in artificially drained soils, have been conducted over a much briefer time (Angle et al., 1993; Rasse et al., The relationships between N fertilizer rate, yield, and NO 3 leaching 1999). In one of the first controlled studies, Baker et need to be quantified to develop soil and crop management practices al. (1975) found that the concentration of NO 3 in tile that are economically and environmentally sustainable. From 1996 through 1999, we measured yield and NO 3 loss from a subsurface drainage water averaged 21 mg N L Ϫ1 and the losses drained field in central Iowa at three N fertilizer rates: a low (L) rate averaged approximately 30 kg N ha Ϫ1 yr Ϫ1 for a N fertilof 67 kg ha Ϫ1 in 1996 and 57 kg ha Ϫ1 in 1998, a medium (M) rate of izer application of 112 kg N ha Ϫ1 on corn grown in 135 kg ha Ϫ1 in 1996 and 114 kg ha Ϫ1 in 1998, and a high (H) rate of rotation with unfertilized oat (Avena sativa L.) or soy-202 kg ha Ϫ1 in 1996 and 172 kg ha Ϫ1 in 1998. Corn (Zea mays L.) bean. In continuous corn production, Randall and Iragaand soybean [Glycine max (L.) Merr.] were grown in rotation with varapu (1995) found flow-weighted NO 3 concentrations N fertilizer applied in the spring to corn only. For the L treatment, during an 11-yr period of applying 200 kg N ha Ϫ1 to NO 3 concentrations in the drainage water exceeded the 10 mg N L Ϫ1 average 13.4 and 12.0 mg N L Ϫ1 for conventional-tillage maximum contaminant level (MCL) established by the USEPA for and no-tillage systems, respectively. In comparing the drinking water only during the years that corn was grown. For the effect of N fertilizer rate, Baker and Johnson (1981) M and H treatments, NO 3 concentrations exceeded the MCL in all found that increasing the fertilizer rate from 100 to 250 years, regardless of crop grown. For all years, the NO 3 mass loss in tile drainage water from the H treatment (48 kg N ha Ϫ1) was significantly kg N ha Ϫ1 on corn, grown in rotation with either soybean greater than the mass losses from the M (35 kg N ha Ϫ1) and L (29 or oat, doubled the NO 3 concentration in tile drainage kg N ha Ϫ1) treatments, which were not significantly different. The from 20 to 40 mg N L Ϫ1. Similar results have been reeconomically optimum N fertilizer rate for corn was between 67 and ported by Gast et al. (1978) for N fertilizer applied to 135 kg ha Ϫ1 in 1996 and 114 and 172 kg ha Ϫ1 in 1998, but the net N
Subsurface drainage is a beneficial water management practice in poorly drained soils but may also contribute substantial nitrate N loads to surface waters. This paper summarizes results from a 15-yr drainage study in Indiana that includes three drain spacings (5, 10, and 20 m) managed for 10 yr with chisel tillage in monoculture corn (Zea mays L.) and currently managed under a no-till corn-soybean [Glycine max (L.) Merr.] rotation. In general, drainflow and nitrate N losses per unit area were greater for narrower drain spacings. Drainflow removed between 8 and 26% of annual rainfall, depending on year and drain spacing. Nitrate N concentrations in drainflow did not vary with spacing, but concentrations have significantly decreased from the beginning to the end of the experiment. Flow-weighted mean concentrations decreased from 28 mg L(-1) in the 1986-1988 period to 8 mg L(-1) in the 1997-1999 period. The reduction in concentration was due to both a reduction in fertilizer N rates over the study period and to the addition of a winter cover crop as a "trap crop" after corn in the corn-soybean rotation. Annual nitrate N loads decreased from 38 kg ha(-1) in the 1986-1988 period to 15 kg ha(-1) in the 1997-1999 period. Most of the nitrate N losses occurred during the fallow season, when most of the drainage occurred. Results of this study underscore the necessity of long-term research on different soil types and in different climatic zones, to develop appropriate management strategies for both economic crop production and protection of environmental quality.
Do extended crop rotations that include forages improve soil quality and are they profitable? Our objectives were to determine (i) how crop rotation affected soil quality indicators, (ii) if those indicator changes were reflected in soil quality index (SQI) ratings when scored and combined using the Soil Management Assessment Framework, and (iii) how SQI values compared with profitability. Soil samples were collected from three long-term studies in Iowa and one in Wisconsin. Bulk density (BD), soil pH, water-stable macroaggregation, total organic C, total N, microbial biomass C, extractable P and K, and penetration resistance were measured. The indicator data were scored using nonlinear curves reflecting performance of critical soil functions (e.g., nutrient cycling, water partitioning and storage, and plant root growth). Profit was calculated by subtracting costs of production from potential income based on actual crop yields and the 20-yr average nongovernment-supported commodity prices. Extended rotations had a positive effect on soil quality indicators. Total organic C was the most sensitive indicator, showing significant measured and scored differences at all locations, while BD showed significant differences at only one location (Kanawha). The lowest SQI values and 20-yr average profit were associated with continuous corn, while extended rotations that included at least 3 yr of forage crops had the highest SQI values. We suggest that future conservation policies and programs reward more diverse and extended crop rotations, as is being done through the Conservation Security Program.
There is a lack of quantitative information describing the impact of farming on water quality at the watershed scale. This study documents the surface water quality of Walnut Creek—a 5130‐ha watershed with about 86% of the land used for crop production. Starting in 1990, flow and concentrations of NO3‐N and four herbicides—atrazine [6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine], alachlor [2‐chloro‐N‐(2,6‐diethylphenyl)‐N‐(methoxymethyl)acetamide], metribuzin [4‐amino‐6‐(1,1‐dimethylethyl)‐3‐(methylthio)‐1,2,4‐triazin‐5(4N)‐one], and metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl) ‐ N ‐ (2 ‐methoxy ‐1‐methylethyl)acetamide]—were measured at eight locations. Nitrate‐N concentrations often exceeded 10 mg L−1 during May, June, and July. Total losses from the watershed ranged from 4 to 66 kg ha−1 yr−1 and represented 6 to 115% of the N applied as fertilizer in any year. Atrazine and metolachlor were detected at concentrations >0.2 µg L−1 in about half of all water samples, while alachlor and metribuzin were seldom detected. Median concentrations for atrazine and metolachlor were below 1 µg L−1 for all locations within the watershed. During runoff events, herbicide concentrations in the stream increased while NO3‐N concentrations decreased. Yearly losses from the watershed ranged from 0.2 to 7.5 g ha−1 for atrazine and from 03 to 6.7 g ha−1 for metolachlor. These losses represent 0.18 to 5.6% of the atrazine and 0.047 to 1.6% of the metolachlor applied in any year.
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