Glyphosate and nutrient retention in preferential flow pathways

Ecología Austral G��������� 29:329-338 Diciembre 2019��������� �� ������������ ���� ��������S ��� �������� Asociación Argentina de Ecología 329 https://doi.org/10.25260/EA.19.29.3.0.855 Glyphosate and nutrient retention in preferential flow pathways G������ C. M. G������₁,*; P���� L������₂; V������� C. A�������₃; J��� L. C����₃ � M����� M. P��������₃ Chacra Experimental Integrada Barrow. INTA. Tres Arroyos, Argentina. 2 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) - Fundación Bariloche. San Carlos de Bariloche, Argentina. 3 Estación Experimental Agropecuaria Balcarce. INTA. Balcarce, Argentina. 1 Abstract. Riparian vegetation strips (RVS) reduce surface runoff volume and retain sediments, pesticides and nutrients that are transported across them from adjacent crop-fields (CF). The ability of these strips to retain glyphosate has been demonstrated using experimental plots, but the spatial variability of that process is unknown. In this work, the influence of microtopography inside the RVS on the retention of glyphosate (and its major metabolite, AMPA), phosphorus and nitrogen were analyzed within a RVS of agricultural landscapes. Retention levels inside and outside preferential flow pathways (PFP) were compared under presence and absence of a tree stratum. Soil glyphosate + AMPA concentration within PFP was 88-fold higher than outside. Phosphorus and nitrogen soil concentrations, clay and bulk density were also higher inside than outside the PFP. The tree stratum did not modify soil concentration of glyphosate + AMPA, phosphorus, nitrogen, clay content, nor the morphometry of the PFP. Bulk density and clay content recorded in adjacent CF and in PFP, in addition to the high glyphosate, phosphorus and nitrogen concentrations in PFP soil, are consistent with a hydraulic connection between the CF and the PFP. These results contrast with some conclusions obtained from experimental studies under uniform plots and emphasize the importance of taking into account the genesis and structure of PFP in the design, evaluation and management of the filtering function of RVS. [Keywords: overland flow, runoff, riparian retention, riparian vegetation strips, ecosystem services of riparian vegetation strip] Resumen. Retención de glifosato y nutrientes en vías de flujo preferencial. Las franjas de vegetación ribereña (RVS) reducen el volumen del flujo de escorrentía superficial y retienen los sedimentos, los pesticidas y los nutrientes que esas aguas transportan desde los campos de cultivo (CF) adyacentes. La capacidad de estas franjas para retener el glifosato se demostró por medio de parcelas experimentales, aunque se desconoce la variabilidad espacial de ese proceso. En este trabajo se analizó la influencia de la microtopografía dentro de las RVS en la retención de glifosato (y su metabolito principal, AMPA), fósforo y nitrógeno dentro de las RVS de paisajes agropecuarios. Se compararon los niveles de retención dentro y fuera de las vías de flujo preferencial (PFP), en presencia y ausencia de un estrato arbóreo. La concentración de glifosato + AMPA en los suelos dentro de las PFP fue 88 veces mayor que fuera de la PFP. Las concentraciones de fósforo y nitrógeno, el contenido de arcilla y la densidad aparente también fueron más altos dentro que fuera de las PFP. La presencia del estrato arbóreo no modificó las concentraciones de glifosato + AMPA, ni de fósforo, ni de nitrógeno, ni el contenido de arcilla, como tampoco la morfometría de las PFP. La densidad aparente y el contenido de arcilla, registrados en los CF y en las PFP, junto con las concentraciones más altas de glifosato, fósforo y nitrógeno en los suelos de las PFP, son consistentes con una conexión hidráulica entre los CF y las PFP. Estos resultados contrastan con algunas conclusiones obtenidas a partir de estudios experimentales en parcelas uniformes y ponen en evidencia la importancia de tomar en cuenta génesis y estructura de las PFP en el diseño, evaluación y manejo de la función de filtrado de las RVS. [Palabras clave: flujo superficial, escorrentía, retención ribereña, franjas de vegetación ribereña, servicios ecosistémicos de las franjas de vegetación ribereña] Editor asociado: Gervasio Piñeiro Recibido: 26 de Noviembre de 2018 * Aceptado: 24 de Junio de 2019 330 GCM GIACCIO ET AL I����������� Riparian vegetation strips (RVS) can play an important role in the maintenance of aquatic environments and water quality in agricultural basins through the retention of nutrients (e.g., Mayer et al. 2007; Hoffmann et al. 2009; Ranalli and Macallady 2010) and pesticides (Syversen 2003; Syversen and Bechmann 2004). Current agricultural production systems have been intensified and transformed into net consumers of external inputs. Among them are pesticides, phosphorus and nitrogen, whose surplus remains in the soil matrix (Okada et al. 2017), with the consequent risk of losses through runoff to surface and groundwater (Kronvang et al. 2012). The retention function of RVS has a renewed interest for Argentina, where the utilization of both fertilizers and pesticides has being dramatically increased throughout the last decades (Orué et al. 2007; Sasal et al. 2010; Giaccio 2017) and where particular concern exists there is a particular concern about the overuse of glyphosate (Aparicio et al. 2015). Glyphosate retention in RVS has been reported by Syversen and Bechmann (2004) and the influence of riparian vegetation structure in the retention of nutrients and glyphosate was analyzed by Giaccio et al. (2016). They found that grassy RVS had a higher glyphosate retention capacity than RVS with trees, while vegetation structure had no effect on phosphorus and nitrogen retention. Although many authors assume that the retention capacity is uniform throughout the channels of RVS, others show that runoff converges and diverges in different places, due to micro-topographic and edaphic differences Ecología Austral 29:329-338 (Dillaha et al. 1989; Sheppard et al. 2006; Hösl et al. 2012). This heterogeneity leads to a preferential flow pathway (PFP), which is probably related to the underlying cause of a heterogeneous capacity of pollutant retention within the RVS (Figure 1). When considering PFP as open channels, its capacity to conduct the surface water flow can be quantified by the hydraulic radius (Rh) (Wobus et al. 2006), and their impact could be related to the hydraulic connection between the CF (source of pollutants) and the water courses where it discharges (Kouwen and Li 1980). Pollutant retention within the PFP may be also related to the surface roughness, which reduces the speed of runoff flow (LeedsHarrison et al. 1999), the volume of surface runoff and, consequently, the detachment and transport of soil sediments (Cogo et al. 1983; Amoah et al. 2013). In this sense, Burwell and Larson (1969) demonstrated a highly positive correlation between soil infiltration capacity and rugosity. Neglecting thi (...truncated)