Rate of fall-applied liquid swine manure: effects on runoff transport of sediment and phosphorus.

Reducing the delivery of phosphorus (P) from land-applied manure to surface water is a priority in many watersheds. Manure application rate can be controlled to manage the risk of water quality degradation. The objective of this study was to evaluate how application rate of liquid swine manure affects the transport of sediment and P in runoff. Liquid swine manure was land-applied and incorporated annually in the fall to runoff plots near Morris, Minnesota. Manure application rates were 0, 0.5, 1, and 2 times the rate recommended to supply P for a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. Runoff volume, sediment, and P transport from snowmelt and rainfall were monitored for 3 yr. When manure was applied at the highest rate, runoff volume and sediment loss were less than the control plots without manure. Reductions in runoff volume and soil loss were not observed for spring runoff when frozen soil conditions controlled infiltration rates. The reduced runoff and sediment loss from manure amended soils compensated for addition of P, resulting in similar runoff losses of total P among manure application rates. However, losses of dissolved P increased with increasing manure application rate for runoff during the spring thaw period. Evaluation of water quality risks from fall-applied manure should contrast the potential P losses in snowmelt runoff with the potential that incorporated manure may reduce runoff and soil loss during the summer.

Role of macropore continuity and tortuosity on solute transport in soils: 2. Interactions with model assumptions for macropore description.

The impact of macropore description on solute transport predictions in soils is not well understood. A 2-D Galerkin finite element model was used to compare different approaches for describing macropore flow in soil. The approaches were: a modification of the hydraulic conductivity function (Hydraulic function), the lumping of all macropores into one single straight macropore (Lumping), the use of an exchange factor between microporosities and macroporosities that occupy the same area (Dual porosity), and a detailed description of each macropore (Full description, base case). Simulated breakthrough curves were obtained with domains that contained one or more macropores of different shapes under both steady state and transient flow conditions. The Hydraulic function approach was not sensitive to macropore continuity and tortuosity. When the macropores were open at the soil surface and the solute was surface applied, the first three approaches underestimated both breakthrough curves and solute distribution in the profile compared to the Full description approach. When the solute was initially incorporated in the soil, the first three approaches overestimated the breakthrough curves compared to the Full description approach. The first three approaches also underestimated the heterogeneity of solute distribution in the profile compared to the Full description approach, mostly when the macropores were tortuous. The differences between predicted breakthrough curves with different approaches decreased with an increase in tortuosity and a decrease in surface continuity. To simplify macropore description, the Dual porosity approach was the better of the first three approaches for predicting breakthrough curves provided the exchange factor between macropores and matrix porosity was available.

Role of macropore continuity and tortuosity on solute transport in soils: 1. Effects of initial and boundary conditions.

Models developed for solute transport vary in their assumptions on macropore continuity and tortuosity. It is unclear how much simplification can be made in computer models to characterize macropore effects on water and solute transport through soils. The objectives of this study were to assess how the importance of macropore continuity and tortuosity varies (1) with various initial and boundary conditions (this paper) and (2) with simplifying model assumptions for macropore description (companion paper). The above assessments were made with a computer model based on 2-D Galerkin finite element solution of Richards' equation for water flow and convective-dispersive equation for solute transport. The model can simultaneously handle macropores of varying length, size, shape, and continuity. Model predictions were in agreement with laboratory data for different macropore shapes and continuities under transient flow conditions. Simulations for various initial and boundary conditions showed that surface connected macropores under ponded conditions and under high intensity rainfalls favored the rapid transport of solutes. However, solute transport was delayed if the solute was initially incorporated in the soil even when macropores were connected to the soil surface. Macropores not connected to the soil surface only slightly accelerated solute transport for any boundary conditions. Macropore tortuosity did not influence breakthrough curves as much as the continuity but greatly influenced solute distribution in the profile. The importance of macropore continuity and tortuosity on preferential transport increased with an increase in solute retardation. General guidelines for simplifying continuity and tortuosity for modeling solute transport are presented for various initial and boundary conditions.

Tillage and nutrient source effects on surface and subsurface water quality at corn planting.

This study quantified the effects of tillage (moldboard plowing [MP], ridge tillage [RT]) and nutrient source (manure and commercial fertilizer [urea and triple superphosphate]) on sediment, NH4+ -N, NO3- -N, total P, particulate P, and soluble P losses in surface runoff and subsurface tile drainage from a clay loam soil. Treatment effects were evaluated using simulated rainfall immediately after corn (Zea mays L.) planting, the most vulnerable period for soil erosion and water quality degradation. Sediment, total P, soluble P, and NH4+ -N losses mainly occurred in surface runoff. The NO3- -N losses primarily occurred in subsurface tile drainage. In combined (surface and subsurface) flow, the MP treatment resulted in nearly two times greater sediment loss than RT (P < 0.01). Ridge tillage with urea lost at least 11 times more NH4+ -N than any other treatment (P < 0.01). Ridge tillage with manure also had the most total and soluble P losses of all treatments (P < 0.01). If all water quality parameters were equally important, then moldboard plow with manure would result in least water quality degradation of the combined flow followed by moldboard plow with urea or ridge tillage with urea (equivalent losses) and ridge tillage with manure. Tillage systems that do not incorporate surface residue and amendments appear to be more vulnerable to soluble nutrient losses mainly in surface runoff but also in subsurface drainage (due to macropore flow). Tillage systems that thoroughly mix residue and amendments in surface soil appear to be more prone to sediment and sediment-associated nutrient (particulate P) losses via surface runoff.

Herbicide banding and tillage system interactions on runoff losses of alachlor and cyanazine.

Herbicides transported to surface waters by agricultural runoff are partitioned between solution and solid phases. Conservation tillage that reduces upland erosion will also reduce transport of herbicides associated with the solid phase. However, transport of many herbicides occurs predominantly in solution. Conservation tillage practices may or may not reduce transport of solution-phase herbicides, as this depends on the runoff volume. Reducing herbicide application rate is another approach to minimize off-site transport. Herbicide banding can reduce herbicide application rates and costs by one-half or more. Our objective was to compare herbicide losses in runoff from different tillage practices and with band- or broadcast-applied herbicides. The herbicides alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl)acetanilide] and cyanazine [2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropionitrile] were broadcast- or band-applied to plots managed in a moldboard plow, chisel plow, or ridge till system. Herbicide concentration in runoff was largest for the first runoff event occurring after application and then decreased in subsequent events proportional to the cumulative rain since the herbicide application. When herbicides were broadcast-applied, losses of alachlor and cyanazine in runoff followed the order: moldboard plow > chisel plow > ridge till. Conservation tillage systems reduced runoff loss of herbicides by reducing runoff volume and not the herbicide concentration in runoff. Herbicide banding reduced the concentration and loss of herbicides in runoff compared with the broadcast application. Herbicide losses in the water phase averaged 88 and 97% of the total loss for alachlor and cyanazine, respectively. Cyanazine was more persistent than alachlor in the soil.