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NITROGEN MANAGEMENT

Corn–Soybean Rotation Effects on Nitrate Leaching

Dep. of Crop and Soil Sci., 116 A.S.I. Bldg., The Pennsylvania State Univ., University Park, PA 16802

^* Corresponding author (yxz117{at}psu.edu)

Received for publication November 8, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
Because agricultural production is a primary nonpoint source of NO^-₃ in the nation's waters, a better understanding of the effects of cropping systems on NO^-₃ leaching is required for developing agricultural production practices that reduce NO^-₃ leaching. A 4-yr experiment was conducted to study the effect of a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation on NO^-₃ leaching using passive capillary fiberglass wick lysimeters. The experiment had five N rates on corn (0–200 kg N ha^-1 in 50-kg increments). Corn was planted in 1997 and 1999, and soybean was planted in 1998 and 2000. The increase of soil residual NO^-₃–N concentrations in the surface 25 cm of soil after crop harvest was not significant (p = 0.05) when N fertilizer rates applied to corn increased from 0 to 100 kg ha^-1 but was significant when N rate increased from 100 to 200 kg ha^-1 in the corn years. The 2-yr average soil residual NO^-₃–N concentrations and annual flow-weighted NO^-₃–N concentrations in leachate were significantly higher (p = 0.05) in soybean years than in corn years at 0 and 100 kg N ha^-1 applied to corn, but the differences at the 200 kg N ha^-1 rate were not significant. The results indicate that at recommended N rates applied to corn in a corn–soybean rotation, NO^-₃ leaching potential is similar for corn and soybean, but at less than 100 kg N ha^-1 rate, a greater NO^-₃ leaching potential exists under soybean than under corn.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
ELEVATED NO^-₃ CONCENTRATIONS in water systems have been an environmental concern in areas of intensive agriculture for many years. As the source of NO^-₃ is mainly agricultural production, developing agricultural production practices that reduce NO^-₃ leaching to water systems is needed. Other than N fertilizer rate, cropping systems may have a greater influence on downward movement of NO^-₃ below the root zone than any other agricultural production practice in nonirrigated areas because different crops have different rooting depths, root densities, water use rates, N requirement characteristics, and N uptake efficiencies (Peterson and Power, 1991; Juergens-Gschwind, 1989). However, there are few reports comparing NO^-₃ leaching from different cropping systems. Most NO^-₃ loss data are from corn fields. It has been repeatedly confirmed that continuous N-fertilized corn usually leads to high NO^-₃ leaching losses because of high N fertilizer inputs year after year, widely spaced rows, and a limited root system during the early growing season (Owens, 1990; Jemison and Fox, 1994; Toth and Fox, 1998; Gast et al., 1978; Andraski et al., 2000). The concentrations of NO^-₃–N in percolate or subsurface drainage from corn fields in all of these studies receiving recommended N rates were usually well above the 10 mg L^-1 drinking water standard.

Incorporating a nonlegume winter cover crop in a cropping system has been found to be quite successful in reducing NO^-₃ leaching because the cover crop can reduce water percolation downward to the ground water and utilize NO^-₃ that would otherwise leach (Staver and Brinsfield, 1998; Brandi-Dohrn et al., 1997; Rasse et al., 2000; Thomsen et al., 1993; Shepherd, 1999; Goss et al., 1998; Francis et al., 1998). Possible problems are that the cover crop may not become established well enough to absorb significant quantities of soil water and mineralized N over winter (Shepherd, 1999; Ritter et al., 1998; Catt et al., 1992), and growth of the succeeding crop may be reduced because of N immobilization following incorporation of a high C/N ratio cover crop (Martinez and Guiraud, 1990; Francis et al., 1998; Wyland et al., 1995).

Rotating corn with legumes may be an alternative to reduce NO^-₃ leaching because legumes use N fixed from atmospheric N₂ and need little or no fertilizer N for their growth. Toth and Fox (1998) using zero-tension pan lysimeters and Randall et al. (1997) using tile drains found that NO^-₃ concentrations in leachate from alfalfa (Medicago sativa L.) fields were usually below 5 mg L^-1 while those from continuous corn fields receiving the recommended N rates were over 15 mg L^-1. Owens (1990) found that percolate NO^-₃–N concentrations from monolith lysimeters planted with an alfalfa (70%)–orchardgrass (Dactylis glomerata) (30%) mixture were <5 mg L^-1 and were 20 to 40 mg L^-1 in leachate from corn receiving 336 kg N ha^-1. However, alfalfa has long, deep roots and easily decomposable, high-N-content residue. After the deep alfalfa roots decompose, the root channels may become deep pathways for leaching; thus, NO^-₃ leaching loss may increase after alfalfa residue is incorporated into soil (Robbins and Carter, 1980; Carter et al., 1995).

However, the effect on NO^-₃–N leaching of introducing the most widely grown legume crop, soybean, in a cropping system is still not clear. Some researchers found that a corn–soybean rotation reduced NO^-₃–N leaching compared with continuous corn, especially when N credits from soybean were taken into account when recommending optimum N rates for corn production (Weed and Kanwar, 1996; Kanwar et al., 1997). Other researchers found that rotating bean crops with corn increased NO^-₃ leaching (Meek et al., 1995; Klocke et al., 1999). Katupitiya et al. (1997) using soil coring to a depth of about 18 m and Randall et al. (1997) using tile drains found that there were essentially no differences in NO^-₃ leaching between continuous corn and a corn–soybean rotation.

When comparing NO^-₃ leaching in corn years and soybean years in a corn–soybean rotation, Owens et al. (1995)(2000) found that leachate volumes were not different between corn years and soybean years, but the flow-weighted NO^-₃–N concentrations were greater in corn years than in soybean years. However, in their experiment, a rye (Secale cereale L.) cover crop was sowed after soybean, which consumed some residual NO^-₃ from soybean and might have reduced NO^-₃ leaching from soybean years. Weed and Kanwar (1996), Kanwar et al. (1997), and Logan et al. (1994) found no significant differences in NO^-₃ leaching between corn and soybean in a corn–soybean rotation.

The above work was mostly done in the Midwest, and the experiments typically only had a single N rate. Studies have shown that NO^-₃ concentrations of ground water are elevated in agricultural areas in Pennsylvania (Swistock et al., 1993). Jemison and Fox (1994) and Toth and Fox (1998) have shown that continuous corn receiving the economic optimum N rate consistently led to high NO^-₃–N leaching losses and that NO^-₃ leaching loss is small in alfalfa years in a corn–alfalfa rotation, thus reducing the NO^-₃ loading of ground water compared with continuous corn. Soybean is the third most widely grown crop in Pennsylvania after hay and corn (Pennsylvania Agric. Stat. Serv., 2000), but there are no data from Pennsylvania on its effect on NO^-₃ leaching in a corn–soybean rotation sequence. In this experiment, we measured and compared NO^-₃ leaching in a corn–soybean rotation under different N rates for corn. We also used passive capillary fiberglass wick lysimeters to determine the NO^-₃ leaching. The wick lysimeters have the advantage in that they sample more representative soil water than do pan lysimeters (Zhu et al., 2002, 2003).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
The experiment site is located at the Russell E. Larson Agricultural Research Center of The Pennsylvania State University in central Pennsylvania. The field soil is a well-drained Hagerstown silt loam (fine, mixed, semiactive, mesic Typic Hapludalf) developed from limestone residuum. The soil basic chemical analyses sampled at planting did not change from 1995 to 2001, and the average Mehlich 3 extractable values from surface 20-cm samples were 48 mg kg^-1 soil for P, 123 for K, 164 for Mg, and 1228 for Ca, with a pH of 6.1 (1:1 water/soil) and a cation exchange capacity of 10.3 cmol_c kg^-1. Other selected soil physical properties were given in Jemison and Fox (1992).

From spring 1988 to spring 1991, the field was used to estimate NO^-₃ leaching from manured and unmanured corn as influenced by N fertilizer rate (Jemison and Fox, 1992, 1994). From April 1991 to April 1994, it was used to study NO^-₃ leaching under continuous N-fertilized corn and alfalfa (Toth and Fox, 1998). The plots that were planted with alfalfa during the years 1991 to 1994 were the manured corn treatments from 1988 to 1990. In April 1994, the alfalfa was killed with herbicide and plowed under, and the entire field was planted with corn. Beginning in 1995, the plots that had been in manured corn and alfalfa became no-till corn, and the plots that had been in tilled continuous corn remained the same chisel-tilled corn.

In 1997, a corn–soybean rotation system was introduced, starting with corn in 1997 and ending with soybean in 2000. The target plant density was 64200 plants ha^-1 with a row spacing of 76 cm for corn and 470000 plants ha^-1 with row spacing of 17 cm for soybean. Grain was harvested in October when black layer had formed in the corn kernel or when the soybean was fully matured and dried in the field (moisture content 10–14%). All plant residue was left on the field. The experiment had two tillage treatments, chisel till to a depth of 25 cm followed by disking to a depth of 10 cm and no-till. Within each tillage treatment, there were five N rates (0–200 kg N ha^-1 in 50-kg increments) applied to corn as ammonium nitrate. No N fertilizer was applied to soybean. The experiment was a partially nested design with the blocking factor nested in the tillage treatments (Neter et al., 1996, p. 1149). Within each tillage treatment, the experiment was a five-N-rate randomized complete block design with three replications. A total of 30 plots each with an area of 10.7 by 15.2 m were laid out in the field.

In April 1995, passive capillary fiberglass wick lysimeters were installed in all plots where corn received 0, 100, or 200 kg N ha^-1 to compare their performance with the zero-tension pan lysimeters already installed and to monitor NO^-₃ leaching as a function of N fertilizer rate and tillage. The wick lysimeters were installed 1.2 m below the soil surface on the opposite side of the lysimeter pits where pan lysimeters had been installed in 1988 (Jemison and Fox, 1992). Because leachate collection efficiency was better with wick lysimeters than with pan lysimeters and there were numerous anomalous high-NO^-₃ concentrations with some pan lysimeters in the latter years of the experiment, presumably due to animal activity (Zhu et al., 2003), only wick lysimeter data are presented in this paper.

The wick lysimeter design was adapted from that published by Holder et al. (1991). A 30- by 30- by 1.3-cm thick plexiglass plate with a 3.2-cm-diam. center hole was attached to a pressure-treated plywood support structure. Cleaned wick material was evenly spread on the top of the plexiglass plate and passed through the center hole of the support structure to 15-L polypropylene carboys. Tunnels for lysimeter installation were excavated 65 cm into soil at 1.2 m below the soil surface to accommodate the lysimeters plus a 30-cm buffer of undisturbed soil between the pit edge and the lysimeters. The vertical distance from the top of the plexiglass to the bottom of the wick in the collecting jugs was 50 cm, which created up to 50 cm of water tension on the soil. Because the lysimeters were installed in plots with 0, 100, and 200 kg ha^-1 N rates under both till and no-till treatments with three replications, a total of 18 plots were installed with lysimeters. The detailed installation procedure can be found in Zhu et al. (2002).

Leachate was collected after rain events that were sufficient to cause leaching to a depth of 1.2 m and on the last day of each month to provide monthly leachate data. Leachate volumes were measured and NO^-₃–N concentrations (including NO^-₂) were determined with a Technicon autoanalyzer (Technicon Instruments Corp., Tarrytown, NY) using a Cd reduction method. Nitrite concentrations, which were measured for several batches of leachate samples, never exceeded 1% of the NO^-₃–N concentrations. Bergstrom (1987) also found that only minimal NO^-₂ existed in leachate samples. Therefore, the results from the Technicon autoanalyzer were considered as NO^-₃. The flow-weighted NO^-₃–N concentrations for different periods were calculated by summing up NO^-₃–N masses collected for the periods divided by the total leachate volume collected in the corresponding periods. For example, 2-yr corn flow-weighted NO^-₃–N concentration is the NO^-₃–N mass collected in leachate during the 2-yr period planted with corn divided by the leachate volume collected in the same 2-yr period. The leaching or crop year was defined as from May through the next April and was named according to the starting year and the crop name. For example, leaching year 1997 or 1997 corn year meant from May 1997 to April 1998.

Annual leachate collection efficiencies for individual lysimeters were used to correct the annual leachate volume and NO^-₃–N mass collected by the lysimeters. The corrected leachate volume and NO^-₃–N mass were calculated by dividing the collected leachate volume and NO^-₃–N mass by the annual leachate collection efficiencies of individual lysimeters. The leachate collection efficiencies were estimated using a water balance method (Zhu et al., 2002). The average leachate collection efficiency of wick lysimeters was approximately 100%.

Plow-layer (0–25 cm) soil samples were taken every year after crop harvest. A composite soil sample of four 1.9-cm-diam. soil cores were taken in each plot where lysimeters were installed. A total of 18 soil samples were taken each year after crop harvest. The soil samples were air-dried and ground to pass a 2-mm sieve. The soil NO^-₃–N was extracted with 2 M KCl solution and analyzed with a Technicon autoanalyzer (Keeney and Nelson, 1982).

The differences in flow-weighted NO^-₃–N concentrations and NO^-₃–N masses in leachate and soil residual NO^-₃–N concentrations in the surface 25 cm of soil between corn and soybean years were analyzed using the paired t test in Minitab software (Minitab, 1997). The differences in soil residual NO^-₃–N concentrations between N rates were analyzed with the General Linear Model in ANOVA in the Minitab package, and the resulting error mean squares were used to calculate the LSD values for mean separation.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
Leachate Volume
It is difficult to compare leachate volumes between soybean years and corn years because of the differences in temperature, precipitation, and evapotranspiration in the different years. However, some generalizations in leachate volumes corrected by leachate collection efficiency from corn and soybean years might be useful. Because tillage treatments had no significant effect on leachate volumes (Zhu et al., 2003), the average leachate volumes for till and no-till for the 4 yr of corn–soybean rotation (1997–2000) along with the 1995 and 1996 continuous corn years are shown in Table 1. Higher leachate volumes were observed in the higher-precipitation years. In 1996 and 1997, when annual precipitation was 1.6 to 2.5 times higher than that of the other 4 yr, the leachate volumes were about 1.4 to 1.7 times higher than the other 4 yr. The annual precipitation for the 1995 and 1999 corn years was similar to that in the 1998 and 2000 soybean years. Leachate volumes were also similar for the two corn and two soybean years. Klocke et al. (1999) found that there was no significant difference in drainage water volumes between corn and soybean in a corn–soybean rotation. Owens et al. (1995)( 2000) and Kanwar et al. (1997) found that differences in percolation water volume were very small between corn and soybean in a corn–soybean rotation. Hattendorf et al. (1988) and Copeland et al. (1993) also found that corn and soybean consumed similar amounts of water for their growth. However, Timlin et al. (1992) observed that soil water potential at 15- and 35-cm depth was lower (more negative) under soybean than under corn, which they attributed to greater canopy cover with soybean than with corn.

Flow-Weighted Nitrate-Nitrogen Concentration in Leachate
Tillage treatments did not have a significant effect on flow-weighted NO^-₃–N concentrations and masses lost in leachate (Zhu et al., 2003). Therefore, average values for the till and no-till treatments for the corn and soybean years are used in the discussion of flow-weighted NO^-₃–N concentrations and masses lost in leachate. Annual flow-weighted NO^-₃–N concentrations in leachate for each of the two soybean years were higher than in each of the two corn years at the 0 and 100 kg N ha^-1 rates (Table 3). Average 2-yr soybean flow-weighted NO^-₃–N concentrations were significantly greater than those for the 2-yr corn at p = 0.01 level for both 0 and 100 kg N ha^-1 rates. For the 200 kg N ha^-1 rate, the annual or overall 2-yr flow-weighted NO^-₃–N concentrations were not different between corn and soybean years. Average annual flow-weighted NO^-₃–N concentration for the 2000 soybean year was the highest in the 4-yr period at the 200 kg N ha^-1 rate, but the annual flow-weighted NO^-₃–N concentration for the 1998 soybean year was the lowest in the 4-yr period. Average flow-weighted NO^-₃–N concentrations for the two soybean years and two corn years at the 200 kg N ha^-1 applied to corn were 26.3 and 22.4 mg L^-1, respectively. The difference was not significant.

Nitrate-Nitrogen Mass in Leachate
Average annual NO^-₃–N mass lost through leaching that was corrected by leachate collection efficiency for two soybean years was significantly (p = 0.05) higher than that for two corn years at the zero-N rate (Table 4). At the 100 and 200 kg N ha^-1 rates applied to corn, average NO^-₃–N mass losses in leachate for the two soybean years and two corn years were not significantly different (p = 0.05). Weed and Kanwar (1996) and Kanwar et al. (1997) using tile drainage found that continuous corn receiving 202 kg N ha^-1 fertilizer lost significantly more NO^-₃–N mass than a corn–soybean rotation with corn receiving 168 kg N ha^-1, but the NO^-₃–N mass losses from corn and soybean were similar in a corn–soybean rotation. Randall et al. (1997) also found that NO^-₃–N mass losses were similar between corn and soybean with corn receiving recommended N rates in a corn–soybean rotation. Results of this and previous studies indicate that at close-to-recommended N rates applied to corn (usually 150–200 kg N ha^-1) in a corn–soybean rotation, NO^-₃ leaching losses during corn years and soybean years are not significantly different at p = 0.05 level, but at low N rates, flow-weighted NO^-₃–N concentrations in leachate and soil residual NO^-₃–N concentrations after crop harvest are greater under soybean than under corn.

	SUMMARY

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
Weather conditions were the main factors affecting leachate volumes in both crops of a corn–soybean rotation. With similar annual precipitation, the differences in leachate volumes were minimal between corn and soybean years. At the 0 and 100 kg N ha^-1 rate applied to corn, annual flow-weighted NO^-₃–N concentrations were greater in soybean years than corn years, but at the 200 kg N ha^-1 rates, annual flow-weighted NO^-₃–N concentrations were not significantly different between corn and soybean years. Annual NO^-₃–N mass losses through leaching were significantly greater in soybean than in corn years at the zero-N rate but were not significantly different at the 100 and 200 kg N ha^-1 rates on corn. Soil residual NO^-₃–N concentrations after crop harvest in the surface 25 cm of soil increased significantly when N rates increased from 100 to 200 kg N ha^-1 in corn years, but the increase was not significant when N rates increased from 0 to 100 kg N ha^-1. In soybean years, soil residual NO^-₃–N concentrations were not significantly affected by increasing N rates applied to the preceding corn. Soil residual NO^-₃–N concentrations after crop harvest were higher in soybean years than in corn years at the 0 and 100 kg N ha^-1 rates but were similar at the 200 kg N ha^-1 rate for corn. The higher soil residual NO^-₃–N concentrations after soybean harvest than after corn harvest and higher flow-weighted NO^-₃–N concentrations in leachate in soybean years than in corn years at low N rates indicates that greater leaching potential exists in soybean years than corn years in a corn–soybean rotation if N application rates to corn are low. The lack of differences in soil residual NO^-₃–N concentrations at 200 kg N ha^-1 rates on corn signifies that the difference in NO^-₃ leaching from corn or soybean in a corn–soybean rotation are probably small when the N fertilizer rate is close to the recommended N rate.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 

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