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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Alfalfa Root and Shoot Mulching Effects on Soil Hydraulic Properties and Aggregation

Crop and Soil Sciences Department, Plant and Soil Sciences Building, Michigan State University, East Lansing, MI 48824-1325 USA

rasse{at}astro.ulg.ac.be

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Although alfalfa (Medicago sativa L.) stands have been reported to improve soil physical properties, little is known about the specific influences of above- and belowground alfalfa components on soil physical properties. A 2-yr study was conducted to investigate alfalfa root and shoot mulch modifications of soil physical properties and water movement in the root zone of a Kalamazoo loam soil (fine-loamy, mixed, mesic Typic Hapludalf) in southwest Michigan. Four treatments were considered: bare fallow (BF), bare fallow with alfalfa shoot mulch (BFSM), alfalfa with shoots removed and roots remaining (AR), and alfalfa with alfalfa shoot mulch (ASM). Volumetric soil water contents were measured by time domain reflectometry (TDR). Development of fine roots was monitored by minirhizotron technology. Alfalfa root systems increased saturated hydraulic conductivity (K_sat) 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 to increased amplitudes of wetting and drying cycles and high rates of root turnover in the Ap horizon. K_sat was significantly correlated with macroporosity . Mean weight diameter (MWD) of aggregates from bare fallow soils was 20% higher when alfalfa shoot mulch was applied. Our results suggest that aggregate stability was more affected by C sources from shoot mulch and root turnover than by factors specific to root activities such as physical enmeshment of aggregates and increased soil wetting and drying cycles.

Abbreviations: AR, alfalfa with shoots removed and roots remaining • ASM, alfalfa with alfalfa shoot mulch • BF, bare fallow • BFSM, bare fallow with alfalfa shoot mulch • K_sat, saturated hydraulic conductivity • MWD, mean weight diameter • MR, minirhizotron • RIM, root-induced macropore • TDR, time domain reflectometry

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
THE ROTATION EFFECT of legume crops has been attributed in part to improved soil physical properties (Folorunso et al., 1992; McVay et al., 1989). Alfalfa is the most frequent perennial legume in rotation with corn (Zea mays L.) in the north-central states (Eberlein et al., 1992). Improved soil structural stability under alfalfa stands has been reported by several authors (Angers, 1992; Perfect et al., 1990; Chantigny et al., 1997). Alfalfa root systems have been reported to increase the K_sat of soils free of previous root channels (Li and Ghodrati, 1994). Meek et al. (1992) observed a sixfold increase of K_sat when compacted sandy loams were planted with alfalfa. Mitchell et al. (1995) reported that alfalfa root systems have the ability to increase the K_sat of swelling soils.

Improvements of soil physical properties by alfalfa stands appear to result from a combination of effects from above- and belowground plant materials (Angers and Caron, 1998). Alfalfa stands have to be destroyed prior to planting the following crop. This operation is generally conducted by spray-killing the stands when substantial amounts of aboveground plant material are present (Baldock and Musgrave, 1980; Robbins and Carter, 1980). Both alfalfa roots and shoots contribute to fresh organic matter inputs into the soil profile, which promotes soil aggregation (Angers and Caron, 1998). Soil structure and the development of soil cracks is influenced by shrinking and swelling cycles (Angers and Caron, 1998; Sissoko and Smucker, 1999, unpublished data). Amplitude of these cycles is increased by plant roots, which absorb water from the soil, and decreased by crop residues, which reduce soil evaporation rates (Prasad and Power, 1991). Although the separate effects on soil physical properties of alfalfa roots and shoots have been suggested in the literature, few studies have investigated the different contributions of these components. A better understanding of the specific influences of above- and belowground alfalfa components on soil physical properties is essential before we can determine the full contribution of alfalfa to the rotation effect. In this study, the influences of alfalfa roots and shoots on soil physical properties and water movement in the root zone were investigated for a loam soil for 2 yr in southwestern Michigan.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Experimental Design and Treatments
A field experiment was conducted at the Long-Term Ecological Research site of the Kellogg Biological Station in southwestern Michigan. Four treatments were evaluated: bare fallow soil (BF); bare fallow soil to which alfalfa shoot mulch was applied following each harvest (BFSM); alfalfa with shoots removed following each harvest and roots remaining (AR); alfalfa with shoot mulch applied to the soil surface following each harvest (ASM). Each treatment was replicated four times in 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. Corn residues were moldboard plowed to a depth of 23 cm. All plots were tilled and trafficked equally. Alfalfa (Pioneer 5246) was planted in one-half of the plots at a rate of 22 kg seed ha^-1 on 30 Aug. 1994. The bare soil plots (BF and BFSM) were also drilled without seeds. The bare soil plots were kept free of weeds by applications of glyphosate [n-(phosphonomethyl)glycine] at 6-wk intervals between April and August of 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 June 1996. Lime was applied at a rate of 2500 kg ha^-1 on 14 June 1996. The alfalfa plots were harvested on 9 June 1995, 24 July 1995, 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 were raked from the AR and ASM plots and weighed. Equal amounts of alfalfa shoots were then applied to all ASM and BFSM plots. At the end of the second growing season, cumulative shoot mulch biomass applications approached 16.4 Mg ha^-1 dry matter. All sampled areas were located at least 1 m from the edge of the plots to avoid border effects. The surface area of each plot was allocated to (i) nondestructive sampling using in situ instruments (2 by 4 m), (ii) a yield assessment area (4 by 4 m), and (iii) a destructive sampling area (2 by 4 m). Plots were separated by surface plastic barriers installed to depths of 10 cm and protruding 5 cm above the soil surface to prevent runoff and runon between plots.

Measurements
Volumetric 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 to intercept the water in the central regions of the Ap, Bt₁, and Bt₂ horizons. Probes were inserted horizontally into undisturbed soil layers from the wall of an access pit, about 0.4 m long by 0.25 m wide, dug in each plot before planting. Soil profiles were described in each access pit, and each horizon was replaced and compacted to its original density. Volumetric soil water contents were collected by an in-line cable tester, TDR meter, model 1502C (Tektronix, Beaverton, OR). Volumetric soil water contents were derived from the TDR-meter readings using the equation developed by Topp et al. (1980).

Alfalfa root demographics were monitored three times each year by minirhizotron technology, using a micro video camera (Bartz Technology, Santa Barbara, CA) as described by Ferguson and Smucker (1989). Three clear polybutyrate tubes (0.05 by 2.4 m) were installed at 45° angles in AR and ASM plots. One control minirhizotron (MR) tube was placed in each of the BF and BFSM plots. Root intersections with the upper surfaces of MR tubes were recorded on identical frame positions, 1.35 by 1.80 cm each. Alfalfa roots that did not display signs of decomposition were hand counted in each of the video frames. Root numbers were added every 10 frames, which represents a vertical depth increment of 9.5 cm. Total root numbers were then reported per square meter by dividing by the cumulative surface of 10 frames. Root turnover rates were assessed by the difference in root populations between consecutive dates. Assumption was made that one root-induced macropore (RIM) was generated each time the decomposition of one root was observed in one MR frame.

Four undisturbed soil cores, 0.076 m in diameter by 0.076 m deep, were collected in each plot in October 1996 for analyzing K_sat 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 K_sat measurements using the constant head method (Klute and Dirksen, 1986). Following K_sat measurements, cores were resaturated and placed in pressure chambers at 0.006 and 0.033 MPa for 4 d, corresponding with macro- and gravimetric porosities, respectively. Cores were dried for 48 h in a forced-air oven at 105°C. Total porosity was calculated from the amount of water contained in the cores at saturation. Macroporosity was calculated from the difference between water contents at saturation and at 0.006 MPa. Microporosity was defined as the difference between total porosity and macroporosity. Gravimetric porosity was calculated from the difference between water contents at saturation and at 0.033 MPa.

Four subsamples of soil were collected from the 0.00- to 0.20-m region of each plot in October 1996. Air-dry aggregates were manually sieved to obtain the 4.75- to 6.30-mm fraction. Aggregates, 25 g per sample, were rewetted by nebulizing for 12 h with distilled water (Sissoko and Smucker, 1999, unpublished data). Aggregate stability of rewetted samples was determined by the wet sieving method (Kemper and Chepil, 1965), using a stack of sieves with 4.000-, 2.000-, 1.000-, 0.500-, 0.250-, and 0.106-mm opening diameters. The results of wet sieving were expressed as the MWD calculated as the mean fraction of soil on each sieve multiplied by the mean diameter of adjacent sieves (Kemper and Chepil, 1965). A correction was applied for primary sand particles.

Disturbed samples for total C analyses were collected from the Ap horizon of each plot to a depth of 0.23 m on 11 Oct. 1996 . Ten subsamples per plot were collected, mixed into one composite 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 Erba Strumentazione, Milano, Italy). Modifications of total soil C by alfalfa and bare fallow systems for this experiment are reported in Rasse et al. (1999). Here soil C is considered only for its interactions with the soil physical properties.

Statistical Analyses
Statistical analyses were conducted using the general linear model of the SAS system (SAS Institute, 1989). Mean separation tests were conducted using Fisher's least significant differences (LSD_0.05) when global F tests were significant. Factorial analyses were conducted with a root factor (i.e., presence 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_sat data were not normally distributed, while logarithmic transformation of these data resulted in a more normal distribution. Consequently, mean-separation tests were performed on log-transformed data. Medians of the nontransformed K_sat data were reported.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Volumetric Soil Water Contents
Variations of soil water contents with time were of greater amplitude under alfalfa stands (AR and ASM) than bare fallows (BF and BFSM) (Fig. 1 and 2) . Soil drying rates were modified by the presence of living alfalfa root systems, as exemplified by the two continuous 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 in AR and ASM plots followed similar trends, becoming progressively lower than water contents for the BF and BFSM treatments. Application of alfalfa shoot mulch to soils under living alfalfa stands appeared to have little impact on soil water contents (Fig. 1 and 2). Alfalfa shoot application to bare fallow soil reduced the rates of water loss from the Ap and Bt₁ horizons in 1995 and 1996. During drier periods in 1995 and 1996, volumetric soil water contents of bare fallow soils were significantly greater when shoot mulch was applied (Fig. 2), suggesting that accumulated amounts of alfalfa shoot mulch reduced soil evaporation, resulting in greater accumulation of soil water in the Ap horizon. This result concurs with other studies reporting that mulch applications to bare fallows reduced evaporation from the soil profile (Prasad and Power, 1991; Steiner, 1994; Walsh et al., 1996).

View larger version (32K): [in this window] [in a new window]   Fig. 1 Volumetric soil water contents in the Ap, Bt1, and Bt2 horizons of soils under bare fallow (open circle), bare fallow with alfalfa shoot mulch (open square), alfalfa with shoots removed and roots remaining (filled circle), and alfalfa with alfalfa shoot mulch (filled square), and daily rainfall measurements in 1995. Fisher's least significant differences (LSD0.05) represented

View larger version (31K): [in this window] [in a new window]   Fig. 2 Volumetric soil water contents in the Ap, Bt1, and Bt2 horizons of soils under bare fallow (open circle), bare fallow with alfalfa shoot mulch (open square), alfalfa with shoots removed and roots remaining (filled circle), and alfalfa with alfalfa shoot mulch (filled square), and daily rainfall measurements in 1996. Fisher's least significant differences (LSD0.05) represented

Root System Distributions
By the end of the second growing season, similar quantities of alfalfa fine roots colonized the soil profile in the Bt₁, Bt₂, and C horizons at depths from 40 to 130 cm (Fig. 3) . Substantial root numbers were observed by MRs to the maximum sampling depth of 140 cm in September 1996. These data confirm a previous report that alfalfa roots grow deeper than 140 cm in stands older than 2 yr (Pietola and Smucker, 1995). High root turnover rates were observed in the surface 9.5 cm of the Ap horizon, where death of fine roots exceeded 75 and 50% during July to October of the 1995 and 1996 growing seasons, respectively (Fig. 4) . A similar pattern of alfalfa fine root turnover has been reported by Goins and Russelle (1996). The high rate of root mortality in the uppermost soil layer appears to result from the large fluctuations of soil water contents within the Ap horizon. Several times, volumetric soil water contents dropped below 12%, which is the wilting point for the Ap horizon of Kalamazoo loam soils (Fig. 1 and 2). These water deficits were not observed in deeper horizons. Alfalfa root turnover rates did not appear to be significantly modified by mulch treatments in any soil horizons (data not reported), as the application of alfalfa shoot mulch did not significantly modify alfalfa fine root numbers for any 9.5-cm depth increment on any given date (Fig. 3 is an example). Investigators have reported no significant differences between alfalfa root production by nodulating and non-nodulating alfalfa varieties, when monitored by MRs (Goins and Russelle, 1996) or by destructive root extractions (Blumenthal and Russelle, 1996; Goins and Russelle, 1996; Lory et al., 1992). These studies suggest that the development of alfalfa root systems may not be sensitive to the availability of N within the soil profile. Accordingly, in this study, availability of inorganic N was increased under alfalfa shoot mulch (Rasse et al., 1999) without inducing significant modification of alfalfa root demographics. Larsson and Jensén (1996) reported that mulching significantly modifies root populations of black current bushes (Ribes nigrum L.) as a result of increases in soil water contents, while mulch-induced modifications of soil temperature had no apparent effects on root growth. In our study, water contents of soils under alfalfa stands were never significantly modified by shoot mulch application (Fig. 1 and 2). Soil temperatures at the three TDR-probe depths recorded on five dates in 1995 were also not significantly different between mulched and nonmulched alfalfa plots (data not reported). In conclusion, application of alfalfa shoot mulch to living alfalfa stands increased soil N (Rasse et al., 1999) but did not significantly modify soil water contents nor temperatures, which resulted in no consistent modifications of alfalfa root distribution patterns or turnover rates.

View larger version (25K): [in this window] [in a new window]   Fig. 3 Minirhizotron observations on 20 Sept. 1996 of fine root distribution patterns in soil profiles in alfalfa plots with shoots removed and roots remaining (filled circle) and with alfalfa shoot mulch applied (filled square) on 20 Sept. 1996. Standard errors given for

View larger version (25K): [in this window] [in a new window]   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 removed and roots remaining and alfalfa with alfalfa shoot mulch were combined because no significant difference between treatments was observed. Standard errors given for

Saturated hydraulic conductivity was significantly higher in soils with alfalfa roots growing than under bare fallows (Tables 1 and 2). This result confirms that alfalfa root systems increase soil K_sat, as previously reported by several authors (Caron et al., 1996; Li and Ghodrati, 1994; Meek et al., 1992; Mitchell et al., 1995). Application of alfalfa shoot mulch to soils under bare fallow and alfalfa stands did not significantly modify K_sat (Table 1). Though little information is available for mulch effects on K_sat, Prasad and Power (1991) estimate that mulching is likely to increase K_sat because of higher soil faunal activity. Lal et al. (1980) reported significant increases in K_sat by mulch application on recently cleared tropical Alfisols. Such an effect was not observed in this study.

Saturated hydraulic conductivities were significantly correlated with macroporosities (Fig. 5A) and gravimetric porosities (Fig. 5B). Root turnover and increased amplitude of wetting and drying cycles under alfalfa stands are potentially responsible for increased K_sat and soil macroporosity in alfalfa systems. Meek et al. (1992) reported that RIMs have to be devoid of alfalfa root tissues before contributing to the preferential flow of water through soils. Living alfalfa roots have the potential to plug empty soil macropores, thereby reducing water fluxes through soils (Gish and Jury, 1983; Rasse and Smucker, 1998; Smucker et al., 1995). Root populations in the upper 9.5 cm of the soil profile were reduced from 4800 roots m^-2 on 26 May 1995 to <1000 roots m^-2 on 8 Aug. 1995, generating 4000 potential RIMs m^-2 (Fig. 4). From summer to fall 1996, root populations were reduced by more than 1000 roots m^-2. Consequently, during the 2-yr period, 5000 potential RIMs m^-2 were opened by alfalfa root turnover, which is four times as much as live root populations observed on 26 Sept. 96. Consequently, increased soil macroporosity and K_sat appeared to result in part from high rates of alfalfa root turnover in the upper 9.5-cm soil profile. Additional soil drying by alfalfa roots potentially increased the number and extent of soil cracks, which also may have contributed to increasing K_sat in alfalfa soils.

View larger version (20K): [in this window] [in a new window]   Fig. 5 Regression of log Ksat 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 with alfalfa shoot mulch (closed square). *** Significant at P 0.001

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Alfalfa root systems increased water flow, as indicated by higher K_sat, total and macroporosities, and water recharge rates of the Ap horizon. Increases in K_sat appeared to have resulted from greater macroporosities. Increases in soil porosity by alfalfa root systems apparently resulted from greater amplitudes of wetting and drying cycles and the formation of RIMs under alfalfa stands. Root turnover, disappearance, and resultant connected porosities associated with RIM formation appeared to have greater impacts on K_sat than living root densities. Minirhizotron technology was successfully used for better understanding modifications of soil water flow induced by growth and decay of alfalfa root systems. It is also concluded that both living root systems and root history (i.e., turnover rates) should be considered when analyzing the impact of root systems on soil physical properties. It is suggested that aggregate stability was more affected by sources of C from shoot mulch and root decomposition than by factors specific to root activities such as physical enmeshment of aggregates and increased soil wetting and drying cycles. Additional research should be conducted to further confirm the specific contributions of above- and belowground components of different plant species to soil aggregation.

	ACKNOWLEDGMENTS

 
This research was supported in part by the NSF/LTER project no. BSR 9527663, the C.S. Mott Foundation Chair for Sustainable Agriculture, and the Michigan Agriculture Experiment Station. Technical assistance by John Ferguson and Mark Halvorson is gratefully acknowledged.

Received for publication December 22, 1998.

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