Article pubs.acs.org/JAFC
Organomineral Interactions and Herbicide Sorption in Brazilian Tropical and Subtropical Oxisols under No-Tillage Eloana J. Bonfleur,*,† Rai S. Kookana,§ Valdemar L. Tornisielo,# and Jussara B. Regitano⊥ †
Department of Soils and Agricultural Engineering, Federal University of Paraná, Rua dos Funcionários 1540, Curitiba, Paraná, Brazil CSIRO Land and Water, Waite Campus, PMB No. 2, Glen Osmond, South Australia 5064, Australia # Center of Nuclear Energy in Agriculture (CENA). Ecotoxicology Laboratory, University of São Paulo (USP), Piracicaba, São Paulo, Brazil ⊥ “Luiz de Queiroz” College of Agriculture, Soil Science Department, University of São Paulo, Avenida Pádua Dias 11, Piracicaba, São Paulo, Brazil §
S Supporting Information *
ABSTRACT: We evaluated the effects of the soil organic matter (SOM) composition, distribution between soil aggregates size, and their interactions with the mineral phase on herbicide sorption (alachlor, bentazon, and imazethapyr) in tropical and subtropical Oxisols under no-till systems (NT). Using soil physical fractionation approach, sorption experiments were performed on whole soils and their aggregates. SOM chemistry was assessed by CP/MAS 13C NMR. The lower sorption observed in tropical soils was attributed to the greater blockage of SOM sorption sites than in subtropical soils. When these sites were exposed upon physical fractionation, sorption of the three herbicides in tropical soils increased, especially for imazethapyr. High amounts of poorly crystallized sesquioxides in these soils may have contributed to masking of sorption sites, indicating that organomineral interactions may lead to blockage of sorption sites on SOM in tropical soils. KEYWORDS: organic matter quality, CP/MAS 13C NMR, iron oxides, aluminum oxides, soil physical fractionation, alachlor, imazethapyr, bentazon
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INTRODUCTION The no-tillage system (NT) is adopted in about 25.5 million hectares in Brazil.1 In the southern region (subtropical), >80% of the cultivated area is currently under NT,2 whereas its implementation is increasing in the midwest region (tropical).3 In general, NT increases soil organic matter (SOM) contents due to enhanced biomass inputs and improved soil aggregation.4−7 Soils under NT usually have higher aggregate stability due to decreasing macroaggregate turnover, which favors microaggregate formation and soil organic carbon (SOC) sequestration.8,9 Oxisols are highly important for crop production in Brazil, in which SOM plays a major role in nutrient availability and is often a major cation exchange capacity (CEC) component.10 However, the presence of sesquioxides and 1:1 clays in abundance also plays an important role in carbon storage due to their reactive surfaces.11 Within Brazil, SOM properties were found to be variable mainly due to the different straw decomposition rates in different climatic regions.12 Therefore, it is important to better understand pesticide behavior in highly weathered soils under NT systems in different climatic regions. SOC content is usually positively correlated with most pesticides sorption.13 However, the simple correlation between SOC and pesticide sorption coefficient (Kd) does not always provide accurate information on the role of SOM in the retention process, mainly due to the diversity in their physicochemical properties.14 Therefore, both SOM chemistry and its physical conformation, as determined by its interaction with the clay and sesquioxide fractions, are in many cases more © XXXX American Chemical Society
important than SOC content to sorption of organic molecules.15−19 The use of soil physical fractionation allows homogeneous SOM fractions to be separated in relation to the nature and function12 and possible organomineral interaction to be inferred.20−23 Furthermore, the use of spectroscopic techniques is an important tool in determining SOM chemistry and its correlations with pesticide sorption.24,25 Recent studies showed that sorption was highest at the smaller soil fractions (
