Soil Science Society of America Journal 63:1845-1847 (1999)
© 1999 Soil Science Society of America
DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION
Soil microbial biomass alterations during the maize silage growing season relative to tillage method
T.E. Staleya
a USDA-ARS-NAA, Appalachian Farming Systems Research Center, 1224 Airport Rd., Beaver, WV 25813-9423 USA
tstaley{at}afsrc.usda.ars.gov
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ABSTRACT
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Tillage method can significantly alter soil microbial populations and activities. Although considerable literature exists on microbial and soil chemical alterations under various tillage methods, little information exists on soil microbial biomass C (SMB) alterations during the growing season, and especially on the relationship of SMB to crop N use. The objective of this study was to determine the effect of no-tillage (NT) or conventional tillage (CT), and soil location, on SMB during the growing season. A maize (Zea mays L.) silage/15N field experiment, under NT or CT for 3 yr before this study, was used during the fourth growing season. Averaged over sampling times and location (within-row or between-row), SMB in the 0- to 3.8-cm and 3.8- to 7.5-cm soil layers under NT was 87 and 33% greater, respectively, than under CT. Linear regression of soil surface layer (0–3.8 cm) SMB on day-of-year revealed a significant (P 
0.10) relationship only within-row and under NT, with a 29% SMB decrease during the growing season. Similar regressions for the other layers and treatments were insignificant (P > 0.10) or had small seasonal differences. SMB was consistently higher in the between-row locations under both tillage methods. Despite substantial tillage method-induced differences in SMB (50% overall, accompanied by small differential seasonal differences) in the more surficial layers, these alterations appear to have been of little practical consequence, since previous work on these plots revealed essentially no differences in silage utilization of either fertilizer N or soil N relative to tillage method. Thus, the importance of SMB in significantly affecting crop N use in this within-row, banded, maize silage system is questioned.
Abbreviations: CT, conventional tillage • DOY, day-of-year • NT, no-tillage • SMB, soil microbial biomass-C
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INTRODUCTION
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TILLAGE method has been shown by numerous workers to significantly alter the microflora of soil. Most often, imposition of conservation tillage, especially no-tillage (NT), has resulted in increased microbial populations and activities in the more surficial layers, compared with conventional tillage (CT). In accord with this observation, soil microbial biomass C (SMB), as a measure of the total microbial tissue of vegetative bacteria and fungi, has also often been found to similarly respond to tillage method (Doran, 1987; Staley et al., 1988; Carter, 1991; McCarty et al., 1995).
Fewer reports have appeared in the literature concerning the temporal effects on SMB under NT, compared with CT. Lynch and Panting (1980, 1982) showed that SMB in the surface layer (0–15 cm) under both direct-drilled (NT) and ploughed (CT) wheat (Triticum aestivum L.) was nearly constant from autumn to spring, increased to a maximum during the summer, then declined to about the autumn concentration. SMB was not significantly different between tillage methods until harvest in late August, when it was 33 to 77% greater under NT than CT. Granatstein et al. (1987), using a wheat–barley (Hordeum vulgare L.)–pea (Pisum sativum L.) rotation site, reported little change in SMB in 5-cm deep soil increments to 30 cm under either tillage method from April to September, then a large increase in the 0- to 5-cm layer in October, but only under NT. In wheat–legume rotations, Van Gestel et al. (1992) found near-linear decreases in SMB from mid-winter to autumn in the 0- to 2.5-cm layer under both CT and NT. To my knowledge, the only study that has examined tillage method effects on SMB (actually, biomass N) specifically and frequently over the growing season is that of Carter and Rennie (1984), who used four sites on Chernozemic soils planted with spring wheat. For the 0- to 5-cm layer, increases in biomass N during the early growing season were greater under zero tillage (NT) than shallow tillage (CT), then declined to about the same level by the end of the growing season.
Because of this paucity of information during the growing season, the response of SMB to crop (root) growth and related soil processes (especially decomposition and crop N availability) under NT and CT remains uncertain, particularly under maize (Zea mays L.). Thus, it was the primary objective of this study to determine the effect of the tillage method (NT or CT for the previous 3 yr) on SMB during the fourth and final growing season of a maize silage–15N experiment. Secondary objectives were to determine the effect of location (distance from fertilizer-N band) on SMB, and the relationship of SMB to previously reported maize N use (Staley and Perry, 1995).
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Materials and Methods
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A field experiment on a moderately well-drained Gilpin silt loam (fine-loamy, mixed, mesic Typic Hapludult) in Cool Ridge, WV (Staley and Perry, 1995) was used during the last year of 4-yr continuous maize silage production. Each year in early June, the CT plots were rototilled, and both the NT and CT plots were planted with 76-cm row spacings. In early July (NH4)2SO4 (15N-depleted) was applied as a 15-cm band, within-row. Silage was cut in late September or early October.
Nine times during the growing season (26 April–6 October), soil cores (1.9-cm Oakfield coring tube) were taken from each of the four replicates of the 112 kg N/ha (near-optimum), high plant density (59 280 plants/ha) treatments. Twelve cores, to depth, were obtained, equidistantly, from each sub-subplot (3.05 x 6.10 m) for both the within-row and between-row locations. Each core was sectioned and pooled by depth (0–3.8, 3.8–7.5, and 7.5–15 cm) for each treatment and location. The samples were immediately placed in a cooler and transported to the laboratory. At ambient field moisture, each sample was sieved to <2 mm, riffled three times, subsampled (visible roots and plant debris removed by hand), and frozen in double plastic bags at -20°C until analyzed in triplicate.
Soil microbial biomass-C was estimated by the alcohol-free, chloroform fumigation–K2SO4 extraction method, which is particularly applicable to acidic soils (Vance et al., 1987). Soil extracts were analyzed on a Model DC–80 Total Carbon Analyzer (Dohrmann, Santa Clara, CA). Soil microbial biomass-C in the soils was calculated by dividing the difference in extractable C from the fumigated and nonfumigated samples by 0.45 (the microbial biomass C mineralization constant), then converted to mg SMB/m3 soil, oven-dried basis.
Linear regression and heterogeneity of slope analyses were done in SAS (SAS Inst., 1990). Significant differences are given at the P 0.05 level, unless otherwise indicated.
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Results and Discussion
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Averaged over locations and sampling times, SMB in the 0- to 3.8-cm layer under NT was significantly increased, being nearly twice (87% greater) that under CT (data not shown). Although only 33% greater, SMB in the 3.8- to 7.5-cm layer was also significantly increased under NT, compared with CT. Unlike the more surficial layers, no significant differences were found between tillage methods in the 7.5- to 15-cm layer, with SMB under NT being only 87% of that under CT in this deepest layer examined. These results corroborate, in general, the numerous reports of other investigators that SMB is increased in the soil surface upon imposition of NT.
Concerning the primary objective of determining the effect of tillage method on SMB over the growing season, a day-of-year (DOY) relationship to SMB was detected and found to be differentially related to tillage method (Fig. 1)
. In the soil surface layers, only under NT and within-row was SMB significantly (P 0.10) affected by DOY, that being a gradual decrease of about 29% from the first sampling in the spring (116 DOY or 29 April) to the last sampling in the fall (279 DOY or 6 October). Although the slopes of the regression equations for the other topsoil treatments were not significant (P > 0.10), all tended to decrease during the sampling period. Similarly significant increases were found in the 3.8- to 7.5-cm layers, between-row, under both NT (30%) and CT (24%). Statistical analyses of all other regressions proved them to be insignificant (P > 0.10).
In addition to DOY effects, location (within-row and between-row) effects on SMB were also examined. Averaged over all sampling periods, SMB was consistently higher (6–64%) for the between-row samples, compared with the within-row samples, in all soil layers under both tillage methods (data not shown). More specifically, except for the 7 July (188 DOY) sampling, between-row SMB in the soil surface layer (0–3.8 cm) was consistently greater than for the within-row samples during the entire growing season under both tillage methods. Interestingly, SMB appeared to temporarily decrease within-row and increase between-row, soon after fertilizer-N band application under both tillage methods (Fig. 1, shaded areas). Fertilizer-N-induced soil acidification within-row, and new root growth (exudation) between-row may explain this observation. Our finding of higher between-row, compared with within-row, SMB is in accord with the findings of Carter and White (1986) after 4-yr continuous, direct-planted maize on a Podzol on Prince Edward Island.
An explanation for these tillage method-induced differences in SMB can only be conjectured at the present time. Previous reports by other workers have suggested root growth differences under NT and CT as positive regulators of SMB (Carter and Rennie, 1984; Lynch and Panting, 1980). Although this explanation cannot be ruled out (root parameters were not measured in the present study), it would seem an unlikely one for this study because of the general trend of decreasing SMB during the growing season found under NT, and the relatively constant concentrations found under CT. In a study on a Delanco silt loam in Maryland, Anderson (1987) found a 90% decrease in root mass at 0.19 m from the maize row at silking under both NT and CT. Decreased root mass between maize rows, coupled with consistently higher SMB for the between-row soil samples in the present study, would seem to argue against the stimulation of SMB by maize roots. However, Coleman et al. (1997) have reported 33 to 44% lower biomass N (and presumably, SMB) in root-excluded, inter-rows of sorghum (Sorghum bicolor L.) in Georgia, which supports the generally held conception that root growth is directly related to SMB.
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Conclusions
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That considerably higher concentrations (50% more) of SMB existed in the more surficial layers under NT, compared with CT, even before planting, and generally persisted during the growing season, suggests that crop N availability should have been affected, likely through immobilization into microbial tissue. In a previous report on the same study site (Staley and Perry, 1995), it was found that neither maize silage production, nor fertilizer-N, nor soil-N use were appreciably affected by tillage method. Thus, the significant SMB differences found in this study, relative to tillage method, seem to have been of little or no consequence to crop growth and N use in this N-banded (within-row) maize silage system.SAS Institute 1990
Received for publication June 8, 1998.
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REFERENCES
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- Anderson E.L. Corn root growth and distribution as influenced by tillage and nitrogen fertilization. Agron. J. 1987;79:544-549.[Abstract/Free Full Text]
- Carter M.R. The influence of tillage on the proportion of organic carbon and nitrogen in the microbial biomass of medium-textured soils in a humid climate. Biol. Fertil. Soils 1991;11:135-139.
- Carter M.R., Rennie D.A. Dynamics of soil microbial N under zero and shallow tillage for spring wheat, using 15N urea. Plant Soil 1984;76:157-164.
- Carter M.R., White R.P. Determination of variability in soil physical properties and microbial biomass under continuous direct-planted corn. Can. J. Soil Sci. 1986;66:747-750.
- Coleman, D.C., P.F. Hendrix, M.H. Beare, and D.A. Crossley, Jr. 1997. Root exclusion effects on microbial biomass-N and 15N in Piedmont fields. p. 208. In Agronomy Abstracts. ASA, Madison, WI.
- Doran J.W. Microbial biomass and mineralizable nitrogen distributions in no-tillage and plowed soils. Biol. Fertil. Soils 1987;5:68-75.
- Granatstein D.M., Bezdicek D.F., Cochran V.L., Elliott L.F., Hammel J. Long-term tillage and rotation effects on soil microbial biomass, carbon and nitrogen. Biol. Fertil. Soils 1987;5:265-270.
- Lynch J.M., Panting L.M. Cultivation and the soil biomass. Soil Biol. Biochem. 1980;12:29-33.
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- SAS Institute. SAS/STAT User's guide, ver. 6.1. Cary, NC: SAS Inst, 1990.
- Staley T.E., Edwards W.M., Scott C.L., Owens L.B. Soil microbial biomass and organic component alterations in a no-tillage chronosequence. Soil Sci. Soc. Am. J. 1988;52:998-1005.[Abstract/Free Full Text]
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ABSTRACT
INTRODUCTION
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