Knowledge of nutrients leached from crop residues will aid in understanding nutrient cycling in agricultural systems and in the development of small watershed chemical transport models. Using a multiple‐intensity rainfall simulator, wheat straw residue (Triticum aestivum L.) was subjected to 25.4 mm simulated rainfall at intensities of 7, 12, 25, 53, and 105 mm h−1. The wheat straw loading rate was 4500 kg ha−1. Runoff was sampled as a function of time and analyzed for PO43−‐P, NH4+‐N, NO3−‐N, and organic carbon (OC). Except for NO3−‐N, nutrient concentrations and losses were greater at the lower rainfall intensities. At each intensity, PO43−‐P, NH4+‐N, and OC concentrations in runoff from the wheat straw increased rapidly to maximum values and then decreased with time. After maximum nutrient concentrations were reached, a power function, Y = aXb, best described the relationship of nutrient concentrations with time, whereas a hyperbolic equation of the form Y = 1/(a + bX) best described the relationship of nutrient concentrations with runoff. The quantity (kg ha−1) of nutrients leached from the wheat straw followed the order C > P > N as (NH4+‐N + NO3−‐N). Amounts of N as (NH4+‐N + NO3−‐N) and C leached from the wheat straw were ≤ 1% of the nutrient content compared with 80 to 140 g kg−1 for P. This study also indicates that the variability in both the leachability and nutrient content of crop residues from different sources will be important, factors in the development of crop residue leaching models.
The adsorption of phosphate, arsenate, MSMA (sodium methanearsonate) and cacodylic acid (hydroxydimethylarsine oxide) by 14 sediments from streams and lakes of the Mississippi River alluvial flood plain was measured using a phosphate fixation test and also dilate (1 g/100 mL) sediment‐water slurries at 50 and 500 µg/L P or As. The fixation experiment results were similar to a previous study with soils from the same geographic area in that sorption of the four species were similar and correlated with clay and extractable Fe and Al content. In contrast with the soils results, phosphate was more strongly adsorbed than the arsenicals because the sediments were richer in Fe and Al oxides than soils of the same clay content and the phosphate sorption index was most dependent on sediment Fe and Al content. Conversely, the arsenicals were most correlated with clay or Fe content. Arsenical fixation also exhibited some sediment‐pH dependence indicative of binding with Ca. In the dilute slurry experiments, the high‐clay sediments were extremely adsorptive of all the species and yielded equilibrium P concentration (EPC) values from 0.07 to 0.3 µg/L P. Cacodylate adsorption was dependent on sediment pH. Those sediments with high clay content had a very large unfilled capacity to adsorb phosphate or arsenate, and would act as sinks for P or As inputs into their associated lakes or streams.
Developing models for predicting amounts of pesticide transported in runoff and suspended sediment from agricultural land to aquatic. habitats has been hampered for foliarly‐applied pesticides by a lack of knowledge about the processes of pesticide washoff from plants by rainfall. A multiple‐intensity rainfall simulator was used to determine the effects of rain intensity and amount on permethrin [(3‐phenoxyphenyl) methyl (±)‐cis, trans‐3‐(2,2‐dichloroethenyl)‐2,2‐dimethylcyclopropanecarboxylate] concentrations and amounts washed from mature cotton (Gossypium hirsutum L.) plants. Permethrin concentration in plant washoff was independent of rain intensity when 25 mm of simulated rain was applied at 6.6, 13.0, 25.7, and 51.3 mm h−1 2 h after permethrin was applied at 0.224 kg ha−1. Concentrations were less when the same amount of simulated rain was applied at 106.4 mm h−1. Permethrin concentrations in plant washoff decreased rapidly during the early phases of washoff. About 35% of the permethrin load on the plants 2 h after application was washed off by 25 mm of rain; an additional 76 mm of rain removed only 11% more of the permethrin from the plants. Rainfall amount had greater influence than rainfall intensity on the amount of permethrin washed from the cotton plants. This information greatly simplifies modeling the movement of permethrin from the plant canopy to soil during natural storms when intensities vary greatly within storms and from storm to storm.
Stormflow from five reforested watersheds (1.5 to 2.8 ha) in northern Mississippi was analyzed during the 1974 water year (Oct. 1973 to Sept. 1974) for phosphorus (P) in solution and in association with suspended sediments. Samples were collected for each storm with Coshocton wheel samplers set below 0.91‐m H‐flumes. For the year, mean concentration of total P in solution was 0.027 mg/liter for the five watersheds. Of this, 45% was hydrolyzable P, 33% ortho‐P, and 22% organic P. Sediment total P concentrations ranged from 274 to 1,067 µg/g and were 2.0 to 8.9 times that in the watershed soils. Increased concentration of P in suspended sediment relative to soil is attributed to selective erosion of fine sediments and/or deposition of coarse sediments in transport. For the five watersheds, solution total P yield during the water year averaged 88 g/ha; whereas sediment total P yield averaged 210 g/ha and accounted for 64 to 76% of the sediment plus solution P yield.
Volatilization and subsequent aerial transport is thought to be a major pathway of pesticide disappearance from application sites. Corroborative evidence obtained for agricultural pesticides under field conditions is scarce. The contribution of volatilization to the overall disappearance of toxaphene (chlorinated camphene) and DDT [1,1,1‐trichloro‐2,2‐bis(p‐chlorophenyl)ethane] from cotton (Gossypium hirsutum L.) was studied under field conditions in the “Delta” section of Mississippi. Drought conditions prevailed throughout most of the study. Measurements were made during two periods: (i) a 10.7‐d period after toxaphene was applied by ground equipment to 50‐cm‐tall cotton plants, and (ii) a 32.7‐d period after a similar application of a mixture of toxaphene and DDT to the same plants. Variable amounts of pesticide were unaccounted for (toxaphene, 17% first application, 54% second application; DDT, 72%) and were apparently lost during application and the following 3‐h period before sampling of air, soil, and plants was begun. The calculated 50% disappearance times of toxaphene (4.7 d, first application; and 10.8 d second application) and DDT (10.3 d) on plants agreed reasonably well with previously reported values. Pesticide disappearance rates were linear functions of the pesticide loads on the plants. The volatile loss of toxaphene (as quantified by the last four chromatographic peaks) during the 10.7‐d test period after the first application was 17% of the amount intercepted by the plants. The comparable volatile loss during the first 10.7 d after the second application (32.7‐d test period) was 33% of the amount on the plants. Total toxaphene and DDT volatile losses during the complete 32.7‐d test period were 53 and 58% of the amounts on the plants, respectively. Because of dry weather, no measurable pesticide volatilization occurred from soil. Disappearance rate changes, as well as volatilization rate changes, for toxaphene and DDT applied at the same time were approximately equal. The study provides additional evidence that post‐application volatilization from plants is a major pathway of pesticide transport.
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