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Soil erosion in Nordic countries – future challengesand research needsBarbro Ulén a , Marianne Bechmann b , Lillian Øygarden b & Katarina Kyllmar aa Department of Soil and Environment, Swedish University of Agricultural Sciences,Uppsala, Swedenb Norwegian Institute for Agricultural and Environmental Research – Bioforsk, Ås, NorwayVersion of record first published: 12 Nov 2012.

To cite this article: Barbro Ulén, Marianne Bechmann, Lillian Øygarden & Katarina Kyllmar (2012): Soil erosion in Nordiccountries – future challenges and research needs, Acta Agriculturae Scandinavica, Section B - Soil & Plant Science,62:sup2, 176-184

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SOIL EROSION IN THE NORDIC COUNTRIES

Soil erosion in Nordic countries � future challenges and research needs

BARBRO ULEN1, MARIANNE BECHMANN2, LILLIAN ØYGARDEN2 &

KATARINA KYLLMAR1

1Department of Soil and Environment, Swedish University of Agricultural Sciences, Uppsala, Sweden, 2Norwegian Institute

for Agricultural and Environmental Research � Bioforsk, As, Norway

In Europe, water, wind and tillage erosion are

common threats to soil quality (Verheijen et al.,

2012). In the Nordic countries the main problem of

erosion is generally that phosphorus (P), pesticides

and other water pollutants attached to eroded

material are lost to recipient waters.

Erosion is monitored from plots, fields, and in

streams and rivers (Boardman & Poensen, 2006).

The amount of soil erosion measured from small

agricultural catchments dominated by silty � clay

soils in Norway and Sweden varied largely in space

(Table I). The range measured at the outlet of the

small streams varied between 0.01 and 3.67 t ha�1

yr�1. In Lithuania, the combined variation in space,

time and topography is huge and soil losses from

0.01 to 19 t ha�1 yr�1 have been measured with

surface runoff from downstream slopes (Kinderiene

& Karcauskiene, 2012). In general, the spatial

variation between different catchments is greater

(varying with a factor of more than hundred) than

the variation between years (varying with a factor of

10). The total P losses in Norway and Sweden varied

between 0.03 and 4.69 kg ha�1 yr�1 (Table I). In

the Swedish catchments, high P losses can be related

to factors such as clay and clay loam soils, medium to

high precipitation and large proportions of annual

crops in the catchments (Kyllmar et al., 2006). In

the Norwegian catchments, the P losses differ

depending on production system, soil types an water

flow pathways (Bechmann et al., 2008). In arable

production systems, erosion is the main P loss

process. At the plot scale, Skøien and Børresen

(2012) found that total P losses are closely correlated

to soil losses, whereas Ulen et al. (2012a) found that

particulate P concentrations can be reasonably well-

predicted from suspended sediment (SS) concentra-

tions above base flow in a Swedish catchment.

Land levelling has caused high erosion from arable

fields in Norway (Øygarden et al., 2006). Roughly

recalculated to arable land based on source appor-

tionment, Norwegian studies indicate that soil losses

from catchments with artificially levelled arable land

may be even higher than 2 t ha�1 yr�1 (Bogen et al.,

1993; Bechmann et al., 2008). At the field scale,

monitoring of soil losses in surface and subsurface

runoff from a land levelled arable field in Norway

showed variations from 0.09 to 3 t ha�1 yr�1

(Bechmann et al., 2011). At the plot scale, measure-

ments of soil losses showed even higher values, up to

7.6 t ha�1 yr�1 (Lundekvam, 2007; Skøien &

Børresen, 2012). The tolerable rate of erosion for

both agricultural production and water quality can

be set to approximately 1 t ha�1 yr�1 (Verheijen

et al., 2012). Standard potential erosion risk in

Norway is defined for areas with the traditional

autumn ploughing. Erosion risk below 500 kg ha�1

yr�1 is defined as low, from 500 to 2000 kg

ha�1yr�1 as medium, from 2000 to 8000 kg ha�1

yr�1 as high and above 8000 kg ha�1 yr�1 as very

high.

The erosion process

Soil and particle mobilization

One criterion for erosive transport is that soil

particles (or soil aggregates) are available for mobi-

lization. Soil physical processes, soil chemical prop-

erties and agricultural management are all important

Correspondence: B. Ulen. E-mail: barbro.ulen@slu.se

Acta Agriculturae Scandinavica Section B � Soil and Plant Science, 2012; 62: Supplement 2, 176�184

ISSN 0906-4710 print/ISSN 1651-1913 online # 2012 Taylor & Francis

http://dx.doi.org/10.1080/09064710.2012.712862

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for the availability for mobilization. Soil erodibility

has been defined as the degree to which external

forces acting on the soil exceed the internal forces

holding the soil together. Particle mobilization may

start as the dissolution of soil aggregates and con-

tinues as slaking of particles when a wetting front

advances horizontally or vertically through the soil,

followed by dispersion of the detached particles in

soil water or ponding water (Legout et al., 2005).

Particle detachment can also occur directly on the

soil surface by splash erosion under the action of

raindrops. The latter is usually of less importance

under Nordic conditions, since rain intensities are

relatively low.

Silt loam and clay loam soils are especially vulner-

able to water erosion, since the silt particles are

relatively small and have low resistance to both

cohesion and friction (Lundekvam & Skøien,

1998). In other words the attractive face-to-edge

forces are low, as are the forces counteracting the

relative motion of the surfaces between particles. For

marine clay soils it is also suggested that uneven

occurrence of ‘gyttja’ (matter of organic origin in

former marine or lake sediment) and oxidized iron

(rust) strengthens crack walls, making them perma-

nent pathways, especially in deeper soil layers (Ulen

et al., 2012b). However, organic matter in associa-

tion with clay and silt acts as a cementing agent and

soil organic matter promotes aggregate formation

and stability of bound aggregates. The improved

aggregate stability because of high organic matter

content may be the most important factor explaining

the differences in soil and P losses from Syverud

compared to the other Norwegian plot studies

described by Skøien and Børresen (2012). Roots

and hyphae enmesh and release the organic com-

pounds that act as a glue to hold particles together

(Bronick & Lal, 2005). Type and amount of organic

matter also influence the rate at which water is

absorbed by the soil and, depending on type, can

improve the soil’s resistance to stress caused by

wetting. Soil concentration of organic matter de-

creases for example in the county of Dalarna,

Sweden but yearly addition of organic matter in the

form of cut grass can improve aggregate stability and

reduce surface soil erosion of P (Ulen & Kalisky,

2005).

Aggregate stability is a measure of the soil’s

vulnerability to the destructive forces of water, which

include swelling/shrinkage of the soil and slaking

processes. Water ponding is particularly damaging,

since it promotes the collapse and dissolution of

soil aggregates (Hillel, 1980). Larger (�2�6 mm)

soil aggregates may break down into smaller micro-

aggregates (B0.25 mm) when they are immersed in

water (Bryan, 1977). A high proportion of water-

stable aggregates in the 2�6 mm fraction facilitate

water infiltration, which affords resistance to erosion.

Grønsten and Børresen (2009) measured aggregate

stability using a wet sieving method and found that

silty soils were ranked as more stable than when using

the standard raindrop impact method. Aggregate

Table I. Dominant soil texture classa, number of observed years (n), mean, minimum and maximum erosion of suspended sediment (SS)

and total phosphorus transport (TotP) in agricultural monitored streams in Norway (1992�2009) and Sweden (2004�2010) using flow-

proportional sampling based on Bechmann et al. (2008) and Kyllmar (2009).

SS (t ha�1 yr�1) TotP (kg ha�1 yr�1)

Catchment n Texture class Mean Min Max Mean Min Max

Norway

Hotran 16 Silty loam 1.63 0.06�3.67 2.34 0.54�4.69

Kolstad 18 Loam 0.11 0.03�0.53 0.34 0.03�1.70

Mørdre 18 Silt, silt clay loam 0.94 0.18�2.60 1.17 0.50�2.17

Skuterud 15 Silty clay loam 0.74 0.18�1.84 1.38 0.36�3.51

Time 8 Silty sand 0.10 0.05�0.15 1.29 0.87�1.86

Volbu 16 Silty sand 0.04 0.01�0.19 0.18 0.05�0.44

Sweden

C6 7 Clay, clay loam 0.34 0.10�0.81 0.46 0.18�0.92

E21 7 Sandy loam 0.01 0.01�0.02 0.10 0.05�0.24

F26 6 Sandy loam 0.15 0.07�0.34 0.61 0.33�0.95

I28 6 Sandy loam 0.03 0.01�0.07 0.30 0.11�0.61

M36 7 Clay, sandy loam 0.25 0.09�0.47 0.53 0.28�0.84

M42 5 Sandy loam, loam 0.07 0.02�0.19 0.42 0.16�0.66

N34 7 Sandy loam 0.12 0.07�0.18 0.42 0.23�0.70

O18 7 Clay, clay loam 1.75 0.31�3.35 1.72 0.51�3.13

Note: In Norway 0.45 mm and in Sweden filters with pore diameter 0.2 mm (Schleicher & Schull GmbH, Dassel, Germany) were used.aThe area close to the stream given first.

Soil erosion in Nordic countries 177

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stability was positively correlated with the soil organic

matter and Al-oxide content and negatively corre-

lated with the silt and very fine sand content (0.002�0.02 mm). The wet sieving method is less laborious

and more widely used elsewhere, but the lower

disruptive effect on silty soils should be kept in

mind when using this method. An aggregate stability

test based on Dexter (1988) has been used in rain

simulation laboratory experiments with soil columns

to evaluate the risk of losses of suspended solids and P

(Ulen & Etana, 2010). The same aggregate stability

test has also been used in plot experiments comparing

glyphosate and P leaching after structure liming and

reduced tillage (Ulen et al., 2012b). Structure liming

of the topsoil was demonstrated to be followed by

more stable aggregates and by decreased glyphosate

concentrations in drain water. Soil compaction and

repeated traffic are other factors influencing both

aggregate strength and infiltration capacity

(Amezketa, 1999).

Slaking occurs when soil aggregates are not strong

enough to withstand internal stresses (Jury et al.,

1991), e.g. from differential swelling of clay particles.

For clay soils the character of swelling/shrinking is in

turn influenced by clay mineralogy in combination

with the cation exchange capacity (CEC) of the soil.

Bivalent Ca2� and polyvalent Al3� and Fe3� ions

improve soil structure through cationic bridging with

clay particles and organic matter. Consequently,

aggregates containing high concentrations of such

ions are highly resistant to slaking (Bronick & Lal,

2005). In contrast, the presence of high concentra-

tions of Mg2� is suggested to result in disruption of

aggregates and increased clay dispersion (Zhang &

Norton, 2002). The extent of the negative effects of

Mg2� depends on the type of clay and electrolyte

concentration in the soil. Clay soil areas with high

soil Mg2� concentrations can be found for example

in the counties of Sodermanland and Ostergotland in

Sweden.

Particle detachment is a dynamic combination of

slaking and dispersion. Forces resulting in particle

dispersion result from changes in ionic strength in

the water and the soil solution (Curtin et al., 1994).

The pore medium chemistry is complex, with

different ions being present in the pore water, and

at different concentrations. Strong links between soil

pH and erodibility have been widely reported (e.g.

Cihacek & Swan, 1994). Free calcium carbonate,

which is more likely to be present above pH 7,

increases the concentration of Ca2� ions and may

inhibit dispersion (Greenland et al., 1975). In

addition, a relatively low pH may dissolve some

Ca-bound P from the clay particles (Devau et al.,

2011). A dispersion test (DESPRAL) of benchmark

soils has been used in relation to surface runoff rain

simulation tests to examine the intrinsic risk of

sediment and P mobilization (Withers et al., 2007).

This method has also been tested for some Swedish

soils (Villa et al., 2012) and the dispersion of

particles has been found to be related to the content

of clay, sand and organic matter. Another dispersion

test (soil suspension turbidity [SST]) has been found

to be related to the content of clay and sand (Villa

et al., 2012).

Surface transport of soil particles

Soil particles may be transported either by surface,

intermediate or vertical runoff and these processes

are locally of different importance in the Nordic

countries. Surface runoff as sheet erosion on the soil

surface may concentrate into rill erosion and gully

erosion (Lundekvam, 2007). In Nordic countries

much runoff occurs during autumn, when the soil

may be saturated after autumn rain and surface

runoff mainly occur as a result of saturation excess.

In addition, the snowmelt period can be most

important under Nordic winter conditions. How-

ever, erosion can be particularly severe in connection

with rain on partially thawed soil (Øygarden, 2000,

2003), when infiltration is restricted and fast water

flow can detach particles from the thawed soil

surface.

Mobilization and transport of eroded particles

depends on permanent soil properties such as soil

texture, mineralogy, organic matter, lime content,

composition of exchangeable cations and pH, and

also on temporary conditions prevailing in the soil,

such as wetting rate, antecedent moisture content

and ageing. Other important factors are presence of

a crop and, for silty loams and sandy loams, soil

crusting. Clay and attached P may be enriched in

eroded material compared with the surface soil from

which it is derived (Sharpley, 1980). This is com-

monly expressed as clay enrichment ration (CER)

and phosphorus enrichment ratio (PER). However,

soil particles may also be transported directly as

aggregates to recipient waters (Sveistrup et al.,

2008). In a similar way, the good efficiency of

constructed sediment ponds (Braskerud, 2001,

2003) and buffer zones (Syversen, 2005) has been

indirectly explained by sedimentation of eroded soil

aggregates.

Vertical transport of eroded particles through the soil

Subsurface runoff occurs more or less vertically and

further via tile drains out into recipient waters. It

may also occur as an interflow, e.g. on a plough pan

(Lundekvam, 2002). Vertical transport is of quanti-

tative importance for drained clay and silty clay soils,

178 B. Ulen et al.

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since surface water can seep quickly through cracks

and macropores (Figure 1), transporting particles via

tile drains to recipient waters.

Transport of eroded and dissolved pollutants

through the soil profile depends on the size and the

continuity of macropores. Tubular pores are often

continuous from the soil surface to tile drains (Jarvis,

2007), but such pores and cracks may be disrupted

by conventional tillage (Larsbo et al., 2009). Cracks

connecting to the drainage system are of significant

importance for erosion and pesticide transport out

via the drains (Jarvis, 2007). Øygarden et al. (1997)

found high particle transport via the drainage system

on clay soils with destroyed structure especially

drained to reduce surface runoff. In tracer studies,

they observed rapid transport through cracks and

macropores directly to drain pipes. Studies of soil

profiles above and between drain pipes revealed the

cracking structure and different pathways for particle

transport. Changing tillage practice from autumn

ploughing to stubble and grass efficiently reduced

particle transport. These results indicate that parti-

cles can be eroded from the plough layer and

transported both laterally and vertically, through

macropores and cracks, into backfill and directly to

drain pipes.

During the erosion process, small, light soil

particles are transported more easily than larger

particles (Sharpley, 1980). When eroded material

passes through a soil it may to some extent be filtered

(Ulen, 2004) and only the smallest particles flow

through, depending on the size of the pores. Such

filtering is important in soils with disturbed structure

(Seta & Karathanasis, 1997) and contributes to the

CER and PER as mentioned for surface transport.

A criterion for fast vertical transport of particles is

the presence of vertical transport pathways (cracks

and biopores) where non-equilibrium flow domi-

nates (e.g. Jensen et al., 1998; Stamm et al., 1998).

Rapid hydrological processes such as preferential

flow are not continuous in time, but occur as a

sequence of discrete episodic events. These are

triggered when hydrological thresholds are exceeded

(McGrath et al., 2007). Critical fast preferential flow

is initiated at the soil surface when the infiltration

capacity of the soil matrix is exceeded. At higher

rainfall intensities larger pores become water-filled

and active in conducting water. This leads to higher

effective pore water velocity and shorter solute travel

times (Edwards et al., 1992; Haws et al., 2004).

Consequently, rainfall intensity, preferential flow

and transport effects are strongly affected by the

antecedent soil moisture conditions. At field scale,

smaller hydrological source areas commonly exist,

critically contributing to erosion and pollutant losses

via drain tiles (Freitas et al., 2008) and changes in

preferential flow may differ between sites and

hydrological conditions (Shipitalo et al., 2000).

Depending on different soil tillage methods, sub-

stantial differences in surface erosion have been

recorded in Norway and Sweden, but for drainage

losses the relationship is less clear. There may even

be negative consequences for P losses via drains

when reduced tillage is applied on flat clay soil areas

(Ulen et al., 2012b).

The impact of climate change on soil erosion

The direct erosive force of rain is determined by

drop size, drop velocity and the intensity and

duration of the rain. The sites at which erosion

studies are taking place in Norway mainly experience

low intensity rainfall (Skøien & Børresen, 2012).

The typical maximum intensity is 15 mm h�1 (mean

drop size 0.5�2 mm) with highest recorded rainfall

intensities of up to 80 mm h�1. Eastern Sweden is

dominated by frequent early summer drought, but

repeated summer rainfall (40�50 mm per week in

June) is quite common and is followed by distinct

peaks in leaching losses of pesticides (Ulen et al.,

2012c). Repeated rainstorms of escalating intensity

only occur once every 10 years in summer in this

Figure 1. Macropore development in a silty clay soil. Large and

small dark spots represent concretions of Fe and Mn-enriched

oxides. The rounder and elongated white spots represent pores,

while the subangular ones represent light colourer minerals as

quarts and feldspars (Photo: Tore Sveistrup).

Soil erosion in Nordic countries 179

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region, but have been reported to cause severe

flooding. In contrast to rain, snowmelt is a more

gentle process. Significant snow accumulation oc-

curs in certain years and may account for the

majority of glyphosate and P leaching when melting

(Ulen et al., 2012b). A low pH in snowmelt water

combined with low electric conductivity influences

the hydrogen bonds of the illite mineral in clay and

may increase the concentrations of dissolved forms

of sorbed P and glyphosate in spring compared with

autumn (Ulen et al., 2012b).

Within the space of a year, there is the continuing

action of freezing�thawing and wetting�drying of the

soil. Phosphorus may thereby be solubilized from the

soil microbial biomass (Blackwell et al., 2010).

The depth to which freezing advances is dependent

upon moisture content, soil type and the freezing

index. In general, the greater the proportion of finer

particles (B0.02 mm), the greater is the suscept-

ibility to frost action.

Climate change may cause changes in the number

and duration of freeze�thaw cycles which has been

recognized to have a considerable impact on the

structure, and thus the geotechnical properties, of

soils. Kim and Daniel (1992) found that soils

compacted dry increased their hydraulic permeabil-

ity by a factor of only 2�6 by freezing, but soils

compacted wet increased their hydraulic permeabil-

ity by a factor of about 100. Studies by, for example,

Viklander and Eigenbrod (2000) and Qi et al. (2006)

showed that freeze�thaw cycles may influence the

particle size distribution by breaking the aggregates.

Correspondingly, Kværnø and Øygarden (2006)

showed that freeze�thaw cycles affect the aggregate

stability and erodibility of both clay and silt soils,

with the most severe effects on silts. They also

compared aggregate stability measured by the

Norwegian standard procedure of rainfall simulation

with a widely used wet sieving technique. Wet sieving

(representing surface runoff) resulted in less aggre-

gate breakdown than the rainfall simulator, which

seemed to be more detrimental on unstable silt soil.

In addition, freezing�thawing had a significant effect

on soil structure. After a number of freeze�thaw

cycles the hydraulic permeability of soils increased,

most likely due to crack development and the large

voids left when ice thawed. In general, dense soils

lose strength after freezing�thawing and the natural

structure of soils is damaged (Kværnø & Øygarden,

2006).

In some erosion-prone areas in Scandinavia the

winters are unstable, with repeated freezing and

thawing, and erosion can be particularly severe

(Øygarden, 2000). Measurements at field scale

showed that when snowmelt occurred on unfrozen

soil, most of the melt water infiltrated in smaller

fields. Larger fields and fields with topographical

depressions had surface runoff and erosion. In

winters with frozen soil all fields had surface runoff

and erosion (Øygarden, 2000), showing the impor-

tance of soil physical conditions for runoff genera-

tion. Events during winter with a combination of

snowmelt and rainfall on partly thawed soils lead to

extreme runoff and erosion (Øygarden, 2003), with

erosion rates of up to 3 t ha�1 during one day. Field

observations revealed rill and gully development and

erosion of more than 100 t ha�1 for fields which

were autumn tilled, while no visible erosion was

observed on fields with stubble. This illustrates the

importance and effect of management and farming

practice for erosion and P losses. Freezing and

thawing decreased the rainfall stability of all soils,

but the effect was most severe on silty soils.

Conditions with soil subjected to many freeze�thaw cycles are expected to occur frequently during

field conditions in unstable winters. Climate change

with more short-term but more frequent freezing of

soils may have profound effects on soil structure and

erodibility, and consequently on runoff erosion and

tile drain erosion.

Soil erosion and sediment transport from

agricultural landscapes

Catchment-scale processes such as sedimentation in

the landscape, retention in buffer zones and wet-

lands, and erosion in topographical depressions,

around hydrotechnical installations and along stream

banks contribute to the total losses of SS from

agricultural catchments (Bogen, 2009). Concen-

trated rill and gully erosion in topographical depres-

sions has been demonstrated to be even higher than

sheet erosion from the same fields (Øygarden, 2003).

Ulen et al. (2012a) suggest that based on their

results, significant erosion along the stream banks

may also occur. A clear clockwise hysteresis effect for

soil particulate phosphorus (PP) concentrations was

shown in a Swedish study of small agricultural

catchments (Ulen et al., 2012a), indicating that

sediment is left over from one event to the next. At

the soil profile scale, Joel et al. (2012) also showed a

hysteresis effect on turbidity in leachate when they

tested two methods for in situ measurement of water

flow and sediment transport.

Modelling erosion and losses of particulate P is a

challenge because of the great spatial and temporal

variation. Surface erosion is commonly calculated

using empirical models such as the Revised Uni-

versal Loss Equation and the Water Erosion Predic-

tion Project. An empirical surface runoff model for

Norwegian conditions with a high proportion of silty

soils is used to estimate erosion risk at a national

180 B. Ulen et al.

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scale (Lundekvam, 2002). USLE or USLE-derived

models give an indication of the risk of erosion and

soil losses, but do not reflect the spatial variation in

erosion and sedimentation in a catchment. Djodjic

and Spanner (2012) tested a range of different

indices for identification of hotspots for erosion

and P losses from a catchment in Sweden. This set

of indices formed the basis for further decisions on

abatement efforts within the catchment.

Mitigating erosion and P losses

Soil tillage operations are important for the erosion

process, as demonstrated in surface runoff experi-

ments on sloping plots (Table II). Results from field

studies showed an 80% reduction in soil losses for

years with no autumn tillage compared to years when

the field was autumn ploughed (Bechmann et al.,

2011). Results also showed reduced losses of soil and

P due to light harrowing or direct drilling in winter

wheat (Øygarden et al., 2006). Kinderiene and

Karcauskiene (2012) found that the mean annual

erosion rate under the grass-grain rotation decreased

by 95% compared with a crop rotation with black

fallow. Similarly, Skøien and Børresen (2012)

showed that soil losses were reduced by 86% for

no autumn tillage compared with autumn tillage. In

their study, the effect of no autumn tillage was

highest for areas with the highest erosion risk. For

long-term grassland, no soil losses were observed in

the study by Kinderiene and Karcauskiene (2012).

In general, the effect on erosion increased when

minimum soil tillage was applied to areas with

steeper slopes.

Integrated crop production systems (Vereijken,

1997; Helander & Dehlin, 2000) are designed to

reduce soil tillage and inputs of fertilizers and

pesticides. Tine cultivation of the superficial topsoil

layer or direct drilling may replace mouldboard

ploughing (inversion of the whole topsoil). Some-

what lower yields but similar profits (based on lower

energy costs) characterize this type of farming

(Holland, 2004). Integrated crop production has

been compared with organic farming in a field-scale

experiment in Sweden (Stenberg et al., 2012). Every

single field of the farm was separately tile-drained

and the field area may act as one complete critical

source area for erosion and P losses, since it has high

connectivity to the recipient stream. Similar studies

were carried out in Norway to identify the difference

between conventional, integrated and organic farm-

ing (Eltun et al., 2002). They concluded that

nutrient losses were very much linked to the propor-

tion of ley in the system, with lower losses the more

ley. However, in the Swedish study, concentrations

of eroded P in drainage water tended to be higher

after ploughing in leys. Stenberg et al. (2012) also

found that disturbance of the soil surface by autumn

tillage and seedbed preparation probably caused

more soil erosion than on non-tilled fields. This is

in accordance with other studies comparing tillage

methods, e.g. Skøien and Børresen (2012).

Knowledge gaps

To be able to identify hot spots of erosion in the

agricultural landscape, knowledge on factors influ-

encing the erodibility of soils are important. Labora-

tory tests of erosion have their limits, although a

number of studies have been conducted to find a

relationship between field- or laboratory-measured

soil properties and soil erodibility. In order to care-

fully classify a soil, its texture may be complemented

with the fine clay fraction (Heathwaite et al., 2005).

Table II. Site in Norway (No), Finland (Fi) or Sweden (SW), main monitoring period and erosion class (Erosion), with mean yearly

erosion via surface runoff from field plot experiments with different slopes and soils based on Bechmann et al. (2011), Skøien and Børresen

(2012), Puustinen et al. (2005) and Ulen and Kalisky (2005).

Site (Country) Period Erosion

AuP

(t ha�1 yr�1)

AuC

(t ha�1 yr�1)

Au har

(t ha�1 yr�1)

SpP

(t ha�1 yr�1)

DiD

(t ha�1 yr�1)

Askima (No) 1987/2006 High 4.2 � � 0.56 �Aurajoki (Fi) 1990/1994 High 2.1 1.76 1.42 0.79 0.62

Bjørnebekka (No) 1994/2004 High 5.2 � � 0.65 �Hedemora (SW) 1994/2001 Medium 0.64 0.40 0.37b 0.22 0.11

Helleruda (No) 2002/2007 Medium 1.5 � 0.40 � �Øsakera (No) 2002/2007 Medium 1.3 � 0.86 � 0.19

Syverud (No) 1994/2000 Low 0.18 � � 0.10 �Vessigebro (SW) 1994/1996 Low 0.015 � 0.007 �

Note: Ulen (1997) compared tillage: Autumn ploughing (AuP); Autumn cultivation (AuC); Autumn harrowing (Au har); Spring ploughing

(SpP) and Direct drilling (DiD).aLevelled land.bShallow cultivation at Hedemora.

Soil erosion in Nordic countries 181

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Furthermore, the particle size distribution may be

complemented with type of organic matter present.

However, there are marked differences in erosion

risk between soils with the same texture and several

other physical/chemical soil factors such as plasticity

and shrinkage/swelling properties of the clay might

be taken into account. In addition, all erosion factors

should be related to soil moisture content.

Soil compaction is a serious problem on some

soils. The most important application of soil testing

should probably be to recognize those soils for which

mechanical stress is likely to lead to structural

damage. More research is needed to identify the

most suitable methods for describing both aggregate

stability and soil dispersion. Yet, such tests only

provide indirect information about soil erodibility

and erosion risk. The coherence class of a soil is not a

measure of the physical properties of the soil as it

exists in the field, but an indication of that soil’s

response to physical stress. Furthermore, variation in

aggregate stability and clay dispersion vary from year

to year and through the seasons which makes it

difficult to set a fixed value for use in erosion models

or calculations based on for example organic matter,

permeability and tendency for crust formation.

Quantification of the importance of erosion pro-

cesses and pathways, and especially subsurface

erosion, is urgent. However, cracking soil, such as

levelled soils and marine clays, usually represent a

natural zone of hotspots with high spatial unpredict-

ability for soil erosion. Inherent soil properties down

to drain depth are decisive for the uniformity of

water flow and determine the risk of preferential

transport of soil particles and associated pollutant.

Hydrological processes occurring when hydrological

thresholds are exceeded appear to be critical for the

transport of colloid-bound and dissolved substances

at such sites. Based on the heterogeneously in

physical properties, an intensive soil sampling is

necessary to capture the full heterogeneity of erosion

parameters. The variation in macropore systems

needs to be determined and crucial rain intensities

and soil water contents that generate most transport

need to be identified from data on preferential flow.

Measures to protect soil organic matter include

returning straw or other crop residues after harvest,

more grasses/leys in crop rotations and shepherding

livestock together with moving forage areas. How-

ever, injudicious application of organic materials as

recycling farm manures, sewage sludge, and com-

posted green wastes in soils may lead to increased

nitrate and phosphate leaching. A future challenge

will be to maintain or (where possible) enhance soil

organic matter while minimizing the polluting effects

of applied organic material. Some other commonly

suggested options to protect the soil from physical

decline and erosion include reduced mechanical

operations on wet ground, planting crops early in

autumn, tillage across slopes where safe to do so,

and use of low ground pressure set-ups on machin-

ery. However, based on the importance of subsur-

face erosion further quantification of the effect of

tillage on erosion from different types of drained

soils is urgently needed in addition to recommenda-

tions of erosion-reducing measures for areas with

subsurface erosion risks. In addition, options such as

buffer strips and filters at surface water inlets and

envelope material around drainage pipes should

be evaluated. The envelope material should keep

particles out and on the same time maintain the

hydrological function of the drains. Conflicting

issues regarding erosion/tillage and use of pesticides

need practical solutions and adaptation to regions,

soil types and cropping system representing other

gaps of knowledge.

Against a background of climate change, the effect

of repeated freezing�thawing should be further

assessed. The erosion risk areas may increase if

winter conditions change from stable to unstable.

More freezing and thawing combined with rainfall

may increase sheet erosion over large areas. Further

research is also needed for measures and combina-

tion of measures to be recommended if changing

climate conditions result in wetter soils for harvest-

ing, tillage and transport on soil, with enhanced risk

of soil compaction.

Beside cost-effective measures on fields, knowl-

edge gaps include the importance and extent of

erosion processes and retention in the landscape.

The importance of various erosion processes and

their transport pathways is unknown in many cases

because of spatial and temporal variation of the

hydrology which is the key driving force. Differences

in macropore topology and susceptibility to prefer-

ential water flow was suggested to explain the

variation in leaching of suspended material and

associated particulate-bound P from a small agricul-

tural plain (Ulen et al., 2012b). Such variation

should be even more crucial in a catchment. How-

ever, development of a distributed model, visualized

on a map, may serve as an opening for discussions

about managements with land-user and other stake-

holders within a catchment (Ulen et al., 2012a). A

change in weather may also change the influence and

importance of different processes and need therefore

to be estimated. If climate change leads to more

extreme rainfall and runoff events, erosion in depres-

sions and streambank erosion may increase which

need to be related to further efforts for measures

against erosion and attached pollutants.

182 B. Ulen et al.

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