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Published online 7 May 2007
Published in J Environ Qual 36:855-863 (2007)
DOI: 10.2134/jeq2006.0355
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Surface Water Quality

Dissolved Organic Carbon in Runoff and Tile-Drain Water under Corn and Forage Fertilized with Hog Manure

Isabelle Royera,*, Denis A. Angersa, Martin H. Chantignya, Régis R. Simard, deceaseda and Daniel Cluisb

a Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd., Québec, Québec, G1V 2J3 Canada
b INRS-ETE, Université du Québec, 490 de la Couronne, Québec, QC, G1K 9A9 Canada

* Corresponding author (royeri{at}agr.gc.ca)

Received for publication September 6, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Dissolved organic carbon (DOC) export from soils can play a significant role in soil C cycling and in nutrient and pollutant transport. However, information about DOC losses from agricultural soils as influenced by management practices is scarce. We compared the effects of mineral fertilizer (MF) and liquid hog manure (LHM) applications on the concentration and molecular size of DOC released in runoff and tile-drain water under corn (Zea mays L.) and forage cropping systems. Runoff and tile-drain water samples were collected during a 2-mo period (October to December 1998) and DOC concentration was measured. Characterization of DOC was performed by tangential ultrafiltration with nominal cut-offs at 3 and 100 kDa. Mean concentration of DOC in runoff water (12.7 mg DOC L–1) was higher than in tile-drain water (6.5 mg DOC L–1). Incorporation of corn residues increased the DOC concentration by 6- to 17-fold in surface runoff, but this effect was short-lived. In runoff water, the relative size of the DOC molecules increased when corn residues and LHM were applied probably due to partial microbial breakdown of these organic materials and to a faster decomposition or preferential adsorption of the small molecules. The DOC concentration in tile-drain water was slightly higher under forage (7.5 mg DOC L–1) than under corn (5.4 mg DOC L–1) even though the application rates of LHM were higher in corn plots. We suggest that preferential flow facilitated the migration of DOC to tile drains in forage plots. In conclusion, incorporation of corn residues and LHM increased the concentration of DOC and the relative size of the molecules in surface runoff water, whereas DOC in tile-drain water was mostly influenced by the cropping system with relatively more DOC and larger molecules under forage than corn.

Abbreviations: DOC, dissolved organic carbon • MF, mineral fertilizer • LHM-S, LHM-F, liquid hog manure applied in the spring and in the fall, respectively • SM, small-sized molecules • MM, medium-sized molecules • LM, large-sized molecules


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
DISSOLVED organic carbon (DOC) is a common constituent of both soil and surface water consisting primarily of organic substances derived from various biological processes. The dissolved fraction represents only a small proportion of total soil organic C (usually <1%) but is the most mobile and therefore plays an important role in many soil processes (Qualls and Haines, 1991; Zsolnay, 2003). Dissolved organic C is defined operationally as a continuum of organic molecules of different size and structure that pass through a filter pore size of 0.45 µm (Kalbitz et al., 2000). The concentrations of DOC in groundwater and in streams usually range from 2 to 50 mg L–1 (Spiteller, 1987; Moore and Dalva, 2001).

The dynamics of DOC has been extensively studied in forest soils (Kalbitz et al., 2000), where it is usually collected with lysimeters (McDowell et al., 1998; Qualls et al., 2000) or piezometers (Glatzel et al., 2003). Although DOC concentration can also be measured directly in the solution of agricultural soils (Simard, 1987), it is most often measured as water-extractable organic C (Chantigny, 2003; Zsolnay, 2003). Many studies have shown that the application of crop residues (McCarty and Bremner, 1992; Campbell et al., 1999a, b; Franchini et al., 2001) and animal manure (Zsolnay and Görlitz, 1994; Rochette and Gregorich, 1998; Chantigny et al., 2002) to agricultural soils can cause an increase in water-extractable organic C.

The study of the molecular size distribution of soluble organic matter can be useful to better understand its physicochemical behavior in soils (Kuiters and Mulder, 1992). Buffle et al. (1978) and many others have used tangential ultrafiltration for the separation and fractionation of organic molecules in fresh and natural waters. Fractionation of DOC by molecular weight is performed using membranes of different pore sizes. It was also reported that there is a link between the molecular weight and the relative biodegradability of DOC, with smaller compounds being more easily degradable than larger ones (Qualls and Haines, 1991). Meyer et al. (1987) reported that DOC < 10 kDa supported a greater amount of bacterial growth than larger molecules (>10 kDa). Also, organic compounds of distinct molecular weights may have different binding capacities to metal ions, which may have an important influence on their mobility and transport (Kuiters and Mulder, 1992; Eyrolle and Benaim, 1999).

Many studies have investigated the role of manure and/or organic residues on the chemical, biological, and physical properties of soils including the dynamics of DOC or water-extractable organic C, but little information is available on the concentrations of DOC in surface runoff and in tile-drain water under various cropping systems. Also, even less is known about the characterization of DOC in waters following corn residues and liquid hog manure (LHM) application. The objective of this study was to evaluate the effects of LHM applications on the concentration and molecular size of DOC released in runoff and tile-drain water under corn and forage cropping systems.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Characteristics
This study was performed during the fall of 1998 and is part of a larger project that was initiated in 1989 at the Agriculture and Agri-Food Canada Research Centre in Lennoxville, Québec, Canada (45°21' N, 71°51' W), on a Coaticook silt loam (loamy, mixed, frigid, Aquic Haplorthod) with a 6% slope. This soil was developed on lacustrine materials and has high crop productivity potential if drainage is improved (Cann and Lajoie, 1943). Mean annual air temperature in the area is 6.2°C, with a mean annual precipitation of 1017 mm. Before the experiment, the site was a meadow composed of timothy (Phleum pratense L.) and red clover (Trifolium pratense L.).

Selected chemical and physical properties of the soil profile at the beginning of the experiment are given in Table 1. In this soil, the silt fraction (50–2 µm) is mainly composed of quartz, illite, chlorite, and feldspath; whereas for clay (<2 µm), illite, chlorite, vermiculite, and interstratified minerals are the main components (Simard et al., 1990). Particle-size analysis was performed using the hydrometer method (Sheldrick and Wang, 1993). Soil pH was measured in 0.01 M CaCl2 with a soil/solution ratio of 1:2. Organic C content was determined by wet oxidation (Tiessen and Moir, 1993).


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  		Table 1. Chemical and physical characteristics of the Coaticook silt loam before the beginning of the experiment (1989).{dagger}

 
Field Experiment
The completely randomized experiment design included six treatments and two replicates for a total of 12 plots. The six treatments were combinations of crops and fertilizer types. The crops were either continuous corn or a mixture of timothy, red clover, and white clover (Trifolium repens L.). The fertilization treatments were: (i) mineral fertilizer (MF) applied in the spring, (ii) MF+LHM applied in the spring (LHM-S), and (iii) MF (spring)+LHM applied in the fall (LHM-F). Each spring, MF was applied to all plots at the recommended rates of 180 kg N ha–1 for corn and 55 kg N ha–1 for forage (AFEQ, 1987). The LHM application rate was based on the manure N content and calculated to provide twice the recommended N rate for the particular crop. This resulted in LHM application rates that were on average (since 1989) 98 m3 ha–1 yr–1 for corn and 30 m3 ha–1 yr–1 for forage. The LHM was obtained from a commercial farm, and the chemical and physical characteristics of the manure varied among years. The composition of the manure used in our experiment (1998) is presented in Table 2. The LHM was surface-applied according to two different schedules: all in the spring (LHM-S) (20 May) or all in the fall (LHM-F) (26 October). The MF and LHM applied in the spring were broadcast 12 to 48 h before corn seeding. In corn plots, LHM was incorporated in the top 10 cm of soil by rototilling within 1 h in the spring and within a day for fall application. The LHM was left on the surface in forage plots. Grain corn was harvested on 17 October. Corn residues were chopped, returned to the soil, and incorporated at 10 cm with a rototiller.


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  		Table 2. Selected characteristics of the liquid hog manure (LHM) used in the present study

 
Each experimental plot (3 m wide by 15 m long) was equipped to collect runoff and tile-drain water separately. At the soil surface, plots were separated from each other by a sodded dike (50 cm wide by 25 cm high) along both sides and at the top, and a trough was installed at the bottom of each plot to collect runoff water. Each plot was also isolated by a 1.2 m high black polyethylene plastic sheet installed on both sides and top. A perforated plastic drain was installed at 0.90-m depth (normal drain depth for >6% slope) at the center of each plot to capture tile-drain water. This system isolates plots from each other and prevents runoff and leaching contamination from other sources. Both the runoff and drainage systems were connected with pipes to individual collection barrels permitting separate water volume measurement and sampling.

Water Sampling
Water sampling was performed from 15 Oct. to 12 Dec. 1998. The precipitation pattern during this period is illustrated in Fig. 1. The samples were collected as grab samples during or immediately after every rainfall to minimize DOC degradation. Water samples were collected using a 250-mL low density polyethylene bottles and no preservative was added to the samples. The frequency of water sampling was a function of the precipitation pattern. Overall, surface runoff water was collected on 12 occasions, whereas tile-drain water was collected on 32 and 33 occasions under forage and corn, respectively. Because of the large quantity of rainfall, more than one sampling was performed on 15 October and 2, 11, 27, and 30 November. Some drains in the plots under corn with LHM and those under forage with MF were clogged at the beginning of experiment, and water samples were not collected from those drains during the first few days of the study.


Figure 1
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  		Fig. 1. Precipitation pattern during the fall of 1998 (from 15 October to 12 December).

 
Analysis and Characterization of Dissolved Organic Carbon
Samples of surface water were first filtered through a 3.0-µm pore size filter (Filter MF-Millipore, mixed cellulose esters) before being filtered at 0.45 µm (Standard MF-Millipore membranes, mixed cellulose esters). All tile-drain water samples were filtered at 0.45 µm. Blank samples were included to correct for possible background release of DOC from cellulose filters. All water samples were filtered on the day of sampling and the samples were refrigerated at 4°C and analyzed for DOC content within 2 d. Dissolved organic C was quantified by UV-persulfate oxidation using an automated C analyzer (Model DC-180, Dohrmann Co., Santa Clara, CA).

The characterization of DOC was performed by molecular size fractionation using tangential ultrafiltration (Amicon TCF10) with nomical cut-offs at 3 kDa and 100 kDa using Ultracel Amicon YM3 and YM100 regenerated cellulose ultrafiltration membranes. This fractionation was performed only at sampling dates for which enough water was available. The hydrophilic tight microstructure of the membranes ensures the highest possible retention with the lowest possible adsorption of macromolecules. They also exhibit sharp cut-off characteristics and are resistant to most common solvents. For each ultrafiltration step, approximately 90% of the samples were allowed to pass the membrane; the remaining retentate was discarded to prevent clogging of the membrane pores (Hartlieb et al., 2001). The ultrafiltration technique of tangential flow fractionation using spiral wound filters limits the problem of concentration polarization and results in faster filtration times (Buffle et al., 1978). The three molecular weight fractions obtained were defined as: small-sized < 3 kDa (SM), medium-sized 3–100 kDa (MM), and large-sized > 100 kDa (LM) molecules. Blank samples were included to correct for possible background release of DOC from cellulose membranes.

Statistical Analysis
The normality of data distribution was first checked for each variable. An analysis of variance with repeated measures was performed to test differences between treatments over sampling dates. The MIXED procedure with the repeated statement on dates was used (SAS Institute, Inc., 1999). The treatments were tested for significant differences using least squares means (LSM). A probability level of ≤ 0.10 was considered significant because of limited degrees of freedom (only two replications per treatment). The data were analyzed separately for each cropping system since the application rates of LHM and fertilizer, as well as agronomic practices, were different. The data for surface and tile-drain waters were also analyzed separately.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Runoff Water
The concentration of DOC in runoff water under corn varied between 2.4 and 205.4 mg L–1 (Fig. 2a). This is in the range of values reported by McDowell et al. (1998) and Raastad and Mulder, (1999) for DOC concentrations in surface soil solution. Cronan et al. (1999) reported lower DOC concentrations for surface water in agricultural watersheds (3.6–7.9 mg L–1). However, their samples were taken in stream water, and therefore were likely diluted as compared with runoff waters.


Figure 2
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  		Fig. 2. Concentrations of dissolved organic carbon (DOC) in runoff water (a) under corn and (b) under forage. Vertical bars represent standard errors (MF, mineral fertilizer; LHM-S, liquid hog manure applied in the spring; LHM-F, liquid hog manure applied in the fall).

 
On average, the DOC concentration of runoff water under corn was similar (P = 0.96) among the MF, LHM-S, and LHM-F treatments. There were no interactions (P = 0.99) between sampling dates and treatments. However, runoff DOC concentrations varied significantly (P < 0.001) among sampling dates (Fig. 2a). Runoff DOC concentrations increased by 6- to 17-fold (average of 176 ± 64 mg L–1) 5 d after the corn residues were chopped and returned to the soil. As observed by Moore and Dalva (2001), DOC can be released through leaching of soluble organic matter from crop residues decomposing in the soil. McCarty and Bremner (1992) also showed that the application of corn residues to the soil surface released water-extractable organic C very rapidly, and noted that this effect was short-lived and could not be detected after a few days. The same phenomenon was observed in our case as DOC concentrations came back to background levels just a few days after residue incorporation (Fig. 2a). Temporary flushing of DOC in runoff water could be also attributed to soil rototilling to incorporate corn residues. Temporary increases in soil dissolved organic matter concentration following soil tillage has been reported by others (Bhogal et al., 2000; Gregorich et al., 2000) and could be attributed to the breakdown of soil aggregates with either the direct release of soluble organic matter or increased microbial activity and production rate of DOC.

The concentration of DOC in runoff water under forage varied with time (P < 0.001) (Fig. 2b). The treatment effects and the interaction between sampling dates and treatments were not significant (P = 0.95 and P = 0.25, respectively), even though DOC concentration tended to be higher for LHM-F than LHM-S and MF on 2 November (7 d after LHM application). A separate ANOVA on data measured on 2 November showed that this difference was significant at P = 0.059. The DOC concentrations in runoff water under forage varied from 2 to 31.7 mg L–1.

The DOC concentration increased under forage on 22 October as seen also under corn, even though no residues were returned to the soil (Fig. 2b). However, this increase was about an order of magnitude lower than under corn. The other significant peaks measured in DOC concentration on 2 and 15 November were also found under both corn and forage, and were of the same magnitude. Those peaks in DOC concentration were all noticed following a rainfall close or above 10 mm (Fig.1), which occurred after 1 or 2 wk without precipitation. We thus suggest that, in addition to residue incorporation in the corn plots, rainfall intensity and wet–dry cycles had a detectable effect on DOC concentration in runoff water from the corn and forage systems. Zsolnay and Görlitz (1994) also noticed that wet–dry cycles had a significant effect on DOC concentration in agricultural soils. After 2 November, DOC concentrations decreased in runoff waters and remained relatively low even after significant rainfalls (Fig. 2). This is possibly because at that time of year (i) colder soil temperatures reduced DOC production rate, (ii) the soil remained relatively wet (data not shown), (iii) soluble materials coming from organic amendments (crop residues or manure) were mostly decomposed, and soil DOC concentrations returned to background levels (McCarty and Bremner, 1992; Jensen et al., 1997).

Application of LHM on 26 October had no significant effect on DOC concentrations in runoff water under corn (Fig. 2a). The peak observed on 2 November was probably not related to LHM application since it was observed in all treatments. Those results are in disagreement with many studies where concentrations of water-extractable organic C were found to be greater when animal manure was applied to the soil (Gregorich et al., 1998; Chantigny et al., 2002). By contrast, Angers et al. (2006) reported a low increase in soil DOC concentration after liquid dairy cattle manure was applied. They argued that rototilling for incorporation favored the adsorption of manure-soluble C to the mineral particles. In addition, Chantigny et al. (2004) observed that volatile fatty acids coming from LHM contributed to soil DOC, but were metabolized within 5 d and did not move below 2-cm depth. Consequently, we assume that the peak in DOC concentration measured on 2 November for all plots were not related to the LHM application, but mainly caused by wet–dry cycles and rainfall before sampling as discussed above.

It is noteworthy that the average DOC concentration in runoff water under forage for all sampling dates (12.8 mg DOC L–1) was similar to the value under corn when disregarding the high value on 22 October (12.5 mg DOC L–1). Thus, even if LHM was applied at lower rates under forage than under corn, DOC concentrations in surface water were overall similar under the two cropping systems. We hypothesize that this is due to the fact that mechanical incorporation of LHM in corn plots is likely to decrease the proportion of manure-soluble C (Angers et al., 2006) susceptible to runoff, whereas the LHM left at the surface of the soil under forage could be more prone to runoff.

Tile-Drain Water
Concentrations of DOC in tile-drain water under corn varied from 3.1 to 10.0 mg L–1 for MF, from 2.2 to 7.4 mg L–1 for LHM-S, and from 3.0 to 20.7 mg L–1 for LHM-F (Fig. 3a). These results are in the same range than values reported by Beauchemin et al. (2003) who found that concentrations of DOC from tile-drain water varied from 1.6 to 6.0 mg L–1 in 43 soils located in the province of Québec, Canada, under a corn–soybean rotation. In Germany, Siemens et al. (2003) found average concentrations of 9 mg DOC L–1 at 90 cm below the soil surface from fine sandy to loamy sand soils cropped to a rotation of maize and cereals, and fertilized with manure. On the contrary, McCarty and Bremner (1992) reported that in Iowa, samples of water from tile drains installed at a depth of 120 cm under continuous corn contained only trace amounts of DOC (<0.3 to 2.9 mg L–1).


Figure 3
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  		Fig. 3. Concentrations of dissolved organic carbon (DOC) in tile-drain water (a) under corn and (b) under forage. Vertical bars represent standard errors (MF, mineral fertilizer; LHM-S, liquid hog manure applied in the spring; LHM-F, liquid hog manure applied in the fall).

 
There was a significant (P < 0.001) treatment x date interaction for DOC concentrations in tile-drain water under corn. Significantly (P ≤ 0.10) higher DOC values were found for LHM-F than LHM-S on 10 out of 19 sampling dates (Fig. 3a). Values of DOC were also higher for LHM-F than MF on six sampling dates after 9 November. A high DOC concentration was measured on 11 November for LHM-F. We suggest that this additional DOC was mainly derived from the recently applied LHM, and reached the tile drains (90-cm depth) 15 d after LHM application. Part of the increase in DOC could also be due to the incorporation of corn residues 3 wk earlier.

The DOC concentrations in tile-drain water under forage varied from 2.6 to 12.5 mg L–1 (Fig. 3b). Our values are in the lower range of those presented by Brye et al. (2001) who reported DOC concentrations from 5 to 20 mg L–1 in lysimeters (1.4-m depth) under prairie cropping systems. There was a significant treatment x date interaction (P < 0.001) for DOC concentrations in tile-drain water under forage. Significantly (P ≤ 0.10) higher DOC concentrations were found for LHM-F than LHM-S and MF on 17 out of 19 sampling dates after 9 November (Fig. 3b), likely reflecting the influence of LHM application on 26 October. The increase in DOC concentration was smaller on 11 November, but lasted over a longer period under forage than corn (Fig. 3). There is no simple explanation for this difference, but it is suggested that the biological, chemical, and physical characteristics of the soil matrix were different between the two cropping systems, which influenced the temporal pattern of DOC release to tile-drain water.

The average DOC concentration in tile-drain water under forage (7.5 mg DOC L–1) was similar to that under corn (5.4 mg DOC L–1). In our study, the forage system received about one-third of the LHM applied to the corn system. Moreover LHM was left on the surface of forage plots, whereas it was incorporated in corn plots. Soils under perennial forage production usually present a network of vertical macropores, originating from soil cracks, fissures, root channels, and earthworms burrows (Ehlers, 1975; Simard et al., 2000), which may enhance preferential water flow to drains and, thereby, could increase the proportion of DOC reaching the tile drains.

For both crops, the concentration in DOC observed in tile-drain water was lower (average of 6.5 mg DOC L–1) than in runoff water (average of 12.7 mg DOC L–1). Several studies have shown the DOC concentration to decrease with depth in mineral soils (Meyer and Tate, 1983; Jardine et al., 1989; Kalbitz et al., 2000). Microbial activity may have promoted the decomposition of corn- and LHM-derived DOC in the soil solution thereby reducing its concentration in tile-drain water (McCarty and Bremner, 1992; Siemens et al., 2003). Alternatively, DOC in surface runoff was in contact only with the A horizon which has little affinity for DOC retention, whereas drainage water has to pass through the B and part of the C horizons which have a greater affinity for DOC adsorption (Kalbitz et al., 2000) thereby reducing the amounts of DOC susceptible to reach tile drains.

Dissolved Organic Carbon Characterization in Runoff Water
The molecular size distribution of DOC in runoff water at the first sampling date (15 October) showed a dominance of SM (<3 kDa) for both cropping systems and all treatments (Fig. 4). The proportions of SM varied from 55 to 63% of total DOC under corn and from 44 to 54% under forage, whereas the proportions of MM (3–100 kDa) and LM (>100 kDa) varied between 6 and 36% of total DOC under corn and between 11 and 35% under forage. Other studies have found that small molecules (approximately 1 kDa) were dominant in DOC from surface runoff (Haygarth et al., 1997) and stream waters (Cronan et al., 1999) in agricultural areas. The SM fraction is comprised of fulvic acids and small molecules such as fatty acids, amino acids, carbohydrates, and hydrophilic acids (Thurman, 1985). These small molecules are believed to be more biodegradable than larger ones (Qualls and Haines, 1991).


Figure 4
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  		Fig. 4. Size fractions of dissolved organic carbon (DOC), obtained by tangential ultrafiltration, in runoff water (a) under corn and (b) under forage at the first sampling date (15 October). Vertical bars represent standard errors (SM, small molecules; MM, medium molecules; LM, large molecules; MF, mineral fertilizer; LHM-S, liquid hog manure applied in the spring; LHM-F, liquid hog manure applied in the fall).

 
The incorporation of corn residues strongly increased the concentrations of SM and MM in surface runoff water (Fig. 5a). In addition, the size distribution of DOC was modified and MM became dominant representing about 60% of total DOC for all treatments. Delprat et al. (1997) reported that maize cultivation promoted the decomposition of native soil organic matter leading to an increase in DOC mostly in the MM fraction. In the present study, DOC was characterized on water samples collected only 5 d after corn residues incorporation. Therefore, the increase in medium- and small-sized DOC may be attributed to (i) the direct release of soluble materials from the freshly incorporated residues, (ii) partial microbial decomposition of corn residue with the release of soluble by-products, (iii) the release of soluble organic matter caused by rototilling.


Figure 5
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  		Fig. 5. Size fractions of dissolved organic carbon (DOC), obtained by tangential ultrafiltration, in runoff water (a) under corn and (b) under forage 5 d after crop residue incorporation (22 October). Vertical bars represent standard errors (SM, small molecules; MM, medium molecules; LM, large molecules; MF, mineral fertilizer; LHM-S, liquid hog manure applied in the spring; LHM-F, liquid hog manure applied in the fall).

 
The MM fraction of DOC in runoff was increased in both cropping systems 3 d after LHM application (Fig. 6). The LHM-derived DOC was mainly composed of SM (Table 2). The very low DOC concentrations in SM in runoff water a few days after LHM addition could be due to the preferential adsorption of LHM-derived SM compared with MM, but also to the more rapid decomposition of SM. Chantigny et al. (2004) reported that small molecules such as volatile fatty acids were completely degraded only 5 d after LHM application.


Figure 6
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  		Fig. 6. Size fractions of dissolved organic carbon (DOC), obtained by tangential ultrafiltration, in runoff water (a) under corn and (b) under forage 3 d after liquid hog manure (LHM) application (26 October). Vertical bars represent standard errors (SM, small molecules; MM, medium molecules; LM, large molecules; MF, mineral fertilizer; LHM-S, liquid hog manure applied in the spring; LHM-F, liquid hog manure applied in the fall).

 
The increase in SM and MM concentration after LHM application was very small (Fig. 6) as compared with the increase measured following the incorporation of corn residues (Fig. 5a). Angers et al. (2006) reported a larger increase in soil DOC following corn harvest than after LHM application. This difference could be explained by either the large amount of soluble C present in fresh corn residues (Recous et al., 1995), which was rapidly released in soil, or a faster decomposition or greater adsorption of LHM- than corn-derived DOC. Indeed, a large proportion of soluble C in LHM is present as volatile fatty acids which are rapidly degraded in soil (Chantigny et al., 2004). At this stage, however, we cannot rule out the possibility that LHM-derived DOC has a greater affinity to adsorb onto soil mineral particles than corn-derived DOC.

Dissolved Organic Carbon Characterization in Tile-Drain Water
In tile-drain water, DOC molecular size fractions were measured on samples collected 16 and 30 d after LHM application (11 and 26 November). On these occasions, water samples were obtained at the beginning of the rainfall and 24 h later. For all treatments under corn, SM was the dominant size fraction with an average of 53% of total DOC, followed by MM at 36% (Fig. 7). By contrast, up to 73% of the total DOC was present in the MM fraction under forage (Fig. 8). The proportion of total DOC accounted for by LM was small in both cases. This difference in the molecular size of DOC reflected the influence of cropping systems on the nature of DOC released to tile drains with relatively smaller molecules under corn than under forage. In general for both cropping systems, the concentration in SM and MM decreased from the beginning to 24 h after the onset of rainfall on 11 November (Fig. 7a and 8a), whereas the contrary was observed on 26 November (Fig. 7b and 8b). Those findings indicate that there was an interaction between the influence of cropping system on the nature of DOC and the effect of rainfall distribution on the temporal pattern of release in DOC in tile-drain water. The results obtained for both rainfalls also showed that despite higher application rates under corn, the concentration of SM and MM in tile-drain water was similar or higher under forage than under corn. It is hypothesized that the presence of macropores in the grassland soil (Ehlers, 1975) facilitated the transport of SM and MM to tile drains. Clearly, more studies are required to elucidate the origin, the nature, and transport mechanisms of DOC to drainage water.


Figure 7
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  		Fig. 7. Size fractions of dissolved organic carbon (DOC), obtained by tangential ultrafiltration, in tile-drain water under corn at the (a) 11 November event and at the (b) 26 November event. Vertical bars represent standard errors (SM, small molecules; MM, medium molecules; LM, large molecules; MF, mineral fertilizer; LHM-S, liquid hog manure applied in the spring; LHM-F, liquid hog manure applied in the fall; 0, beginning of rainfall; 24, 24 h after beginning of rainfall).

 

Figure 8
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  		Fig. 8. Size fractions of dissolved organic carbon (DOC), obtained by tangential ultrafiltration, in tile-drain water under forage at the (a) 11 November event and at the (b) 26 November event. Vertical bars represent standard errors (SM, small molecules; MM, medium molecules; LM, large molecules; MF, mineral fertilizer; LHM-S, liquid hog manure applied in the spring; LHM-F, liquid hog manure applied in the fall; 0, beginning of rainfall; 24, 24 h after beginning of rainfall).

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The incorporation of corn residues greatly increased DOC concentrations in surface runoff water, but this effect was short-lived. In addition, the application of LHM had little detectable effects on DOC concentration in surface water. A possible explanation for these findings is that the DOC derived from corn residues and LHM was adsorbed or rapidly biodegraded following application to the soil.

In tile-drain water under forage, the DOC concentrations increased after LHM application in the fall, but little change was observed under corn. Also, the average DOC concentrations in tile-drain water under forage were equal or slightly higher than those measured under corn despite smaller manure application rate. These findings were attributed to preferential flow under forage.

This study showed that both the DOC concentration and the molecular-size fractions vary in runoff and tile-drain water according to management practices and cropping systems, and to the temporal pattern of precipitation. Overall, the incorporation of corn residues and LHM increased the concentration and molecular size of DOC present in surface runoff water, whereas the cropping system (corn vs. forage) had a determinant influence on the amounts and nature of DOC in tile-drain water. More studies are required to understand the impact of agricultural management practices on the amounts and nature of soil DOC exported to water bodies and their consequences on environmental quality.


   ACKNOWLEDGMENTS
 
The authors thank Dr. Gordon M. Barnett of the Dairy and Swine Research and Development Centre of Lennoxville, QC for providing access to the field plots. The authors are also grateful to Laurence Delprat for her assistance in the field and technical support. We deeply regret that Dr. Régis Simard will never see the final result of this work.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
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