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Tissue carbon sources for Pontoscolex corethrurus (Oligochaeta: Glossoscolecidae) in a sugarcane ecosystem
Soil
Bd.
Pnnted
Biochem. in Great
Vol. Britain.
21. All
No.
5. pp.
rights
703-706.
1990
0038-0717/90 Copyright
reserved
33.00
‘5: 1990 Pcrgamon
+ 0.00 Press pk
TISSUE CARBON SOURCES FOR PONTOSCOLEX CORETHRURUS (OLIGOCHAETA: GLOSSOSCOLECIDAE) IN A SUGARCANE ECOSYSTEM A. V. SPAIN Davies
Laboratory,
CSIRO,
PMB.
PO Aitkenvale,
Qld 4814. Australia
P. G. SAFFIGNA Division
of Australian
Environmental
Studies. Griffith
University.
Nathan.
Qld 41
I I,
Australia
A. W. Woov CSR Ltd, Victoria
Mill. c/o Post OtTice, Ingham, (Accepfed
I5 lunuury
Qld 4850,
Australia
1990)
Summary-The pantropical. geophagous earthworm species Ponroscolex corefhrurus (Miiller) is common in lowland soils supporting suparcane in notheastern Queensland. In comparison with situations where harvest residues are burned. its populations are substantially increased under cultural treatments in which harvest residues are retained as a surface mulch or are mechanically incorporated into the topsoil. The 6°C values of stem and leaf materials. soil organic matter and the earthworm whole-body tissues and casts wcrc determined. It is unlikely that P. corrrhrurus assimilates much of its tissue C from the more complex fractions of soil organic matter or directly from decomposing residues. at least until a late stage of breakdown. No dillircnccs in S”C values were apparent in samples of this species from the ditTcrent cultural trcatmcnts. From field observations of an intimate association between P. cowthrurus and the sugarcanc roots, WC suggest that this species may derive much of its tissue C from rhizospherc sources.
beyond an amelioration of the physical environment of the surface soil (Wood, 1986). Howcvcr. the retention of sorghum crop residues under semi-arid tropical conditions in Australia resulted in greater quantities of C. N and P being present in the soil microbial population than where above-ground residues were removed (Safiigna er al., 1989) and this may well pertain in sugarcane crops. P. corethrums is a polyhumic endogeic species with a pantropical distribution; Lavelle ef ul. (1987) have recently reviewed the biology of this parthenogenetic, near-surface dwelling species. The individuals of this species are small and tolerate cultivation well. Where populations are large, substantial masses of soil may pass annually through the gut of this species (Lavelle er al.. 1983). From observation of the soil beneath surface mulches of harvest residues, P. corethrums has a marked effect on the structure of the underlying soil. Although it is normally geophagous and casts within the soil, it casts actively into the layers of the residues during the later stages of decomposition. We report the results of a single sampling of the populations and biomasses of P. corethrums on the 29-31 July 1986 at a time judged close to peak population numbers from studies made elsewhere in the area (A. V. Spain er al.. in preparation) and demonstrates the substantial increase in populations of this species that occurs when harvest residues arc retained. The 6°C values of sugarcane plant the soils and the whole-bodies of material, P. corerhrurus were determined on samples collected at the same time. The 6°C value of a consumer’s
INTKOtyUCTION Despite the importance of the sugarcanc crop in northcastcrn Australia, little is yet known of the agents mediating organic matter breakdown and thus controlling the rates of nutrient cycling and C flow in soils supporting this crop. Studies of the biology of these soils (A. V. Spain er ul.. in preparation) indicate that ~~‘on~oscole.r cwerhrurus (Miiller) is the major earthworm species in the coastal lowland areas supporting sugarcane between lngham (latitude IS’ 42’S) and lnnisfail (latitude 17’ 32’S). The climate of the area is humid but distinctly seasonal; >70% of the rainfall occurs in the months December to April (Gordon. 1971). Sugarcanc is harvested in the drier months immediately preceding the above period. Rcccnt widcsprcad changes in the cultural system of growing sugarcane have involved the cessation of preharvest crop burning and retention of the substantial masses of harvest residues (to at least I5 t ha-‘) as a surface mulch, or their incorporation into the mineral soil. The changes have Icd to increased crop yields (Wood, 1986). substantially rcduccd soil erosion in hilly areas (Prove et al., 1986) and a possible reduction in soil compaction. Despite the warm humid climate sugarcane residues retained as a surface mulch persist for substantial periods. A. V. Spain L’I al. (in preparation) in estimated that 19% of initial residue dry matter was still present 338 days after harvest and ascribed this to the high initial C:N ratio (> 100). Nonetheless, little is known of the biological consequences of preserving these rcsiducs 703
A. V. SPAINe! al.
704
tissues is related closely to that of C it assimilates from its diet (Fry and Arnold, 1982). Previous sampling of field populations of this species had revealed an intimate association with the roots of sugarcane and we wished to test the hypothesis that P. corethrum derives much of its dietary C from rhizosphere sources. Thus, we here examine the major organic components of a sugarcane ecosystem in relation to their potential as tissue C sources for P. corerhrurus.
MATERIALS
AND
After harvest of the above-ground parts, samples were taken of the roots and all individuals of P. corethrums in the top IOcm of the rings were collected. The specimens of P. corerhrurus were counted, dried and weighed and subsamples ashed for 3 h at SOO’C to allow expression of the biomass results on an ash-free basis. Soils were sampled at IOcm intervals to 2Ocm depth 2Ocm intervals to 60cm depth and at 30cm intervals below this. The 6°C values were determined on a VG Isogas SIRA IO mass spectrometer and expressed as per mil (?&) units according to the equation:
METHODS
6 “C = [(R sample/R
The study was conducted at Coldwater, Queensland (IS. 29’S 145’ 58’E) which has an average annual rainfall of 2033 mm, based on that of Abergowrie Bridge, some 5 km distant. The soil is a sandy loam in texture and is classified as a Tropudult (Soil Survey Staff. 1975) or as a Red Earth (State ef al., 1968). N and C concentrations in the surface soils (O-IOcm) at the site are. respectively 0.06 and 0.8%. The populations of P. corerhrurus studied here were collected on 29-31 July 1986 from soil within the top IO cm of steel cylinders (100 cm dia) which had been inserted 50 cm into the soil of an established fourth ratoon (fifth year of growth) crop of sugarcane (Succhurun~ officinnrum var. ‘Pindar’). The crop had been planted in 1981, following a leguminous green manure crop of Dolichos luhbh and harvest residues had since been retained as a surface mulch. There wcrc three trcatmcnts and three cylinders per trcatmcnt. The cylinders wcrc installed early in midAugust I985 after harvesting the previous ratoon crop. The trcatmcnts imposed wcrc.
I.
Sug.rrc;lne m
yields
(1 cane
‘) and biomasxs
dry
ma~kx
frz oraanic
ha
mitllcr
‘) ror
m
each
treatment
logcthcr
:) of P.rorrrhruru.~ in the
with
populabons
three rrcarmenls
(n = 3)
P. cnrurhrurw Crop yield Treatment
P
I. R.ake and burn 2. Surke mulch 3. H.ncst
restduer
7u.4
110.8 incornorated
139.8
I]. IO’
Table I prcscnts the cstimatcd populations and biomasscs of P. corethrurus collcctcd from the top IO cm of the rings. For both the populations and the biomasses, Mann-Whitney tests showed that levels in the rake and burn treatment were significantly lower (P c 0.10) than those from the treatments where harvest residues were retained. There were no signilicant differences (P > 0.10) between the two treatments where residues were retained and, for both measures, the differences between these two treatments combined and the rake and burn treatment were significant (P ~0.03). Table I also shows that commercial yields of sugarcane are substantially increased by retaining the harvest residues. It is clear that large increases in populations of P. corethrurus resulted from the presence of the harvest residues, whether these are retained as a mulch or incorporated into the topsoil; this accords with results from more detailed sampling made elsewhere in northeastern Queensland (A. V. Spain cr ul.. in preparation). Figure I prcscnts the d”C values as
The first treatment simulated the conventional practice of burning both the crop before harvest and remaining residues after harvest. The other treatments simulated the two common conservation tillage prncticcs of the local industry. The treatments were provided with l’N-labelled urea in order to estimate losses from this form of fertilizer and the distribution of the labclled N in the plant, soil and other components of the ccosystcm. This information will bc prcsentcd clscwherc.
T.Mc
-
WKst:I.TS
&kc, untlburn. The crop was harvcstcd without burning and the rcsiducs were carefully raked and burned in situ. Sur/ilce rrsichre. The crop was harvested without burning and the residues were rctaincd as a surface mulch. Itrcarporarecl residues. The crop was harvested without burning and residues were incorporated mechanically into the surface of the mineral soil.
kidi\idu&
standard)
where R is the ratio “C/“C. Samples were prepared for analysis using a modification of the method of Le Feuvre and Jones (1988). For the vegetation and earthworm samples, cupric oxide was used as the oxygen donor. Due to the low C contents of the soil samples, vanadium pentoxide (0.5 g) was used instead of the cupric oxide because of the large sample volumes required to obtain sufficient C for analysis. The standard error of a single measurement of homogenous material is less than or equal to 0.07%. All values were indirectly refcrcnced to the international standard Pee Dee Belcmnitc (PDB) and corrected for “0: intcrfcrencc (Craig, 1957). The d “C values of P. rorerhrurus wcrc determined on whole spccimcns and soil organic matter in the gut may have contributed to the values prcscntcd.
Populalion S
Biomass
?’
5
?’
9. 1.7
7.5 199
4x
1.3x
0.X
63
3.73
0.94
31.9
276
5.IR
I.50
I
II3
5
P. corerhrurus
in a sugarcane ecosystem
they change down the profile. These values reflect organic matter largely derived from plants with the C, photosynthetic pathway (Smith and Epstein. 1971). In addition. the distinctly lower values of the topsoil C (&IO cm _T? = - 14.94, s = 0.51, n = 6; IO-20cm f = - 16.07. s = 0.35, n = 3) are attributed to dilution of the predominantly C, organic matter by that of the C, leguminous green manure crop incorporated before planting the current crop. Figure 2 presents the 6°C values of the stem material. the body tissues and casts of P. corefhrurus from the three treatments. The sample of stem material from the rake and burn treatment (7 = - 12.32. s = 0.35) was significantly (P = 0.037) more negative than that from the combined harvest residue retention treatments (y = -12.2, s =0.06. n = 5). A single sample of green leaf material was analyzed from each of the three treatments; the 6°C values and were - I’.‘1 __ and - 12.24 for the incorporated surface mulch residue retention treatments respectively, and - 12.59 for that of the rake and burn treatment. It is not known why the d”C values should vary between these treatments. The whole-body tissue values of P. cordrrurus did not differ bctwccn the three treatments (P > 0.10. Kruskal-Wallis test) and clearly hnvc greater S”C values than those of the plant tissues. soils or casts. Since the prcscncc of soil in the gut would dcprcss the d”C value of the earthworm, it is likely that the d’!C value of the tissue is higher than that prosontcd in Fig. 2. Similarly the plant tissues also have grcatcr 6°C vulucs than those of the casts and surface soils. The 6’!C values of the casts of P. core~lrrwu.s (.f = - 11.45, s = 0.37, n = 6) arc slightly but signilicantly (P < 0.10) greater than those of the surface soils. DISCUSSION
On a world scale. populations of P. coredwurus at site arc high where harvest rssiducs arc retained. This implies the passage of a large mass (200-400 t ha-‘) of soil annually through the gut of
the present
S13C
.
Y
Fig. I. Variation in the 6°C values with depth in the profile (w. rake and burn; A. surface residues: 0. incorporated residues).
705
-
kuadP.amhmnq
s&c-1c.m
mm .16
.
I
!
I
I
I
I
,
.15
-14
.13
.12
-11
-10
-9
8
6% Fig. 2. S ‘?C values of selected organic components of the sugarcane ecosystem at the study site. this species (Lavelle et al.. 1983). The species appears to have important effects on the structure of the soils it inhabits. From field observations of surface and near-surface soils, a substantially-increased quantity of structural aggregates is apparent in the treatments where harvest residues are retained. Aggregates formed by earthworms are frequently more water stable than others (Swaby, 1949) and further study of the soil structural efTccts of this geophagous earthworm is warranted. It is not known precisely from what sources P. corehms assimilates its tissue C in this environmum. Barois and Lavcllc (1986) have suggcstcd that this spccics may assimilate C from the more complex fractions of soil organic matter through a priming cll’cct occurring in the earthworm gut. liowcvcr, the wide difTcrcncc in 5°C values bctwccn the body tissues and soils (5.93%) suggests that the resistant fraction of soil organic matter contributes little to tissue C. Since P. corcfhrur~t.s is not a surface-feeding spccics, it is unlikely to obtain its nutrition directly from the decomposing surface rcsiducs and this is supported by the lack of a significant diffcrcncc in the 6°C values of the tissues of P. corehurus bctwecn trcatmcnts where harvest residues were and wcrc not rctaincd. Roots sampled at another study site wcrc similar in S”C values to those of the aerial parts of the plant and also seem unlikely to provide a direct source of tissue C. Both incorporated and surface-retained residues may eventually provide a direct source of tissue C although P. corerhrurus is unlikely to ingest harvest residues until a late stage of decomposition. However, decomposition products of these residues may indirectly provide a source of assimilable tissue C. P. corethrurus may also derive tissue C from the microbial biomass, particularly from the more prolific microbiota of the rhizosphcre (Foster, 1988). The increased microbial biomass resulting from the prcscncc of a mulch (Safigna er ~1.. 1989) may also comprise an additional source of tissue C. P. corethruru.~ may also derive part of its tissue C from root cxudatcs, lysatcs, mucilages and other rhizosphcrc products and thus participate in the ‘fast’ cycle of organic matter turnover associated with the rhizosphcrc (Coleman er al., 1983). The hypothesis that the rhizosphere is the major source of tissue C in P. corerhrurus is supported by field observations of larger earthworm populations in the row (as opposed to the inter-row spaces) and the intimate association of individuals of this species with sugarcanc roots observed during field sampling.
A. V. SPaiNet at.
706
Because of the seasonality of climate and crop production, it is likely that soil C biomass will undergo temporal changes. However, no seasonal variation in the d’?C values of P. corerhrurus tissues was noted at one other sampling site where this was measured and suggests a constant source of dietary C. De Niro and Epstein (1978) showed that wholeanimal 6°C is cu 1!% enriched relative to the dietpresumably due to preferential loss of “CO2 during respiration. Greater differences in 6°C values have been recorded between adjacent trophic levels (Rounick and Winterbourn. 1986) but these have mainly come from freshwater and marine environments. Thus. the 3%0higher 6 ‘)C values of earthworm tissue relative to plant tissue warrants further investigation. There are several possible reasons for the increased yields of sugarcane crops under harvest residue retention systems. Tomati et at. (1988) have shown that considerable chemically-based stimulation of plant growth results from the presence of earthworms, in addition to that due to an increased mineralization. A substantial amelioration of near-surface soil temperature and moisture regimes is recorded by Wood (1986). Because of improved structure due to carthworm working of the upper part of the mineral horizons. a readier acceptance of rainfall occurs and this may be of importance in stimulating early season growth. LA (1987) states that high C:N rcsiducs such as those from sugarcanc may dcplctc soil N and depress crop yields: this does not appear to occur hcrc, perhaps bccausc of relatively heavy fertilizer N application. The occurrcncc of N, fixation in dccomposing harvest residues (Patriquin. have a bearing on this.
1982) may also
Foster R. (1988) Microenvironments of soil microorganisms. Biology and Fertility of Sods 6, 189-203. Fry B. and Arnold C. (1982) Rapid “C/‘zC turnover during growth of brown shrimp @eMpus uzrecus). Oecdogia 54, 200-204. Gordon B. (1971) Ctimaric Surrey; Northern, Region 16: Queensland. Bureau of Meteorology. Commonwealth of Australia, Melbourne. Lal R. (1987) Tropical Ecology and Edaphotogy. Wiley, Chichester. Lavelle P., Rangel P. and Kanyonyo J. (1983) Intestinal mucus production by two speciesof tropical earthworms: Mittsoniu tumfoiunna (Megascolecidaef and Pontoscolex corerhrurus (Glossoscolecidae). In New TrenrLr in Soils Siotogy (P. Lebrun, H. M. Andre. A. De Medts, C. Gregoire-Wibo and G. Wauthy, Eds). pp. 405-410. Dieu-B&hart, Ottignies-Louvain-la-Neuve. Lavelle P.. Barois 1, Crut I., Fragoso C.. Hemandez A. Pinesa A. and Rangel P. (1987) Adaptive strategies of Pontoscotex corethruras (Gtossoscolecidae, Oligochaeta), a peregrine geophagous earthworm of the humid tropics. Biology and Ferfilirv of Soils 5. 188-194. Le Feuvre R. P. and Jones R. J. (1988) Static combustion of biological samples sealed in glass tubes as a preparation for 6°C determination. Analvst 113. 817-823. Patriquin D. G. (1982) Nitrogen.fixation in sugarcane litter. Biujogical
Agriculture
and-Hor#i~at:ure
1, $9-64.
Prove B. G.. Truonn P. N. and Evans D. S. (1986) Strategies for contr&ling &eland erosion in the wit trdpical coast of Queensland. Proceedings of the Australiun Saciefy of Sugar Cane Technologists 77-84.
Rounick S. S. and Winterbourn M. S. (1986) Stable carbon isotopes and carbon flow in ecosystems. Bioscience 36, 171-177. SafIignl P. G.. Powlson D. S.. Brookes P. C. and Thomas G. A. (1989) Inlluencc of sorghum residues and tillage on soil organic matter and soil microbial biomass in an Australian vcrtisol. Soil Biology & Biochemistry 21, 759-765.
lic.X.nttw.frt~~&,~~f‘~~.~ -We thank Mr L. Mackee of Coldwater for the use of his property during the experiments and Mr R. P. Le Feuvre of CSIRO for the carbon isotope analysts.
Smith B. N. and Epstein S. (1971) Two categories of ‘JC/‘zC ratios for higher plants. Ptunr ph.v.~i~~to~y47, 380-384.
Soil Survey Staff. (1975) Soil Tuxonomy: REFERENCFS
Barois i. and Lavelle P. (1986) Changes in respiration rate and some physi~~hemi~l properties of a tropical soil during transit through fonroscolrs curc*rhruru.s.Suit Biulogy and 5iochcmsifry 18, 539.-54 I Coleman D. C., Reid C. P. P. and Cole C. V. (1983) Biological strategies of nutrient cycling in soil systems. Ad-unces in Ecologicut Rcsrurch 13, l-55. Craig H. f 1957) Isotopic srandards for carbon and oxygen and correction factors lor mass-s~trophotometric analyszsof carbon dioxide. G~&~inric\a n Cusmochimica Acra It, 133-149. De Niro M. S. and Epstein S. (1978) Influence of diet on the distribution of carbon isotopes in animals. Crochimica Cwmoc~himiw
ACM 42. 495 -506.
for
Muking
and hrerpreting
A 5usic Sysrem Suil Surwyr. U.S. Depart-
ment of Agriculture Handbook 436. Government Printer, Washington. D.C. State H. C. T.. Hubble G. D.. Brewer R., Northcote H. K., S&man J. R., Mulcahy M. I. and Hallsworth E. G. (1968) A Handbook of Ausrrutian Soils. Rellim Technical Publications, Glenside. South Australia. Swaby R. J. (1949) The influence of earthworms on soil aggrcgalion. Juurnul of Soil Science 1, 195-198. Tomati U., Grapelli A. and Galii E. (1988) The hormonelike erect of earthworm casts on plant growth. &logy and Ferritify
of Soils 5, 288-294.
Wood A. W. (1986) Green cane trash management in the Herbert Valley. Preliminary results and research priorities. Proceedings of the Ausrraliun Society of Sugar Cane Technologists ES-94.
Bd.
Pnnted
Biochem. in Great
Vol. Britain.
21. All
No.
5. pp.
rights
703-706.
1990
0038-0717/90 Copyright
reserved
33.00
‘5: 1990 Pcrgamon
+ 0.00 Press pk
TISSUE CARBON SOURCES FOR PONTOSCOLEX CORETHRURUS (OLIGOCHAETA: GLOSSOSCOLECIDAE) IN A SUGARCANE ECOSYSTEM A. V. SPAIN Davies
Laboratory,
CSIRO,
PMB.
PO Aitkenvale,
Qld 4814. Australia
P. G. SAFFIGNA Division
of Australian
Environmental
Studies. Griffith
University.
Nathan.
Qld 41
I I,
Australia
A. W. Woov CSR Ltd, Victoria
Mill. c/o Post OtTice, Ingham, (Accepfed
I5 lunuury
Qld 4850,
Australia
1990)
Summary-The pantropical. geophagous earthworm species Ponroscolex corefhrurus (Miiller) is common in lowland soils supporting suparcane in notheastern Queensland. In comparison with situations where harvest residues are burned. its populations are substantially increased under cultural treatments in which harvest residues are retained as a surface mulch or are mechanically incorporated into the topsoil. The 6°C values of stem and leaf materials. soil organic matter and the earthworm whole-body tissues and casts wcrc determined. It is unlikely that P. corrrhrurus assimilates much of its tissue C from the more complex fractions of soil organic matter or directly from decomposing residues. at least until a late stage of breakdown. No dillircnccs in S”C values were apparent in samples of this species from the ditTcrent cultural trcatmcnts. From field observations of an intimate association between P. cowthrurus and the sugarcanc roots, WC suggest that this species may derive much of its tissue C from rhizospherc sources.
beyond an amelioration of the physical environment of the surface soil (Wood, 1986). Howcvcr. the retention of sorghum crop residues under semi-arid tropical conditions in Australia resulted in greater quantities of C. N and P being present in the soil microbial population than where above-ground residues were removed (Safiigna er al., 1989) and this may well pertain in sugarcane crops. P. corethrums is a polyhumic endogeic species with a pantropical distribution; Lavelle ef ul. (1987) have recently reviewed the biology of this parthenogenetic, near-surface dwelling species. The individuals of this species are small and tolerate cultivation well. Where populations are large, substantial masses of soil may pass annually through the gut of this species (Lavelle er al.. 1983). From observation of the soil beneath surface mulches of harvest residues, P. corethrums has a marked effect on the structure of the underlying soil. Although it is normally geophagous and casts within the soil, it casts actively into the layers of the residues during the later stages of decomposition. We report the results of a single sampling of the populations and biomasses of P. corethrums on the 29-31 July 1986 at a time judged close to peak population numbers from studies made elsewhere in the area (A. V. Spain er al.. in preparation) and demonstrates the substantial increase in populations of this species that occurs when harvest residues arc retained. The 6°C values of sugarcane plant the soils and the whole-bodies of material, P. corerhrurus were determined on samples collected at the same time. The 6°C value of a consumer’s
INTKOtyUCTION Despite the importance of the sugarcanc crop in northcastcrn Australia, little is yet known of the agents mediating organic matter breakdown and thus controlling the rates of nutrient cycling and C flow in soils supporting this crop. Studies of the biology of these soils (A. V. Spain er ul.. in preparation) indicate that ~~‘on~oscole.r cwerhrurus (Miiller) is the major earthworm species in the coastal lowland areas supporting sugarcane between lngham (latitude IS’ 42’S) and lnnisfail (latitude 17’ 32’S). The climate of the area is humid but distinctly seasonal; >70% of the rainfall occurs in the months December to April (Gordon. 1971). Sugarcanc is harvested in the drier months immediately preceding the above period. Rcccnt widcsprcad changes in the cultural system of growing sugarcane have involved the cessation of preharvest crop burning and retention of the substantial masses of harvest residues (to at least I5 t ha-‘) as a surface mulch, or their incorporation into the mineral soil. The changes have Icd to increased crop yields (Wood, 1986). substantially rcduccd soil erosion in hilly areas (Prove et al., 1986) and a possible reduction in soil compaction. Despite the warm humid climate sugarcane residues retained as a surface mulch persist for substantial periods. A. V. Spain L’I al. (in preparation) in estimated that 19% of initial residue dry matter was still present 338 days after harvest and ascribed this to the high initial C:N ratio (> 100). Nonetheless, little is known of the biological consequences of preserving these rcsiducs 703
A. V. SPAINe! al.
704
tissues is related closely to that of C it assimilates from its diet (Fry and Arnold, 1982). Previous sampling of field populations of this species had revealed an intimate association with the roots of sugarcane and we wished to test the hypothesis that P. corethrum derives much of its dietary C from rhizosphere sources. Thus, we here examine the major organic components of a sugarcane ecosystem in relation to their potential as tissue C sources for P. corerhrurus.
MATERIALS
AND
After harvest of the above-ground parts, samples were taken of the roots and all individuals of P. corethrums in the top IOcm of the rings were collected. The specimens of P. corerhrurus were counted, dried and weighed and subsamples ashed for 3 h at SOO’C to allow expression of the biomass results on an ash-free basis. Soils were sampled at IOcm intervals to 2Ocm depth 2Ocm intervals to 60cm depth and at 30cm intervals below this. The 6°C values were determined on a VG Isogas SIRA IO mass spectrometer and expressed as per mil (?&) units according to the equation:
METHODS
6 “C = [(R sample/R
The study was conducted at Coldwater, Queensland (IS. 29’S 145’ 58’E) which has an average annual rainfall of 2033 mm, based on that of Abergowrie Bridge, some 5 km distant. The soil is a sandy loam in texture and is classified as a Tropudult (Soil Survey Staff. 1975) or as a Red Earth (State ef al., 1968). N and C concentrations in the surface soils (O-IOcm) at the site are. respectively 0.06 and 0.8%. The populations of P. corerhrurus studied here were collected on 29-31 July 1986 from soil within the top IO cm of steel cylinders (100 cm dia) which had been inserted 50 cm into the soil of an established fourth ratoon (fifth year of growth) crop of sugarcane (Succhurun~ officinnrum var. ‘Pindar’). The crop had been planted in 1981, following a leguminous green manure crop of Dolichos luhbh and harvest residues had since been retained as a surface mulch. There wcrc three trcatmcnts and three cylinders per trcatmcnt. The cylinders wcrc installed early in midAugust I985 after harvesting the previous ratoon crop. The trcatmcnts imposed wcrc.
I.
Sug.rrc;lne m
yields
(1 cane
‘) and biomasxs
dry
ma~kx
frz oraanic
ha
mitllcr
‘) ror
m
each
treatment
logcthcr
:) of P.rorrrhruru.~ in the
with
populabons
three rrcarmenls
(n = 3)
P. cnrurhrurw Crop yield Treatment
P
I. R.ake and burn 2. Surke mulch 3. H.ncst
restduer
7u.4
110.8 incornorated
139.8
I]. IO’
Table I prcscnts the cstimatcd populations and biomasscs of P. corethrurus collcctcd from the top IO cm of the rings. For both the populations and the biomasses, Mann-Whitney tests showed that levels in the rake and burn treatment were significantly lower (P c 0.10) than those from the treatments where harvest residues were retained. There were no signilicant differences (P > 0.10) between the two treatments where residues were retained and, for both measures, the differences between these two treatments combined and the rake and burn treatment were significant (P ~0.03). Table I also shows that commercial yields of sugarcane are substantially increased by retaining the harvest residues. It is clear that large increases in populations of P. corethrurus resulted from the presence of the harvest residues, whether these are retained as a mulch or incorporated into the topsoil; this accords with results from more detailed sampling made elsewhere in northeastern Queensland (A. V. Spain cr ul.. in preparation). Figure I prcscnts the d”C values as
The first treatment simulated the conventional practice of burning both the crop before harvest and remaining residues after harvest. The other treatments simulated the two common conservation tillage prncticcs of the local industry. The treatments were provided with l’N-labelled urea in order to estimate losses from this form of fertilizer and the distribution of the labclled N in the plant, soil and other components of the ccosystcm. This information will bc prcsentcd clscwherc.
T.Mc
-
WKst:I.TS
&kc, untlburn. The crop was harvcstcd without burning and the rcsiducs were carefully raked and burned in situ. Sur/ilce rrsichre. The crop was harvested without burning and the residues were rctaincd as a surface mulch. Itrcarporarecl residues. The crop was harvested without burning and residues were incorporated mechanically into the surface of the mineral soil.
kidi\idu&
standard)
where R is the ratio “C/“C. Samples were prepared for analysis using a modification of the method of Le Feuvre and Jones (1988). For the vegetation and earthworm samples, cupric oxide was used as the oxygen donor. Due to the low C contents of the soil samples, vanadium pentoxide (0.5 g) was used instead of the cupric oxide because of the large sample volumes required to obtain sufficient C for analysis. The standard error of a single measurement of homogenous material is less than or equal to 0.07%. All values were indirectly refcrcnced to the international standard Pee Dee Belcmnitc (PDB) and corrected for “0: intcrfcrencc (Craig, 1957). The d “C values of P. rorerhrurus wcrc determined on whole spccimcns and soil organic matter in the gut may have contributed to the values prcscntcd.
Populalion S
Biomass
?’
5
?’
9. 1.7
7.5 199
4x
1.3x
0.X
63
3.73
0.94
31.9
276
5.IR
I.50
I
II3
5
P. corerhrurus
in a sugarcane ecosystem
they change down the profile. These values reflect organic matter largely derived from plants with the C, photosynthetic pathway (Smith and Epstein. 1971). In addition. the distinctly lower values of the topsoil C (&IO cm _T? = - 14.94, s = 0.51, n = 6; IO-20cm f = - 16.07. s = 0.35, n = 3) are attributed to dilution of the predominantly C, organic matter by that of the C, leguminous green manure crop incorporated before planting the current crop. Figure 2 presents the 6°C values of the stem material. the body tissues and casts of P. corefhrurus from the three treatments. The sample of stem material from the rake and burn treatment (7 = - 12.32. s = 0.35) was significantly (P = 0.037) more negative than that from the combined harvest residue retention treatments (y = -12.2, s =0.06. n = 5). A single sample of green leaf material was analyzed from each of the three treatments; the 6°C values and were - I’.‘1 __ and - 12.24 for the incorporated surface mulch residue retention treatments respectively, and - 12.59 for that of the rake and burn treatment. It is not known why the d”C values should vary between these treatments. The whole-body tissue values of P. cordrrurus did not differ bctwccn the three treatments (P > 0.10. Kruskal-Wallis test) and clearly hnvc greater S”C values than those of the plant tissues. soils or casts. Since the prcscncc of soil in the gut would dcprcss the d”C value of the earthworm, it is likely that the d’!C value of the tissue is higher than that prosontcd in Fig. 2. Similarly the plant tissues also have grcatcr 6°C vulucs than those of the casts and surface soils. The 6’!C values of the casts of P. core~lrrwu.s (.f = - 11.45, s = 0.37, n = 6) arc slightly but signilicantly (P < 0.10) greater than those of the surface soils. DISCUSSION
On a world scale. populations of P. coredwurus at site arc high where harvest rssiducs arc retained. This implies the passage of a large mass (200-400 t ha-‘) of soil annually through the gut of
the present
S13C
.
Y
Fig. I. Variation in the 6°C values with depth in the profile (w. rake and burn; A. surface residues: 0. incorporated residues).
705
-
kuadP.amhmnq
s&c-1c.m
mm .16
.
I
!
I
I
I
I
,
.15
-14
.13
.12
-11
-10
-9
8
6% Fig. 2. S ‘?C values of selected organic components of the sugarcane ecosystem at the study site. this species (Lavelle et al.. 1983). The species appears to have important effects on the structure of the soils it inhabits. From field observations of surface and near-surface soils, a substantially-increased quantity of structural aggregates is apparent in the treatments where harvest residues are retained. Aggregates formed by earthworms are frequently more water stable than others (Swaby, 1949) and further study of the soil structural efTccts of this geophagous earthworm is warranted. It is not known precisely from what sources P. corehms assimilates its tissue C in this environmum. Barois and Lavcllc (1986) have suggcstcd that this spccics may assimilate C from the more complex fractions of soil organic matter through a priming cll’cct occurring in the earthworm gut. liowcvcr, the wide difTcrcncc in 5°C values bctwccn the body tissues and soils (5.93%) suggests that the resistant fraction of soil organic matter contributes little to tissue C. Since P. corcfhrur~t.s is not a surface-feeding spccics, it is unlikely to obtain its nutrition directly from the decomposing surface rcsiducs and this is supported by the lack of a significant diffcrcncc in the 6°C values of the tissues of P. corehurus bctwecn trcatmcnts where harvest residues were and wcrc not rctaincd. Roots sampled at another study site wcrc similar in S”C values to those of the aerial parts of the plant and also seem unlikely to provide a direct source of tissue C. Both incorporated and surface-retained residues may eventually provide a direct source of tissue C although P. corerhrurus is unlikely to ingest harvest residues until a late stage of decomposition. However, decomposition products of these residues may indirectly provide a source of assimilable tissue C. P. corethrurus may also derive tissue C from the microbial biomass, particularly from the more prolific microbiota of the rhizosphcre (Foster, 1988). The increased microbial biomass resulting from the prcscncc of a mulch (Safigna er ~1.. 1989) may also comprise an additional source of tissue C. P. corethruru.~ may also derive part of its tissue C from root cxudatcs, lysatcs, mucilages and other rhizosphcrc products and thus participate in the ‘fast’ cycle of organic matter turnover associated with the rhizosphcrc (Coleman er al., 1983). The hypothesis that the rhizosphere is the major source of tissue C in P. corerhrurus is supported by field observations of larger earthworm populations in the row (as opposed to the inter-row spaces) and the intimate association of individuals of this species with sugarcanc roots observed during field sampling.
A. V. SPaiNet at.
706
Because of the seasonality of climate and crop production, it is likely that soil C biomass will undergo temporal changes. However, no seasonal variation in the d’?C values of P. corerhrurus tissues was noted at one other sampling site where this was measured and suggests a constant source of dietary C. De Niro and Epstein (1978) showed that wholeanimal 6°C is cu 1!% enriched relative to the dietpresumably due to preferential loss of “CO2 during respiration. Greater differences in 6°C values have been recorded between adjacent trophic levels (Rounick and Winterbourn. 1986) but these have mainly come from freshwater and marine environments. Thus. the 3%0higher 6 ‘)C values of earthworm tissue relative to plant tissue warrants further investigation. There are several possible reasons for the increased yields of sugarcane crops under harvest residue retention systems. Tomati et at. (1988) have shown that considerable chemically-based stimulation of plant growth results from the presence of earthworms, in addition to that due to an increased mineralization. A substantial amelioration of near-surface soil temperature and moisture regimes is recorded by Wood (1986). Because of improved structure due to carthworm working of the upper part of the mineral horizons. a readier acceptance of rainfall occurs and this may be of importance in stimulating early season growth. LA (1987) states that high C:N rcsiducs such as those from sugarcanc may dcplctc soil N and depress crop yields: this does not appear to occur hcrc, perhaps bccausc of relatively heavy fertilizer N application. The occurrcncc of N, fixation in dccomposing harvest residues (Patriquin. have a bearing on this.
1982) may also
Foster R. (1988) Microenvironments of soil microorganisms. Biology and Fertility of Sods 6, 189-203. Fry B. and Arnold C. (1982) Rapid “C/‘zC turnover during growth of brown shrimp @eMpus uzrecus). Oecdogia 54, 200-204. Gordon B. (1971) Ctimaric Surrey; Northern, Region 16: Queensland. Bureau of Meteorology. Commonwealth of Australia, Melbourne. Lal R. (1987) Tropical Ecology and Edaphotogy. Wiley, Chichester. Lavelle P., Rangel P. and Kanyonyo J. (1983) Intestinal mucus production by two speciesof tropical earthworms: Mittsoniu tumfoiunna (Megascolecidaef and Pontoscolex corerhrurus (Glossoscolecidae). In New TrenrLr in Soils Siotogy (P. Lebrun, H. M. Andre. A. De Medts, C. Gregoire-Wibo and G. Wauthy, Eds). pp. 405-410. Dieu-B&hart, Ottignies-Louvain-la-Neuve. Lavelle P.. Barois 1, Crut I., Fragoso C.. Hemandez A. Pinesa A. and Rangel P. (1987) Adaptive strategies of Pontoscotex corethruras (Gtossoscolecidae, Oligochaeta), a peregrine geophagous earthworm of the humid tropics. Biology and Ferfilirv of Soils 5. 188-194. Le Feuvre R. P. and Jones R. J. (1988) Static combustion of biological samples sealed in glass tubes as a preparation for 6°C determination. Analvst 113. 817-823. Patriquin D. G. (1982) Nitrogen.fixation in sugarcane litter. Biujogical
Agriculture
and-Hor#i~at:ure
1, $9-64.
Prove B. G.. Truonn P. N. and Evans D. S. (1986) Strategies for contr&ling &eland erosion in the wit trdpical coast of Queensland. Proceedings of the Australiun Saciefy of Sugar Cane Technologists 77-84.
Rounick S. S. and Winterbourn M. S. (1986) Stable carbon isotopes and carbon flow in ecosystems. Bioscience 36, 171-177. SafIignl P. G.. Powlson D. S.. Brookes P. C. and Thomas G. A. (1989) Inlluencc of sorghum residues and tillage on soil organic matter and soil microbial biomass in an Australian vcrtisol. Soil Biology & Biochemistry 21, 759-765.
lic.X.nttw.frt~~&,~~f‘~~.~ -We thank Mr L. Mackee of Coldwater for the use of his property during the experiments and Mr R. P. Le Feuvre of CSIRO for the carbon isotope analysts.
Smith B. N. and Epstein S. (1971) Two categories of ‘JC/‘zC ratios for higher plants. Ptunr ph.v.~i~~to~y47, 380-384.
Soil Survey Staff. (1975) Soil Tuxonomy: REFERENCFS
Barois i. and Lavelle P. (1986) Changes in respiration rate and some physi~~hemi~l properties of a tropical soil during transit through fonroscolrs curc*rhruru.s.Suit Biulogy and 5iochcmsifry 18, 539.-54 I Coleman D. C., Reid C. P. P. and Cole C. V. (1983) Biological strategies of nutrient cycling in soil systems. Ad-unces in Ecologicut Rcsrurch 13, l-55. Craig H. f 1957) Isotopic srandards for carbon and oxygen and correction factors lor mass-s~trophotometric analyszsof carbon dioxide. G~&~inric\a n Cusmochimica Acra It, 133-149. De Niro M. S. and Epstein S. (1978) Influence of diet on the distribution of carbon isotopes in animals. Crochimica Cwmoc~himiw
ACM 42. 495 -506.
for
Muking
and hrerpreting
A 5usic Sysrem Suil Surwyr. U.S. Depart-
ment of Agriculture Handbook 436. Government Printer, Washington. D.C. State H. C. T.. Hubble G. D.. Brewer R., Northcote H. K., S&man J. R., Mulcahy M. I. and Hallsworth E. G. (1968) A Handbook of Ausrrutian Soils. Rellim Technical Publications, Glenside. South Australia. Swaby R. J. (1949) The influence of earthworms on soil aggrcgalion. Juurnul of Soil Science 1, 195-198. Tomati U., Grapelli A. and Galii E. (1988) The hormonelike erect of earthworm casts on plant growth. &logy and Ferritify
of Soils 5, 288-294.
Wood A. W. (1986) Green cane trash management in the Herbert Valley. Preliminary results and research priorities. Proceedings of the Ausrraliun Society of Sugar Cane Technologists ES-94.