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AbstractLeaching of nutrients through agricultural soils is a priority water quality concern on the Atlantic Coastal Plain. This study evaluated the effect of tillage and urea application on leaching of phosphorus (P) and nitrogen (N) from soils of the Delmarva Peninsula that had previously been under no-till management. Intact soil columns (30 cm wide × 50 cm deep) were irrigated for 6 wk to establish a baseline of leaching response. After 2 wk of drying, a subset of soil columns was subjected to simulated tillage (0–20 cm) in an attempt to curtail leaching of surface nutrients, especially P. Urea (145 kg N ha-1) was then broadcast on all soils (tilled and untilled), and the columns were irrigated for another 8 wk. Comparison of leachate recoveries representing rapid and slow flows confirmed the potential to manipulate flow fractions with tillage, albeit with mixed results across soils. Leachate trends in the finer-textured soil suggest that tillage impeded macropore flow and forced greater matrix flow. Despite significant vertical stratification of soil P that suggested tillage could prevent leaching of P via macropores from the surface to the subsoil, tillage had no significant impact on P leaching losses. Relatively high levels of soil P below 20 cm may have served as the source of P enrichment in leachate waters. However, tillage did lower losses of applied urea in leachate from two of the three soils, partially confirming the study’s premise that tillage would destroy macropore pathways transmitting surface constituents to the subsoil.

Phosphorus and Nitrogen Leaching Before and After Tillage and Urea Application

Kun Han, Peter J.A. Kleinman,* Lou S. Saporito, Clinton Church, Joshua M. McGrath, Mark S. Reiter, Shawn C. Tingle, Arthur L. Allen, L.Q. Wang, and Ray B. Bryant

Leaching of nutrients through agricultural soils is a priority water quality concern on the Delmarva Peninsula that abuts the Chesapeake Bay. Accelerated

eutrophication of the Chesapeake Bay has prompted regulatory action (USEPA, 2010) to reduce nitrogen (N) and phospho-rus (P) loadings from agriculture. Approximately 8% of broiler chickens (Gallus gallus domesticus) in the United States are pro-duced on the Delmarva Peninsula (Nachman et al., 2005), with 563 million broilers raised in 2011 alone (Delmarva Poultry Industry, 2012), producing approximately 713,000 Mg of poul-try manure in the form of dry litter (Kleinman et al., 2011). Although a valuable and highly sought after fertilizer source, land application of poultry litter is seen as a major source of N and P to runoff water. Consequently, nutrient management guidelines and regulations have sought to minimize poultry litter applica-tion to soils with ample P content for crop production (Magdoff and Amaddon, 1980). Given the overall high P concentrations of Delmarva soils that have historically received poultry litter or inorganic fertilizers in excess of crop requirement (Feyereisen et al., 2010; International Plant Nutrition Institute, 2010), strong interest exists in developing remedial strategies to minimize off-site losses of P once additions have been curtailed (Boesch et al., 2001).

Nutrient losses from agricultural soils of the Delmarva Peninsula occur primarily via subsurface pathways, enhanced by the Delmarva Peninsula’s extensive artificial drainage networks. Kleinman et al. (2007) concluded that >90% of P transport from fine-textured Delmarva soils to field ditches occurred in subsurface flow, reflecting the dominance of this pathway as a source of drainage water and the extremely high P status of

Abbreviations: DP, dissolved phosphorus; ICP–OES, inductively coupled plasma–optical emission spectrometry; PP, particulate phosphorus; TP, total phosphorus.

P.J.A. Kleinman, L.S. Saporito, C. Church, and R.B. Bryant, USDA–ARS, Pasture Systems and Watershed Management Research Unit, University Park, PA 16802-3702; K. Han and L.Q. Wang, Northwest A&F Univ., College of Resources and Environment, Yangling, Shaanxi China, 712100; J.M. McGrath, Univ. of Kentucky, Plant and Soil Sciences, Lexington, KY 40546; M.S. Reiter, Virginia Tech, Eastern Shore Agricultural Research and Extension Center, Painter, VA 23420; C. Tingle, Univ. of Delaware, Carvel Research & Education Center, Georgetown, DE 19947; A.L. Allen, Univ. of Maryland Eastern Shore, 11868 Academic Oval, 3111 John T. Williams Hall, Princess Anne, MD 21853; K. Han, current address: Agronomy College, Shandong Agriculture Univ., Shandong Province, Tai’an, Rd. No. 61, China. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. Assigned to Associate Editor Zach Easton.

Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. J. Environ. Qual. 44:560–571 (2015) doi:10.2134/jeq2014.08.0326 Received 3 Aug. 2014. Accepted 8 Oct. 2014. *Corresponding author (peter.kleinman@ars.usda.gov).

Journal of Environmental QualityPHOSPHORUS FATE, MANAGEMENT, AND MODELING IN ARTIFICIALLY DRAINED SYSTEMS

SPECIAL SECTION

Published March 11, 2015

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the soils. A substantial body of research exists on subsurface P transport, with macropore flow considered the primary pathway of transport, bypassing the reactive matrix of the subsoil (Kleinman et al., 2007). Indeed, P leaching has been documented in a wide array of soils, from fine to coarse textured (Hansen and Djurhuus, 1997; Sharpley and Smith, 1994; Kleinman et al., 2007), although the inherent integrity and continuity of macropores can profoundly affect P leaching potential, particularly for applied sources (Djodjic et al., 2004; Kleinman et al., 2015). In contrast to P, subsurface N transport is generally assumed to be dominated by matrix flow (Kladivko et al., 2004), which accounts for a considerably larger proportion of flow in drainage waters than does macropore flow (e.g., Cullum, 2009). This association reflects the predominance of nitrate (NO3) in groundwater and the low reactivity of NO3 with soil constituents. However, other N fractions, such as ammonium (NH4) and urea [CO(NH2)2], an emerging contaminant of concern to the Bay’s growing frequency of harmful algal blooms (Sallade and Sims, 1994; Guertal and Howe, 2012), may be more likely to move in macropore flow given their exchange with negatively charged soil particles (NH4) or their rapid hydrolysis (urea).

In flat regions, such as the Atlantic Coastal Plain of the Delmarva Peninsula, interest exists in the judicious use of tillage as a tool for controlling nutrient losses from agriculture, particularly P. Tillage impacts on soil structure, including the temporary destruction of macropores, have been shown to lower the transmission of dissolved P (DP) that is applied to the surface of soils (Kleinman et al., 2009; Mueller et al.1984; Sharpley and Smith, 1994), although some studies document an increase in particulate P (PP) losses in tile-drained soils that were tilled (Chichester and Richardson, 1992; Sharpley and Smith, 1994; Schelde et al., 2006). Tillage may also serve to lower the P concentration at the soil surface in soils where P is vertically stratified by mixing subsoil that has high P sorption capacity with surface soil (Sharpley and Smith, 1994). Conclusions regarding tillage effects on N leaching are inconsistent, ranging from greater leaching (Harris and Colbourne, 1986; Goss et al., 1988) to lesser leaching (Tyler and Thomas, 1977; Goss et al., 1990) of NO3 in comparison with no-till. In addition, concern was raised regarding potential water quality trade-offs in the prescription of tillage for nutrient management because known impacts on soil erosion in surface runoff can readily overwhelm potential benefits from lesser leaching losses.

Soils of the Delmarva Peninsula range widely in physical and chemical properties, from well-drained Psamments to poorly drained Aqualfs (Matthews and Hall, 1966). Even though subsurface transport of nutrients is a consistent concern across agricultural lands of the Delmarva Peninsula, understanding soil-specific responses to alternative management strategies is key to quantifying their benefits and minimizing their unintended consequences. Building on the initial effort of Kleinman et al. (2015) to characterize edaphic and applied sources of P in leachate, the current study was designed to assess the effect of tillage on nutrient leaching from no-tilled soils of the Delmarva Peninsula, evaluating low and high nutrient concentrations of common agricultural soil series. Specifically, we examined P and N leaching forms through intact soil columns before and after the application of urea with and without tillage.

Materials and MethodsSoil Selection and Column Collection

Three soils were selected from the poultry producing region of the Delmarva Peninsula (Fig. 1): Bojac (coarse-loamy, mixed, semiactive, thermic Typic Hapludult), Evesboro (mesic, coated Lamellic Quartzipsamment), and Sassafras (fine-loamy, siliceous, semiactive, mesic Typic Hapludult). The Evesboro soil is excessively drained; the Bojac and Sassafras soils are well drained. Textures range from sand (Evesboro) to sandy loam (Sassafrass) to sandy clay loam (Bojac), providing different structural characteristics that likely favor matrix (Evesboro, Sassafrass) or preferential flow (Bojac), as inferred from leaching of surface-applied P in poultry litter (Kleinman et al., 2015). To minimize the role of field management variables on physical processes affecting P leaching, soils were selected from fields that had been in no-till corn production for the season before column extraction. For each soil, two members representing lower and higher antecedent soil fertility (as indicated by surface soil P concentration) were included in the study.

Five intact columns of 50 cm length and 30 cm diameter were collected from each soil and fertility level combination. To collect the columns, schedule 80 PVC cylinders were driven into the soil with a 2-Mg drop weight following the method of Kleinman et al. (2015). Compaction was avoided by not allowing the weight to contact the soil surface. Consequently, the PVC cylinders extended approximately 2 cm above the surface of the soil columns. No visual evidence of surface soil compaction was observed after cylinder insertion.

Soil columns were removed by digging away the soil next to the submerged cylinder and then tilting the cylinder to cleanly break contact between the bottom of the soil column and the underlying subsoil. To prepare the columns for the leaching experiments, columns were covered with a plate and inverted to prevent surface residue and surface soil from moving during the preparation period. A pick was used to break away peds from the exposed bottom of the soil column, removing enough soil to create a level surface at roughly the base of the PVC cylinder. Clean sand was poured onto this surface to fill the <1.0 cm void that would otherwise exist between the soil column and a cloth-covered, perforated (~60 0.2-cm holes) PVC plate that was used to keep the bottom of the column intact. The plate was secured to the PVC cylinder using silicone caulk and stainless steel screws. A cap fitted with a 1-cm PVC nipple was attached to the plate to serve as a funnel for drainage water. Columns were inverted back to their upright position before travel.

For each set of soil columns, horizon morphology was described from the profiles exposed by the excavation of the columns. In addition, bulk soil samples were collected from the exposed profiles to represent major soil horizons or regular depth increments (0–5, 5–15, 15–30, 30–45, and 45–50 cm). All columns were transported to a climate-controlled facility (20–35°C).

Column Leaching ExperimentsSoil columns were irrigated indoors (22–26°C) to evaluate

leaching for 6 wk before and 8 wk after urea application. To ensure that simulated rainwater possessed a comparable ionic content to natural rainfall, distilled water was spiked to

562 Journal of Environmental Quality

approximate the average of the 2008 annual rainwater quality observed at the Wye (MD13) and Smith Island (MD15) stations, which are along the windward edge of the study area (National Atmospheric Deposition Program, 2009a, 2009b). Simulated rainwater possessed the following properties: Ca, 0.105 mg L-1; Mg, 0.087 mg L-1; K, 0.039 mg L-1; Na, 0.679 mg L-1; NH4, 0.22 L-1; NO3, 0.82 mg L-1; Cl, 1.27 mg L-1; SO4, 1.34 mg L-1; pH 4.67.

Pre-Leaching to Draw Down NO3–NBefore initiating experiments, columns were irrigated

approximately three times weekly (20 mm per irrigation event) to draw down high antecedent levels of NO3–N. This pre-leaching was performed over a period of roughly 6 wk, at which point the CV (SD mean-1) in NO3–N between all columns was <0.2. At that point, comparisons of leachate trends, before and after the tillage/urea treatment, were initiated.

Weeks 1–6: Leaching to Assess Antecedent TrendsAfter the NO3–N draw-down period was concluded,

columns were irrigated on a once-weekly basis to assess leaching losses for 6 wk before tillage and urea application. The irrigation regime (20 mm) represented the average weekly rainfall for the Delmarva Peninsula. Simulated rainwater was applied evenly over the surface of each soil column on Mondays. Leachate samples were obtained 24 h after irrigation (Tuesday) using

acid-washed, 1.5-L plastic containers. A subsample of leachate was promptly filtered through a glass filter (0.70 mm) to obtain a subsample for urea analysis and then through a paper filter (0.45 mm) for dissolved P (DP), NH4, and NO3 analyses. Filtered and unfiltered subsamples were stored at 4°C until analyzed.

Weeks 8–16: Leaching to Assess Trends after Tillage and Urea Application

After the initial 6 wk of irrigation to assess trends before tillage and urea application, soil columns were allowed to dry at room temperature for 2 wk. Once dry, three of five soil columns from each soil and fertility level combination (e.g., Evesboro “low-fertility” soil) were selected for simulated tillage (the other two were left untilled to represent no-till management). A single soil sample was obtained from the three columns where tillage was to be performed for depths of 0 to 2, 2 to 10, and 10 to 20 cm. To simulate tillage, the upper 20 cm of soil was removed from the PVC cylinder, mixed thoroughly (but gently so as not to destroy micro-aggregates) in a clean plastic bucket and then placed back in the cylinder. A sample of the mixed soil (0–20 cm) was obtained. After tillage simulation, 145 kg N ha-1 urea was broadcast to the surface of all columns (tilled and untilled).

Once tillage and urea application were completed, all soil columns were subjected to 8 wk of irrigation. During this period, columns were irrigated once weekly (20 mm). Soils were irrigated

Fig. 1. Map of the Delmarva Peninsula in the mid-Atlantic United States showing the location and soil series for the intact soil columns collected with relatively low and high fertility.

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on Mondays. Two sets of leachate samples were collected to assess quick/fast-flow and slow-flow processes. Specifically, leachate samples were collected 24 h after application (Tuesday) and on the following Monday (before subsequent irrigation), representing 7 d of leaching after irrigation. Leachate samples were processed as described previously.

Laboratory AnalysesAll soil samples were air dried and sieved (2 mm) before

analysis. Total P (TP) in soil was determined by digesting 0.5 g of soil in aqua regia solution (USEPA Method 3050b) (Kimbrough and Wakakuwa, 1989). Mehlich-3 extraction was conducted by shaking 2.0 g of soil with 20 mL of Mehlich-3 solution for 5 min (Mehlich, 1984) followed by filtration (Whatman 1). Aqua regia digests and Mehlich-3 extracts were analyzed for P by inductively coupled plasma–optical emission spectrometry (ICP–OES). Inorganic N was extracted from soil samples with 2 mol L-1 KCl (Mulvaney, 1996), with NH4–N and NO3–N determined by flow injection analysis (Lachat Instruments, 2001, 2003). Particle size analysis was conducted by the pipette method (Day, 1965). Soil pH was determined by mixing air-dry soil with distilled water (solution:soil = 1:1).

Filtered (0.7 mm for urea and 0.45 mm for DP, NH4, and NO3) and unfiltered water samples were used to measure dissolved and total constituents in the leachate. Total P was measured in unfiltered samples by aqua regia digestion (Kimbrough and Wakakuwa, 1989) followed by P determination with ICP–OES. Dissolved P and iron (Fe) were determined on filtered samples by ICP–OES. Particulate P was estimated by the difference between TP and DP. Filtered water samples were analyzed colorimetrically for urea, NH4, and NO3 by flow injection analysis (Price and Harrison, 1987; Lachat Instruments, 1990).

Data AnalysisTrends in leachate hydrology and water quality were analyzed

with parametric statistics after initial confirmation of Gaussian distribution. Data were segregated into two groups: weeks 1 to 6 (before tillage) and weeks 9 to 16 (after tillage). Differences in leachate properties between soil series (Evesboro, Sassafras, and Bojac) were evaluated by ANOVA with Tukey’s pairwise comparison. Differences in leachate properties by relative fertility level (“high” vs. “low”) and tillage practice (tilled vs. untilled) were evaluated with Student’s t test. Relationships between variables were assessed with Pearson’s correlation analysis. All

data were analyzed using SAS v. 9.2 (SAS Institute). Statements of statistical significance in the text reflect an a of 0.05.

Results and DiscussionSoil Properties

Our results confirmed that a wide range of soil properties are likely important to nutrient leaching potential. For all soils (“low fertility” and “high fertility”), Mehlich-3 P values were above the agronomic optimum of 50 mg kg-1 (Table 1), justifying remedial practices such as tillage to address P leaching concerns (Kleinman et al., 2015). The designations of “low fertility” and “high fertility” are relative within the series evaluated in this study and do not apply to soil fertility recommendations (Beegle, 2011). Based on Mehlich-3 P values after tillage, the “high-fertility” Bojac soil possessed soil P concentrations that were 33% higher than the “low-fertility” soil, whereas the “high-fertility” Sassafras and Evesboro soils had soil P concentrations that were 75 and 340% higher in Melich-3 P than the “low-fertility” soils, respectively. Phosphorus was vertically stratified in all of these no-till soils, but stratification was most strongly expressed in the “low-fertility” soils. High concentrations of extractable NO3–N and NH4–N (Table 1) justified the need to preleach the columns to draw down high antecedent levels before applying urea. Clay contents in the finer-textured Bojac soil were roughly double those in the Sassafras soil, which were roughly double those in the sandy Evesboro soil (Table 1).

Hydrologic TrendsUnderstanding hydrologic trends is critical to assessing

variability in nutrient losses in leachate. Total leachate recoveries during weeks 1 to 6 were more variable than in weeks 9 to 16 after tillage and urea application (Table 2), suggesting that irrigation during weeks 1 to 6 served to wet all columns to similar moisture contents, resulting in more similar hydrologic characteristics. With the exception of the “low-fertility” Evesboro soil, comparisons of 24-h leachate recoveries between “low-fertility” and “high-fertility” columns within the same soil and tillage treatment were very similar in the 9- to 16-wk period.

When trends were evaluated by soils, relative fertility level, and tillage treatment, no significant differences in the total recovery of leachate water were detected for weeks 1 to 6 or weeks 9 to 16 (Table 2). However, for weeks 9 to 16 (after tillage of some of the columns and urea application to all of the columns), significant differences and interactions were observed when rapid-flow (24-h collection) and slow-flow (7-d

Table 1. Soil properties by horizon and by soil sampling depth before and after simulated tillage and urea application.

Soil series

Relative fertility

level

Mehlich-3 P KCl-extractable NO3 + NH4 Clay content

Before tillageAfter

tillage

Before tillageAfter

tillage

Before tillage0–2 cm 2–10 cm 10–20 cm 0–20 cm 0–2 cm 2–10 cm 10–20 cm 0–20 cm 0–5 cm 5–15 cm 15–30 cm 30–45 cm 45–50 cm

—————————————————— mg kg-1 —————————————————— ————————— % —————————Bojac low 141 (14)† 135 (12) 129 (10) 123 (7) 14.1 (2.6) 4.0 (0.3) 3.3 (0.6) 4.6 (1.7) 9.6 9.1 16.9 22.3 21.8

high 168 (28) 177 (16) 158 (12) 164 (17) 14.5 (6.2) 4.6 (1.5) 3.4 (0.6) 4.7 (0.7) 13.4 17.2 15.7 27.9 27.4Evesboro low 124 (6) 65 (3) 55 (2) 61 (4) 9.7 (0.6) 4.1 (1.2) 3.8 (1.4) 4.2 (0.4) 4.0 3.9 5.8 7.0 5.8

high 284 (32) 283 (11) 268 (17) 284 (15) 11.8 (1.7) 5.8 (0.9) 3.1 (1.0) 6.5 (1.3) 3.7 3.9 5.6 6.6 5.9Sassafras low 161 (11) 154 (39) 137 (53) 144 (51) 14.9 (4.0) 5.6 (2.7) 3.0 (1.9) 5.6 (3.2) 5.7 7.9 9.7 11.9 7.9

high 238 (111) 252 (7) 242 (5) 252 (2) 19.7 (12.9) 9.6 (2.4) 10.0 (1.1) 9.5 (0.8) 6.6 7.0 8.2 6.5 6.7† Numbers in parentheses represent SD.

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collection) recoveries were analyzed separately. Lesser recovery of rapid flow (24-h collection) and greater recovery of slow flow (7-d collection) were observed in Bojac and “low-fertility” Evesboro columns in weeks 9 to 16. This change in hydrology was not observed in the Sassafras or the “high-fertility” Evesboro columns. The “low-fertility” Evesboro columns yielded the least proportion of rapid flow of soils in the study, suggesting that these soils would be the least affected by practices aimed at disrupting macropore flow. Fine-textured lamellae were observed near the base of the “low-fertility” Evesboro columns, potentially affecting drainage patterns. Indeed, there was a slightly higher clay content in the “low-fertility” Evesboro soil at 30 to 45 cm than there was at that depth in the “high-fertility” Evesboro soil (Table 1). In addition, Subler and Kirsch (1998) and Bohlen et al. (1997) document significantly lower macroinvertbrate activity/macropore contribution in low-fertility soils with low organic matter, a possibility with soils of the current study.

Comparison of leachate recoveries from tilled and untilled columns for weeks 9 to 16 confirmed the potential to manipulate flow fractions, albeit with mixed results across soils (Table 2). In the finer-textured Bojac soil columns, rapid flow was significantly lower from tilled columns than from untilled columns, whereas slow flow was significantly greater. Our findings are consistent with those reported by Golabi et al. (1995) and Cullum (2009), who observed that tillage destroyed soil macropores, thereby impeding preferential flow and forcing greater matrix flow. Tillage had no influence in shifting the recovery of flows from rapid to slow fractions for the coarser-textured Sassafras and for the “high-fertility” Evesboro soil columns. We had hypothesized that the

high proportion of rapid flow recovered from these soils would be susceptible to tillage manipulation. However, tillage of the upper 20 cm may not have been deep enough to disrupt critical macropores controlling rapid flow. Specifically, the finest textures in the Sassafras soil were associated with the subsoil. Furthermore, soils were collected from sites that had a range of no-till histories, including a minimum of 1 yr. In soils with shorter periods of no-till management, extensive macropore networks may not have been as developed as in soils with longer histories of no-till. Tillage to 20 cm merely mixed materials that were relatively unstructured.

Nutrient Losses in LeachateAs expected, trends in DP, PP, urea-N, NH4–N, and NO3–N,

differed before and after tillage and urea application. Differences during the first 6 wk were observed as a result of soil and fertility level factors, consistent with date for P leaching in the study by Kleinman et al. (2015). After tillage and urea application, differences in leachate collected 24 h and 7 d after irrigation offer insight into the likely role of rapid flow via macropores (reactive or particulate compounds) and slow flow via the soil matrix (conservative solutes) on nutrient fate. In addition, discrete periods of urea-N, NH4–N, and NO3–N loss in leachate point to the role of biological transformations in N loss.

Trends before Urea Application and Tillage Implementation (Weeks 1–6)Phosphorus

Over the first 6 wk of the study, DP dominated TP in leachate, with DP and PP contributing 65 to 94% and 6 to 35%,

Table 2. Average leachate recoveries from soil columns as a function of irrigation input before (weeks 1–6) and after (weeks 9–16) simulated tillage and urea application.

Soil series Relative soil fertility

Weeks 1–6 Weeks 9–16

All columns, 24-h collection only†

Untilled columns Tilled columns24-h 7-d 24-h 7-d

————————————————— (%) mm mm-1 per collection —————————————————Bojac low 73 (17)‡ 64 (1) 12 (2) 54 (1) 19 (2)Bojac high 48 (19) 64 (9) 8 (6) 51 (2) 24 (4)

Evesboro low 60 (7) 48 (2) 26 (20) 33 (5) 37 (5)Evesboro high 94 (6) 72 (3) 5 (3) 71 (2) 5 (1)

Sassafras low 90 (8) 69 (7) 7 (2) 70 (2) 6 (3)Sassafras high 90 (6) 70 (2) 4 (1) 70 (2) 5 (1)

† From weeks 1 to 6, irrigation water was applied three times weekly (20 mm per event)

‡ Numbers in parentheses represent SD.

Table 3. Average cumulative losses of dissolved P, particulate P, urea-N, NH4–N, and NO3–N in leachate before simulated tillage and urea application (weeks 1–6).

Analyte† Leachate collection period

Bojac Evesboro SassafrasHigh fertility Low fertility High fertility Low fertility High fertility Low fertility

———————————————————————— kg ha-1 ————————————————————————DP 24-h flow 0.03 (0.04)‡ 0.42 (0.24) 4.38 (0.94) 0.04 (0.01) 0.27 (0.07) 0.12 (0.09)PP 24-h flow 0.02 (0.03) 0.09 (0.03) 0.28 (0.13) 0.01 (0.01) 0.10 (0.01) 0.07 (0.03)Urea-N 24-h flow 0.01 (0.02) 0.04 (0.02) 0.08 (0.02) 0.03 (0.01) 0.03 (0.01) 0.03 (0.01)NH4–N 24-h flow 0.03 (0.01) 0.04 (0.01) 0.06 (0.00) 0.04 (0.00) 0.05 (0.00) 0.06 (0.02)NO3–N 24-h flow 13.78 (1.62) 16.93 (2.76) 16.93 (2.46) 14.45 (3.45) 34.76 (9.19) 20.95 (5.03)

† DP, dissolved P; PP, particulate P.

‡ Numbers in parentheses represent SD.

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respectively, to TP. During this period, losses of P in leachate ranged from 0.04 to 4.31 kg ha-1 for DP and from 0.01 to 0.28 kg ha-1 for PP (Table 3). When trends were evaluated by soil series and fertility level, significant differences in DP and PP loss were observed. The greatest losses of DP and PP in leachate were observed from the Evesboro soil columns (averaging 1.92 and 0.20 kg ha-1, respectively), as compared with Bojac (averaging 0.10 and 0.08 kg ha-1, respectively) and Sassafras (averaging 0.13 and 0.10 kg ha-1, respectively). With the exception of the “high-fertility” Evesboro columns, PP loss was very low throughout the first 6 wk (Fig. 2). Phosphorus loss from the “high-fertility” columns averaged about 10 times more than columns with lower fertility (Table 3).

Trends in concentrations of DP in leachate reveal several likely controls of DP loss during the weeks before tillage implementation (Fig. 3). Across all soils, there was a strong correlation between average DP and soil P concentration, pointing to greater P desorption with higher concentrations of Mehlich-3–extractable P. This relationship varied to some extent depending on the depth of soil sampling. For Evesboro soils, the DP concentration in leachate was strongly related to Mehlich-3 P concentrations of soil obtained from all depths within the

column, including a mixed 0- to 20-cm sample. However, for Bojac and Sassafras, the DP concentration in leachate was better related to Mehlich-3 P in subsurface soil samples (2–10 cm and 10–20 cm) than in surface soil samples (0–2 cm). Relationships with depth of soil sampling suggest that uniform tillage of the surface soil may have a greater effect on the Evesboro soil than on the other soils.

Leachate DP concentrations from the Evesboro “high-fertility” columns were an order of magnitude greater than other soil columns and increased with time (Fig. 3). All columns had received relatively recent poultry litter additions before this experiment, and DP may have been produced by mineralization of that litter (Oehl et al., 2004), but that was not observed in any of the other columns. Reductive dissolution of Fe-bound P may also have contributed to losses of DP during weeks 1 to 6 because we observed a concomitant increase in dissolved Fe in leachate in the Evesboro “high-fertility” columns, best represented by quadratic model (r = 0.85) that was generally not observed in the other treatments (Fig. 4), although a weak correlation was observed between these variables in the Bojac “low-fertility” treatment. The high rates of irrigation during the first 6 wk of the study, which were

Fig. 2. Particulate P (PP) concentrations in leachate from intact soil columns before and after simulated tillage and urea application. Note the different scale for Evesboro particulate P concentrations.

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intended to draw down nutrient reserves, may have resulted in water logging and anoxic soil conditions that promoted the process of Fe reduction and Fe-bound P release. Vadas and Sims (1998) documented reductive dissolution of Fe-bound P in comparable soils from the same region but did not observe the strong correlation between solution Fe and P found in the current study.Nitrogen

During the 6 wk before tillage and urea application, losses of N were primarily as NO3–N (13.7–33.9 kg N ha-1), NH4–N (0.03–0.06 kg N ha-1) and urea (0.01–0.06 kg N ha-1) losses were low (Table 3). Unlike P in leachate, which was related to extractable soil P, no significant relationship was observed between extractable soil N and N in leachate. Concentrations of urea-N and NH4–N remained relatively constant during this period, whereas NO3–N concentrations declined, confirming the depletion of soil NO3 over the initial 6 wk due to leaching or denitrification (Fig. 5–7). Leachate losses of all N fractions differed significantly by soil series, with relative differences between series ranging widely. Nitrate-N loss for Sassafras (26.89 kg ha-1) averaged 1.7-fold higher than for Bojac (15.44 kg ha-1), which did not differ significantly from Evesboro (15.29 kg ha-1). Ammonium-nitrogen loss for Sassafras (0.06 kg ha-1) averaged 1.5-fold higher than Bojac (0.04 kg ha-1) and Evesboro (0.04 kg ha-1), which did not differ significantly.

Fig. 3. Dissolved P (DP) concentrations in leachate from intact soil columns before and after simulated tillage and urea application. Note different scale for Evesboro dissolved P concentrations.

Fig. 4. Relationship between dissolved P (DP) and Fe concentrations in leachate from intact columns of the Evesboro soil with high relative fertility. Data are from weeks 1 to 6, before the simulation of tillage and application of urea.

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Urea-nitrogen loss for Evesboro (0.05 kg ha-1) averaged 1.7-fold higher than Bojac (0.03 kg ha-1) and Sassafras (0.03 kg ha-1). Leachate NO3–N loss differed significantly between soil fertility levels (“high fertility” averaged 22.2 kg ha-1; “low fertility” averaged 18.5 kg ha-1), but these differences were not observed with NH4–N and urea-N. It is likely that residual N in the soil from historical poultry litter applications accounts for the differences between “high-fertility” and “low-fertility” soils. Previously, Parfitt et al. (2009) observed that field soils with high fertility contributed to greater NO3 leaching in 3 yr, compared with lower fertility, but they did not observe any significant differences in leachate NH4–N.

Trends after Tillage Implementation and Urea Application (Weeks 9–16)Phosphorus

Significant interactions in the effect of tillage, soil series, and antecedent fertility level on P loss in leachate were observed during weeks 9 to 16. For tilled columns, significant increases in PP concentration were observed in the first few weeks after simulated tillage, but those concentrations declined after 3 wk (Fig. 2). As a result, DP accounted for 43 to 90% of TP in leachate from tilled columns, whereas DP accounted for 55 to 94% of TP in leachate from untilled columns. Losses of P in leachate ranged widely (0.05–3.74 kg DP ha-1; 0.05–0.46 kg PP ha-1). Most P in

leachate was collected in the first 24-h sample, accounting for 51 to 98% of DP and 64 to 100% of PP collected each week. Lower concentrations of Fe suggest that soil columns became aerobic as they dried in weeks 7 and 8 and remained aerobic in weeks 9 to 16, likely due to the reduced frequency of irrigation that served to change the oxidation status of the soils.

Contrary to expectations, tillage had no significant influence on cumulative DP loss in leachate (Table 4). Only in the Evesboro columns with a “low fertility” level was DP concentration significantly different in leachate from tilled and untilled treatments during the first 2 wk of leaching (Fig. 3). However, in these soils, DP concentrations in the 24-h leachate samples (i.e., rapid flow) were higher from the tilled columns. Although the hydrologic trends discussed above had indicated that the “low-fertility” Evesboro columns should be the least affected by practices aimed at destroying macropore flow and reducing nutrient transport from the surface via macropores, an increase in DP concentration after tillage was unexpected. This unexpected result may have resulted from stochastic processes during simulated tillage, such as the placement of high-P surface soil at the entrance of a subsoil macropore below the tillage zone. More importantly, this study does not support the hypothesis that mixing of surface soil with tillage significantly reduces the source of DP to leaching water in these

Fig. 5. Urea-nitrogen concentrations in leachate from intact soil columns before and after simulated tillage and urea application.

568 Journal of Environmental Quality

coastal plain soils. Most likely, this is due to the very high soil P status in all soils, even in the 10- to 20-cm soil depth. Our results suggest that the controlling source of DP in leachate was below the 20-cm depth of tillage.

Not only was no reduction in DP leaching loss detected with tillage, but significantly greater PP losses were found in leachate from the full population of tilled columns than from untilled columns (Table 4). When soils were evaluated individually, significant differences were only detected in leachate from the Evesboro columns with “high fertility” levels. The inconsistent significance in difference between the full population of columns and the subset of columns of similar series and fertility level is likely due to the small number of replicates with the latter category (three tilled columns and two untilled columns). Elevated PP losses in drainage waters have been documented elsewhere; Schelde et al. (2006) observed increases in PP in tile drain effluent after tillage, pointing to the direct transfer of particles from disturbed aggregates at the soil surface.Nitrogen

As with P, significant interactions in the effect of tillage, soil series, and antecedent fertility level on N in leachate were detected during weeks 9 to 16. Immediately after the application of urea, spikes in urea concentration in leachate from most soils were observed for 1 wk, with the exception of the Evesboro high,

which did not spike for several weeks (Fig. 5). After these spikes, urea concentrations dropped to background levels, reflecting hydrolysis of the remaining urea. Although no-till columns tended to yield the highest mean concentrations of urea during this period, the “low-fertility” Sassafras and Bojac soils were the only ones where losses of urea were significantly lower with tillage (Table 4). Our results highlight the considerable spatial variability in urea leaching, a function of the likely transport of urea via macropores transiting the length of the soil column. Our results also confirm the initial premise of the study that tillage can curtail the transfer of constituents in leachate from the soil surface to lower in the soil profile.

Trends in NH4–N leaching were somewhat similar to those in urea leaching, with NH4–N concentrations peaking briefly after urea application to the columns, reflecting urease hydrolysis of applied urea fertilizer to NH4–N (Fig. 6). Despite some apparent elevations in NH4–N concentrations in leachate from no-till columns, no significant differences in NH4–N concentrations were observed with tillage treatment. Differences in cumulative loads of NH4–N were only observed in the “low-fertility” Bojac soils (Table 4). However, differences were detected for the first 4 wk after urea application between the tilled and untilled Sassafras “low-fertility” columns. When 24-h and 7-d leachate collections were compared, differences

Fig. 6. Ammonium-nitrogen concentrations in leachate from intact soil columns before and after simulated tillage and urea application.

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in NH4–N loads were detected in the 24-h samples but not in the 7-d samples, reflecting the likely role of quick-responding macropore flow in NH4–N transport. Elsewhere, Silva et al. (2000) concluded that NH4–N leaching occurs primarily via mixed/macropore flow.

Unlike urea-N and NH4–N leaching trends, NO3–N concentrations in leachate increased gradually over the first 6 to 7 wk after urea application, signifying the microbial conversion of NH4–N to NO3–N by nitrification (Fig. 7). In general, no significant differences were observed in NO3–N concentrations between tilled and untilled soils, although contrasting trends

Fig. 7. Nitrate-nitrogen concentrations in leachate from intact soil columns before and after simulated tillage and urea application.

Table 4. Average cumulative dissolved P, particulate P, urea-N, NH4–N, and NO3–N losses in leachate after simulated tillage and urea application (weeks 9–16).

Analyte†Leachate collection

period

Bojac Evesboro SassafrasHigh fertility Low fertility High fertility Low fertility High fertility Low fertility

No-till Till No-till Till No-till Till No-till Till No-till Till No-till Till

————–————–——–——–——–——–——–———–————– kg ha-1 ————–————–——–——–——–——–——–——–—————–DP 24-h flow 0.05 (0.02)‡ 0.04 (0.01) 0.13 (0.07) 0.10 (0.05) 3.52 (0.43) 3.61 (0.72) 0.05 (0.01) 0.05 (0.02) 0.29 (0.12) 0.23 (0.04) 0.08 (0.08) 0.04 (0.03)

7-d flow 0.01 (0.00) 0.03 (0.01) 0.02 (0.00) 0.05 (0.03) 0.22 (0.16) 0.12 (0.10) 0.03 (0.01) 0.05 (0.01) 0.00 (0.00) 0.01 (0.01) 0.01 (0.00) 0.01 (0.00)PP 24-h flow 0.05 (0.03) 0.05 (0.04) 0.08 (0.00) 0.07 (0.03) 0.25 (0.12) 0.45 (0.16) 0.02 (0.00) 0.03 (0.01) 0.14 (0.04) 0.19 (0.05) 0.07 (0.05) 0.06 (0.02)

7-d flow 0.00 (0.00) 0.02 (0.02) 0.03 (0.03) 0.03 (0.01) 0.00 (0.00) 0.00 (0.00) 0.01 (0.01) 0.02 (0.00) 0.00 (0.00) 0.01 (0.00) 0.00 (0.00) 0.00 (0.00)Urea-N 24-h flow 7.03 (9.72) 1.28 (1.39) 5.21 (0.42) 1.29 (1.11) 0.88 (0.28) 0.93 (0.89) 0.10 (0.11) 0.02 (0.00) 0.06 (0.06) 0.03 (0.02) 5.41 (0.95) 0.23 (0.14)

7-d flow 0.01 (0.00) 0.09 (0.13) 0.38 (0.16) 0.03 (0.02) 0.01 (0.00) 0.04 (0.06) 0.02 (0.01) 0.01 (0.01) 0.00 (0.00) 0.00 (0.00) 0.01 (0.01) 0.01 (0.01)NH4–N 24-h flow 1.17 (1.03) 0.09 (0.05) 1.43 (0.42) 0.55 (0.28) 0.21 (0.19) 1.31 (0.89) 0.06 (0.02) 0.26 (0.37) 0.32 (0.38) 0.08 (0.04) 1.71 (0.69) 0.49 (0.93)

7-d flow 0.03 (0.04) 0.05 (0.06) 0.03 (0.02) 0.03 (0.02) 0.02 (0.02) 0.05 (0.05) 0.04 (0.00) 0.19 (0.22) 0.00 (0.00) 0.04 (0.04) 0.01 (0.01) 0.10 (0.19)NO3–N 24-h flow 54.9 (3.1) 44.8 (11.7) 52.5 (0.5) 68.7 (9.6) 72.6 (1.5) 90.0 (9.7) 44.1 (5.8) 34.4 (13.9) 47.4 (7.1) 48.8 (5.3) 60.6 (9.9) 58.2 (6.0)

7-d flow 3.5 (0.0) 18.5 (1.6) 8.8 (0.3) 20.8 (0.4) 3.6 (3.4) 2.6 (1.7) 19.7 (2.7) 38.2 (9.5) 0.7 (0.4) 3.4 (1.1) 4.3 (3.4) 3.6 (2.3)

† DP, dissolved P; PP, particulate P.

‡ Numbers in parentheses represent SD.

570 Journal of Environmental Quality

were detected for “low-fertility” Bojac soils (no-till > till) and “high-fertility” Bojac soils (till > no-till). Although results from the current study are too varied to suggest a consistent trend toward greater NO3–N leaching with either no-till or tillage, cumulative NO3–N loads were significantly greater for the tilled members of the “low-fertility” Bojac soil and the “high-fertility” Evesboro soil. The absence of soil N uptake by a growing crop during the study period may have contributed to these differences.

ConclusionsThis study sought to ascertain differences in N and P

leaching after tillage of coastal plain soils with sufficient antecedent P to warrant application of N fertilizer only. Tillage had no significant impact on P leaching losses despite significant vertical stratification of soil P that suggested tillage could prevent macropore transport of P from the surface to the subsoil. However, tillage did curtail losses of applied urea in leachate in the Sassafras and Bojac soils, confirming the study’s premise that tillage would destroy macropore pathways transmitting surface constituents to the subsoil. Relatively high levels of soil P below the 20-cm depth of simulated tillage may have served as the source of P enrichment of leachate waters, although variability in leachate P between columns likely also contributed. Further work is needed to determine whether deeper tillage could better address P leaching losses.

AcknowledgmentsWe thank the staff of the USDA–ARS Pasture Systems and Watershed Management Research Laboratory for their contributions to this study. Soil columns were collected by David Otto and Mike Reiner. Laboratory analyses were conducted by Sarah Fishel, Charles Montgomery, and Paul Spock. This project was funded by the Chinese Service Center for Scholarly Exchange and the USDA Agricultural Research Service.

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