Soil & Tillage Research 166 (2017) 100–107
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Soil & Tillage Research
journal homepage: www.elsevier.com/locate/still
Residual plastic mulch fragments effects on soil physical properties and
water flow behavior in the Minqin Oasis, northwestern China
Xiao Jin Jianga,* , Wenjie Liua , Enheng Wangb , Tingzhang Zhouc , Pei Xinc
a
b
c
Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun, Yunnan 666303, China
School of Forestry, Northeast Forestry University, Harbin 150040, Heilongjiang, China
State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, China
A R T I C L E I N F O
Article history:
Received 26 January 2016
Received in revised form 20 September 2016
Accepted 25 October 2016
Available online 11 November 2016
Keywords:
Dye tracer
Heterogeneous water infiltration
Maize root (densely rooted) zone
Biochemistry properties
A B S T R A C T
The covering of complete plastic films on soil would affect physical and biochemical properties of the soil
microenvironment. Residual plastic film fragments (RPFF), which is left behind after retrieving the most
of plastic films, could affect the flow behavior in the arable layer. This study was designed to assess the
potential effects of RPFF on soil physical properties, water infiltration and distribution in the soil.
Treatments with and without residual plastic film fragments (RPFF and NRPFF, respectively) were
performed. A dye tracer was introduced to track the water movement, and the physical properties of the
soil and the dynamic behavior of water transport in the soil column of the arable layer (0–20 cm) were
determined. The initial gravimetric water content, bulk density, total porosity in 0–20 cm was
significantly different between RPFF and NRPFF treatment. The dark blue area in maize root and densely
rooted zones under RPFF decreased by 99% and 4%, respectively, relative to that under NRPFF. The sharply
changing state of infiltration and outflow indicated that water flowed along a preferential path, e.g.,
macro-pores in NRPFF and residual plastic film pieces in RPFF. After clearing the RPFF, the time of
equilibrium in 0–5, 5–10, 10–15, and 15–20 cm soil columns decreased by 48%, 50%, 49%, and 45%,
respectively. Our results highlighted that the presence of RPFF significantly influenced soil physical
properties, altered soil water distribution, and decreased the matching degree of the flow distribution
region and the maize root (densely rooted) zone. The present study demonstrated that the importance of
clearing the RPFF for irrigation water management in agriculture produces.
ã 2016 Elsevier B.V. All rights reserved.
1. Introduction
Low temperature at seedling stage as well as insufficient
precipitation and high evaporation during growth period affect the
growth, development, and yield of crops in arid and semiarid
regions (Beltrano et al., 1999; Boyer and Westgate, 2004). These
issues have been intensified by global climate change (Zhao et al.,
2014) and global population increase (Lisson et al., 2016).
According to the prediction of FAO (2009), the world population
would increase from 6.8 billion in 2010 to over 9.1 billion by 2050,
with an expected increase in food demand of 70%. This presents a
major challenge for agricultural production, especially in areas
where crop production is likely to be adversely affected by
unreasonable agricultural management practices.
* Corresponding author.
E-mail address: [email protected] (X.J. Jiang).
http://dx.doi.org/10.1016/j.still.2016.10.011
0167-1987/ã 2016 Elsevier B.V. All rights reserved.
Numerous management strategies have been adopted to solve
these issues over the past decades. Recently, plastic film fully
mulched ridge-furrow system has been widely adopted to
significantly improve grain yield in regions where irrigation is
unavailable (Gan et al., 2013); in this system, narrow ridges are
alternated with wide ridges, and mulching is completed with
plastic films with furrow planting (Zhao et al., 2012). Studies have
shown that plastic film fully mulched ridge-furrow system exhibits
several advantages. This system has reduced harvest time by up to
nine days (Andrew et al., 1976) and has almost doubled grain yield
(Hopen, 1965) as a result of the improved physical and biochemical
properties of the soil microenvironment. This system increases the
water content and temperature of the soil (Chakraborty et al.,
2008; Cook et al., 2006), stabilizes the topsoil (De Silva and Cook,
2003), decreases evaporation and weed competition (Li et al.,
2013a), and reduces soil compaction and erosion, enhances soil
water infiltration (Gan et al., 2013), activates soil nutrients (Li et al.,
2004a), collects rainwater and improves its use efficiency (Jiang
and Li, 2015; Liu et al., 2016). The system also increases the water
X.J. Jiang et al. / Soil & Tillage Research 166 (2017) 100–107
stability of macro-aggregates and volume of macropores (Zhang
et al., 2013a); seed fertility and individual plant growth (Zhou et al.,
2009); and root and shoot biomass. Moreover, the plastic film fully
mulched ridge-furrow system exerts direct or indirect effects on
the biochemical properties of the soil. For instance, this system
alleviates the decline of soil organic carbon (Zhang et al., 2013b),
increases N mineralization (Ghosh et al., 2006), reduces leaching of
N fertilizers (Romic et al., 2003), improves plant nitrogen
availability (Wang et al., 2015), and promotes microbial biomass
C but decreases soil organic C (Li et al., 2004b). However, the
disadvantages of plastic film fully mulched ridge-furrow system
have attracted the attention of researchers and farmers. Grain yield
decreases because of changes in the microclimate, including
consistently increasing temperatures (Hou et al., 2010; Wang et al.,
2009), poor soil aeration resulting from high CO2 concentrations
(Tiquia et al., 2002), excessive N mineralization (Ghosh et al.,
2006), depletion of soil organic carbon (Liu et al., 2013), increased
emission of two major greenhouse gases (N2O and CH4) (Cuello
et al., 2015), accelerated decomposition of soil organic matter, and
deterioration of the soil structure (Zhang et al., 2013b). Researchers
and farmers have made efforts to overcome these negative factors.
Overall, the benefits of mulching outweigh its disadvantages;
therefore, the plastic film fully mulched ridge-furrow system has
been widely applied and promoted in the arid and semiarid zones
of China. For instance, the area where the plastic film fully mulched
ridge-furrow system was applied reached 193 103 ha in 2008 in
Gansu Province; this area occupies about 7% of the total cropland
and contributes to 20% of the total grain production (Wang et al.,
2009).
However, after one or few seasons, the residual plastic
fragments become problematic when the plastic films must be
replaced by new ones. Soils are heavily contaminated with these
films which were disposed by farmers through on-site land filling
and burning (Gonzalez-Sanchez et al., 2014). This negative effect
on soil will last for many years because plastic film fragments
cannot decompose biologically. Moreover, plastic film fragments
are discarded and buried in the arable layer. The loose characteristic of soil in this layer could lead to matrix flow. Other flow types
resulting from heterogeneous structures also appear in the arable
layer, including macropore flow caused by preferential paths (e.g.,
cracks, plant roots, worm holes, and voids between peds) (Beven
and Germann, 1982) and lateral flow induced by inclined
hydraulically restrictive layers, such as bedrocks (Allaire et al.,
2009) and tillage pans (Jiang et al., 2015). RPFF that random
appeared in the soil could not only interrupt the soil structure but
also serve as a water-resistant layer or an inclined hydraulically
restrictive layer. The random characteristic of RPFF induced water
to either bypass the water-resistant layer or follow the laterally
hydraulically restrictive layer. Thus RPFF could affect the flow
behavior in the same layer. The proposed physical process and the
significance of residual plastic films to soil water distribution have
not been experimentally reported in the literature, although many
studies have focused on the relationship between plants, soil and
plastic film mulching. Therefore, in the present study a traditional
method involving a dye tracer was performed on two soil plots
with different management practices. This study aims to assess the
effect of residual plastic film fragments on the water distribution
and flow processes in the soil.
2. Materials and methods
2.1. Experimental site
Minqin Oasis, is located in the arid inland of Northwest China,
whose agricultural production accounts for 80% of the gross
regional domestic product (Feng et al., 2011) and 76% of its
101
population is engaged in agricultural production (Statistical
Bureau of Minqin County, 2009). From 2005 to 2010, the farmland
area of the Minqin Oasis has shrunken by 12.89 km2. The study area
was located on XueBai Farm in the MinQin county of Gansu
Province, China (between 101490 –104120 E and 38 030 –39 280 N).
The east, west, and north sides of Minqin County are surrounded by
the Tengger Desert and Badain jaran Desert. The area has an arid
continental climate with an average annual temperature of 7.8 C.
The mean precipitation in the area is 110.5 mm yr1, and the annual
average evaporation is 2 646.4 mm yr1. The soil under study was
classified as “irrigated desert soil” in accordance with the Chinese
Soil Classification System (Gansu Provincial Soil Survey Office,
1992) and similar to Anthropic Camborthids according to Soil
Taxonomy (Soil Survey Staff, 1998).
2.2. Experimental design
2.2.1. Dye tracing
The plastic film fully mulched ridge-furrow system was applied
to plant corn and sunflower by local farmers (Fig. 1A). The entire
land surface was tightly covered with polyethylene film (colorless
and transparent, 0.008 mm thick and 1200 mm wide) to collect
rainwater and prevent heat and water loss and the perforations
(around 1 cm in diameter, 30 cm apart, matches the plant spacing
of 30 cm in a row) were drilled through the film by using a
handheld device during sowing time. Simultaneously, we found
lots of residual plastic film fragments left in the plough layer
(Fig. 1B).
Given the spatial heterogeneity, two contacting fields (10 m
20 m) were chosen as research fields. In each research field,
0.8 kg polyethylene film was used to cover soil during sowing; the
crops for these two fields were alternated in a maize-wheat
rotation from 2000 to 2015. From 2012 to 2015, these two fields
exhibited several common agricultural management practices
after harvesting. The large pieces of polyethylene films (about
0.5 kg in one field) were cleared and removed and the small pieces
of polyethylene films (about 0.3 kg were left behind in one field)
were randomly broken into fragments with different sizes when
the arable layer (0–20 cm) of research fields was plowed using a
rotary cultivator. The difference in agricultural practice was that
the residual plastic film fragments in the first field were retained,
whereas those in the second field were picked and cleared during
plowing. Agricultural practices (e.g., irrigation, plowing, sowing)
conducted on the two fields before harvesting in the second year
was expected to reduce the effect of the picking and cleaning
process on soil. Generally, besides the picking and cleaning
process of residual plastic film fragments in the second field,
other agricultural practices were conducted in the two fields.
Namely the two treatments in this study were residual plastic
film fragments (RPFF) and no residual plastic film fragments
(NRPFF).
Notice that the experiments were conducted from October 6th
to 15th, 2015 before the maize was harvested. Three plots were
randomly selected as replicates in each field. The dye-stained test
plots were prepared by carefully removing the polyethylene film
and a thin layer of the soil (less than 2 cm) to ensure a levelled
horizontal surface. Each replicate plot was surrounded by a PVC
quadrat (height = 0.3 m, diameter = 0.25 m). The bottom edges of
the PVC quadrats with watertight sidewalls and edges were
inserted 0.05 m into the soil. All eight plots were irrigated with
4.1 L of water dye solution containing 3.0 g L1 Brilliant Blue FCF
(Flury and Flühler, 1995). The infiltrated area was covered with a
vinyl film to prevent evaporation and dilution by rainfall (Jiang
et al., 2012) between the end of the infiltration and the beginning
of excavation. The eight dye-stained soil vertical sections of 40 cm
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X.J. Jiang et al. / Soil & Tillage Research 166 (2017) 100–107
Fig. 1. Pattern of plastic film fully mulched ridge–furrow cropping was applied to planting maize and sunflower in the Minqin Oasis (A) and residual plastic film fragments in
the vertical soil section of plough layer (B).
(length) 35 cm (width) were carefully excavated from the center
of the ring by using a spatula (Weiler and Flühler, 2004) 24.0 h
after the infiltration. The calibrated frames for soil vertical
sections were placed in the soil profile to assist subsequent image
correction. The entire pit area was placed beneath a black
umbrella to provide soft light conditions. All soil sections were
photographed using a digital camera (Canon EOS Rebel T3, Japan)
under daylight conditions. The distance from the camera to the
center of the vertical sections was 25 cm, whereas that to the
lower boundary of the soil sections was 20 cm. Six dye-stained
soil vertical sections were photographed. Five dye calibration
patches were prepared and photographed for the soil horizons in
our previous study to define the concentration categories for dyestained soil regions (Jiang et al., 2015).
Image processing was conducted using ERDAS IMAGINE version
9.0 (Jiang et al., 2015), following the procedures developed by
Forrer et al. (2000) and described in detail by Cey and Rudolph
(2009). Images were developed with the following procedure:
geometric correction, background subtraction, color adjustment,
histogram stretching, dye classification, and final visual checking.
The resulting images were grouped into dyed and non-dyed
regions, with the dyed regions further divided into three relative
classes based on the intensity of dye staining. Dye-stained soil
calibration patches were used to determine the concentration
categories for the three dye-stained soil classes (0.05–0.5, 0.5–2.0,
and >2.0 g L1). The maximum dye-stained depth (cm) and width
(cm) in the soil vertical sections was measured using a tapeline
when photographing. To elucidate the effect of water infiltration
and distribution on maize growth, we assumed that the maize root
(and densely rooted) zone (fan-shaped, below the surface) on the
soil section had a radius of 10 (and 5) cm, with the location of seed
placement (5 cm in depth) as the center. Under this premise, the
areas (cm2) of the different concentration categories (light, dark
blue) in the maize root (and densely rooted) zone of each
photographed soil vertical section were estimated (Jiang et al.,
2012). Applying these two parameters, we intended to illustrate
the effect of residual plastic film fragments on water distribution.
2.2.2. Water infiltration and outflow of soil column
After the dye tracer infiltration twelve undisturbed soil
columns of the arable layer (prepared at the interval of 5 cm from
the surface to 20 cm soil depth, three replicates in each soil layer)
were extracted from each plot by using cutting rings (inner
diameter = 50.46 mm, height = 50 mm). These twelve undisturbed
soil columns were used to measure the soil physical properties
(e.g., gravimetric water content, soil bulk density, total porosity,
and saturated hydraulic conductivity). The procedures of obtaining
bulk soil samples and determining soil physical properties in
NRPFF were following the methods described by Chen (2005). It is
worth noting that when the bulk soil samples in RPFF were
obtained, the attached plastic film was cut off from the samples by
a sharp scraper. According to the abovementioned method,
another twelve undisturbed soil columns of the arable layer were
obtained and then the columns were used to measure the
infiltration and outflow of water from falling head (height = 50
mm) and constant head (height = 50 mm) permeameter measurement. This measurement was also used to measure the saturated
hydraulic conductivity. A simple apparatus was designed for this
experiment. The top cover of the cutting ring (containing the soil
column) was removed and replaced by a circular wire mesh
(diameter = 50.46 mm). A hollow cutting ring was spliced on the
top of the cutting ring that contained the soil column and poured
with distilled water. A burette was used to ensure that the water
level was as high as the height of the cutting ring, and a filter cup
was placed on the bottom of the cutting ring containing the soil
column to collect the outflow. The volume of infiltration and
outflow was recorded using a burette and a measuring cylinder,
respectively, for every 2.0 min until steady-state conditions were
reached. In this experiment, infiltration refers to the flow of water
into a soil section, outflow refers to the water flow out of the soil
section, and permeation means water flow through a soil column.
The rate of infiltration and outflow was calculated using the
following formula:
V¼
Q
TS
X.J. Jiang et al. / Soil & Tillage Research 166 (2017) 100–107
where V is the rate of infiltration and outflow (mm min1), Q is the
volume of infiltration and outflow (mm3), T is the time interval
(min), and S is the area of soil section (mm2).
2.3. Statistical analysis
One-way analysis of variance (ANOVA) was used to assess the
effect of different treatments on the dyed parameters (e.g.,
maximum dye-stained depth and area of each concentration
category), as well as the measured soil physical properties based on
the average values of three replicates. Significant differences
between means were detected using the least significant difference (LSD) at P < 0.05. All statistical procedures were performed in
SPSS 17.0.
3. Results
3.1. Soil physical properties
Soil physical properties that could determine soil water flow
and distribution were more affected by RPFF compared with that
by NRPFF from two perspectives (Table 1). First, the emergence of
RPFF in the same soil layer significantly changed soil physical
properties at different depths. Under RPFF, the initial soil water
content (%) in 5–10 and 15–20 cm columns, bulk density (g cm3)
in 10–20 cm column, and saturated hydraulic conductivity
(103 cm s1) in 5–10 cm column were significantly decreased,
whereas total porosity (%) in 10–20 cm column significantly
induced compared with those under NRPFF. Second, as observed
from the same soil section, the RPFF in the soil resulted in physical
properties (except saturated hydraulic conductivity) sharing the
same change tendency from the soil surface to the 20.0 cm
column. For instance, the soil bulk density consistently increased
from 1.36 g cm3 at the soil surface to 1.66 g cm3 at 20.0 cm
depth in the column, whereas the total porosity (%) consistently
decreased from 48.55% on the soil surface to 37.11% in 20.0 cm
column in the RPFF plots. In summary, the initial gravimetric
water content, bulk density, total porosity in 0–20 cm was
significantly different between RPFF and NRPFF treatment. Higher
CV (coefficient of variation) indicated that these parameters
largely varied under RPFF treatment because the residual plastic
film fragments were randomly distributed in the soil.
103
3.2. Flow distribution patterns
The water flow paths and distributions were directly interpreted from the six classified vertical dye-stained patterns of
30.0 cm (length) 25.0 cm (depth) (Fig. 2). The corresponding
stain characteristics (e.g., maximum dye stained depths, widths,
and areas) are summarized in Table 2. The dark blue areas, light
blue areas, and green areas were heavily, slightly, and very slightly
stained with Brilliant Blue FCF dye, respectively.
The appearance of preferential flow was induced by RPFF in the
plough layer; thus, the number, distribution characters and
connectivity of preferential flow paths were directly illustrated
from the three vertical soil sections. In a similar soil section of RPFF,
the dye stained patterns had more or less difference between any
two tests. The dye was less uniformly distributed, as evidenced by
an unstained fraction within the staining area. Moreover, the dark
blue regions were broken up into separated and isolated patches.
The RPFF in the soil appeared randomly and destroyed the soil
structure. As a water-resistant layer, the RPFF in the soil would
obstruct the initial orientation of water flow; simultaneously, if the
RPFF in the soil served as an inclined hydraulically restrictive layer,
it would induce the appearance of lateral flow. This random
characteristic caused the water to either bypass the water-resistant
layer or follow the inclined hydraulically restrictive layer. These
basic postulated flow phenomena caused the matching area of the
flow distribution region and maize root (densely rooted) zone to
shrink abruptly. After removing the RPFF, the soil structure was
restored to its loose and macropore-rich characteristic. Therefore,
matrix flow was the dominant flow type in the NRPFF plot.
Furthermore, the matching degree of the flow distribution region
and the maize root (densely rooted) zone was improved.
The maximum dye-stained depths between RPFF and NRPFF
were not significantly different. However, maximum dye-stained
widths significantly differed between RPFF and NRPFF treatments;
in particular, the entire stained area was significantly less in RPFFtreated (271.54 cm2) than that in NRPFF-treated soil (368.15 cm2)
(Table 2). The removal of the residual plastic film not only
significantly decreased the green and light blue areas but also
significantly increased the dark blue area. For instance, the light
blue area decreased by 24%, and the dark blue area increased by
101% under NRPFF compared with those under RPFF treatment.
Maize root distribution range increased from the maize densely
rooted zone to the maize root zone along with maize growth;
Table 1
Soil physical properties (mean SE, n = 3) of the two sites.
Soil depth (cm)
Treatments
Initial gravimetric water content
Bulk density
Value (%)
CV (%)
Value (g cm3)
CV (%)
Total porosity
Value (%)
CV (%)
Saturated hydraulic conductivity
Value (103 cm s1)
CV (%)
0–5
RPFF
NRPFF
(7.96 0.08)a
(8.03 0.01)a
1.84
0.26
(1.41 0.06)a
(1.36 0.01)a
7.79
1.12
(46.67 1.25)a
(48.55 0.33)a
4.63
1.19
(4.20 0.07)a
(4.33 0.04)a
3.06
1.77
5–10
RPFF
NRPFF
(8.51 0.07)b
(8.75 0.01)a
1.35
0.23
(1.37 0.06)a
(1.41 0.01)a
6.96
1.78
(48.30 2.08)a
(46.67 0.55)a
7.45
2.03
(3.97 0.08)b
(4.27 0.03)a
3.53
1.07
10–15
RPFF
NRPFF
(8.99 0.07)a
(9.15 0.02)a
1.41
0.29
(1.42 0.05)b
(1.60 0.01)a
6.68
0.95
(46.29 2.07)a
(39.50 0.33)b
7.75
1.46
(3.53 0.14)a
(3.53 0.05)a
6.99
2.32
15–20
RPFF
NRPFF
(7.43 0.07)b
(9.62 0.01)a
1.63
0.10
(1.45 0.05)b
(1.66 0.01)a
6.23
0.92
(45.41 1.96)a
(37.11 0.33)b
7.49
1.55
(3.11 0.05)a
(3.25 0.03)a
3.05
1.70
0–20
RPFF
NRPFF
(8.23 0.07)b
(8.89 0.01)a
1.55
0.26
(1.41 0.03)b
(1.51 0.00)a
3.12
0.34
(46.67 0.73)a
(42.96 0.11)b
2.71
0.43
(3.70 0.05)a
(3.84 0.02)a
2.49
1.01
CV, coefficient of variation.
RPFF, residual plastic film fragments. NRPFF, no residual plastic film fragments.
Small letters a and b within a column of the same depth indicate significant difference at the 0.05 level.
104
X.J. Jiang et al. / Soil & Tillage Research 166 (2017) 100–107
Fig. 2. Flow distribution patterns in the vertical soil sections of plough layer as visualized by Brilliant Blue FCF dye-tracing under two different treatments 24.0 h after the
infiltration of dye tracer. RPFF, residual plastic film fragments; NRPFF, no residual plastic film fragments. Three profile sections in each sample plot are shown by tests 1, 2, and
3. As water infiltrated the soil, flow paths were stained with different concentrations of Brilliant Blue FCF dye. The dark blue areas, light blue areas, and green areas were
heavily, slightly, and very slightly stained with Brilliant Blue FCF dye; the associated concentration was >2.0, 0.5–2.0, and 0.05–0.5 g L1, respectively. The maize root (and
densely rooted) zone was encircled in each of the six pictures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of
this article.)
Table 2
Dye stain characteristics (mean SE, n = 3) in the classified vertical dye patterns for the two sample plots.
Treatments
LBA
(cm2)
DBA
(cm2)
GA
(cm2)
ESA
(cm2)
Area of LBA in
MRZ
(cm2)
Area of LBA in
MDRZ
(cm2)
Area of DBA in
MRZ
(cm2)
Area of DBA in
MDRZ
(cm2)
MDSD
(cm)
MDSW
(cm)
RPFF
(94.38 1.09)
a
(76.28 2.45)
b
(120.32 2.97)
b
(241.95 1.28)
a
(56.84 2.32)
a
(49.93 1.33)
b
(271.54 2.13)
b
(368.15 4.59)
a
(78.05 0.90)
a
(35.85 1.15)
b
(43.13 0.50)a
(115.51 2.86)
b
(229.85 1.22)
a
(56.61 3.11)a
(18.89 0.45)
a
(18.19 0.26)
a
(21.28 0.40)
b
(26.80 0.11)
a
NRPFF
(3.34 0.70)b
(58.66 0.26)a
RPFF, residual plastic film fragments. NRPFF, no residual plastic film fragments.
Small letters a, b, and c within a column indicate a significant difference at the 0.05 level.
LBA, light blue area; DBA, dark blue area; GA, green area; ESA, entire stained area; MRZ, maize root zone; MDRZ, maize densely rooted zone; MDSD, maximum dye-stained
depth; MDSW, maximum dye-stained width.
therefore, the matching degree of water distribution range and
maize root distribution region was important for maize growth and
development. The light blue area in the maize root and the densely
rooted zone decreased by 54% and 92%, respectively, whereas the
dark blue area increased by 99% and 4%, respectively, under NRPFF
compared with those under RPFF.
3.3. Dynamic behavior of water transport
Dye tracer in the field could track water distribution in the soil.
Experiments on water infiltration and outflow of soil columns were
conducted on each column in the laboratory to characterize the
dynamic behavior of water transport. Fig. 3 shows the common
features between NRPFF and RPFF involving infiltration and
outflow trend in soil columns. First, infiltration rates decreased
rapidly as classical theory predicted. Second, water started to drain
from the columns soon after the start of infiltration (16–20 min in
NRPFF and 22–34 min in RPFF). The sharply changing stages of
infiltration and outflow indicated that the water flow along a
preferential path, e.g., macropore in NRPFF and fragments in RPFF.
The steady state of infiltration and outflow implied that the water
exchanged between the preferential path and the soil matrix
arrived at a stable or equilibrium condition when the soil columns
were saturated by water. Third, the outflow rate from the column
remained smaller than the measured infiltration rate most of the
time during the measurement period. Furthermore, the outflow
rates measured from the columns in NRPFF and RPFF increased
slowly with time and exhibited fluctuations before the emergence
of equilibrium flow. The fluctuation indicated that the exchange of
water between macropore and soil matrix, slowly faded when the
equilibrium flow arrived.
Several differences between NRPFF and RPFF were also
observed from the infiltration and outflow rate in the columns.
The fluctuations before the steady state under NRPFF exhibited a
smaller range and shorter time compared with RPFF (Table 3). In
addition, the time of outflow in 0–5, 5–10, 10–15, and 15–20 cm soil
X.J. Jiang et al. / Soil & Tillage Research 166 (2017) 100–107
105
14
Rate (mm 炙 min-1)
12
RPFF
0--5 cm
RPFF
15--20 cm
Infiltration
Outflow
Infiltration
Outflow
Infiltration
Outflow
8
RPFF
10--15 cm
RPFF
5--10 cm
10
Infiltration
Outflow
6
4
2
0
0
20
40
60
80
100
120
140
160
180 0
20
40
60
Time (min)
80
100
120
140
160
180 0
20
40
60
Time (min)
80
100
120
140 160
180 0
20
40
60
80
100
120
140 160
180
Time (min)
Time (min)
14
NRPFF
0--5 cm
Rate (mm 炙 min-1)
12
NRPFF
10--15 cm
NRPFF
5--10 cm
NRPFF
15--20 cm
10
Infiltration
Outflow
8
Infiltration
Outflow
Infiltration
Outflow
Infiltration
Outflow
6
4
2
0
0
20
40
60
80
100
120 0
20
40
60
80
100
0
120
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min)
Time (min)
Time (min)
Time (min)
Fig. 3. Water infiltration and outflow measured during the column experiments from the two treatment plots; the value was averaged by three replicate columns. RPFF,
residual plastic film fragments; NRPFF, no residual plastic film fragments. Infiltration and outflow rate of water were measured from falling head (height = 50 mm) and
constant head (height = 50 mm) permeameter measurement.
Table 3
Time of outflow and equilibrium flow in different soil columns for the two treatments.
Treatments 0–5 cm
RPFF
NRPFF
5–10 cm
10–15 cm
15–20 cm
Time of
outflow (min)
Time of equilibrium
flow (min)
Time of
outflow (min)
Time of equilibrium
flow (min)
Time of
outflow (min)
Time of equilibrium
flow (min)
Time of
outflow (min)
Time of equilibrium
flow (min)
22
18
142
74
26
16
148
74
30
16
148
76
34
20
152
84
RPFF, residual plastic film fragments. NRPFF, no residual plastic film fragments.
column under NRPFF decreased by 18%, 38%, 47%, and 41%,
respectively, relative to that under RPFF (Table 3). After clearing the
RPFF, the time of equilibrium in 0–5, 5–10, 10–15, and 15–20 cm
soil columns decreased by 48%, 50%, 49%, and 45%, respectively
(Table 3).
4. Discussion
This study implied that the presence of RPFF in soil caused
differing soil physical properties and water flow phenomena,
which confirmed the results of previous reports from other places
of China. Although different places exhibited different soil texture,
the effect of residual plastic film on soil shared some common
features. As early as 1992, in research conducted in Changji City of
Xinjiang, China, Wen et al. (1992) found that residual plastic film
mulch affected soil physical properties by reducing soil bulk
density and increasing soil porosity. Results from potted plant
simulations and field experiments in Shanxi province also showed
that residual plastic film fragments not only increased soil bulk
density but also reduced the velocity of soil water flow (Xie et al.,
2007). A result from Inner Mongolia (Li et al., 2013b) reported that
RPFF changed the path of water transport, reduced the rate of
infiltration, and increased both transport velocity and distance of
wetted volume. Besides, in Qinyang City of Gansu, China, Zhang
et al. (2014) reported that the gravitational movement of water was
blocked when soil pore continuity was altered or cut off by the
residual plastic film fragments. In the present study, the residual
plastic film fragments in the soil enlarged the CV of soil physical
properties (gravimetric water content, soil bulk density, total
porosity, and saturated hydraulic conductivity) and induced the
emergence of heterogeneous water infiltration in the soil column.
More important, the residual plastic film fragments not only
reduced the dye-stained region but also remarkably decreased the
matching degree of water distribution region and maize root
(densely rooted) zone. The unevenly distributed dye-stained
patterns in the present study were equivalent to the wetted
volume in other studies. We noticed that the distribution of the
wetted volume in the vertical directions was much larger in the
NRPFF treatments than in RPFF treatments. The wetted front was
uneven and irregular, and the size of the wetted volume was
reduced because some water bypassed the border of the RPFF and
redistributed in the soil; the extent of unevenness and irregularity
increased along with the amount of RPFF. In addition, soil water
content was affected by the water distribution, namely the vertical
downward flow which was affected by the residual plastic film
caused water to spread unevenly across the vertical soil section.
Such flow characteristics became more complicated, although the
location of RPFF and the contact extent of RPFF and soil were
difficult to confirm. It is worth noting that the water unevenly
distributed in the wetted volume and water content exhibited
different values. This result disagreed with the finding of Li et al.
(2013b). Their results showed that there was no difference of water
content inside the wetted volume. Thus we concluded that the
greater the amount of residual plastic film fragments, the lesser
was the rate and amount of infiltration into soil. Such predicament
would be worse for crops with fibrous root systems. This finding
implied that a preferred irrigation strategy should consider root
development status and distribution characteristics, i.e., taproots
106
X.J. Jiang et al. / Soil & Tillage Research 166 (2017) 100–107
or fibrous roots. The taproot system of cotton can reach deep soil
layers to absorb water and nutrients by penetrating or bypassing
the residual film plastic mulch fragments. However, the fibrous
roots of wheat and corn could only be distributed to shallow parts
of the soil where residual film plastic mulch fragments are left. The
production and the planted survival rate were much lower for
undeveloped root species than for developed root species when
field soil was contaminated with RPFF, although they belonged to
one species of cotton. The water exchange between the preferential
path and the soil matrix was an important water flow phenomenon
in soil (Cey and Rudolph, 2009). In the present study, water was
supplied by a constant head permeameter. As long as water
infiltrated into soil matrix (matrix flow) and part of water migrated
in the preferential flow path (macropore flow), there would be
water exchange between the preferential path and the soil matrix.
The outflow water was the water that was transported in
preferential flow paths while bypassing the soil matrix. Some
water infiltrated from those preferential paths into the soil matrix
before the soil column was saturated. Thus, the outflow rate
remained smaller than the infiltration rate during the measurement period. Water exchange would reach an equilibrium
condition when the soil column was saturated, thus the outflow
rate equal to the infiltrated rate. Besides, we speculated that
different preferential paths (macropores in NRPFF and residual
plastic film pieces in RPFF) were responsible for the temporal
fluctuations of water infiltration and outflow. Water transport in
soil under RPFF exhibited the more dominant temporally
heterogeneous flow behavior, which could affect the exchange
of water and nutrient elements between preferential flow path and
soil matrix. For instance, RPFF destroyed the path for migrating
water and nutrients and reduced the water and nutrients use
efficiency, ultimately affecting the crop growth. One problem that
involves the methodology used in the study is that the process of
cleaning the plastic fragments in NRPFF could disturb the soil
sample and create some voids within the soil profile and further
increase the void ratio and macropores in the soil, however this
negative effect was restrained to the lowest degree by some
agriculture practices (e.g., irrigation, plowing, sowing). Another
insufficiency was that the cutting ring was not big enough to obtain
the precise mass and volume of plastic fragments, so that the mass
and volume of plastic fragments was not measured. Our future
work will focus on the relationship between soil physicochemical
properties (total nitrogen, available P, microbial biomass carbon,
etc.) and specified plastic existence percentage.
Soil physical conditions could indirectly influence the biochemical properties of the soil. For instance, Dong et al. (2013) found
that poor ambient conditions reduced the available phosphate and
nitrogen in the soil by 55% and 60%, respectively. Poor physical and
biochemical soil properties had negative impact on crop growth;
Zhang et al. (2014) found that the yields of wheat, corn, and cotton
were heavily reduced by 1–22%, 2–27%, and 1–8%, respectively.
More seriously, more than 1 million ton of plastic film was still
used to promote agricultural production in arid and semiarid zones
in China each year, and larger parts remained in fields as fragments.
These residual mulching plastic films fragments will continue to
release phthalate esters into the soil environment (Chen et al.,
2013), and could persist in the soil for hundreds of years (up to
200–400 years). In summary, the residual mulching plastic films
fragments have negative effects on the soil which may result in
poor physical and biochemical soil properties. And this would
introduce harmful pollutants into the agroecosystem. One of the
ways to mitigate the effects of this problem is to incinerate the
fragments under controlled conditions. However, from an environmental and sustainable point of view, Gonzalez-Sanchez et al.
(2014) advised that mechanical recycling of agricultural plastic
film wastes would be a better solution. Simultaneously, he stated
that agricultural plastic waste could be used as matrix for
composite materials. Although such methods seem preferable,
recycling agricultural plastic film fragments from fields is
challenging, especially those from the arable layer. From the point
of view of plant productivity and disease management, plastic
mulches are critical tools for weed suppression, soil moisture
retention, and soil temperature control. The sustainability of
agricultural practices in the long run requires not only agricultural
but also economic and ecological benefits. Minqin Oasis located
adjacent to the Badanjilin Desert in the west and Tengger Desert in
the east, caused it to be an ecological fragile region. Related
agricultural measures (water saving irrigation, drip irrigation
project, etc.) on water resources should prefer comprehensive
agricultural management measures. Agricultural sustainable
development should prefer the whole development of agriculture,
economy, ecology, and society. Brodhagen et al. (2015) recently
proposed that biodegradable plastic mulch films are an exciting
option for sustainable development if a perfect collaboration
among polymer engineers, microbiologists, agricultural scientists,
toxicologists, and soil ecologists could be reached.
In general, quantitatively and qualitatively analyzing soil
physical properties, water distribution and transport in soil
columns demonstrated the importance of clearing RPFF for soil
quality. Notwithstanding that the way RPFF affects soil biochemical
properties and agricultural production requires future investigations, the present study has shed light on the effect of residual
plastic film fragments on soil physical properties, water distribution and transport. The findings from the present study have the
following implications for soil quality and agricultural management: 1) To ensure crop growth and development, soil quality
must be optimized to favorable physical and biochemical properties. Any impurities and foreign materials that would disturb water
flow and mass transport in soil must be cleared out. 2) The
characeristic of root systems should be associated with the
distribution of water and nutrient elements. Taproot systems
can reach deeper soil layers while fibrous root systems only reach
shallower soil layers. The best strategy of water irrigation, is to use
little amount of water that could saturate the crop root zone, rather
than using plenty of water which may infiltrate into deeper soil
and become unavailable for plants. 3) The favorable exchange of
water and nutrient elements between preferential flow path and
soil matrix could promote the use efficiency of water and nutrients.
5. Conclusion
This study demonstrated that the presence of RPFF in the soil
significantly changed the soil physical properties, altered soil water
distribution, caused heterogeneous water infiltration, and remarkably reduced the matching degree of flow distribution region and
the maize root zone.
Acknowledgements
We highly appreciate the valuable comments from two
anonymous reviewers which were very constructive for the
improvement of the manuscript. This work was supported by
the National Nature Science Foundation of China (31570622),
Nature Science Foundation of Yunnan Province (2013FA022 and
2014HB042).
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