Biol Fertil Soils (1995) 19:10-14 �9 Springer-Verlag 1995

D.E. Clay �9 S.A. Clay �9 Z. Liu �9 S.S. Harper

Leaching of dissolved organic carbon in soil following anhydrous ammonia application

Received: 10 October 1993

Abstract The transport of anhydrous NH3-solubilized soil organic matter from surface to subsurface soils may affect subsurface microbial activity. In the present study we determined the impact of anhydrous NH3-N fertilizer on organic C solubilization and the propensity of solubilized C to leach with percolating water. In fertilized treatments, anhydrous NH3 was subsurface-banded at 20 g N m-Z in ridge or valley areas of a ridge tillage sys- tem. In contol treatments, 0 g N m -2 was banded into the valley area of a ridge tillage system. Rainfall (17 cm) was applied with a drop-type artificial rainfall simulator 3, 10, and 24 days after the fertilizer application. The treatments were replicated twice. Grid lysimeters (15 by 15 cm) were placed 75 cm below the soil surface of a Brandt silty clay loam (fine-silty over sandy or sandy skel- etal mixed Pachic Udic Haploboroll). Lysimeters were used to collect percolating water temporally and spatially. The application of N fertilizer increased dissolved organic C concentrations in percolating water when rainfall was applied 3 days after the fertilizer application. However, when the rainfall was applied 24 days after the fertilizer application the dissolved organic C concentrations in per- colating water was not influenced by anhydrous NH 3 ap- plication. The smaller dissolved organic C concentrations in percolating water with a longer incubation time were most likely the result of microbial assimilation or respira- tion of solubilized C.

Key words Water quality �9 Water-soluble C �9 N fertilizer Atrazine �9 Anhydrous ammonia �9 Ridge tillage system

D.E. Clay (~) �9 S.A. Clay �9 Z. Liu �9 S.S. Harper Plant Science Department, South Dakota State University, Brookings, SD 57007, USA Tennessee Valley Authority, Muscle Shoals, AL 35660, USA

Introduction

The application of NH3-based fertilizer increases soil pH and dissolved organic C (Nommik and Nelson 1963; Tomasiewiez and Henry 1985; Norman et al. 1987; Myers and Thien 1988; Norman et al, 1988). Substantial amounts of solubilized C can be produced. For example, Myers and Thien (1988) reported that the application of NH4OH-N at rates of 0, 600, 1200, and 2400 mg N kg -1 to soil resulted in dissolved organic matter concentrations of 62, 199, 389, and 564mgkg -1 leachate, respectively, collected 2.5 cm above and below the application zone. Norman et al. (1987) reported that anhydrous NH 3 solubilized up to 4~ of the organic matter in the center of the fertilizer application zone.

Solublized C may be degraded by microbes, leached with percolating water, and may become less soluble as pH values decrease in the fertilizer band. Zsolnay and Steindl (1991) reported that C extracted from soil by 0 .01MCaClz (2:1 solution to soil ratio) and filtered through 1.0 gm polycarbonate filters contained a non- biodegradable fraction with an average concentration of 3.9 gg C g-1 of soil and a fraction that was 85~ biode- gradable after 90 days at 25 ~ C. These results were inde- pendent of soil depth.

If fertilizer-solubilized dissolved organic C leaches with percolating water, then this C source may influence denitrification or herbicide degradation in C-limited sub- surface environments (Parkin and Meisinger 1989; Mc- Carthy and Bremner 1992; Yeomans et al. 1992). The ob- jective of the present study was to determine the impact of subsurface-banded N fertilizer in the form of anhy- drous NH 3 on organic C solubilization and the propensi- ty of the solubilized C to leach with percolating water.

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Materials and methods

Field experiment

A field experiment was conducted within an established ridge tillage system where N fertilizer had not been applied for 2 years previous- ly, at Aurora, South Dakota. The soil was a Brandt silty clay loam (fine-silty over sandy or sandy skeletal, mixed Pachic Udic Haploboroll). Selected physical characteristics and additional re- sults from this experiment on the impact of fertilizer placement on inorganic N and atrazine transport have been presented by Clay et al. (1994a, b).

Anhydrous NH 3 was banded between 5 and 15 cm below the soil surface at the rate of 20 g N m -2 to either ridge or valley areas of a ridge tillage system on 3 June 1991 by a nine-row commercial anhydrous NH 3 applicator. In control treatments, 0 g N m -2 was banded into the valley area of a ridge tillage treatment. Rainfall (17 cm) was applied 3, 10, or 24 days after the fertilizer application at the rate of 4.86 cm h -1 with a drop-type artificial rainfall simu- lator. The simulated rainfall mimicked the effects of single-event rains observed in 1991 (10 cm), 1992 (8 cm), and 1993 (15 cm). Plas- tic sheets were used to prevent water infiltration during natural rain- fall events and metal barriers were used to prevent runoff. Each treatment was replicated twice.

Five sets of three lysimeters were placed next to each other 75 cm below undisturbed fertilizer-treated soil (Fig. 1). This arrangement resulted in each position location being replicated 3 times. The lysimeters were installed from a pit dug beside the plot following the method of Clay et al. (1994a, b). Sample-collection bottles were connected to each of the 15 lysimeters. This arrangement allowed spatial and temporal fractionation of the leachate.

The collection bottles were changed each time 400ml of leachate was collected. The leachate was cooled immediately to 4 ~ C, the volume recorded, and the sample split for dissolved organ- ic C, inorganic N, and pH analysis.

Correlation analyses were conducted by combining research results from all rainfall events.

Laboratory experiment

The effects of the addition of N and the length of incubation on C solubilization were determined under laboratory conditions. Ten grams of surface soil were placed in centrifuge tubes and either NH4OH-0.01 MCaC12 or NaOH-0.01 MCaC12 was used to adjust

Fig. 1 system

Grid lysimeters 3

10 Relative placement of grid lysimeters within ridge tillage 23

the pH values to 5.2 (control), 7.7, and 8.9. The water content of the soil was increased to 50~ (g water g-i dry soil) by adding the appropriate amount of 0.01MCaC12. After the base had been added, centrifuge tubes were shaken for 24 h, and allowed to incu- bate for 0 or 8 days. After incubation the slurry was centrifuged, the supernatant was collected, and solution pH and dissolved or- ganic C concentrations were determined.

Laboratory analysis

Inorganic N (NO~- +NH~--N) was determined on a Wescan ammo- nia analyzer with a Zn column to reduce NO3- to NH 3. The pH of leachate samples was determined directly using a pH electrode. Soil pH was determined in 2:1 0.01 MCaC12 solution:soil ratio. Solu- tions were prepared for dissolved organic C analysis by centrifuging to remove particulates. The dissolved organic C concentration in the supernatant was determined on a Dohrmann Carbon Analyzer (Rosemount Analytical Inc., Santa Clara, Calif., USA).

Results and discussion

Field study

The average dissolved organic C concent ra t ion in leachate collected under ridge, side ridge, and valley areas of 0 N controls was less t han 4.1 m g l - i on each rainfall day (Table 1). W h e n rainfall was applied 3 days after 0 N ap- pl icat ion the dissolved organic C concent ra t ion in leachate collected under the valley area was slightly lower than the average in leachate collected under the ridge or side ridge areas. W h e n rainfall was applied 10 and 24 days after the fertilizer appl ica t ion the dissolved organic C concentra t ions in leachate collected under ridge, side ridge, and valley areas were similar. Clay et al. (1994a, b) have reported that the fertilizer slots in this experiment acted as a macropore and inf luenced both fertilizer and atrazine movement through the soil. Therefore, the fertil- izer slot in the 0 N control may have provided a channel that bypassed the surface soil, leaving less dissolved or- ganic C in the percolating water.

When rainfall was applied 3 days after the fertilizer ap- pl icat ion the dissoved organic C concentra t ions increased with the a m o u n t of water collected in lysimeters located

Table 1 Mean dissolved organic C concentrations (mg 1 1) and confidence intervals (CI) in leachate collected under ridge, side ridge, and valley areas when rainfall was applied 3, 10, and 24 days after 0 g N m -2 was subsurface-banded in valley areas

Rainfall Sample location timing (days) Ridge Side ridge Valley

Mean 95% CI Mean 95% CI Mean 95~ CI

4.01 0.63 4.06 0.66 2.68 0.43 4.08 0.83 4.13 0.54 3.99 0.53 3.05 0.20 3.00 0.23 2.99 0.31

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under the fertilizer bands in ridge and valley areas (Fig. 2, Table 2). In the lysimeters not located under fertilizer bands, but in the fertilized plots, the amount of water col- lected and the dissolved organic C concentrations were not correlated. When rainfall was applied 10 days after the fertilizer application, the dissolved organic C concen- tration and the amount of water collected were correlated when fertilizer was applied on the ridge but not when ap- plied in the valley. When rainfall was applied 24 days after the fertilizer application, the dissolved organic C concen- trations in leachate collected under ridge and valley areas were not correlated with the amount of water collected.

When rainfall was applied 3 and 10 days after the fer- tilizer application, the mean leachate pH values were not affected by the application of N (Table 3). However, when the rainfall was applied 24 days after the fertilizer applica- tion, the leachate pH was higher when collected under fertilizer bands than in lysimeters not located under fertil- izer bands.

The application of fertilizer did not influence the mean leachate inorganic N concentration when rainfall was applied 3 days after the fertilizer application (Clay et al. 1994a). However, the inorganic N concentration in the leachate was increased when rainfall was applied I0 and 24 days after the fertilizer application. These findings correspond to overall decreases measured in the percolate pH with time, and therefore nitrification was most likely responsible for the changes in pH and NO 3 concentra- tions with time.

The pH and dissolved organic C in percolate collected under fertilizer bands and from lysimeters not located un- der fertilizer bands in the fertilized treatments were posi- tively correlated with each other, while inorganic N con- centrations were negatively correlated with pH and dis- solved organic C (Table 4). These results suggest that pH controlled DOC concentrations in percolating water.

20

15

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N r i d g e 20 ~and to No Iband i �9

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0 10 20 30

days (b) I I I

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0 10 20 30 0 10 20

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Fig. 2 Influence of fertilizer treatment on dissolved organic C (DOC) transport with percolating water 3 (a), 10 (b), and 24 (e) days after fertilizer application

Table 2 Regression equations relating water collected (cm) and dissolved organic C (ppm) concentrations in lysimeters located under banded and no-band areas of plots fertilized with 20 g N m -2. The relationship between water and dissolved organic C

was not linear for 3 day ridge band treatment. The regression equa- tion was y = 3.78+ 1.55x-O.155x2+O.OO5x 3. The regression equation reported in Table 2 for this treatment does not include data with greater than 10 cm of water collected

Fertilizer Days prior to rainfall treatment

3 10 24

y Slope r y Slope r y Slope r

Ridge No band 3.09 0.05 0.20 3.41 0.004 0.01 3.01 0.025 0.09 Band 5.67 0.41 0.39" 4.08 0.081 0.47"* 3.59 0.002 0.01

Valley No band 2.52 0.14 0.68 ** 2.96 0.101 0.35 * 3.23 0.010 0.04 Band 1.71 1.29 0.68 ** 4.10 - 0.040 - 0.19 2.42 0.004 0.09

* P<0 .05 , ** P<0 .01

Table 3 Mean pH, inorganic N (NH~-N + NO~--N), and Rainfall dissolved organic C (DOC) in timing water samples collected in (days) lysimeters located under band- ed and no-bands areas of fer- tilized plots (LSD least signifl- 3 cant difference) I0

24

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pH Sample location

NH~-N + NO 3-N (mg 1-1) Sample location

DOC (mg 1-1) Sample location

Band No band LSD Band No band LSD Band No band LSD

7.92 7.90 7.86 7.92 7.73 7.29

NS 32.6 30.8 NS 6.96 3.07 1.11 NS 36.3 27.6 4.43 4.47 3.70 0.75 0.15 83.6 27.6 6.84 3.29 3.06 NS

Laboratory study

A laboratory study was conducted to determine the effect of pH on dissolved organic C concentrations in the soil solution. The pH of the control treatment remained con- stant during the incubation. Treatment of the soil with NH4OH and NaOH increased the solution pH (Table 5). Maximum pH values were observed on day 0. Increasing the incubation length from 0 to 8 days resulted in similar pH decreases with each base. The pH decrease when NaOH was applied most likely resulted from the soil buf- fering capacity slowly reducing pH. Because both bases produced similar pH decreases, the soil buffering capaci- ty most likely controlled the soil pH decrease during the NH4OH treatment from 0 to 8 days.

The application of NH4OH and NaOH increased the solution dissolved organic C concentrations (Table 5). With similar pH values, NaOH was more efficient at dis- solving organic C than NH4OH. As the incubation tength increased from 0 (maximum pH values) to 8 days, the dissolved organic C values decreased with the solution pH when NaOH was applied. However, when NH4OH was applied, dissolved organic C increased slightly with the length of incubation. These results show that pH in- fluenced dissolved organic C in different ways when NaOH and NH4OH were added and that pH by itself most likely did not control these organic C concentra- tions. Norman et al. (1988) reported similar results and suggested that the fertilizer-solubilized C in the period immediately following fertilizer application was not con- trolled by pH readjustment, and that solubilized C was readily mineralized by microbial populations. The present study expands these findings and shows that solubilized C readily leached with percolating water.

Fertilizer-solubilized C may have a significant impact on the amount of dissolved organic C in the aquifer. Wa- ter samples collected from monitoring wells located in the aquifer below the study site generally contain less than 1 ppm dissolved organic C. However, higher concentra- tions have been observed following groundwater recharge events. If leached organic C provides an energy source for subsurface microbial populations, then subsurface agrichemicaI detoxification may be increased by the co- leaching of dissolved organic C with agrichemicals. Cor- relations between C and inorganic N leaching suggest that fertilizer-derived dissolved organic C and inorganic N were not cotransported. However, if fertilizer slots act as macropores, channelling water flow through them (Clay

Table 4 Correlation matrix between pH, dissolved organic C (DOC), and inorganic N (NH~-N+NO--N) in combined leachates collected 3, 10, and 24 days after fertilizer application and artificial rainfall, under banded and no-band areas

Band No band

pH DOC pH DOC

pH DOC 0.56'* 0.33"* Inorganic N -0.77** -0.57** -0.40** -0.22*

* Significant at 0.05 ** The number of comparisons for the N-applied treatment was 250. The number of comparisons for no band was 144. Therefore, for the fertilized treatment, values greater than 0.165 or less than -0.165 were significant at 0.01. For the no band treatment values greater than 0.228 or less than -0.228 were significant at 0.01

Table 5 Influence of base type and incubation (inc.) length on soil solution pH and dissolved organic C (DOC) concentrations (SEM in parentheses)

pH treatment pH DOC (gg ml)

0 days inc. 8 days inc. 0 days inc. 8 days inc.

Control 0 5.2 (0.07) 5.2 (0.05) 23 (9) 19 (5) NH4OH Low 7.6 (0.02) 7.1 (0.06) 110 (46) 180 (45)

High 8.8 (0.01) 8.0 (0.03) 280 (42) 400 (59) NaOH Low 7.7 (0.08) 7.1 (0.06) 400 (180) 210 (10)

High 8.9 (0.06) 8.0 (0.08) 3200 (1360) t100 (190)

et al. 1994a, b) then the potential for co-leaching of her- bicides and dissolved organic C or herbicides and inor- ganic N may be high.

In summary, fertilizer additions initially increased dis- solved organic C concentrations in percolating water. As the incubation length increased, these organic C concen- trations in the percolating water decreased. Lower dis- solved organic C concentrations in the percolating water with increasing incubation length were most likely caused by microbial assimilation or respiration of solubilized C.

Acknowledgements Partial support was provided by USGS-104, USGS-105 Grant Program award No 14-08-0001-G1908, South Dakota Ground Water Research and Education Program, and TVA. The views and conclusions are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied of the United States Government. South Dako- ta State University Experiment Station No. 2743.

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References

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