Ralph A. Leonard

Riparian Forests as Nutrient Filters in Agricultural Watersheds

Riparian (streamside) vegetation may help control transport of sediments and chemicals to stream channels. Studies of a coastal plain agricultural watershed showed that riparian forest ecosystems are excellent nutrient sinks and buffer the nutrient discharge from surrounding agroecosystems. Nutrient uptake and removal by soil and vegetation in the riparian forest ecosystem prevented outputs from agricultural uplands from reaching the stream channel. The riparian ecosystem can apparently serve as both a shortand long-term nutrient filter and sink if trees are harvested periodically to ensure a net uptake of nutrients. (Accepted for publication 28 November 1983)

Nutrient budgets for agricultural watersheds in the southeastern coastal plain

Watershed-level agroecosystem studies are essential to relate land management to the external environmental effects produced by agricultural nutrients and to enhance our understanding of agricultural nutrient cycles. Inputs and outputs of N, P, K, Ca, Mg, and Cl were determined for four subwatersheds of the Little River in the Georgia Coastal Plain from 1979 through 1981. The four watersheds had 40, 36, 54, and 50%, respectively, of their land in agricultural uses (row crop and pasture). Precipitation inputs and streamflow outputs were determined by field sampling of water volumes and nutrient concentrations. Agronomic inputs (from fertilizer and symbiotic N-fixation) and outputs in harvested material were estimated from land use data; countywide averages of fertilizer applications and crop yield; and plot studies on peanuts and soybeans. All elements except C1 had greater inputs than outputs on each watershed each year. The general order of streamflow loads was Cl > Ca > K > Mg > N > P. Fertilizer inputs exceeded precipitation inputs for all elements on all watersheds. Outputs of N, P, and K in harvest generally exceeded streamflow loads, but harvest outputs of Ca, Mg, and Cl were generally lower than streamflow loads. The two watersheds with more agri- cultural land had consistently higher loads of N, K, Ca, Mg, and Cl in streamflow and had NO3-N loads 1.5 to 4.4 times higher than loads from the less agricultural watersheds. Streamflow loads on the Little River watersheds were similar to those on other Coastal Plain agricultural watersheds with comparable land use and discharge volumes. Budgets for the upland portion of one of the watersheds indicated that large amounts of N, P, K, Ca, and Mg were not accounted for. About 56 kg ha-1 yr- of N were retained or lost to gaseous emissions from the uplands. Apparently, a large percentage of the nutrients applied to these watersheds was being retained somewhere in the watershed or being lost in some unquantified way.

TRACER STUDIES OF SUBSURFACE FLOW PATTERNS IN A SANDY LOAM PROFILE

A study designed to provide a better understanding of the mechanisms of agrichemical transport to groundwater was conducted on a 0.81-ha agricultural corn field near Plains, Georgia. The objectives were to: (1) characterize vadose zone flow paths of water and agrichemicals under normal climatic and management conditions and evaluate their spatial and temporal variability; and (2) relate spatial and temporal transport patterns to geophysical properties of the soil and climatic conditions. Agrichemical transport was assessed over a five-year period from 1989 to 1994 through analysis of collected soil and groundwater samples. A bromide (Br –) tracer was applied at 78 kg ha–1 in 1989 and at 105 kg ha–1 in 1991. Chloride (Cl–) and nitrogen were applied with fertilizer each year except 1994. Soil characterization tests indicated a dramatic decrease in the saturated hydraulic conductivity associated with a large increase in clay content in a zone from 1 to 4 m below the soil surface. As a result of this soil feature, Br – concentrations in the vadose zone below 4 m were normally less than 2 mg kg–1 throughout the study. Aquifer chemical concentrations indicated nitrate nitrogen (NO3 – N) and Cl– applied to the soil surface in the spring were transported to the groundwater at 9 m by that same fall. Bromide concentrations in ground water peaked at 0.65 mg L–1 while NO3 – N concentrations peaked at 6.9 mg L–1 and Cl– at 4.0 mg L–1. Agrichemical transport and variability were controlled by climatic patterns and soil hydraulic characteristics. Transport to groundwater increased when precipitation and irrigation volumes in the first 30 days after spring fertilization and planting exceeded twice the normal precipitation. If large spring thunderstorms occur soon after chemical application, the likelihood of groundwater contamination by agrichemicals is substantially increased. These data provide the means to relate transport of agrichemicals in and through the vadose zone to geophysical characteristics and irrigation and precipitation inputs.

ATRAZINE AND CARBOFURAN TRANSPORT THROUGH THE VADOSE ZONE IN THE CLAIBORNE AQUIFER RECHARGE AREA

A 1-ha field plot with a sandy surface soil, located near Plains, Georgia, was studied for three years (from 1993 to 1995) to evaluate pesticide transport in the vadose zone. Vadose zone soil samples were collected 23 times: prior to the initial 1993 pesticide application, each year at approximately 1, 3, 7, 14, 28, and 44 days after pesticide application, each fall after harvest, and in the spring of 1995 prior to planting. The samples were analyzed for atrazine, carbofuran, deethylatrazine (DEA), and deisopropylatrazine (DIA). Atrazine and carbofuran in the active root zone (< 100 cm) degraded rapidly. Overall, the higher concentration levels of atrazine, DEA, DIA, and carbofuran were limited to the top 25 cm of the profile and to the period from 1 to 30 days after application. On the average, by 30 days after application 83% of the atrazine and 96% of the carbofuran had degraded. By 44 days after application, virtually all of the pesticides in the top 250 cm of the soil had degraded. Atrazine was found to be more persistent than was carbofuran with a half life approximately twice that for carbofuran. A two-stage model with a variable dissipation rate for the period up to 44 days after pesticide application and a second dissipation rate for periods greater than that was found to fit the data better than a single stage model. For the first 44 days after application, the first-order decay rate with a half life of 12 days was found to fit the field data for atrazine within the soil profile. A first-order decay rate with a half life of approximately 6 days fit the observed carbofuran data best. The dissipation rate decreased rapidly after the first 44 days. When a two-stage dissipation process was assumed, the dissipation rate coefficient decreased from 0.059 to 0.006 (days -1 ) for atrazine, while for carbofuran it decreased from 0.110 to 0.018 (days -1 ). Observed levels of the atrazine metabolites DIA and DEA were highest in the top 1 cm of the soil. There appeared to be some movement or creation of the metabolites at lower depths in the profile later in the growing season, but not at large concentrations. Keywords. Soils, Aquifers, Pesticide transport, Water quality, Atrazine, Carbofuran.

Simulating processes in nonpoint source pollution

The paper describes development of a mathematical model to evaluate nonpoint pollution from diffuse agricultural and forestry sources. Although the model includes numerous physical and chemical processes, a generalized flow chart is used to present the entire system along with more detailed components. The nitrogen cycling and pesticide elements of the chemistry components are presented. The interactions of complex processes are described relative to climate, soil, and agricultural management practices.