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A review: long-term effects of agricultural systems on soil biochemical and microbial parameters
Agriculture, Ecosystems and Environment, 40 (1992) 25-36
25
Elsevier Science Publishers B.V., Amsterdam
A review: long-term effects of agricultural systems on soil biochemical and microbial parameters Richard P. Dick 202 Strand Agrwultural Hall, Department of Crop and Soil Science, OregonState Umversity, Corvallis, OR 97331, USA
ABSTRACT Dick, R.P., 1992. A review: long-term effects of agricultural systems on soil biochemical and microbial parameters. Agric. Ecosystems Environ., 40: 25-36. This paper pro~;des a review of recent developments on assessing the effect of agricultural systems on long-term productivity of soils. Cultivation of soils, besides affecting so;I chemistry and structure, reduces biological activity due to the reduction of macroaggregates which provides an important microhabitat for microbial activity. Indirect evidence suggests that soil amendments such as animal and green manures, and plant diversity (crop rotations) may be more important in maintaining soil microbial activity/diversity than conservation tillage in monocultural systems. There is increasing evidence that crop rotation promotes crop productivity by suppressing deleterious microorganisms that flourish under monoculture. Additions of inorganic fertilizers can increase soil biological activit,, because of an increased plant biomass production which upon incorporation stimulates soil biGlogical activity. Conversely, limited evidence suggests that repeated applications of inorganic fertilizer nutrients can suppress production of certain soil enzymes that are involved in cycling of a given nutrient. The observed transitory decrease in crop productivity during conversion from chemical intenswe input to alternatwe systems (greater reliance on biological resources) may be due to the initial diminished biological potentials of conventionally managed soils to efficiently cycle and mineralize organic nutrient sources. This review reaffirms the continuing need for the maintenance of existing long-term experimental sites and establishment of new studies in major agroecosystems throughout the world.
INTRODUCTION
There is increasing concern about the long-term productivity of soils as a resource base to provide food and fiber for the ever increasing world population. Because of population pressure in developing countries and economic considerations in developed countries, over the past 50 years agricultural systems have evolved which have less biodiversity and are more intensively managed (Harwood, 1990). Historically, much attention has focused on impacts of agricalture on soil erosion and depletion of organic matter. More Correspondence to: Richard P. Dick, 202 Strand Agricultural Hall, Department of Crop and Soil Science, O-egon State University, Corvallis, OR 97331, USA.
© 1992 Elsevier Science Publishers B.V. All rights resel~,ed 0167-8809/92/$05.00
26
R.P. DICK
recently additional attention has been focused on long-term impacts of agriculture on soil biology and biochemical parameters in soils (Doran et al., 1987; Dick et al., 1988). Biologically mediated processes in soils are central to the ecological function of soils. Soil biotic activity is the driving force in the degradation and conversion of exogenous plant material and anthropogenic depositions, transformations of organic matter, and evolution and maintenance of soil structure. Energy obtained by the primary decomposers of organic matter supports the activity of a number of trophic levels in soils. In turn this activity plays a primary function in nutrient cycling and support of plant life. To study biological processes in soils various parameters have been used. Because of the complex dynamics of soil ecosystems, no one parameter is satisfactory. Methods for the estimation of microbial growth are microbial counts (microscopy and via plate counts) and total microbial biomass (Jenkinson and Ladd, 1981 ). The microbial biomass is a small but labile source nf major nutrients (C, N, P, and S) and many transformations of these nutrients occur in the biomass. However these measurements give no indication of activity. Specific indices of activity include microcalorimetry (Spading, 1981 ), respiration rates (Nannipieri et al., 1979), measurements of ATP in soil extracts (Brookes et al., 1983), and enzyme activities (Frankenberger and Dick, 1983). Soil enzymes are the biological catalysts of innumerable reactions in soils. Although some enzymes (e.g. dehydrogenase) are only found in viable cells most soil enzymes can also exist as exoenzymes secreted by microorganisms or as enzymes originating from microbial debris and plant residue that are stabilized in complexes of clay minerals and humic colloids. Since it is difficult to extract enzymes from soils, enzymes are studied indirectly by measuring the activity via assays. Because assays are done in vitro under controlled conditions (temperature, buffers, excess substrate, etc.) it is difficult to relate activities to those occurring in situ. Nonetheless, studying soil enzyme activities provides insight into biochemical processes in soils and is sensitive as a biological index (Frankenberger and Dick, 1983 ). Long-term experimental sites provide opportunities to study the impacts of various agricultural systems on soil biology which is important in projecting the long-term productivity of soils. In addition these studies provide information on ecological processes that are important in the development of alternative agricultural systems that have increased dependency on biological interactions. The primary focus of this paper is ~!~ :ffect~ cf "oag-term agricultural systems on soil microbial and biochemical parameters. Although organisms from higher trophic levels of soil fauna are important in soil biology, they are not dealt with in this review. The major topics to be covered are tillage, crop rotation, and inorganic and
AGRICULTURAL SYSTEMS AND LONG-TERM PRODUCTIVITY OF SOILS
27
organic soil amendments. Pesticide effects on non-target soil microorganisms is potentially another area of concern. However, research to date suggests that the impacts of pesticides are minor and short lived (see review Somerville and Greaves, 1987). This was further confirmed in a more recent 2 year field study by Schuster and Schr~der (1990) who reported that simultaneous and sequential applications of eight pesticides and a growth regulator caused slight and short lived side-effects on the soil microflora. TILLAGE
Many studies on classic long-term sites have shown that long-term cultivation in the absence of organic amendments usually causes decreases in organic C and total N content (see Tare, 1987). Associated with this decline in organic matter is a decrease in soil air space, aggregation and water holding capacity. Concurrent with the change in aggregation is a distinct change in the distribution of organic matter among the soil particle fractions. Tiessen and Stewart ( 1983 ) showed that after only 4 years cultivation of grassland, 43% of the C loss was associated with the greater than 50 pm particles which they attributed to disruption of the macroaggregates. Compared with organic matter losses, less is known about the effects of cultivation on soil biology. Work by Gupta and Germida (1988) suggests that reduction in macroaggregates due to cultivation also affects soil biology and biochemical processes. They compared native grassland soil with an adjacent site where the soil had been under cultivation for 69 years. The effects of cultivation on the amounts of microbial biomass-C and -N was greater for macroaggregates than microaggregates (Table l ). Cultivation also decreased aryisulfatase and acid phosphatase, enzymes involved in S and P transformations (Fig. l ). Further microscopy observations in this same study showed that macroaggregates (greater than 1.00 pm) from native soil had extensive growth of fungal mycelium, whereas little fungal gro vth was detected in macroaggregates from the cultivated soil. The effects of cultivation on soil biology can be modified by the type of tillage management that is used. Conservation tillage practices that have various degrees of soil disturbance and that leave significant amounts of plant residt~e on the soil surface can affect biological properties of soils. No-tiUage systems result in an increase in the concentration of nutrients, organic matter and pesticides at the soil surface (Dick and Daniel, 1987). Soil urease, acid phosphatase, and protease activities in the 0-10 cm depth (Klein and Koths, 1980), and acid phosphatase, dehydrogenase (Doran, 1980), phosphatase, arylsulfatase, invertase, amidase, and urease (Dick, 1984) in the 0-7.5 cm depth, were significantly higher in soils with no-till than ploughed soils. However, in the study by Dick (1984) this was offset in the 22.5-30 cm depth
28
R.P. DICK
TABLE 1 Effect of 69 years of culuvation on nutrient ratios of different aggregate size fraction (adapted from
Gupta and Germida, 1988) Microbial biomass ratios
Size class (mm)
C/N
C/N/S
Native > !.00 0.50-1,00 0.25-0,50 0.10-0.25 <0.10
143:13:! 274:23:i 170:16:i 190:19:1 131:15:1
11:1 12:1 11:1 10:l 9:1
143:13:1 86:8:1 156:16:1 141:17:1 140:17:1
10:l 10:l 9.5:1 8:1 S:l
(~dtt~tt,d(69),ears) > 1.00 0.50-1.00 0.25-0.50 0.10-0.25 <0,10
100
I.LI~ U)"
80 []
so
2O 0
m
0 25-2,00 o 1.0 25 oos.o 1
LI
Native Cultivated Fig. I. Relative distribution of arylsulfatase activity in ! g soil of different aggregate size fractions from native and cultivated (69 years) soils (adapted from Gupta and Germida, 1988 ).
where the ploughed soil had equal, or in some eases higher, enzyme activity than the no-till soils. El-Harts et al. (1983) compared the effect of moldboard and no-till systems on N mineralization potential (No) in a wheat system in semi-arid region of the US Pacific Northwest. They found that whereas No in the 0-5 cm depth was greater in no-till than moldboard ploughed soil it was less in the 515 cm depth which resulted in no net effect of tillage on No in the 0-15 cm depth, Similar results shown in Table 2 were reported by Carter (1986) at Prince Edward island, Canada. Here again CO2 respiration and microbial
29
AGRICULTURAL SYSTEMS AND LONG-TERM PRODUCTIVITY OF SOILS
TABLE 2 Changes in microbial biomass C and N (kg h - ' ) under moldboard plowed and zero-tillage systems on Prince Edward Island, Canada (adapted from Carter, 1986) Soil depth (cm)
0-5 5-10 Total
Moldboard plow
Zero tillage
Activity~
C
N
Activity I
C
N
35 412 76
111
23
2472
462
682 20 88
1822 164 346
352 32 67
358
69
~CO2-C respired. 2 p > 0.05 between tillage systems for the same biomass parameter.
biomass levels were higher in :he surface soil (0-5 cm) but comparison of the totals for these biological properties in the O-10 cm depth were similar for moldboard and zero tillage. CROP ROTATION
The effect of crop rotation on disrupting disease and arthropod cycles is well established (Francis and Clegg, 1990). However the focus of this section will be on how crop rotation affects soil biodiversity and how this might impact disease interactions, nutrient cycling and crop yields. Studies have shown that crop rotations have significantly higher levels of microbial biomass (McGill et al., 1986) and soil enzyme activities (Khan, 1970; Dick, 1984) than cropping sequences that are either continuously monocultured or have more limited crop rotations. Addition of a green manure crop (Austrian winter pea, P i s u m arvense L. ) to wheat-based systems in the semi-arid region of the Pacific North-West over a 30 year period caused a significant increase in urease, phosphatase, and dehydrogenase activities, N flush (following chloroform fumigation) and in microbial biomass (Bolton et al., 1985). However this same study showed little significant differences due to green manure for microbial-plate and most-probable-number counts, indicating that microbial biomass (fumigation/incubation method) and soil enzyme activities can be more sensitive biological indices for discriminating between management systems. Martyniuk and Wagner (1978) compared microbial counts in treatments of continuous corn or wheat and crop rotation (corn-oat-wheat-red clover) which were sampled on a monthly basis at the Sanborn Field in Missouri, USA. These plots have been in place since 1888. In the absence of animal manure or inorganic fertilizer treatments bacterial counts were higher with crop rotation. This was attributed to the red clover crop in the crop rotation treatment which would provide some additional N over continuous wheat or
30
R.P. D I C K
corn. However, in the presence of N, P and K applications or animal manure treatments, bacterial counts were higher in the continuous crop treatments. In the case of fungi, counts were generally lower in the rotation plots regardless of manure or fertilizer treatments. Also crop rotation decreased the level of Fusarium, a genus commonly associated with several plant diseases. Although this study had some other confounding factors such as pH effects, crop rotation appears to have supported more biodiversity which suppressed Fusarium. Continuous monoculturing of a single crop species typically results in reduction of crop yields in comparison to the same species in rotation (Dick and Van Doren, 1985; Griffith et al, 1988) and these reductions usually are not associated with fertility or pest interactions. Although it has been suggested that alleopathic toxins derived from decomposing plant residues may inhibit yields, this has yet to be clearly established (Breakwell and Turco, 1990). There is growing evidence that the 'rotation effect' is due to the suppression of deleterious rhizobacteria that build up under continuous cropping. Work by Fredrickson and Eiliott ( 1985 ) and Suslow and Schr6th (1982) indicates that these organisms may not be directly pathogenic. Rather, it seems that there may be certain bacteria primarily Pseudomonas which are thought to lessen plant vigor, reduce root length and increase the susceptibility of plants to fungal pathogens. Use of a soil fumigant under field conditions provides further evidence for the suppression of deleterious microorganisms under crop rotation because it is a non-selective biocide that reduces the overall microbial population with16, I
1ii8 6
;:q
Q .J -W~,,
16
Fumioate d r=n Non-fum m
1987
Z nO
o
0
-
-
No- ,ll Contm.
No-tili Rotate
Plowed Contin.
Plowed Rotate
Fig. 2. Effect of fumigation, rotation and tillage on yields of corn for the years 1986 (upper) and 1987 (lower) (fumigation increased yield significantly for continuous corn systems at P<0.05 and P<0.1 for 1986 and 1987, respectively) (adapted from Turco et aL, 1990).
AGRICULTURAL SYSTEMSAND LONG-TERM PRODUCTIVITY OF SOILS
31
out altering toxins. Thus methyl bromide fumigation allows for differentiation between the effects of toxins and deleterious microorganisms. Fumigation of soils prior to planting for crops grown continuously under monoculture does simulate the rotation effect (O'Sullivan and Reyes, 1980; Turco et al., 1990) which is illustrated in Fig. 2 where fumigation maintains corn yields similar to those of corn in rotation (i.e. no significant difference (P<0.05) between fumigated continuous cropping and non-fumigated rotated soils). In this same study 130 bacterial isolates were tested in a bioassay for root growth on germinating corn roots. Approximately 22% of the bacterial isolates inhibited root growth and of these, 72% were isolated from soil under continuous cropping, suggesting that continuous cropping promotes deleterious rhizobacteria. INORGANIC AND ORGANIC AMENDMENTS
Long-term management of plant nutrients and organic amendments does affect soil biological properties. In general, management practices that increase inputs of organic residue, plant or animal manures, increase biological activity. Addition of farmyard manure (FYM) usually increases microbial biomass (Schniirer et al., 1985; McGill et al., 1986; Rasmussen et al., 1989) and soil enzyme activities (Khan, 1970; Verstraete and Voets, 1977; Dick et al., 1988) over soils that have not received any organic or inorganic amendments. Other indexes that increase with long-term FYM applications are N mineralization potential and soil respiration (Verstraete and Voets, 1977). Although Campbell et al. (1986) found no significant increases of microbial biomass C or CO2 respiration, due to FYM soil amendments, they did find higher rates of N mineralization potential in FYM soils. They attributed these results to the soil type which was relatively high in organic matter. Itowever, when comparisons have been made between soils amended with FYM or inorganic fertilizers, there have been mixed results which vary with cropping system and biological index. Usually there is a strong correlation between soil organic C content and soil microbial biomass (Biederbeck et al., 1984; Schniirer et al., 1985 ). Thus management practices that increase incorporation of organic residue typically increase biological activity. Use of inorganic fertilizer can increase the plant biomass production which in turn increases the amount of residue returned to the soil each year and stimulates biological activity. In addition Lynch and Panting (1980) reported that soil microbial biomass increased with root growth and rooting density of the crop which are typical responses of plants to recommended fertilization rates. This increased root growth would not only provide more root mass for decomposition but would also provide root exudates and a favorable environment for microbial activity during the growing season.
32
R.P. DICK
Exemplifying this effect is world"reported by Martyniuk and Wagner (1978) who found bacteria, actinomycetes, and fungal counts to be higher for N P K fertilizer treatment than the control. This was also shown in a study by ElHaris et aL (1983) who compared rates of N fertilization (0-2?0 kg N h a - ' applied for 6 years) on N mineralization parameters (lab incubation techniques) under various tillage methods and crop rotations. They reported that although there were no changes in total C, increasing N fertilization rates in general increased cumulative N mineralized, N mineralization potential, and rate of N mineralized. This could be attributed to greater inputs of plant biomass and possibly incorporation of fertilizer N into the labile N soil pool at high rates of N fertilization. Research on long-term plots (58 years) under a wheat-fallow system in the semi-arid region of Oregon, USA provides evidence that inorganic fertilizer does affect selected biochemical processes (Dick et al., 1988). This study compared the effects of applications of green manure, animal manure and varying rates of N fertilizer on six soil enzymes. Figure 3 has plots of enzyme activity versus net N inputs which show that the form of N added to soils is important for the enzymes involved in N cycling (urease and amidase). Organic amendments (manure and pea vine) stimulated activity but increasing rates (0-90 kg N h a - i ) of inorganic N decreased activity of these two enzymes. Since 1944, N fertilizer plots have routinely received NH~ which is the end-product for these two enzymes and apparently this has inhibited microbial synthesis of these enzymes. For the other enzymes tested which are not in the N cycle, these relationships were not noted. An example of this is shown in Fig. 3 for arylsulfatase (involved in S cycle) which had a non-significant relationship of activity with inorganic N inputs. This research indiArylsulfatase 6¢ r,0.0?"
&
4C 3C
110( •
I
A 60(: Urease Y, 160-3.94X r ,0.00"
A]
I
400 II
ILl ~_~ 20 l i e e e
30C
m
9o(
..:. ,o" 0
30( Amldue ym747"7.34X r ,o.so'"
7o( :
Ngo
1 0 0 ~ _
rlgo I
;6 4
-,e "8 o' 8' ; , 2 .
.8-8
o 8 ;o2.
N E T N I T R O G E N INPUT (kg h a " y e a r " )
Fig, 3, Regressionanalysisof enzymeactivity and net N input excluding manure ( & ) and peavine residue ([]) treatments (where arylsulfataseactivity as #g ofp-nitrophenol releasedg- ~soil 2 h - s; urease activity expressed as/~gNH~"-N5 g- t soil 4 h- t; amidase activity expressed as/~g ~+-N3 g- t soil 4 h- ~) (adapted from Dick et al.. ! 988).
AGRICULTURAL SYSTEMS AND LONG-TERM PRODUCTIVITY OF SOILS
33
cates that management practices that minimize organic inputs diminish the potential for enzymatic activity, which is likely to affect the ability of the soil to cycle and provide nutrients for plant growth. In addition, inorganic fertilizer amendments can selectively affect certain biochemical processes but not others. PERSPECTIVES
Cultivation of soils, besides affecting soil chemistry and structure, also affects soil biology. Tillage reduces biological activity and there is evidence that this is due to the reduction of macroaggregates with long-term cultivation practices. Macroaggregates provide an important microhabitat for microbial activity. Conservation tillage practices that keep residue on the surface can maintain biological activity in the surface soil but subsurface activity may be equal or lower in these systems compared with tilled soils. Indirect evidence suggests that soil amendments such as animal and green manures, and plant diversity (crop rotations) may be more important in maintaining soil microbial activity/diversity than conservation tillage in monocultural systems. There is increasing evidence that crop rotation affects crov productivity via suppressing deleterious microorganisms that flourish under monoculture. This also has implications for suppressing root disease organisms where practices that promote soil biodiversity may inhibit certain disease organisms. Additions of inorganic fertilizers can increase soil biological activity because of increased plant biomass production which upon incorporation stimulates soil biological activity. Conversely, limited evidence suggests that repeated applications of inorganic fertilizer nutrients can suppress production of certain soil enzymes that are involved in cycling of a given nutrient (e.g. amidase in N cycle). This has implications for cnnversion from conventional systems to alternative systems that have greater reliance on biological interactions. Typically there is a drop in crop productivity during conversion from chemical intensive input systems to alternative systems that have crop rotations, cover crops, and animal manure inputs (Culik, 1983). Evidence from long-term sites suggests that this is because these soils (those that have received inorganic nutrients and limited organic inputs) have reduced biological potential to efficiently cycle and mineralize organic sources of nutrients. This has resulted in diminished N availability during transition to alternative cropping systems (Doran et al., 1987 ). The work reviewed is largely from comparative, site-specific research. These studies have been useful in assessing the long-term effects in terms of how agricultural practices change the soil biology and to some extent underlying mechanisms on a location-by-location basis. There is interest in developing a universal 'soil quality index' that could be used to assess the 'health' of a given soil. As shown by this review, soil biological indices can be sensitive indica-
34
R.P. DICK
tors to management practices and measurable differences can occur much sooner than most chemical indices such as total C, N, etc. However, because soil biological parameters naturally vary widely among soil types it is necessary to have a reference point in time or by a companion soil treatment (i.e. a control to make biological indices meaningful). Therefore at this time it is not possible to simply measure a series of soil biological parameters independent of a comparative control or treatment at a given site to determine the 'soil health'. This reaffirms the continuing need for the maintenance of existing long-term experimental sites and establishment of new studies in major agroecosystems throughout the world.
REFERENCES
Biederbeck, V.O., Campbell, C.A. and Zentner, R.P., 1984. Effect of crop rotation and fertilization on some biological properties of a loam in southwestern Saskatchewan. Can. J. Soil Sci., 64: 355-367. Bolton, H., Jr., Elliot, L.F., Papendick, R.I. and Bezdicek, D.F., 1985. Soil microbial biomass and selected soil enzyme activities: Effect of fertilization and cropping practices. Soil Biol. Biochem., i 7: 297-302. Breakweil, D.P. and Tureo, R.F, 1990. Nutrient and phytotoxic contributions of residues to soil in no-till continuous corn ecosystems. Biol. Fertil., 8: 328-334. Brookes, P.C., Tare, K.R. and Jenkinson, D.S., 1983. The adenylate energy charge of the soil microbial biomass. Soil Biol. Biochem, 15: 9-16. Campbell, C.A, Schnitzer, M., Stewart, J.W.B., Biederbeck, V.O. and Selles, F., 1986. Effect of manure and P fertilizer on properties of a black Chernozem in southern Saskatchewan. Can. J. Soil Sci., 66: 601-613. Carter, M.R., 1986. Microbial biomass as an index for tillage-induced changes in soil biological properties. Soil Tillage Res., 7: 29-40. Culik, M.N., 1983. The conversion experiment: reducing farming costs. J. Soil Water Conserv., 38: 333-335. Dick, R.P., Rasmussen, P.E. and Kerle, E.A., 1988. Influence of long-term residue management on soil enzyme activities in relation to soil chemical properties of a wheat.fallow system. Biol. Fertil. Soils, 6: 159-164. Dick, W.A., 1984. Influence ofiong-term tillage and crop rotation combinations on soil enzyme activities. Soil Sci. Soc. Am. J., 48: 569-574. Dick, W.A. and Daniel, T.C., 1987. Soil chemical and biological properties as affected by conservation tillage: Environmental impacts. In: T.J. Logan et al. (Editors), Effects of Conservation Tillage on Groundwater Quality: Nitrates and Pesticides. Lewis Publishers, Inc., Chelsen, MI, USA. Dick, W.A. and Van Doren, Jr., D.M., 1985. Continuous tillage and rotation combinations effects on cor~, soybean, and oat yields. Agron., J., 77: 459-465. Doran, J.W., 1980. Soil microbial and biochemical changes associated with reduced tillage. Soil Sci. Soc. Am. J., 44:765-77 I. Doran, J.W., Fraser, D.G., Culik, M.N. and Liebhardt, W.C., 1987. Influence ofalternative and conventional agricultural management on soil microbial processes and nitrogen availability. Am. J. Alternative Agric., 2: 99-106. EI-Haris, M.K., Cochran, V.L., Elliot, L.F. and Bezdicek, D.F., 1983. Effect of tillage, cropping,
AGRICULTURAL S" "STEMS AND LONG-TERM PRODUCTIVITY OF SOILS
35
and fertilizer management on soil nitrogen mineralization potential. Soil Sci. Soc. Am. J., 47:1157-1161. Francis, C.A. and Ciegg, M.D., 1990. Crop rotations in sustainable agricultural systems. In: C.A. Edwards et ai. (Editors), Sustainable Agriculture Systems. Soil and Water Conserv., Soc., Ankeny, IA, USA, pp. 107-122. Frankenberger, W.T. Jr. and Dick, W.A., 1983. Relationships between enzyme activities and microbial growth and activity indices in soil. Soil Sci. Soc. Am. J., 47:945-951. Fredrickson, J.K. and Elliott, L.F., 1985. Effects on winter wheat seedling growth by toxinproducing rhizobacteria. Plant Soil, 83: 399-409. Griffith, D.R., Kladivko, E.J., Mannering, J.V., West, T.D. and Parsons, S.D., 1988. Long-term tillage and rotation effects on corn growth and yield on high and low organic matter, poorly drained soil. Agron. J., 80: 599--605. Gupta, V.V.S.R. and Germida, J.J., 1988. Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation. Soil Biol. Biochem., 20: 777786. Harwood, R.R., 1990. A history of sustainable agriculture. In: C.A. Edwards et al. (Editors), Sustainable Agriculture Systems. Soil and Water Conserv., Soc., Ankeny, IA, USA, pp. 3-19. Jenkinson, D.S. and Ladd, J.N., 1981. Microbial biomass in soil: measurement and turnover. In: E.A. Paul and J.N. Ladd (Editors), Soil Biochemistry. Vol. 5. Marcel Decker, New York, pp. 415-471. Khan, S.U., 1970. Enzymatic activity in a gray wooded soil as influenced by cropping systems and fertilizers. Soil Biol. Biochem., 2:137-139. Klein, T.M. and Koths, J.S., 1980. Urease, protease, and phosphatase in soil continuously cropped to corn by conventional or no-tillage methods. Soil Biol. Biochem., 12: 293-294. Lynch, J.M. and Panting, L.M., 1980. Cultivation and the soil biomass. Soil Biol. Bioehem., 12: 29-33. Martyniuk, S. and Wagner, G.H., 1978. Quantitative and qualitative examination of soil microflora associated with different management systems. Soil Sci., 125: 343-350. McGill, W.B., Cannon, K.R., Robertson, J.A. and Cook, ED., 1986. Dynamics of soil microbial biomass and water-soluble organic C in Breton L after 50 years of cropping to two rotations. Can. J. Soil Sci., 66: 1-19. Nannipieri, P., Pedrazzini, F., Arcara, P.G. and Piovanelli, C., 1979. Changes in amino acids, enzyme activities, and biomasses during soil microbial growth. Soil Sci., 127: 26-34. O'Sullivan, J. and Reyes, A.A., 1980. Effects of soil fumigation, rotation, and nitrogen on yield, petiole NO3-N and verticilium wilt of potatoes. Am. Soc. Holt' . Sci., 105: 809-812. Rasmussen, P.E., Collins, H.P. and Smiley, R.W., 1989. Long-t¢ management effects on soil productivity and crop yield in semi.arid regions ofeastern Oregon. Star. Bull. 675. Columbia Basin Ag. Res. Ctr., USDA-ARS and Oregon State University, Pendleton, OR. Schniirer, J., Clarholm, M. and Rosswall, T., 1985. Microbial biomass and activity in an agricultural soil with different organic matter contents. Soil Biol. Biochem., 17:611-618. Schuster, E. and Schr~der, D., 1990. Side effects of sequentially-applied pesticides on non-target soil microorganisms: Field experiments. Soil Biol. Biochem., 22: 367-373. L. Somerville and M.P. Greaves (Editors), 1987. Pesticide Effects on Soil Microflora. Taylor and Francis, London/New York/Philadelphia, 240 pp. Spading, G.P., 1981. Microcalorimetery and other methods to assess biomass and activity in soil. Soil Biol. Biochem., 13: 93-98. Suslow, T.V. and Schr6th, M.N., 1982. Role of deleterious rhizobacteria as minor pathogens in reducing crop growth. Phytopathology, 72:111-115. Tare, R.L., Jr. (Editor), 1987. Soil Organic Matter Biological and Ecological Effects. John Wiley, New York, pp. 37-42. Tiessen, H. and Stewart, J.W.B., 1983. Particle-size fractions and their use in studies of soil
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organic matter. II Cultivation effects on organic matter composition in size fractions. Soil Sci. Soc. Am. J., 47:509-5 ! 4. Turco, R.F., Bischoff, M., Breakwell, D.P. and Griffith, D.R., 1990. Contribution of soil-borne bacteria to the rotation effect in com. Plant Soil, 122: 115-120. Verstraete, W. and Voets, J.P,, 1977. Soil microbial and biochemical characteristics in relation to soil management and fertility. Soil Biol. Biochem., 9: 253-258.
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Elsevier Science Publishers B.V., Amsterdam
A review: long-term effects of agricultural systems on soil biochemical and microbial parameters Richard P. Dick 202 Strand Agrwultural Hall, Department of Crop and Soil Science, OregonState Umversity, Corvallis, OR 97331, USA
ABSTRACT Dick, R.P., 1992. A review: long-term effects of agricultural systems on soil biochemical and microbial parameters. Agric. Ecosystems Environ., 40: 25-36. This paper pro~;des a review of recent developments on assessing the effect of agricultural systems on long-term productivity of soils. Cultivation of soils, besides affecting so;I chemistry and structure, reduces biological activity due to the reduction of macroaggregates which provides an important microhabitat for microbial activity. Indirect evidence suggests that soil amendments such as animal and green manures, and plant diversity (crop rotations) may be more important in maintaining soil microbial activity/diversity than conservation tillage in monocultural systems. There is increasing evidence that crop rotation promotes crop productivity by suppressing deleterious microorganisms that flourish under monoculture. Additions of inorganic fertilizers can increase soil biological activit,, because of an increased plant biomass production which upon incorporation stimulates soil biGlogical activity. Conversely, limited evidence suggests that repeated applications of inorganic fertilizer nutrients can suppress production of certain soil enzymes that are involved in cycling of a given nutrient. The observed transitory decrease in crop productivity during conversion from chemical intenswe input to alternatwe systems (greater reliance on biological resources) may be due to the initial diminished biological potentials of conventionally managed soils to efficiently cycle and mineralize organic nutrient sources. This review reaffirms the continuing need for the maintenance of existing long-term experimental sites and establishment of new studies in major agroecosystems throughout the world.
INTRODUCTION
There is increasing concern about the long-term productivity of soils as a resource base to provide food and fiber for the ever increasing world population. Because of population pressure in developing countries and economic considerations in developed countries, over the past 50 years agricultural systems have evolved which have less biodiversity and are more intensively managed (Harwood, 1990). Historically, much attention has focused on impacts of agricalture on soil erosion and depletion of organic matter. More Correspondence to: Richard P. Dick, 202 Strand Agricultural Hall, Department of Crop and Soil Science, O-egon State University, Corvallis, OR 97331, USA.
© 1992 Elsevier Science Publishers B.V. All rights resel~,ed 0167-8809/92/$05.00
26
R.P. DICK
recently additional attention has been focused on long-term impacts of agriculture on soil biology and biochemical parameters in soils (Doran et al., 1987; Dick et al., 1988). Biologically mediated processes in soils are central to the ecological function of soils. Soil biotic activity is the driving force in the degradation and conversion of exogenous plant material and anthropogenic depositions, transformations of organic matter, and evolution and maintenance of soil structure. Energy obtained by the primary decomposers of organic matter supports the activity of a number of trophic levels in soils. In turn this activity plays a primary function in nutrient cycling and support of plant life. To study biological processes in soils various parameters have been used. Because of the complex dynamics of soil ecosystems, no one parameter is satisfactory. Methods for the estimation of microbial growth are microbial counts (microscopy and via plate counts) and total microbial biomass (Jenkinson and Ladd, 1981 ). The microbial biomass is a small but labile source nf major nutrients (C, N, P, and S) and many transformations of these nutrients occur in the biomass. However these measurements give no indication of activity. Specific indices of activity include microcalorimetry (Spading, 1981 ), respiration rates (Nannipieri et al., 1979), measurements of ATP in soil extracts (Brookes et al., 1983), and enzyme activities (Frankenberger and Dick, 1983). Soil enzymes are the biological catalysts of innumerable reactions in soils. Although some enzymes (e.g. dehydrogenase) are only found in viable cells most soil enzymes can also exist as exoenzymes secreted by microorganisms or as enzymes originating from microbial debris and plant residue that are stabilized in complexes of clay minerals and humic colloids. Since it is difficult to extract enzymes from soils, enzymes are studied indirectly by measuring the activity via assays. Because assays are done in vitro under controlled conditions (temperature, buffers, excess substrate, etc.) it is difficult to relate activities to those occurring in situ. Nonetheless, studying soil enzyme activities provides insight into biochemical processes in soils and is sensitive as a biological index (Frankenberger and Dick, 1983 ). Long-term experimental sites provide opportunities to study the impacts of various agricultural systems on soil biology which is important in projecting the long-term productivity of soils. In addition these studies provide information on ecological processes that are important in the development of alternative agricultural systems that have increased dependency on biological interactions. The primary focus of this paper is ~!~ :ffect~ cf "oag-term agricultural systems on soil microbial and biochemical parameters. Although organisms from higher trophic levels of soil fauna are important in soil biology, they are not dealt with in this review. The major topics to be covered are tillage, crop rotation, and inorganic and
AGRICULTURAL SYSTEMS AND LONG-TERM PRODUCTIVITY OF SOILS
27
organic soil amendments. Pesticide effects on non-target soil microorganisms is potentially another area of concern. However, research to date suggests that the impacts of pesticides are minor and short lived (see review Somerville and Greaves, 1987). This was further confirmed in a more recent 2 year field study by Schuster and Schr~der (1990) who reported that simultaneous and sequential applications of eight pesticides and a growth regulator caused slight and short lived side-effects on the soil microflora. TILLAGE
Many studies on classic long-term sites have shown that long-term cultivation in the absence of organic amendments usually causes decreases in organic C and total N content (see Tare, 1987). Associated with this decline in organic matter is a decrease in soil air space, aggregation and water holding capacity. Concurrent with the change in aggregation is a distinct change in the distribution of organic matter among the soil particle fractions. Tiessen and Stewart ( 1983 ) showed that after only 4 years cultivation of grassland, 43% of the C loss was associated with the greater than 50 pm particles which they attributed to disruption of the macroaggregates. Compared with organic matter losses, less is known about the effects of cultivation on soil biology. Work by Gupta and Germida (1988) suggests that reduction in macroaggregates due to cultivation also affects soil biology and biochemical processes. They compared native grassland soil with an adjacent site where the soil had been under cultivation for 69 years. The effects of cultivation on the amounts of microbial biomass-C and -N was greater for macroaggregates than microaggregates (Table l ). Cultivation also decreased aryisulfatase and acid phosphatase, enzymes involved in S and P transformations (Fig. l ). Further microscopy observations in this same study showed that macroaggregates (greater than 1.00 pm) from native soil had extensive growth of fungal mycelium, whereas little fungal gro vth was detected in macroaggregates from the cultivated soil. The effects of cultivation on soil biology can be modified by the type of tillage management that is used. Conservation tillage practices that have various degrees of soil disturbance and that leave significant amounts of plant residt~e on the soil surface can affect biological properties of soils. No-tiUage systems result in an increase in the concentration of nutrients, organic matter and pesticides at the soil surface (Dick and Daniel, 1987). Soil urease, acid phosphatase, and protease activities in the 0-10 cm depth (Klein and Koths, 1980), and acid phosphatase, dehydrogenase (Doran, 1980), phosphatase, arylsulfatase, invertase, amidase, and urease (Dick, 1984) in the 0-7.5 cm depth, were significantly higher in soils with no-till than ploughed soils. However, in the study by Dick (1984) this was offset in the 22.5-30 cm depth
28
R.P. DICK
TABLE 1 Effect of 69 years of culuvation on nutrient ratios of different aggregate size fraction (adapted from
Gupta and Germida, 1988) Microbial biomass ratios
Size class (mm)
C/N
C/N/S
Native > !.00 0.50-1,00 0.25-0,50 0.10-0.25 <0.10
143:13:! 274:23:i 170:16:i 190:19:1 131:15:1
11:1 12:1 11:1 10:l 9:1
143:13:1 86:8:1 156:16:1 141:17:1 140:17:1
10:l 10:l 9.5:1 8:1 S:l
(~dtt~tt,d(69),ears) > 1.00 0.50-1.00 0.25-0.50 0.10-0.25 <0,10
100
I.LI~ U)"
80 []
so
2O 0
m
0 25-2,00 o 1.0 25 oos.o 1
LI
Native Cultivated Fig. I. Relative distribution of arylsulfatase activity in ! g soil of different aggregate size fractions from native and cultivated (69 years) soils (adapted from Gupta and Germida, 1988 ).
where the ploughed soil had equal, or in some eases higher, enzyme activity than the no-till soils. El-Harts et al. (1983) compared the effect of moldboard and no-till systems on N mineralization potential (No) in a wheat system in semi-arid region of the US Pacific Northwest. They found that whereas No in the 0-5 cm depth was greater in no-till than moldboard ploughed soil it was less in the 515 cm depth which resulted in no net effect of tillage on No in the 0-15 cm depth, Similar results shown in Table 2 were reported by Carter (1986) at Prince Edward island, Canada. Here again CO2 respiration and microbial
29
AGRICULTURAL SYSTEMS AND LONG-TERM PRODUCTIVITY OF SOILS
TABLE 2 Changes in microbial biomass C and N (kg h - ' ) under moldboard plowed and zero-tillage systems on Prince Edward Island, Canada (adapted from Carter, 1986) Soil depth (cm)
0-5 5-10 Total
Moldboard plow
Zero tillage
Activity~
C
N
Activity I
C
N
35 412 76
111
23
2472
462
682 20 88
1822 164 346
352 32 67
358
69
~CO2-C respired. 2 p > 0.05 between tillage systems for the same biomass parameter.
biomass levels were higher in :he surface soil (0-5 cm) but comparison of the totals for these biological properties in the O-10 cm depth were similar for moldboard and zero tillage. CROP ROTATION
The effect of crop rotation on disrupting disease and arthropod cycles is well established (Francis and Clegg, 1990). However the focus of this section will be on how crop rotation affects soil biodiversity and how this might impact disease interactions, nutrient cycling and crop yields. Studies have shown that crop rotations have significantly higher levels of microbial biomass (McGill et al., 1986) and soil enzyme activities (Khan, 1970; Dick, 1984) than cropping sequences that are either continuously monocultured or have more limited crop rotations. Addition of a green manure crop (Austrian winter pea, P i s u m arvense L. ) to wheat-based systems in the semi-arid region of the Pacific North-West over a 30 year period caused a significant increase in urease, phosphatase, and dehydrogenase activities, N flush (following chloroform fumigation) and in microbial biomass (Bolton et al., 1985). However this same study showed little significant differences due to green manure for microbial-plate and most-probable-number counts, indicating that microbial biomass (fumigation/incubation method) and soil enzyme activities can be more sensitive biological indices for discriminating between management systems. Martyniuk and Wagner (1978) compared microbial counts in treatments of continuous corn or wheat and crop rotation (corn-oat-wheat-red clover) which were sampled on a monthly basis at the Sanborn Field in Missouri, USA. These plots have been in place since 1888. In the absence of animal manure or inorganic fertilizer treatments bacterial counts were higher with crop rotation. This was attributed to the red clover crop in the crop rotation treatment which would provide some additional N over continuous wheat or
30
R.P. D I C K
corn. However, in the presence of N, P and K applications or animal manure treatments, bacterial counts were higher in the continuous crop treatments. In the case of fungi, counts were generally lower in the rotation plots regardless of manure or fertilizer treatments. Also crop rotation decreased the level of Fusarium, a genus commonly associated with several plant diseases. Although this study had some other confounding factors such as pH effects, crop rotation appears to have supported more biodiversity which suppressed Fusarium. Continuous monoculturing of a single crop species typically results in reduction of crop yields in comparison to the same species in rotation (Dick and Van Doren, 1985; Griffith et al, 1988) and these reductions usually are not associated with fertility or pest interactions. Although it has been suggested that alleopathic toxins derived from decomposing plant residues may inhibit yields, this has yet to be clearly established (Breakwell and Turco, 1990). There is growing evidence that the 'rotation effect' is due to the suppression of deleterious rhizobacteria that build up under continuous cropping. Work by Fredrickson and Eiliott ( 1985 ) and Suslow and Schr6th (1982) indicates that these organisms may not be directly pathogenic. Rather, it seems that there may be certain bacteria primarily Pseudomonas which are thought to lessen plant vigor, reduce root length and increase the susceptibility of plants to fungal pathogens. Use of a soil fumigant under field conditions provides further evidence for the suppression of deleterious microorganisms under crop rotation because it is a non-selective biocide that reduces the overall microbial population with16, I
1ii8 6
;:q
Q .J -W~,,
16
Fumioate d r=n Non-fum m
1987
Z nO
o
0
-
-
No- ,ll Contm.
No-tili Rotate
Plowed Contin.
Plowed Rotate
Fig. 2. Effect of fumigation, rotation and tillage on yields of corn for the years 1986 (upper) and 1987 (lower) (fumigation increased yield significantly for continuous corn systems at P<0.05 and P<0.1 for 1986 and 1987, respectively) (adapted from Turco et aL, 1990).
AGRICULTURAL SYSTEMSAND LONG-TERM PRODUCTIVITY OF SOILS
31
out altering toxins. Thus methyl bromide fumigation allows for differentiation between the effects of toxins and deleterious microorganisms. Fumigation of soils prior to planting for crops grown continuously under monoculture does simulate the rotation effect (O'Sullivan and Reyes, 1980; Turco et al., 1990) which is illustrated in Fig. 2 where fumigation maintains corn yields similar to those of corn in rotation (i.e. no significant difference (P<0.05) between fumigated continuous cropping and non-fumigated rotated soils). In this same study 130 bacterial isolates were tested in a bioassay for root growth on germinating corn roots. Approximately 22% of the bacterial isolates inhibited root growth and of these, 72% were isolated from soil under continuous cropping, suggesting that continuous cropping promotes deleterious rhizobacteria. INORGANIC AND ORGANIC AMENDMENTS
Long-term management of plant nutrients and organic amendments does affect soil biological properties. In general, management practices that increase inputs of organic residue, plant or animal manures, increase biological activity. Addition of farmyard manure (FYM) usually increases microbial biomass (Schniirer et al., 1985; McGill et al., 1986; Rasmussen et al., 1989) and soil enzyme activities (Khan, 1970; Verstraete and Voets, 1977; Dick et al., 1988) over soils that have not received any organic or inorganic amendments. Other indexes that increase with long-term FYM applications are N mineralization potential and soil respiration (Verstraete and Voets, 1977). Although Campbell et al. (1986) found no significant increases of microbial biomass C or CO2 respiration, due to FYM soil amendments, they did find higher rates of N mineralization potential in FYM soils. They attributed these results to the soil type which was relatively high in organic matter. Itowever, when comparisons have been made between soils amended with FYM or inorganic fertilizers, there have been mixed results which vary with cropping system and biological index. Usually there is a strong correlation between soil organic C content and soil microbial biomass (Biederbeck et al., 1984; Schniirer et al., 1985 ). Thus management practices that increase incorporation of organic residue typically increase biological activity. Use of inorganic fertilizer can increase the plant biomass production which in turn increases the amount of residue returned to the soil each year and stimulates biological activity. In addition Lynch and Panting (1980) reported that soil microbial biomass increased with root growth and rooting density of the crop which are typical responses of plants to recommended fertilization rates. This increased root growth would not only provide more root mass for decomposition but would also provide root exudates and a favorable environment for microbial activity during the growing season.
32
R.P. DICK
Exemplifying this effect is world"reported by Martyniuk and Wagner (1978) who found bacteria, actinomycetes, and fungal counts to be higher for N P K fertilizer treatment than the control. This was also shown in a study by ElHaris et aL (1983) who compared rates of N fertilization (0-2?0 kg N h a - ' applied for 6 years) on N mineralization parameters (lab incubation techniques) under various tillage methods and crop rotations. They reported that although there were no changes in total C, increasing N fertilization rates in general increased cumulative N mineralized, N mineralization potential, and rate of N mineralized. This could be attributed to greater inputs of plant biomass and possibly incorporation of fertilizer N into the labile N soil pool at high rates of N fertilization. Research on long-term plots (58 years) under a wheat-fallow system in the semi-arid region of Oregon, USA provides evidence that inorganic fertilizer does affect selected biochemical processes (Dick et al., 1988). This study compared the effects of applications of green manure, animal manure and varying rates of N fertilizer on six soil enzymes. Figure 3 has plots of enzyme activity versus net N inputs which show that the form of N added to soils is important for the enzymes involved in N cycling (urease and amidase). Organic amendments (manure and pea vine) stimulated activity but increasing rates (0-90 kg N h a - i ) of inorganic N decreased activity of these two enzymes. Since 1944, N fertilizer plots have routinely received NH~ which is the end-product for these two enzymes and apparently this has inhibited microbial synthesis of these enzymes. For the other enzymes tested which are not in the N cycle, these relationships were not noted. An example of this is shown in Fig. 3 for arylsulfatase (involved in S cycle) which had a non-significant relationship of activity with inorganic N inputs. This research indiArylsulfatase 6¢ r,0.0?"
&
4C 3C
110( •
I
A 60(: Urease Y, 160-3.94X r ,0.00"
A]
I
400 II
ILl ~_~ 20 l i e e e
30C
m
9o(
..:. ,o" 0
30( Amldue ym747"7.34X r ,o.so'"
7o( :
Ngo
1 0 0 ~ _
rlgo I
;6 4
-,e "8 o' 8' ; , 2 .
.8-8
o 8 ;o2.
N E T N I T R O G E N INPUT (kg h a " y e a r " )
Fig, 3, Regressionanalysisof enzymeactivity and net N input excluding manure ( & ) and peavine residue ([]) treatments (where arylsulfataseactivity as #g ofp-nitrophenol releasedg- ~soil 2 h - s; urease activity expressed as/~gNH~"-N5 g- t soil 4 h- t; amidase activity expressed as/~g ~+-N3 g- t soil 4 h- ~) (adapted from Dick et al.. ! 988).
AGRICULTURAL SYSTEMS AND LONG-TERM PRODUCTIVITY OF SOILS
33
cates that management practices that minimize organic inputs diminish the potential for enzymatic activity, which is likely to affect the ability of the soil to cycle and provide nutrients for plant growth. In addition, inorganic fertilizer amendments can selectively affect certain biochemical processes but not others. PERSPECTIVES
Cultivation of soils, besides affecting soil chemistry and structure, also affects soil biology. Tillage reduces biological activity and there is evidence that this is due to the reduction of macroaggregates with long-term cultivation practices. Macroaggregates provide an important microhabitat for microbial activity. Conservation tillage practices that keep residue on the surface can maintain biological activity in the surface soil but subsurface activity may be equal or lower in these systems compared with tilled soils. Indirect evidence suggests that soil amendments such as animal and green manures, and plant diversity (crop rotations) may be more important in maintaining soil microbial activity/diversity than conservation tillage in monocultural systems. There is increasing evidence that crop rotation affects crov productivity via suppressing deleterious microorganisms that flourish under monoculture. This also has implications for suppressing root disease organisms where practices that promote soil biodiversity may inhibit certain disease organisms. Additions of inorganic fertilizers can increase soil biological activity because of increased plant biomass production which upon incorporation stimulates soil biological activity. Conversely, limited evidence suggests that repeated applications of inorganic fertilizer nutrients can suppress production of certain soil enzymes that are involved in cycling of a given nutrient (e.g. amidase in N cycle). This has implications for cnnversion from conventional systems to alternative systems that have greater reliance on biological interactions. Typically there is a drop in crop productivity during conversion from chemical intensive input systems to alternative systems that have crop rotations, cover crops, and animal manure inputs (Culik, 1983). Evidence from long-term sites suggests that this is because these soils (those that have received inorganic nutrients and limited organic inputs) have reduced biological potential to efficiently cycle and mineralize organic sources of nutrients. This has resulted in diminished N availability during transition to alternative cropping systems (Doran et al., 1987 ). The work reviewed is largely from comparative, site-specific research. These studies have been useful in assessing the long-term effects in terms of how agricultural practices change the soil biology and to some extent underlying mechanisms on a location-by-location basis. There is interest in developing a universal 'soil quality index' that could be used to assess the 'health' of a given soil. As shown by this review, soil biological indices can be sensitive indica-
34
R.P. DICK
tors to management practices and measurable differences can occur much sooner than most chemical indices such as total C, N, etc. However, because soil biological parameters naturally vary widely among soil types it is necessary to have a reference point in time or by a companion soil treatment (i.e. a control to make biological indices meaningful). Therefore at this time it is not possible to simply measure a series of soil biological parameters independent of a comparative control or treatment at a given site to determine the 'soil health'. This reaffirms the continuing need for the maintenance of existing long-term experimental sites and establishment of new studies in major agroecosystems throughout the world.
REFERENCES
Biederbeck, V.O., Campbell, C.A. and Zentner, R.P., 1984. Effect of crop rotation and fertilization on some biological properties of a loam in southwestern Saskatchewan. Can. J. Soil Sci., 64: 355-367. Bolton, H., Jr., Elliot, L.F., Papendick, R.I. and Bezdicek, D.F., 1985. Soil microbial biomass and selected soil enzyme activities: Effect of fertilization and cropping practices. Soil Biol. Biochem., i 7: 297-302. Breakweil, D.P. and Tureo, R.F, 1990. Nutrient and phytotoxic contributions of residues to soil in no-till continuous corn ecosystems. Biol. Fertil., 8: 328-334. Brookes, P.C., Tare, K.R. and Jenkinson, D.S., 1983. The adenylate energy charge of the soil microbial biomass. Soil Biol. Biochem, 15: 9-16. Campbell, C.A, Schnitzer, M., Stewart, J.W.B., Biederbeck, V.O. and Selles, F., 1986. Effect of manure and P fertilizer on properties of a black Chernozem in southern Saskatchewan. Can. J. Soil Sci., 66: 601-613. Carter, M.R., 1986. Microbial biomass as an index for tillage-induced changes in soil biological properties. Soil Tillage Res., 7: 29-40. Culik, M.N., 1983. The conversion experiment: reducing farming costs. J. Soil Water Conserv., 38: 333-335. Dick, R.P., Rasmussen, P.E. and Kerle, E.A., 1988. Influence of long-term residue management on soil enzyme activities in relation to soil chemical properties of a wheat.fallow system. Biol. Fertil. Soils, 6: 159-164. Dick, W.A., 1984. Influence ofiong-term tillage and crop rotation combinations on soil enzyme activities. Soil Sci. Soc. Am. J., 48: 569-574. Dick, W.A. and Daniel, T.C., 1987. Soil chemical and biological properties as affected by conservation tillage: Environmental impacts. In: T.J. Logan et al. (Editors), Effects of Conservation Tillage on Groundwater Quality: Nitrates and Pesticides. Lewis Publishers, Inc., Chelsen, MI, USA. Dick, W.A. and Van Doren, Jr., D.M., 1985. Continuous tillage and rotation combinations effects on cor~, soybean, and oat yields. Agron., J., 77: 459-465. Doran, J.W., 1980. Soil microbial and biochemical changes associated with reduced tillage. Soil Sci. Soc. Am. J., 44:765-77 I. Doran, J.W., Fraser, D.G., Culik, M.N. and Liebhardt, W.C., 1987. Influence ofalternative and conventional agricultural management on soil microbial processes and nitrogen availability. Am. J. Alternative Agric., 2: 99-106. EI-Haris, M.K., Cochran, V.L., Elliot, L.F. and Bezdicek, D.F., 1983. Effect of tillage, cropping,
AGRICULTURAL S" "STEMS AND LONG-TERM PRODUCTIVITY OF SOILS
35
and fertilizer management on soil nitrogen mineralization potential. Soil Sci. Soc. Am. J., 47:1157-1161. Francis, C.A. and Ciegg, M.D., 1990. Crop rotations in sustainable agricultural systems. In: C.A. Edwards et ai. (Editors), Sustainable Agriculture Systems. Soil and Water Conserv., Soc., Ankeny, IA, USA, pp. 107-122. Frankenberger, W.T. Jr. and Dick, W.A., 1983. Relationships between enzyme activities and microbial growth and activity indices in soil. Soil Sci. Soc. Am. J., 47:945-951. Fredrickson, J.K. and Elliott, L.F., 1985. Effects on winter wheat seedling growth by toxinproducing rhizobacteria. Plant Soil, 83: 399-409. Griffith, D.R., Kladivko, E.J., Mannering, J.V., West, T.D. and Parsons, S.D., 1988. Long-term tillage and rotation effects on corn growth and yield on high and low organic matter, poorly drained soil. Agron. J., 80: 599--605. Gupta, V.V.S.R. and Germida, J.J., 1988. Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation. Soil Biol. Biochem., 20: 777786. Harwood, R.R., 1990. A history of sustainable agriculture. In: C.A. Edwards et al. (Editors), Sustainable Agriculture Systems. Soil and Water Conserv., Soc., Ankeny, IA, USA, pp. 3-19. Jenkinson, D.S. and Ladd, J.N., 1981. Microbial biomass in soil: measurement and turnover. In: E.A. Paul and J.N. Ladd (Editors), Soil Biochemistry. Vol. 5. Marcel Decker, New York, pp. 415-471. Khan, S.U., 1970. Enzymatic activity in a gray wooded soil as influenced by cropping systems and fertilizers. Soil Biol. Biochem., 2:137-139. Klein, T.M. and Koths, J.S., 1980. Urease, protease, and phosphatase in soil continuously cropped to corn by conventional or no-tillage methods. Soil Biol. Biochem., 12: 293-294. Lynch, J.M. and Panting, L.M., 1980. Cultivation and the soil biomass. Soil Biol. Bioehem., 12: 29-33. Martyniuk, S. and Wagner, G.H., 1978. Quantitative and qualitative examination of soil microflora associated with different management systems. Soil Sci., 125: 343-350. McGill, W.B., Cannon, K.R., Robertson, J.A. and Cook, ED., 1986. Dynamics of soil microbial biomass and water-soluble organic C in Breton L after 50 years of cropping to two rotations. Can. J. Soil Sci., 66: 1-19. Nannipieri, P., Pedrazzini, F., Arcara, P.G. and Piovanelli, C., 1979. Changes in amino acids, enzyme activities, and biomasses during soil microbial growth. Soil Sci., 127: 26-34. O'Sullivan, J. and Reyes, A.A., 1980. Effects of soil fumigation, rotation, and nitrogen on yield, petiole NO3-N and verticilium wilt of potatoes. Am. Soc. Holt' . Sci., 105: 809-812. Rasmussen, P.E., Collins, H.P. and Smiley, R.W., 1989. Long-t¢ management effects on soil productivity and crop yield in semi.arid regions ofeastern Oregon. Star. Bull. 675. Columbia Basin Ag. Res. Ctr., USDA-ARS and Oregon State University, Pendleton, OR. Schniirer, J., Clarholm, M. and Rosswall, T., 1985. Microbial biomass and activity in an agricultural soil with different organic matter contents. Soil Biol. Biochem., 17:611-618. Schuster, E. and Schr~der, D., 1990. Side effects of sequentially-applied pesticides on non-target soil microorganisms: Field experiments. Soil Biol. Biochem., 22: 367-373. L. Somerville and M.P. Greaves (Editors), 1987. Pesticide Effects on Soil Microflora. Taylor and Francis, London/New York/Philadelphia, 240 pp. Spading, G.P., 1981. Microcalorimetery and other methods to assess biomass and activity in soil. Soil Biol. Biochem., 13: 93-98. Suslow, T.V. and Schr6th, M.N., 1982. Role of deleterious rhizobacteria as minor pathogens in reducing crop growth. Phytopathology, 72:111-115. Tare, R.L., Jr. (Editor), 1987. Soil Organic Matter Biological and Ecological Effects. John Wiley, New York, pp. 37-42. Tiessen, H. and Stewart, J.W.B., 1983. Particle-size fractions and their use in studies of soil
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R.P. D I C K
organic matter. II Cultivation effects on organic matter composition in size fractions. Soil Sci. Soc. Am. J., 47:509-5 ! 4. Turco, R.F., Bischoff, M., Breakwell, D.P. and Griffith, D.R., 1990. Contribution of soil-borne bacteria to the rotation effect in com. Plant Soil, 122: 115-120. Verstraete, W. and Voets, J.P,, 1977. Soil microbial and biochemical characteristics in relation to soil management and fertility. Soil Biol. Biochem., 9: 253-258.