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a Department of Soil, Water, and Climate, University of Minnesota, St. Paul, MN 55108
b Previously with the Department of Soil, Water, and Climate, University of Minnesota, St. Paul, MN 55108
c Previously with the Department of Soil Science, University of Wisconsin, Madison, WI 53706
* Corresponding author (sgupta{at}umn.edu).
Received for publication June 6, 2003.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: GS, growing season • NGS, nongrowing season
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The soils in this region are relatively shallow and have some inter-pedal fractures. There is also some presence of earthworm macropores (Hanewall, 1996). Since manure application to land favors the proliferation of earthworms and development of macropores (D. Fuchs and D. Linden, personal communication, 1988; Munyankusi et al., 1994), there is concern that manure application may increase nutrient transport to ground water through preferential transport.
In a survey of the 500 drinking water wells tested in 51 counties of Minnesota, Klaseus et al. (1988) reported the presence of pesticides and nitrate in 33 and 43% of the wells, respectively. A large number of contaminated wells were in the karst region of southeastern Minnesota. A major hypothesis proposed to explain this contamination was that inter-pedal fractures and macropores were facilitating water and chemical transport through soils to fractured bedrock and then to ground water.
An analysis of 303 water samples collected from wells in the 12 midcontinental states showed that NO3 concentrations were in excess of the USEPA (2000) maximum drinking water standard (10 mg L–1) in 6% of the samples and over the upper bound of natural NO3 concentration (3 mg L–1) in 29% of the samples (Burkart and Kolpin, 1993). Both the above surveys simply attest to the substantial impact of anthropogenic sources of NO3–N, such as manure and fertilizer, on ground water quality in the U.S. Midwest.
It is well known that the nutritional benefits of manure to plants are maximized when manure is incorporated into the soil with some type of tillage. Moldboard plow–based tillage systems maximize manure incorporation but leave very little crop residue at the soil surface for erosion control. Because of the concern for accelerated erosion from moldboard plowing, this practice is rarely used for manure incorporation on steep landscapes such as the karst region of the Upper Midwest. Instead, conservation tillage systems that leave some residue on the soil surface are commonly used. These systems include chisel-plowing and disking along contours. The other less common conservation tillage system in the karst region is no-till.
Tillage and residue management systems greatly affect earthworm activities and thus earthworm macropore development. Zachmann et al. (1987) and Steenhuis et al. (1990) showed that earthworm macropores in tilled and residue incorporated plots were not as continuous as in no-tillage plots. Kemper et al. (1987) showed that less intense tillage not only kept the crop residue at the soil surface but it also increased the activity of surface-feeding earthworms, which in turn leads to the presence of numerous surface-connected macropores and higher infiltration.
In the karst region of the Upper Midwest, manure is generally spread on the soil surface during fall and spring but often during winter over frozen soils. Fall- and spring-applied manure is incorporated in the soil with tillage right after its application. On the other hand, winter-applied manure remains on the soil surface until spring when it is incorporated into the soil with primary tillage except in no-till fields. Since there is increased potential for contact between winter-applied manure and spring snowmelt or spring rainfall runoff, there is some potential for nutrient dissolution from manure in runoff water and then their transport to water bodies.
A few studies have reported on the effect of winter application of manure on water quality (Young and Mutchler, 1976; Young and Holt, 1977; Converse et al., 1976; Witzel et al., 1969; Hensler et al., 1970). However, all of these studies have characterized water quality effect only in terms of surface water quality. Furthermore, many of these studies quantified manure application effects on water quality under a tillage system where there was some mixing of manure with soil. The goal of this study was to quantify the effect of manure vs. no manure, and the timing of manure application under two conservation tillage systems (some soil–manure mixing, no soil–manure mixing) on surface and subsurface water quality during both the nongrowing and growing seasons. In this paper, we report the results of these treatments on subsurface water quality as reflected by mineral N leaching. In a subsequent paper, we will discuss the effect of the above treatments on runoff water quality.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The soil at the site is a Rozetta silt loam, which has a succession of Bt horizons at depths varying from 33 to 154 cm and a massive soil structure at depths varying from 70 to 102 cm (De Bonis, 1996). The organic matter content of the surface soil is 0.025 g g–1. Ponded infiltration rates at the soil surface (12.0 m d–1) and at a 90-cm depth (9.1 m d–1) were lower compared with the infiltration rates at a 45-cm depth (28.4 m d–1). An extremely cherty stone line was present at about a 2.5-m depth. The site was under alfalfa (Medicago sativa L.) in 1983–1984, under corn (Zea mays L.) in 1985–1988, again in alfalfa in 1989–1991, and under corn in 1992. The site has approximately 12% slope and faces 5° northeast.
Experimental Design
A randomized complete block design in a split-split-split-plot arrangement was used with two replications in 1993–1994 and three replications in 1994–1995. The main plot was season and the sub-plot was tillage. Manure application was the sub-sub-plot and the two sampling devices (wick and pan samplers) were the sub-sub-sub-plots in the manure subplots.
The plots were 18.3 m long and 4.9 m wide. The plots were isolated from the surrounding area using galvanized corrugated steel that was pounded into the ground to a 15-cm depth with a sledge hammer.
Tillage and Manure Treatments
The treatments were two types of tillage (chisel and no-till) with and without manure application. Manure was either applied in fall (before the onset of snow) or in winter (over snow with frozen soil underneath).
In spring of each year a starter fertilizer was applied to all plots. In 1994, the starter fertilizer (9–23–30) was applied at 15 kg N ha–1 whereas in 1995 the starter fertilizer (8–32–17) was applied at 12.5 kg N ha–1. Other than the starter fertilizer there was no other fertilizer application to no-manure plots. All tilled plots were chisel-plowed in fall and then disked in spring. For the chisel-plowed fall manure plots, plowing was done after manure application in the fall. For the no-till there was no mixing of manure with soil either for the fall- or winter-applied manure treatment.
Manure was a daily scrap and haul liquid dairy manure. Table 1 lists the amount of manure applied during 1993–1995 and the associated total Kjeldahl N, organic N, and available mineral N. The available N was estimated as total mineral N plus 30% of the organic N (Sutton et al., 1985). The manures for the 1994–1995 nongrowing season and 1995 growing seasons were from two different dairies and thus resulted in different estimated available N application rates.
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Construction and Installation of Samplers
Pan and wick samplers were installed at a 60-cm depth to represent a shallow soil profile overlying the bedrock or the residuum. Pan samplers were installed to collect water from macropore flow whereas wick samplers were installed to collect water from both macropore and matrix flow. Jordan (1968) and Parizek and Lane (1970) have described the construction and functioning of pan samplers whereas Holder et al. (1991) and Boll et al. (1992) have described the construction and functioning of wick samplers. Percolation and chemical concentration data from these samplers can be used to differentiate between macropore and matrix leaching.
The samplers were installed in two replications in 1993–1994 and three replications in 1994–1995. The pan or the wick samplers consisted of a 79- x 36-cm PVC sheet with a 2.5-cm-wide PVC strip glued along the edges (Fig. 1). A 2.5-cm hole drilled in the middle of the PVC sheet allowed the leachate to move into the collection vessel. A fiber-glass wick (Papperell Braiding Co., Pepperell, MA) facilitated the movement of the percolate solution from the PVC sheet to the collection vessel. The top end of the fiber-glass wick was unwound and its strands spread and then glued to the PVC sheet. The base of the PVC sheet was then covered with a fiber-glass cloth (Industrial Arts Supply Mail Order Co., St. Louis Park, MN) to ensure that all water collected on the PVC sheet was channeled to the collection vessel. The fiber-glass cloth was then covered with clean fine sand to filter out any clay particles that may be appearing with the percolate solution.
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The difference between the pan and wick samplers was that in the pan sampler we placed a plastic grille over the fiber-glass cloth and sand (Fig. 2a). The grille was also covered with a plastic window screen to prevent soil chunks from falling into the pan sampler. The plastic grille in the pan sampler prevented the fiber-glass cloth and sand from coming in direct contact with the soil above the sampler. Therefore, the water that percolated into the pan samplers was that which had dripped from the soil matrix when it was saturated or the water that came through the macropores, root channels, and inter-pedal voids.
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Pan and wick samplers were installed by digging a 1.8-m-wide and 1.8-m-deep trench on one side of the plot with a back hoe, excavating a small tunnel (about the same size as the sampler) in the side wall of the pit (Fig. 2b), and then jacking the sampler against the top face of the tunnel with sliding wooden wedges. The tunnel was dug first by cutting the tunnel area with a chain saw and then digging the soil with a hand shovel.
The collection bucket was positioned next to the pit wall with a slight tilt toward the wall (Fig. 2b). Tilting was done in such a way that the water-carrying neoprene tube was at the lowest point in the bucket. The bucket was also positioned such that the vertical distance from the top of the sampler to the bottom of the wick was 50 cm. This length provided a suction of 50 cm to the soil above the wick sampler. Soil was tightly packed around the samplers, the wedges, and the buckets. All tubes were buried below the depth of tillage and the ends brought to the edge of the plots. The access trench for sampler installation was back-filled first with subsoil and then with surface soil. All the samplers were primed with distilled water just to ensure that they were working and to determine the inaccessible volume in buckets during pumping. The inaccessible volume was too small to significantly affect the nutrient concentration in the leachate.
Leachate Sampling and Chemical Analysis
Water was pumped from the buried collection vessel through the 0.44-mm-diameter neoprene tube. The pumping setup consisted of a 30-mL plastic vial connected to the water extraction tube, which in turn was connected to a 4-L Erlenmeyer flask and then on to an electric pump. There was some mixing of the leachate in the 30-mL vial as it was pumped out from the collection vessel to the Erlenmeyer flask. The 30-mL leachate sample in the plastic vial was frozen until its chemical analysis. The volume of the leachate collected in the Erlenmeyer flask was measured and then discarded. If the volume of leachate collected in the collection bucket was greater than 4 L, pumping was repeated until no more leachate came out of the collection bucket. A manifold setup was used to pump all four samplers in each plot at the same time.
At each sampling time, leachate samples were analyzed for mineral and ammonium N using the conductimetric procedure of Carlson (1978)( 1986). Nitrate N concentrations were calculated as the difference between the mineral and ammonium N. Ammonium N concentrations were relatively small compared with nitrate N concentrations. Mineral N leaching was calculated by multiplying the mineral N concentration of the leachate with the volume of the leachate (percolation) collected from each sampler. The leachate volume was converted into an equivalent depth of water by dividing it with the cross-sectional area of the sampler (2868 cm2). Within each year, percolation, mineral N concentration of the leachate and mineral N leaching losses were grouped by periods with and without plant cover. The period with plant cover was designated as a growing season whereas the period without plant cover was designated as the nongrowing season. The nongrowing season started from the time of fall harvest to planting, whereas the growing season ran from planting to fall harvest. Dates for the two nongrowing seasons (NGS) and two growing seasons (GS) are as follows: NGS1, 8 Nov. 1993 through 10 May 1994; GS1, 11 May through 6 Oct. 1994; NGS2, 7 Oct. 1994 through 15 May 1995; and GS2, 16 May through 3 Oct. 1995.
Data Analysis
All data were tested for homogeneity of variance and normality. Concentration data were log-transformed to meet requirements for ANOVA. The data were analyzed using the general linear models procedure of SAS (SAS Institute, 1999). Treatment means were compared using the Waller–Duncan test with a k ratio of 50, which corresponds to the 10% probability of making a Type I error (Steel et al., 1997).
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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There was less precipitation during NGS1 than NGS2 (202 vs. 357 mm). However, there was more precipitation during GS1 than GS2 (523 vs. 303 mm). Higher precipitation during GS1 and NGS2 resulted in greater percolation during these periods. Detailed percolation analysis showed instantaneous water percolation in response to bursts of rain events during the nongrowing season.
The effects of season, tillage, timing of manure application, and sampler type on percolation (cm), mineral N concentration of the leachate (mg L–1), and mineral N leaching (kg ha–1) are summarized in Table 2.
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The no-till treatment had a significantly higher annual water percolation than the chisel-plow treatment (24.0 vs. 17.5 cm). This increase in percolation in the no-till system possibly reflects greater numbers of earthworm burrows, and undisturbed root channels and inter-pedal voids in the no-till than the chisel-plow treatment. Hanewall (1996) showed that at this site the total number of earthworms was significantly (P = 0.05) higher in the no-till than the chisel-plow treatment. For example, the total number of worms in the no-till treatment was 51, 154, and 81 m–2 compared with 9, 73, and 16 m–2 for chisel-plow treatment in spring–summer 1995, fall 1995, and spring–summer 1996, respectively. Even though there was no statistical effect of tillage on number of Lumbricus terrestris, a deeper-burrowing and surface-feeding worm, their numbers were consistently higher in the no-till treatment (7, 18, and 10 m–2) than the chisel-plow treatment (1, 4, and 5 m–2). These earthworms generally form vertical borrows that are open at the surface. In general, in soils that are annually disturbed, such as under the chisel-plow system, continuity of the burrows and previous crop root channels is broken, thus making them less effective in carrying any surface water to deeper depths (Kemper et al., 1987; Zachmann et al., 1987). Conversely, under no-till systems, the macropores that are initially open at the soil surface remain open after planting operations and provide a direct conduit for surface water to percolate to deeper depths (Ehlers, 1975; Edwards et al., 1993). On average, 25% of the precipitation percolated to a 60-cm depth in the chisel-plow compared with 35% under the no-till treatment.
Annual water percolation was significantly higher in the wick samplers (22.0 cm) than the pan samplers (17.3 cm). This is expected since water percolation to the pan is mainly through macropores or when the soil is saturated, whereas percolation in the wick samplers reflects additional water percolation due to suction applied by the hanging wick (Holder et al., 1991). In our case, suction resulting from the hanging wick was equivalent to 50 cm. The amount of water collected in the wick and the pan samplers at a 60-cm depth was as high as 63 and 41% of the precipitation, respectively, during NGS2.
Percolation was much higher from the wick than the pan sampler during the nongrowing season but there was no difference in percolation between sampler types during the growing season (Fig. 3). This is expected considering that the soils are near saturation during late fall or early spring (the nongrowing season) in the Upper Midwest and thus any suction applied from the presence of the wick (50 cm) will substantially drain water from the soil matrix, leading to significant differences in percolation between sampler types. Conversely, soils during the growing season are relatively unsaturated (near field capacity or drier) and thus application of additional suction from the wick samplers will not add much to growing-season percolation.
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Mineral N concentration of the leachate was mostly above the drinking water standard of 10 mg L–1 set by the USEPA (USEPA, 2000). Leachate mineral N concentration (data not shown) increased with the onset of the nongrowing season (fall) and decreased with the onset of the growing season (spring). The decrease in mineral N concentration during the growing season was mainly due to greater water (thus less percolation) and associated N uptake by plants as a result of evapotranspiration. There were significant differences in mineral N concentration of the leachate among the manure treatments (P < 0.01) and between the sampler types (P < 0.01). Furthermore, there were significant tillage by manure (P = 0.04), tillage by sampler type (P = 0.04), and season by sampler type (P < 0.01) interactions on leachate mineral N concentrations.
Manure application resulted in significantly higher mineral N concentration (17.3 mg L–1) of the leachate compared with the no-manure plots (12.4 mg L–1). The differences in mineral N concentration occurred mostly during NGS2. In other seasons, there was not much difference in mineral N concentration of the leachate between the manure and no-manure treatments. Mineral N concentrations of the leachate were generally higher for the fall-applied manure (19.4 mg L–1) treatment compared with the winter-applied (15.2 mg L–1) or no-manure (12.4 mg L–1) treatments. This effect of the timing of manure application on mineral N concentration in the percolating solution was much more evident during NGS2 (data not shown). Higher mineral N concentration for the fall- compared with winter-applied manure was possibly due to greater mineralization of manure organic N during early fall, especially in the chisel-plow system due to soil mixing.
Leachate mineral N concentration was highest for the chisel-plow treatment with fall manure application (23.3 mg L–1) (Fig. 4). This appears to be due to increased mineralization of manure following incorporation into the soil during fall tillage. Because of the lack of mixing as well as lack of manure addition, mineral N concentration was lowest in the no-till system with no manure application (11.1 mg L–1). Mineral N concentration in the leachate from the chisel-plow, no-manure treatment (13.9 mg L–1) was slightly higher than the no-till, no-manure treatment (11.1 mg L–1). This difference reflects the increased contribution from soil mineralization due to fall chiseling. There was no difference in the leachate mineral N concentration between the chisel-plow winter manure application (15.0 mg L–1) and the chisel-plow, no-manure application (13.9 mg L–1) treatments. This suggests very little contribution from mineralization of surface-applied manure in the winter. There was no difference in mineral N concentration of the leachate between fall (16.1 mg L–1) and winter-applied manure (15.3 mg L–1) in the no-till system. This is expected considering the contribution of manure mineralization from surface application is relatively minimal, and the fall- and winter-applied manure remain at the soil surface and are not mixed with soil in the no-till treatment. Leachate mineral N concentration from the winter-applied manure in both chisel-plow (15.0 mg L–1) and no-till (15.3 mg L–1) were also similar. This is expected considering that there was no mixing of manure with soil in either tillage system before planting.
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Mineral N concentration of the leachate was slightly higher (P = 0.12) in the chisel-plow (16.9 mg L–1) than in the no-till treatment (14.0 mg L–1), possibly due to greater N mineralization under the chisel-plow treatment.
Mineral Nitrogen Losses
Mineral N losses by leaching were significantly different among manure treatments (P < 0.01) and between sampler types (P < 0.01) (Table 2). There were also significant season by sampler type (P = 0.04), manure by sampler type (P = 0.07), and season by manure by sampler type (P = 0.08) interactions on mineral N leaching.
Mineral N losses tended to be higher (P = 0.11) during the nongrowing season (30.6 kg ha–1) than the growing season (8.5 kg ha–1), possibly due to higher percolation losses from late fall to early spring when precipitation exceeded evapotranspiration. Among the growing seasons, there were greater mineral N losses during GS1 than GS2 because of higher precipitation during the growing season of 1994 than 1995. Among the nongrowing seasons, mineral N losses were higher in NGS2 than NGS1, again because of higher precipitation and higher percolation during the 1994–1995 nongrowing season and possibly due to higher N application rate.
Under the northern states climate, some mineral N leaching is inevitable under arable cropping systems during the nongrowing season. This is because the soils in the region have high organic matter content, which in turn means that a large pool of organic N is available if optimal conditions exist for its mineralization. Furthermore, plant N uptake is nearly zero during the nongrowing season and precipitation always exceeds evapotranspiration.
Mineral N loss by leaching was higher from manure application (43.6 kg ha–1) than from the no-manure treatment (28.3 kg ha–1). This higher mineral N leaching is mainly due to higher mineral N concentration of the percolate water from the manure than the no-manure treatment. Mineral N leaching was significantly greater from the fall- (51.5 kg ha–1) than the winter-applied manure treatment (37.7 kg ha–1). This difference in mineral N leaching was due to the difference in mineral N concentration of the percolate solution and not due to the differences in the amount of percolation. The fall-applied manure treatment had significantly higher mineral N concentrations in the leachate (19.4 mg L–1) compared with both the winter-applied manure (15.2 mg L–1) and the no-manure (12.4 mg L–1) treatments.
Generally, there was not much difference in mineral N leaching between the fall- and winter-applied manure treatments during the growing season (Fig. 6). Most of the differences in annual mineral N leaching between the fall- and winter-applied manure treatments were during the nongrowing seasons (Fig. 6). Higher mineral N leaching in fall-applied manure compared with winter-applied manure treatment during the nongrowing season is because of greater organic N mineralization in fall-applied manure treatment mainly in the chisel-plow with fall manure application treatment. Since there was no standing crop in the fall after manure application, the mineralized N accumulated in the soil and then leached with snowmelt or rain water either during the fall or in early spring.
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Mineral N losses by leaching were higher from the wick sampler (24.7 kg ha–1) than the pan sampler (14.4 kg ha–1). This was because percolation (during nongrowing season) and mineral N concentration (during growing season) in the percolate solutions were higher from the wick than the pan sampler due to additional contributions from the soil matrix facilitated by the hanging wick. However, the difference in mineral N losses between the two sampling methods differed with manure treatment and season, as indicated by the significant season by manure by sampler type interaction (P = 0.10). Mineral N leaching was highest from fall application followed by winter application and no manure application (Fig. 6). Most of these N losses occurred during the nongrowing season when the water and mineral N uptake was minimal. Highest mineral N losses (68.9 kg ha–1 yr–1) occurred in wick samplers in the fall-applied manure treatment and 79% of these losses were during the nongrowing season. These higher mineral N losses from wick samplers during nongrowing season are mainly due to increased percolation as a result of 50-cm suction from the hanging wick (Fig. 3).
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A significant (28.3 kg ha–1) amount of mineral N leaching occurs through the karst soils even without the application of fertilizer or manure. This mineral N is mainly from soil organic matter mineralization because there had not been any manure application for at least 10 yr. The field was under alfalfa two years before the start of our experiment. There is some likelihood that some N leaching in no manure may have been from alfalfa-fixed nitrogen. Mineral N losses will increase as one moves from north to south in the U.S. Midwest due to higher temperatures and less time these soils are frozen.
Winter manure application had little effect on mineral N leaching from these soils, but fall manure application resulted in higher mineral N leaching (51.5 kg ha–1 yr–1) than the winter manure application (37.7 kg ha–1 yr–1) or the no manure application. Higher N leaching in the fall-applied manure treatment is mainly due to soil and manure organic N mineralization as a result of fall tillage.
On an annual basis, more than two-thirds of the percolation and N leaching occur during the nongrowing season from these karst soils. Mineral N concentration of the leachate is mostly above the drinking water standard of 10 mg L–1.
Mineral N leaching from chisel-plow and no-till treatments was similar because higher mineral N concentrations in the leachate from chisel-plow plots offset the higher percolation losses from the no-till plots.
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ACKNOWLEDGMENTS |
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NOTES |
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