soil-resistant carbon in 160 days in the no-till and chisel tillage soils were high at ... Ke~~wor& Soil organic matter; Microbial biomass; Soil respiration; Metabolic ...
Soil & Tillage Research33 (1995) 17-28
soila Tillage Research
Soil organic carbon, microbial biomass and C02-C production from three tillage systems Roberto Alvarez’,*,Ratil A. Diaz’, Nidia Barberob, OscarJ. Santanatogliaa, Luis Blotta’ aLaboratorio de Radioihtopos, Facuitad de Agronomia, Universidad de Buenos Aires, Avda. San Martin 4453, (1417), Buenos Aires, Argentina ‘C&tedra de Producei& Vegetal, Facultad de Agronomia, Universidad de Buenos Aires, Avda. San Martin 4453, (1417), Buenos Aires, Argentina ‘Estacih ExperimentalAgropecuaria Pergamino, Institute National de Tecnologia Agropecuaria, CC 31(2700), Pergamino, Argentina
Accepted7 September 1994
Abstract Organic matter is a major soil component which is influenced by tillage. This paper quantities the effect of no-till, chisel tillage and plow tillage on the content and depth distribution of organic carbon and microbial biomass after 12 years of each tillage system. The soil was typical of the Argentine Rolling Pampa. The resistance of organic matter to mineralization was evaluated by means of an incubation test. In the no-till and chisel tillage systems, crop debris accumulated within the top 5 cm of soil, especially in the no-till system. Consequently, organic carbon was 42-50°h higher (P=O.Ol ) in the no-till soil than in the soil from the plow and chisel tillage systems. Biomass carbon and soil basal respiration (O-10 day period) were noticeably stratified under no-till and chisel tillage, while they were uniform from 0 to 15 cm in the plowed soil. The metabolic quotient of the biomass (basal respiration/biomass) was regulated in all casesby the coarse plant debris content of the soil ( r2 = 0.79, P= 0.0 1). A doubled exponential model was fitted to C02-C values produced during 160 days of incubation ( r2>0.95). This shows that soil carbon dynamics can be described as being composed of two pools: one labile, and one resistant to microbial attack. The proportion of total carbon mineralized and the decomposition of soil-resistant carbon in 160 days in the no-till and chisel tillage soils were high at the soil surface, but decreased with depth. In plowed soil, these parameters were constant from 0 to 20 cm. The organic matter at the soil surface under the no-till and chisel tillage systems was more readily degradable than under plow tillage in the laboratory experiment. Carbon inputs from crops were estimated to be similar between tillage systems. Consequently, in situ accumulation of labile forms of organic matter under a no-till system may be ascribed * Correspondingauthor. 0167-1987/95/$09.50 @ 1995 ElsevierScienceB.V. All rights reserved SSDIOl67-1987(94)00432-3
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R. Alvarezet al. /Soil & Tillage Research33 (1995) 17-28
to a decrease in the mineralization intensity of the soil organic matter. Soil temperature determinations suggested that plowed plots were warmer than no-tilled plots, and this phenomenon could lead to a decrease of microbial respiration in straw-covered soil. Ke~~wor&Soil organic matter; Microbial biomass; Soil respiration; Metabolic quotient; Carbon mineralization
1. Introduction Conservationtillage systems,especiallyno-till, result in the accumulation of organicmatter in the first few centimetresof the soil profile (Follett and Schimel, 1989;Karlen et al., 1991) . In contrast, plow tillage distributes organic matter more evenly within the plowedlayer. After severalyears,soils which haveundergonereducedtillage or no-till have a higher carbonconcentrationin the surface layer than plowed soils. On the other hand, carbon levels at lower depths are similar in both systems,or slightly higher under plow tillage (Carter and Kunelius, 1986;Follet and Schimel, 1989;Karlen et al., 1991). Microbial biomassin non-plowedsoils showsa stratification pattern similar to that of organic carbon,although there is generallymore in the upper layer than in plowed soil (Carter and Kunelius, 1986;Follet and Schimel, 1989). Microbial biomass differencesas a result of soil managementare sometimesgreaterthan the differencesin organic matter. Therefore,microbial biomass may be a better indicator of changesinduced by tillage (Carter, 1986;Safflgnaet al., 1989) . Soil respiration is relatedto carbonavailability in the biomass,and is generally higherat the soil surfaceunderno-till becauseof greaterbiologicalactivity (Carter, 1986;Follet and Schimel, 1989).Higher production of COZ-Chasbeenrecorded in the field under no-till than under conventional tillage (Hendrix et al., 1988). Conversely,tilIed soils, becausethey have lower organic carbon contents, can sometimesgenerategreateramounts of COZ-Cand sustaina microbial biomass with a higher metabolic quotient (basal respiration/biomass) than non-plowed soils (Safflgnaet al., 1989). The different accessibilityof the carbonsubstrateto microorganismsor metabolic changesin the flora could be responsiblefor these results. Our aim was to quantify the effectsproducedby three tillage systems(no-till, chisel tillage and plow tillage) on: ( 1) the accumulation and distribution of organic matter and microbial biomass, and (2) the relationship betweencarbon availability for microorganismsand the intensity of organic matter mineralization. The study wasperformed in a typical soil of the Argentine Rolling Pampa. The soils of the region are Mollisols, and are the most productive soils of the country (HalI et al., 1992) .
R. Alvarez et al. /Soil & Tillage Research 33 (1995) 17-28
19
2. Materials and methods The experiment was performed on a Typic Argiudol (Pergamino series) located in the Estacion Experimental AgropecuariaPergamino-INTA (33O 56’S; 60’ 34’W). In the O-20 cm layer, its main characteristicswere 27% clay, 16% sand, 1.94%organic carbon, 0.15%total nitrogen, 11 p.p.m. available phosphorus (Bray-Kurtz), and pH (soil/water = l/2.5) 5.8. The experiment started in 1979to assessthe influence of different tillage systemson the most widespread cropping sequencein the region:corn followed by double-croppedwheatand soybean.The rotation started with corn, which was followed by wheat and soybean in the following year.The samesequencewas repeatedfor 10years.Three tillage systemswereapplied: ( 1) plow tillage, i.e. tilling the soil with a moldboard plow to a depth of 14-16 cm and then using a vibrocultivator; (2) chisel tillage, i.e. chisellingthe soil to a depthof 18-20 cm and then tilling it with a vibrocultivator; (3) no-till, i.e. implementing direct drilling. The experiment was designedas a randomizedblock with four replicatesof eachtillagetreatment (plot size 14m X 45 ml. In December 1991,after the wheat crop harvest (southern hemisphere), soil sampleswere collectedfrom beneaththe stubble. Soil was sampledto a depth of 20 cm with a 4 cm diameter auger.Each core samplewasdivided into four 5 cm layers.Composite samplesof eachplot wereobtained by mixing ten subsamples collectedat random. Within 48 h after sampling, the freshsoil was homogenized with a knife and large straw pieceswere discarded.Microbial biomass determinations were made using a chloroform-fumigation technique (Jenkinson and Powlson, 1976b). Soil (equivalent to 100g on a dry basis) was incubatedin 400 ml flasks. The production of CO+ in non-fumigatedsoil was usedas a control during the 10-20 days and 40-70-day periods of incubation. A k factor of 0.45 was applied for the conversionof CO#Z to biomass carbon (Oadesand Jenkinson, 1979). At the time of incubation, the water content of the sampleswas adjusted to 50% of their water-holding capacity, and the incubation temperature was 25OC.The metabolic quotient was defined as the ratio betweenthe COZ-C generatedby non-fumigatedsamplesduring the lirst 10daysof incubation (basal respiration) and the initial biomass carbon content. The evolution of COZ-Cin control soil was monitored for 160 days in order to determine the intensity of organicmatter mineralization in eachtreatment. Coarseplant debris (plant debris> 500pm) wasdetermined by flotation and wet-sievingof the soil (Bohn, 1979). Carbon analysisof plant debriswas carried out by digestion (Amato, 1983). Soil organic carbonwas determinedon ground samples ( < 500 pm) by the Walkley-Black technique (Nelson and Sommers, 1982). Carbon in the soil light fraction (density< 2 Mg rnm3)was evaluatedby densimetric separationon soil samplessievedthrougha 250pm meshscreen(Richter et al., 1975). All the determinations were performed in duplicate and the resultsexpressedon an oven-dry soil basis ( 105‘C) . Carbon inputs from cropsfor the 12 yearperiod were estimatedby calculating straw and root production on the basis of grain yields and the straw:grain and
20
R. Alvarez et al. 1 Soil & Tillage Research 33 (1995) 17-28
straw:roots ratios. Straw:grain and straw:roots ratios were establishedby harvestingtwo microplots of 1 m* per plot at the phenologicalstageof physiological maturity. Roots weremeasuredin 8 cm diameter soil samplestakento a depth of 20 cm, both in the furrows and betweenfurrows at various distancesfrom the rows. Roots were separatedfrom the soil by wet-sieving (Bohn, 1979). After washing,root datawereintegrated.Plant material was dried (7O’C) and carbon concentrationwas determined by digestion (Amato, 1983). Soil temperatures (at midday) were measuredat a depth of 5 cm during both corn and wheat emergencein plowed and no-till plots. Data were analyzedusing the analysisof variancetechnique. Differencesbetweenmeansweretestedusingthe Duncan test. Regressionanalysiswasusedto relate parameters,and the significanceof eachparameterwas assessedby the Ftest. COZ-Cproduction data were titted with a double exponential model to describe the processof carbon degradation.This model was similar to the model proposedby Bonde et al. ( 1988) to describethe mineralization of soil nitrogen: where Cminis mineralized carbon, Cl is the labile carbon pool, Cr is the resistant carbonpool, k1is the Cl mineralization constant,k2 is the C1mineralization constant and t is the period of incubation. The model was fitted to data on carbonmineralization by nonlinear regression using the algorithm of Marquardt ( 1963) . The curve fitting was performed in a three-stepprocessto avoid convergence.In the tirst step,initial approximations of C, and kI (the labile carbonpool and its rate of mineralization) wereobtained by fitting data of CO*-C production for the 0-70-day period of incubation to the simple negativeexponentialmodel. In the secondstep,the initial approximations of Cr and k2 (the residual carbon pool and its rate of mineralization) were obtained by fitting data of CO*-C production for the 70-l 60-dayperiod to the negative exponentialmodel. Theseinitial approximationswereusedto fit the double model by successiveiterations until maximum r* wereobtained. 3. Results and discussion 3.1. Distribution
of plant residues and organic matter
No-till and chisel tillage resultedin an accumulation of coarseplant debris at the soil surface(O-5 cm) (Fig. 1(A) ). In no-till soil, the amount of plant debris at the soil surfacewas approximately 2.5 times higher (P~0.01) than that for chisel tillage, which in turn was approximately 3 times higher (P=O.Ol ) than that for plow tillage. There wereno differencesbetweentillage systemsin coarse plant debris content at greaterdepths.The content of coarseplant debris in the top 20 cm of soil under no-till was twice that observedunder the other tillage systems(P=O.O5). The mean coefficient of variation of the coarseplant debris determination was 20% for the three tillage systems.Under no-till, higher levels
R. Alvarez et al. /Soil & Tillage Research 33 (1995) 17-28 COARSE
PLANT
21
DEBRiS
bkgC g4soil 1 00
1
2
3
4 NT
ORGANiC
CARBON
(mgC g-‘soil)
r
00 I l!L LIJ OE 2.E $?
5.
10
20
30
q
lo15-
20Fig. 1. Distribution in depth of (A) coarse plant debris and (B) organic carbon, NT, no-till; CT, chisel tillage; PT, plow tillage.
of coarseplant debriswerefound comparedwith plowedsoil becauseburied straw generallydecomposesfasterthan residuesleft on the surface(Holland and Coleman, 1987). The organiccarboncontentat a depthof O-5 cm was42-50% greater(P= 0.01) in no-till than in the other treatments (Fig. 1(B ) ). From the surfaceto a depth of 20 cm, organiccarbondecreasedsignikantly (P= 0.05) with soil depth in notill. In plowed and chiseledsoil, organic carbon remained almost constant to a depth of 15 cm. For the first 20 cm layer, organic carbon was 5-S% higher (P=O.O5) in chisel tillage and no-till than in plowed soil. The variability of organic carbonwaslow (coefficient of variation 2%) for all tillage systems. For no-till and chisel tillage, carbon from coarseplant debris, as a fraction of total carbon,was severaltimes higherat the soil surfacethan in the 5-20 cm soil layer. At the soil surfacein no-till plots, carbon from coarseplant debris representsabout 11%of total carboncomparedto about7% in chiseledplots. At greater depth, the proportion of total carbon in plant debris decreaseto l-2%. Under plow tillage, the fraction of total carbonin plant debrisdid not vary significantly from a mean value of 2% betweensoil layers.Similar trends were also observed for the carboncontent of the soil light fraction (data not presented),which was closelycorrelatedto the coarseplant debris level (r2 = 0.98;P= 0.0 1).
22
3.2. Microbial
U. Alvarez et a[. /Soil &! Tillage Research 33 (‘1995) 17-28
biomass and activity
Microbial biomass determination by the chloroform fumigation-incubation technique is basedon the differencebetweenthe CO& production of a fumigatedsoil sampleand that of a control soil sample.In the former, CO& is produced from non-biomasssubstratesand dead microorganisms, whereasin the control CO& is producedfrom non-biomasscarbon substancesonly. The production of CO& from a non-fumigated soil sample in a lo-20-day period of incubation is usually usedasa control. In the literature, the first 10 daysof incubation are discardedto avoid sample handling effects,and the data from Days 10-20 are used.During this period, soil C02-C production becomeslinear with time (Jenkinsonand Powlson, 1976a). In this experiment,the estimation of microbial biomassusing the CO& production from non-fumigated soil in Days 10-20 of incubation as the control showederratic results,with extremely low or negativevaluesin somecases.The reasonfor this seemsto be the occurrenceof plant material in the samples,especially from no-till surfacesoil, as sampleswere not sievedbeforeanalysis.These materials are attackedby microorganismswith much lessintensity in fumigated soil than in non-fumigatedsoil (Martens, 1985) , and consequentlythe mineralization of non-biomasscarbonsubstratesis more intensein the control soil than in the fumigated samples.As a result, very low or negativevaluesfor biomasscan be achievedwhensubtractingCO& production by control soil from production by fumigated soil. In our case,COZ-Cgenerationbecamelinear after 40 daysincubation for all the samples( r2> 0.95). Therefore,the rateof C02-C production between40 and 70 dayswas usedas the control to assessbiomasscarbon. Onethird of the C02-C evolvedfrom the non-fumigatedsoil during this 30-dayperiod (i.e. 10 days’production) wastaken asthe control. Under no-till and chisel tillage, the microbial biomasslevel in the O-5 cm soil layer was abouttwice (P= 0.01) that under plow tillage (Fig. 2 (A) ) . No sign& cant differencesweredetectedbetweentillage systemsat increasingdepths.Biomasscarbondecreasedin the subsurfacelayer (P= 0.05) in the no-till and chisel tilIage systems,and was constantdown to 15 cm in plowed soil. The mean coefficient of variation for biomassdeterminationswas 19%.This distribution of microbial carboncan be attributed to the movement of plant residuesby the tillage systemsbecausesoil microbial biomass level is regulatedby the amount of residuesaddedto the soil (Insam et al., 1991). The distribution of basalrespirationwas similar to that shownby biomass (Fig. 2 (B ) ), althoughdifferencesweregreaterbetweenthe tillage systems.Surfacesoil from no-till and chisel tillage conditions generatedabout three times more C02C than that from plow tillage (P~0.01) during laboratory incubation. From 10 to 15cm deepthe oppositetrend wasobserved,with greaterbiological activity in the plow tilIage treatment (P= 0.05) . The relationship betweenmicrobial carbon and respiration wasvery closefor all the samples( r2= 0.98,P= 0.0 1). The metabolic quotient wasnearly constantwith depth in plowed soi down to 20 cm, but wasmore stratitied for the other tillage treatments (Fig. 2 (C) ). How-
R. Alvarez et al. /Soil & Tillage Research 33 (I 995) 17-28
23
MiCROBiAL BiOMASS (flgC g-'soil) o"-
1 IL w -lOOE 2.2 s
150
300
450
600
5-
15201 BASAL RESPiRATiON (,ugC g-'soil) OOY
100
200
300
400
METAEIOLiC QUOTiENT LugC&Cbiom.d') oo
a025
o,o50
a075
at00 NT
5x ki w-10OE .-10 t?
15201
Fig. 2. Distribution in depth of (A) microbial biomass, (B) basal respiration quotient. NT, no-till; CT, chisel tillage; PT, plow tillage.
and (C) the metabolic
ever, this index of microbial metabolism intensity only differed (P=O.O5) between no-till and plow tillage in the O-5 cm layer. It was closely associated with the coarse plant debris content of the soil ( r2= 0.79; P= 0.01). Basal respiration showed a lower variability than microbial biomass, with a mean coefficient of variation of 5%. Metabolic quotient variability was similar to that of microbial biomass. The incorporation of crop residues in the soil increases the metabolic quotient (Ocio and Brookes, 1990), which is associated with a more active microbial en-
24
R. Alvarez et al. /Soil & Tillage Research 33 (199s) 17-28
vironment. Nevertheless,there is an inverse relationship betweenbiomasslevel in soil and metabolic quotient (Santruckova and Straskraba,1991) . This suggeststhat, in somecases,higherlevelsof carboninputs to the soil increasemicrobial biomassbut not metabolic activity, which in turn lowers the metabolic quotient (Insam et al., 1991) . Our resultsindicate that parameterssuchas microbial biomassand respiration are sensitiveindices of the effect of cultural practiceson the soil microbiological environment. 3.3.Mneralization intensity of organicmatter C02-C production in all samplesdecreasedduring incubation. After 5 months, C02-C generatedby the soils was between50 and 85% lower than the amount measuredduring the first 10 days of incubation. Basal and cumulative respiration ( 160days) werehighly correlatedbetweeneachother (r* = 0.95, P= 0.01). The intensity of total carbonmineralization (mineralized C : total C) varied with the tillage system.It was constantunder plow tillage for the different soil layers sampled,while it was much higher (P~0.01) at the soil surfaceand decreased rapidly with depthunder no-till and chisel tillage (Fig. 3). The cumulative CO*-C production over 160days could be fitted to a double exponentialmodel (r* > 0.95) for all treatmentsand layersof the soil. This model describesthe decomposition of two separatepools of substrates,one labile and the other resistantto microbial attack. It hasbeenusedpreviously to analyzesoil nitrogen mineralization (Bonde et al., 1988), and a simplified version of this method has beenapplied to the study of organiccarbon mineralization in longterm incubations (Nicolardot, 1988) . The labile carbonmineralization constant (k, ) did not showa clear pattern of variation in relation to depth and tillage treatment (Fig. 4 (A) ). However, the resistantcarbonmineralization constant (k2) wassimilar in all layersunderplow tillage but decreasedwith depth in no-till and chisel tillage (Fig. 4 (B ) ) . Organic matter resistanceto microbial attack,asjudged by its rate of decompositiondurCO2-C RESPiRED
160 d-l: TOTAL-C
Fig. 3. Variation in the fraction of total carbon respired after 160 days of incubation depths. NT, no-till; CT, chisel tillage; PT, plow tillage.
at different
R. Alvarez et al. 1 Soil & Tillage Research 33 (1995) 17-28 K
25
cd-’ lo-‘)
K (d-l
lO-4 1
Fig. 4. Variation in the mineralization constant of (A) labile and (B ) stable forms of organic carbon (/c2) at different depths. NT, no-till; CT, chisel tillage; PT, plow tillage.
ing incubation, appearedto be lower in the soil surfaceof the no-till and chisel tillage systemsand increasedwith depth. In samplestaken from 0 to 5 cm under no-till, the proportion of total carbon mineralized was three times greater (P= 0.0 1) than in samplescoming from plowed soil; the differencesdisappeared, however,below 5 cm depth. This could be attributed to surfacedeposition and accumulation of plant residuesin the soil. The correlation coefficient between the fraction of carbon respired and the fraction of carbon in plant residuesaccountedfor 97% (P= 0.01) of the variation. Carbonin the soil light fraction was also relatedto cumulative CO& production ( r2= 0.93,P= 0.01). The intensity of soil organic matter mineralization is regulatedby the quality of the substrate(DIaz-Ravina et al., 1988). Part of the carbonand nitrogenwhich is mineralizedin long-termincubationscomesfrom the microbial biomass (Bonde et al., 1988;Nicolardot, i988), and microbial biomass is relatedto the rate of decomposition of the soil-resistantcarbon (Nicolardot, 1988). Increasesin the labile carbon pool, representedby the sand-sizefraction, have beenobservedin no-till as comparedwith plow tillage (Cambardellaand Elliott, 1992). A higher rate of mineralization of carbon in samplestaken from no-till soils than in samples from plowed soils has also beendetectedin laboratory experiments (Follett and Schimel, 1989). Our resultsshowthat organicmatter in the first 5 cm of soils
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R. Alvarez et al. 1 Soil & Tillage Research 33 (I 995) 17-28
subjectedto no-till and chiseltillage is composedof more easilydecomposedmaterials than those in plowed soils. Thesematerials are mineralized more rapidly during laboratory incubations when samplesfrom different tillage systemshave the sametemperatureand water content, The estimations of carbon inputs to the soil from crops did not differ among tillage systems(Table 1) . Consequently,the accumulation of organicmatter under no-till cannotbe ascribedto higher carbon inputs to the soil. A lower mineralization intensity of soil organicmatter under no-till appearedto be the causeof the organiccarbonaccumulation. It had been observedthat when crop residuesare not buried, fungal growth predominatesover bacterialgrowthcomparedwith buried material (Holland and Coleman, 1987). As a result, the efficiency of the transformation of plant carbon into microbial carbon can rise becausefungi have a lower maintenanceenergy requirementthan bacteria,and this may leadto the formation of more humilied organicmatter. In the sameway, when straw is not mixed with soil under no-till it meansthat residual carbon is accumulatedin the lirst few centimetresof the prolile, which are more exposedto desiccationand wherethe breakdownof organicmatter canbe slowerthan at greaterdepths. Mulched soils are often coolerthan plowed soils (Buss&e and Cellier, 1984). In plowed land, soil temperaturesduring wheat and corn emergencehave been observedto be 2.4 and 5‘C higher,respectively,than under no-till in this experiment. Thesedata suggestthat under lield conditions, no-till plots have a lower mineralization intensity than plots under plow tillage. The labile organicform of carbon may thus accumulatein no-till partly as a result of lower temperatures owing to residuecover.A build-up of organicmatter canbeexpectedundertillage systemsthat leave large amounts of stubble on the soil surface,even when dry matter inputs from cropsare the sameasunder plow tillage. Table 1 Estimated dry matter (t ha-’ year-r) production of crops under different tillage systems and different carbon inputs to the soil. Values are means for 12 years Crop
Tillage system No-till Straw
Wheat Soybean Corn
4.52 5.99 9.74
Plow tillage
Chisel tillage Root 1.91 2.84 4.81
Total
Straw
6.43 8.83 14.6
5.41 5.52 9.84
Root 2.29 2.62 4.86
Total
Straw
?.I0 8.14 14.7
5.99 5.81 9.61
Root 2.53 2.75 4.14
Total a.52 8.56 14.4
Mean annual input (t ha-’ ) Dry matter Carbon
14.9 5.7
15.3 5.8
15.7 6.0
R. Alvarez et al. /Soil & Tillage Research 33 (1995) 17-28
21
References Amato, M., 1983. Determination of carbon i2C and 14C in plant and soil. Soil Biol. Biochem., 15: 61 l-612. Bohn, W., 1979. Methods of Studying Root Systems. Ecological Studies, Vol. 33. Springer, Berlin, pp. 115-124. Bonde, T.A., Schnilrer, J. and Rosswall, T., 1988, Microbial biomass as a fraction of potentially mineralizable nitrogen in soils from long-term field experiments. Soil Biol. Biochem., 20: 447-452. Buss&e, F. and Cellier, P., 1994. Moditication of the soil temperature and water content regimes by a crop residue mulch: Experiment and modelling. Agric. For. Meteorol., 68: l-28. Cambardelia, C.A. and Elliott, E.T., 1992. Particulate soil organic-matter changes across a grassland cultivation sequence. Soil Sci. Sot. Am. J., 56: 777-783. Carter, M.R., 1986. Microbial biomass as an index for tillage-induced changes in soil biological properties. Soil Till. Res., 7: 29-40. Carter, M.R. and Kunelius, H.T., 1986. Comparison of tillage and direct drilling for Italian ryegrass on the properties of a tine sandy loam soil. Can. J. Soil Sci., 66: 197-207. Diaz-Raviiia, M., Carballas, T. and Acea, M.J., 1988. Microbial biomass and metabolic activity in four acid soils. Soil Biol. Biochem., 20: 8 17-823. Follett, R.F. and Schimel, D.S., 1989. Effect of tillage practices on microbial biomass dynamics. Soil Sci. Sot. Am. J., 53: 1091-1096. Hall, A.J., Rebella, C.M., Ghersa, CM. and Culot, J.P., 1992. Field-crop systems of the Pampas. In: C.J. Pearson (Editor), Field Crop Ecosystems. Elsevier, Amsterdam, pp. 413-450. Hendrix, P.F., Chun-Ru, H. and Groffman, P.M., 1988. Soil respiration in conventional and no-tillage agroecosystems under different winter cover crop rotations. Soil Till. Res., 12: 135- 148. Holland, E.A. and Coleman, D.C., 1987. Litter placement effects on microbial and organic matter dynamics in an agroecosystem. Ecology, 68: 425-433. Insam, H., Mitchel, C.C. and Dormaar, J.F., 199 1. Relationship of soil microbial biomass and activity with fertilization practice and crop yield on three Ultisols. Soil Biol. Biochem., 23: 459-464. Jenkinson, D.S. and Powlson, D.S., 1976a. The effects of biocidal treatments on metabolism in soil. I. Fumigation with chloroform. Soil Biol. Biochem., 8: 167- 177. Jenkinson, D.S. and Powlson, D.S., 1976b. The effects of biocidal treatments on metabolism in soil. V. A method for measuring soil biomass. Soil Biol. Biochem., 8: 209-2 13. Karlen, D.L., Berry, E.C., Calvin, T.S. and Kanwar, RX, 1991. Twelve-year tillage and crop rotation effects on yields and soil chemical properties in Northeast Iowa. Commun. Soil Sci. Plant Anal., 22: 1985-2003. Marquardt, D.W., 1963. An algorithm for least-squares estimation of nonlinear parameters. J. Sot. Ind. Appl. Math., 11: 431-441. Martens, R., 1985. Limitations in the application of the fumigation technique for biomass estimations in amended soils. Soil Biol. Biochem., 17: 57-63. Nelson, D.W. and Sommers, L., 1982. Total Carbon, Organic Carbon, and Organic Matter. Methods of Soil Analysis. Part 2. American Society of Agronomy, USA, Agronomy 9, pp. 539-579. Nicolardot, B., 1988. Evolution du niveau de biomasse microbienne du sol au tours dune incubation de longe duree: relations avec la mineralisation du carbone et de l’azote organique. Rev. Ecol. Biol. Sol. 25: 287-304. Oades, J.M. and Jenkinson, D.S., 1979. Adenosine triphosphate content ofthe soil microbial biomass. Soil Biol. Biochem., 11: 201-204. Gcio, J.A. and Brookes, P.C., 1990. An evaluation of methods for measuring the microbial biomass in soils following recent additions of wheat straw and characterization of the biomass that develops. Soil Biol Biochem., 22: 685-694. Richter, M., Mizuno, I., Aranguez, S. and Uriarte, S., 1975. Densimetric fractionation of soil organ+ mineral complexes. J. Soil Sci., 26: 112-l 13. Saffigna, P.G., Powlson, D.S., Brookes, P.C. and Thomas, G.A., 1989. Influence of sorghum residues
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and tillage on soil organic matter and soil microbial biomass in an Australian Vertisol. Soil Biol. Biochem., 21: 759-765. Santruckova, H. and Straskraba, M., 1991. On the relationship between specific respiration activity and microbial biomass in soils. Soil Biol. Biochem., 23: 525-532.
