Persistence of fluazinam in soil under boreal conditions.

Fluazinam, a widely used pesticide in conventional potato cultivation, is effective against epidemics of the fungal disease late blight. To assess fluazinam persistence in soil, laboratory experiments were conducted with fluazinam added to soil as a pure chemical or contained in the commercial product Shirlan®. In a follow-up experiment, the persistence was monitored under constant temperature and water content conditions during a maximum period of 1 year. In an annual climatic rotation experiment, fluazinam added to soil was exposed to the year-round temperature and water content conditions occurring in the boreal zone. A third experiment was undertaken to clarify the effect of soil organic matter (SOM) on the recovery of fluazinam. In the follow-up and annual climatic rotation experiments, more than half of the added fluazinam was recovered after 1 year of incubation. The estimated half-life of fluazinam ranged between 355 and 833 days. The degradation of fluazinam was enhanced by an abundance of SOM, a warm temperature, and wetness. Additionally, in over half of soil samples collected from fields where potato had been intensively cultivated for many years, varying concentrations of fluazinam were detected. Fluazinam can carry over to the next growing season in professional potato production.

Nitrogen stocks and flows in an acid sulfate soil.

Besides causing acidification, acid sulfate (AS) soils contain large nitrogen (N) stocks and are a potential source of N loading to waters and nitrous oxide (N2O) emissions. We quantified the stocks and flows of N, including crop yields, N leaching, and N2O emissions, in a cultivated AS soil in western Finland. We also investigated whether controlled drainage (CD) and sub-irrigation (CDI) to keep the sulfidic horizons inundated can alleviate N losses. Total N stock at 0-100 cm (19.5 Mg ha-1) was smaller than at 100-200 cm (26.6 Mg ha-1), and the mineral N stock was largest below 170 cm. Annual N leaching (31-91 kg N ha-1) plus N in harvested grain (74-122 kg N ha-1) was 148% (range 118-189%) of N applied in fertilizers (90-125 kg N ha-1) in 2011-2017, suggesting substantial N supply from soil reserves. Annual emissions of N2O measured during 2 years were 8-28 kg N ha-1. The most probable reasons for high N2O emission rates in AS soils are concomitant large mineral N pools with fluctuating redox conditions and low pH in the oxidized subsoil, all favoring formation of N2O in nitrification and denitrification. Although the groundwater level was higher in CD and CDI than in conventional drainage, N load and crop offtake did not differ between the drainage methods, but there were differences in emissions. Nitrogen flows to the atmosphere and drainage water were clearly larger than those in non-AS mineral soils indicating that AS soils are potential hotspots of environmental impacts.

Sediment from Agricultural Constructed Wetland Immobilizes Soil Phosphorus.

Phosphorus (P) losses from agricultural soils impair the quality of receiving surface waters by enhancing eutrophication. This study tested the potential of using sediment from agricultural constructed wetlands (CWs) to immobilize soil P using two soils differing in texture and soil test P (STP). A silty clay soil (SIC) with high STP (24 mg ammonium acetate-extractable P [P] L) and a sandy loam soil (SL) with excessive STP (210 mg P L) were incubated with increasing amounts of clayey CW sediment. The soil-sediment mixtures were studied with the quantity/intensity (Q/I) technique, using chemical extractions, and by exposing the mixtures to simulated rainfall. In both Q/I and simulated rainfall tests, P solubility steadily decreased with increasing sediment proportion in the mixtures. However, in chemical extractions this effect was observed only at high sediment addition rates (10 or 50% [v/v] sediment). At a practically feasible sediment addition rate of 5%, dissolved reactive P (DRP) in percolating water from simulated rainfall decreased by 55% in SIC and by 54% in SL ( < 0.001 in both cases). Particulate P (PP) also showed a decreasing trend with increasing sediment addition rate. Upon prolonged simulated rainfall, the decreasing effect of sediment on DRP and PP declined somewhat. The effects of sediment addition can be attributed partly to increased salt concentrations in the sediment, which have a short-term effect on P mobilization, but mostly to increased concentrations of Al and Fe (hydr)oxides, increasing long-term P sorption capacity. Adding CW sediment at a rate of up to 5% of surface soil volume to soils could provide an alternative to chemical treatment (e.g., with metal salts) for immobilizing P in small, high-risk P leaching areas, such as around drinking troughs in pastures.

Ethiopian agriculture has greater potential for carbon sequestration than previously estimated.

More than half of the cultivation-induced carbon loss from agricultural soils could be restored through improved management. To incentivise carbon sequestration, the potential of improved practices needs to be verified. To date, there is sparse empirical evidence of carbon sequestration through improved practices in East-Africa. Here, we show that agroforestry and restrained grazing had a greater stock of soil carbon than their bordering pair-matched controls, but the difference was less obvious with terracing. The controls were treeless cultivated fields for agroforestry, on slopes not terraced for terracing, and permanent pasture for restrained grazing, representing traditionally managed agricultural practices dominant in the case regions. The gain by the improved management depended on the carbon stocks in the control plots. Agroforestry for 6-20 years led to 11.4 Mg ha-1 and restrained grazing for 6-17 years to 9.6 Mg ha-1 greater median soil carbon stock compared with the traditional management. The empirical estimates are higher than previous process-model-based estimates and indicate that Ethiopian agriculture has greater potential to sequester carbon in soil than previously estimated.

High-performance liquid chromatography (HPLC) as a tool for monitoring the fate of fluazinam in soil.

Fluazinam is a widely used pesticide employed against the fungal disease late blight in potato cultivation. A specific, repeatable, and rapid high-performance liquid chromatography (HPLC) method utilizing a diode array detector (DAD) was developed to determine the presence of fluazinam in soil. The method consists of acetonitrile (ACN) extraction, clean-up with solid-phase extraction (SPE), and separation using a mobile phase consisting of 70% ACN and 30% water (v/v), including 0.02% acetic acid. HPLC was performed with a C18 column and the detection wavelength was 240 nm. The method was successfully applied to an incubation experiment and to soil samples taken from potato fields where fluazinam had been applied two to three times during the on-going growing season. In the 90-day incubation experiment, analytical standard fluazinam and the commercial fungicide Shirlan(®) were added to soil samples that had never been treated with fluazinam, and were then extracted with ACN and 0.01 M calcium chloride (CaCl2). Fluazinam was not extractable with CaCl2, indicating that it does not leach to watercourses in the dissolved form. Recovery with ACN extraction for sandy soils was 72-95% immediately after application and 53-73% after 90 days of incubation. Out of the eight potato field soil samples, fluazinam was found in two samples at concentrations of 2.1 mg kg(-1) and 1.9 mg kg(-1), well above the limit of quantification (0.1 mg kg(-1)).

Phylogenetic diversity of Rhizobium species recovered from nodules of common beans (Phaseolus vulgaris L.) in fields in Uganda: R. phaseoli, R. etli, and R. hidalgonense.

A total of 75 bacterial isolates were obtained from nodules of beans cultivated across 10 sites in six agro-ecological zones in Uganda. Using recA gene sequence analysis, 66 isolates were identified as members of the genus Rhizobium, while nine were related to Agrobacterium species. In the recA gene tree, most Rhizobium strains were classified into five recognized species. Phylogenetic analysis based on six concatenated sequences (recA-rpoB-dnaK-glnII-gyrB-atpD) placed 32 representative strains into five distinct Rhizobium species, consistent with the species groups observed in the recA gene tree: R. phaseoli, R. etli, R. hidalgonense, R. ecuadorense, and R. sophoriradicis, with the first three being the predominant. The rhizobial strains grouped into three nodC subclades within the symbiovar phaseoli clade, encompassing strains from distinct phylogenetic groups. This pattern reflects the conservation of symbiotic genes, likely acquired through horizontal gene transfer among diverse rhizobial species. The 32 representative strains formed symbiotic relationships with host beans, while the Agrobacterium strains did not form nodules and lacked symbiotic genes. Multivariate analysis revealed that species distribution was influenced by the environmental factors of the sampling sites, emphasizing the need to consider these factors in future effectiveness studies to identify effective nitrogen-fixing strains for specific locations.

Leaching of nitrogen, phosphorus and other solutes from a controlled drainage cultivated peatland in Ruukki, Finland.

Cultivated peatlands are important for grass production in Northern Europe, but the potential impact of nutrients leaching to surface waters is a major concern. Due to a lack of data on nitrogen (N), phosphorus (P) and organic carbon leaching, a monitoring programme was established at Ruukki (Siikajoki, Finland), an agricultural, subsurface drained peat site with a peat thickness of 20-80 cm. Concentrations and loading of N, P, and total organic carbon (TOC) were monitored, along with other water quality parameters for the field discharge, in 2018-2021. We observed N leaching from subsurface discharge to be 25 kg N ha-1 year-1 (range 11-40 kg N ha-1 year-1, 74 % as nitrate NO3-N). The least N leaching was recorded from plots of thinner peat topsoil and those with grass cover, while the majority of N leaching originated from thicker peat plots (bare or under barley) in spring. Leaching of N strongly decreased during periods of thick grass cover. Significant N leaching also occurred during the mild winter of 2019-2020, characterized by alternating freeze and thaw periods. Annual P loading from subsurface drainage was 0.30 kg P ha-1 (0.20-0.43 kg P ha-1), low compared to that of average cultivated soils in Finland. It was estimated that 13 % of the total N leaching and 50 % of the total P leaching occurred in surface runoff. Leaching of TOC was significant at 87 kg ha-1 year-1 (31-137 kg ha-1 year-1). Leaching of dissolved P and TOC increased with peat thickness. Abundant loading of sulfur and acidity indicates the oxidation of sulfidic material in the subsoil. Leaching concentrations correlated with discharge quantity, suggesting that mobilization processes during the dry periods resulted in leaching during high discharge periods. The results show the importance of avoiding bare peat soil for NO3-N leaching reduction, even during wintertime in cultivated peatlands.

Phylogenetically diverse Bradyrhizobium genospecies nodulate Bambara groundnut (Vigna subterranea L. Verdc) and soybean (Glycine max L. Merril) in the northern savanna zones of Ghana.

A total of 102 bacterial strains isolated from nodules of three Bambara groundnut and one soybean cultivars grown in nineteen soil samples collected from northern Ghana were characterized using multilocus gene sequence analysis. Based on a concatenated sequence analysis (glnII-rpoB-recA-gyrB-atpD-dnaK), 54 representative strains were distributed in 12 distinct lineages, many of which were placed mainly in the Bradyrhizobium japonicum and Bradyrhizobium elkanii supergroups. Twenty-four of the 54 representative strains belonged to seven putative novel species, while 30 were conspecific with four recognized Bradyrhizobium species. The nodA phylogeny placed all the representative strains in the cosmopolitan nodA clade III. The strains were further separated in seven nodA subclusters with reference strains mainly of African origin. The nifH phylogeny was somewhat congruent with the nodA phylogeny, but both symbiotic genes were mostly incongruent with the core housekeeping gene phylogeny indicating that the strains acquired their symbiotic genes horizontally from distantly related Bradyrhizobium species. Using redundancy analysis, the distribution of genospecies was found to be influenced by the edaphic factors of the respective sampling sites. In general, these results mainly underscore the high genetic diversity of Bambara groundnut-nodulating bradyrhizobia in Ghanaian soils and suggest a possible vast resource of adapted inoculant strains.

Thickness of peat influences the leaching of substances and greenhouse gas emissions from a cultivated organic soil.

The off-site effects of agricultural organic soils include the leaching of N, P, and organic carbon (OC) to watercourses and CO2, CH4, and N2O emissions into the atmosphere. The aim of this study was to quantify how the thickness of organic layers affects these loads. A 19.56-ha experimental field drained by subsurface pipes was established in Ruukki, northwestern Finland. Three plots had a 60-80 cm-thick sedge peat layer and three others had a thickness of 20 cm or less. The drainage pipes lie in mineral soil that, in this field, contains sulfidic material. This study documents the experimental settings and reports on the leaching of substances in the first two years, as well as CO2, CH4 and N2O emissions during eight weeks in one summer. Total N (TN) and OC loads were higher from the thicker peat plots. The mean TN loads during a hydrological year were 15.4 and 9.2 kg ha-1 from the thicker and thinner peat plots, respectively, with organic N representing 36% of TN load. Total P (TP) load averaged 0.27 kg ha-1 yr-1. Dissolved P load represented 63 and 36% of TP in the thicker peat area and only 23 and 13% in the thinner peat area, and was thus increased upon peat thickness. These N and P loads through the subsurface drainage system represented roughly 83% of TN and 64% of TP loads from this field. There were no clear differences in greenhouse gas emissions among the plots during the eight-week monitoring period. Slowly oxidizing sulfide in the subsoil resulted in annual leaching of 147 kg S ha-1, almost ten times that of non-sulfidic soils. Our first results emphasize the effect of the peat thickness on the leaching of substances and warn about considering all organic soils as a single group in environmental assessments.

Genetically diverse lentil- and faba bean-nodulating rhizobia are present in soils across Central and Southern Ethiopia.

In total 196 bacterial isolates were obtained from root nodules of lentil (Lens culinaris) and faba bean (Vicia faba) grown on soil samples collected from 10 different sites in central and southern parts of Ethiopia. All isolates were identified as members of the genus Rhizobium by using recA gene sequence analysis. In the recA phylogenetic tree 195 rhizobial strains were classified into nine genospecies. The phylogeny of symbiotic genes nodC and nifH revealed five and six distinct groups respectively, largely dominated by symbiovar viciae. A multivariate analysis showed that environmental variables of the sampling sites considered in this study had more effect on the distribution and composition of the genospecies than the host legumes of the strains. Twenty representative strains, selected based on their isolation site, host plant and nodC group, were able to nodulate all lentil, faba bean, field pea (Pisum abyssinicum) and grass pea (Lathyrus sativus) plants in a greenhouse test in axenic conditions. The majority of the rhizobial strains were effective nitrogen-fixing symbionts for all tested legumes, indicating their potential to serve as broad host-range inoculants in agriculture. The present work suggests the presence of taxonomically and symbiotically diverse rhizobial species for legumes in the Viciae tribe in Ethiopia.

Low black carbon concentration in agricultural soils of central and northern Ethiopia.

Soil carbon (C) represents the largest terrestrial carbon stock and is key for soil productivity. Major fractions of soil C consist of organic C, carbonates and black C. The turnover rate of black C is lower than that of organic C, and black C abundance decreases the vulnerablility of soil C stock to decomposition under climate change. The aim of this study was to determine the distribution of soil C in different pools and impact of agricultural management on the abundance of different species. Soil C fractions were quantified in the topsoils (0-15cm) of 23 sites in the tropical highlands of Ethiopia. The sites in central Ethiopia represented paired plots of agroforestry and adjacent control plots where cereal crops were traditionally grown in clayey soils. In the sandy loam and loam soils of northern Ethiopia, the pairs represented restrained grazing with adjacent control plots with free grazing, and terracing with cereal-based cropping with adjacent control plots without terracing. Soil C contained in carbonates, organic matter and black C along with total C was determined. The total C median was 1.5% (range 0.3-3.6%). The median proportion of organic C was 85% (range 53-94%), 6% (0-41%) for carbonate C and 6% (4-21%) for black C. An increase was observed in the organic C and black C fractions attributable to agroforestry and restrained grazing. The very low concentration of the relatively stable black C fraction and the dominance of organic C in these Ethiopian soils suggest vulnerability to degradation and the necessity for cultivation practices maintaining the C stock.

Effects of Break Crops on Yield and Grain Protein Concentration of Barley in a Boreal Climate.

Rotation with dicotyledonous crops to break cereal monoculture has proven to be beneficial to successive cereals. In two fields where the soil had been subjected to prolonged, continuous cereal production, two 3-year rotation trials were established. In the first year, faba bean, turnip rape and barley were grown, as first crops, in large blocks and their residues tilled into the soil after harvest. In the following year, barley, buckwheat, caraway, faba bean, hemp and white lupin were sown, as second crops, in each block and incorporated either at flowering stage (except barley) or after harvest. In the third year, barley was grown in all plots and its yield and grain protein concentration were determined. Mineral N in the plough layer was determined two months after incorporation of crops and again before sowing barley in the following year. The effect of faba bean and turnip rape on improving barley yields and grain protein concentration was still detectable two years after they were grown. The yield response of barley was not sensitive to the growth stage of second crops when they were incorporated, but was to different second crops, showing clear benefits averaging 6-7% after white lupin, faba bean and hemp but no benefit from caraway or buckwheat. The effect of increased N in the plough layer derived from rotation crops on barley yields was minor. Incorporation of plants at flowering stage slightly increased third-year barley grain protein concentration but posed a great potential for N loss compared with incorporation of crop residues after harvest, showing the value of either delayed incorporation or using catch crops.

Use of sequential extraction to assess metal partitioning in soils.

The state of heavy metal pollution and the mobility of Cd, Cu, Ni, Cr, Pb and Zn were studied in three texturally different agricultural soil profiles near a Cu-Ni smelter in Harjavalta, Finland. The pseudo-total concentrations were determined by an aqua regia procedure. Metals were also determined after division into four fractions by sequential extraction with (1) acetic acid (exchangeable and specifically adsorbed metals), (2) a reducing agent (bound to Fe/Mn hydroxides), (3) an oxidizing agent (bound to soil organic matter) and (4) aqua regia (bound to mineral structures). Fallout from the smelter has increased the concentrations of Cd, Cu and Ni in the topsoil, where 75-90% of Cd, 49-72% of Cu and 22-52% of Ni occurred in the first two fractions. Slight Pb and Zn pollution was evident as well. High proportions of mobile Cd, Cu and Ni also deeper in the sandy soil, closest to the smelter, indicated some downward movement of metals. The hydroxide-bound fraction of Pb dominated in almost all soils and horizons, while Ni, Cr and Zn mostly occurred in mineral structures. Aqua regia extraction is usefully supplemented with sequential extraction, particularly in less polluted soils and in soils that exhibit substantial textural differences within the profiles.

Response of pore water Al, Fe and S concentrations to waterlogging in a boreal acid sulphate soil.

Environmental hazards caused by acid sulphate (AS) soils are of worldwide concern. Among various mitigation measures, waterlogging has mainly been studied in subtropical and tropical conditions. To assess the environmental relevance of waterlogging as a mitigation option in boreal AS soils, we arranged a 2.5-year experiment with monolithic lysimeters to monitor changes in the soil redox potential, pH and the concentrations of aluminium (Al), iron (Fe) and sulphur (S) in pore water in response to low and high groundwater levels in four AS soil horizons. The monoliths consisted of acidic oxidized B horizons and a reduced C horizon containing sulphidic material. Eight lysimeters were cropped (reed canary grass, Phalaris arundinacea) and two were bare without a crop. Waterlogging was conducive to reduction reactions causing a slight rise in pH, a substantial increase in Fe (Fepw) and a decrease in Al (Alpw) in the pore water. The increase in Fepw was decisively higher in the cropped waterlogged lysimeters than in the bare ones, which was attributable to the microbiologically catalysed reductive dissolution of poorly ordered iron oxides and secondary minerals. In contrast to warmer climates, Fepw concentrations remained high throughout the experiment, indicating that the reduction was poised in the iron range, while sulphate was not reduced to sulphide. Therefore, the precipitation of iron sulphide was negligible in the environment with a low pH and abundant with poorly ordered Fe oxides. Increased Fe in pore water counteracts the positive effects of waterlogging, when water is flushed from fields to watercourses, where re-oxidation of Fe causes acidity and oxygen depletion. However, waterlogging prevented further oxidation of sulphidic materials and decreased Alpw to one-tenth of the initial concentrations, and even to one-hundredth of the levels in the low water table lysimeters.

The microbial communities and potential greenhouse gas production in boreal acid sulphate, non-acid sulphate, and reedy sulphidic soils.

Acid sulphate (AS) soils along the Baltic coasts contain significant amounts of organic carbon and nitrogen in their subsoils. The abundance, composition, and activity of microbial communities throughout the AS soil profile were analysed. The data from a drained AS soil were compared with those from a drained non-AS soil and a pristine wetland soil from the same region. Moreover, the potential production of methane, carbon dioxide, and nitrous oxide from the soils was determined under laboratory conditions. Direct microscopic counting, glucose-induced respiration (GIR), whole cell hybridisation, and extended phospholipid fatty acid (PLFA) analysis confirmed the presence of abundant microbial communities in the topsoil and also in the deepest Cg2 horizon of the AS soil. The patterns of microbial counts, biomass and activity in the profile of the AS soil and partly also in the non-AS soil therefore differed from the general tendency of gradual decreases in soil profiles. High respiration in the deepest Cg2 horizon of the AS soil (5.66 µg Cg(-1)h(-1), as compared to 2.71 µg Cg(-1)h(-1) in a top Ap horizon) is unusual but reasonable given the large amount of organic carbon in this horizon. Nitrous oxide production peaked in the BCgc horizon of the AS and in the BC horizon of the non-AS soil, but the peak value was ten-fold higher in the AS soil than in the non-AS soil (82.3 vs. 8.6 ng Ng(-1)d(-1)). The data suggest that boreal AS soils on the Baltic coast contain high microbial abundance and activity. This, together with the abundant carbon and total and mineral nitrogen in the deep layers of AS soils, may result in substantial gas production. Consequently, high GHG emissions could occur, for example, when the generally high water table is lowered because of arable farming.

A multi-scale comparison of dissolved Al, Fe and S in a boreal acid sulphate soil.

Acid sulphate (AS) soils are most prevalent in the tropics, but the acidic discharge from cultivated AS soils also threatens water bodies under boreal conditions. Feasible options to reduce the acid load are needed. In this study, the groundwater of an AS field was monitored for 3.5 years, and the efficiency of waterlogging in mitigating the environmental risks caused by acidic discharge was investigated in a 2.5-year experiment with 10 monolithic lysimeters taken from the same field. In order to unravel the transferability of the results from lysimeters to the field scale, the Al, Fe and S concentrations in discharge water from the lysimeters were compared with those in the groundwater of the AS field (pedon and field scale), and in pore water (pedon and horizon scale). In the waterlogged bare lysimeters (HWB), the Al, Fe and S concentrations in discharge waters were broadly similar to those measured in the groundwater and followed the changes in the pore water. In the waterlogged cropped (reed canary grass, Phalaris arundinacea) lysimeters (HWC), in contrast, the discharge waters were markedly higher in Fe and lower in Al than the groundwater in the field. This outcome was attributable to the reduction of Fe(3+) to the more soluble Fe(2+) and the reduction-induced increase in pH, which enhanced the formation of Al(3+) hydroxy species. Lowering of the water table (LWC) caused soil ripening, which resulted in increased saturated hydraulic conductivity and porosity and enhanced the oxidation of sulphidic materials and acid formation. The responses of Al, Fe and S in drainage waters from HWC and LWC lysimeters resembled previous findings in AS soils. Based on this and the similarity between dissolved element concentrations in the discharge water of HWB lysimeters and groundwater in the field, we conclude that our monolithic lysimeters yielded realistic results concerning the efficiency of various methods in mitigating environmental risks related to cultivated AS soils.

Dispersal, environment, and floristic variation of western Amazonian forests.

The distribution of plant species, the species compositions of different sites, and the factors that affect them in tropical rain forests are not well understood. The main hypotheses are that species composition is either (i) uniform over large areas, (ii) random but spatially autocorrelated because of dispersal limitation, or (iii) patchy and environmentally determined. Here we test these hypotheses, using a large data set from western Amazonia. The uniformity hypothesis gains no support, but the other hypotheses do. Environmental determinism explains a larger proportion of the variation in floristic differences between sites than does dispersal limitation; together, these processes explain 70 to 75% of the variation. Consequently, it is important that management planning for conservation and resource use take into account both habitat heterogeneity and biogeographic differences.