~ Soil & ~' ~ T H l a g e .

K e s e ~ n E LS EV 1ER Soil & Tillage Research 32 (1994) 223-236

Impacts of antecedent moisture and soil surface mulch coverage on water and chemical transport

through a no-till soil

A.V. Granovsky a, E.L. McCoy a'*, W.A. Dick a, M.J. Shipitalo b, W.M. E d w a r d s b

aDepartment of Agronomy, The Ohio State University, OARDC, 1680 Madison Avenue, Wooster, OH 44691, USA

b USDA-ARS North Appalachian Experiment Watershed, Coshocton, OH 43812, USA

Accepted 26 April 1994

Abstract

Flow in macropores of no-tillage soils is often implicated as a principal mechanism re- sponsible for accelerated movement of agrochemicals into groundwater. The objective of this study was to assess the impact of a surface mulch coverage and antecedent water con- tent on water and chemical transport characteristics in a Typic Hapludult soil. SrBrz-6H20 and atrazine were surface-applied to four undisturbed 0.3 m X 0.3 m X 0.3 m surface soil blocks. Three simulated 30 mm rains were applied to the block surfaces, and leachate was collected from 64 cells at the bottom of each block. Leachate volume, chemical amounts, and conducting macropore areas were determined for each cell and block. A parameter, m, found by fitting sorted cumulative outflow curves to an exponential function, was used to describe the degree of flow preference in a block. The dominant factor producing trans- port differences between the four blocks was pre-rain moisture content, which correlated negatively with degree of flow preference and positively with total leachate volume in each block. In a drier soil only the more rapid flow pathways, marked by high cell leachate volumes, contributed to the flow, while the slower pathways having greater interaction with the bulk soil were mostly truncated. This resulted in a higher degree of flow prefer- ence, smaller total leachate volumes and smaller block-averaged concentrations of Br, Sr and atrazine in soil with lower pre-rain moisture content. The peak of chemical transport was observed aCter the first simulated rain regardless of pre-rain moisture and surface mulch coverage. Following the second and third rains the chemical transport was reduced two- tbld for the less reactive Br, three-fold for the more reactive atrazine and ten-fold for St, apparently due to the by-pass of chemicals by subsequent leaching events. Mulch had little effect on water movement, but slightly enhanced the Sr and atrazine transport through the

* Corresponding author.

(J167° 1987/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0167-1987(94)00411-7

224 A. K Granovsky et al. / Soil & Tillage Research 32 (1994) 223-236

blocks, most likely by prolonging the chemical contact with infiltrating water at the soil surface.

Keywords: Maeropore; Preferential flow; Br transport; Sr transport; Atrazine transport

1. Introduction

Intense research has been conducted on soil macropores during the past 15 years because of their presumed role in the rapid transport of agrochemicals to the groundwater (Kanwar et al., 1985; White et al., 1986; Zachmann et al., 1987; Shipitalo et al., 1990; Edwards et al., 1992). This research has been motivated partly by the increased use of no-tillage practices in many climatic regions of the USA (Conservation Tillage Information Center (CTIC), 1986) because such practices result in the formation of macropore networks resulting from uninter- rupted earthworm activity (Edwards, 1982; Zachmann et al., 1987; Shipitalo et al., 1990). Although the mechanisms of preferential chemical transport in macro- pores are not yet clear, numerous results on breakthrough times and chemical concentrations in leachate suggest that this process depends on pre-rain moisture content (White et al., 1986; Kluitenberg and Horton, 1990), chemical reactivity (Kanwar et al,. 1985; White et al., 1986 ), rain intensity and duration (Shipitalo et al., 1990; Edwards et al., 1992 ), and characteristics of the soil surface (Trojan and Linden, 1992).

Preferential water flow in soils containing macropores is said to occur when some part of the soil volume is by-passed by infiltrating water (Beven and Ger- mann, 1982; White et al., 1986; Kluitenberg and Horton, 1990). The ratio be- tween soil volumes conducting versus by-passed by water flowmtermed here the degree of flow preference--needs to be quantified to implement environmentally sound management practices. Research techniques providing some estimate of flow preference include the dye staining of macropore flow pathways (Andreini and Steenhuis, 1990; Trojan and Linden, 1992), calculating the macroporosity occupied by water flow from tension infiltration (Jarvis et al., 1987; Wilson and Luxmoore, 1988 ), and finding the ratio of mobile to immobile water from fitting mathematical models to chemical breakthrough curves (Nkedi-Kizza et al., 1984; Andreini and Steenhuis, 1990; Kluitenberg and Horton, 1990). However, these techniques are either indirect (tension infiltration) or model dependent (break- through curve fitting), suggesting that an alternative procedure must be sought to provide flow preference comparisons among soils under various conditions.

A series of studies have recently been conducted exploring the lateral non-uni- formity of water and chemical transport through large undisturbed soil samples as related to initial storm effects (Shipitalo et al., 1990), rain intensity (Edwards et al., 1992 ), and tillage (Andreini and Steenhuis, 1990; Granovsky et al., 1993 ). Lateral non-uniformity of water flow in these experiments was assessed by col-

A.V. Granovsky et al. ~Soil & Tillage Research 32 (1994) 223-236 225

lecting outflow at several loci (cells) organized in a spatial grid and recording which loci produced leachate. The number of cells that produced leachate in a sample, as well as the distribution of cell leachate volumes, reflect the non-uni- formity of water flow and the water flow preference at the soil block level (An- dreini and Steenhuis, 1990; Quisenberry and Phillips, 1991; Edwards et al., 1992 ). In this case fewer leachate producing cells and more skewed distributions of cell leachate volumes indicate more preferential flow.

The character of local water flow is closely related to chemical transport at the individual outflow cell. As was concluded by Shipitalo et al. (1990), a 'contin- uum' from faster, more preferential to slower, matrix-type flow occurs in soil, the 'faster' pathways being marked by cells with shorter water breakthrough times and larger leachate volumes. The 'faster' versus 'slower' pathways were also con- trasted by less chemical interaction with the surrounding soil matrix. The differ- ence between the faster and slower flow pathways was evident when surface-ap- plied chemicals were introduced into the soil by a previous rain. Water from the subsequent rains which flowed through the faster pathways had high concentra- tions of Br, Sr and atrazine, while water flowing along the slower pathways mostly by-passed the more reactive species Sr and atrazine which were strongly retained by the matrix (Shipitalo et al., 1990; Granovsky et al., 1993). Therefore, the blocks with a higher proportion of the faster pathways transported larger amounts of reactive chemicals following the second simulated rain compared with the blocks having fewer of such pathways. Degree of water flow preference derived from the distribution of cell leachate volumes is thus related to the total chemical transport at the soil block level.

Beside facilitating the creation of macropore networks, conservation tillage al- ters the soil hydrologic properties by leaving at least 30% of soil surface covered with a layer of crop residue (CTIC, 1986). This crop residue mulch is used pri- marily for reducing runoff and erosion, which in turn increases water infiltration and storage (Meyer et al., 1970; World Meteorological Organization, 1975; On- stad and Otterby, 1979; Laflen and Colvin, 1981; Gilley et al., 1986). Mulch reduces runoff by diminishing the raindrop impact on soil aggregates and me- chanically slowing down the surface water flow. In one study as little as 0.56 Mg ha-~ of straw mulch (30% surface coverage), by slightly reducing the runoff, di- minished three-fold the soil loss from the plowed 13-15 ° slopes (Meyer et al., 1970).

Although the effect of surface coverage on reducing soil erosion is relatively well understood (Laflen and Colvin, 1981; Edwards, 1982; Gilley et al., 1986), its impact on water and chemical flow in the soil macropores is not yet clear. The quality of mulch residue as a food source for soil earthworms was shown to influ- ence the establishment and activity of the earthworm population. Therefore, mulch has an indirect effect on 'short-circuiting' of chemicals to groundwater by influencing the formation of earthworm channels (Edwards, 1982; Mackay and Kladivko, 1985; Zachmann et al., 1987 ). The surface coverage by the mulch layer should also have a direct effect on preferential flow in soil macropores. Most probably this direct effect is through prevention of surface sealing and alteration

226 A.V. Granovsky et al. /Soil & Tillage Research 32 (I 994) 223-236

to some extent of the surface microrelief, the role of which on preferential flow has been demonstrated (Trojan and Linden; 1992).

The purpose of this study was to examine the direct impact of surface coverage on preferential flow of water and deep chemical transport. In the course of anal- ysis, however, it appeared that pre-rain moisture content had a stronger effect on these processes.

2. Materials and methods

Four undisturbed soil blocks (0.3 m × 0.3 m × 0.3 m ) were removed from Wa- tershed 188 of the North Appalachian Experimental Watershed. Watershed 188 contains Rayne silt loam (fine-loamy, mixed, mesic Typic Hapludult) soil and has had continuous no-till corn grown since 1970. The first two blocks were sam- pled on 14 July 1989 and another two on 14 August 1989. Cubic pedestals of soil were carved and enclosed on four sides with plywood boxes of a larger size. Liq- uid polyurethane foam was injected into the 1 cm gap between the soil and the box and, after allowing the foam 1 day to cure, the encased blocks were carefully cut at their bases and brought to the laboratory. On both sampling dates, soil cores were taken from the soil surface to the depth of 30 cm at 5 cm increments, and moisture content and soil bulk density were determined gravimetrically. All visible macropores (> 2 mm in diameter) on the bottom side of each soil block were mapped on a clear plastic sheet, and their size and position were recorded. The mulch layer was removed from one of the blocks taken in July and from one taken in August prior to chemical and rainfall application (approximately 100 g of mulch, or 70-80% surface coverage).

One hour before the first simulated rain, the 0.09 m 2 surface of each block was uniformly treated with 8.57 g of granular SrBr2.6H20 (48.25 mmol Br, 24.13 mmol Sr) used to simulate Ca (NO3)2, and 15 ml of water containing 71 mg of atrazine 4L (6-chloro-N-ethyl-N'- ( l-methylethyl)- 1,3,5-triazine-2,4-diamine) was applied dropwise. The amount of Br was equivalent in terms of moles per unit area to the amount of nitrate in a 150 kg ha- 1 application ofNH4NO3. Atra- zinc application (0.323 mmol) was equivalent to the rate of 7.7 kg a.i. ha- 1.

Each block then received a 30 mm rain (Rain 1 ) in 30 min applied using the device of Shipitalo et al. (1990) consisting of an array of hypodermic needles. The second and third rains (Rains 2 and 3) were applied after the blocks had returned to their 'pre-rain' weight, approximately 1 week later. The block surface was 100 mm below the hypodermic needles, although the actual kinetic energy of the droplets was not assessed. The blocks were weighed prior to each simulated rain, and pre-rain volumetric moisture contents were calculated.

During each rain, the blocks were positioned on a collection rack consisting of an 8X8 grid of 64 37.5 mmX37.5 mm funnel-shaped square cells divided by knife edges made of aluminium strips (width 10 mm, thickness 1 mm ). The cells were connected via Tygon tubing to 64 separate collection vessels, in which leach-

A. V. Granovsky et al. / Soil & Tillage Research 32 (1994) 223-236 227

ate volumes and chemical concentrations were determined after each rain. Con- centrations of Br and Sr in the cell leachate were measured by ion chromato- graphy and atomic absorption, respectively, and atrazine was analysed using gas chromatography. Using the maps of macropores, the area of macropores in the cells that produced leachate was calculated as a measure of conducting macro- pore area. Only a few cells that produced leachate had more than one visible ma- cropore, which implies that our estimate of the area of conducting macropores has an accuracy of + 10 %. Procedures for sampling the blocks, chemical appli- cation, rain simulation device and chemical analyses are described in more detail by Shipitalo et al. (1990).

The degree of flow preference was assessed by sorting cell leachate volumes in decreasing order and then plotting the cumulative outflow in a block against the cumulative outflow area, which is proportional to the number of cells. If all 64 cells equally contributed to outflow, the resulting dependence would be a straight line. A non-uniform distribution of cell leachate volumes would generate a con- vex upward curve (Quisenberry and Phillips, 1991 ). In our study these sorted cumulative outflow curves were fitted to the exponential function:

FLW= 1 - e x p ( - m × A R ) ( 1 )

where FLW is the cumulative outflow divided by total outflow (dimensionless), and AR is the cumulative relative outflow area, measured as cumulative number of cells divided by the total number of cells (also dimensionless). The exponent, m, was found as a slope of the linear regression [In( 1 - F L W ) ] vs. AR using only the rising part of the curve and disregarding the tail of 1 's. This linear estimation provided r z values from 0.995 to 0.999. Fitting Eq. ( 1 ) to the outflow data for Rain 1 is shown in Fig. 1. The parameter m computed for each block and rain was treated as the degree of flow preference, since its higher values reflect more non-uniform flow with fewer percolating cells (Fig. 1 ). Also, the sorted cumula- tive outflow curves offer a good visualization of the ratio between the cells with large versus small leachate volumes, which are represented by the curve segments with a steeper and a more gradual rise, respectively.

o

m

E

1.0

0.8

0.6

0.4

0.2

0.0 0.0

RAIN 1 [

• mulch t o no mulch

0 . 2 0 . 4 0 . 6 0 . 8 1 .0

C u m u l a t i v e R e l a t i v e A r e a

Fig. 1. Sorted cumulat ive outflow curves for the four blocks following Rain 1. The lines show fitting data to Eq. ( I ). Numbers refer to blocks.

228 A.V. Granovsky et al. / Soil & Tillage Research 32 (I 994) 223-236

Correlation coefficients were calculated between the cell leachate volumes and chemical concentrations in the cell leachates separately for each soil block and rain. For each chemical these coefficients had the same sign and similar magni- tude regardless of mulch treatment. Then the four blocks with and without mulch were pooled together in order to examine a joint effect on chemical concentration of the cell leachate volume and antecedent moisture content. Multiple regression analysis was run with two independent variables: cell leachate volume, LV (1), and antecedent moisture content taken as constant across the cells within a block, 0o (m 3 m-3) . The concentration of each chemical in the cell leachate separately ( [Br], [Sr], or [atrazine] in mmol 1-1 ) was taken as a dependent variable. Thus nine multiple regression equations were generated for the three rains and three chemicals. Multiple regression coefficients, their significance, and coefficients of determination, R 2, were computed using STATISTICA (StatSoft Inc., 1991 ).

3. Results and discussion

3.1. Character o f water f low

Using maps of the bottom block surfaces, 20-41 macropores having diameters of 2-11 mm were counted per block, corresponding to 222-456 pores m-2. Sim- ilar large pore densities have been reported elsewhere (Edwards, 1982; Mackay and Kladivko, 1985; Wilson and Luxmoore, 1988). The four blocks did not dif- fer considerably with regard to the total area of large visible pores at the 0.3 m depth (Table 1 ). This indicates that potentially the blocks had similar capacities for the macropore flow.

Water outflow in our experiment was not uniform. For all blocks and rains only 5-30 out of 64 cells produced leachate; single cells producing the greatest leachate volumes (50-140 ml) accounted for 20-40% of total outflow in a particular block. In two-thirds of the cases, one or several macropores were mapped in a cell that produced large leachate volumes ( > 50 ml). For most other cells producing large leachate volumes, macropores were mapped in the cells immediately adjacent. These observations suggest that flow of large water amounts was predominantly associated with the visible large pores.

For the cells with smaller leachate volumes ( < 50 ml) there was less associa- tion with macropores: in one-half of the cases no macropores were mapped in these or adjacent cells. Besides macropore flow, small leachate volumes could result both from slower, matrix-type pathways, or from water moving via large pores down to some depth and then seeping through the bottom layer containing no macropores (Granovsky et al., 1993). Using an experimental design with water interception at one particular depth, only the manifestly faster preferential flow pathways, marked by large cell leachate volumes, could be localized and related to macropores. When larger and smaller leachate volumes, i.e. faster and slower pathways, are considered, their relationship with mapped macropores should be treated in terms of statistical correlations at the level of an entire block.

Tab

le 1

T

otal

lea

chat

e vo

lum

e, t

otal

che

mic

al t

rans

port

, ar

ea o

f co

nduc

ting

mac

ropo

res,

and

the

exp

onen

t, m

, in

eac

h bl

ock

afte

r ea

ch s

imul

ated

30

mm

rai

n ;~

Mul

ch

Blo

ck

Tot

al v

isib

le

Con

duct

ing

Exp

onen

t m

L

each

ate

No.

m

acro

pore

s m

acro

pore

s vo

lum

e (%

bot

tom

are

a)

(% b

ott

om

are

a)

(dim

ensi

onle

ss)

(1)

Pre

-rai

n B

r S

r A

traz

ine

moi

stur

e tr

ansp

ort

tran

spor

t tr

ansp

ort

(m 3

m -

3)

(mm

ol)

(m

mo

l)

(mm

ol)

Rai

n 1

No

mul

ch

Mul

ch

Rai

n 2

No

mul

ch

Mul

ch

Rai

n 3

No

mul

ch

Mul

ch

• 1

0.27

4

0.40

2

0.39

3

0.50

1 0.

27

4 0.

40

2 0.

39

3 0.

50

1 0.

27

4 0.

40

2 0.

39

3 0.

50

0.24

7.

3 1.

04

0.05

61

.0

0.21

0.

17

11.4

0.

74

0.16

24

.7

0.97

0.22

11

.1

0.92

0.

09

63.0

0.

36

0.15

19

.4

0.61

0.

22

18.6

0.

83

0.21

11

.2

0.84

0.

09

68.2

0.

31

0.19

11

.1

0.71

0.

19

16.1

0.

64

0.42

9.

80

1.15

0.

0260

-,,

0.

25

1.03

0.

14

0.00

43

0.41

5.

66

0.62

0.

0190

0.

35

8.67

1.

82

0.02

91

0.42

2.

89

0.16

2 0.

0120

~

0.25

0.

42

0.05

4 0.

0045

~

0.39

1.

72

0.03

9 0.

0096

~,

0.

35

1.83

0.

055

0.01

10

t.,a

x~

0.41

2.

02

0.06

2 0.

0095

0.

22

0.61

0.

039

0.00

29

0.40

1.

69

0.02

5 0.

0076

ix

a 0.

37

1.08

0.

030

0.00

48

b,J

230 A.V. Granovsky et al. /Soil & Tillage Research 32 (1994) 223-236

Counting macropores in the cells that produced leachate revealed that on av- erage one-half of all macropores visible at the bottom surfaces of soil blocks were hydraulically active. Areas of those macropores were, however, two to eight times less than the total area of visible macropores (Table 1 ), because many larger pores 7-11 mm were mapped in the cells with no percolation. Areas of conduct- ing macropores obtained by simply counting large pores in the leachate produc- ingcells need to be interpreted with caution, since water flow may not occupy the entire cross-sectional area of these pores (Beven and Germann, 1982 ). However, a significant positive correlation was found between the area of conducting ma- cropores and total leachate volume in a block (r= 0.98) (Fig. 2). While at the level of individual cells the outflow was not always associated with the presence of a macropore in a cell, at the block level the magnitude of preferential water flow resulting from both the faster and slower pathways seems to be well corre- lated with conducting macroporosity.

To link percolation patterns in each block to the presence of soil surface cov- erage, the degree of flow preference and total leachate volume were compared among the blocks (Table 1 ). Surface cover had no apparent effect on the values of m and total leachate volume. Subsequently, the 12 leaching events (4 blocks × 3 rains) were pooled and tested for the effect of pre-rain moisture content. One soil block (No. 4), probably sampled on a micro-elevation, was much drier than the other three. Since this block had similar values of bulk density and number of visible large pores at the 0.3 m depth, and fitted well into the regression between total leachate volume and conducting macroporosity (Fig. 2 ), its inclusion in our analyses was justified.

Pre-rain moisture content was significantly negatively correlated with the de- gree of flow preference, even if block No. 4 was not considered (Fig. 3 ). It can be concluded that the drier pre-rain conditions resulted in a more preferential flow of water corresponding to less leachate producing cells in a block. Blocks with lower antecedent moisture content (Nos. 3 and 4) had reduced numbers of cells that produced small leachate volumes, implying that predominantly the slower,

0~ t= o o.

g

0 . 3

0.2

0 . 1

• m u l c h

o n o m u l c h ~ ~

0.0 ~ L i i i

0 . 2 0 . 4 0 . 5 0 . 7 0 . 9 1 . 0 1 . 2

Total leachate v o l u m e , L

Fig. 2. Relationship between total leachate volume in a block (L) and area of conducting macropores (% of bottom area) counted in the cells that produced leachate obtained for four blocks following three simulated rains.

A. I~ Granovsky et al. / Soil & Tillage Research 32 (1994) 223-236 231

80 • v

60

40

20

0 - - 0.20

"\~\ 4 - . o r---0.98 • m u l c h

" ' . . ~ o no mulch I \ \x . . \ r

1 -t r=-0.85 -

0.25 0.30 0.35 0.40 0.45

Pre-rain moisture content, cm3cm 3

Fig. 3. Dependence o f the exponent m, calculated for each block and rain, on the pre-rain moisture content. Numbers refer to three runs for each block. Regression lines are fitted to nine data points for Blocks l, 2 and 3 ( r = -0.85 ), and to 12 data points for all the blocks ( r = - 0 . 9 8 ).

,.d d

_=

,.d

1.2

1.0

0.8

0.6

0.4

0.2

0.0 0.20

1 • mulch

1 o 0 no m u l c h • 0

3 • 0 2

0 0

0 4

0.25 0.30 0.35 0.40 0.45

Pre-rain moisture content, cm3cm

Fig. 4. Relat ionship between the pre-rain moisture content and the total leachate vo lume in a block. Numbers refer to three runs for each block.

matrix-type flow pathways were truncated because of water sorption by the drier soil matrix.

All blocks taken together, pre-rain moisture content had a significant effect on the total leachate volume (Fig. 4). Little correlation could be discerned, how- ever, when excluding the driest block No. 4, or when the three runs for the same block were compared separately. More simulations are needed with dry and in- termediate pre-rain soil moisture contents to further explore the moisture-out- flow relationship. Yet, at this point it is quite clear that waiting for the blocks to return to their pre-rain weights yielded percolation patterns for each block that were consistent among the three rains. This is evidenced by little fluctuation in conducting macropore areas, the values of m, and (except for block No. 3 ) total leachate volumes.

3.2. A m o u n t s o f chemicals transported

To relate local character of water flow through the no-till soil blocks to the transport of surface-applied chemicals, the joint effect of cell leachate volumes

232 A. V. Granovsky et al. ~Soil & Tillage Research 32 (1994) 223-236

and pre-rain moisture contents on chemical concentration in cell leachate was examined using multiple regression analysis (Table 2). In general, cell leachate volumes were positively correlated with chemical concentrations (statistically significant positive regression coefficients for leachate volume), except for Br after Rains 2 and 3. Therefore, the faster pathways marked by the larger cell leachate volumes were characterized by higher chemical concentrations of Br after Rain 1 and of Sr and atrazine following all three rains. The less reactive Br was expected to reflect the movement of water (Andreini and Steenhuis, 1990; Tro- jan and Linden, 1992 ), so that its concentration would be similar across the cells with larger and smaller percolation. Since the chemicals were surface-applied, however, some mixing with soil water apparently took place along the slower paths, resulting in higher Br concentrations in the cells producing larger leachate vol- umes. The correlation between Br concentrations and cell leachate volumes be- came insignificant for Rains 2 and 3 (Table 2), implying similar Br concentra- tions in the faster and slower pathways. It can be deduced that Br was distributed throughout the profile as a result of Rain 1 and moved like a typical conservative species during the subsequent rains. A similar change in the pattern of Br trans- port following the first and subsequent rains was reported by Shipitalo et al. (1990).

A strong relationship between the concentrations of Sr and atrazine with cell leachate volume following all three rains implied that greater interaction with the soil matrix in the slower paths diminished concentrations of the reactive Sr and atrazine. Similar findings on the differences in transport of the less and more reactive chemicals at the cell level have been reported (Shipitalo et al., 1990; Edwards et al., 1992; Granovsky et al., 1993).

The local character of chemical movement determined the differences in the total deep chemical transport among the soil blocks. Although the leachate vol-

Table 2 Multiple regression equations relating chemical concentration in cell leachate (mmol 1 - l ) tO cell leachate volumes LV (1) and overall block pre-rain moisture content 8o ( m 3 m - 3 )

Rain 1 [Br ] = - 2.52 + 2 3 . 2 L V ' + 21.200 [Sr] = - 0 . 3 0 + 8 . S L V + 1.50o [ At raz ine ] = 0.015 + 0 .075LV + 0.00800

R 2 = 0 . 3 2 ** R 2 = 0 . 4 2 ** R 2 = 0 . 3 2 **

Rain 2 [Br ] = 1 , 8 - 0 . 7 7 L V + 2.58o [Sr ] = - 0.06 + 0.70LV + 0.2600 [ Atraz ine ] = 0.010 + 0.026LV + 0.0048o

R 2 = 0 . 0 2 R 2 = 0 . 2 7 ** R 2 = 0 . 1 4 *

Rain 3 [Br ] = - 1 . 1 7 - 2 . 4 L V + 9.580 [Sr] = - 0.004 + 0.32LV + 0.098o [ At raz ine ] = 0.002 + 0 ,019LV + 0.0150o

R 2 = 0 . 1 2 * R 2 = 0 . 2 7 ** R 2 = 0 . 1 5 *

R 2 is the coefficient of determination. * Coefficient of determination significant at P = 0.05. ** Coefficient of determination significant at P = 0.01. "Underlined values are significant coefficients.

A.V. Granovsky et al. ~Soil & Tillage Research 32 (1994) 223-236 2 3 3

umes were quite similar from rain to rain for each individual block, the largest amounts of the surface-applied chemicals were transported after the first rain (Table 1 ). Of the approximately 48 mmol of Br, 24 mmol of Sr, and 0.33 mmol of atrazine applied, the amounts leached by Rain 1 represented 2-20% of Br, 0.6- 7.5% of Sr, and I-9% of atrazine among the blocks. Prior to Rains 2 and 3, only 2-20% less chemicals were available for leaching, yet the chemical transport was reduced two-fold for the less reactive Br and three- to ten-fold for the more reac- tive atrazine and Sr, respectively. Apparently, chemicals were spread throughout the larger soil volumes and were by-passed by the preferential flow during the subsequent leaching events. This selectivity of chemical transport has been also reported by Kanwar et al. ( 1985 ), Shipitalo et al. (1990), Edwards et al. ( 1992 ) and Trojan and Linden ( 1992 ). The peak of deep chemical transport following Rain 1 has important environmental implications, as the very first intense rain following the surface application of agrochemicals will necessarily be the most damaging, especially in wetter soils.

To account for different total leachate volumes, flow-weighted chemical con- centrations (total chemical amounts transported divided by total leachate vol- ume) were compared instead of the total chemical amounts (Fig. 5 ). Among the four blocks, flow-weighted concentrations of the less reactive Br decreased in the same sequence as the total leachate volumes. As established earlier, this sequence was determined predominantly by the antecedent moisture content. The wetter

@

i M

1.5

l.O

0.5

0.0

• m u l c h

0 no m u l c h

~ . . . . . . . o

_ - - 0 . . . . . . . . 0 4 0 - - - -

~,~ lO

s

6 @

~ 2

~ o

1 f~ ~- • m u l c h 3

2 • " ' " ~ , O n o m u l c h

I 2 3 1 2 3

Rain No. Rain No.

~,a io

= fl

0.1

0.01

• m u l c h

i O ~ © no m u l c h

" ""':~ .... : : - - - 0

ff

0.03

0.02

0.01

. malo 21 O, " \ , ~ , ~ " O n o m u l c h

0.00 1 2 3 1 2 3

R a i n N o . R a i n N o .

Fig. 5. Total leachate vo lumes and flow-averaged chemical concentrations (total chemical transport divided by leachate v o l u m e ) in each block following the three rains.

234 A.V.. Granovsky et al. / Soil & Tillage Research 32 (1994) 223-236

soil blocks were characterized by a larger number of cells having percolate with high chemical concentrations and large volumes, which increased the flow- weighted concentrations of Br. However, the same sequence was not observed for the flow-weighted concentrations of Sr and atrazine after Rain 1, as the blocks with mulch tended to produce higher concentrations of reactive chemicals than would be predicted from pre-rain moisture conditions alone.

It should be noted here that direct statistical comparison between chemical amounts or concentrations in blocks with and without mulch are not valid be- cause of the strong effect of the pre-rain moisture and rather small number of replicates. Nonetheless, the tendency for the mulch layer to enhance Sr and atra- zine transport should be noted for future research. This result was quite unex- pected, because mulch was considered chemically active, causing increased sorp- tion reactions. On the other hand, it was shown by Martin et al. ( 1978 ) that the first 5 mm of a 40 mm rain washed off 60-80% of the herbicides applied on the corn residue. If in fact the mulch layer is ineffective in binding reactive chemicals like Sr and atrazine, it might enhance downward transport of chemicals by pro- longing their contact with infiltrating water at the soil surface. Further studies using carefully controlled moisture levels and attention to the surface microrelief as affected by mulch are warranted to validate this tendency.

4. Summary

The preferential flow in our experiments was closely associated with large po- res ( > 2 mm diameter) as evidenced by significant correlation between the area of macropores in leachate producing cells and total leachate volume in a soil block. Not all macropores visible at the 0.3 m depth were active hydraulically, and the ratio of the area of conducting versus total visible large pores varied for the same block depending on the pre-rain moisture content.

The parameter m, found by fitting the exponential function (Eq. ( 1 ) ) to the sorted cumulative outflow curves, was a convenient measure of the degree of flow preference in a block. For the lower pre-rain moisture contents, the values of m were higher, because only a few more rapid pathways contributed to the total outflow. The slower pathways were truncated under drier conditions, presumably as a result of water sorption by the soil matrix. Higher antecedent moisture con- tent yielded a less preferential flow with a larger number of leachate producing cells. Pre-rain moisture was positively (insignificantly) correlated with the total outflow. The effect of surface coverage on water flow was apparently oversha- dowed by the impact of pre-rain moisture content.

Chemical concentrations were higher in cells with more leachate, because less interaction with bulk soil took place along the more rapid pathways. The peak total chemical transport was observed following the first simulated rain in blocks with and without mulch. In the driest block, the chemical concentrations in leach- ate after the first rain were still high, implying that the first intense rain following the field surface application of agrochemicals will have a maximal effect on the

A. V. Granovsky et al. / Soil & Tillage Research 32 (I 994) 223-236 235

deep chemical transport. Flow from the subsequent rains by-passed the chemicals distributed throughout the profile following the first simulated rain. As a result, the chemical transport following the subsequent rains was reduced compared to the amounts transported after Rain 1. This reduction was stronger for the more reactive Sr (ten-fold) and atrazine (three-fold) than for the less reactive Br (two- fold).

The mulch layer, rather than binding the applied chemicals, slightly enhanced the transport of the more reactive Sr and atrazine after the first rain, perhaps by prolonging their contact with water at the soil surface. More research is required to substantiate this finding.

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