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TECHNICAL REPORTS

Surface Water Quality

Nitrate Losses in Subsurface Drainage from a Corn–Soybean Rotation as Affected by Time of Nitrogen Application and Use of Nitrapyrin

^a University of Minnesota Southern Research and Outreach Center, 35838 120th Street, Waseca, MN 56093-4521
^b Dow AgroSciences, 750 Double Spring Road, Almond, NC 28702

^* Corresponding author (grandall{at}soils.umn.edu).

Received for publication May 27, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Subsurface drainage, a water management practice used to remove excess water from poorly drained soils, can transport substantial amounts of NO₃ from agricultural crop production systems to surface waters. A field study was conducted from the fall of 1986 through 1994 on a tile-drained Canisteo clay loam soil (fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquoll) to determine the influence of time of N application and use of nitrapyrin [NP; 2-chloro-6-(trichloromethyl) pyridine] on NO₃ losses from a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation. Four anhydrous ammonia treatments [fall N, fall N + NP, spring preplant N, and split N (40% preplant and 60% sidedress)] were replicated four times and applied at 150 kg N ha^-1 for corn on individual drainage plots. Sixty-two percent of the annual drainage and 69% of the annual NO₃ loss occurred in April, May, and June. Flow-weighted NO₃–N concentrations in the drainage water were two to three times greater in the two years following the three-year dry period compared with preceding and succeeding years. Nitrate N concentrations and losses in the drainage from corn were greatest for fall N with little difference among the other three N treatments. Nitrate losses from soybean were affected more by residual soil NO₃ following corn than by the N treatments per se. Averaged across the four rotation cycles, flow-normalized NO₃–N losses ranked in the order: fall N > split N > spring N = fall N + NP. Under these conditions NO₃ losses from a corn–soybean rotation into subsurface drainage can be reduced by 13 to 18% by either applying N in the spring or using NP with late fall–applied ammonia.

Abbreviations: ET, evapotranspiration • NP, nitrapyrin • RSN, residual soil nitrate nitrogen

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
ARTIFICIAL DRAINAGE through subsurface tile lines is a common water management practice in highly productive agricultural areas with poorly drained soils that have seasonally perched water tables or shallow ground water. This management practice increases crop productivity, reduces risk, and improves economic returns to producers, particularly in the north-central region of the USA (Zucker and Brown, 1998). Research has shown that substantial amounts of N, particularly NO₃, can be transported in tile drainage from the landscape to surface waters (Baker and Johnson, 1981; David and Gentry, 2000; Fenelon and Moore, 1998; Jaynes et al., 1999; Kladivko et al., 1991). Nitrate concentrations in the Mississippi River are generally greatest in the tributaries emanating from Illinois, Iowa, and Minnesota (Antweiler et al., 1995) where artificially drained soils planted to corn and soybean dominate the landscape (Burkart and James, 1999). Nitrate transported in the Mississippi River to the Gulf of Mexico has been linked indirectly to hypoxia off the coast of Louisiana (Mitsch et al., 2001; Rabalais et al., 1996; Turner and Rabalais, 1994).

Monitoring of subsurface tile drainage water can be useful in assessing the influence of agricultural management practices on surface and ground water quality (Baker and Johnson, 1981; Gast et al., 1978; Hallberg et al., 1986; Kanwar et al., 1988; Randall and Goss, 2001). Long-term drainage studies to assess management practices are necessary if accurate estimates of nutrient losses and performance evaluation of the practices are desired, because short-term results obtained under unusually wet or dry conditions could be misleading if used by themselves (Jaynes et al., 1999; Randall and Iragavarapu, 1995). The fruitfulness of long-term research was also emphasized by Goldstein et al. (1998) when evaluating complex systems integrating several measures to reduce NO₃ pollution found in drainage waters from the widespread corn–soybean rotation found in the U.S. Corn Belt.

Numerous field studies have shown that the corn–soybean rotation contributes significant losses of NO₃ to subsurface drainage waters (Dinnes et al., 2002; Goldstein et al., 1998). In a four-year drainage study in Minnesota, flow-weighted NO₃–N concentration averaged 28 mg L^-1 for continuous corn, 23 mg L^-1 for a corn–soybean rotation, and <2 mg L^-1 for alfalfa (Medicago sativa L.) and grass perennial crops (Randall et al., 1997). Edge-of-field NO₃ losses for this period were 9% lower for the corn–soybean rotation compared with continuous corn, but were still 30 to 50 times greater than from the perennial crops. A four-year study in Ohio showed NO₃–N concentrations in the subsurface drainage water from soybean to be as high or higher than with corn in a corn–soybean rotation, especially in the spring (Logan et al., 1994). They concluded that a significant portion of the NO₃ in tile drainage is due to N carried over from the previous corn crop. Mean NO₃–N concentration in five tile drain systems in an Iowa watershed dominated by corn and soybean ranged from 8.6 to 13.4 mg L^-1 during 1992–1995, with maximum concentrations ranging from 19.7 to 29.3 mg L^-1 in samples collected weekly (Jaynes et al., 1999). Annual NO₃–N losses in the tile drainage ranged from 5 to 51 kg ha^-1 during this four-year period (Cambardella et al., 1999).

Management of N to minimize NO₃ leaching losses in row-crop farming is based on a simple concept—avoiding excess NO₃ in the root zone when the soil is vulnerable to leaching by excess rainfall, usually spring and fall (Keeney and Follett, 1991). Spring applications of N are frequently superior to fall application because N loss is less between time of N application and uptake by the crop (Randall and Goss, 2001). However, many U.S. corn growers, especially in the northern part of the Corn Belt, desire to apply N in the fall because they usually have more time and field conditions are better. Early planting of corn as soon as the soils are fit in the spring is desirable for greatest yields and profit (Randall and Schmitt, 1998). Six separately drained 2-ha parcels in Illinois were monitored for one year, and NO₃–N concentrations for fall-applied N were 58% greater than for the same N rate applied in the spring (Smiciklas and Moore, 1999). Nitrification inhibitors (e.g., nitrapyrin), which when added to ammonium fertilizers slow the conversion to NO₃, have been shown to reduce NO₃ losses in subsurface drainage water from continuous corn (Owens, 1987). In the six Illinois drainage parcels, NO₃–N concentration in the drainage water was decreased by 9% where NP was used compared with not using NP.

Much of the previous research on NO₃ losses to subsurface tile drainage as affected by N management practices has been conducted in continuous corn systems. The objectives of this eight-year, subsurface, tile drainage study were to determine (i) the relationship between time and amount of precipitation on NO₃ losses from a corn–soybean rotation and (ii) the effect of time of N fertilizer application and NP for corn on NO₃ losses from both crops in the rotation.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A field experiment was conducted on a poorly drained Canisteo clay loam glacial till soil from 1987 through 1994 at the University of Minnesota's Southern Research and Outreach Center in Waseca, MN. The site was located on a 0 to 1% slope, and the organic matter content of the Ap horizon was 5.5%. Thirty-six individual subsurface tile drainage plots were installed in 1976. Each plot, measuring 9.1 by 6.1 m, has a separate drain outlet and is isolated to a depth of 1.8 m by trenching and installation of a 12-mil-thick plastic sheeting. A tile line (6.1 m in length) is located 1.5 m from one end of each plot (spaced to simulate a 15.2-m spacing) and placed 1.2 m deep. After completing a research project using this drainage facility in 1983, the experimental area was cropped to corn with a blanket N rate (190 kg ha^-1) in 1984 and 1985 to establish uniformity.

Beginning in 1986, a corn–soybean rotation was started with corn planted on one-half of the experimental area while soybean was planted on the other half. Thirty-two plots (a set of 16 for corn and a set of 16 for soybean) with the most uniform drainage were selected for the study. For drainage purposes, the experimental design consisted of a 4 x 4 Latin square where the rows and columns were based on annual tile discharge from each plot during 1977–1983. Within each set of 16 plots, four plots with the greatest discharge were assigned to Column A, the next four to Column B, the next four to Column C, and the four plots with the lowest discharge to Column D. Within each column the plot with the greatest discharge was assigned to Row A, and following the same procedure as for columns, the plot with the least discharge was assigned to Row D. The four N treatments [fall N, fall N + NP, spring, and split (40% preplant and 60% sidedress at V8 stage)] were then assigned within each row and column to minimize drainage variability among the treatments. Spatially, however, the plots were arranged in a randomized complete-block design with four replications and restricted randomization (Randall et al., 2003). The N treatments were not re-randomized each year; thus, each treatment for corn occupied the same plots in 1987, 1989, 1991, and 1993 for one set of 16 plots and in 1988, 1990, and 1992 for the other set of 16 plots.

Anhydrous ammonia was applied for corn at a rate of 150 kg N ha^-1 for all N treatments. Nitrapyrin (N-Serve; Dow AgroSciences, Indianapolis, IN) was applied at the recommended rate of 0.56 kg a.i. ha^-1. Dates of application ranged from 19 to 28 October for the fall treatments, 18 April to 21 May for the spring preplant treatment, and 15 June to 12 July for the sidedress treatment. Soil temperatures at the 15-cm depth on the day of fall application were 10°C in six of eight years and averaged between 4.4 and 9.4° in the 10-d period following application, indicating that soil temperatures were cooling to minimize nitrification. Specific fertilization information and cultural practices used in the establishment and production of the corn and soybean are shown in Randall et al. (2003).

The experiment was conducted under ambient precipitation. Precipitation data collected daily at a site 0.5 km from the drainage site were summarized as monthly and seasonal totals from 1987 through 1994 (Table 1).

View larger version (43K): [in this window] [in a new window]   Fig. 1. Hydrograph showing daily drain flow from corn plots in 1991.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Growing Season Precipitation and Drain Flow
Growing season precipitation (April–October) during the eight-year period was highly variable, ranging from 402 mm (65% of normal) in 1989 to 958 mm (155% of normal) in 1993 (Table 1). Rainfall was below normal during the first three years of the study, which resulted in limited amounts (<20 mm) of subsurface drainage in 1987 and 1988 and no drainage in 1989 (Table 2). In the last five years of the study (1990–1994), growing season rainfall was above normal, especially in 1991 and 1993, and subsurface drainage through the tile system was plentiful (Table 2). Monthly rainfall was above normal in April each year and in May in 1990, 1991, and 1993. In the northern Corn Belt, soils are generally frozen until late March, and corn and soybean are not planted until late April. Thus, evapotranspiration (ET) from a corn–soybean cropping system until mid-June is small compared with July and August, and the potential for percolation and drainage of excess water through subsurface tiles is great. Table 3 shows that the majority of annual subsurface drainage occurred in April, May, and June in five of seven years. In 1987, all of the drainage (43 mm) occurred in early August following a very wet July and 102 mm rainfall between 4 and 9 August. Significant drainage in the fall occurred only in 1992 and 1994. In both years October rainfall totaled >200% of normal, which recharged the soil moisture profile to above field capacity during this period of low ET. For the eight-year period, 62% of the annual subsurface drainage occurred in April through June. Surface runoff from this very flat site was not measured, but was thought to be <2% of the annual precipitation.

Nitrate Nitrogen Concentrations in Tile Drainage
Flow-weighted NO₃–N concentrations in the tile water for both the corn and soybean phases of the rotation were greatly influenced by the dry weather conditions from 1987–1989 (Table 4). Highest NO₃–N concentrations in the drainage water, ranging from 27 to 35 mg L^-1 from the corn plots and 15 to 22 mg L^-1 from the soybean plots, occurred in 1990 following three dry years. These dramatic increases over previous and succeeding years were similar to those reported by Randall and Iragavarapu (1995) and were probably due to carryover of unused fertilizer N and mineralized soil N from the dry years. Nitrate N concentrations in 1991 were reduced by about 50% in the corn plots while concentrations in the soybean plots remained about the same as in 1990. Some fertilizer N applied for corn in 1990 apparently carried over and was leached into the drainage water from soybeans in 1991. This was especially likely because most drainage in 1991 occurred from April through July before the soybean plants were large enough to take up residual soil nitrate nitrogen (RSN). During the third and fourth years of significant drainage (1992 and 1993), NO₃–N concentrations were reduced further to between 8 and 13 mg L^-1 for corn and 5 and 11 mg L^-1 for soybean. This was probably due to purging of NO₃ from the soil profile, greater uptake of fertilizer N by the crops, and some denitrification during this four-year wet period (Randall et al., 1997).

Temporal variation of monthly flow-weighted NO₃–N concentration for corn during the 1990 and 1992 seasons is shown in Fig. 2a and during the 1991 and 1993 seasons is shown in Fig. 2b. Highest NO₃–N concentrations in the drainage water were found with the fall N treatment throughout 1990 and 1993 and for most of 1991 and 1992. In 1990, NO₃–N concentration for the fall N treatment was consistently higher throughout the year and was significantly greater than the other treatments in June and August. Nitrate N concentrations for the split N treatment did not vary greatly throughout the season.

View larger version (33K): [in this window] [in a new window]   Fig. 2. Flow-weighted nitrate N concentration in subsurface tile drainage as affected by time of N application and nitrapyrin (NP) for corn in four cycles of a corn–soybean rotation for (a) corn grown in even years (1990 and 1992) and (b) corn grown in odd years (1991 and 1993). All months shown had drainage exceeding 10 mm. Vertical bars above each monthly data set indicate the LSD (0.10). Absence of vertical bars above a monthly data set indicates no significant difference among flow-weighted nitrate N concentrations.

In 1992, when 69 mm or 41% of the annual drainage occurred in March and April, little difference in NO₃–N concentration was seen early in the season among the four treatments for corn (Fig. 2a). Nitrate N increased slightly for all treatments (about 1–4 mg L^-1) from April to June but decreased about 5 mg L^-1 from June to November when 36% (about 60 mm) of the annual drainage occurred. Significant differences (P = 0.10 level) occurred in April and June when NO₃–N concentrations were greatest for fall N, intermediate for fall N + NP and split N, and lowest for spring N.

Excessive rainfall during April, May, and June 1993 had a large temporal effect on NO₃–N concentration in subsurface drainage from corn (Fig. 2b). Moreover, 81% of the annual drainage occurred during these months. Monthly flow-weighted NO₃–N concentration for the fall N treatment increased from 8 mg L^-1 in April to 18 mg L^-1 in June and July. Drainage water from the fall N + NP and spring N treatments averaged 7 mg L^-1 in April and increased slowly to 13 and 11 mg L^-1, respectively, in July. Nitrate N concentration from the split N treatment never exceeded 10 mg L^-1 during the year. At the end of the drainage season (August), NO₃–N concentrations averaged about 12 mg L^-1 for the two fall N treatments and 9 mg L^-1 for the spring and split N treatments.

Monthly flow-weighted NO₃–N concentrations in subsurface drainage from the soybean phase of the rotation are shown in Fig. 2a for 1991 and 1993 and in Fig. 2b for 1990, 1992, and 1994. In general, temporal variation of NO₃–N was less during the soybean phase than during the corn phase of the rotation, and there was little consistent effect of N treatment across the four years. However, the dry years (1987–1989) did result in higher NO₃–N concentrations for all treatments in both 1990 and 1991. At the beginning of the drainage season, NO₃–N concentrations for the four treatments ranged from 15 to 22 mg L^-1 in 1990 and 17 to 23 mg L^-1 in 1991. In 1990, this was probably due to higher levels of RSN resulting from reduced uptake of fertilizer N by the crop and no losses via drainage in 1989. In 1991, greater RSN due to the dry years and unused fertilizer N from the treatments applied for the 1990 corn crop probably caused the higher NO₃–N concentrations. Also, monthly NO₃–N concentrations in 1990 generally increased from May through July and then declined by as much as 50% in August, whereas NO₃–N concentrations decreased throughout the 1991 season. The elevated NO₃–N concentrations of almost 30 mg L^-1 in July 1990 for the fall + NP and spring treatments were not significantly greater than the other treatments due to lower flow and high variability in flow rate among plots. By August each year, NO₃–N concentration had declined to about 15 mg L^-1 for all N treatments.

Month-to-month variation in NO₃–N concentration within each of the four N treatments was minimal in 1992 and 1993 and can be characterized as declining about 20% from June to November in 1992 (Fig. 2b) and 25% from April to August in 1993 (Fig. 2a). The declining NO₃–N concentration observed each year was probably due to a combination of NO₃ being purged from the profile by leaching and uptake of NO₃ from June through August by soybean. Nitrate N concentrations in April and June 1992 were significantly greater for the split and spring applications compared with the fall applications.

In summary, the four-year data clearly indicate the tremendous effect that high amounts of March through June rainfall have on NO₃–N concentrations in subsurface water being drained from fall-applied N, compared with spring- and split-applied N in the year corn is being grown. This was not obvious in the year soybean was grown. Carryover of RSN from mineralized soil N and from previous fertilizer N treatments in dry years with minimal leaching, regardless of application time, had the greatest affect on NO₃–N concentration in subsurface drainage from soybean. Although not statistically significant in most years, beginning in 1991 and continuing through 1993, NO₃–N concentrations in the drainage from soybean tended to be consistently greatest when the N was split-applied for corn the previous year. This is consistent with results reported for Iowa by Baker and Melvin (1994) and supports the observation that delaying fertilizer N application until the eight-leaf stage of corn increases the carryover of N in these glacial till soils and the potential for NO₃ leaching in the fall and following spring (Jokela and Randall, 1989).

Nitrate Nitrogen Losses in Tile Drainage
Annual NO₃–N losses (flux), the product of water flow multiplied by NO₃–N concentration, were affected substantially by growing season precipitation and NO₃–N concentration in the drainage water in both the corn and soybean phases of this study (Table 5). Nitrate N losses from the corn phase increased from a range of 0 to 5 kg ha^-1 yr^-1 for the four N treatments in the dry years (1987–1989) to between 62 and 122 kg ha^-1 yr^-1 in the two succeeding wet years, but then declined to between 19 and 35 kg ha^-1 yr^-1 in 1992 and 1993 as wet conditions continued. The same trend, but with slightly lower losses, occurred in the soybean phase. Nitrate N losses from the four N treatments applied for the previous corn crop ranged from 56 to 79 kg ha^-1 yr^-1 in 1990 and 1991, but declined to between 8 and 25 kg ha^-1 yr^-1 in the following three wet years. Nitrate N losses from the soybean phase were not determined in 1988 and 1989 because of very limited tile flow. These results dramatically show the strong effect of wet and dry climatic cycles on NO₃ losses from soils through subsurface drainage and agree well with other studies (Lucey and Goolsby, 1993; Randall, 1998). The value of long-term studies becomes quite apparent in research where climate plays such a significant role, especially when the findings may affect environmental policy development.

Averaged across treatments, 68 and 70% of the NO₃ lost in the drainage water during this eight-year period occurred in April through June for corn and soybean, respectively (data not shown). For the four years when NO₃ losses were measured for both crops (1990–1993), 55% of the total NO₃ lost from the corn–soybean rotation occurred from the corn plots and 45% from the soybean plots (Table 5).

Nitrate N losses, normalized for annual flow volume and expressed on a per-centimeter basis, are shown in Table 6 for both the corn and soybean phases, the two-year rotation average for each cycle, and the four-cycle rotation average. In all years, flow-normalized NO₃–N losses were greater in the corn phase than in the soybean phase for all N treatments. The two fall treatments tended to give greater flow-normalized losses than the spring and split treatments in the corn phase, but the opposite was true in the soybean phase of each cycle. When averaging both the corn and soybean phases across the four cycles of the rotation, flow-normalized NO₃–N losses ranked in the order: fall N > split N > spring N = fall N + NP. Averaged across this period with drain flow each year, NO₃–N losses in the drainage water from the corn–soybean rotation were reduced 18, 17, and 13% by the fall N + NP, spring N, and split N treatments, respectively, compared with the fall N treatment without NP. These data emphasize the fact that fall-applied N without NP for corn is much more susceptible to loss via subsurface drainage compared with fall N + NP, spring N, or split N applications. However, contrary to public opinion, the data also suggest that N split-applied between April (40%) and June (60%) is susceptible to losses equal to or greater than spring N or fall N + NP, when applied at equal N rates in a corn–soybean rotation. The primary causes for this rather consistent finding may be inadequate uptake by corn or reduced immobilization of the split-applied N, resulting in late-season losses in the corn phase when fall precipitation is plentiful, or loss in spring drainage during the succeeding year when soybean is grown.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Accurately assessing the effect of various crop and soil management practices on loss of NO₃ in subsurface drainage and then extrapolating these findings from plot- or field-size scales to watershed- or basin-size scales is difficult due to the complex nature of climate–soil–drainage interactions. Factors such as annual precipitation, temporal distribution of precipitation, climate during the nongrowing season, crops grown and their ET rates, soil texture, hydraulic conductivity, slope, drain tile depth, and tile spacing all can affect drainage losses, their interpretation, and their applicability for wide-spread adoption. For these reasons, long-term studies with detailed measurements are necessary to smooth year-to-year climatic variability, thereby providing more meaningful information.

This eight-year study (four cycles of a corn–soybean rotation), conducted across a range of dry to very wet years, provides information on the hydrologic effects governing losses of NO₃ from four N application strategies for corn grown in a rotation with soybean. The primary findings are:

• Sixty-two percent of the annual subsurface drainage occurred in April through June. Precipitation frequently exceeds evapotranspiration during these months, leading to substantial percolation and drainage. This uncontrollable factor presents a tremendous challenge for management of fall-applied N. If significant nitrification of fall-applied N occurs either in the fall or in the spring before crop uptake of N or increased ET, losses of NO₃ in drainage water can be large. On the other hand, few drainage events occurred in the fall, resulting in only 11% of the annual drainage lost in October and November. Thus, little RSN is likely to be leached from the soil profile into drainage water in the fall.
  
• Sixty-eight and 70% of the NO₃ lost annually to subsurface drainage from corn and soybean, respectively, occurs in April, May, and June. This indicates that the loss of NO₃ is proportionately greater than the removal of water through the drains over this period. It suggests that fall-applied N for corn and mineralization of soil organic matter and soybean residue, including roots, could contribute greatly to leaching losses in the corn phase of the rotation. In the soybean phase, RSN from the previous crop is very susceptible to percolation into subsurface drains in the spring. During July and August, uptake of N is significant for both crops and ET generally exceeds rainfall, leading to few and small losses of NO_3. These findings agree with drainage studies reported by Jaynes et al. (1999) and NO₃ accumulations in the Mississippi River reported by Antweiler et al. (1995).
  
• When averaged across all four N treatments, 55% of the NO₃ in the drainage occurred during the corn phase and 45% during the soybean phase. Thus, when evaluating the effect of N management practices for corn on NO₃ losses to subsurface drainage, both the corn and soybean phases need to be included in the analysis. This is especially true if the N is applied at higher rates or later in the growing season or if normal to less-than-normal amounts of precipitation occur during the growing season of the corn phase of the rotation.
  
• Compared with late fall–applied N as anhydrous ammonia, losses of NO₃ in subsurface drainage from a corn–soybean rotation can be reduced by 18% if including NP with late fall–applied ammonia, by 17% with spring preplant–applied ammonia, and by 13% with N split-applied between April (40%) and June (60%). Losses of NO₃ in the soybean phase tended to be somewhat greater for the split-applied treatment compared with the fall and spring preplant treatments.
  
• Corn production (yield and profit) can be improved and NO₃ losses to subsurface drainage waters can be reduced by using NP with late fall–applied N or applying N in the spring as a preplant or split (preplant plus sidedress) treatment compared with late fall applications without NP.
  

	ACKNOWLEDGMENTS

 
The authors thank the technicians on the soils crew for the collection of the data, Brian Anderson for the annual data tabulation and analyses, and Arielle Balak for preparation of this manuscript. Partial funding of this research was provided by Dow AgroSciences and is greatly appreciated.

	REFERENCES

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