Predicting Pesticide Transport in Mulch-Amended Soils: A Two-Compartment Model

cation as the layby treatment. Such herbicide treatments follow the last cultivation before harvest. Shading and With adoption of combine harvest technology, massive sugarcane competition from sugarcane plants usually control weeds (Saccharum Spp. Hyb.) residue is left on the soil surface following each harvest, which could affect the environmental fate and efficacy developing after canopy closure. Atrazine is recomof applied pesticides in subsequent cropping seasons. The objective mended as a pre-emergence treatment to control winter of this study was to quantify the retention characteristics and mobilor early spring weeds or as a post-emergence treatment ity of a commonly used herbicide, atrazine (2-chloro-4-ethylamine-6-isofor layby or fallow fields. propylamino-S-triazine), in soils when sugarcane mulch residue was The role of crop residue on the fate of applied agripresent. Specifically, adsorption and desorption kinetic batch experichemicals in conservation tillage systems is not well ments were performed to quantify atrazine retention by the mulch understood. The effect of crop residue on interception, residue over time. Atrazine sorption exhibited time-dependent behavsubsequent wash-off, and movement of herbicide in the ior and was followed by slow release regardless of input concentration. soil profile is the primary focus associated with conserA kinetic—equilibrium model based on a second-order two-site (SOTS) vation measures in today’s agriculture. Several conserformulation was successful in describing atrazine adsorption versus time by the mulch residue. One set of model parameters was capable vation production systems are characterized by the presof describing atrazine release based on six successive desorption steps. ence of mulch residue left on the soil surface to protect As a test for the applicability of the model, data sets from two other it from water and soil erosion. In fact, several studies on experiments where sorption and release was measured for extended best management practices (BMP) have shown distinct time periods (1348 and 2476 h) were successfully predicted by the advantages of minimum or no-till systems (Dao, 1991, SOTS model. Miscible displacement methods were used to measure 1995; Banks and Robinson, 1982). the mobility of atrazine in packed columns where the mulch residue A literature search revealed that few studies have was mixed with a reference sand material or a Sharkey clay soil (very been performed on the fate and transport behavior of fine, smectitic, thermic Chromic Epiaquerts). Use of batch-measured herbicides in the presence of mulch residue. One exammodel parameters did not adequately describe atrazine mobility in ple was the column study by Dao (1991) where packed the sand-mulch column. In contrast, for a Sharkey-mulch soil column, a two-compartment SOTS model was successful in predicting atrazine wheat straw was subjected to herbicide pulses under difbreakthrough results where independently estimated parameters for ferent velocities. Based on the shape of the breakthrough the mulch residue and the soil matrix were used as the input paramecurves (BTCs), Dao (1991) concluded that metribuzin ters. Results also showed that mixing of mulch with soil created physi(4-amino-6-tert-butyl-4,5-dihydro-3-methyltio-1,2,4-trical non-equilibrium condition in the columns, which was responsible azin-5-one) was more retarded than S-ethyl-metribuzin. for errors in model predictions. The strong affinity of wheat (Triticum aestivum) straw residue for metribuzin resulted in asymmetrical BTCs for all combinations of straw ages (straws collected at S 1995, the sugarcane industry in Louisiana gradudifferent times after harvesting). Breakthrough curves ally adopted a new harvesting system that involves that exhibit asymmetry, which indicated slow herbicide the use of a combine harvester that separates leaf-materelease, were more pronounced at low pore-water velocrial from billets. The sugarcane residue is deposited ities. In contrast to metribuzin retention based on batch directly on the soil surface and may intercept chemical sorption experiments by the wheat straw, significantly spray and thus reduce the efficacy of soil-applied herbilower affinities of wheat straw for metribuzin during cide (Banks and Robinson, 1982; Ghadiri et al., 1984; transport was observed. Such differences were attribCrutchfield et al., 1985). Atrazine remains a major heruted to possible chemical non-equilibrium conditions of bicide that is used extensively in sugarcane production the sorption-desorption processes during transport in (Gianessi and Puffer, 1991). Chemical weed control prothe column experiments. Nevertheless, Dao (1991) congrams for sugarcane usually require two herbicide applicluded that the straw mulch is a temporary storage mecations, one before crop emergence and another postdium that alters herbicide retention patterns and may emergence before the crop canopy closes. Sugarcane be a significant mechanism of retardation of the moveproducers in southern Louisiana refer to the latter appliment of applied agricultural chemicals to the subsurface under conservation tillage. L. Ma, USDA-ARS, Great Plains Systems Research Unit, 2150 Centre Field investigations on a Norge loam (fine-silty, Avenue, Building D, Fort Collins, CO 80526. H.M. Selim, Dep. of mixed, active, thermic Udic Paleustolls) soil were later Agronomy and Environmental Management, Louisiana State Univ., performed by Dao (1995) to determine the effect of Baton Rouge, LA 70803. Contribution from the Louisiana Agricultural Experiment Station, LSU Agricultural Center as Manuscript no. placement of wheat straw on the mobility of metribuzin 03-14-0293, and USDA-ARS, Great Plains Systems Research Unit. versus depth. Results of measured metribuzin concenReceived 7 June 2004. *Corresponding author ([email protected]. edu). Abbreviations: BMP, best management practices; BTCs, breakthrough curves; CDE, the convective-dispersive equation; LSS, liquid Published in Soil Sci. Soc. Am. J. 69:318–327 (2005). © Soil Science Society of America scintillation spectroscopy; MRTM, multi-reaction transport model; RMSE, root mean square error; SOTS, second-order two-site. 677 S. Segoe Rd., Madison, WI 53711 USA