长期施用生物有机肥土壤的氮矿化

1997年在中国农业大学曲周试验站设置了施用EM堆肥、传统堆肥、化肥和对照处理的田间试验,在2004年和2005年取样测定土壤的矿质氮和氮矿化率.结果表明:在小麦生长的前期矿质氮含量是:化肥处理 > EM堆肥处理 > 传统堆肥处理 > 对照;但是在施用化肥前和小麦生长的后期矿质氮含量是: EM堆肥处理 > 传统堆肥处理 > 化肥处理 > 对照.土壤的氮矿化率是:EM堆肥处理 > 传统堆肥处理 > 化肥处理 > 对照.小麦的产量是:EM堆肥处理显著高于传统堆肥处理,传统堆肥处理显著高于化肥处理,化肥处理显著高于对照.与传统堆肥相比,EM堆肥提高了土壤的矿质氮含量,提高了土壤的氮矿化率,增加了小麦产量.     

添加氮素对沙质草地土壤氮素有效性的影响

通过氮素添加(20g.m-2.a-1)试验,研究了科尔沁沙地东南部沙质草地生态系统土壤氮矿化及有效氮的季节变化。对2006年生长季的观测发现,添加氮素显著提高了沙质草地生长季土壤铵态氮、硝态氮、矿质氮的含量以及9月1日至10月15日的净氮矿化速率与硝化速率;添加氮素导致土壤有效氮的季节变异增大,净氮矿化(1.29～11.60mg.kg-1.30d-1)与硝化(-4.15～11.20mg.kg-1.30d-1)速率随时间呈上升趋势,铵态氮含量逐渐降低,硝态氮与矿质氮(6.49～20.66mg.kg-1)含量的变化呈"V"型,最小值出现在生物量生长高峰期的7月中旬。该沙质草地土壤氮的有效性较低,施氮肥可明显提高土壤供氮能力。     

长期施用化肥及秸秆还田对砂姜黑土细菌群落的影响

【目的】在施用化肥的基础上进行秸秆还田是提高砂姜黑土肥力的有效措施,以往的研究只注重秸秆还田对土壤结构、肥力等物理化学性状方面的研究,缺少施肥对砂姜黑土微生物群落影响的研究。本研究以安徽蒙城典型的砂姜黑土为研究对象,以期揭示长期施用化肥和秸秆还田对砂姜黑土细菌群落的影响。【方法】采用454高通量测序对砂姜黑土不同农业施肥措施下的细菌群落进行分析研究,并通过生物信息学的分析方法揭示影响砂姜黑土细菌群落的主要因素。【结果】通过对454高通量测序数据的分析,发现砂姜黑土主要的细菌门类为放线菌、变形菌、酸杆菌、绿弯菌和拟杆菌。长期施用化肥显著提高了砂姜黑土肥力和作物产量,但导致了细菌群落结构的显著变化和多样性的显著降低。秸秆还田有利于土壤肥力的进一步提高,但是并没有缓解长期施用化肥对土壤细菌群落产生的不利影响。分析发现土壤pH的变化是导致土壤细菌群落变异的主要因素。【结论】在施用化肥的基础上进行秸秆还田有利于砂姜黑土肥力的提升,然而并没有缓解由施肥导致的土壤酸化对土壤细菌群落组成和多样性产生的不利影响。这暗示秸秆还田可能并未对砂姜黑土微生物生态产生根本性的有益影响,对于秸秆农田的利用方式还需要进一步研究,以达到农业生产效益和生态效益的并重。     

牧鸡密度与取样时间对沙质草地土壤氮素有效性的影响

以科尔沁沙质草地为对象,设置5个牧鸡密度D1(10羽·200 m-2)、D2(10羽·400 m-2)、D3(10羽·600 m-2)、D4(10羽·800 m-2)、CK(0羽·200 m-2),研究牧鸡密度与取样时间对土壤无机氮、氮矿化与硝化、微生物生物量等的影响.结果表明:与对照相比,高牧鸡密度(D1、D2)显著增加了8月、10月土壤潜在净氮矿化、硝化速率,而低牧鸡密度(D3、D4)对土壤潜在净氮矿化、硝化速率的影响均不显著;牧鸡显著提高了土壤硝态氮含量、土壤潜在净氮矿化、硝化速率,对土壤铵态氮含量影响不显著;土壤无机氮含量、潜在净氮矿化、硝化速率对取样时间的响应均达到显著水平,对牧鸡密度与取样时间交互作用的响应不显著.牧鸡对土壤微生物生物量碳、氮及微生物生物量碳/氮的影响均不显著.研究认为,牧鸡可以增加土壤的供氮能力,且随牧鸡密度的增加呈上升趋势,最适牧鸡密度为250 ～500羽·hm-2.     

肥沃耕层构建对白浆土土壤肥力和玉米产量的影响

为探究肥沃耕层构建技术对白浆土土壤肥力和玉米产量的影响,本研究在黑龙江省富锦市的白浆土上进行了3年的定位试验,试验设置常规整地方法(T15)和肥沃耕层构建方法[秸秆深混还田(T35+S)、有机肥深混施用(T35+M)、秸秆与有机肥深混还田(T35+S+M)、深翻35 cm+秸秆深混+有机肥+化肥(T35+S+M+F)]共5个处理。结果表明：1)肥沃耕层构建处理均比常规对照显著增加了玉米产量,增幅15.4%～50.9%。2)与常规对照相比,肥沃耕层构建前两年对土壤pH值影响不显著,到第3年显著提高了耕层(0～15 cm)土壤pH值;T35+S+M+F、T35+S+M、T35+M处理显著提高了亚耕层(15～35 cm土层)土壤pH值,T35+S处理对亚耕层土壤pH值影响不显著。3)肥沃耕层构建处理(T35+S+M+F、T35+S+M、T35+M、T35+S)可提高耕层和亚耕层土壤养分含量,其中亚耕层土壤有机质、全氮、有效磷、碱解氮和速效钾含量分别显著提高了3.2%～46.6%、9.1%～51.8%、17.5%～130.1%、4.4%～62.8%、22.2%～68.7%,导致亚耕层肥力丰富度...     

秸秆还田与施肥对土壤酶活性和作物产量的影响

通过大田定位试验,在小麦-玉米轮作条件下,以小麦品种‘西农889’和玉米品种‘郑单958’为供试作物,采取不施肥秸秆不还田(CK)、秸秆还田(S)、秸秆还田+腐熟有机肥(SM)、秸秆还田+氮肥(SN)、秸秆还田+氮肥+磷肥(SNP)共5种处理,对不同处理下土壤电导率、蔗糖酶活性和脲酶活性的动态变化及作物产量进行了研究。结果显示:(1)秸秆还田后土壤的电导率变化呈先上升后下降趋势,不同处理间周年电导率平均值表现为SNP>SN>SM>S>CK,且差异显著。(2)秸秆还田配合施用氮肥处理的土壤蔗糖酶活性和脲酶活性最高,蔗糖酶活性最大值(70.62mg.g-1.d-1)为对照的1.36倍,脲酶活性最大值(3.58mg.g-1.d-1)比对照提高了9.15%。(3)土壤有机碳含量在S、SM处理之间差异不显著,而S、SM处理与CK、SN、SNP处理之间差异显著,SM处理比对照处理提高了8.91%。(4)土壤全氮含量在不同处理之间差异显著,并以SNP处理最高,其次是SM处理,S、SN处理再次之,且SNP、SM、S、SN处理土壤全氮含量分别比对照提高了19.8%、11.1%、9.88%和7.41%。(5)秸秆还田处理的作物产量显著高于CK,并以秸秆配施氮磷肥处理的小麦产量最高,比CK提高了50.6%;秸秆配施氮肥处理的玉米产量最高,比CK提高了34.3%。研究表明,秸秆还田配施有机肥、无机肥可以有效促进有机物矿质化,显著增加土壤养分含量,增强土壤酶活性,提高土壤有机碳含量,从而促进作物增产。     

南方地区秸秆还田对土壤综合肥力和作物产量的影响

以安徽、江西、湖南、湖北、四川、重庆、广西等省94个秸秆还田定位试验为基础,运用数值化理论综合评价我国南方地区秸秆还田的土壤肥力变化和培肥增产效应.结果表明:研究区土壤肥力综合指数(SFI)和作物产量具有明显的区域差异,但SFI整体以中等水平的三等地(该等级土地面积占研究区土地面积的69.1％)和较低水平的四等地(占21.3％)为主;秸秆还田后,SFI和产量较秸秆不还田平均提高了6.8％和4.4％;水稻产量与SFI之间呈极显著正相关.SFI可真实反映研究区土壤肥力综合水平.在当前农艺水平下,秸秆还田是有效提高我国南方地区土壤综合肥力的重要举措,应大面积推广秸秆还田腐熟.     

长期保护性耕作对黄土高原旱地土壤水分及作物叶水势的影响

通过长期定位试验研究了黄土高原西部旱农区,传统耕作方式和5种保护性耕作措施对豌豆-小麦(P→W)和小麦-豌豆(W→P)轮作序列的耕层土壤水分和作物叶水势的影响。结果表明,与传统耕作(T)处理相比,保护性耕作都能不同程度地提高0—30 cm土层土壤含水量,增幅为3.29%—28.67%,其中免耕+秸秆覆盖(NTS)处理的土壤含水量在整个生育期内均为最高。豌豆和春小麦在不同生育期,叶水势的日变化趋势大致相同,均为清晨6:00最高,然后随着时间的推移而下降,大约在12:00—14:00之间达到最低,随后逐渐回升。春小麦各处理在拔节期和抽穗期的叶水势相对较高,孕穗期和开花期次之,灌浆期最低;叶片相对含水量在拔节期和抽穗期最高,开花期次之,灌浆期最低。豌豆各处理的叶水势均在出苗期和孕蕾期达到了最大值,分枝和开花期结荚期次之,灌浆成熟期相对较低;叶片相对含水量均随生育期的进程而呈下降趋势。整个生育期春小麦和豌豆各处理10:00的叶水势与0—30 cm平均土壤含水量之间显著相关,当土壤水分含量较低时,春小麦和豌豆叶水势与耕层土壤含水量的相关性达极显著水平。与传统耕作(T)相比,免耕+秸秆覆盖(NTS)、免耕+地膜覆盖(NTP)、免耕(NT)、传统耕作+秸秆还田(TS)、地膜覆盖(TP)5种保护性耕作措施能不同程度的提高作物叶水势、叶片相对含水量和作物产量,其中免耕+秸秆覆盖(NTS)的优势最明显。     

耕作方式与秸秆还田对土壤微生物数量、酶活性及作物产量的影响

通过两年田间裂区设计试验,研究了不同土壤耕作方式(常规耕作、深耕、深松)与秸秆还田(秸秆还田、秸秆不还田)对冬小麦一夏玉米一年两熟农田土壤微生物数量、酶活性和作物产量的影响.结果表明:深松(耕)和秸秆还田不仅降低了土壤容重,提高了土壤有机碳含量,而且增加了土壤微生物数量、土壤酶活性和作物产量,且二者对夏玉米季的影响大于冬小麦季.与常规耕作+无秸秆还田相比,深耕+秸秆还田、深松+秸秆还田处理的20～30 cm土壤容重分别降低8.5％和6.6％,土壤有机碳含量分别提高14.8％和12.4％,土壤微生物数量、土壤酶活性分别提高45.9％、33.9％和34.1％、25.2％,作物产量分别提高18.0％和19.3％,且两处理间无显著差异.说明土壤深松(耕)结合秸秆还田有利于作物产量、土壤微生物数量和酶活性的提高.     

Nitrogen recoveries from organic amendments in crop and soil assessed by isotope techniques under tropical field conditions

The integration of multipurpose legumes into low-input tropical agricultural systems is needed because they are a nitrogen (N) input through symbiotic fixation. The drought-tolerant cover legume canavalia (Canavalia brasiliensis) has been introduced for use either as forage or as a green manure into the crop-livestock system of the Nicaraguan hillsides. To evaluate its impact on the subsequent maize crop, an in-depth study on N dynamics in the soil-plant system was conducted. Microplots were installed in a 6-year old field experiment with maize-canavalia rotation. Direct and indirect ¹⁵N-labelling techniques were used to determine N uptake by maize from canavalia residues and canavalia-fed cows?? manure compared to mineral fertilizer. Litter bags were used to determine the N release from canavalia residues. The incorporation of N from the amendment into different soil N pools (total N, mineral N, microbial biomass) was followed during the maize cropping season. Maize took up an average of 13.3 g?N?m^?2, within which 1.0 g?N?m^?2 was from canavalia residues and 2.6 g?N?m^?2 was from mineral fertilizer, corresponding to an amendment N recovery of 12% and 32%, respectively. Recoveries in maize would probably be higher at a site with lower soil available N content. Most of the amendment N remained in the soil. Mineral N and microbial N were composed mainly of N derived from the soil. Combined total ¹⁵N recovery in maize and soil at harvest was highest for the canavalia residue treatment with 98% recovery, followed by the mineral fertilizer treatment with 83% recovery. Despite similar initial enrichment of soil microbial and mineral N pools, the indirect labelling technique failed to assess the N fertilizer value of mineral and organic amendments due to a high N mineralization from the soil organic matter.     

长期施肥和不同生态条件下我国作物产量可持续性特征

采用产量可持续性指数（SYI）法,研究了我国不同生态条件下20个长期试验点8个肥料处理的水稻、玉米和小麦产量的可持续性.结果表明：作物SYI值因施肥、作物种类和水热因子不同而呈显著差异.长期不施肥（CK）条件下,水稻、玉米和小麦的SYI值较低,分别为0.55、0.44和0.43;施肥尤其是NPK化肥配施有机肥可显著提高作物产量的可持续性,水稻、玉米和小麦的SYI值分别为0.66、0.58和0.57;单施N肥或NK肥的玉米和小麦的SYI值在0.36～0.47.SYI值大于0.55表明可持续性较好,小于0.45表明可持续性差.经纬度和气象因子对作物SYI也有不同程度的影响,3种作物不施肥时,水稻SYI变异较小,与各因子间没有显著相关性,玉米SYI变异最大且与各因子间存在显著的相关关系,小麦介于两者之间.因此,NPK配施有机肥有利于作物高产稳产,是维持系统可持续性的最优施肥模式.     

黑土长期施肥及养分循环再利用的作物产量及土壤肥力质量变化Ⅰ.作物产量

在一丰P黑土(Olsen P25.8mg·kg^-1)上进行13年中长期田间试验的结果表明,氮肥平均增产率为275%,年增产粮食(大豆、玉米、小麦混合)724kg·hm^-2,1kgN增产粮食9.4kg;磷肥前期增产不明显,13年平均增产率为7%,年增产粮食241kg·hm^-2,1kgP增产粮食12.7kg.每年施用循环回田猪圈肥(以处理区收获产品的80%喂猪、垫圈经堆制而成),粮食平均年增产量在不施化肥、施N、施NP基础上,分别为268、258、255kg·hm^-2,相应的增产率为9.8%、7.6%和7.0%.试验期间循环回田猪圈肥的增产效果有逐渐增长趋势,表明存在着猪圈肥残效的叠加效应.     

Turnover of organic nitrogen in soils and its availability to crops

The root development of barley seedlings grown for one week in an aerated nutrient solution was studied in the presence of dissolved organic matter from an aqueous chestnut leaf litter extract. In particular, the different effects of low and high molecular weight fractions (small molecules: molecular weight <1000; large molecules: >10,000) of the leaf litter extract were examined. In the presence of large molecules root growth was inhibited, an irregular root tip morphology was observed, and Ca and Mg concentrations in the shoots were lower than in control plants. These phytotoxic effects were not caused by the formation of an impermeable layer of large molecules on the root surfaces that lower accessibility for nutrient cations as inferred from voltammetric experiments. A germination assay using spruce seeds, however, indicated allelochemical effects of large molecules, which exhibit a higher aromaticity than the small molecules as indicated by spectroscopic characterisation. In the growth experiments with small molecules, no influence on the root development of barley was evident, but an increase of Ca and Mg in the shoots was detected. During these growth experiments, a large amount of the small molecules, mainly simple phenols and amino acids, disappeared from the nutrient solution. The loss of small molecules was most likely the effect of mineralisation.Abbreviations DOC dissolved organic carbon - DOM dissolved organic matter - LLE leaf litter extract - MW molecular weight - HMDE hanging mercury drop electrode     

Comparison of Nitrogen mineralization from 15N-labeled organic amendments under flooded and upland conditions

Knowledge of N availability from organic amendments is a key to improve N use efficiency and reduce environmental pressure from agriculture. Nitrogen mineralization from ¹⁵N-labeled cattle dung compost and rapeseed cake was investigated under flooded and upland (60% of water holding capacity) conditions in an incubation experiment for 63 d at 25 °C. The relative abundance of N in the cattle dung compost by the simple step-wise acid hydrolysis method was in the following order: labile N (37% of total N, refluxing with 1 M HCl for 3 h, H1-N) > non-hydrolyzable N (32%) > recalcitrant N (18%, 3 M HCl for 3 h, H2-N). There was no significant difference in the ¹⁵N abundance between total N and N in each fraction of the cattle dung compost. For the rapeseed cake, the H1-N accounted for 81% of total N and the ¹⁵N abundance of total N and H1-N was higher than the ¹⁵N abundance of H2-N and non-hydrolyzable N. In the cattle dung treatment, inorganic ¹⁵N was the highest at 21 d of incubation and then decreased thereafter under flooded conditions, whereas it remained constant from 21 to 63 d under upland conditions. In the rapeseed cake treatment, inorganic ¹⁵N was the highest at 42 d under flooded conditions and inorganic ¹⁵N increased until 42 d and remained stable thereafter under upland conditions. The N mineralization rate from the cattle dung compost was slow both under flooded and upland conditions. More than half of N in the rapeseed cake was mineralized during the incubation period both under flooded and upland conditions. There was no significant difference in ¹⁵N recovery in the soil between flooded and upland conditions at 63 d in the cattle dung treatment, while the ¹⁵N recovery in the soil at 63 d was higher under upland than under flooded conditions in the rapeseed cake treatment. Although N mineralization from the rapeseed cake was greater under flooded conditions than upland conditions, there was no significant difference in N mineralization from the cattle dung compost between both conditions. Therefore, N mineralization from organic amendments is not always more rapid under flooded than upland conditions depending on the amendment type.     

Vertical transport of dissolved organic C and N under long-term N amendments in pine and hardwood forests

At the Harvard Forest, Massachusetts, a long-term effort is under way to study responses in ecosystem biogeochemistry to chronic inputs of N in atmospheric deposition in the region. Since 1988, experimental additions of NH₄NO₃ (0, 5 and 15 g N m^–2 yr^–1) have been made in two forest stands:Pinus resinosa (red pine) and mixed hardwood. In the seventh year of the study, we measured solute concentrations and estimated solute fluxes in throughfall and at two soil depths, beneath the forest floors (Oa) and beneath the B horizons.Beneath the Oa, concentrations and fluxes of dissolved organic C and N (DOC and DON) were higher in the coniferous stand than in the hardwood stand. The mineral soil exerted a strong homogenizing effect on concentrations beneath the B horizons. In reference plots (no N additions), DON composed 56% (pine) and 67% (hardwood) of the total dissolved nitrogen (TDN) transported downward from the forest floor to the mineral soil, and 98% of the TDN exported from the solums. Under N amendments, fluxes of DON from the forest floor correlated positively with rates of N addition, but fluxes of inorganic N from the Oa exceeded those of DON. Export of DON from the solums appeared unaffected by 7 years of N amendments, but as in the Oa, DON composed smaller fractions of TDN exports under N amendments. DOC fluxes were not strongly related to N amendment rates, but ratios of DOC:DON often decreased.The hardwood forest floor exhibited a much stronger sink for inorganic N than did the pine forest floor, making the inputs of dissolved N to mineral soil much greater in the pine stand. Under the high-N treatment, exports of inorganic N from the solum of the pine stand were increased >500-fold over reference (5.2 vs. 0.01 g N m^–2 yr^–1), consistent with other manifestations of nitrogen saturation. Exports of N from the solum in the pine forest decreased in the order NO₃-N> NH₄-N> DON, with exports of inorganic N 14-fold higher than exports of DON. In the hardwood forest, in contrast, increased sinks for inorganic N under N amendments resulted in exports of inorganic N that remained lower than DON exports in N-amended plots as well as the reference plot.     

The potential of cropping systems and soil amendments for carbon sequestration in soils under long-term experiments in subtropical India

An understanding of the dynamics of carbon (C) stock in soils, as impacted by management strategies, is necessary to identify the pathways of C sequestration in soils and for maintaining soil organic C (SOC) at a level critical for upkeeping soil health and also for restraining global warming. This is more important in tropical and subtropical region where soils are inherently low in organic C content and the production system is fragile. We evaluated the long‐term role of crop residue C inputs to soil in SOC sequestration and also the critical value of C inputs for maintenance of SOC across five different rice‐based cropping systems and four soil management practices including a fallow (no cultivation since initiation of the experiments) using five long‐term (7–36 years) fertility experiments in subtropical India. Cropping per se always caused a net depletion of SOC. Such depletion was inversely proportional to the amount of crop residue C incorporated into the soils (r=−0.92, P=0.001). Balanced fertilization with NPK, however, caused an enrichment (9.3–51.8% over the control) of SOC, its extent being influenced by the cropping systems. Long‐term application of organic amendments (5–10 Mg ha⁻¹ yr⁻¹) through farmyard manure (FYM) or compost could increase SOC hardly by 10.7% constituting only 18% of the applied C, the rest getting lost through oxidation. The total quantity of soil C sequestered varied from −11.5 to 14.5 Mg C ha⁻¹ and was linearly related (r²=0.40, P=0.005) with cumulative crop residue C inputs to the soils. On an average, the rate of its conversion to SOC came out to be 6.4%. This was more in presence of added organics (6.9%) than in its absence (4.2%). For sustenance of SOC level (zero change due to cropping) we found that a minimum quantity of 2.9 Mg C is required to be added per hectare per annum as inputs. The cropping systems and the management practices that could provide C input higher than the above critical level are likely to sustain the SOC level and maintain good soil health in the subtropical regions of the Indian subcontinent.     

Modelling soil organic matter levels after long-term applications of crop residues,and farmyard and green manures

A computer simulation model for long-term soil organic matter dynamics was developed and evaluated with data from long-term field trials in Belgium, Germany and The Netherlands. The model distinguishes four pools of soil organic components (including a microbial biomass pool) with different chemical properties. Transformation rates are described by (pseudo) first order kinetics. Effects of temperature and soil moisture tension were included. Simulation results were in agreement with experimental data from arable farming practices where common input rates were applied. Model calculations overestimated soil organic matter levels when green manures or exceptionally high input rates were applied. Inadequate experimental estimations of organic matter input rates and insufficient modelling of the soil preservation capacity for organic matter and biomass are likely reasons. After changes in the soil organic matter-input management it may take more than a century to reach new equilibrium levels.     

The influence of organic soil amendments on sulfate adsorption and sulfur availability in a Brazilian Oxisol

Soil management practices that involve additions of organic materials may influence plant sulfur availability in highly-weathered, acid soils. This study evaluated the effects of organic additions on sulfate adsorption and sulfur availability in a limed (3,4 t ha^-1) and unlimed Typic Haplustox soil of the Cerrado Region of Brazil. In unlimed soil, the proportion of applied sulfate (600 kg S ha^-1 as gypsum) that was adsorbed temporarily decreased over two cropping seasons by incorporation of 10 t dry matter ha^-1 crop^-1 of guinea grass (Panicum maximum Jacq.) but not when a similar quantity of a tropical legume, feijâo de porco (Canavalia ensiformis L.), was added. Liming reduced sulfate adsorption and resulted in sulfate leaching to a depth of 30 to 45 cm. Both plant materials temporarily reduced sulfate adsorption in laboratory studies when added to an unlimed soil at a rate equivalent to 40 t ha^-1. Analysis of soil properties affected by organic additions and liming showed significant correlations between sulfate adsorption and soil pH, extractable aluminum, calcium and magnesium, and surface charge. Maize dry matter yields increased by 1.3 to 3.5 t ha^-1 with addition of both organic materials. However, only the feijâo de porco treatment resulted in increases in sulfur uptake for the years in which organic materials were applied. Determining the effects of organic material additions on plant sulfur availability is complicated by the combined effects of sulfur mineralization, sulfate adsorption, and the plant's ability to utilize adsorbed subsoil sulfate.Joint contribution of Cornell University and CPAC-EM- BRAPA. This research was supported by USAID through the Title XII CRSP subgrant SM-CRSP-10 from North Carolina State University     

Organic amendments and land management affect bacterial community composition, diversity and biomass in avocado crop soils

Background and aims

The avocado-producing area of southern Spain includes conventional orchards and organic orchards that use different organic amendments. To gain insight into the effects of these amendments, physicochemical properties and microbial communities of the soil were analysed in a representative set of commercial and experimental orchards.

Methods

The population size of several groups of culturable microorganisms was determined by plating on different selective media. Bacterial community structure was studied by denaturing gradient gel electrophoresis (DGGE)

Results

Commercial composts showed the largest effects, especially the animal compost, enhancing the population sizes of some microbial groups and affecting bacterial community structure in superficial and deep soil layers. Moreover, animal and vegetal compost, manure and blood meal addition are related to high bacterial diversity in the superficial soil layer.

Conclusions

All of the organic amendments used in this study affect soil properties in one or more of the characteristics that were analysed. Culturable microbial population data revealed the most evident effects of some of the organic treatments. However, molecular analysis of soil bacterial communities by DGGE allowed the detection of the influence of all of the analysed amendments on bacterial community composition. This effect was stronger in the superficial layer of the avocado soil.