Changes in soil properties and humic substances after long‐term amendments with manure and crop residues in dryland farming systems

After 16 years of periodical applications of either farmyard manure or crop wastes at two levels of mineral N fertilization to a Calcic Haploxeralf in the semiarid central Spain, we found significant changes in chemical fertility levels and in the concentration, chemical composition, and carbon mineralization rates of soil organic matter (SOM). The changes in SOM quality were related to significant improvements of soil physical properties, mainly aggregate stability and water retention. Such changes were related to the increased concentration of humic colloids in soil, the mineral N dose, and the type of organic matter applied. When compared with the control plots, the organic matter accumulated in the amended plots tended to be less transformed, and its total concentration and humification degree decreased with increasing external N‐inputs. Humic acids from the amended plots showed a more marked aliphatic character (mainly after N addition) than those from control plots. Farmyard manure led to a significant improvement of soil physical properties, but had a comparatively small effect in promoting biodegradation and humification of crop wastes. This could be due to the high biological stability of the manure used which, in semiarid Mediterranean fields, usually leads to an accumulation of little transformed SOM.     

RothC simulation of carbon accumulation in soil after repeated application of widely different organic amendments

Multi-compartment soil carbon (C) simulation models such as RothC are widely used for predicting changes in C stocks of arable soils. However, rigorous routines for establishing entry pools that account for the diversity of exogenous organic matter (EOM) applied to croplands are still lacking. We obtained data on changes in soil C stocks after repeated applications of EOM from four long-term experiments (LTEs): Askov K2 (Denmark, 31 yrs), Qualiagro (France, 11 yrs), SERAIL (France, 14 yrs) and Ultuna (Sweden, 52 yrs). The adjustment of the partition coefficients of total organic C in EOM (EOM-TOC) into the labile, resistant and humified entry pools of RothC (f_DPM, f_RPM, f_HUM, respectively) provided a successful fit to the accumulation of EOM-derived C in the LTE soils. Equations estimating the EOM partition coefficients in the RothC model were based on an indicator (I_ROC) of the EOM-TOC potentially retained in soil. I_ROC was derived from the C found in the soluble, lignin + cutin-like and cellulose-like Van Soest fractions and the proportion of EOM-TOC mineralized during 3 days of incubation. Using the EOM partition coefficients derived from these laboratory analyses resulted in RothC simulations with only slightly larger errors than simulations based on partition coefficients fitted from LTE soil data, except for EOMs that caused very large accumulations of C in soil (e.g. peat) possibly due to factors not accounted for in the RothC model, such as change in soil pH. The proposed partitioning of EOM-TOC allows the potential soil C storage after EOM applications to be predicted regardless of field location and specific composition of EOMs.     

不同辅料生物菌剂堆肥发酵层温度变化

该文研究了添加不同辅料猪粪高温(＞60℃)堆肥发酵层的温度变化规律,结果表明随着堆肥发酵时间不同,堆肥高温层的位置和厚度呈动态变化,有明显的变化规律,即：各辅料处理的堆体高温发酵层出现在离顶部20 cm和离底部20 cm之间,高温发酵层厚度随着发酵期而递增。辅料对猪粪生物堆肥过程高温发酵期开始及其厚度、高温发酵期延续时间有显著影响,从辅料对猪粪高温堆肥发酵层温度变化的影响看,猪粪生物菌剂堆肥生产中选用20%左右砻糠和木屑作为辅料较适宜。     

不同土壤改良剂对酸化土壤微生物群落结构的影响

酸化土壤中微生物多样性降低是土壤养分减少、土传病害高发、作物产量和品质降低的重要原因。施用碱性土壤改良剂是改善酸化土壤理化特性、减少土传病害发生、提高作物产量的有效手段。为了明确不同碱性土壤改良剂对土壤微生物群落结构及多样性的影响,通过盆栽试验比较施用生石灰（S1）及氨基酸生态肥（S2）和黄腐酸水溶肥（S3）对酸化土壤微生物群落结构的改良效果。结果表明,施用3种土壤改良剂后土壤细菌丰富度和多样性均降低,而真菌群落物种丰富度没有显著变化。改良后土壤的主导菌门与改良前相同,而主导菌属存在较大差异。Iamia、Peroneutypa为S1处理特有主导菌属,节核细菌属（Arthrobacter）、德巴利酵母属（Debaryomyces）、Paraphaeosphaeria为S2处理特有主导菌属。根霉菌属（Rhizopus）、裂褶菌属（Schizophyllum）为S3处理特有主导菌属。各改良土壤中,S1处理对细菌的筛选作用最强,S2处理对真菌群落的影响最大,而S3处理相对丰度增加的菌属最多。3种土壤改良剂均能够抑制有害土壤微生物的生长和定殖,但抑制机制可能存在差异。综上所述,3种土壤改良剂均能...     

Soil loss and run‐off characteristics under different soil amendments and cropping systems in the semi‐deciduous forest zone of Ghana

Soil erosion is a major constraint to crop production on smallholder arable lands in Sub‐Saharan Africa (SSA). Although different agronomic and mechanical measures have been proposed to minimize soil loss in the region and elsewhere, soil management practices involving biochar‐inorganic inputs interactions under common cropping systems within the framework of climate‐smart agriculture, have been little studied. This study aimed to assess the effect of different soil and crop management practices on soil loss characteristics under selected cropping systems, typical of the sub‐region. A two‐factor field experiment was conducted on run‐off plots under different soil amendments over three consecutive cropping seasons in the semi‐deciduous forest zone of Ghana. The treatments, consisting of three soil amendments (inorganic fertilizer, biochar, inorganic fertilizer + biochar and control) and four cropping systems (maize, soyabean, cowpea, maize intercropped with soyabean) constituted the sub‐plot and main plot factors, respectively. A bare plot was included as a soil erosion check. Seasonal soil loss was greater on the bare plots, which ranged from 9.75–14.5 Mg ha^?1. For individual crops grown alone, soil loss was 31%–40% less under cowpea than under maize. The soil management options, in addition to their direct role in plant nutrition, contributed to significant (p < 0.05) reductions in soil loss. The least soil loss (1.23–2.66 Mg ha^?1) was observed under NPK fertilizer + biochar treatment (NPK + BC) over the three consecutive cropping seasons. Biochar in combination with NPK fertilizer improved soil moisture content under cowpea crops and produced considerably smaller bulk density values than most other treatments. The NPK + BC consistently outperformed the separate mineral fertilizer and biochar treatments in biomass yield under all cropping systems. Biochar associated with inorganic fertilizers gave economic returns with value–cost ratio (VCR) > 2 under soyabean cropping system but had VCR < 2 under all other cropping systems. The study showed that biochar/NPK interactions could be exploited in minimizing soil loss from arable lands in SSA.     

Impact of organic‐manure combinations on the productivity and soil quality in different cropping systems in central India

In a field experiment, the effect of combination of different organic manures on the productivity of crops and soil quality were evaluated in deep vertisols of central India. Combinations of cattle dung manure (CDM), poultry manure (PM), and vermicompost (VC) vis‐à‐vis mineral fertilizers were tested in four cropping systems involving soybean (Glycine max L.), durum wheat (Triticum durum Desf.), mustard (Brassica juncea L.), chickpea (Cicer arietinum L.), and isabgol (Plantago ovata Forsk). The organic manures were applied based on the N‐equivalent basis and nutrient requirement of individual crop. The grain yields of durum wheat and isabgol were higher in the treatment that received a combination of CDM + VC + PM whereas in mustard, CDM + PM and in chickpea, CDM + VC recorded the higher yields. The yield levels in these organic‐manure combinations were similar to the yields obtained with mineral fertilizers. Among the cropping systems, soybean–durum wheat and among the nutrient sources, the combination of CDM + VC + PM recorded the highest total productivity. At the end of the 3‐year cropping cycle, application of organic manures improved the soil‐quality parameters viz., soil organic carbon (SOC), soil available nutrients (N, P, and K), soil enzymes (dehydrogenase and alkaline phosphatase), and microbial biomass C in the top 0–15 cm soil. Bulk density and mean weight diameter of the soil were not affected by the treatments. Among the cropping systems, soybean–durum wheat recorded the highest SOC and accumulated higher soil available N, P, and K. In conclusion, the study clearly demonstrated that the manures applied in different combinations improved the soil quality and produced the grain yields which are at par with mineral fertilizers.     

Long-term manure amendments enhance neutral sugar accumulation in bulk soil and particulate organic matter in a Mollisol

Manure application generally increases soil organic matter (SOM) and particulate organic matter (POM) content in soil. Free and occluded POM (fPOM and oPOM) can be quantified by combining density and ultrasonic dispersion approaches, but it remains unclear which fraction of POM is more responsive to manure application, and whether manure treated soils have a more pronounced effect on POM content than unmanured soils (no or chemical fertilizer treated soils). Because neutral sugars in POM can be attributed to either plant- or microbial-derived compounds, we analyzed the pattern and ratio of different neutral sugars to clarify effects of different fertilizations on quality of POM in a study over two decades. Soil samples (0–20 cm) were collected from six fertilization treatments in a 25-year long fertilization experiment including no fertilizer (CK), low manure (M1), high manure (M2), chemical nitrogen, phosphorus and potassium fertilizers (NPK), and combined manure and chemical fertilizers (M1NPK, M2NPK). Our results showed that manure application generally led to higher organic carbon concentrations in bulk soil (M2NPK > M2 > M1NPK > M1 > CK > NPK), fPOM (M2NPK > M2 > M1 > M1NPK > NPK > CK) and oPOM (M1 > M2 > M1NPK > M2NPK > NPK > CK), respectively. As compared with unmanured treatments, manure amendments induced 48, 21 and 107% greater increases on average in neutral sugar concentrations in bulk soil, fPOM and oPOM, respectively. More plant-derived organic compounds were enriched in fPOM than oPOM and bulk soil, and the enrichment was more pronounced in manure treated soils than the unmanured soils. This study suggests that long-term use of manure enhanced microbial routing of specific monosaccharides into different POM fractions. Clearly, manure amendments improved labile SOM content and SOM quality in the Mollisol thus maintaining soil productivity over decades.     

不同种类绿肥翻压对土壤肥力的影响

通过模拟试验,研究了旱地翻压箭筈豌豆（Vicia sativa L.）、苕子（Vicia villosa Roth. Var.）和山黧豆（Lathyrus palustris l. Var. Pilosus ledeb）三种绿肥对土壤肥力特性的影响。试验结果表明,在绿肥翻压腐解过程中,土壤pH（原始pH值为5.95）均呈现出先升高后降低最后趋于平缓的趋势,在翻压0～25 d内高于不翻压绿肥对照,之后始终低于对照。翻压绿肥处理的土壤有机碳含量均高于对照,增幅为1.6%～6.2%。土壤无机氮含量显著高于对照,无机氮形态0～20 d以铵态氮为主,30～70 d以硝态氮为主,翻压70 d时,硝态氮含量较对照增加了15倍以上。翻压绿肥后土壤速效磷的含量均高于对照,增幅为11.9%～37.7%。翻压绿肥的土壤速效钾含量在翻压15 d时达最高且均比对照增加1.5倍以上。不同绿肥对土壤养分特性的影响不同,山黧豆处理的土壤pH变化幅度最大,土壤有机碳和无机氮含量最高,箭筈豌豆处理的土壤速效钾和速效磷含量最高。本试验条件下,三种绿肥翻压均能够改变土壤pH值,提高土壤硝态氮和速效钾含量,土壤有机碳和速效磷含量略有增加。     

Changes in amount of organic and inorganic fractions of nitrogen in an Eutrochrept soil after long‐term cropping with different fertilizer and organic manure inputs

The influence of 30 years of cropping with different fertilizer and farmyard manure (FYM) inputs on the contents and depth distribution of organic C, total N (N_t), soil mineralizable N, and organic and inorganic N fractions was investigated in an Eutrochrept. Continuous application of 100 %NPK(+S), 150 %NPK(+S), and 100 %NPK(+S)+FYM led to a marked increase in organic C, total N, hydrolyzable N (viz., amino acid N, hydrolyzable NH₄‐N, hexose amine N, and unidentified hydrolyzable N), and nonhydrolyzable N as compared to an adjacent fallow. The contents of the various organic N fractions were largest in surface soil and thereafter decreased with the depth. However, at 30 – 45 cm depth the content of organic C was not affected by the different treatments except 100 %NPK(+S)+FYM. On the other hand, continuous cropping without fertilization resulted in a depletion of total hydrolyzable N in control over fallow by 27.2 % (0–15 cm), 19.6 % (15–30 cm), and 4.7 % (30–45 cm). The incorporation of FYM with 100 %NPK(+S) resulted in greater contents of soil mineralizable N as compared to 100 %NPK(+S) (0–15, 15–30 cm). The proportion of hydrolyzable N (57–76 % of N_t) decreased and that of nonhydrolyzable N (22–40 % of N_t) increased with depth. The proportion of amino acid N (19–26 % of N_t), hexose amine N (2.1–3.5 % of N_t) and unidentified hydrolyzable N (17–27 % of N_t) decreased with depth. All organic soil N fractions including even nonhydrolyzable N in surface and subsurface soils were highly significantly correlated with soil mineralizable N derived from incubations under waterlogged and aerobic conditions. The best correlation to mineralizable N was found for amino acid N and the least significant correlation for nonhydrolyzable N.     

The response of soil organic matter to manure amendments in a long-term experiment at Ultuna, Sweden

In a long-term field experiment started in 1956 on a clay loam soil at Uppsala, Sweden, changes of organic carbon in the topsoils receiving various organic amendments at the rate of 200 kg C ha^'1 year^'1 were studied to determine soil organic matter characteristics, variations of δ¹³C in the soil and to estimate a carbon balance. Fallow and mineral fertilizer without N led to a significant decrease of soil organic matter (SOM) in the soil, green manure maintained the SOM content, and animal manure and peat increased the SOM content significantly. The stable portion of the added organic materials after 37 years of continuous input was 12·8, 27·3, and 56·7%, for green manure, animal manure and peat, respectively. This was reflected by half-lives of organic carbon originating from the amendments between 3·0 (green manure) and 14·6 years (peat). The isotopic composition of SOM changed both due to mineralization (continuous fallow) and the addition of amendments is topically different from soil humus (green manure, animal manure). The isotopic effect was used to calculate the percentage of carbon derived from animal manure present for the year 1993. This value (55·4%) was larger than that derived from the carbon balance, which indicated a priming effect of the animal manure on the initial soil humus. Mineralization of microbially available organic substances led to an increase in the degree of humification on plots not receiving organic amendments. Adding peat and animal manure resulted in a decrease of the humification index due to the continuous input of poorly humified material. The extinction ratio (E₄/E₆) and ratio of fulvic acid to humic acid changed considerably in the peat treated plots. Fourier transform infrared (FTIR)-measurements of the extracts showed that peat characteristics can be detected in peat treated soils. The other amendments did not alter the characteristics of the extractable humic substances.     

天津地区不同冬绿肥培肥土壤的效果

  【目的】  研究天津地区长期冬绿肥–春玉米轮作体系对土壤性质的影响,探讨该模式对土壤综合肥力的贡献。  【方法】  田间定位试验于2012—2019年在天津进行,供试作物为春玉米,冬绿肥处理包括冬绿肥二月兰(Orychophragmus violaceus L.)、毛叶苕子(Vicia villosa Roth L.)、黑麦(Secale cereale L.)、黑麦草(Lolium L.)、毛叶苕子二月兰混播、毛叶苕子黑麦混播及冬闲对照,共7个处理。测定土壤理化性质及酶活性,并通过主成分分析方法分析种植不同冬绿肥及其组合对土壤综合肥力的贡献。  【结果】  冬绿肥–春玉米轮作均显著增加了土壤有机质、全氮、全磷、有效磷、速效钾、微生物量碳、微生物量氮含量,同时增加了土壤饱和持水量,降低了土壤EC值。与冬闲–春玉米处理相比,冬绿肥–春玉米轮作土壤细菌数量、真菌数量、放线菌数量分别提高26.67%～75.89%、61.9%～97.9%和51.4%～92.1%,土壤脲酶活性显著增加6.59%～20.47%。毛叶苕子黑麦混播、毛叶苕子、黑麦处理显著提高土壤磷酸酶活性,黑麦、黑麦草处理显著提高土壤蔗糖酶活性,毛叶苕子二月兰混播、毛叶苕子处理显著提高土壤过氧化氢酶活性,毛叶苕子、黑麦草、二月兰和黑麦处理显著提高土壤多酚氧化酶活性。主成分分析结果表明冬绿肥种植显著提高土壤综合肥力,特别是冬绿肥混播种植,提升效果从高到低排序为毛叶苕子二月兰混播>毛叶苕子黑麦混播>毛叶苕子>二月兰>黑麦草>黑麦。  【结论】  华北春玉米种植区,长期冬绿肥种植显著改善了土壤理化性质和生物学性质,提高了土壤综合肥力水平,冬绿肥混播处理对土壤综合肥力贡献高于冬绿肥单播处理。     

四种土壤调理剂对镉、铅的吸附效果研究

采用等温吸附试验研究了主要原料为麦饭石(M)、蒙脱石(T)、牡蛎壳(O)和硅钙矿(S)4种土壤调理剂对溶液中镉和铅的吸附效果。结果表明,4种土壤调理剂对镉和铅的吸附均可用Langmuir方程进行描述;相比而言,4种土壤调理剂对镉吸附量的大小顺序为O调理剂>S调理剂>M调理剂>T调理剂,对铅吸附量的大小顺序为M调理剂>S调理剂>O调理剂>T调理剂;随溶液pH值的增大各调理剂的吸附量逐渐减小,尤其当Cd溶液pH值>8,Pb溶液pH值>6,各调理剂的吸附量迅速降低。     

Temporal variation of soil nutrients under the influence of different organic amendments

Maintenance of tropical soil quality for crop production without damaging the environment is a challenge and thus the development of an efficient nutrient-management technique is important. Soil amendment by organic manures has been widely accepted as an efficient nutrient-management technique in tropical agriculture. In this study, a long-term laboratory experiment was conducted to investigate the influence of organic manures of different sources (e.g. cattle manure, poultry manure, vermicompost and oil cakes) on changes in pH, electrical conductivity, organic C and nutrient content (mineral N, available phosphorous and potassium) in tropical alluvial soil. Applications of organic amendments in this study indicated considerable changes in the basic soil physico-chemical properties and different nutrient levels. Soil pH declined slightly, whereas organic C and all the other nutrients increased distinctly, due to the application of organic manures. Thus, the magnitude of changes in the soil properties was dependent on the nature of the organic manure.     

Use of soil color meter for aqueous iron and ammonium measurements

Abstract

In this paper, we proposed a new approach for on-site colorimetric analysis of ferrous ions (Fe²⁺) and ammonium-nitrogen (NH₄ ⁺-N) using a soil color meter as an alternative method to conventional spectrophotometry. The soil color meter we used can express solution color numerically on the basis of L*a*b* color space. After coloring of water by the 1, 10 phenanthroline method and the Indophenol blue method, the color of solution was measured by the soil color meter. A linear relationship between Fe²⁺ and a* or b* values, and systematic change of NH₄ ⁺-N with L* value, enable us to make a calibration curve. The Fe²⁺ and NH₄ ⁺-N concentrations in groundwater samples (Fe²⁺: 0.3–1.3 mg L^?1; NH₄ ⁺-N: 0.02–0.62 mg L^?1) determined by the proposed method agreed well with those determined by conventional spectrophotometry with the difference being ± 0.05 mg L^?1 and ± 0.02 mg L^?1, respectively. Since a similar apparatus is widely used in the soil science field, this technique would facilitate field surveys.     

Effects of cover crops,compost, and manure amendments on soil microbial community structure in tomato production systems

Soil microbial community structure and crop yield was investigated in field tomato production systems that compared black polyethylene mulch to hairy vetch mulch and inorganic N to organic N. The following hypotheses were tested: (1) hairy vetch cover cropping increases crop yield and significantly affects soil microbial community structure when compared to the standard plastic mulch and synthetic fertilizer-based system; (2) within plastic mulch systems, organic amendments will increase crop yield and significantly affect soil microbial community structure when compared to synthetic fertilizer; (3) crop yields and microbial community structure will be similar in the hairy vetch cover cropping and the organic amended plasticulture systems. Treatments consisted of ammonium nitrate (control), hairy vetch cover crop, hairy vetch cover crop and poultry manure compost (10 Mg/ha), three levels of poultry manure compost (5, 10, and 20 Mg/ha), and two levels of poultry manure (2.5 and 5 Mg/ha). Black polyethylene mulch was used in all treatments without hairy vetch. Fatty acid analysis was used to characterize the total soil microbial community structure, while two substrate utilization assays were used to investigate the community structure of culturable bacteria and fungi. Crop yield was not significantly increased by hairy vetch cover cropping when compared to black polyethylene mulch, although microbial community structure was significantly affected by cover cropping. Under black polyethylene mulch, crop yields were significantly increased by the highest levels of compost and manure when compared to inorganic fertilizer, but there was no detectable effect on soil microbial community structure. When cover cropping was compared to organic amended plasticulture systems, crop yields were similar one year but dissimilar the next. However, hairy vetch cover cropping and organic amendments under black plastic mulch produced significantly different soil microbial community structure.     

Estimating soil stress distribution by using depth‐dependent soil bulk‐density data

Depth‐dependent soil bulk density (BDS) is usually affected by soil‐specific factors like texture, structure, clay mineralogy, soil organic‐matter content, soil moisture content, and composition of soil solution and is also affected by external factors like overburden‐stress history or hydrological fluxes. Generally, the depth‐dependent BDS cannot be predicted or extrapolated precisely from a limited number of sampling depths. In the present paper, an easy method is proposed to estimate the state of soil mechanical stress by analyzing the packing characteristics of the profile using soil bulk‐density data. Results for homogeneous loess profiles exposed to the site‐specific climatic conditions show that the depth‐dependent relation of void ratio vs. weight of overburden soil can be described systematically so that deviations from the noncompacted reference state can be detected. We observed that precompaction increased from forest soils (reference) to agricultural soils with decreasing depth.     

The effect of four calcium‐based amendments on soil aggregate stability of two sandy topsoils

The structural stability of soil is a physical characteristic that affects soil degradation processes. Calcium‐based amendments, such as calcium carbonate, calcium sulfate, and calcium oxide/hydroxide, have been shown to improve the stability of soil aggregates. This study seeks to determine which calcium‐based soil amendments, and at what concentration, are the most efficient in improving aggregate stability of sandy topsoils derived from granitic and metamorphic parent materials, and to analyze the mechanisms involved. In the pot experiment, soils amended with CaCO₃, CaCl₂, and CaSO₄ did not present significant differences in aggregate stability compared to the control or among each other. In contrast, Ca(OH)₂ soil amendment brought the greatest stability to the soil aggregates. A dose of 1% Ca(OH)₂ significantly increased the stability of soil aggregates. This effect is due to the reaction of Ca(OH)₂ with atmospheric CO₂ which leads to the formation of CaCO₃, a delayed reaction not showed by the other soil amendments tested. Likewise, the greater solubility of Ca(OH)₂ compared to CaCO₃ exerts a greater aggregation effect on soil. Thus, the mechanism of action of Ca(OH)₂ is related to cementation, rather than flocculation. Future studies should be carried out to demonstrate the effectiveness of Ca(OH)₂ under field conditions.     

Effect of long‐term swine‐manure application on soil hydraulic properties and heavy metal behaviour

This study evaluated the effect of 13 years of swine‐manure application on the changes in soil hydraulic properties, and as associated physicochemical properties, with a focus on heavy metal mobility. Various soil hydraulic properties were measured, including soil water retention (SWR), saturated field hydraulic conductivity (K_fs) and unsaturated field hydraulic conductivity (K_funsat) using a disc infiltrometer. Heavy metal mobility was evaluated with a sequential extraction procedure. At 0–30 cm soil depth in the heavily manured plot (SMhigh plot), SWR at 0 to ?100 kPa was significantly larger than in plots amended with a standard amount of manure (SMstd plot) or with chemical fertilizer (CF plot). K_fs and K_funsat values in both manure‐amended plots were less than in the CF plot under dry soil conditions but greater than those of the CF plot under wet soil conditions. Furthermore, K_fs and K_funsat did not necessarily increase with manure application rates. On the other hand, high‐mobility metal fractions, such as the exchangeable fraction of Zn, and the CH₃CO₂Na‐extractable fraction of Zn and Mn, and the metal–organic complex fractions of Zn, Cu and Mn, increased with the greater manure application rate. In addition, low‐mobility metal fractions, the organically bound fractions of Zn, Cu and Mn in the high SM plot and the easily reducible metal oxide fraction of Mn in both manure‐amended plots were probably affected and released into high‐mobility fractions. This indicated that manure application changed the soil redox conditions by improving the soil structure, depending on the water content of soil pores. Despite the reduction of K_fs and K_funsat by heavy manure application, the transport of high‐mobility metal fractions with either surface water flow or infiltration water flow could be controlled by soil water content at the beginning of a rain or irrigation event.     

Soil microbial community responses to sulfadiazine-contaminated manure in different soil microhabitats

Veterinary antibiotics such as sulfadiazine (SDZ) are applied with manure to agricultural soil. Antimicrobial effects of SDZ on soil microbial community structures and functions were reported for homogenized bulk soils. In contrast, field soil is structured. The resulting microhabitats are often hot spots that account for most of the microbial activity and contain strains of different antibiotic sensitivity or resilience. We therefore hypothesize that effects of SDZ are different in diverse soil microhabitats. We combined the results of laboratory and field experiments that evaluated the fate of SDZ and the response of the microbial community in rhizosphere, earthworm burrow, and soil macroaggregate microhabitats. Microbial communities were characterized by phenotypic phospholipid fatty acid (PLFA) and genotypic 16S rRNA gene patterns (DGGE) and other methods. Data was evaluated by principle component analyses followed by two-way ANOVA with post-hoc tests. Extractable SDZ concentrations in rhizosphere soil were not clearly different and varied by a factor 0.7–1.2 from those in bulk soil. In contrast to bulk soil, the extractable SDZ content was two-fold larger in earthworm burrows, which are characterized by a more hydrophobic organic matter along the burrow surface. Also, extractable SDZ was larger by up to factor 2.6 in the macroaggregate surface soil. The rhizosphere effect clearly increased the microbial biomass. Nonetheless, in the 10 mg SDZ kg⁻¹ treatment, the biomass deceased by about 20% to the level of uncontaminated bulk soil. SDZ contamination lowered the total PLFA concentrations by 14% in the rhizosphere and 3% in bulk soil of the field experiment. Structural shifts represented by Pseudomonas DGGE data were larger in SDZ-contaminated earthworm burrows compared to bulk soils. In the laboratory experiment, a functional shift was indicated by a four-fold reduced acid phosphatase activity in SDZ-contaminated burrows compared to bulk soil. Structural and functional shifts after SDZ contamination were larger by a factor of 2.5 in the soil macroaggregate surface versus interior, but this relation reversed over the long-term under field conditions. Overall, the combined effects of soil microhabitat, microbial community composition, and exposure to SDZ influenced the microbial susceptibility towards antibiotics under laboratory and field conditions.