥处理主要是通过增加穗粒数和百粒重而达到高产.对玉米产量-配施纯氮量关系进行曲线拟合发现,该地区秸秆全量还田配施氮肥的最佳用量为260 kg·hm^-2.研究结果可为该地区土壤培肥和滴灌玉米生产提供重要依据.     

不同秸秆还田量和氮肥配施对玉米田土壤CO2排放的影响

探讨不同秸秆还田量和氮肥量配施对辽西北半干旱区玉米田土壤CO₂排放的影响,可为固碳减排和黑土地保护计划的实施提供理论支撑。本试验主区设置3个秸秆还田水平,分别为3000(S₁)、6000(S₂)和9000 kg·hm^-2(S₃,秸秆全量还田);副区设置3个氮肥施用水平,分别为105(N₁)、210(N₂,常规施氮量)和420 kg N·hm^-2(N₃),另设置不施氮肥不添加秸秆的对照处理(CK),共10个处理。采集定位试验4年后玉米田间土壤,通过培养试验,探究不同处理对玉米田土壤CO₂排放的影响及CO₂排放与土壤溶解性有机碳(DOC)和微生物生物量碳(MBC)的关系。结果表明: 秸秆还田和氮肥施用均会促进玉米田土壤CO₂排放,并随秸秆还田量和施氮量的增加而显著增加,其中氮肥施用是促进玉米田土壤CO₂排放的最主要因素;秸秆还田与氮肥配施通过促进微生物生物量增加并加剧DOC消耗来促进玉米田土壤CO₂排放;MBC和DOC含量显著刺激玉米田土壤CO₂排放,且主要受两者培养前期含量的影响。从保障秸秆还田培肥地力同时减少CO₂排放的角度考虑,210 kg N·hm^-2常规施氮量与6000 kg·hm^-2秸秆还田配合施用(N₂S₂)是本试验条件下辽西北半干旱区最有潜力的田间施肥模式。     

玉米与蚕豆秸秆配施对秸秆分解及土壤养分含量的影响

采用室内培养试验研究了禾本科作物玉米秸秆和豆科作物蚕豆秸秆单施及其不同比例配施后的秸秆分解及土壤养分含量.结果表明：单施玉米秸秆,土壤有机碳的矿化量和秸秆有机碳的矿化速率都较低,土壤矿质态氮被固持的时间也最长;玉米秸秆与蚕豆秸秆配合施用促进了秸秆有机碳和土壤固持矿质态氮的矿化.两种秸秆单施和配施均显著增加土壤微生物生物量碳、氮含量.禾本科作物秸秆与豆科作物秸秆配合施用,可以加快秸秆的分解,协调养分供应.     

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。     

Linking soil bacterial biodiversity and soil carbon stability

Native soil carbon (C) can be lost in response to fresh C inputs, a phenomenon observed for decades yet still not understood. Using dual-stable isotope probing, we show that changes in the diversity and composition of two functional bacterial groups occur with this ‘priming'' effect. A single-substrate pulse suppressed native soil C loss and reduced bacterial diversity, whereas repeated substrate pulses stimulated native soil C loss and increased diversity. Increased diversity after repeated C amendments contrasts with resource competition theory, and may be explained by increased predation as evidenced by a decrease in bacterial 16S rRNA gene copies. Our results suggest that biodiversity and composition of the soil microbial community change in concert with its functioning, with consequences for native soil C stability.Substrate inputs can stimulate decomposition of native soil organic carbon (SOC; Kuzyakov et al., 2000), a phenomenon known as the ‘priming effect'' (Kuzyakov, 2010), and is considered large enough to influence ecosystem C balance (Wieder et al., 2013). Two functionally distinct groups of microorganisms are postulated to mediate priming: one that grows rapidly utilizing labile C, and one that grows slowly, breaking down recalcitrant SOC (Fontaine et al., 2003; Blagodatskaya et al., 2007). However, distinguishing these groups is technically challenging. Here, we used dual-stable isotope probing with ¹³C-glucose and ¹⁸O-water to identify bacteria in these two groups growing in response to single and repeated pulses of glucose. Organisms that utilize labile C for growth assimilate both ¹³C-glucose and ¹⁸O-water into their DNA, whereas organisms that grow using SOC incorporate only ¹⁸O-water. Differential isotope incorporation leads to a range of DNA densities separable through isopycnic centrifugation, which can then be characterized by sequencing (Radajewski et al., 2000).We sequenced fragments of bacterial 16S rRNA genes following single and repeated glucose pulses. We hypothesized that the single pulse of labile C would stimulate growth of opportunistic organisms, thus immobilizing nutrients and suppressing growth and diversity of the SOC-utilizing community, decreasing SOC decomposition (negative priming), a response analogous to that observed in plant communities in response to chronic N additions (Tilman, 1987; Clark and Tilman, 2008). We hypothesized that multiple glucose additions would stimulate growth of a more diverse bacterial community, including more native SOC-utilizing organisms that possess enzymes to decompose recalcitrant compounds, causing positive priming (Fontaine et al., 2003; Kuzyakov, 2010).Soil from a ponderosa pine ecosystem was amended weekly for 7 weeks with 500 μg C-glucose per gram soil (2.65 atom % ¹³C) in 100 μl deionized water or with 100 μl deionized water (n=5). Measurements of δ¹³C–CO₂ and [CO₂] enabled the partitioning of CO₂ into that derived from added glucose or from native SOC (C_SOC):where C_total is CO₂–C from glucose-amended samples, δ_total is the δ¹³C–CO₂ from glucose-amended samples, δ_glucose is the δ¹³C of the added glucose and δ_SOC is the δ¹³C–CO₂ evolved from the non-amended samples. Priming was calculated as the difference between SOC oxidation of the amended and non-amended samples. With this approach, any evolved CO₂ carrying the ¹³C signature of the added glucose is considered respiration of glucose, including ¹³C-labeled biomass and metabolites derived from prior glucose additions. Thus, this approach quantifies priming as the oxidation of SOC present at the beginning of the experiment, consistent with many other studies of priming (Cheng et al., 2003; De Graaff et al., 2010).In a parallel incubation for dual-stable isotope probing, the repeated-pulse samples received unlabeled glucose (500 μg C-glucose per gram soil) for 6 weeks while the non-amended and single-pulse samples received sterile deionized water. In week 7, samples received one of four isotope treatments (n=3): 97 atom % H₂ ¹⁸O (non-amended soil), 99 atom % ¹³C-glucose and 97 atom % H₂ ¹⁸O (single- and repeated-pulse soil), ¹²C-glucose and 97 atom % H₂ ¹⁸O (repeated-pulse soil) or ¹²C-glucose and H₂ ¹⁶O (repeated-pulse soil). After incubating for 7 days, soil was frozen at −40 °C. DNA was extracted, separated through isopycnic centrifugation, and two density ranges were sequenced for the bacterial 16S rRNA gene (Supplementary Figure 1): 1.731–1.746 g ml⁻¹ (hereafter called the SOC-utilizing community) and 1.759–1.774 g ml⁻¹ (hereafter called the glucose-utilizing community).Amplicons of the V3–V6 16S rRNA region were bar coded with broad-coverage fusion PCR primers and pooled before sequencing on a Genome Sequencer FLX instrument. These sequence data have been submitted to the GenBank database under accession number SRP043371. Data were checked for chimeras (Edgar et al., 2011), demultiplexed and quality checked (Caporaso et al., 2010). Taxonomy was assigned to genus at the ⩾80% bootstrap confidence level (Cole et al., 2009).We used the Shannon''s diversity index (H′), commonly used in microbial systems (Fierer and Jackson, 2006), to assess changes in microbial diversity. Analysis of variance was used to compare the amount of DNA within densities between isotope treatments (Supplementary Figure 2) and to test the effects of the treatments on the Shannon''s diversity (Figure 2) and Pielou''s evenness (Supplementary Figure 3) of the active bacterial communities, with post hoc Student''s t-tests, α=0.05. PRIMER 6 and PERMANOVA were used to create the nonmetric multidimensional scaling ordination and to compare bacterial communities between glucose treatments and the two sequenced density ranges.The single pulse of glucose suppressed SOC oxidation, whereas repeated pulses increased SOC oxidation (Figure 1). Few experiments to date have examined priming in response to repeated substrate amendments (Hamer and Marschner, 2005; Qiao et al., 2014), even though in nature soil receives repeated substrate pulses from litterfall and rhizodeposition. Our results demonstrate the dynamic response of SOC decomposition to repeated labile C inputs.Open in a separate windowFigure 1Weekly priming rates calculated as the difference in SOC respired between glucose-amended and non-amended soil (n=5).Dual-stable isotope probing was able to separate the growing bacteria into two groups with distinct DNA densities (P<0.001, PERMANOVA; Figure 3a), indicating differential uptake of ¹³C-glucose and ¹⁸O-water. In response to the initial glucose addition, the diversity of the growing glucose- and SOC-utilizing bacterial communities declined compared with the non-amended community (P<0.001, t-tests; Figure 2), driven by a strong decrease in evenness (Supplementary Figure 3). In the SOC-utilizing community, where DNA was labeled with ¹⁸O only, the relative abundance of Bacillus increased 4.9-fold compared with the non-amended control to constitute 31.6% of the community (Figure 3b). Bacillus survives well under low-nutrient conditions (Panikov, 1995), and is able to synthesize a suite of extracellular enzymes capable of degrading complex substrates (Priest, 1977), traits that are conducive for using SOC for growth. In the glucose-utilizing community, where DNA was labeled with both ¹³C and ¹⁸O, Arthrobacter increased 67.7-fold relative to the non-amended control to constitute 75.5% of the growing bacteria (Figure 3b). In culture experiments, Arthrobacter can rapidly take up and store glucose for later use (Panikov, 1995) and here we find it dominating the high-density DNA fractions, signifying that it is using the labeled glucose to grow. The increased biomass of Arthrobacter may have resulted in greater resource competition, thus reducing the diversity of the growing community, as is frequently found in plant communities (Bakelaar and Odum, 1978; Clark and Tilman, 2008).Open in a separate windowFigure 2Shannon''s diversity index (H′) of the non-amended, single-pulse, and repeated-pulse treatments (n=3) in the SOC- (mid-density) and glucose-utilizing (high-density) communities. Treatments with the same letter are not significantly different from each other (Student''s t, α=0.05).Open in a separate windowFigure 3(a) Nonmetric multidimensional scaling ordination showing differences in growing bacterial communities at the genus taxonomic level in the SOC-utilizing (mid-density; open symbols) and glucose-utilizing (high-density; closed symbols) groups of non-amended (Δ), single-pulse (○) and repeated-pulse (□) treatments (n=3). (b) Pie charts of genera in the SOC- and glucose-utilizing communities of the single- and repeated-pulse treatments (n=3). Genera with relative abundances >5% are listed in the figure legend.After repeated glucose amendments, the diversity of the growing community recovered to non-amendment levels (Figure 2) without strongly dominant organisms (Figure 3b and Supplementary Figure 3). The higher diversity found after repeated glucose pulses may be explained by trophic interactions where predators graze on prey populations that have been enlarged by resource addition, suppressing competition between prey species and causing secondary mobilization of nutrients (Clarholm, 1985). The decrease in total bacterial 16S rRNA gene copies in the repeated-pulse—compared with the single-pulse—treatment (Supplementary Figure 4) supports predation as a potential mechanism explaining the observed diversity increase after repeated glucose pulses.The recovery of diversity after repeated glucose pulses contrasts with resource competition theory (Tilman, 1987). When chronic additions of a limiting resource are applied, species diversity and evenness typically decrease (Bakelaar and Odum, 1978; Clark and Tilman, 2008) because competitive organisms become dominant. We observed this after the single glucose pulse, but not after repeated pulses. This diversity response may be the result of community shifts facilitated by short bacterial life cycles and the tens to hundreds of generations expected during the 7-week incubation (Behera and Wagner, 1974). In contrast, systems on which most ecological theory is based (for example, plants) might achieve perhaps 20 generations in a multi-decadal field experiment (Bakelaar and Odum, 1978; Clark and Tilman, 2008). With more generations, more community dynamics can occur, including increased resource cascades in which extracellular enzymes, metabolites or lysed cells of one functional group increase substrates for another (Blagodatskaya and Kuzyakov, 2008). Our results highlight the opportunity to test ecological theories in microbial ecosystems (Prosser et al., 2007), particularly as the short life cycles of microbes makes them well suited for pursuing ecological questions in an evolutionary framework (Jessup et al., 2004).The priming effect is ubiquitous, yet its drivers remain elusive. Our results suggest that changes in the diversity and composition of the growing bacterial community contribute to priming, and thus that ecosystem properties such as soil C storage may be sensitive to soil microbial biodiversity.     

Distinct impacts of reductive soil disinfestation and chemical soil disinfestation on soil fungal communities and memberships

Applied Microbiology and Biotechnology - Soil disinfestation is an important agricultural practice to conquer soil-borne diseases and thereby ensure crop productivity. Reductive soil disinfestation...     

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.     

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.     

Crop diversification reinforces soil microbiome functions and soil health

Plant and Soil - Intensive agriculture with continuous monocropping and massive chemical inputs has adversely affected belowground microbial composition and functions, resulting in soil sickness...     

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.     

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.     

Harvester ant nests, soil biota and soil chemistry

Many ant species accumulate organic debris in the vicinity of their nests. These organic materials should provide a rich resource base for the soil biota. We examined the effect of harvester ant nests (Pogonomyrmex barbatus) on the soil community and soil chemistry. Ant nest soils supported 30-fold higher densities of microarthropods and 5-fold higher densities of protozoa than surrounding, control soils. The relative abundances of the major groups of protozoa differed as well: amoebae and ciliates were relatively overrepresented, and flagellates underrepresented, in ant nest versus control soils. Densities of bacteria and fungi were similar in the two soil types. Concentrations of nitrate, ammonium, phosphorus, and potassium were significantly higher in ant nest soils, while concentrations of magnesium, calcium, and water were similar in nest and control soils. Ant nest soils were marginally more acidic than controls. The results demonstrate that P. barbatus nests constitute a significant source of spatial heterogeneity in soil biota and soil chemistry in arid grasslands. Received: 17 March 1997 / Accepted: 10 June 1997