Non-sustainable agricultural practices can alter the quality of soil and water. A sustainable soil management requires detailed understanding of how tillage affects soil quality, erosion, and leaching processes. Agricultural soils in the Haean catchment (South Korea) are susceptible to erosion by water during the monsoon. For years, erosion-induced losses have been compensated by spreading allochthonous sandy material on the fields. These anthropogenically modified soils are used for vegetable production, and crops are cultivated in ridges using plastic mulches. To evaluate whether the current practice of ridge cultivation is sustainable with regard to soil quality and soil and water conservation, we (i) analysed soil properties of topsoils and (ii) carried out dye tracer experiments. Our results show that the sandy topsoils have a very low soil organic matter content and a poor structure and lack soil burrowers. The artificial layering induced by spreading sandy material supported lateral downhill water flow. Ridge tillage and plastic mulching strongly increased surface runoff and soil erosion. We conclude that for this region a comprehensive management plan, which aims at long-term sustainable agriculture by protecting topsoils, increasing soil organic matter, and minimizing runoff and soil erosion, is mandatory for the future.
Ecosystem services and agriculture are closely related and affect each other. On the one hand, ecosystems used for agriculture produce food, reduce hunger, and improve public health—services that become more and more important in view of a growing world population. On the other hand, agricultural mismanagement can reduce the ability of ecosystems to provide these goods and services [
Soils play a key role in providing supporting and regulating services such as soil fertility, soil retention, nutrient cycling, and carbon sequestration [
Agricultural production in South Korea faces an enormous pressure due to its limited arable land of about 22% of the total area [
To determine the pathways of agricultural pollutants and sediments, we have to identify the impact of agricultural practices on water flow on the soil surface as well as in the soils. In general, there are quite different flow phenomena in the soils. Water can percolate slowly through the soil matrix (uniform flow) or it can move rapidly through preferred pathways and bypass a fraction of the porous soil matrix. This preferential flow can occur in root channels, earthworm burrows, fissures or cracks (macropore flow), or along textural boundaries (funnel flow) [
In South Korea, the distribution of allochthonous soil material on dryland fields has become a widespread method to compensate the erosion-induced soil loss. Although this method has recently been prohibited by the government, the artificial layering induced by this practice still persists in the soil profiles. To our knowledge, the influence of this management practice on the water flow in soils has never been investigated. On most of these anthropogenically modified dryland fields, crops are cultivated in ridge tillage systems using plastic mulch. Ridge tillage and plastic mulching were found to have positive effects on crop yield and weed control [
The aim of our study is to evaluate whether the current practice of ridge cultivation in the Haean catchment in South Korea is sustainable. We focus on soil quality and soil and water conservation. Therefore, we (i) analysed the soil properties of the anthropogenically modified topsoils from various dryland fields distributed over the Haean catchment. Additionally, to (ii) qualitatively describe the effects of tillage on flow patterns and to (iii) quantify surface runoff and soil erosion, we carried out four dye tracer experiments under flat conventional tillage, ridge tillage, ridge tillage with plastic mulch, and ridge tillage with plastic mulch cropped with potato plants.
The Haean-myun catchment (128°1′33.101′′E, 38°28′6.231′′N), also called “the Punchbowl,” is located in the mountainous northeastern part of South Korea and has a total area of approximately 64 km2. The characteristic bowl-shaped topography subdivides the catchment into three major land use zones. The steep hillslopes are mostly forested (58%), moderate slopes at the forest edges are dominated by dryland farming (22%), and rice paddies (8%) are characteristic for the flat central area of the catchment. The remainder is occupied by residential areas, grassland, field margins, and farm roads.
The annual precipitation in the Haean catchment equals 1599 mm (13-year average from 1999 to 2011) with more than 60% of the annual rainfall occurring during the monsoon season from June to August. The annual temperature averages 8.5°, ranging from −6.8° in January to 21.5° in July (13-year monthly averages from 1999 to 2011).
The bedrock in the catchment is made up mainly of granite which is strongly weathered due to the high precipitation rates. It constitutes the parent material for Cambisols—the most widely spread soil type in the study area.
The dominant agricultural practice for row crops on Korean dryland fields is ridge tillage with polyethylene covers (plastic mulch) (Figure SF4 in the Supplementary Material available online at
At the beginning of the growing season, between April and May (depending on the crop type), a granulated mineral fertiliser is applied, fields are ploughed, and the fertiliser is mixed into the top soil. Subsequently, ridges (approximately 15 cm high and 30 cm wide) are created primarily perpendicular to the main slope direction with approximately 70 cm spacing. The ridges are covered with black polyethylene sheets perforated with 25–30 cm spaced planting holes (5 cm diameter), while furrows between ridges remain uncovered. After ridges and furrows are created, depending on the crop type, seeds are sown or juvenile plants are planted into the planting holes. Several times during the growing season, herbicides and pesticides are applied, and mineral fertilisers are spread a second time on the fields, depending on the crop type. Finally, harvesting begins usually between August and September.
The main row crops cultivated on dryland fields are cabbage, radish, potato, and beans [
In 2009, we took samples of topsoils on 32 dryland fields and on 16 forest sites in the Haean catchment. The dryland fields included the four major crops cultivated in the catchment, namely, cabbage, radish, potato, and bean fields. At each agricultural site, five samples (from the four corners and the center of the field) were taken and mixed together. After sampling, soil texture, C, N and soil organic matter (SOM) were analysed in the laboratory.
In 2010, we carried out four irrigation experiments at two potato fields (
Soil physical properties of the experimental sites.
Horizon (WRB) | Deptha (cm) | Clay (%) | Silt (%) | Sand (%) | Soil texture class | Bulk density (g cm−3) | |
---|---|---|---|---|---|---|---|
Site 1 | Ap | 0–25 | 3.2 | 16.4 | 80.3 | Loamy sand | 1.43 |
2Apbb | 25–50 | 20.2 | 53.4 | 26.4 | Silt loam | 1.45 | |
Bwb | 50–100 | 24.8 | 46.6 | 28.6 | Loam | 1.38 | |
| |||||||
Site 2 | Ap1 | 0–35 | 1.9 | 14.5 | 83.6 | Loamy sand | 1.41 |
Ap2 | 35–45 | 8.1 | 28.9 | 63.0 | Sandy loam | 1.66 | |
Ap3 | 45–55 | 7.6 | 27.9 | 64.5 | Sandy loam | 1.61 | |
2Apb | 55–70 | 20.9 | 58.2 | 20.9 | Silt loam | 1.28 | |
2Bwb | 70–100 | 13.6 | 38.9 | 47.5 | Loam | 1.56 |
We carried out the first two experiments on field site 1 and the last two on field site 2. The first experiment (CT) took place after ploughing and before ridges were created, so that the soil surface was flat and represented conventional tillage. The second one (RT) was carried out after the creation of ridges. At field site 2, potato crops were planted in ridges covered with black plastic mulch, and we conducted the third experiment (RTpm) in the early season when the potatoes were just sown. Finally, the last irrigation experiment (
Before irrigation, we installed FDR soil moisture sensors (Decagon devices, Inc., Pullman, WA 99163, USA) to monitor the volumetric water content
We irrigated a surface of 2 m2 with a tracer solution containing 5 g L−1 of Brilliant Blue FCF using an automated sprinkler. Because this tracer can be retarded compared to the infiltrating water [
One day after the irrigation we excavated 8–10 soil profiles of
The soil profiles were sampled to determine soil physical properties. We carefully scraped soil material from different profiles and analysed the texture in a laser particle size analyzer (Mastersizer S “MAM 5044,” Malvern instruments GmbH, Herrenberg, Germany). Additionally, we took undisturbed samples with small soil core rings (diameter 2.8 cm, height 1 cm) in different horizons. They were weighed, dried for 24 hours at 105°C in a drying oven, and weighed again to calculate the bulk density.
We corrected the images for perspective and radial distortion such that they corresponded to pictures taken by an ideal camera looking perpendicularly onto the profiles. The transformation was calculated by
Subsequently, we transformed the images from RGB to HSI (hue, saturation, intensity) color space and classified the pixels into Brilliant Blue stained (black) and nonstained (white) ones to obtain binary images. The transformation is necessary because the HSI color space is more suitable for color-based segmentations of images taken under varying illumination. More details on image transformation and classification are given in Bogner et al. [
For the experiments RT, RTpm, and
Processing of images of dye-stained soil profiles: (a) rectified dye tracer image, (b) background image with the soil coded black and the background between ridges white, and (c) final binary image used to calculate image indices with dye-stained pixels coded black and nonstained ones coded white.
We used the binary images to assess differences between the tillage management systems. The first two experiments (CT and RT) show the influence of soil surface topography on flow patterns in general. By comparing the experiments RT and RTpm, we can infer the effect of plastic mulch. Finally, we can extract information about the impact of the potato canopy and root system on flow patterns by comparing the images of RTpm with those of
To effectively analyse the flow patterns in binary images, we calculated image index functions or simply indices. An index is a real-valued function of a binary vector
The
The distribution of run lengths can be summarized in a robust manner by their 5%, 50%, and 95% quantiles. In our experiments, however, we only used the 95% quantile, that we call the
The indeterminate case where there are no stained pixels in a row and
Last but not least, we want to evaluate the information contained in an image row
To apply (
More detailed structures can be captured by considering words of length
Special care should be taken when calculating image index functions for soils with an uneven soil surface. Actually, pixels not belonging to the soil should be excluded from the analysis. Therefore, to differentiate between soil and nonsoil on the ridged surface of RT, RTpm, and
The analysis of the topsoils revealed large differences between soils in the forest and on dryland fields (Table
The average topsoil properties of 32 dryland fields and 16 forest sites in the Haean catchment.
Parameter | Dryland sites | Forest sites | ||
---|---|---|---|---|
Mean (%) | Std. dev. (%) | Mean (%) | Std. dev. (%) | |
N | 0.06 | 0.02 | 0.41 | 0.17 |
C | 0.53 | 0.30 | 5.77 | 2.47 |
SOMa | 0.98 | 0.53 | 9.92 | 4.25 |
Clay | 5.43 | 2.98 | 12.27 | 4.87 |
Silt | 22.32 | 7.10 | 38.81 | 9.79 |
Sand | 72.26 | 9.97 | 48.91 | 14.01 |
Total amount of irrigation, infiltration, surface runoff, sediment load, and erosion during the dye tracer experiments.
Experiment | Total amount of irrigated water (L) | Infiltration | Runoff | Sediment load (g) | Erosion (g m−2) | ||
---|---|---|---|---|---|---|---|
(L) | (%) | (L) | (%) | ||||
CTa | 87 | 69 | 79 | 18 | 21 | 151.41 | 75.71 |
RTb | 74 | 46 | 62 | 28 | 38 | 66.59 | 33.30 |
|
81 | 41 | 50 | 41 | 50 | 322.45 | 161.23 |
|
91 | 63 | 69 | 28 | 31 | 54.02 | 27.01 |
These changes are also evident when comparing topsoil and subsoil properties of the experimental sites (Table
We observed the largest infiltration and the smallest runoff on CT (Table
At the beginning of the dye tracer experiment on CT the water content in 5 cm depth was lower compared to 20 cm depth (Figure
The dynamics of water content in different depths during the dye tracer experiments CT, RT, and RTpm. The grey area indicates the time of irrigation.
On plots RT and RTpm we found larger water contents in furrows at the beginning of irrigation. This was probably caused by previously preferentially infiltrated water due to topography effects. In 5 cm depth on RT the water content rose first in furrows, since the runoff from the ridges accumulated there and then in ridges. It increased only slightly in 20 cm depth.
During the irrigation on RTpm the dynamics of water content was comparable to RT except on ridges that were covered with plastic mulch. There, it increased slightly only in 20 cm depth probably due to water which infiltrated primarily in the furrows and was subsequently funnelled laterally to the ridges.
The dye tracer experiments revealed that firstly, tillage produced zones of preferential infiltration, namely, furrows and planting holes, and zones of no infiltration, namely, plastic mulched ridges (Figure
Example images of excavated soil profiles and their binary images. From top to bottom: CT, RT, RTpm, and
Secondly, the layer boundary between the spread topsoil and the subsoil was the most important feature for water movement in these agricultural soils. This was clearly shown by the decrease of all indices to zero in approximately 25–35 cm depth (i.e., between the horizons Ap and Bwb on site 1 and between the horizons Ap1 and Ap2 on site 2, resp.) (Figure
Image index functions and their 25% and 75% quantiles (colored areas): dye coverage
Thirdly, the shape of the index curves showed that in our experiments water flow occurred in the topsoil and was funnelled preferentially above the layer boundary. Actually, the vertical propagation to the deeper soil horizons via macropores, fissures, and cracks was absent. This was also confirmed by comparing the Brilliant Blue stained patterns to the iodide patterns. The propagation of the iodide tracer solution was similar to that of Brilliant Blue FCF.
The effect of the
The effect of the
The effect of the
Ridge tillage and plastic mulching created typical infiltration zones (i.e., furrows and planting holes) and noninfiltration zones (i.e., plastic covered ridges) resulting in soil moisture differences between furrows and ridges. Our results agree well with Saffigna et al. [
Additionally, we could show that ridge tillage and plastic mulching increased surface runoff and soil erosion substantially in the early growing season. The largest surface runoff and soil erosion under RTpm were confirmed by Arnhold et al. [
We want to highlight the former practice (that still takes place today occasionally) to distribute sandy soil material on agricultural fields prior to planting and the subsequent ploughing. Indeed, the distribution of sandy soil material to counterbalance erosion loss in the Haean catchment strongly influences the flow processes. This management practice created an artificial layering with different soil physical properties. A cohesive, denser, and finer textured subsoil is overlain by a topsoil consisting of a noncohesive and coarse material. As a result, an important textural boundary is created between the horizons. These structural differences between the topsoil and the subsoil are responsible for the funnel flow above the layer boundary. This was also reported by Petersen et al. [
Several authors reported that fissures, cracks, and earthworm burrows could act as preferential flow paths especially in fine textured subsoils [
Our results demonstrate that the risk of a vertical propagation of agrochemicals to groundwater is generally relatively low because of lack of macropores in the sandy soils. However, the lateral downhill water flow above the layer boundary between the anthropogenically modified topsoil and the subsoil seems to be ecologically relevant. Especially during the East Asian summer monsoon, when rainfall can reach more than 100 mm per day [
We have shown that surface runoff, soil erosion, and subsurface water flow are reduced in the adult stage of the crop development. This also means that the leaching and erosion risk are especially high at the beginning of the growing season when the plants are juvenile and the fertilisers are recently applied. This is supported by Kettering et al. [
However, in our experiments, the runoff still constituted one-third of the total irrigation even in the later season, when the crop canopy was well developed. We assume that the widespread usage of plastic mulching in combination with heavy monsoon events is partly responsible for higher phosphorus leaching in the Haean catchment because phosphorus is predominately transported via surface runoff. Kim et al. [
There are several options to reduce the risk of surface runoff and soil erosion. Arnhold et al. [
Finally, soil management in terms of a sustainable agriculture should improve the soil quality. This includes, for example, the enhancement of soil fertility and soil structure, soil carbon sequestration and the support of soil biota and bioturbation [
Hence, agricultural practices in the Haean catchment should aim now at long-term sustainable improvement of the degraded soils. This can be achieved by soil amendments like, for example, crop residues [
A sustainable agriculture aims at both, increasing crop yield and minimizing the impact on natural resources. Therefore, to improve or at least to maintain the quality of water and soil is of great importance. When focusing on soil and water conservation, agricultural management practices should minimize runoff, control erosion and reduce nonpoint source pollution [
In our study we found that the impact of ridge cultivation with or without plastic mulch on the subsurface water flow is relatively low compared to the increase in surface runoff. Therefore, to reduce it, we suggest (i) to encourage crop production in ridge cultivation with perforated biodegradable plastic mulch and (ii) to enhance infiltration in furrows by either minimizing herbicide input into furrows or by protecting the furrows with crop residues. These practices will diminish the risk of erosion and leaching of agrochemicals, especially in the early season when crops are juvenile. Perforated plastic mulch can still maintain a positive effect on crop yield by increasing the temperature in the root zone and by weed control.
Furthermore, a particular attention should be paid to the risk of lateral downhill leaching of agrochemicals and fertilisers induced by artificial layering, especially on field sites located directly next to the stream network. Thus, we propose (iii) to promote the establishment of riparian buffer zones between dryland farming fields and the rivers.
Moreover, a sustainable long-term development of fertile topsoils by protection from erosion and return of organic material is necessary. Therefore, we suggest further research (iv) to identify the potential of soil amendments such as biochar to improve the soil carbon stock in sandy soils under monsoonal conditions and (v) to identify appropriate species of winter cover crops that could increase the SOM content of topsoils and protect the bare soil from erosion and leaching after the harvest in the autumn and winter season.
The aurhors are grateful to Professor Baltasar Trancón y Widemann for technical assistance and intensive discussions. They would like to thank Andreas Kolb for his invaluable technical support and Bora Lee and Heera Lee for translation and negotiating permissions for their experiments. Furthermore the authors would like to thank all TERRECO members, who helped them with soil sampling and Professor Ok and his laboratory assistants for analysing the topsoil samples. This study was carried out as part of the International Research Training Group TERRECO (GRK 1565/1) funded by the Deutsche Forschungsgemeinschaft (DFG) at the University of Bayreuth, Germany, and the Korean Research Foundation (KRF) at Kangwon National University, Chuncheon, South Korea.