The environmental importance of iron speciation in soils: evaluation of classic methodologies.

Iron is an essential mineral and one of the most abundant in soils, presenting itself in the environment as ferrous and ferric ions. As each oxidation state of iron has a different role in the environment, its speciation in environmental studies is important. The determination of ferrous iron received great attention from soil chemists because of its important role in agriculture, in redox processes, and as an electron acceptor in the catalysis of organic matter. Methodologies with the use of colorimetric reagents to determine ferrous iron are divergent and not very clear. In this study, we compared two colorimetric reagents (1,10-phenanthroline and ferrozine) to determine the total concentration of iron, ferrous and ferric ions in soil, using simple and low-cost methodologies. The determination of ferrous and total iron with 1,10-phenanthroline colorimetric reagent, following published instructions, did not correlate with ferrozine method, presenting an erroneous quantification. After neutralizing the extract of 1,10-phenanthroline with NaOH, both colorimetric methods allowed to quantify with precision and high yield the amount of ferrous and total iron extracted from the soil. The oxidation states of iron have a different contribution and importance to the environment. In this sense, the improvement of a widely used methodology is crucial for the better study of iron speciation in soil.

Organic management increases soil nitrogen but not carbon content in a tropical citrus orchard with pronounced N2O emissions.

The use of organic amendments is important for the sustainability of organic farming, with implications for soil organic matter turnover, nutrient cycling and greenhouse gases (GHGs) emissions to the atmosphere. Here, we investigated how long-term citrus organic farming influenced carbon sequestration and GHG emissions under organic and conventional management. We assessed the effects of management systems on soil organic matter dynamics and GHG emissions, focusing on N2O direct emissions from fertilizers. Soil stable isotope C and N compositions (0-100 cm) were used as parameters to assess changes in soil organic matter dynamics, with native forest as the reference. After the conversion from forest to orange orchard, stocks of soil C increased approximately 40 Mg ha-1, whereas stocks were similar in the organic and conventional treatments. Enrichment of 13C through the entire soil profile showed that organic matter from fertilizer replaced the original soil C by at least 20%, considering that poultry was fed only with C4 plants. By contrast, organic farming increased soil N stocks and inorganic N. Nitrogen emission factors for inorganic and organic fertilizers were 1.47 and 3.14, respectively. Organic management increased soil GHG emissions, primarily N2O emissions. Carbon emissions either as CO2 or CH4 were greater at the mid-rows than those under the crop canopy. We conclude that organic management did not promote C sequestration after six years of management. Moreover, organic management increased N2O emissions, and the GHG balance was more negative for organic than that for conventional farming when the ratio between crop harvest and emissions was determined.

Spatial modeling applied to environmental monitoring: identifying sources of potentially toxic metals in aquatic system.

Aquatic contamination by potentially toxic metals is a problem that has been aggravated, especially due to the quantity and the diversity of sources. Locating these sources is not always an easy task, especially because of the wide variety of possibilities. In this context, the application of geostatistical methods may represent an excellent tool to find out sources of metal contaminants in aquatic systems. Thus, the objective of this work was to elaborate an approach to identify sources of potentially toxic metals (Zn, Ba, Pb, Cr, Mn and Fe), by relating their spatial-temporal variations with the local land use patterns, along a longitudinal profile of the Pirapora River, located in the State of Sao Paulo, Brazil. For this purpose, water samples were collected at different points, taking into consideration each specific land use pattern and quantifying the metals contents by microwave plasma atomic emission spectrometry. In this work, thirteen land use patterns have been identified: mining, forestry, abandoned pasture, water, urban area, human occupation, floodplain, bare soil, temporary crop, roads, forest, streets and pasture. The results revealed temporal variations for the metals Ba, Cr, Fe, and Pb and spatial for Zn and Mn, making possible to correlate the presence of these two latter metals with mining and forestry, the most proeminent activities in the region. Overall, this work proposes a model which brings together geoprocessing and analytical methods, in order to correlate spatial-temporal variations of potentially toxic metals with specific land use patterns of a determined region, aiming the environmental monitoring.

Recycling organic residues in agriculture impacts soil-borne microbial community structure, function and N2O emissions.

Recycling residues is a sustainable alternative to improve soil structure and increase the stock of nutrients. However, information about the magnitude and duration of disturbances caused by crop and industrial wastes on soil microbial community structure and function is still scarce. The objective of this study was to investigate how added residues from industry and crops together with nitrogen (N) fertiliser affect the microbial community structure and function, and nitrous oxide (N2O) emissions. The experimental sugarcane field had the following treatments: (I) control with nitrogen, phosphorus, and potassium (NPK), (II) sugarcane straw with NPK, (III) vinasse (by-product of ethanol industry) with NP, and (IV) vinasse plus sugarcane straw with NP. Soil samples were collected on days 1, 3, 6, 11, 24 and 46 of the experiment for DNA extraction and metagenome sequencing. N2O emissions were also measured. Treatments with straw and vinasse residues induced changes in soil microbial composition and potential functions. The change in the microbial community was highest in the treatments with straw addition with functions related to decomposition of different ranges of C-compounds overrepresented while in vinasse treatment, the functions related to spore-producing microorganisms were overrepresented. Furthermore, all additional residues increased microorganisms related to the nitrogen metabolism and vinasse with straw had a synergetic effect on the highest N2O emissions. The results highlight the importance of residues and fertiliser management in sustainable agriculture.

Impacts of sugarcane agriculture expansion over low-intensity cattle ranch pasture in Brazil on greenhouse gases.

Sugarcane is a widespread bioenergy crop in tropical regions, and the growing global demand for renewable energy in recent years has led to a dramatic expansion and intensification of sugarcane agriculture in Brazil. Currently, extensive areas of low-intensity pasture are being converted to sugarcane, while management in the remaining pasture is becoming more intensive, i.e., includes tilling and fertilizer use. In this study, we assessed how such changes in land use and management practices alter emissions of greenhouse gases (GHG) such as CO2, N2O and CH4 by measuring in situ fluxes for one year after conversion from low-intensity pasture to conventional sugarcane agriculture and management-intensive pasture. Results show that CO2 and N2O fluxes increased significantly in pasture and sugarcane with tillage, fertilizer use, or both combined. Emissions were highly variable for all GHGs, yet, cumulatively, it was clear that annual emissions in CO2-equivalent (CO2-eq) were higher in management-intense pasture and sugarcane than in unmanaged pasture. Surprisingly, tilled pasture with fertilizer (management-intensive pasture) resulted in higher CO2-eq emissions than conventional sugarcane. We concluded that intensification of pasture management and the conversion of pasture to sugarcane can increase the emission factor (EF) estimated for sugarcane produced in Brazil. The role of management practices and environmental conditions and the potential for reducing emissions are discussed.