Protection of soil carbon by microaggregates within earthworm casts
Heleen Bossuyt a,*,Johan Six b ,Paul F.Hendrix a,c
a
Institute of Ecology,University of Georgia,Athens,GA 30602,USA
b
Department of Agronomy and Range Science,University of California,Davis,CA 95616,USA c
Department of Crop and Soil Sciences,University of Georgia,Athens,GA 30602,USA Received 2January 2003;received in revid form 18February 2004;accepted 10July 2004
Abstract
Earthworms are known to play a role in aggregate formation and soil organic matter (SOM)protection.However,it is still unclear at what scale and how quickly earthworms manage to protect SOM.We investigated the effects of Aporrectodea caliginosa on aggregation and aggregate-ass
ociated C pools using 13C-labeled sorghum (Sorghum bicolor (L.)Moench)leaf residue.Two incubations were t up.The first incubation consisted of soil samples crushed !250m m to break up all macroaggregates with three treatments:(i)control soil;(ii)soil C 13C-labeled residue and (iii)soil C 13C-labeled residue C earthworms.Earthworms were added after 8d and 12d (days)later,aggregate size distribution was measured together with total C and 13C in each aggregate fraction.A cond incubation was made to assay protected versus unprotected total C and 13C from 21-d laboratory incubations of intact and crushed large (O 2000m m)and small (250–2000m m)macroaggregates and microaggregates (53–250m m).Eight different pools of aggregate-associated C were quantified:(1)and (2)unprotected C pools in large and small macroaggregates,(3)unprotected C pools in microaggregates,(4)and (5)protected C pools in large and small macroaggregates,(6)protected C pool in microaggregates,and (7)and (8)protected C pools in microaggregates within large and small macroaggregates.In the prence of earthworms,a higher proportion of large macroaggregates was newly formed and the aggregates contained more C and 13C compared to bulk soil.There were no significant differences between the samples with or without earthworms in the C pool-sizes protected by macroaggregates,microaggregates or microaggregates within small macroaggregates.However,in the prence of earthworms,the C protected by microaggregates within large macroaggregates was a significant pool and 22%of this C pool was newly added C.In co
nclusion,the results clearly indicate the direct involvement of earthworms in providing protection of soil C in microaggregates within large macroaggregates leading to a possible long-term stabilization of soil C.q 2004Elvier Ltd.All rights rerved.
Keywords:Aggregation;Microaggregates;Carbon;Earthworms;Carbon protection
1.Introduction
Soil aggregation has a great influence on the physical characteristics of the soil.Well-aggregated soils posss a larger pore space,a higher infiltration rate and better gaous exchange between soil and atmosphere than poorly-aggregated soils,leading to enhanced microbial activity (Lynch and Bragg,1985).Soil aggregation and soil organic matter (SOM)dynamics are cloly linked.Aggregates are thought to play an important role in the physical protection
of SOM and at the same time,SOM binds with mineral particles to form aggregates of different sizes (Tisdall and Oades,1982).The prervation of SOM is desirable for land u since SOM is widely recognized as a key component in nutrient cycling.Furthermore,the retention of organic C in soil is becoming more important since the ri in atmospheric CO 2and global warming are recent concerns (Schlesinger,1997).
Earthworms are considered to improve soil aggregation and they are known to promote the cycling of nutrients (Lee and Foster,1991;Edwards and Bohlen,1996).They play a crucial role in the removal of plant litter and other organic materials from the soil surface and the incorporation of the organic materials into soil aggregates (Martin,1991).Earthworms ingest organic matter and mix it with inorganic
0038-0717/$-e front matter q 2004Elvier Ltd.All rights rerved.
doi:10.1016/j.soilbio.2004.07.035
Soil Biology &Biochemistry 37(2005)251–258ein
/locate/soilbio
*Corresponding author.Address:Laboratory for Soil and Water Management,Katholieke Universiteit Leuven,Kasteelpark Arenberg 20,B-3001Heverlee,Belgium.Tel.:C 3216329676;fax:C 3216321997.
E-mail address:heleen.bossuyt@agr.kuleuven.ac.be (H.Bossuyt).
soil material.This mixture pass through their gut and is excreted as a cast,which contributes to soil aggregation. Casts occur mostly in the upper0–20cm of the soil(Lee and Foster,1991)and contain more water-stable aggregates than surrounding soils(Shipitalo and Protz,1988;Marinisn, 1994).The formation of water-stable macroaggregates (O250m m)depends primarily on temporary binding agents (Tisdall and Oades,1982).Earthworms play a role in the formation of the binding agents(Martin and Marinisn, 1993)through cretion of mucus in their gut.Microbial polysaccharides and other organic products in the casts may strengthen bonds between organic and mineral components, resulting in a protection against microbial degradation. Martin(1991)found a decrea in SOM decomposition in the long term when earthworms were prent,possibly due to the physical protection of SOM in water-stable aggregates.
Earthworms may also have a profound effect on soil aggregation and structure at the microaggregate scale. Several studies have indicated that during gut transit of the soil,the old microstructure is completely destroyed,but new microaggregates are formed within the casts(Shipitalo and Protz,1988;Barois et al.,1993;Jongmans et al.,2001).In an incubation study,Bossuyt et al.(2004)confirmed that new microaggregates are very rapidly formed(!20d (days))within newly-excreted casts.However,the C protective capacity of the newly formed microaggregates was not investigated.
Our objective was to investigate the effects of earth-worms on(i)soil macro-and microaggregate formation,and (ii)protection of C at a microaggregate scale inside of their casts.We ud13C-labeled sorghum leaves,to follow the incorporation of newly-added residue into different aggre-gate-associated C fractions.Carbon mineralization rates of formed intact aggregates and crushed aggregates were determined to asss the amount of C protection from decomposition exerted by the different aggregate size class.
儿童学英语2.Materials and methods
2.1.Site description and soils
Surface(0–10cm)no-tillage soil samples were collected with a shovel from the long-term agricultural experimental site(Horshoe Bend)near Athens,GA, which is located in the Piedmont of the southern Appalachian Mountains(338540N,838240W).The soil is a well-drained sandy clay loam(66%sand,13%silt, 21%clay)in the Hiwa ries(fine kaolinitic thermic typic Kanhapludult).The area receives a mean annual precipitation of1270mm.The experimental plots(0.1ha) were established in1978with replicated tillage treat-ments assigned in a complete randomized design.Details on the treatment histories at the Horshoe Bend Rearch Area Site can be found in Beare et al.(1994) and Hendrix(1997).
After collection,the soil was air-dried(moisture content after air-drying1–2%)and forced through a 250m m sieve.The250–1000m m sized sand plus particu-late organic matter fractions were kept and re-mixed with the soil after sieving.Stones larger than1000m m were discarded.
2.2.Formation of aggregates and earthworm casts
Thisfirst experiment was designed to measure the effects of earthworms on the formation and distribution of soil aggregates.The incubation was conducted as described by Bossuyt et al.(2004).Briefly,the250m m sieved soil was subjected to three treatments,each with four replicates(n Z
4):(i)no plant material and no earthworms;(ii)13C labeled plant material,but no earthworms;and(iii)13C labeled plant material and six adult earthworms[Aporrectodea caliginosa(Savigny 1826)].Carbon-13labeled sorghum(Sorghum bicolor(L.) Moench)leaves were ud as the labeled plant material. All treatments consisted of150g soil.In the last two treatments,1.2g of plant material was mixed in with the soil;the mixture was brought tofield capacity(11%water content)and put in glass jars.Tests showed that maximum aggregation occurred after20d.Therefore,samples were incubated at208C for20d.Earthworms were added after 8 d.Respiration(total CO2and13CO2)was measured every day for7d and every other day afterwards for up to 20d.At d20,the earthworms were taken out of the jars and the soil was air-dried for3d to allow earthworm casts to stabilize(Marinisn and Dexter,1990).It could be en that the earthworms had procesd a large amount of the soil in the jars.Aggregate size distribution was determined by wet sieving the capillary rewetted soil (Elliot,1986).A ries of three sieves was ud to obtain four aggregate size fractions:(i)O2000m m(large macroaggregates);(ii)250–2000m m(small macroaggre-gates);(iii)53–250m m(microaggregates);(iv)!53m m (silt and clay fraction).Following wet sieving,the aggregate size fractions O53m m were dried on the sieves in a dehumidifying chamber(108C).Particles!53m m were collected in a bucket,total volume was measured and stirred and a subsample of a known volume was taken for analysis.
2.3.Protection of C inside newly formed aggregates
and earthworm casts
This cond experiment was made to determine the effects of earthworm activity on the protection of C within earthworm casts by biologically determining the protected C and13C pools inside the newly-formed casts and macro-and microaggregates.Eight different ts of incubations were conducted:(1)and(2)intact large and small
H.Bossuyt et al./Soil Biology&Biochemistry37(2005)251–258 252
macroaggregates,(3)and(4)large and small macroaggre-gates crushed to!250m m,(5)and(6)large and small macroaggregates crushed to!53m m;(7)microaggregates and,(8)microaggregates crushed to!53m m.Aggregates were crushed until!250m m by gentle pressure with mortar and pestle and until!53m m by grinding in a grinder(Spex 8000Mixer/Mill,Spex Industries,Inc.,Edison,NJ,USA). For all eight intact and crushed aggregate fractions from the first experiment(e Section2.2),dry subsamples(12–15g) were weighed into plastic cups,and deionized water was added to achievefield capacity.The subsamples were incubated(308C)in aled jars with lids containing pta for gas sampling.Samples were incubated for21d since C and N mineralization assays are most often done f
or21d. Gas samples were taken on d3,11,and21.
2.4.Analys
Total CO2evolved during the incubations was analyzed on a Varian Star3600CX(Varian Analytical Instruments, Sugar Land,Texas)gas chromatograph,which determined concentration bad on thermal conductivity.Variations in 13C of the CO
2
evolved during the incubations were determined using a micromass VG optima mass spectro-meter(Micromass UK Ltd.,Manchester,UK).Results are expresd as:
13C‰Z½ð13R
sample
=13R standardÞK1 !1000
where13R Z13C=12C and the standard is the international Pee Dee Belemnite(PDB).
The amount of CO2–C derived from the sorghum residue (Q p)was calculated using the following mass balance:
Q t!d t Z Q p!d p C Q s!d s C Q b!d b
where Q t,the total amount of CO2–C;d t,its isotopic composition;Q p,the amount of CO2–C derived from the sorghum;d p,its isotopic composition(296G2.7‰);Q s,the amount of CO2–C derived from the soil;d s Z its isotopic composition(K24.76G0.18‰);Q b,blank CO2–C amount;d b,its isotopic composition(K7.5G0.59‰). The control samples(no sorghum added)were ud to measure Q s and d s,with the assumption of no priming effect.
Total C and13C from the aggregate size fractions was determined using a Finnigan Delta C Mass Spectrometer coupled to a Carlo Erba,NA1500,CHN Combustion Analyzer via Finnigan’s Conflo II Interface.
2.5.Calculations
The results of the aggregate incubations were ud to define eight C pools in aggregates(Bossuyt et al.,2002):(1) and(2)unprotected C pools in large and small macro-aggregates,(3)unprotected C pool ifamous
n microaggregates,(4)and(5)protected C pools in large and small macroaggregates,(6)protected C pool in microaggregates and;(7)and(8)protected C pools in microaggregates within large and small macroaggregates.The different pools were calculated as follows(e also Fig.1):
(1)and(2)Unprotected C pools in large and small
macroaggregates Z intact macroaggregate C min
(3)Unprotected C pool in microaggregates Z intact
microaggregate C min
(4)and(5)Protected C pools in large and small
macroaggregates Z!250m m crushed macroaggregate
C min K intact macroaggregate C min
(6)Protected C pool in microaggregates Z!53m m
crushed microaggregate C min K intact microaggregate
C min
(7)and(8)Protected C pools in microaggregates within
large and macroaggregates Z!53m m crushed macro-aggregate C min K macroaggregate-protected C K Unpro-tected macroaggregate C
[or,replacing macroaggregate-protected C(e Eq.(4) and(5))
Z!53m m crushed macroaggregate C min K(!250m m crushed macroaggregate C min K intact macroaggregate
C min)K intact macroaggregate C min
Z!53m m crushed macroaggregate C min K!250m m crushed macroaggregate C min
with C min the cumulative C mineralized after21d of incubation.
2.6.Statistical analysis
The data were analyzed,using the SAS statistical package for analysis of variance(ANOVA-PROC G
LM, SAS Institute,1990).Separation of means was tested with the PDIFF option of the LSMEANS statement (means paration option:TUKEY)with a significance level of P!0.05.
3.Results
3.1.Formation of aggregates and earthworm casts
we will rock you 歌词
3.1.1.Water-stable aggregates
There were significant effects of earthworm activity on the distribution of water-stable aggregates(Fig.1).Large macroaggregates(O2000m m)made up the largest pro-portion(w37%on average)of the whole soil for samples with earthworms and the proportion of large macroaggre-gates was on average3.6times greater than samples without earthworms.When no earthworms and no residue was added,no large macroaggregates were formed.The smaller
H.Bossuyt et al./Soil Biology&Biochemistry37(2005)251–258253
aggregate size class (250–2000,53–250and !53m m)made up a greater proportion of the soil without earthworms or residues.
3.1.2.Respiration
There was no significant difference in total or residue-derived cumulative respiration between samples with or without earthworms.Total respiration was 6.33mg g K 1soil and residue-derived respiration was 4.41mg K g K 1soil.The residue-derived respiration accounted for approximately 70%of the total respiration.
3.1.3.Total C and 13C concentrations
Total C and 13C were significantly influenced by earthworm activity.Total C and 13C in large macroaggre-gates were significantly higher in the prence of earth-worms (Figs.2and 3).There was no significant difference in total C or 13C in small macroaggregates.In the other aggregate size class,total C and 13C were higher in the abnce of earthworms.There were no significant differ-ences in total C between the control samples and the samples without earthworms,except for the silt and clay fraction (!53m m)in which total C was significantly higher when no residue was added.
3.2.Protection of C inside newly formed aggregates and earthworm casts
3.2.1.Unprotected and protected C and 13C pools in aggregates
The amount of C mineralized after 21d is shown in Table 1as cumulative respired C (mg C kg K 1
soil).
Fig.1.Visualization of unprotected and protected C pools in macroaggregates and
microaggregates.
Fig.2.Effects of earthworm activity on aggregate size distribution.Values followed by a different lowerca letter within aggregate size class are significantly different between sample treatments.
H.Bossuyt et al./Soil Biology &Biochemistry 37(2005)251–258
254
The unprotected C pools were always significantly larger than the corresponding protected C pools.The unpro-tected C pool in small macroaggregates was highest in the abnce of earthworms and lowest in the abnce of both earthworms and residue.The unprotected C pool in microaggregates was significantly higher in samples without earthworms than in tho with earthworms.In the control soil,this pool was not significantly different from the other treatments.The protected C pools in small macroaggregates,microaggregates and microaggregates within small macroaggregates did not differ between treatments.The protected C pool in microaggregates within large macroaggregates was a large pool in the prence of earthworms and this pool was about 2.5times higher than the protected C pool in the micro-aggregates within small macroaggregates.In contrast,the protected C pool in the microaggregates within large
macroaggregates was not detectable in the samples without earthworms becau few large macroaggregates were prent.
The amount of 13C or residue-derived C mineralized after 21d is shown in Table 1as cumulative respired 13C (mg 13C kg K 1soil).The unprotected 13C pool in small macroaggregates was higher in the abnce of earthworms.There was no difference between the treatments for the unprotected 13C pool in microaggregates.There was no protected 13C found in the large macroaggregates,small macroaggregates or microaggregates-within-small-macro-aggregates.The protected 13C pool in microaggregates-within-large-macroaggregates was a substantial pool in the prence of earthworms.Of the C in microaggregates-within-large-macroaggregates,22%was newly added residue C.Earthworm activity had no effect on the protected 13
C pool in microaggregates.
4.Discussion
4.1.Water-stable aggregates
arkeThe prence of the endogeic earthworm A.caliginosa ,had significant effects on the formation of lar
ge macro-aggregates.In the samples where earthworms were added,higher amounts of water-stable large macroaggregates (O 2000m m)were found.Several rearchers have described the positive influence of earthworms on the formation and stability of soil aggregates.Martin and Marinisn (1993)described how earthworms play an important role in the production of binding agents responsible for the formation of water-stable macroaggre-gates.van Rhee (1977),De Vleesschauwer and Lal (1981)and McKenzie and Dexter (1987)showed a higher stability in earthworm casts than in the surrounding soil aggregates.Earthworms ingest large quantities of organic materials that are mixed and excreted as casts (Parmelee et al.,
mou1990;
Fig. 3.Effects of earthworm activity on total aggregate-associated C concentrations.Values followed by a different lowerca letter within aggregate size class are significantly different between sample treatments.
Table 1
C and 13C pools associated with aggregates in soil samples with or without earthworms
Total carbon mineralized (mg kg K 1soil)13
Carbon mineralized (mg kg K 1soil)
C Worms
K Worms K Residue C Worms K Worms (1)
Large macroaggregates (O 2000m m)Unprotected C 260.8n.d*0177n.d Protected C
6.0n.d 00.0n.d (2)艾美奖2011
神户大学
Small macroaggregates (250–2000m m)Unprotected C 179.5b 283.7a 129.8c 137b 206a Protected C
1.2a 0.0a 10.7a 1.1a 0.0a (3)
初二英语上册期中试卷Microaggregates (53–250m m)Unprotected C 168.8b 260.9a 238.4ab 63.3a 72.9a Protected C初中英语单词记忆
55.8a 72.4a 70.8a 8.8a 6.3a (4)Microaggregate within large macroaggregates Protected C
161.7n.d 035.2n.d (5)
Microaggregate within small macroaggregates Protected C
62.4a
42.5a
44.1a
0.0a
0.0a
Values followed by a different lowerca letter across the table are significantly different between sample treatments.*n.d,not detectable.
H.Bossuyt et al./Soil Biology &Biochemistry 37(2005)251–258255
