2003---Dissolved and water-extractable organic matter in
Dissolved and water-extractable organic matter in
soils:a review on the influence of land use and
management practices
Martin H.Chantigny *
Soils and Crops Research and Development Centre,Agriculture and Agri-Food Canada,2560,
Hochelaga Boulevard,Sainte-Foy,Quebec,Canada G1V 2J3
Received 30January 2002;accepted 9December 2002
Abstract
Despite that dissolved organic matter (DOM)and water-extractable organic matter (WEOM)
represent only a small part of soil organic matter,they appear to be involved in many soil processes.
This review intends to compile the current information on the influence of land use and management
practices on soil DOM and WEOM,and to identify the gaps in our knowledge that sometimes
preclude from drawing general conclusions.The literature on DOM and WEOM dynamics in
different ecosystems is derived mainly from studies on temperate forest soils.Although less
abundant,literature on temperate grassland and arable soils is also available,whereas very few
studies have been published on tropical ecosystems.Similarly,studies on DOM and WEOM have
focused mainly on the carbon fraction,whereas nitrogen and phosphorus have received much less
attention.On the short term,temporal and spatial variations in DOM and WEOM are complex and
influenced by environmental https://www.sodocs.net/doc/84678602.html,boratory studies have shown that management practices,
such as liming and N fertilization,can induce marked fluctuations in DOM and WEOM.Under field
conditions,however,the net effect of management practices often remains unclear because many soil
properties,which can interact and counterbalance,are influenced at the same time.Changes in DOM
and WEOM upon management practices are generally of short duration,whereas long-term effects
are more related to vegetation type and to the amount of plant litter returned to the soil.So far,
research on soil DOM and WEOM as influenced by land use and management practices has offered
fragmented and sometimes contradictory information.It is concluded that the standardization of
collection and extraction methods for DOM and WEOM measurements is crucial to reduce the
uncertainty when comparing results from different studies.More research would also be required (i)
under field conditions and especially in tropical ecosystems,(ii)to determine the biological
significance of measured fluctuations in DOM and WEOM concentration and composition,and (iii)
0016-7061/02/$-see front matter D 2002Elsevier Science B.V .All rights reserved.doi:10.1016/S0016-7061(02)00370-1
*Tel.:+1-418-657-7980;fax:+1-418-648-2402.
E-mail address:chantignym@agr.gc.ca (M.H.Chantigny).
https://www.sodocs.net/doc/84678602.html,/locate/geoderma
Geoderma 113(2003)357–
380
to elucidate the mechanisms determining DOM/WEOM dynamics following changes in land use and
management practices.
D 2002Elsevier Science B.V .All rights reserved.
Keywords:Animal manure;Crop residues;Dissolved organic matter;Fertilization;Liming;Tillage;Water-
extractable organic matter
1.Introduction
Dissolved organic matter (DOM)and water-extractable organic matter (WEOM)
account for only a small proportion of the total organic matter in the soil (McGill et al.,
1986).Nevertheless,it is now recognized that those molecules influence soil biological
activity (Paul and Beauchamp,1989;Xu and Juma,1993;Flessa et al.,2000),affect the
transport of metals and organic pollutants (Chiou et al.,1986;Ro ¨mkens and Dolfing,
1998;Schnitzer and Khan,1972),and contribute to mineral weathering (Pohlman and
McColl,1988;Raulund-Rasmussen et al.,1998)and podzolization (Dawson et al.,1978;
van Hees and Lundstro ¨m,2000).The DOM and WEOM are composed of an array of
molecules generally reflecting the composition of total soil organic matter,since the
soluble phase tends to be in equilibrium with the solid phase of soil organic matter
(Zsolnay,1996).A detailed description of compounds that can be found in the soil solution
is beyond the scope of the present article,but the subject has been reviewed by Thurman
(1985)and Stevenson (1994).Numerous biotic and abiotic factors control the temporal
and spatial dynamics of DOM and WEOM (Kalbitz et al.,2000;Murphy et al.,2000).
Briefly,soil properties such as pH and ionic strength of the water phase determine organic
matter solubility,whereas Al and Fe oxides and clay minerals determine the sorption/
desorption equilibrium between the dissolved phase and the solid phase of soil organic
matter.The production and the consumption of DOM and WEOM are dependent mainly
on microbial activities and the equilibrium with the solid phase of soil organic matter.
Land use and related management practices affect soil properties,and thereby are
likely to influence DOM and WEOM.However,their impacts on the amount and
composition of DOM and WEOM have not been extensively studied,and the information
appears fragmented and sometimes contradictory.In general,DOM and WEOM concen-
trations vary in the order forest soils>grassland soils>arable soils,mostly due to different
vegetation types (Saviozzi et al.,1994;Delprat et al.,1997;Ellert and Gregorich,1995;
Quideau and Bockheim,1997;Haynes,2000).However,the size and the composition of
this organic matter pool are also influenced by management practices such as liming
(Simard et al.,1988;Andersson et al.,1994,2000),organic amendment (Zsolnay and
Go ¨rlitz,1994;Leinweber et al.,1995;Chantigny et al.,2000),and mineral fertilization
(McDowell et al.,1998;Rochette and Gregorich,1998;Chantigny et al.,1999).Many
studies report significant fluctuations in DOM and WEOM following a change in land use
or management practices.However,the processes driving the effects are largely unknown.
Both short-term and long-term effects of management practices remain poorly understood
under field conditions,since many environmental and soil factors interact at the same
https://www.sodocs.net/doc/84678602.html,paring data from different studies is often delicate since large discrepancies
M.H.Chantigny /Geoderma 113(2003)357–380
358
M.H.Chantigny/Geoderma113(2003)357–380359 pertaining to the methodological approaches used to characterize DOM and WEOM in different ecosystems exist(Herbert and Bertsch,1995;Zsolnay,1996).The main objectives of this review are to report on the short-term and long-term effects of land use and management practices on soil DOM and WEOM,and to identify the issues that should be addressed in future research to improve our general understanding of the processes involved.
2.Definitions and general considerations
The organic matter present in soil solution and passing a filter pore size of0.4–0.6 A m is operationally defined as DOM(Herbert and Bertsch,1995;Zsolnay,1996). However,this material is distributed among different pore sizes in soil,and the fraction present in mesopores and micropores might not be collectable without disturbing the soil structure.The fraction of DOM collected by lysimeters or suction cups is mostly located in soil macropores(Zsolnay,1996).By contrast,the organic matter extracted by agitating soil samples with aqueous solutions(WEOM)is considered to include DOM present in the macropores and some DOM located in smaller pores,since it results in disturbance of soil structure.The magnitude of WEOM is thus generally larger than DOM(Zsolnay,1996).Moreover,when using distilled water or when the extraction procedure induces an extensive breakdown of soil aggregates,WEOM is likely to contain additional material yielded by the lysis of living cells and by the desorption of organic matter from mineral surfaces.In this latter case,WEOM includes some potential DOM as defined by Tipping(1998).For those reasons,it is not possible to define a common term for DOM and WEOM.Therefore,in the present review,the acronym DOM/WEOM is used as a general term.When referring to specific research
results, Array Fig.1.Schematic of the influence of land use,management practices,and climatic and soil factors on DOM/ WEOM.The level of influence increases as factors are closer to the centre.
Table1
Selected literature values a for changes(D)in the concentration and amounts of DOC and WEOC induced by changes in land use and management practices Change in land use/
management practices
Extract type b D Concentration D Amount/export c Comments Source
1–2years after clear-cut
Clear-cutting of DOC/ZTL+7mg là1+140kg haà1O horizon Qualls et al.,2000 forest+17mg là1O horizon Hughes et al.,1990 DOC/SC+15mg là1+170kg haà1Mineral horizon Qualls et al.,2000
+156mg là1A horizon Delprat et al.,1997 DOC/stream no effect Slash not removed Moore and Jackson,1989
à4.2kg haà1yearà1Slash removed Meyer and Tate,1983
3–10years after clear-cut
DOC/stream+1.2mg là1+6kg haà1yearà1Slash burned Moore,1989
+4.2mg là1+86kg haà1yearà1Slash not burned Moore,1989
à8mg là1à111kg haà1yearà1Slash not burned Moore and Jackson,1989 Afforestation DOC/ZTL+1to+4mg là136years after afforestation Hughes et al.,1990 DOC/SC+20to+27mg là130–40years after afforestation Quideau and Bockheim,
1996,1997
Cultivation of DOC/SC+36mg là1After3years of cultivation Delprat et al.,1997 forest soilà7mg là1After26years of cultivation Delprat et al.,1997 WEOC/water+3mg là1After3years of cultivation Delprat et al.,1997
à43mg là1After26years of cultivation Delprat et al.,1997
+200to
+220mg kgà1+240to528kg haà1After>50years of cultivation
to maize
Boyer and Groffman,1996
Cultivation of grassland DOC/SC+8.2mg là1After>50years of cultivation
to maize
Quideau and Bockheim,
1996
WEOC/waterà20toà44mg kgà1à39toà62kg haà1After1–9years of cultivation Haynes,2000
à56mg kgà1à136kg haà1After40years of cultivation Saviozzi et al.,1994 WEOC/0.5M K2SO4à17to+12mg kgà1à61to+43kg haà1After>80years of cultivation DeLuca and Keeney,1993à5toà17mg kgà1à18toà61kg haà1After>80years of cultivation DeLuca and Keeney,1994 WEOC/125mM K2SO4à2toà27mg kgà1à5toà65kg haà1After4–37years of cultivation Gregorich et al.,2000
Crop rotations WEOC/water+1.9kg haà1Wheat–legume rotations vs.
continuous wheat
Campbell et al.,1999b
+6to+15mg kgà1+7to+18kg haà1Legume vs.gramineae Table2,this paper
+3to+37mg kgà1+4to+44kg haà1With vs.without legume trees Mazzarino et al.,1993M.H. Chantigny / Geoderma 113 (2003) 357–380 360
Liming(forest)DOC/stream+2.5mg là1Average of5years after liming Grieve,1990 DOC/leachate+317to+483mg kgà1Mor humus;23years after
liming
Andersson et al.,2000 WEOC/water+485to+847mg kgà1+17to+30kg haà1O horizon;60h after liming Erich and Trusty,1997
no effect11weeks after liming Cronan et al.,1992 Liming(arable)DOC/stream+6.4kg haà1yearà115years after liming Karlik,1995 DOC/Cà8toà12mg kgà1à19toà29kg haà13weeks after liming Simard et al.,1988
N fertilization (forest)DOC/ZTL+7to+16mg là1+70to+100kg haà1
yearà1
50–150kg haà1yearà1N McDowell et al.,1998
à17to+58mg là1à30to+105kg haà1
yearà1
50–150kg haà1yearà1N Currie et al.,1996 +6to+13mg là150–150kg haà1yearà1N Yano et al.,2000
No effect35–75kg haà1yearà1N Emmett et al.,1998 DOC/SC No effect35kg haà1yearà1N Raastad and Mulder,1999 DOC No clear response35kg haà1yearà1N Gundersen et al.,1998 WEOC/water No effect11weeks after adding
150kg haà1N
Cronan et al.,1992
N fertilization (grassland)DOC/LP+63kg haà1345kg haà1yearà1N;
undrained plots
McTiernan et al.,2001
+20kg haà1345kg haà1yearà1N;
drained plots
McTiernan et al.,2001
N fertilization (arable)DOC/ZTL+0to+3mg là1200kg haà1N;anhydrous
ammonia
Clay et al.,1995
WEOC/water+284to
+334mgà1kgà1
+426to+501kg haà11Mg mà3anhydrous
ammonia
Norman et al.,1987
à3toà9mgà1kgà1à3.3toà9.4kg haà1180kg haà1yearà1N Chantigny et al.,1999
à23to+10mgà1kgà1à53to+23kg haà1170–400kg haà1yearà1N Liang et al.,1998
+3.1kg haà1Long-term plots;46kg haà1
yearà1N
Campbell et al.,1999b à42mgà1kgà1à50kg haà1150kg haà1yearà1N Mazzarino et al.,1993
WEOC/VS+130to
+502mgà1kgà1
+286to+1104kg haà1600–2400mg kgà1N;
NH4OH
Myers and Thien,1988
(continued on next page)
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Change in land use/management practices
Extract type b
D Concentration D Amount/export c Comments Source N fertilization
(arable)WEOC/4mM CaSO 4
no effect Long-term plots;150kg ha à1year à1N Zsolnay and Go ¨rlitz,1994WEOC/0.5M
K 2SO 4
+4.2mg à1kg à1+5.2kg ha à1200kg ha à1year à1N Rochette and Gregorich,1998Amendment (forest)DOC/ZTL
+335mg l à1225Mg ha à1sawdust Cronan et al.,1992Amendment (grassland)DOC/C
+100to +3320mg l à150m 3ha à1synthetic urine Shand et al.,2000Amendment WEOC/0.5M
+14kg ha à18Mg ha à1rape straw Jensen et al.,1997(arable)K 2SO 4+37to +135mg à1kg à1+45to +161kg ha à1
112Mg ha à1dairy manure Gregorich et al.,1998+96to +129mg à1kg à1+106to +133kg ha à1
100Mg ha à1dairy manure Rochette and Gregorich,1998WEOC/4mM CaSO 4+7to +28mg à1kg à1+11to +44kg ha à1
Long-term plots;50–200kg ha à1year à1N as FYM d Zsolnay and Go ¨rlitz,1994WEOC/HW +422mg à1kg à1+1013kg ha à1Long-term plots;15Mg ha à1
year à1FYM
Leinweber et al.,1995+51to +107mg à1kg à1+80to +168kg ha à150–100Mg ha à1PS d
Chantigny et al.,2000Tillage (grassland)WEOC/water +2to +32mg à1kg à1+1to +19kg ha à1Tillage depth 5–25cm
Fig.4,this paper Tillage (arable)WEOC/water à32to à84kg ha à1Compared to no-till soils;
0–7.5cm depth
Linn and Doran,1984à8to +22kg ha à1Compared to no-till soils;
7.5–15cm depth
Linn and Doran,1984Drainage (grassland)
DOC/LP à33kg ha à12months sampling period McTiernan et al.,2001a For the sake of clarity,values given in this table have often been averaged over several sampling dates,soil depths,soil textures,or treatments and thus may differ from those found directly in the cited papers.Values given are the difference between treatments and untreated controls and are thus presented as an increase (+)or a decrease (à)in the concentration (mg l à1or mg kg à1)or amount (kg ha à1)of carbon in the soil solution following treatment.
b DOC =dissolved organi
c carbon;WEOC =water-extractable organic carbon;ZTL =zero-tension lysimeter;SC =suction cup;C =centrifugation;LP =lysimeter plot;VS =vacuum suction;HW =hot water.
c In many papers,only the concentration values were given.Where sufficient information was provided,amounts in kilograms per hectare carbon were calculate
d using soil bulk density and sampling depth as indicated in th
e papers.I
f not provided,soil bulk density of 1.2M
g m à3for mineral soil and 0.14Mg m à3for organic soil were used.In some papers,values were given as kilograms per hectare per year DOC and represent estimated carbon exports from the study sites.
d FYM =farm yard manure;PS =paper sludge.
Table 1(continued )
M.H.Chantigny /Geoderma 113(2003)357–380362
M.H.Chantigny/Geoderma113(2003)357–380363 values for dissolved organic carbon(DOC),dissolved organic nitrogen(DON),and dissolved organic phosphorus(DOP),and for water-extractable organic carbon(WEOC), water-extractable organic nitrogen(WEON),and water-extractable organic phosphorus (WEOP)are also presented.
In the present review,land use is considered as the factor with the greatest influence on soil DOM/WEOM,since it determines the type of vegetation grown on the soil,and since plant litter is the primary source of soil organic matter in the ecosystem(Fig.1).Forest, grassland,and arable soils from temperate climate are considered here because most of the available literature concerns these ecosystems.Characteristics of DOM/WEOM and the short-term and long-term impacts of a change in land use are reviewed.Management practices are envisioned as secondary factors affecting DOM/WEOM since they are determined by the type of land use.Both the short-term and long-term effects of plant species,liming,mineral N fertilization,organic amendments,and tillage/drainage are considered here.Environmental factors such as climate,landscape,and soil hydrology and texture represent a tertiary level of influence on DOM/WEOM,since they influence the dynamics of soil organic matter at the local and global scales.The effects of environmental factors are not discussed here,but have been partly reviewed by Hope et al.(1994)and Kalbitz et al.(2000).
Land use and management practices influence DOM/WEOM content in the soil. However,the extent of fluctuations in DOM/WEOM as influenced by land use and management practices is not well known.The values presented in Table1compile part of the available literature and provides ranges of increase(+)or decrease(à)in DOC and WEOC content following changes in land use and management practices.
https://www.sodocs.net/doc/84678602.html,nd use
3.1.DOM/WEOM characteristics in temperate ecosystems
Although the figures in the literature range widely,Zsolnay(1996)indicated that DOM/ WEOM concentration tends to be larger in forest than in agricultural soils.He reported that in forest floor,DOC concentration ranges from5to440mg là1,whereas WEOC content ranges from1000to3000mg là1.In agricultural soils,he reported values varying from0 to70mg là1for DOC and from5to900mg là1for WEOC.Several studies that have compared forest to agricultural soils using either paired or unrelated sites have confirmed that the forest floor produces larger amounts of DOM/WEOM and generally shows higher concentrations than the A horizon of arable(Ne′meth et al.,1988;Ellert and Gregorich, 1995;Quideau and Bockheim,1996;Delprat et al.,1997)or grassland soils(Hughes et al., 1990;Quideau and Bockheim,1997;Khomutova et al.,2000).Other studies have also indicated higher DOM/WEOM concentrations in grassland than in arable soils(Saviozzi et al.,1994;Flessa et al.,2000;Gregorich et al.,2000;Haynes,2000).Only a few studies reported no clear difference among ecosystems(DeLuca and Keeney,1993;Corre et al., 1999),or higher DOC(Quideau and Bockheim,1996)and WEOC content(Boyer and Groffmann,1996)in arable soils.The mechanisms responsible for those differences between forest and agricultural soils have not been clearly identified.Fungi are thought to
play a significant role in the production of DOM (Guggenberger et al.,1994;M?ller et al.,1999).The greater fungal biomass present in forest floor compared to agricultural soils (Alexander,1977)could partly explain higher DOM/WEOM content in the former.Nevertheless,on the long term,DOM/WEOM content tends to be proportional to the whole soil organic matter content (Saviozzi et al.,1994;Delprat et al.,1997;Gregorich et al.,2000),suggesting that DOM/WEOM production and concentration should be determined primarily by the amount of organic matter present in soil (Zsolnay,1996;Kalbitz et al.,2000).
A few studies have compared the composition of soluble organic matter in forest and agricultural soils.It was found that large molecules and organo-metal complexes are more abundant in DOM from forest floors (Hughes et al.,1990;Strobel et al.,1999),whereas agricultural soils contain a greater proportion of smaller molecules,such as fulvic and hydrophilic acids,carbohydrates,and amino acids (Delprat et al.,1997;Leinweber et al.,2001).In agreement,Cronan et al.(1999)found that at the watershed level,the proportion of high-molecular-weight DOM present in streams decreases as the proportion of agricultural land increases.A possible reason for the greater proportion of organic complexes in forest floor than in agricultural soil would be that the material returned to the soil by tree canopy contains more lignin and other recalcitrant compounds,such as tannins and phenolic acids,than agricultural crop residues.Quideau and Bockheim (1997)suggested that the large organic acids present in afforested soils are not directly produced by tree canopy but rather originate from the forest floor during the degradation of tree litter.The frequent disturbances caused by tillage,liming,amendment,and fertilization in arable soils have been pointed out as a factor impeding the formation of large organo-metal complexes (Delprat et al.,1997).Increased management intensity has been found to increase the humification index (Kalbitz,2001)and the proportion of highly decomposed and microbially derived (Leinweber et al.,2001)DOM in soil.
The biodegradability of WEOC was found to be higher in maize than in forest soils (Boyer and Groffmann,1996).However,very few studies have directly compared the biodegradability of various tree and agricultural crop residues.Fox et al.(1990)found that the decomposition of herbaceous legumes was faster than tree legumes,and was negatively correlated to the (lignin +polyphenol)-to-N ratio of the residues.Ellert and Gregorich (1995)reported greater decay rate constant for buried wheat straw (à0.73year à1)than for pine needles (à0.40year à1)or maple leaves (à0.62year à1).If wheat straw were left on the soil surface,however,its decay rate constant was much lower (à0.18year à1)than with other litter types,indicating a strong interaction between intrinsic biodegradability and litter placement.
Overall,most studies indicate that DOM/WEOM concentration is in decreasing order:forest floor>grassland Ah>arable Ah,whereas soluble organo-metal complexes are more abundant in forest than in agricultural soils.The amount of WEON and the proportion of amino compounds also appear to be higher under forest and grassland than under arable
crops (Ne
′meth et al.,1988;DeLuca and Keeney,1993,1994;Mengel et al.,1999).Higher lignin content and C-to-N ratio of forest litter compared to agricultural crop residues could partly explain those differences.However,more information about the mechanisms responsible and about the relative biodegradability of agricultural vs.forest litters is required.
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M.H.Chantigny/Geoderma113(2003)357–380365 3.2.Change in land use:clear-cutting and afforestation
Converting forest to agriculture first implies clear-cutting and often tillage to incorpo-rate the organic horizon into the mineral soil.Some studies have investigated the fluctuations in DOM concentration following clear-cutting(Table1),and mostly reported an initial mobilization of DOC ranging from10to150mg là1(Hughes et al.,1990; Delprat et al.,1997;Qualls et al.,2000).Following clear-cutting,DON content in forest floor has been found to increase by1.4times,and by3.6times for DOP(Qualls et al., 2000).Those increases in DOM content have been attributed either to soil disturbance, increased water flux,accumulation of decaying wood debris on the soil,or stimulation of microbial activity.This initial increase in DOM has been found to last from less than2 years(Meyer and Tate,1983)and up to10years(Moore,1989).Conversely,some authors reported little change in DOM after clear-cutting(Moore and Jackson,1989;McDowell and Likens,1988),or found a decrease in DOC exports from clear-cut sites(Meyer and Tate,1983;Ellert and Gregorich,1995).This discrepancy among studies could reflect the amount of wood debris left on the soil.For instance,Moore(1989)reported that DOC mobilization was lower in a clear-cut site if wood debris were burnt rather than left on the soil.However,dissimilarities could also be related to the successional state of vegetation at time of sampling(Meyer and Tate,1983),or to differences in soil hydrology and climate (Moore and Jackson,1989;Hope et al.,1994;Ellert and Gregorich,1995).A progressive decline in the amount of DOM has been reported a few years after clear-cutting.This decline was primarily attributed to the stabilization of the remaining organic matter (Delprat et al.,1997)and to a lower organic matter input to the soil(Meyer and Tate,1983; Qualls et al.,2000).
The soluble material mobilized by clear-cutting is mostly composed of medium to large humic acids,colloids,and organo-metal complexes(Hughes et al.,1990;Delprat et al., 1997).However,Delprat et al.(1997)found that when a clear-cut site is cultivated to maize,those molecules are gradually replaced by smaller substances such as fulvic and hydrophilic acids,carbohydrates,and amino acids.As opposed to cultivation of forest soils,afforestation of agricultural soils appears to induce a significant increase in DOC concentration(Table1).This increase in DOC is caused mainly by an increase in the amount of organic–Al complexes and chelating organic acids(Hughes et al.,1990; Quideau and Bockheim,1997).Most studies thus indicate that clear-cutting mobilizes DOM from the forest floor,which is enriched in large organo-metal complexes.Those organics are replaced gradually with smaller and less complexed molecules if the soil is cultivated.On the contrary,afforestation would,in the long term,stimulate the production of new large organo-metal complexes.
3.3.Change in land use:grasslands vs.arable soils
Even though scarce,the available literature strongly indicates that when grassland soils are switched to arable cropping,their WEOC content decreases(Table1).Moreover,the extent of decline in WEOC appears to be more pronounced as the number of years under arable cropping increases(Gregorich et al.,2000;Haynes,2000),apparently due to a gradual depletion in the whole soil organic matter(Saviozzi et al.,1994).In crop rotations
including annual and perennial crops,soil WEOC content tends to increase with the number of years under perennial crops (Campbell et al.,1999b;Haynes,2000).Similarly,higher WEON content was recorded in a sandy soil under permanent grassland (55kg ha à1)than under continuous wheat (20kg ha à1)(Bhogal et al.,2000;Murphy et al.,2000).The quality of DOM/WEOM has also been suggested to decrease when grassland soils are converted to arable cropping,since there is generally less water-extractable carbohydrates and amino–N compounds in the latter (DeLuca and Keeney,1993,1994).
Studies on the influence of land use on soil DOM/WEOM are still scarce,especially at the catchment level.Nevertheless,there are indications that in the long term,vegetation type and the amount of organic matter returned to the soil are major factors in determining the amount and composition of DOM/WEOM in soils (Meyer and Tate,1983;Saviozzi et al.,1994;Delprat et al.,1997;Campbell et al.,1999a,b;Qualls et al.,2000).Significant amounts of DON (0.1–5mg l à1)and DOP (0.02–5mg l à1)have been measured in forest (Currie et al.,1996;McDowell et al.,1998;Qualls et al.,2000)and agricultural soils (Chardon et al.,1997;Beauchemin et al.,1998).From 10to 30kg ha à1WEON have also been measured in agricultural soils (Jensen et al.,1997;Bhogal et al.,2000;Murphy et al.,2000).However,detailed information on the nature,bioavailability,and fate of the mobilized DOM/WEOM following a change in land use is still lacking.
4.Management practices
4.1.Plant species—crop rotations
As previously discussed,land use effects on soil DOM/WEOM are partly determined by vegetation type (e.g.,tree vs.herbaceous plants).However,plant canopies may be composed of various species present at the same time or in rotation.Natural forests may contain mixed tree species,and the quality of forest floor and DOM/WEOM is determined by the dominant plant species (Kuiters,1993).Reported DOC contents are generally 1–45mg l à1(Currie et al.,1996;McDowell et al.,1998;Yano et al.,2000)or 9–24kg ha à1(Smolander and Kitunen,2002)higher in coniferous than in deciduous forest floor.Similarly,DON content has been reported to be 0.1–1.6mg l à1(Currie et al.,1996;McDowell et al.,1998;Yano et al.,2000)or 0.3–1kg ha à1(Smolander and Kitunen,2002)higher in coniferous than in deciduous forest floor.The reason for this difference has not been clearly established.Coniferous litter may accumulate more than deciduous litter,since its decomposition rate can be slower than deciduous litter (Kuiters,1993;Currie and Aber,1997).Therefore,higher DOC concentration in coniferous than in deciduous soils could be the reflection of a greater accumulation of decomposing litter at the soil surface of coniferous forest.Root exudation can also be a significant source of DOM (Yano et al.,2000),and greater inputs from coniferous roots are possible,especially during winter period when no photosynthesis occurs in deciduous stands.
Since the amount and the type of C input to the soil are dependent on plant species (Zsolnay,1996;Campbell et al.,1999a,b),crop rotations in agricultural soils may influence DOM/WEOM concentration from year to year.It is known that plant roots exude easily metabolizable material (Barber and Martin,1976;Xu and Juma,1993)and
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M.H.Chantigny/Geoderma113(2003)357–380367 that the nature and the amount of the released material may vary among crop species and cultivars(Xu and Juma,1993).Very few studies have directly compared the influence of crop rotations and plant species on in situ DOM/WEOM concentrations during the growing season(Table1).However,cropping to legumes or including them into the rotation appears to increase the amount of WEOC present in the soil by2–44kg haà1 (Mazzarino et al.,1993;Campbell et al.,1999b).In a field experiment(Chantigny et al., 1997),different crop species were established on a silty clay loam and a clay loam,and WEOC content was measured monthly in the top20cm of soil for two consecutive growing seasons(Table2).On average,WEOC concentration was generally higher under legumes than under gramineae species.This difference could reflect different root exudation patterns among crop species.It is possible that legumes exude significantly more soluble molecules than gramineae to signal their presence to Rhizobia and to rapidly initiate the formation of root nodules and nitrogen fixation.This hypothesis remains to be verified.No study was found on the possible influence of crop species or rotations on DON/WEON in the soil.Overall,the available information consistently suggests that both in forest and agricultural soils,plant species influences DOM/WEOM.Nevertheless, additional research is required to better understand how plant species influences DOM/ WEOM concentration and composition.
4.2.Soil liming
Liming has been found to influence DOM/WEOM(Table1).An increase is generally recorded following liming of forest(Grieve,1990;Andersson et al.,1994;Driscoll et al., 1996;Erich and Trusty,1997;Hildebrand and Schack-Kirchner,2000)and agricultural soils(Murayama and Ikono,1975;Karlik,1995).Various mechanisms,such as increased organic matter solubility(Murayama and Ikono,1975;Andersson et al.,1994;Erich and Trusty,1997),increased microbial activity and production of soluble molecules(Guggen-berger et al.,1994),and displacement of the previously adsorbed DOM by other mobilized anions(Kalbitz et al.,2000),could explain this phenomenon.However,other mechanisms, such as microbial consumption of DOM/WEOM(Andersson,1999;Andersson et al., 1994;Karlik,1995)and DOM flocculation or adsorption by cation bridging due to high Ca2+concentration(Ro¨mkens and Dolfing,1998),might decrease DOM/WEOM con-centration.It is likely that under field conditions,many of those processes occur at the same time upon liming,and the net effect of liming on DOM/WEOM depends on the balance between different counteracting processes.This could explain why some studies Table2
Average WEOC content(mg kgà1)in the rooting zone of various gramineae and legume species grown in a silty clay loam and a clay loam
19901991
SCL a CL SCL CL Gramineae62F1b69F4b42F4b50F3b Legumes71F4a75F1a50F4a65F7a Numbers within a column followed by different letters are statistically different at P<0.05.
a SCL=silty clay loam;CL=clay loam.
found no clear effect (Cronan et al.,1992;Smolander et al.,1995;Ponette et al.,1996)or a significant decrease in DOM/WEOM content (Simard et al.,1988)after liming.
Liming has been found to affect the composition of DOM/WEOM by increasing the proportion of hydrophobic acids (Andersson et al.,2000),humic acids (Cronan et al.,1992),and carboxylic groups (Karlik,1995)in DOC,and by precipitating high-molecular-weight DOC with Ca 2+(Ro ¨mkens and Dolfing,1998),while releasing smaller molecules less complexed to metals (Erich and Trusty,1997).Soil respiration significantly increases upon liming (Andersson,1999;Andersson et al.,1994;Karlik,1995),indicating that the mobilized DOM could be particularly biodegradable (Erich and Trusty,1997;Andersson et al.,2000).However,it is not known whether the mobilized material is transformed into microbial metabolites or simply mineralized.
Even though many laboratory and field studies report increased DOM mobilization,the net effect under field conditions might be hard to predict since liming mediates both mobilization and precipitation processes.At present,the factors determining the dominant process in the field are not known.Both DON (Andersson,1999)and DOP (Ron Vaz et al.,1993)mobilization might occur upon liming,and their behavior in soil could be different than DOC (Andersson,1999).However,the amount,nature,and fate of the mobilized DOM/WEOM remain unclear.
4.3.Inorganic nitrogen fertilization
Inorganic fertilizers may influence DOM/WEOM content (Table 1).Urea-based and ammonium-based fertilizers temporarily solubilize soil organic matter and can induce a marked increase in DOC/WEOC content due to an increase in soil pH (Hartikainen and Yli-Halla,1996;Liu et al.,1995;Myers and Thien,1988).In a laboratory experiment,Myers and Thien (1988)showed that injection of ammonium hydroxide in the soil caused an immediate and significant increase in DOC concentration in the few millimeters of soil surrounding the injection zone.However,this effect has been found to be short-lived (Clay et al.,1995),suggesting that the solubilized organic matter is readily biodegradable (Norman et al.,1987,1988;Yano et al.,2000)and rapidly consumed by soil micro-organisms.
Under field conditions,many authors have reported no consistent effect of N fertilization on DOM in forest soils (Emmett et al.,1998;Gundersen et al.,1998;Raastad and Mulder,1999;Stuanes and Kj?nass,1998).Others have found no effect on DOC,but a significant increase in DON concentration ranging from 1to 16kg ha à1with N fertilizer rates from 50to 150kg ha à1(Currie et al.,1996;McDowell et al.,1998).The nature of the mobilized DON has not been https://www.sodocs.net/doc/84678602.html,paring laboratory and field results,McDowell et al.(1998)proposed that N fertilization of forest soils would stimulate DOC consumption by microbes,but under field conditions,this material would be replaced at the same rate by the decomposition of fresh litter,root exudation,and microbial by-products.Accordingly,Cronan et al.(1992)proposed that N fertilization of forest soils would stimulate microbial DOC degradation with preferential use of low-molecular-weight acids and other substrates with carboxylic groups.
Conflicting results are reported under field conditions on the effect of N fertilization on WEOM in agricultural soils (Table 1).Some studies report no significant effect (Zsolnay
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M.H.Chantigny/Geoderma113(2003)357–380369 and Go¨rlitz,1994;Rochette and Gregorich,1998),whereas others report a decrease (Mazzarino et al.,1993;Liang et al.,1998;Chantigny et al.,1999)or an increase in WEOC content(Campbell et al.,1999b;McTiernan et al.,2001).Chantigny et al.(1999) noticed that soil WEOC content decreased markedly soon after applying180kg haà1N as ammonium nitrate to corn(Fig.2).However,the effect was transient and WEOC content increased as soil mineral N content was reduced by crop uptake.They argued that N fertilization enhances WEOC consumption by soil microbes,thereby decreasing its concentration,and that this stimulation disappears as soil mineral N content returns to background https://www.sodocs.net/doc/84678602.html,ing14C in a laboratory experiment,Merckx et al.(1987)demon-strated that under N-limited conditions,inorganic N addition to the soil stimulates microbial mineralization rate of easily degradable WEOC.However,Rochette and Gregorich(1998)and Chantigny et al.(1999)found no significant change in soil respiration after mineral N addition,suggesting that WEOC is not necessarily mineralized, but could be immobilized in microbial biomass or released in the soil as microbial metabolites.Finally,it is also possible that mineral N fertilizers favour microbial degradation of both DOM/WEOM and solid organic matter,the latter resulting in the production of DOM(Guggenberger et al.,1994;Kalbitz et al.,2000).This would partly explain that some authors found no effect or an increase in DOC/WEOC following inorganic N input.
Over the long term,repeated inorganic N applications have not been found to significantly influence the amount of DOM in forest(Gundersen et al.,1998;Yano et al.,2000)or in agricultural soils(Zsolnay and Go¨rlitz,1994).Nevertheless,comparing various cropping systems with and without N fertilization,Campbell et al.(1999a,b) reported a greater WEOC concentration in long-term plots with N additions.They attributed this increase to a greater crop residue input in fertilized soils than in unfertilized soils.In agricultural soils,inorganic N fertilization has also increased the amount of
DON Array Fig.2.Changes in soil soluble mineral N and WEOC as influenced by mineral N fertilization of a sandy loam cropped to maize.Nitrogen was applied on July1st at180kg haà1as ammonium nitrate.Modified after Chantigny et al.(1999).
by 1–22kg ha à1(Mercik and Ne
′meth,1985;McDonald et al.,2001).However,the origin of this soluble organic N and the reason for this increase in concentration are not known.It appears that both in forest and agricultural soils,the effects of N fertilization on DOM/WEOM have not yet been clearly established.Similarly to liming,inorganic N fertilization could stimulate both DOC/WEOC production and consumption processes at the same time.Therefore,the net effect remains hard to predict under field conditions.Finally,the nature of organic compounds that are dissolved by ammonium fertilizers and the fate of the mobilized material are unknown.
https://www.sodocs.net/doc/84678602.html,anic amendments
Amending the soil always induces an increase in DOM/WEOM (Table 1).DOC has been found to increase in forest floor after sawdust amendment (Cronan et al.,1992).However,much more literature is available on amendment of agricultural soils.Many studies have found immediate and significant increases in soil WEOC content upon amendment with crop residues (McCarty and Bremner,1992;Jensen et al.,1997;Franchini et al.,2001),animal manure (Kirchmann and Lundvall,1993;Zsolnay and Go ¨rlitz,1994;Gregorich et al.,1998;Rochette and Gregorich,1998;Martin-Olmedo and Rees,1999,Chantigny et al.,2002),or industrial wastes (Gigliotti et al.,1997;Chantigny et al.,2000).This immediate increase is generally attributed to the presence of soluble materials in the amendments (Reinertsen et al.,1984;Paul and Beauchamp,1989;Kirchmann and Lundvall,1993;Chardon et al.,1997;Martin-Olmedo and Rees,1999;Chantigny et al.,2002).However,this soluble material appears to be rapidly decomposed in soil,and WEOC content may return rapidly to background level (McCarty and Bremner,1992;Kirchmann and Lundvall,1993;Jensen et al.,1997;Franchini et al.,2001).It was also found that WEOC coming from crop residues is degraded too rapidly to reach deep soil layers (Dietz and Bottner,1981;McCarty and Bremner,1992).In a field experiment,liquid hog manure was surface-applied at 90m 3ha à1on a silty clay loam,and the concentration of water-extractable volatile fatty acids was monitored in thin soil layers (Fig.3;Chantigny et al.,unpublished).In accordance with previous studies on crop residues (Dietz and Bottner,1981;McCarty and Bremner,1992),it clearly appeared that volatile fatty acids coming from the manure were metabolized within 5days in the soil and did not penetrate below 2–3cm depth (Fig.3).Jensen et al.(1997)reported that,on average,WEON content increased by 1kg ha à1following the addition of 8mg ha à1oilseed rape straw.However,this N was rapidly assimilated by soil microbial biomass.Changes in soil WEON content varying from à1to +6mg kg à1were measured following amendment with various crop residues (Murphy et al.,2000).The negative values were attributed to microbial N immobilization in the presence of crop residues with high C-to-N ratio.Even though the increase in soil DOM/WEOM content after amendment might be short-lived,long-term trials have indicated that,compared to conventional farming,organic farming may increase the amount of WEOC present in the soil by 11–1013kg ha à1(Leinweber et al.,1995;Zsolnay and Go ¨rlitz,1994),and WEON by 1–91kg ha à1(Leinweber et al.,1995;Murphy et al.,2000;McDonald et al.,2001),possibly due to repeated inputs of organic matter (Lundquist et al.,1999).
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The composition of soil DOM/WEOM is also influenced by the type of amendment.Ohno and Crannell (1996)reported that WEOM in plant residues mostly has molecular weight ranging from 710to 850,whereas it varies from 2000to 2800for animal manure.They also found that WEOM in plant residues was more reactive with soil Al and P than in animal manure.Crop residues and animal manure also contain DON (Kirchmann and Lundvall,1993;Jensen et al.,1997)and DOP (Gerritse,1981;Chardon et al.,1997),which can modify the amount and composition of soil DOM/WEOM.Both laboratory and field experiments have indicated that in soil amended with pig slurry,WEOP and DOP are the dominant form of soluble P present in soil (Chardon et al.,1997).In some cases,high amendment rates may induce a steady increase in soil WEOM for several weeks to years (Gregorich et al.,1998;Rochette and Gregorich,1998;Chantigny et al.,2000).However,even where soil WEOM content remains stable for a long period of time after amendment,its composition gradually changes,and plant-derived molecules are mostly replaced by microbial metabolites (Chantigny et al.,2000).
The effects of grazing on WEOM dynamics in grassland soils are not well documented.However,due to the patchy distribution of faeces and urine (Haynes and Williams,1993),it is likely that WEOM concentration and composition show large spatial variations.When urine is deposited on the soil,urea is rapidly hydrolyzed,which locally increases soil pH and causes DOM mobilization (Shand et al.,2000).Using 15N-labeled synthetic urine without phosphorus,Shand et al.(2000)measured a 30-fold increase in DOC,a 25-fold increase in DON,and a 10-fold increase in DOP content following urine deposition.They also determined that the mobilized DON and DOP were essentially coming from indigenous soil organic matter.Similarly,Bol et al.(1999)reported that dung deposition increased DOC leaching by 10-fold in a grassland soil,and up to 50%of the mobilized DOC was of soil origin.They suggested that dung application either produced a
priming Fig.3.Changes in the concentration of volatile fatty acids (VFAs)in a bare silty clay loam amended with 90m 3ha à1liquid hog manure as a function of soil depth and time after amendment.Vertical bars indicate standard error of the mean (Chantigny et al.,unpublished).
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effect on DOC production from native soil C,or that dung-derived DOC partly displaced or substituted soil DOC adsorbed on the mineral phase.
Both laboratory and field studies on arable soils consistently demonstrate that WEOM already present in organic amendments is highly biodegradable and rapidly consumed by microbes,which may result in transient increases in soil DOM/WEOM content.However,by changing soil properties such as pH,animal manure might dissolve or desorb indigenous DOM (Bol et al.,1999;Shand et al.,2000).Information about the mechanisms by which amendments affect the concentration and composition of soil DOM/WEOM is still fragmented,especially regarding the amendment of forest soils and the effects of amendments on native soil DOM/WEOM.In addition,few studies report on changes in DOM/WEOM composition after amendment,and on the long-term effects of organic farming on the composition of DOM/WEOM.
4.5.Soil tillage and drainage
Soil tillage has been found to influence DOM/WEOM (Table 1).As compared to no-till plots,Linn and Doran (1984)measured lower WEOC content in the top 7.5cm of conventionally tilled soils.However,there was no significant difference in deeper soil layers.On the contrary,in corn-cropped soils (0–20cm),Gregorich et al.(2000)found more WEOC where corn residues were incorporated.They suggested that by loosening the soil and mixing with corn residues,tillage would stimulate microbial degradation of the residues,thereby increasing WEOC production.In accordance,Leinweber et al.(2001)found that an increase in tillage intensity altered soil WEOC composition.They suggested that increased tillage intensity enhanced oxidative microbial activity.No literature was found addressing the effect of tillage depth on soil DOM/WEOM.In a field experiment (Lapierre et al.,unpublished),an old grassland soil was tilled in the late autumn either by harrowing (0–5cm),rototilling (0–15cm),or by moldboard plowing (0–25cm).In the next spring,WEOC concentration (0–5cm)was increased,but the extent of this increase tended to be inversely related to tillage depth (Table 1;Fig.4).It is suggested that the organic matter incorporated to the soil was more diluted as tillage depth increased,and WEOC concentration was proportional to the amount of organic matter present in the sampled (0–5cm)soil layer.Following tillage of grassland soils,21–184kg ha à1WEON was mobilized (0–20cm depth),but this effect disappeared within a year as the soil was reseeded to grass and no more cultivated (Bhogal et al.,2000).The fate of the mobilized DOW/WEOM has not been determined.
Drainage generally improves water flow through the soil profile and could influence DOM/WEOM transport to the streams (Eckhardt and Moore,1990;Beauchemin et al.,1998).In lysimeter plots,McTiernan et al.(2001)reported DOC exports 33kg ha à1lower from drained plots than from undrained plots.Tile drainage has been pointed out to increase DOP leaching and exports from clayed soils,especially for those with medium to high P status (Beauchemin et al.,1998).Nevertheless,there is insufficient information available to conclude on those aspects.By modifying soil hydrology and the distribution of organic matter,drainage and tillage are likely to affect DOM/WEOM dynamics.Much work remains to be done to determine the effects of those management practices.For tillage studies,soil sampling depth appears critical (Angers et al.,1997),since by
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modifying residue distribution,tillage could influence more the location of DOM/WEOM production than the production itself.
5.Concluding remarks
It is now more obvious that land use and management practices may significantly influence the amount and the composition of DOM/WEOM in soil,but the processes involved remain largely unknown.Although laboratory studies help to isolate and better define these processes,the net effect of management practices can be poorly predictable under field conditions because various soil properties are modified at the same time,resulting in confounding and counteracting effects on DOM/WEOM.Environmental factors (not discussed in the present review)are also thought to significantly interact with land use and management practices in determining DOM/WEOM dynamics (Fig.1),since they affect organic matter cycling.However,the available literature hardly allows to compare similar land use under different climate or soil types.Clearly,more research is also required on soil DON and DOP since they appear to represent a significant part of the global N (Bhogal et al.,2000;Murphy et al.,2000)and P budgets (Schoenau and Bettany,1987;Chardon et al.,1997;Beauchemin et al.,1998)of many ecosystems.
We have seen that changes in land use and management practices have both short-term and long-term effects on DOM/WEOM that might appear contradictory.For example,converting forest to agriculture can markedly increase DOM/WEOM mobilization on the short term,but decreases it on the longer term as total soil organic matter stabilizes at
lower Fig.4.Concentration of WEOC following tillage of an old grassland,clayed soil either by harrowing (HA),rototilling (RO),or moldboard plowing (MP).Concentrations were measured in the 0–5cm depth (Lapierre et al.,unpublished).
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levels.Few studies aimed at determining the long-term effects of land use and management practices on DOM/WEOM,but there are consistent indications that over a long period of time,DOM/WEOM concentration and composition are determined mainly by vegetation type and the amount of litter returned to the soil.Repeated applications of animal manure or industrial wastes could also significantly influence both the amount and the composition of soil DOM/WEOM as they contribute a significant part of the whole soil organic matter.
Many studies report significant variations in DOM/WEOM concentration and compo-sition following a change in land use or management practices (Table 1).However,the real dynamics of DOM/WEOM is generally inferred,since most studies fail at determining whether the recorded changes are caused by an alteration in the production or in the consumption of soluble organic matter.Additionally,the bioavailability of the mobilized DOM/WEOM is often not investigated.Recent developments in sophisticated analytical approaches,such as liquid (Chantigny et al.,1997,2000)and gas chromatography (Amelung et al.,1996),13C nuclear magnetic resonance (Kaiser et al.,2001;Leinweber et al.,2001)and UV–visible spectra (Cronan et al.,1992;Kalbitz,2001),and fluorescence (Senesi et al.,1991;Erich and Trusty,1997)and Fourier transform infrared spectroscopy (Kaiser et al.,1997),will provide crucial insights on the composition and turnover of DOM/WEOM after changes in land use and management practices.
Some gaps remain to be filled before we can make general conclusions.A large discrepancy exists in the investigation methods reported in the literature.In forest soils,DOM collection is often performed by centrifugation,suction cups,or zero-tension lysimeters.In arable soils,shaking duration and intensity of soil samples to extract WEOM vary from 1min by hand stirring to overnight on a mechanical shaker.Moreover,numerous extractant types,such as distilled water,4mM CaSO 4,0.5M K 2SO 4,and 2M KCl,are used.All those methodological options have been found to significantly influence the amount and nature of the collected DOM and WEOM (Zsolnay,1996and this issue).Developing standardized extraction methods would help to reduce the uncertainty when comparing results from different studies.
In the future,more field work would be required on agricultural soils,and especially tropical soils,where specific cropping systems such as alley cropping and paddy fields have been overlooked.The effects of tillage and drainage also need to be addressed in more detail because they significantly influence soil hydrology and organic matter placement.A few authors report that earthworms induce DOM mobilization (Subler et al.,1997;Briones et al,1998;Cole et al.,2000),and the mobilized DOM appears to be particularly enriched in nitrogen (Subler et al.,1997;Briones et al.,1998).Soil fauna is highly sensitive to land use and management practices (Wardle et al.,1999;Filser et al.,2002),and the significance of this interaction in determining DOM/WEOM dynamics should be investigated.
Acknowledgements
I gratefully thank Dr.Denis Angers and Dr.Saran Sohi for helpful discussions and comments.I also thank Dr.Philippe Rochette and Claude Lapierre for kindly sharing unpublished results.
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