alfalfa root and shoot mulching effects on soil hydraulic properties and aggregation

Alfalfa Root and Shoot Mulching Effects on Soil Hydraulic Properties and AggregationDaniel P. Rasse,* Alvin J. M. Smucker, Djail Santos

ABSTRACTAlthough alfalfa (Medicago saliva L.) stands have been reported

to improve soil physical properties, little is known about the specificinfluences of above- and belowground alfalfa components on soilphysical properties. A 2-yr study was conducted to investigate alfalfaroot and shoot mulch modifications of soil physical properties andwater movement in the root zone of a Kalamazoo loam soil (fine-loamy, mixed, mesic Typic Hapludalf) in southwest Michigan. Fourtreatments were considered: bare fallow (BF), bare fallow with alfalfashoot mulch (BFSM), alfalfa with shoots removed and roots remaining(AR), and alfalfa with alfalfa shoot mulch (ASM). Volumetric soilwater contents were measured by time domain reflectometry (TDR).Development of fine roots was monitored by minirhizotron technol-ogy. Alfalfa root systems increased saturated hydraulic conductivity(/£„,) by 57%, total and macroporosities by 1.7 and 1.8%, respectively,and water recharge rate of the soil profile by as much as 5.4% per day.These effects of alfalfa roots on soil porosity were mainly attributed toincreased amplitudes of wetting and drying cycles and high rates ofroot turnover in the Ap horizon. A'̂ , was significantly correlated withmacroporosity (r = 0.90, P < 0.01). Mean weight diameter (MWD)of aggregates from bare fallow soils was 20% higher when alfalfashoot mulch was applied. Our results suggest that aggregate stabilitywas more affected by C sources from shoot mulch and root turnoverthan by factors specific to root activities such as physical enmeshmentof aggregates and increased soil wetting and drying cycles.

THE ROTATION EFFECT of legume crops has been attrib-uted in part to improved soil physical properties

(Folorunso et al., 1992; McVay et al., 1989). Alfalfa isthe most frequent perennial legume in rotation withcorn (Zea mays L.) in the north-central states (Eberleinet al., 1992). Improved soil structural stability underalfalfa stands has been reported by several authors(Angers, 1992; Perfect et al., 1990; Chantigny et al.,1997). Alfalfa root systems have been reported to in-crease the ATsat of soils free of previous root channels(Li and Ghodrati, 1994). Meek et al. (1992) observed asixfold increase of K^ when compacted sandy loamswere planted with alfalfa. Mitchell et al. (1995) reportedthat alfalfa root systems have the ability to increase theKw, of swelling soils.

Improvements of soil physical properties by alfalfastands appear to result from a combination of effectsfrom above- and belowground plant materials (Angersand Caron, 1998). Alfalfa stands have to be destroyedprior to planting the following crop. This operation isgenerally conducted by spray-killing the stands whensubstantial amounts of aboveground plant material arepresent (Baldock and Musgrave, 1980; Robbins andCarter, 1980). Both alfalfa roots and shoots contribute

D.P. Rasse, A.J.M. Smucker, and D. Santos, Crop and Soil SciencesDepartment, Plant and Soil Sciences Building, Michigan State Univer-sity, East Lansing MI 48824-1325. Received 22 Dec. 1998. *Corre-sponding author ([email protected]).

Published in Soil Sci. Soc. Am. J. 64:725-731 (2000).

to fresh organic matter inputs into the soil profile, whichpromotes soil aggregation (Angers and Caron, 1998).Soil structure and the development of soil cracks isinfluenced by shrinking and swelling cycles (Angers andCaron, 1998; Sissoko and Smucker, 1999, unpublisheddata). Amplitude of these cycles is increased by plantroots, which absorb water from the soil, and decreasedby crop residues, which reduce soil evaporation rates(Prasad and Power, 1991). Although the separate effectson soil physical properties of alfalfa roots and shootshave been suggested in the literature, few studies haveinvestigated the different contributions of these compo-nents. A better understanding of the specific influencesof above- and belowground alfalfa components on soilphysical properties is essential before we can determinethe full contribution of alfalfa to the rotation effect. Inthis study, the influences of alfalfa roots and shoots onsoil physical properties and water movement in the rootzone were investigated for a loam soil for 2 yr in south-western Michigan.

MATERIALS AND METHODSExperimental Design and Treatments

A field experiment was conducted at the Long-Term Eco-logical Research site of the Kellogg Biological Station in south-western Michigan. Four treatments were evaluated: bare fal-low soil (BF); bare fallow soil to which alfalfa shoot mulchwas applied following each harvest (BFSM); alfalfa with shootsremoved following each harvest and roots remaining (AR);alfalfa with shoot mulch applied to the soil surface followingeach harvest (ASM). Each treatment was replicated four timesin a randomized complete block design. Experimental plots,6 by 10 m, were installed in a Kalamazoo loam soil (fine-loamy, mixed, mesic Typic Hapludalf) in late August 1994.The preceding crop was corn, fertilized at 123 kg N ha"1. Cornresidues were moldboard plowed to a depth of 23 cm. Allplots were tilled and trafficked equally. Alfalfa (Pioneer 5246)was planted in one-half of the plots at a rate of 22 kg seedha"1 on 30 Aug. 1994. The bare soil plots (BF and BFSM)were also drilled without seeds. The bare soil plots were keptfree of weeds by applications of glyphosate [n-(phosphono-methyl)glycine] at =6-wk intervals between April and Augustof each year. No N fertilizer was applied to the research plots.Potash was applied to all of the plots at a rate of 280 kg ha"1

of K2O equivalents, together with 2.2 kg ha"1 of B, on 13 June1996. Lime was applied at a rate of 2500 kg ha"1 on 14 June1996. The alfalfa plots were harvested on 9 June 1995, 24 July1995. 31 Aug. 1995, 31 May 1996, 3 July 1996, and 21 Aug.1996. Plants were cut 5 cm above the soil surface by a 90-cm-wide sickle-bar mower. After cutting, the alfalfa shoots wereraked from the AR and ASM plots and weighed. Equalamounts of alfalfa shoots were then applied to all ASM and

Abbreviations: AR, alfalfa with shoots removed and roots remaining;ASM, alfalfa with alfalfa shoot mulch; BF, bare fallow; BFSM, barefallow with alfalfa shoot mulch; K^, saturated hydraulic conductivity;MWD, mean weight diameter; MR, minirhizotron; RIM, root-inducedmacropore; TDR, time domain reflectometry.

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726 SOIL SCI. SOC. AM. J., VOL. 64, MARCH-APRIL 2000

BFSM plots. At the end of the second growing season, cumula-tive shoot mulch biomass applications approached 16.4 Mgha"1 dry matter. All sampled areas were located at least 1 mfrom the edge of the plots to avoid border effects. The surfacearea of each plot was allocated to (i) nondestructive samplingusing in situ instruments (2 by 4 m), (ii) a yield assessmentarea (4 by 4 m), and (iii) a destructive sampling area (2 by4 m). Plots were separated by surface plastic barriers installedto depths of 10 cm and protruding 5 cm above the soil surfaceto prevent runoff and runon between plots.

MeasurementsVolumetric soil water contents were assessed at three

depths by TDR measurements. Stainless steel TDR probes(28.5 cm long) were inserted at 15-, 35-, and 60-cm depths tointercept the water in the central regions of the Ap, Btb andBt2 horizons. Probes were inserted horizontally into undis-turbed soil layers from the wall of an access pit, about 0.4 mlong by 0.25 m wide, dug in each plot before planting. Soilprofiles were described in each access pit, and each horizonwas replaced and compacted to its original density. Volumetricsoil water contents were collected by an in-line cable tester,TDR meter, model 1502C (Tektronix, Beaverton, OR). Volu-metric soil water contents were derived from the TDR-meterreadings using the equation developed by Topp et al. (1980).

Alfalfa root demographics were monitored three times eachyear by minirhizotron technology, using a micro video camera(Bartz Technology, Santa Barbara, CA) as described by Fer-guson and Smucker (1989). Three clear polybutyrate tubes(0.05 by 2.4 m) were installed at 45° angles in AR and ASMplots. One control minirhizotron (MR) tube was placed ineach of the BF and BFSM plots. Root intersections with theupper surfaces of MR tubes were recorded on identical framepositions, 1.35 by 1.80 cm each. Alfalfa roots that did notdisplay signs of decomposition were hand counted in each ofthe video frames. Root numbers were added every 10 frames,which represents a vertical depth increment of 9.5 cm. Totalroot numbers were then reported per square meter by dividingby the cumulative surface of 10 frames. Root turnover rateswere assessed by the difference in root populations betweenconsecutive dates. Assumption was made that one root-in-duced macropore (RIM) was generated each time the decom-position of one root was observed in one MR frame.

Four undisturbed soil cores, 0.076 m in diameter by 0.076 mdeep, were collected in each plot in October 1996 for analyzingKsat and soil porosity. Cores were collected on the soil surface,after gently removing the top 0.5 cm of soil and residues.Cores were saturated from underneath for 48 h prior to Kiaimeasurements using the constant head method (Klute andDirksen, 1986). Following K^ measurements, cores were re-saturated and placed in pressure chambers at 0.006 and0.033 MPa for 4 d, corresponding with macro- and gravimetricporosities, respectively. Cores were dried for 48 h in a forced-air oven at 105°C. Total porosity was calculated from theamount of water contained in the cores at saturation. Macro-porosity was calculated from the difference between watercontents at saturation and at 0.006 MPa. Microporosity wasdefined as the difference between total porosity and macropo-rosity. Gravimetric porosity was calculated from the differencebetween water contents at saturation and at 0.033 MPa.

Four subsamples of soil were collected from the 0.00- to0.20-m region of each plot in October 1996. Air-dry aggregateswere manually sieved to obtain the 4.75- to 6.30-mm fraction.Aggregates, 25 g per sample, were rewetted by nebulizingfor 12 h with distilled water (Sissoko and Smucker, 1999,unpublished data). Aggregate stability of rewetted samples

was determined by the wet sieving method (Kemper andChepil, 1965), using a stack of sieves with 4.000-, 2.000-, 1.000-,0.500-, 0.250-, and 0.106-mm opening diameters. The resultsof wet sieving were expressed as the MWD calculated as themean fraction of soil on each sieve multiplied by the meandiameter of adjacent sieves (Kemper and Chepil, 1965). Acorrection was applied for primary sand particles.

Disturbed samples for total C analyses were collected fromthe Ap horizon of each plot to a depth of 0.23 m on 11 Oct.1996 . Ten subsamples per plot were collected, mixed into onecomposite sample, air dried, and finely ground (<0.5 mm).Analyses were conducted by dry combustion method (Kirsten,1983) using a C-N analyzer NA1500 series 2 (Carlo ErbaStrumentazione, Milano, Italy). Modifications of total soil Cby alfalfa and bare fallow systems for this experiment arereported in Rasse et al. (1999). Here soil C is considered onlyfor its interactions with the soil physical properties.

Statistical AnalysesStatistical analyses were conducted using the general linear

model of the SAS system (SAS Institute, 1989). Mean separa-tion tests were conducted using Fisher's least significant differ-ences (LSDo.os) when global F tests were significant. Factorialanalyses were conducted with a root factor (i.e., presence

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31 May 20 July 8 Sept 28Oct 17 DecFig. 1. Volumetric soil water contents in the Ap, Bti, and Bt2 horizons

of soils under bare fallow (open circle), bare fallow with alfalfashoot mulch (open square), alfalfa with shoots removed and rootsremaining (filled circle), and alfalfa with alfalfa shoot mulch (filledsquare), and daily rainfall measurements in 1995. Fisher's leastsignificant differences (LSD0.o5) represented.

RASSE ET AL.: ALFALFA ROOT AND SHOOT EFFECTS ON SOIL PROPERTIES 727

or absence of living alfalfa roots) and a mulch factor (i.e.,application or no application of alfalfa shoot mulch at harvest).Normality tests indicated that the K^ data were not normallydistributed, while logarithmic transformation of these dataresulted in a more normal distribution. Consequently, mean-separation tests were performed on log-transformed data. Me-dians of the nontransformed KM data were reported.

RESULTS AND DISCUSSIONVolumetric Soil Water Contents

Variations of soil water contents with time were ofgreater amplitude under alfalfa stands (AR and ASM)than bare fallows (BF and BFSM) (Fig. 1 and 2). Soildrying rates were modified by the presence of livingalfalfa root systems, as exemplified by the two continu-ous soil drying periods of 3 Aug. 1995 to 13 Oct. 1995(Fig. 1) and of 29 July 1996 to 13 Sept. 1996 (Fig. 2).During these two drying periods, soil water contents inAR and ASM plots followed similar trends, becomingprogressively lower than water contents for the BF andBFSM treatments. Application of alfalfa shoot mulchto soils under living alfalfa stands appeared to have little

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10 Apr 30 May 19 July 7 Sept 27Oct 16 DecFig. 2. Volumetric soil water contents in the Ap, Bti, and Bt2 horizons

of soils under bare fallow (open circle), bare fallow with alfalfashoot mulch (open square), alfalfa with shoots removed and rootsremaining (filled circle), and alfalfa with alfalfa shoot mulch (filledsquare), and daily rainfall measurements in 1996. Fisher's leastsignificant differences (LSDIMI5) represented.

impact on soil water contents (Fig. 1 and 2). Alfalfashoot application to bare fallow soil reduced the ratesof water loss from the Ap and Bti horizons in 1995 and1996. During drier periods in 1995 and 1996, volumetricsoil water contents of bare fallow soils were significantlygreater when shoot mulch was applied (Fig. 2), sug-gesting that accumulated amounts of alfalfa shoot mulchreduced soil evaporation, resulting in greater accumula-tion of soil water in the Ap horizon. This result concurswith other studies reporting that mulch applications tobare fallows reduced evaporation from the soil profile(Prasad and Power, 1991; Steiner, 1994; Walsh et al.,1996).

Water-recharge rates of the Ap horizon were signifi-cantly increased by alfalfa root systems. For example,from 8 July to 9 July 1996, increases in soil water contentof the Ap horizon of unmulched plots were significantlygreater (P < 0.05) under alfalfa (+12.6%) than underbare fallow (+7.2%). Similarly, precipitation between26 July and 29 July 1996 increased water contents ofthe Ap horizon by 11.9% in the AR plots, which wassignificantly higher (P < 0.001) than the 5.7% increaseobserved in the BF plots. These different increases insoil water contents following rainfall events suggest al-falfa roots augmented soil porosity during the first 2 yrof treatment. Maximum soil water contents in the Ap,Bti, and Bt2 horizons in 1995 were observed in the BFSMplots, while in 1996 maximum soil water contents werereached in the AR and ASM plots. Consequently, alfalfaroot systems appeared to have increased soil porositywith resultantly greater infiltration rates in alfalfa treat-ments than for the bare soil treatments.

Root System DistributionsBy the end of the second growing season, similar

quantities of alfalfa fine roots colonized the soil profilein the Bti, Bt2, and C horizons at depths from 40 to130 cm (Fig. 3). Substantial root numbers were observed

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0 2000 4000 6000 8000

Roots (no. m"2)Fig. 3. Minirhizotron observations on 20 Sept. 1996 of fine root distri-

bution patterns in soil profiles in alfalfa plots with shoots removedand roots remaining (filled circle) and with alfalfa shoot mulchapplied (filled square) on 20 Sept. 1996. Standard errors given forn = 4.


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Fig. 4. (A) Minirhizotron root counts and (B) root turnover rates in the upper 9.5 cm of alfalfa plots. Data from alfalfa with shoots removedand roots remaining and alfalfa with alfalfa shoot mulch were combined because no significant difference between treatments was observed.Standard errors given for « = 8.

by MRs to the maximum sampling depth of 140 cm inSeptember 1996. These data confirm a previous reportthat alfalfa roots grow deeper than 140 cm in standsolder than 2 yr (Pietola and Smucker, 1995). High rootturnover rates were observed in the surface 9.5 cm ofthe Ap horizon, where death of fine roots exceeded 75and 50% during July to October of the 1995 and 1996growing seasons, respectively (Fig. 4). A similar patternof alfalfa fine root turnover has been reported by Coinsand Russelle (1996). The high rate of root mortality inthe uppermost soil layer appears to result from the largefluctuations of soil water contents within the Ap hori-zon. Several times, volumetric soil water contentsdropped below 12%, which is the wilting point for theAp horizon of Kalamazoo loam soils (Fig. 1 and 2).These water deficits were not observed in deeper hori-zons. Alfalfa root turnover rates did not appear to besignificantly modified by mulch treatments in any soilhorizons (data not reported), as the application of alfalfashoot mulch did not significantly modify alfalfa fine rootnumbers for any 9.5-cm depth increment on any givendate (Fig. 3 is an example). Investigators have reportedno significant differences between alfalfa root produc-tion by nodulating and non-nodulating alfalfa varieties,when monitored by MRs (Coins and Russelle, 1996) orby destructive root extractions (Blumenthal and Rus-selle, 1996; Coins and Russelle, 1996; Lory et al., 1992).These studies suggest that the development of alfalfaroot systems may not be sensitive to the availability ofN within the soil profile. Accordingly, in this study,availability of inorganic N was increased under alfalfa

shoot mulch (Rasse et al., 1999) without inducing signifi-cant modification of alfalfa root demographics. Larssonand Jensen (1996) reported that mulching significantlymodifies root populations of black current bushes (7?/i>esnigrum L.) as a result of increases in soil water contents,while mulch-induced modifications of soil temperaturehad no apparent effects on root growth. In our study,water contents of soils under alfalfa stands were neversignificantly modified by shoot mulch application (Fig.1 and 2). Soil temperatures at the three TDR-probedepths recorded on five dates in 1995 were also notsignificantly different between mulched and non-mulched alfalfa plots (data not reported). In conclusion,application of alfalfa shoot mulch to living alfalfa standsincreased soil N (Rasse et al., 1999) but did not signifi-cantly modify soil water contents nor temperatures,which resulted in no consistent modifications of alfalfaroot distribution patterns or turnover rates.

Soil Physical PropertiesTotal, macro-, and gravimetric soil porosities were

significantly higher when alfalfa root systems were pres-ent, while no overall significant effect of shoot mulchwas observed (Table 1). Total porosity was significantlyhigher by 2.3% in unmulched alfalfa compared withbare fallow treatments (Table 2). Macro- and gravimet-ric porosities were significantly higher, by 3.0 and 2.8%,respectively, in AR than in BFSM treatments. Micropo-rosity was significantly higher when shoot mulch wasapplied to bare fallow, although the factorial analysis


Table 1. Factorial analyses for root and mulch effects on saturatedhydraulic conductivity (A'snl); mean weight diameter (MWD);total, gravimetric, micro-, and macroporosities; and total Ccontents in the Ap horizon of soils under bare fallow (BF),bare fallow with alfalfa shoot mulch added (BFSM), alfalfa(AR), and alfalfa with alfalfa shoot mulch added (ASM), fol-lowing 2 yr of treatment.

Source ofvariation

Porosity"sal MWD Total Micro Gravim. Macro

- Probability of a greater F -Root <0.001 0.102 0.032 0.772 0.012 0.027 0.145Mulch 0.437 0.003 0.667 0.068 0.135 0.116 0.849Root x mulch 0.360 0.091 0.414 0.105 0.461 0.776 0.553

indicated that overall root and mulch factors were non-significant (Tables 1 and 2). Total porosities (TP) werecorrelated to total soil C contents (C) and root numbers(R) in the Ap horizon. The regression equations were:TP = 28 + 13.0CTP = 39 + 5.0 X 10~4CTP = 30 + 9.8C + 3.1 X 1C

(r = 0.76, P = <0.001)(r = 0.68, P = £0.01)

R (r = 0.84, P = <0.001)Total soil C and root numbers independently increasedtotal porosity, as demonstrated by the absence of a sig-nificant correlation between soil C contents and rootnumbers (r = 0.45), and by the improved coefficient ofcorrelation when both factors were considered in theregression. Though a trend towards greater soil C con-tents under alfalfa stands was observed, these differ-ences could not be proven significant for the 2-yr period(Tables 1 and 2). These observations imply that totalporosity was modified faster than total soil C contentsby alfalfa root systems. Increased total, macro, andgravimetric soil porosities by alfalfa root systems poten-tially resulted from an increase in the amplitude of wet-ting and drying cycles (Fig. 1 and 2), which have alsobeen reported to enhance the development of soil cracks(Angers and Caron, 1998).

Saturated hydraulic conductivity was significantlyhigher in soils with alfalfa roots growing than underbare fallows (Tables 1 and 2). This result confirms thatalfalfa root systems increase soil Ksu, as previously re-ported by several authors (Caron et al., 1996; Li andGhodrati, 1994; Meek et al., 1992; Mitchell et al., 1995).Application of alfalfa shoot mulch to soils under barefallow and alfalfa stands did not significantly modify,Ksat (Table 1). Though little information is available formulch effects on Kwt, Prasad and Power (1991) estimatethat mulching is likely to increase K^ because of highersoil faunal activity. Lal et al. (1980) reported significantincreases in K^ by mulch application on recently clearedtropical Alfisols. Such an effect was not observed inthis study.

Saturated hydraulic conductivities were significantlycorrelated with macroporosities (Fig. 5A) and gravimet-ric porosities (Fig. 5B). Root turnover and increasedamplitude of wetting and drying cycles under alfalfastands are potentially responsible for increased K^t andsoil macroporosity in alfalfa systems. Meek et al. (1992)reported that RIMs have to be devoid of alfalfa roottissues before contributing to the preferential flow of

Table 2. Saturated hydraulic conductivities (/£«,,); mean weightdiameter (MWD); total, gravimetric, micro, and macroporosi-ties; and total C contents in soils under bare fallow (BF), barefallow with alfalfa shoot mulch added (BFSM), alfalfa (AR),and alfalfa with alfalfa shoot mulch added (ASM), following2 yr of treatment._________________________

PorosityJC»,t MWD Total Micro Gravim. Macro Carbon

BFBFSMARASM

0.86b1.64a1.50a

3.76b4.57a4.26a4.56a

38.7b39.0ab41.0a40.1ab

27.3b29.0a27.9ab28.1ab

13.9ab12.5b15.3a14.8a

11.4ablO.flb13.0a12.0ab

0.90a0.92a0.96a0.95a

t Median reported; means separation test conducted on log-transformeddata.

I Means and medians followed by the same letter are not significantlydifferent by Fisher's LSD(U15.

water through soils. Living alfalfa roots have the poten-tial to plug empty soil macropores, thereby reducingwater fluxes through soils (Gish and Jury, 1983; Rasseand Smucker, 1998; Smucker et al., 1995). Root popula-tions in the upper 9.5 cm of the soil profile were reducedfrom 4800 roots m~2 on 26 May 1995 to <1000 rootsm~2 on 8 Aug. 1995, generating =4000 potential RIMsm~2 (Fig. 4). From summer to fall 1996, root populationswere reduced by more than 1000 roots m~2. Conse-quently, during the 2-yr period, =5000 potential RIMsm~2 were opened by alfalfa root turnover, which is fourtimes as much as live root populations observed on26 Sept. 96. Consequently, increased soil macroporosityand ATsat appeared to result in part from high rates ofalfalfa root turnover in the upper 9.5-cm soil profile.Additional soil drying by alfalfa roots potentially in-creased the number and extent of soil cracks, which alsomay have contributed to increasing Ksat in alfalfa soils.

The stability of soil aggregates, ranging from 4.75 to6.30 mm in diameter and sampled from 0.00- to 0.20-msoil depths was significantly improved, as indicated bycomparisons of MWD, when alfalfa shoot mulch wasapplied to the soil surface (Table 1). No significant effecton soil aggregation by alfalfa roots was observed whenmulched and unmulched plots were considered together(Table 1). Nevertheless, when unmulched plots wereconsidered separately, aggregate stability was signifi-cantly higher in the Ap horizon of soils with alfalfa roots(AR) than when devoid of root systems (BF) (Table2). Although alfalfa improvements of soil structure havebeen reported (Angers, 1992; Perfect et al., 1990; Chan-tigny et al., 1997), this is the first report of the separateand combined contributions of alfalfa shoots and rootsfor improving soil aggregate stability. Chantigny et al.(1997) reported that alfalfa promotes soil aggregationto a lesser extent than canary grass (Phalaris arundina-cea L.) and timothy (Pleum pratense L.). When incorpo-rated, alfalfa improved the stability of soil structuremore than poultry manure or sewage sludge (Martensand Frankenberger, 1992). In this study, soil aggregatestability was not significantly correlated with K^t, bulkdensity, porosity, total C, root numbers, or root turnoverrates (data not reported), suggesting that other factorsbesides these soil and plant root parameters were modi-


0.8

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-0.4

Log K , = -1.06 + 0.098 x porosity r = 0.90 ***

10 12

Macroporosity (%)14 16

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Log Ksat = -1.47 + 0.110 x porosity r = 0.91 *** B

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Gravimetric porosity (%)Fig. 5. Regression of log KM vs. (A) macroporosity and (B) gravimetric porosity for the upper Ap horizon of plots under bare fallow (open

circle), bare fallow with alfalfa shoot mulch (open square), alfalfa with shoots removed and roots remaining (closed circle), and alfalfa withalfalfa shoot mulch (closed square). *** Significant at P < 0.001.

fying the mechanisms associated with forming stableaggregates. The absence of correlation between rootpopulations and aggregate stability suggests that thephysical enmeshing of soil aggregates and enhancedwetting and drying cycles by alfalfa roots were not theprimary mechanisms of structural stabilization by alfalfaroots. Similar conclusions were reported by Degens etal. (1994), who found no significant stabilization of mac-roaggregates by clover roots in sandy soils, while oppo-site results were obtained by Perfect et al. (1990), whoreported significant correlations between wet aggregatestability and root parameters. Wetting and drying cyclescan increase soil fragmentation (Angers and Caron,1998). Highest MWD of aggregates in the BFSM treat-ment corresponded with the lowest amplitudes of wet-ting and drying cycles. Nevertheless, greatest amplitudesof wetting and drying cycles were observed in the alfalfaplots, although aggregate stabilities did not significantlydiffer among AR, ASM, and BFSM plots. In addition,lowest MWD was observed in the BF plots, where soilwater varied less over time than in AR and ASM plots.Therefore, modifications to the soil water regime did

not appear to be the driving mechanism for differencesin MWD between treatments in this study. Transientsoluble C pools and associated microbial biomass activi-ties, as reported by Haynes et al. (1991), Angers andMehuys (1989), and Sissoko and Smucker (1999, unpub-lished data), may have been the primary stabilizing fac-tors for aggregates during these field studies.

CONCLUSIONSAlfalfa root systems increased water flow, as indicated

by higher Ksa, total and macroporosities, and water re-charge rates of the Ap horizon. Increases in Ksa ap-peared to have resulted from greater macroporosities.Increases in soil porosity by alfalfa root systems appar-ently resulted from greater amplitudes of wetting anddrying cycles and the formation of RIMs under alfalfastands. Root turnover, disappearance, and resultantconnected porosities associated with RIM formation ap-peared to have greater impacts on Ksat than living rootdensities. Minirhizotron technology was successfullyused for better understanding modifications of soil wa-


ter flow induced by growth and decay of alfalfa rootsystems. It is also concluded that both living root systemsand root history (i.e., turnover rates) should be consid-ered when analyzing the impact of root systems on soilphysical properties. It is suggested that aggregate stabil-ity was more affected by sources of C from shoot mulchand root decomposition than by factors specific to rootactivities such as physical enmeshment of aggregatesand increased soil wetting and drying cycles. Additionalresearch should be conducted to further confirm thespecific contributions of above- and belowground com-ponents of different plant species to soil aggregation.

ACKNOWLEDGMENTSThis research was supported in part by the NSF/LTER

project no. BSR 9527663, the C.S. Mott Foundation Chairfor Sustainable Agriculture, and the Michigan AgricultureExperiment Station. Technical assistance by John Fergusonand Mark Halvorson is gratefully acknowledged.

alfalfa root and shoot mulching effects on soil hydraulic properties and aggregation

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