abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter...

10
Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production Ehsan R. Toosi a, * , Timothy A. Doane b , William R. Horwath b a School of Forestry, University of Canterbury, Christchurch, New Zealand b Department of Land, Air, and Water Resources, University of California, Davis, CA 95616, USA article info Article history: Received 3 June 2011 Received in revised form 20 February 2012 Accepted 23 February 2012 Available online 17 March 2012 Keywords: Dissolved organic matter Organic matter solubilization Microbial activity Abiotic factors Abiotic solubilization abstract Dissolved organic matter (DOM) is a small but reactive pool of the soil organic matter (SOM) that contributes to soil dynamics including the intermediary pool spanning labile to resistant SOM fractions. The solubilization of SOM (DOM production) is commonly attributed to both microbially driven and physico-chemically mediated processes, yet the extent to which these processes control DOM production is highly debated. We conducted a series of experiments using 13 C-ryegrass residue or its extract ( 13 C-ryegrass-DOM) separately under sterile and non-sterile conditions to demonstrate the importance of DOM production from microbial and physico-chemical processes. Soils with similar properties but differing in parent material were used to test the inuence of mineralogy on DOM production. To test the role of the source of C for DOM production, one set of soils was leached frequently with 13 C-ryegrass-DOM and in the other set of soils 13 C-ryegrass residue was incorporated at the beginning of the experiment into the soil and soils were leached frequently with 0.01 mol L 1 CaCl 2 solution. Leaching events for both treatments occurred at 12-d intervals over a 90-day period. The amount of dissolved organic C and N (DOC and DON) leached from residue-amended soils were consistently more than 3 times higher in sterile than non-sterile soils, decreasing with the time. Despite changes in the concentration of DOC and DON and the production of CO 2 , the proportion of DOC derived from the 13 C-ryegrass residue was largely constant during the experiment (regardless of microbial activity), with the majority (about 70%) of the DOM originating from native SOM. In 13 C-residue-DOM treatments, after successive leaching events and regardless of the sterility conditions i) the native SOM consistently supplied at least 10% of the total leached DOM, and ii) the contribution of native SOM to DOM was 2e2.9 times greater in 13 C-residue-DOM amended soils than control soils, suggesting the role of desorption and exchange reactions in DOM production in presence of fresh DOM input. The contribution of the native SOM to DOM resulted in higher aromaticity and humication index. Our results suggest that physico-chemical processes (e.g. exchange or dissolution reactions) can primarily control DOM production. However, microbial activity affects SOM solubilization indirectly through DOM turnover. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction One of the most elusive soil C pools is arguably dissolved organic matter (DOM), which is considered to be an important link among the continuum of labile to stable SOM pools (von Lützow et al., 2007). DOM is a highly mobile and reactive pool of SOM (Neff and Asner, 2001; von Lützow et al., 2007). Though dwarfed in size by total SOM, it plays an important intermediary role between physically stable and labile C pools through its fast turnover rate, high mobility and broad reactivity in the soil (Ellert and Gregorich, 1995; Neff and Asner, 2001; Boddy et al., 2007). Although little is known on actual rate of DOM ux, it has been suggested to be an important component of terrestrial C cycling (Kindler et al., 2011; Neff and Asner, 2001). Numerous studies have addressed the dynamics of DOM in terrestrial ecosystems, yet there is still much debate on mecha- nisms regulating DOM production. The dearth of information on DOM production is largely attributed to the lack of knowledge on mechanisms controlling SOM solubilization. The native SOM (vs. fresh OM input) is increasingly known as the primary source of DOM within the soil (Fröberg et al., 2006; Hagedorn et al., 2004). Currently, the consensus is that the rate of SOM solubilization (primary source of DOM) is controlled by interactions among * Corresponding author. Current address: Department of Agronomy, Iowa State University, Ames, IA 50011, USA. Tel.: þ1 5152901198; fax: þ1 515 294 3163. E-mail addresses: [email protected], [email protected] (E.R. Toosi). Contents lists available at SciVerse ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2012.02.033 Soil Biology & Biochemistry 50 (2012) 12e21

Upload: ehsan-r-toosi

Post on 12-Sep-2016

214 views

Category:

Documents


1 download

TRANSCRIPT

Page 1: Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

at SciVerse ScienceDirect

Soil Biology & Biochemistry 50 (2012) 12e21

Contents lists available

Soil Biology & Biochemistry

journal homepage: www.elsevier .com/locate/soi lbio

Abiotic solubilization of soil organic matter, a less-seen aspect of dissolvedorganic matter production

Ehsan R. Toosi a,*, Timothy A. Doane b, William R. Horwath b

a School of Forestry, University of Canterbury, Christchurch, New ZealandbDepartment of Land, Air, and Water Resources, University of California, Davis, CA 95616, USA

a r t i c l e i n f o

Article history:Received 3 June 2011Received in revised form20 February 2012Accepted 23 February 2012Available online 17 March 2012

Keywords:Dissolved organic matterOrganic matter solubilizationMicrobial activityAbiotic factorsAbiotic solubilization

* Corresponding author. Current address: DepartmUniversity, Ames, IA 50011, USA. Tel.: þ1 5152901198

E-mail addresses: [email protected], erazavy@yah

0038-0717/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.soilbio.2012.02.033

a b s t r a c t

Dissolved organic matter (DOM) is a small but reactive pool of the soil organic matter (SOM) thatcontributes to soil dynamics including the intermediary pool spanning labile to resistant SOM fractions.The solubilization of SOM (DOM production) is commonly attributed to both microbially drivenand physico-chemically mediated processes, yet the extent to which these processes control DOMproduction is highly debated. We conducted a series of experiments using 13C-ryegrass residue or itsextract (13C-ryegrass-DOM) separately under sterile and non-sterile conditions to demonstrate theimportance of DOM production from microbial and physico-chemical processes. Soils with similarproperties but differing in parent material were used to test the influence of mineralogy on DOMproduction. To test the role of the source of C for DOM production, one set of soils was leached frequentlywith 13C-ryegrass-DOM and in the other set of soils 13C-ryegrass residuewas incorporated at the beginningof the experiment into the soil and soilswere leached frequentlywith 0.01mol L�1 CaCl2 solution. Leachingevents for both treatments occurred at 12-d intervals over a 90-day period. The amount of dissolvedorganic C and N (DOC and DON) leached from residue-amended soils were consistently more than 3 timeshigher in sterile than non-sterile soils, decreasing with the time. Despite changes in the concentration ofDOC and DON and the production of CO2, the proportion of DOC derived from the 13C-ryegrass residuewaslargely constant during the experiment (regardless of microbial activity), with the majority (about 70%) ofthe DOM originating from native SOM. In 13C-residue-DOM treatments, after successive leaching eventsand regardless of the sterility conditions i) the native SOM consistently supplied at least 10% of the totalleached DOM, and ii) the contribution of native SOM to DOMwas 2e2.9 times greater in 13C-residue-DOMamended soils than control soils, suggesting the role of desorption and exchange reactions in DOMproduction in presence of fresh DOM input. The contribution of the native SOM to DOM resulted in higheraromaticity and humification index. Our results suggest that physico-chemical processes (e.g. exchange ordissolution reactions) can primarily control DOM production. However, microbial activity affects SOMsolubilization indirectly through DOM turnover.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

One of themost elusive soil C pools is arguably dissolved organicmatter (DOM), which is considered to be an important link amongthe continuum of labile to stable SOM pools (von Lützow et al.,2007). DOM is a highly mobile and reactive pool of SOM (Neffand Asner, 2001; von Lützow et al., 2007). Though dwarfed insize by total SOM, it plays an important intermediary role betweenphysically stable and labile C pools through its fast turnover rate,

ent of Agronomy, Iowa State; fax: þ1 515 294 3163.oo.com (E.R. Toosi).

All rights reserved.

high mobility and broad reactivity in the soil (Ellert and Gregorich,1995; Neff and Asner, 2001; Boddy et al., 2007). Although little isknown on actual rate of DOM flux, it has been suggested to be animportant component of terrestrial C cycling (Kindler et al., 2011;Neff and Asner, 2001).

Numerous studies have addressed the dynamics of DOM interrestrial ecosystems, yet there is still much debate on mecha-nisms regulating DOM production. The dearth of information onDOM production is largely attributed to the lack of knowledge onmechanisms controlling SOM solubilization. The native SOM(vs. fresh OM input) is increasingly known as the primary source ofDOM within the soil (Fröberg et al., 2006; Hagedorn et al., 2004).Currently, the consensus is that the rate of SOM solubilization(primary source of DOM) is controlled by interactions among

Page 2: Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

Table 1Properties of the soils used in the experiment.

Soil pHa CEC(C mol Kg�1)

ECa C (%) N (%) Textureb Soil classificationc

Sand Silt Clay

Yolo Land 5.7 15 860 1.6 0.16 26 51 23 Mixed, thermic Mollic HaploxeralfSierra Field Station 5.5 16 1280 2.4 0.18 36 45 19 Mixed, thermic Mollic Haploxeralf

a Saturated paste.b Hydrometer method.c USDA soil classification system.

E.R. Toosi et al. / Soil Biology & Biochemistry 50 (2012) 12e21 13

abiotic (environmental conditions, soil properties, and OM quality)and biotic processes (Fierer et al., 2009; Kuzyakov, 2010). Therelative significance of biotic and abiotic controls on DOMproduction remains unresolved (Guggenberger et al., 1994;Kemmitt et al., 2008; Moore et al., 2008; Neff and Asner, 2001).

The concept that size, activity and composition of microbialcommunity largely control decomposition of SOM pools is wide-spread in the literature (Fontaine and Barot, 2005; Kalbitz et al.,2000a,b). In an investigation on the role of the decomposercommunity in regulating DOM production, Kemmitt et al. (2008)identified strong physico-chemical controls on DOM production,challenging the concept of microbial control on SOM mineraliza-tion. They proposed the “regulatory gate” hypothesis, positing thatabiotic factors primarily control DOM production. The hypothesisstates that SOM is mineralized during a two-step process with non-bioavailable compounds initially transformed to bioavailable OM(DOM), solely as a result of abiotic processes. The mobilized(bioavailable) compounds are subsequently metabolizable by thedecomposer community (the second step). Desorption, diffusion,and a range of chemical reactions were proposed as possible abioticmechanisms that regulate SOM solubilization (Kemmitt et al.,2008). The separation of SOM decomposition and microbialactivity had been previously proposed (e.g. Schimel andWeintraub,2003) in order to explain the observed lack of relationship betweenSOM mineralization and microbial activity (e.g. Garcia-Pausas andPaterson, 2011; Griffiths et al., 2000). However, the regulatorygate hypothesis remains controversial mainly because it ignoresthe role of biological factors in controlling the mineralization ofSOM (Kuzyakov et al., 2009; Paterson, 2009).

The regulatory gate hypothesis states that in the absence of freshorganic matter input, DOM production from native SOM occursindependent of biological activity of the soil (Kemmitt et al., 2008).We tested the regulatory gate hypothesis, using a 13C isotope pooldilution approach to determine the influence of addition of twoforms of fresh OM on DOMproduction from native SOM. Sterile andnon-sterile soil conditions were imposed to determine the role ofmicrobial activity in the production of DOM. Factors influencingDOM production were further studied by examining the role of soilmineralogy and OM input (litter vs. litter extract). Previous studieshave shown that mineralogy and source of OM considerably affectthe SOM turnover and thus, its solubilization (Moorhead andSinsabaugh, 2006; Rasmussen et al., 2007). We selected i) twosimilar soils with different dominant mineralogy and ii) two formsof OM input i.e. plant residue and its extract (plant DOM). We

Table 2Properties of ryegrass residue and DOM added to the soils.

Source C N P C/N

Ryegrass 38.9b 1.41b 0.26b 27.6DOMa 382 � 10c 25.6 � 0.7c 2.3 � 0.17c 15 �a Data are mean � SE of 9 extraction sets.b %.c mg L�1.

assumed that if the regulatory gate hypothesis functions under theexperimental conditions (fresh OM input), regardless of soilmineralogy and the form of OM input, the size of the solubilized OMpool will be largely independent of microbial activity. The objectiveof this studywas to evaluate the role of the microbial activity on thenet production of DOM from the native SOM in the presence ofadded plant residue or its extract.

2. Materials and methods

2.1. Soil sampling

Soils were selected to reflect differences in mineralogy but werelargely similar in their other properties (Table 1). Samples werecollected from soils under i) oak woodland (Sierra Foothill RangeField Station, 39.278 N and 121.289 W) and ii) permanent pasture(Yolo County Land, 38.650 N and 122.066 W). While both soilscontain 2:1 clay minerals, the oak woodland (Sierra) soil is derivedfrom schist (metavolcanic), dominated by mica and chlorite, witha high iron oxide content. In contrast, the pasture (Yolo) soil isderived from mixed alluvium and its secondary mineralogy isdominated by montmorillonite. The soils were sampled from the0e15 cm depth from multiple locations within an area of 0.5 ha ateach site and composited. The visible litter of the soils was removedand the soils were sieved (2 mm). Soils were adjusted to 50% of thewater holding capacity (WHC) and pre-incubated for 10 d at 22 �Cbefore the start of the experiment.

2.2. 13C-ryegrass and 13C-ryegrass-DOM

Ryegrass (Lolium perenne L.) was grown in a temperaturecontrolled chamber and labeled weekly (10 wk) with 10% atomenriched 13CeCO2 (See Bird et al., 2003). The above-ground part ofryegrass was harvested, oven dried at 50 �C, ground and sieved(mesh size 40). Two experiments were conducted to examine therole of plant residue (A) or plant residue extract (B) on DOMproduction in soil. Residue extract (ryegrass-DOM) was preparedby extracting plant residue with 0.01 mol L�1 CaCl2 (0.5:100 ratio)at 75 �C for 6 h followed by shaking (15 min). The extract wasfiltered through a 0.45 mm filter. The ryegrass-DOM was preparedfresh before each leaching event (see Experiment B). Table 2shows the properties of the 13C-ryegrass residue and 13C-ryegrass-DOM.

13C (atom %) SUVA (L mg�1eC m�1) HI

1.819 _ _0.6 1.681 � 0.01 0.86 � 0.03 0.61 � 0.02

Page 3: Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

E.R. Toosi et al. / Soil Biology & Biochemistry 50 (2012) 12e2114

2.3. Incubation experiments

2.3.1. Experiment A: 13C-ryegrass residueThe pre-incubated soils were mixed with sand (0.4:1, sand-to-

soil ratio, dry based) to ensure proper infiltration and leachingduring the incubation. The sand-soil mixture was wetted to 50% ofWHC and incubated for 7 d before the start of the experiment. 13Clabeled ryegrass residue at a ratio of 1% (w/w, dry based) wasuniformly mixed into the soil-sand mixture and 70 g of the mixturewas packed into a 100 mL polyethylene leaching column(2.5 � 13 cm) with a fixed glass-fiber filter at the bottom to retainthe soil. A subset of soils containing the 13C labelled plant residuewas sterilized by autoclaving at 121 �C (1 h). Soil without addedryegrass served as controls. Each treatment was independentlyreplicated three times. Each leaching column was placed in a one-quart (946 mL) Mason jar containing about 5 mL water to preventsoil desiccation, sealed and placed in the dark at 22 �C. The lids ofthe jars contained septa for headspace sampling to monitor CO2production using an infrared gas analyzer (Qubit CO2 analyzer,model S-151, Qubit systems, Kingston, ON, Canada). The duration ofincubation was 90 d (78 d for control soils) to provide sufficienttime for the assessment of DOM production dynamics. Thesampling of headspace CO2 occurred regularly to ensure the CO2

concentration did not exceed 2% by volume. At each CO2 samplingpoint, a sample of 12 mL for 13CO2 was transferred to a Vacutainerand analyzed on a SerCon Cryoprep TGII trace gas concentrationsystem interfaced on a PDZ Europa 20e20 isotope ratio massspectrometer (Sercon Ltd., Cheshire, UK). After each CO2 sampling,the jars were aerated. Headspace samples from blank jars wereused to correct the sample CO2 concentration.

The soil columns were leached on day 1 and after 6, 18, 30, 42,54, 66, 78 and 90 days with CaCl2 (0.01 mol L�1) at a ratio of 1:1(solution-to-soil) to maintain the soil structure and uniformleaching characteristics. To maintain the sterility of the autoclavedsoils during the incubation, HgCl2 (0.7 mg-Hg g-soil�1; Wolf andSkipper, 1994) was added to the leaching solution. Following eachleaching event, the soil columns were placed under 1 atm ofvacuum, weighed and additional water added as necessary tomaintain 50% of WHC throughout the incubation. The leachate wasfiltered through a 0.45 mm filter (DOM) and stored at 4 �C (<7 d)until analyzed.

2.3.2. Experiment B: 13C-ryegrass-DOMThe same soil preparation, sterilization procedures, leaching

columns, and CO2 and DOM analysis was carried out as inExperiment A, but instead of 13Ceryegrass residue, its extract(ryegrass-DOM) was added to the soil at a ratio of 1:1 (ryegrass-DOM-to-Soil; w/w) (part 2.2.). The ryegrass-DOM (Table 2) wasapplied during the leaching events described above. HgCl2(0.7 mg-Hg g-soil�1) was added to the ryegrass-DOM before eachapplication to sterile soil treatments. Prior to the experiment itwas demonstrated that no interaction (precipitation or fluctua-tion) occurred between Hg and DOM at the concentration used(data not shown).

2.4. Analytical methods

Total soil C was determined on an elemental analyzer (CostechECS 4010, Valencia, CA). Dissolved organic C was determined byUV-persulfate digestion (Teledyne-Tekmar Phoenix 8000). Totaldissolved nitrogen was determined by persulfate oxidation(Cabrera and Beare, 1993). Mineral N (NO�

3 and NHþ4 ) was deter-

mined colorimetrically (Verdouw et al., 1978; Doane and Horwath,2003). Organic N was calculated as the difference between theconcentrations of total N and total mineral N.

UV absorbance of leached DOM was determined in 1 cm quartzcuvettes at room temperature, on a Shimadzu UV spectrophotom-eter (UV-Mini 1240) at 280 nm to estimate the average degree ofaromaticity of DOM. Specific UV absorption (SUVA) was calculatedafter normalizing UV absorption value for the DOC concentration(Weishaar et al., 2003). A SUVA value more than 0.3 was diluted toensure comparability of results. No interference by Hg on DOM UVabsorption at 280 nm was observed following separate addition ofHgCl2 (data not shown).

Fluorescence measurements were carried out in 1 cm quartzfluorescence cells at room temperature, using a Cary Eclipse spec-trofluorometer (Varian Inc, CA, USA). The samples containing morethan 10 mg L�1 DOC were diluted before fluorescence analysis. Theexcitationwavelength and the emissionwavelength range were setat 280 nm and 400e470 nm (5 nm increments), respectively.Humification Index (HI) was calculated as the ratio of fluorescenceintensity at 470:400 nm (Kalbitz et al., 2000a,b).

The DOM samples were analyzed for 13C content using a TOCanalyzer (1010 OI Analytical, College Station, TX, USA) coupled toa PDZ Europa 20e20 isotope ratio mass spectrometer (Sercon Ltd.,Cheshire, UK). Each run of samples included working DOC stan-dards prepared with glucose of known 13C enrichment in order toensure accuracy of the 13C determinationwithin and between runs.At each CO2 sampling point, a headspace sample was taken for13CO2 as described in part 2.3.1. The fraction (F) of C originatingfrom added 13C-ryegrass residue or 13C-ryegrass-DOM in leachateand CO2 was calculated according to the following isotope mixingmodel:

F ¼�d13Csample � d13Ccontrol

�.�d13Csubstrate � d13Ccontrol

(1)

Where d13Csample is the d13C value of the leached or respired CO2from treated soils, d13Ccontrol is the d13C value in the leachate orrespired CO2 from control soils, and d13Csubstrate is the d13C value ofthe input 13C-ryegrass residue or 13C-ryegrass-DOM. All data arereported as mean � standard error of three replicates.

3. Results

3.1. CO2 efflux in all experiments

We observed regular increase in CO2 production correspondingto leaching events in non-sterile soils in both experiments (Fig. 1).No CO2 was emitted in the sterile soils in any treatment (data notshown). The ryegrass amended (non-sterile) and correspondingcontrol soils showed a declining valley and peak cycle in CO2production subsequent to leaching events (Fig. 1a). In non-sterileryegrass-DOM amended soils, CO2 production was characterizedby consistent valley and peak cycles as a result of the addedryegrass-DOM at each leaching event (Fig. 1b). In contrast toryegrass-DOM amended soils, the respiration rate of ryegrassamended soils declined considerably (89% and 75% in Sierra andYolo soil, respectively) by the end of the incubation period (Fig. 1a).Similarly, the respiration rate of control soils declined 76% in Sierraand 72% in Yolo soil. The CO2 production was higher in Yolo thanSierra soil in control soils (468 vs. 357 mg-C g�1 soil), and bothryegrass amended and ryegrass-DOM amended soils (Fig. 1 andTable 3). A considerable portion (46e52%) of the added ryegrassresidue was mineralized to CO2, amounting to 67 and 71% of thetotal respired C in Yolo and Sierra soils, respectively (Table 3). Incomparison, a smaller proportion (14e30%) of ryegrass-DOM wasrespired as CO2, equivalent to 42% and 27% of the total C loss (CO2)in Yolo and Sierra soils, respectively.

Page 4: Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

Fig. 2. Changes in DOC concentration in ryegrass residue amended (a) and ryegrass-DOM amended (b) soils in living and sterile conditions during incubation period (St:sterile soil; data are mean � SE, n ¼ 3).

Fig. 1. Dynamics of CO2 production in control, ryegrass residue amended (a), andryegrass-DOM amended (b) soils in living and sterile conditions.

E.R. Toosi et al. / Soil Biology & Biochemistry 50 (2012) 12e21 15

3.2. Experiment A: ryegrass residue amended soils

The concentration of the DOC leached from sterile soils wasconsistently higher than non-sterile soils (Fig. 2a). By Day 50, DOCleached from sterile soils stabilized, but remained about 10 mg-C g�1 soil higher than non-sterile soils. Despite the overall lowerconcentrations of DOC leached from control than ryegrass amen-ded soils, the pattern of DOC changes during experiment wassimilar (Fig. 2a). In sterile control soils, DOC decreased substantially(86 and 90% in Yolo and Sierra, respectively), but it changedconsiderably less in non-sterile control soils (28 and 37% in Yoloand Sierra, respectively). Overall, the amounts and pattern of DOC

Table 3The amounts of C respired from native SOM vs. added ryegrass, and the contribution of Cand leached at the end of experiment.

Soil Total respired C(mg-C g�1 soil)

SOM respired C(mg-C g�1 soil)

Added OM(mg-C g�1 s

Ryegrass residue amended soilYolo 3028 995 2033Yolo (St) e e e

Yolo (Cl) 468 468 e

Sierra 2525 744 1781Sierra (St) e e e

Sierra (Cl) 357 357 e

Ryegrass-DOM amended soilYolo 2387 1395 992Yolo (St) e e e

Sierra 1714 1246 468Sierra (St) e e e

St: sterileCl: control

release during the experiment was similar for both Yolo and Sierrasoils (Fig. 2a).

The pattern of DON release during the experiment was similarto DOC (Fig. 3a). We observed a substantial decrease in theconcentration of DON leached from both non-sterile (88% and 95%in Yolo and Sierra soil, respectively) and sterile soils (91% and 92% inYolo and Sierra soils, respectively) during the incubations.

Apart from a few exceptions, the proportion of the DOC derivedfrom the added ryegrass residue was less than 30% of the total DOCleached from the soils (Fig. 4). Despite slight fluctuations in the

derived from 13C-ryegrass residue or 13C-ryegrass-DOM to respired, retained in soil

respired Coil)

Respired (%) Retained (%) Leached (%)

52.4 � 0.7 37.2 � 0.7 10.4 � 0.4e 85.5 � 0.4 14.5 � 0.4e e e

45.9 � 2.3 46.2 � 2.1 8.0 � 0.2e 87.9 � 0.2 12.1 � 0.2e e e

30.1 � 2.1 17.0 � 0.4 52.9 � 2.5e 27.7 � 0.6 72.3 � 0.614.2 � 0.9 16.6 � 0.5 68.0 � 1.2e 28.8 � 0.9 71.2 � 0.9

Page 5: Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

Fig. 3. Changes in DON concentration in ryegrass residue amended (a) and ryegrass-DOM amended (b) soils in living and sterile conditions during incubation period (St:sterile soil; data are mean � SE, n ¼ 3).

E.R. Toosi et al. / Soil Biology & Biochemistry 50 (2012) 12e2116

proportion of the DOC derived from the added ryegrass in non-sterile soils, this proportion was largely constant in sterile soilsduring the experiment. Only 8e14.5% of the total DOC was derivedfrom the added ryegrass residue (Table 3). At the end of theexperiment the amount of remaining C in the soil from ryegrassresidue was almost 2 times higher in the sterile than non-sterilized

Fig. 4. Changes in the proportion of DOC derived from added ryegrass residue orryegrass-DOM in living and sterile soils during incubation period (St: sterile; data aremean � SE, n ¼ 3).

soils. The difference was accounted for as respired C (46% in Sierraand52% in Yolo) from ryegrass residue in non-sterile soils.

The pattern of changes in the C/N ratio of leached DOM betweennon-sterile soils differed prior to Day 30, but by the end of theexperiment, C/N ratio was more stable and similar between soils(15.0 and 14.3 in Yolo and Sierra soil, respectively) (Fig. 5a). Insterile soils, the initial C/N ratio ranged from 13.0 to 16.8 andincreased sharply to>20 for both soils by the end of the experiment(Fig. 5a).

The SUVA values were constantly greater in DOM leached fromnon-sterile than sterile soils, indicating a higher degree of aroma-ticity (Fig. 5c). The proportion of SUVA values of sterile soils grad-ually increased during the incubation period (48% and 83%, in Yoloand Sierra, respectively). In non-sterile soils, SUVA values fluctu-ated, increasing (115%) in Sierra soil or decreasing (16%) in Yolo soil,but by the end of the experiment, SUVA values of DOM obtainedfrom both soils reached a similar level (Fig. 5c).

Similar to SUVA, HI of DOMwas constantly higher in non-sterilethan sterile soils (Fig. 5e). The HI of DOM leached from non-sterilesoils was almost stable. In contrast, HI of sterile soils graduallyincreased (14% for Yolo and 12% for Sierra) during the incubation(Fig. 5e).

3.3. Experiment B: ryegrass-DOM amended soils

The concentration of DOC leached from sterile soils (both Yoloand Sierra) was consistently higher than non-sterile soils (Fig. 2b).In contrast to sterile soils, the pattern of the DOC release in non-sterile soils differed, with the Sierra soil releasing higher DOCafter day 20 of the experiment. By the end of the experiment, DOCconcentration was similar between Yolo and Sierra in sterileor non-sterile soils. In sterile soils DOC release increased slightly(6% and 17% in Yolo and Sierra soil, respectively) during theexperiment (Fig. 2b). In contrast, in non-sterile soils DOC releaseincreased substantially (89% and 93% in Yolo and Sierra soil,respectively) during the incubation period.

A similar pattern of DON release to that of DOC (Fig. 3b) occurredsubsequent of ryegrass-DOM addition. The concentration of theDON leached from non-sterile soils increased substantially (from2.4 to 18.0 and 5.5 to 17.5 mg-N g-soil�1 in Yolo and Sierra soil,respectively) following frequent application of ryegrass-DOM(Fig. 3b). This led to a similar concentration of the DON leachedfrom sterile and non-sterile soils at the end of the experiment. Theconcentration of DON leached from sterile soils was consistentlyless than the concentration of N of ryegrass-DOM (25.6 � 0.7 mg-N g-soil�1, Table 2).

The proportion of DOC derived from ryegrass-DOM was fairlyconstant for both soils and sterility status during the incubation(Fig. 4). We observed that in non-sterile soils at least 92.0% (Yolo)and 88.7% (Sierra) of the DOC was derived from ryegrass-DOM(Fig. 4). This proportion was less in sterile soils, but increasedslightly (18% in both soils) during the experiment. The differencebetween proportions of DOC derived from ryegrass-DOM in sterileand non-sterile soils decreased during the incubation experiment(Fig. 4). Between 53% to 72% of DOC derived from ryegrass-DOMwas recovered from both sterile and non-sterile soils (Table 3).The proportion of ryegrass-DOM that remained in soils at the end ofexperiment was similar between Yolo and Sierra soils (about 17%),but substantially smaller in non-sterile than the sterile soils (28 and29% in Yolo and Sierra, respectively). The difference was accountedfor as respired C in non-sterile soils (14e30% of added ryegrass-DOM).

Despite some differences in the initial C/N ratio of leached DOM,this was largely constant during the incubation in both soilsregardless of sterility status (Fig. 5b). The C/N ratio of the leached

Page 6: Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

Fig. 5. Changes in the C/N ratio, specific UV absorption (SUVA), and humification index (HI) of DOM in ryegrass residue amended soils (a, c, e) and ryegrass-DOM amended soils (b,d, f) in sterile and non-sterile conditions (St: sterile soil; data are mean � SE, n ¼ 3).

E.R. Toosi et al. / Soil Biology & Biochemistry 50 (2012) 12e21 17

DOM was slightly higher than that of ryegrass-DOM (mean15 � 0.6, Table 2).

We observed a gradual increase in SUVA values of the leachedDOM during the incubation period (Fig. 5d). However, the overallincrease was greater in DOM leached from non-sterile soils (26%and 23% in Yolo and Sierra soil, respectively) compared to sterilesoils (7% and 21% in Yolo and Sierra soil, respectively). The SUVAvalues of the leached DOM remained less than that of ryegrass-DOM (0.86� 0.03 L mg-Cm�1, Table 2) throughout the experiment.

The HI of leached DOM did not change in non-sterile soils, but itincreased 18% in Yolo and 19% in Sierra soils in sterile soils (Fig. 5f).The HI of DOM leached from non-sterile soils was greater thansterile soils throughout the experiment. The HI of ryegrass-DOM(0.61 � 0.02, Table 2) increased after passing through the soil inboth sterile and non-sterile soils.

4. Discussion

4.1. Soil respiration

Yolo and Sierra residue amended soils both respired a similarproportion (about 50%) of the added ryegrass. In contrast, consid-erably less proportion of C input (14% in Sierra and 30% in Yolo) wasmineralized in ryegrass-DOM amended soil. This suggests thecontrasting role of soil mineralogy in C mineralization after addi-tion of different forms of OM input (residue vs. extract), in agree-ment with Rasmussen et al. (2007). The addition of the ryegrassresidue or ryegrass-DOM resulted in increase in the mineralizationof native SOM. Compared to control soils, the amount of respired Cfrom SOM (CO2 primed) was 2.1 times larger in residue amendedsoils (both Yolo and Sierra) and 3.0e3.5 times larger in ryegrass-

Page 7: Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

E.R. Toosi et al. / Soil Biology & Biochemistry 50 (2012) 12e2118

DOM amended soils. The accelerated decomposition of native SOMin response to added labile OM, termed the priming effect, has longbeen observed (e.g. Löhnis, 1926; Bingeman et al., 1953), but itsimportance has been of recent focus as complicated interactionsbetween living and dead OM pools (Kuzyakov, 2010).

In residue amended soils the majority (w70%) of leached DOC(see Section 4.3) was derived from the humified OM. In contrast,the majority (67e71%) of mineralized C was derived from ryegrassresidue. Similarly, Hagedorn et al. (2004) reported that fresh Cinput was preferentially respired as CO2, with the majority of DOCoriginating from the humified SOM. This is in line with recentevidence suggesting that the solubilized SOM does not necessarilyserve as the primary source of assimilated C (Christ and David,1996; Chow et al., 2006).

4.2. Dynamics of leached DOC and DON

The initially higher concentration of DOC and DON in sterileryegrass amended or ryegrass-DOM amended soils (Figs. 2 and 3)was likely due to the (i) release of lyzedmicrobial biomass (Warcup,1957), (ii) release/detachment of the physically bound/trapped OM(Berns et al., 2008), and (iii) solubilization of the OM after auto-claving (Alef, 1995; Salonius et al., 1976). The initial few leachingevents appeared sufficient to remove DOC and DON produced asa result of the autoclaving. Thus, the subsequent lower concentra-tions of DOC and DON leached from non-sterile soils is attributed toC and N uptake during microbial assimilation and respiration (e.g.Stutter et al., 2007).

The soils had similar texture and other characteristics (Table 1),but differed in their dominant mineralogy. We posit that thedifferent trends of DOC and DON release in ryegrass-DOM amendedSierra compared to Yolo soil was likely due to its more complexmineralogy, particularly its high iron oxides content. However,similar amounts of DOC and DON were being leached from bothsoils by the end of the incubation. Other studies have shown theconsiderable influence of mineralogy on DOM adsorption (e.g.Benke et al., 1999). In addition to mineralogy, different patterns ofDOC and DON release (and CO2 efflux) may be due to differentdecomposer community of the soils. Decomposition rate of addedplant residue (grass vs. forest) has been shown to be affected bynative microbial community of the soil (Cookson et al., 1998).Therefore, at the initial stage of the incubation, the microbialcommunity of woodland (Sierra) soil may have been less efficientthan grassland (Yolo) soil to decompose ryegrass extract (DOM).

4.3. Abiotic solubilization of SOM

In ryegrass residue amended soils, apart from a few exceptionsthe contribution of C derived from the added residue was not morethan 30% of leached C (DOC); instead, themajority (60e80%) of DOCoriginated from native SOM, irrespective of sterility status (Fig. 4).The lack of a considerable difference in the proportion of solubi-lized C between sterile and non-sterile soils highlights the role ofabiotic factors in OM solubization. Paterson et al. (2007) quantifiedthe accelerated solubilization of the native SOM bymicroorganismsin the presence of labile OM input and showed that after 4 weeksmore than 80% of bacterial biomass C within the rhizosphereoriginated from native SOM. Other studies have shown thatcompared to native SOM, fresh OM input (throughfall, rhizodepo-sition, litter layer) has a minor contribution to DOM production intopsoil (e.g. Hagedorn et al., 2002) and specifically subsoil (e.g.Fröberg et al., 2006). For example, Fröberg et al. (2006) using 14Cand Sanderman et al. (2008) with 14C and 13C-NMR techniquesshowed that the origin of solubilized OM (DOM) within the mineralsoil was the humified SOM and not fresh OM input from upper

horizons. They concluded that DOM is formed during (i) substantialmicrobial processing of OM (biotic pathway) and particularly (ii)exchange (sorptionedesorption) reactions between incoming DOMand humified SOM (abiotic pathway).

Even after successive addition of ryegrass-DOM, 10e20% of DOCin sterilized soils was consistently derived from the native SOM(Fig. 4), implying the role of exchange/desorption reactions in SOMsolubilization. In addition and regardless of sterility conditions, thecontribution of native SOM to DOC was 2.0e2.9 times greater inryegrass-DOM amended than control soils. This is in agreementwith Marx et al. (2007) and signifies the role of DOM input insolubilizing native SOM via different sorption-desorption mecha-nisms. The accelerated solubilization of native SOM subsequent toDOM input (e.g. rhizodeposits, litter leachate, etc.) has been sug-gested as a mechanism of the release of primed C (Marx et al.,2007). In ryegrass-DOM amended soils, the total amount of DOCwas consistently larger in sterile than non-sterile soils (Fig. 2b), buta larger proportion of DOC was derived from SOM in sterile soils(Fig. 4). This was accounted to be due to C release from SOM andmicrobial biomass after autoclaving (Alef, 1995; Berns et al., 2008)and partial exchange of the released OMwith the applied ryegrass-DOM. This suggests that microbial activity reduced the contributionof native SOM to DOM, through immobilization and respiration (seeSection 4.1.). This along with the results observed in the residueamended soils (see above) highlight the contribution of abioticprocesses (e.g. exchange and accelerated solubilization/desorptionof native SOM) in DOM production from SOM.

Our results did not show a considerable difference over time inthe proportion of leached DOC derived from ryegrass residue orryegrass-DOM, regardless of declining soil microbial activity(measured as CO2; Fig. 4). However, the presence of microorgan-isms initially enhanced the production of DOM, suggesting thatdecomposers, may partly, control the rate-limiting step of SOMmineralization (Garcia-Pausas and Paterson, 2011). However, theinitial stronger impact of microbial activity was largely attenuatedover incubation time as evidenced by only a small differencebetween the proportions of the DOC derived from either ryegrassresidue or ryegrass-DOM in sterile and non-sterile soils. Includingmicrobial activity into OM decomposition (main source of DOM)models has been suggested to improve the simulation of SOMturnover (Lawrence et al., 2009). This is consistent with the notionthat a combination of biotic and abiotic factors determines DOMproduction in the soil (Kuzyakov et al., 2009). Our findings supportprevious studies that have indicated that the process of DOMexport within the soil is controlled primarily by abiotic factors(Michalzik and Matzner, 1999; Neff and Asner, 2001). For example,diffusion of OM from the immobile to mobile phase (desorption/sorption) has been suggested to act as a constant DOM replenishingprocess in soil (Tipping, 1998; Stutter et al., 2007). The diffusionprocess itself is largely regulated by soil water flux (Park andMatzner, 2003) and temperature. Comparing of a large number ofsoil samples collected across a global latitudinal gradient, Joneset al. (2009) reported that the conversion of high- to low-molecular weight OM is the rate-limiting step in SOM breakdownand is primarily controlled by abiotic factors (moisture, tempera-ture, etc.). In contrast, Pansu et al. (2010) compared six contrastingecosystems and reported that decomposition of OM is primarilycontrolled bymicrobial activity (but not community size). However,they acknowledged the role of non-biological factors as the mainregulator of OM transformations.

Our results do not contradict the close relationship betweenDOC production and microbial activity in the soil, but clearlyindicate the strong influence of physico-chemical processes incontrolling DOC production. Despite some ambiguities (Kuzyakovet al., 2009) raised from the experimental conditions in the study

Page 8: Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

E.R. Toosi et al. / Soil Biology & Biochemistry 50 (2012) 12e21 19

undertaken by Kemmitt et al. (2008), our findings are consistentwith Garcia-Pausas and Paterson (2011) and in line with theregulatory gate hypothesis proposed by Kemmitt et al. (2008). Thishypothesis states that in the absence of fresh OM, the solubilizationof the native SOM is regulated primarily by abiotic factors. Ourresults showed that even in the presence of fresh OM, the solubi-lization of native SOM (DOC production) was largely unaffected bymicrobial activity (Fig. 4). Microbial activity; however, increasedthe total solubilized OM pool (DOM) through its consumption,resulting in a higher diffusion rate of newly solubilized OM toreplenish DOM. Therefore, it appears that microbial activity affectsSOM solubilization indirectly through the turnover of DOM.

In defense of the biological pathway, the production of DOM inthe sterile soils could have resulted from the activity of residualextracellular enzymes. The activity of residual extracellularenzymes, released from (i) the lyzed microbial biomass during thesterilization processes, and/or (ii) disruption of the aggregates aftersuccessive leaching experiments (wettingedrying), could havebeen attributed to abiotic factors. In this case, one could expecta decrease in the trend of the DOC production from either ryegrassor ryegrass-DOM amended soils because of the depletion andturnover of the enzymes during the experiment. However, thealmost constant trend of DOC production from ryegrass-DOM andspecifically ryegrass residue amended soils suggests that extracel-lular enzymes played a minor role in the decomposition of OMunder the experimental conditions. The extracellular enzymeswere largely denatured during autoclaving (Carter et al., 2007;Tiwari et al., 1988). Furthermore, given the strong interference ofHg with extracellular enzymes through binding mechanisms(Baldrian, 2003), the continuous addition of Hg in the sterile soilsshould have further reduced any activity of extracellular enzymesremaining in the soils after autoclaving.

4.4. DOM properties

The initial lower C/N ratios of DOM from non-sterile ryegrassamended Yolo soil (Fig. 5a) likely resulted from greater SOMmineralization due to higher microbial activity. The lack of thistrend of C/N ratio in DOM from non-sterile Sierra soil (ryegrassamended) appears to be related to lower overall microbial activityand the greater DOM adsorption capacity of Sierra soil (Benke et al.,1999; Kaiser and Zech, 2000). Although the initial C/N ratio of DOMin sterile ryegrass amended soils (Fig. 5a) was almost stable, itappears that the C/N of DOM likely decreased due to the declineand turnover of microbial biomass with low C/N ratio (Vance et al.,1987). The increase in the C/N ratio of both sterile soils at the end ofthe experiment (Fig. 5a) is likely due to the greater contribution ofthe humified OM (the major source of DOM).

The C/N ratio of DOM leached from ryegrass residue amendedsoils was considerably less than the C/N ratio of applied ryegrassresidue (C/N: 27.6) (Table 2, Fig. 5a). In fact, given that the majorityof DOM leached from ryegrass amended soils originated from thebulk SOM, the range of C/N ratio of leached DOMwas similar to theC/N ratio of soils (10 and 13.7 in Yolo and Sierra soil, respectively).Previous studies have also shown that the C/N ratio of DOMresembles that of the SOM compared to fresh OM input (Michalzikand Matzner, 1999; Park and Matzner, 2003; Smolander andKitunen, 2002).

The C/N ratio of DOM leached from ryegrass-DOM amendedsoils was similar to C/N ratio of the applied DOM (C/N: 15 � 0.6,Table 2). This is consistent with the large contribution of ryegrass-DOM to the leached DOM in both sterile and non-sterile soils(Fig. 4). Indeed, it appears that the any microbial effect on C/N ratioof the leached DOM was largely attenuated by the high DOM inputand its short contact time with soils, resulting in the lack of

difference in the C/N ratio of DOM leached from sterile and non-sterile soils.

Specific UV absorption (SUVA) at 280 nm has been widelyshown to correlate strongly with the aromatic properties of DOM(Chin et al., 1994). We observed that SUVA values of DOM leachedfrom sterile and non-sterile soils (either ryegrass or ryegrass-DOMamended) gradually increased during the experiment (Fig. 5c andd). This likely reflects the constant release of DOM from humifiedSOM (Schaumann et al., 2000). The SUVA values of DOM leachedfrom sterile soils increased during subsequent leaching eventslikely due to the gradual depletion of the lyzed microbial biomasscomponents (low aromatic content and C/N ratio). The higher SUVAvalues of DOM leached from non-sterile soils compared to sterilesoils appears to be a result of the selective microbial uptake oflow-molecular compounds of DOM and their turnover, leavingmore aromatic compounds in the leached DOM (Ogawa et al., 2001;Stutter et al., 2007).

The humification index (HI) has been used as a sensitive indi-cator for DOM characterization and its source (Lombardi andJardim, 1999; Zsolnay et al., 1999). This property is believed toreflect the degree of polycondensation of DOM (Kalbitz et al.,2000a,b). Similar to SUVA, HI of DOM in ryegrass amended soilswas higher in non-sterile than sterile soils (Fig. 5e). The initial lowerHI of the DOM leached from sterile ryegrass amended soils isassumed to be partly associated with the presence of the lyzedmicrobial biomass, representing low HI (Akagi et al., 2007; Bernset al., 2008). While HI values of DOM in non-sterile ryegrass orryegrass-DOM amended soils were largely constant, the HI of DOMfrom sterile soils gradually increased (<20%) during the incubationperiod (Fig. 5e and f). This is likely due to the gradual removal of thelyzed microbial biomass (low HI) and/or larger contribution of thesolubilized native SOM (high HI) in DOM. The overall greater SUVAand HI properties of DOM in ryegrass amended compared toryegrass-DOM amended soils (Fig. 5cef) supports the properties ofthe major source of the leached DOM (humified SOM vs. ryegrass-DOM, respectively). The similar trends of HI and SUVA values are inagreement with other studies (Kalbitz et al., 2000a,b).

4.5. Conclusions

DOM production in soil is regulated by a range of processes thatare affected by biotic and abiotic interactions. The commonlyaccepted paradigm of microbial activity being the mediators of thesolubilization of the native SOM has been recently challenged byKemmitt et al. (2008) who proposed the regulatory gate hypothesis(microbial activity is primarily controlled by “abiotically solubilizedOM”). We observed that in contrast to abiotic processes, microbialactivity did not have a considerable or sustained effect on theproportion of the DOM produced from the two added forms of OMinput (ryegrass residue or its DOM) during a 90-day incubationexperiment. The effect of varied mineralogy did not influence theresults. Under the experimental conditions, desorption of nativeSOM, solubilization of SOM due to its binding or exchange withincoming DOMwere important abiotic processes observed. Despiteits limited contribution to the pool size of the solubilized OM(DOM), microbial activity did influence the properties of the solu-bilized OM (DOM).

Acknowledgment

Authors would like to thank Peter J. Hernes for providing thefluorescence spectrophotometer. The first author was funded bythe University of Canterbury and New Zealand Postgraduate StudyAbroad scholarships. We acknowledge the J. G. Boswell EndowedChair in Soil Science funding to support this research.

Page 9: Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

E.R. Toosi et al. / Soil Biology & Biochemistry 50 (2012) 12e2120

References

Akagi, J., Zsolnay, Á., Bastida, F., 2007. Quantity and spectroscopic properties of soildissolved organic matter (DOM) as a function of soil sample treatments: air-drying and pre-incubation. Chemosphere 69, 1040e1046.

Alef, K., 1995. Sterilization of soil and inhibition of microbial activity. In: Alef, K.,Nannipieri, P. (Eds.), Methods in Applied Soil Microbiology and Biochemistry.Academic Press, San Diego, USA, pp. 52e55.

Baldrian, P., 2003. Interactions of heavy metals with white-rot fungi. Enzyme andMicrobial Technology 32, 78e91.

Benke, M.B., Mermut, A.R., Shariatmadari, H., 1999. Retention of dissolved organiccarbon from vinasse by a tropical soil, kaolinite, and Fe-oxides. Geoderma 91,47e63.

Berns, A.E., Philipp, H., Narres, H.D., Burauel, P., Vereecken, H., Tappe, W., 2008.Effect of gamma-sterilization and autoclaving on soil organic matter structureas studied by solid state NMR, UV and fluorescence spectroscopy. EuropeanJournal of Soil Science 59, 540e550.

Bingeman, C.W., Varner, J.E., Martin, W.P., 1953. The effect of the addition of organicmaterials on the decomposition of an organic soil. Soil Science Society ofAmerica Proceedings 17, 34e38.

Bird, J.A., van Kessel, C., Horwath, W.R., 2003. Stabilization and 13C-Carbon andimmobilization of 15N-Nitrogen from rice straw in humic fractions. Soil ScienceSociety of America Journal 67, 806e816.

Boddy, E., Hill, W.P., Farrar, J., Jones, D.L., 2007. Fast turnover of low molecularweight components of the dissolved organic carbon pool of temperate grass-land field soils. Soil Biology and Biochemistry 39, 827e835.

Cabrera, M.L., Beare, M.H., 1993. Alkaline persulfate oxidation for determining totalnitrogen in microbial biomass extracts. Soil Science Society of America Journal57, 1007e1012.

Carter, D.O., Yellowlees, D., Tibbett, M., 2007. Autoclaving kills soil microbes yet soilenzymes remain active. Pedobiologia 51, 295e299.

Chin, Y.P., Aiken, G., O’Loughlin, E., 1994. Molecular weight, polydispersity andspectroscopic properties of aquatic humic substances. Environmental Scienceand Technology 28, 1853e1858.

Chow, A.T., Tanji, K.K., Gao, S., Dahlgren, R.A., 2006. Temperature, water content andwet-dry cycle effects on DOC production and carbon mineralization in agri-cultural peat soils. Soil Biology and Biochemistry 38, 477e488.

Christ, M.J., David, M.B., 1996. Temperature and moisture effects on the productionof dissolved organic carbon in a Spodosol. Soil Biology and Biochemistry 28,1191e1199.

Cookson, W.R., Beare, M.H., Wilson, P.E., 1998. Effects of prior crop residuemanagement on microbial properties and crop residue decomposition. AppliedSoil Ecology 7, 179e188.

Doane, T.A., Horwath, W.R., 2003. Spectrophotometric determination of nitrate witha single reagent. Analytical Letters 36, 2713e2722.

Ellert, B.H., Gregorich, E.G., 1995. Management-induced changes in the activelycycling fractions of soil organic matter. In: McFee, W.M., Kelly, J.M. (Eds.),Carbon Forms and Functions in Forest Soils. Soil Sci. Soc. Am., Inc., Madison,pp. 119e138.

Fierer, N., Grandy, A.S., Six, J., Paul, E.A., 2009. Searching for unifying principles insoil ecology. Soil Biology and Biochemistry 41, 2249e2256.

Fontaine, S., Barot, S., 2005. Size and functional diversity of microbe populationscontrol plant persistence and long-term soil carbon accumulation. EcologyLetters 8, 1075e1087.

Fröberg, M., Berggren, D., Bergkvist, B., Bryant, C., Mulder, J., 2006. Concentrationand fluxes of dissolved organic carbon (DOC) in three Norway spruce standsalong a climatic gradient in Sweden. Biogeochemistry 77, 1e23.

Garcia-Pausas, J., Paterson, E., 2011. Microbial community abundance and structureare determinants of soil organic matter mineralisation in the presence of labilecarbon. Soil Biology and Biochemistry 43, 1705e1713.

Griffiths, B.S., Ritz, K., Bardgett, R.D., Cook, R., Christensen, S., Ekelund, F.,Sørensen, S.J., Bååth, E., Bloem, J., de Ruiter, P.C., Dolfing, J., Nicolardot, B., 2000.Ecosystem response of pasture soil communities to fumigation-inducedmicrobial diversity reductions: an examination of the biodiversity-ecosystemfunction relationship. Oikos 90, 279e294.

Guggenberger, G., Zech, W., Schulten, H.R., 1994. Formation and mobilizationpathways of dissolved organic matter: evidence from chemical structuralstudies of organic matter fractions in acid forest floor solutions. OrganicGeochemistry 21, 51e66.

Hagedorn, F., Blaser, P., Siegwolf, R., 2002. Elevated atmospheric CO2 and increasedN deposition effects on dissolved organic carbon-clues from 13C signature. SoilBiology and Biochemistry 34, 355e366.

Hagedorn, F., Saurer, M., Blaser, P., 2004. A 13C tracer study to identify the origin ofdissolved organic carbon in forested mineral soils. European Journal of SoilScience 55, 91e100.

Jones, D.L., Kielland, K., Sinclair, F.L., Dahlgren, R.A., Newsham, K.K., Farrar, J.F.,Murphy, D.V., 2009. Soil organic nitrogen mineralization across a global lat-itudinal gradient. Global Biogeochemical Cycles 23. doi:10.1029/2008GB003250.

Kaiser, K., Zech, W., 2000. Dissolved organic matter sorption by mineral constitu-ents of subsoil clay fractions. Journal of Plant Nutrition and Soil Science 163,531e535.

Kalbitz, K., Geyer, S., Geyer, W., 2000a. A comparative characterization of dissolvedorganic matter by means of original aqueous samples and isolated humicsubstances. Chemosphere 40, 1305e1312.

Kalbitz, K., Solinger, S., Park, J.H., Michalzik, B., Matzner, E., 2000b. Controls onthe dynamics dissolved organic matter in soils: a review. Soil Science 165,277e304.

Kemmitt, S.J., Lanyon, C.V., Wen, W.Q., Addiscott, T.M., Bird, N.R.A., O’Donnell, A.G.,Brookes, P.C., 2008. Mineralization of native soil organic matter is not regulatedby the size, activity or composition of the soil microbial biomass e a newperspective. Soil Biology and Biochemistry 40, 61e73.

Kindler, R., Siemens, J., Kaiser, K., Walmsley, D.C., Bernhofer, C., Buchmann, N.,Cellier, P., Eugster, W., Gleixner, G., Grunwald, T., Heim, A., Ibrom, A., Jones, S.K.,Jones, M., Klumpp, K., Kutsch, W., Larsen, K.S., Lehuger, S., Loubet, B.,McKenzie, R., Moors, E., Osborne, B., Pilegaard, K., Rebmann, C., Saunders, M.,Schmidt, M.W.I., Schrumpf, M., Seyfferth, J., Skiba, U., Soussana, J.F., Sutton, M.A.,Tefs, C., Vowinckel, B., Zeeman, M.J., Kaupenjohann, M., 2011. Dissolved carbonleaching from soil is a crucial component of the net ecosystem carbon balance.Global Change Biology 17, 1167e1185.

Kuzyakov, Y., Blagodatskaya, E., Blagodatsky, S., 2009. Comments on the paper byKemmitt et al. (2008) Mineralization of native soil organic matter is not regu-lated by the size, activity or composition of the soil microbial biomass e a newperspective’ [Soil Biology & Biochemistry 40, 61e73]. Soil Biology andBiochemistry 41, 435e439.

Kuzyakov, Y., 2010. Priming effects: interactions between living and dead organicmatter. Soil Biology and Biochemistry 42, 1363e1371.

Lawrence, C.R., Neff, J.C., Schimel, J.P., 2009. Does adding microbial mechanisms ofdecomposition improve soil organic matter models? A comparison of fourmodels using data from a pulsed rewetting experiment. Soil Biology andBiochemistry 41, 1923e1934.

Löhnis, F., 1926. Nitrogen availability of green manures. Soil Science 22, 253e290.Lombardi, A.T., Jardim, W.F., 1999. Fluorescence spectroscopy of high performance

liquid chromatography fractionated marine and terrestrial organic materials.Water Research 33, 512e520.

Marx, M., Buegger, F., Gattinger, A., Zsolnay, A., Munch, J.C., 2007. Determination ofthe fate of C-13 labelled maize and wheat exudates in an agricultural soil duringa short-term incubation. European Journal of Soil Science 58, 1175e1185.

Michalzik, B., Matzner, E., 1999. Dynamics of dissolved organic nitrogen and carbonin a Central European Norway spruce ecosystem. European Journal of SoilScience 50, 579e590.

Moore, T.R., Paré, D., Boutin, R., 2008. Production of dissolved organic carbon inCanadian forest soils. Ecosystems 11, 740e751.

Moorhead, D.L., Sinsabaugh, R.L., 2006. Theoretical models of litter decay andmicrobial interaction. Ecological Monographs 76, 151e174.

Neff, J.C., Asner, G.P., 2001. Dissolved organic carbon in terrestrial ecosystems:synthesis and a model. Ecosystems 4, 29e48.

Ogawa, H., Amagai, Y., Koike, I., Kaiser, K., Benner, R., 2001. Production of refractorydissolved organic matter by bacteria. Science 292, 917e920.

Pansu, M., Sarmiento, L., Rujano, M.A., Ablan, M., Acevedo, D., Bottner, P., 2010.Modeling organic transformations by microorganisms of soils in six contrastingecosystems: validation of the MOMOS model. Global Biogeochemical Cycles 24.doi:10.1029/2009GB003527.

Park, J.H., Matzner, E., 2003. Controls on the release of dissolved organiccarbon and nitrogen from a deciduous forest floor investigated by manip-ulations of aboveground litter inputs and water flux. Biogeochemistry 66,265e286.

Paterson, E., Gebbing, T., Abel, C., Sim, A., Telfer, G., 2007. Rhizodeposition shapesrhizosphere microbial community structure in organic soil. New Phytologist173, 600e610.

Paterson, E., 2009. Comments on the regulatory gate hypothesis and implicationsfor C-cycling in soil. Soil Biology and Biochemistry 41, 1352e1354.

Rasmussen, C., Southard, R.J., Horwath, W.R., 2007. Soil mineralogy affects coniferforest soil carbon source utilization and microbial priming. Soil Science Societyof America Journal 71, 1141e1150.

Salonius, P.O., Robinson, J.B., Chase, F.E., 1976. A comparison of autoclaved andgamma-irradiated soils as media for microbial colonization experiments. Plantand Soil 27, 239e248.

Sanderman, J., Baldock, J.A., Amundson, R., 2008. Dissolved organic carbon chem-istry and dynamics in contrasting forest and grassland soils. Biogeochemistry89, 181e198.

Schaumann, G.E., Siewert, C., Marschner, B., 2000. Kinetics of the release of dis-solved organic matter (DOM) from air-dried and pre-moistened soil material.Journal of Plant Nutrition and Soil Science 163, 1e5.

Schimel, J.P., Weintraub, M.N., 2003. The implications of exoenzyme activity onmicrobial carbon and nitrogen limitation in soil: a theoretical model. SoilBiology and Biochemistry 35, 549e563.

Smolander, A., Kitunen, V., 2002. Soil microbial activities and characteristics ofdissolved organic C and N in relation to tree species. Soil Biology andBiochemistry 34, 651e660.

Stutter, M.I., Lumsdon, D.G., Thoss, V., 2007. Physico-chemical and biologicalcontrols on dissolved organic matter in peat aggregate columns. EuropeanJournal of Soil Science 58, 646e657.

Tipping, E., 1998. Modelling properties and behaviour of dissolved organic matterin soils. Mitteilungen der Deutschen Budenkundlichen Gesellschaft 87,237e252.

Tiwari, S.C., Tiwari, B.K., Mishra, R.R., 1988. Enzyme activities in soils: effects ofleaching, ignition, autoclaving and fumigation. Soil Biology and Biochemistry20, 583e585.

Page 10: Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

E.R. Toosi et al. / Soil Biology & Biochemistry 50 (2012) 12e21 21

Vance, E.D., Brookes, P.C., Jenkinson, D.S., 1987. An extraction method for measuringsoil microbial biomass C. Soil Biology and Biochemistry 19, 703e707.

Verdouw, H., Van Echteld, C.J.A., Dekkers, E.M.J., 1978. Ammonia determinationbased on indophenol formation with sodium salicylate. Water Research 12,399e402.

von Lützow, M., Kögel-Knabner, I., Ekschmitt, K., Flessa, H., Guggenberger, G.,Matzner, E., Marschner, B., 2007. SOM fractionation methods: relevance tofunctional pools and to stabilization mechanisms. Soil Biology and Biochemistry39, 2183e2207.

Warcup, J.H., 1957. Chemical and biological aspects of soil sterilization. Soils andFertilizers 20, 1e5.

Weishaar, J.L., Aiken, G.R., Bergamaschi, B.A., Fram, M.S., Fujii, R., Mopper, K., 2003.Evaluation of specific ultraviolet absorbance as an indicator of the chemicalcomposition and reactivity of dissolved organic carbon. Environmental Scienceand Technology 37, 4702e4708.

Wolf, D.C., Skipper, H.D., 1994. Soil sterilization. In: Weaver, R.V., Angle, S.,Bottomley, P., Bezdiecek, D., Smith, S., Tabatabai, A., Wollum, A., Mickleson, S.H.(Eds.), Methods of Soil Analysis. Part 2. Microbiological and Biochemical Prop-erties. Soil Science Society of America, Inc., Madison, WI, pp. 41e51.

Zsolnay, A., Baigar, E., Jimenez, M., Steinweg, B., Saccomandi, F., 1999. Differentiatingwith fluorescence spectroscopy the sources of dissolved organic matter in soilsubjected to drying. Chemosphere 38, 45e50.