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a Univ. of Vermont Extension, 278 S. Main St, St. Albans, VT 05478
b Dep. of Horticulture, Oregon State Univ., 4017 ALS, Corvallis, OR, 97331
c School of Natural Resources, Ohio State Univ., Columbus, OH
* Corresponding author (heather.darby{at}uvm.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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12.1 g cm–3, FDA activity was 2.88 µg FDA min–1 g–1 dry wt, and microbial biomass was 91.6 µg C g–1 dry wt, and these levels were proposed as suppressive thresholds. Only the FDA threshold held up over all sampling times.
Abbreviations: diH2O, deionized water • DO, damping-off • FDA activity, rate of hydrolysis of fluorescein diacetate • fPOM, free particulate organic matter • MS, separated dairy manure solids • MSC, composted separated dairy manure solids • oPOM, occluded particulate organic matter • POM, particulate organic matter • SOM, soil organic matter • WSA, water stable aggregates
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Increasing the level of total or "active" soil organic matter (SOM) content typically improves overall soil quality (Herrick and Wander, 1997). Three different SOM pools are recognized: active, protected, and stable (Herrick and Wander, 1997). The active pool is composed mainly of plant residues in different stages of decomposition. This is the smallest and youngest pool of organic matter and has a very short resident time in soil (Carter, 1996). The active pool is the most labile (microbially active) organic matter pool and should therefore be the pool most involved in microbially mediated processes such as N mineralization and disease suppression. The active pool can be estimated by physical fractionation techniques (Christensen, 1992). The fPOM fraction, an indirect measure of the active pool, is not associated with mineral particles and consists mostly of incompletely decomposed organic residues (Herrick and Wander, 1997). The oPOM fraction, an indirect measure of the protected pool, is thought to include fPOM and/or low-density mineral-associated organic matter protected from microbial degradation within aggregates (Herrick and Wander, 1997).
Organic matter that is lightly decomposed and colonized by a diversity of microbial species is typically suppressive to diseases caused by Pythium spp. (Hoitink and Boehm, 1999). The sphagnum peat model is the best-researched example of this phenomenon. Only peats harvested from the top layers of the bog (very lightly decomposed peat moss, or light peat) are suppressive to Pythium DO. Light peat is typically suppressive for up to 7 wk, but as it decomposes it loses its ability to suppress Pythium DO. This gradual loss of suppression is related to a decline in microbial activity (rate of hydrolysis of FDA), culturable rhizosphere biocontrol agents, and carbohydrate content (Boehm et al., 1997). Peats of FDA level above 3.2 µg min–1 g–1 dry wt potting mix are reliably suppressive to Pythium DO (Boehm and Hoitink, 1992).
As a step toward understanding these relationships in the field, the impact of POM decomposition on Pythium DO was investigated in sand amended with composted dairy manure solids (Stone et al., 2001). The composition of compost-derived POM suppressive to Pythium DO in compost-amended sand was similar to that of soil fPOM as determined by 13C CPMAS NMR spectroscopy (Golchin et al., 1994; Stone et al., 2001). Compost-derived POM was suppressive to DO for approximately 1 yr. Compost-derived POM which had decomposed to the point where it could no longer support suppression of Pythium DO was compositionally similar to the slightly more decomposed field soil oPOM (Golchin et al., 1994; Stone et al., 2001). Stone et al. (2001) concluded that suppression was sustained by degradation of the less-decomposed POM, or the "active" organic matter. At this time there are no reports to our knowledge on the relationship between the content or decomposition level of fPOM and disease suppression in field soils.
Indirect measures of SOM composition or decomposition level such as FDA activity may be the better, or more easily measured, indicators of the disease suppressive potential of soil, as they have been successfully used as indicators in container systems (Boehm et al., 1997; Stone et al., 2001). However, there are currently few reports on the relationships between FDA activity or other indirect indicators (such as microbial biomass) and disease suppression in field systems (Workneh et al., 1993).
The primary objective of this work was to determine the effect of fresh and composted dairy manure amendments on (A) severity of root rot of sweet corn (causal agents Drechslera sp., Phoma terrestris, and Pythium arrhenomanes), root rot of snap bean (causal agents Fusarium spp. and Pythium spp.), and DO of cucumber (casual agents, Pythium spp.); and (B) soil biological and physical characteristics. The secondary objective was to identify relationships between the severity of these diseases and soil biological and physical characteristics as a step toward identifying indicators of soil quality that could also be used as indicators of SOM-mediated disease suppressive potential.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The experimental design was a randomized complete block (eight replications). Amendment type/rate, including a non-amended control, was the main effect. The plot size was 6.1 m by 9.2 m. The treatments were separated dairy manure solids (MS) and composted dairy manure solids (MSC), each applied at two rates, and a non-amended control. In the spring of each year, amendments were applied on a weight basis, spread manually, and incorporated with a rotovator to a soil depth of 15 cm. On 15 May 2001, MS was applied at 16.8 and 33.6 dry Mg ha–1 and MSC at 28 and 56 dry Mg ha–1. On 15 May 2002, both amendments were reapplied to the same field plots at 16.8 and 33.6 dry Mg ha–1. The MS and MSC amendments were purchased from a local dairy. The MS was the solid fibrous fraction obtained from liquid dairy manure using a screen separator. The MSC was made from MS composted without additives in windrows for 2 mo. The windrow was turned twice a week for 1 mo and once a week for an additional month with a windrow turner. Materials were not applied on a mineral nutrient basis, but on a C basis, to increase the potential for disease suppression. The chemical characteristics and nutrient content of the amendments were determined each spring before application (Table 1). Amendments were applied 4 wks before planting to permit sufficient residue decomposition before planting, as raw residues may increase severity of diseases caused by Pythium and Rhizoctonia (Lumsden et al., 1983; Grunwald et al., 2000).
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Soil Sampling
Soils were sampled 2 and 12 mo after the first amendment and 2 and 6 mo after the second amendment during the growing seasons of 2001 and 2002. Soils collected 6 mo after the second amendment were only assayed for severity of root rot of sweet corn and snap bean and DO of cucumber. Soils were sampled at 6 instead of 12 months after amendment in the second year as in the first year the twelve-month soils were not suppressive. From each plot a composite of 10 soil cores (2.5 cm diam., 15 cm depth) were sieved through a 2-mm mesh sieve, stored at 4°C, and assessed for FDA and arylsulfatase activities and microbial biomass C (Dick et al., 1996). A 10-g subsample was used to determine gravimetric water content. A separate set of 10 cores (5 cm diam., 15 cm depth) was composited, passed through an 8-mm mesh sieve, and air-dried (48 h at 25°C) for determination of fPOM and oPOM (Wander and Yang, 2000). In addition, 10 soil wedges (approximately 13 cm by 5 cm by 15 cm) were taken with an AMS Soil Sampling Sharpshooter Shovel (AMS Inc., American Falls, ID). Soil wedges were composited and a subsample was passed through a 4.75-mm sieve and air-dried as above for WSA analyses (Buller, 1999). The remaining soil was passed though a 2.54-cm screen and used for disease assessment in the greenhouse.
Soil Analyses
Microbial activity was assessed on field moist soil by measuring FDA and arylsulfatase activities within 48 h of soil collection. The rate of FDA hydrolysis was assessed by modification of procedures proposed by Dick et al. (1996). Briefly, 3 g of field moist soil were added to each of four Erlenmeyer flasks. Fifty milliliters of 60 mM sodium phosphate buffer (pH 7.8) containing fluorescein diacetate (3', 6' diacetyl fluorescein) substrate was added to each of three flasks. The fourth flask, to which only buffer was added, served as a control. Reaction flasks were incubated for 3 h on a rotary shaker (178 rev min–1) at room temperature (25°C). The reaction was then terminated by the addition of 2 mL of acetone to each flask. The extracts were centrifuged for 5 min at 31 000 x g, filtered (Whatman #42), and the quantity of FDA hydrolyzed was determined in filtrates at 490 nm with a spectrophotometer (Beckman Model 34, Beckman Industries Inc., Irvine, CA). Activity was calculated as µg fluorescein hydrolyzed min–1 g–1 dry wt soil by comparing absorbance against a standard curve. Background absorbance was corrected for each treatment with the control sample.
Arylsulfatase activity was determined as described by Tabatabai (1994). In brief, 1 g of soil was placed into a 50-mL Erlenmeyer flask, and 0.25 mL of toluene, 4 mL of acetate buffer (ph 5.8), and 1 mL of 
-nitrophenyl sulfate solution were added. Samples were incubated for 1 h at 37°C. After incubation, 1 mL of CaCl2 and 4 mL of 0.5 M NaOH were added to each flask. Samples were filtered (Whatman #2) and absorbance was measured at 410 nm. Two analytical replicates and one control were used per sample. The results were calculated as the activity of -nitrophenol min–1 g–1 dry wt soil by comparing absorbance to a standard curve. The absorbance of the control was subtracted.
Microbial biomass C was measured by the chloroform-fumigation incubation method (Jenkinson and Powlson, 1976). A 10-g sample of field moist soil was weighed into glass scintillation vials. Vials were placed into a desiccator with a 50-mL beaker containing 40 mL of ethanol-free chloroform. The desiccator was evacuated and the soil was exposed to chloroform vapor for 24 h. Soils were transferred to 125-mL Erlenmeyer flasks, stoppered, and incubated in the dark at 25°C for 10 d. The amount of CO2 produced was measured on a Varian gas chromatograph (Varian, Palo Alto, CA).
The fPOM and oPOM were determined by densiometric separation of the light fraction by modification of the methods proposed by Golchin et al. (1994) and Puget and Drinkwater (2001). Twenty grams of air-dried soil (<8 mm) were placed in a 250-mL Nalgene centrifuge bottle (Nalge Nunc International Corp., Naperville, IL) with 75 mL of sodium polytungstate solution (1.9 g cm–3) (Geoliquids, Chicago, IL). The solution was shaken for 1 h at 100 rpm. The soil suspension was allowed to settle for 16 h. The free POM on the top of the solution was aspirated into a 500-mL Erlenmeyer flask. Free particulate organic matter was recovered on a 0.45-µm MAGNA nylon filter (Osmonics, Inc., Minnetonka, MN). The fPOM was washed with 100 mL of deionized water (diH20) and then rinsed off the filter with diH20 into aluminum pans and dried overnight at 80°C. The sodium polytungstate filtrate was returned to the 250-mL centrifuge bottle and used to generate the oPOM fraction by centrifuging at 2460 x g in a Beckman model TJ-6 centrifuge (Beckman Industries Inc., Irvine, CA). The supernatant was poured into the same filtration unit and rinsed with 100 mL of diH2O and the oPOM recovered as described for fPOM.
Water stable aggregation was measured by wet sieving (Kemper and Rosenau, 1986). First the soil was sieved to remove fractions that were <1 mm. Four grams of the remaining soil (1 mm and 
4.75 mm) was placed in a screened cup (3.6-cm diam. with 0.250-mm stainless steel screen). Unstable aggregates were removed by cycling the soil through 100 mL of diH2O for 3 min at 35 cycles min–1. Stable aggregates were than recovered by cycling the remaining sample through a dispersion solution (sodium polyphosphate, 2 g L–1) until only sand particles remained on the screen. Percentage of WSA was calculated as follows:
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Disease Bioassays
Disease severity was assessed with bioassays as it is not possible to grow these crops in the field at all the time points required. Bioassays were conducted in the greenhouse or growth chamber. Soil samples collected from the field (as described above) were placed into 550-mL cone tubes (Stuewe & Sons Inc., Corvallis, OR) and assayed for their potential to suppress DO of cucumber and root rot of snap bean and of sweet corn. Each bioassay was replicated twice per plot (n = 16).
Cucumber Damping-Off Bioassay
A cucumber bioassay was conducted as in Boehm and Hoitink (1992). In brief, five untreated cucumber seeds (cv Straight Eight) were planted 1 cm deep in soil. Cone tubes were incubated in a growth chamber for 10 d at 20°C and 16-h illumination. Plants were watered to keep soil moisture levels near field capacity. Disease severity was rated 10 d after planting where 1 = symptomless; 2 = emerged but wilted or with visible lesions on the hypocotyls; 3 = post-emergence DO; and 4 = pre-emergence DO. A mean disease severity rating (n = 5) was determined for each container (n = 16).
Root Rot of Bean and Corn Bioassays
Snap bean (cv Oregon 91G) and sweet corn (cv Golden Jubilee) seeds, treated with Captan [N-(trichloromethylthio)-4-cyclohexane-1, 2,-dicarboximide], which does not control root rot of sweet corn, were surface disinfested with 10% sodium hypochlorite for 5 min and rinsed in diH2O before planting. Three bean or two corn seeds were planted in soil 2.5-cm deep per cone tube. After emergence, corn was thinned to one plant per cone tube. Plants were grown in a greenhouse at 21°C (day) and 15°C (night) with a 14-h photoperiod. Plants were watered daily to keep soil moisture contents near field capacity, and they were fertilized every 2 wk with a water-soluble fertilizer mixture (N-P-K/20–20–20). Snap beans were harvested and roots were evaluated for severity of root rot 4 wk after planting. Corn roots were harvested and rated at the sixth leaf stage (Ritchie et al., 1996).
Bean root rot evaluations were based on the extent of necrosis of the rootball on a 0 to 5 scale where, 0 = healthy, 1 = 1 to 10%, 2 = 11 to 25%, 3 = 26 to 50%, 4 = 51 to 75%, and 5 = 75 to 100% of the rootball was necrotic. Root rot of corn was evaluated by visually assessing the proportion of the radicle that was necrotic. The radicle rating was based on a 0 to 4 scale where 0 = healthy, 1 = 1 to 9%, 2 = 10 to 50%, 3 = 51 to 99%, and 4 = 100% of the radicle was necrotic (Hoinacki, 2003). Raw disease ratings were converted to percentage radicle necrosis (the mean necrosis value for each numeric rating) for statistical analysis.
Statistical Analyses
Mixed-model analysis for each individual sample time was calculated using the PROC MIXED procedure of SAS (SAS Institute, 1999). Treatment means were separated by the LSD procedure when the F-test was significant (P < 0.05). Regression analysis examined relationships between soil biological and physical characteristics and the severity of root diseases. Regression analysis was performed across sample dates 2 and 12 mo after the first amendment and 2 mo after the second amendment. Regression coefficients were described when significant (P < 0.05). Pearson correlations were performed to examine the relationship between fPOM and other measured soil biological and physical characteristics.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Soil Biological Characteristics
Two months after the first amendment, all amended soils with the exception of the low rate of MS had significantly higher levels of FDA activity and microbial biomass C than the non-amended control (Table 2). High MSC had significantly higher levels of FDA activity than all other treatments. Treatments did not differ in arylsulfatase activity at 2 mo after the first amendment (Table 2).
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Soil Physical Characteristics
Two months after the first amendment all amended treatments had significantly higher soil fPOM contents than the non-amended control (Table 3). Soils amended with the high rate of MSC had 14.55 mg cm–3 (a 593% increase) more soil fPOM contents than the non-amended control. By 12 mo, only soils amended with the high rate of MSC had significantly higher fPOM contents than the control. Once the soils were re-amended, all amended treatments had significantly higher soil fPOM contents than the non-amended control (Table 3). Soil oPOM contents were not significantly different among treatments at any sampling time (Table 3).
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Relationships between Soil Properties and Disease Severity
In general, across all sampling times, significant negative linear relationships between soil biological and physical properties and severity of cucumber DO and root rot of snap bean were observed (Fig. 2
and 3)
. In addition, significant negative linear relationships between FDA activity and corn root rot severity, and slightly less significant relationships between fPOM and microbial biomass C and corn root rot severity, were also observed (Fig. 4
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In the first year, a statistically significant level of disease suppression was observed across all diseases when fPOM content was 12.1 mg cm–3, FDA activity was 2.88 µg hydrolyzed FDA min–1 g–1 dry wt, microbial biomass was 91.6 µg C g–1 dry wt, and percentage of WSA was 43.6% (Fig. 2, 3, and 4), and these levels were proposed as preliminary, statistically derived, disease suppressive thresholds. The data points representing the high rate of MSC fell above the proposed thresholds for both fPOM and microbial biomass C at the 12 mo sampling time, but no suppression was observed compared to the non-amended control (Fig. 2, 3, and 4).
Microbial activity, microbial biomass C, and the percentage of WSA showed a positive linear response to soil fPOM contents (Fig. 5 ). The relationship was strongest for microbial biomass C (Fig. 5).
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The low rate of MS was not suppressive to any of the three diseases until after the second year of amendment, at which time it was suppressive to all three diseases. The lack of suppression in the first year was accompanied by low soil fPOM contents and FDA activity. It took 2 yr of amendment at the low rate to bring the fPOM content and FDA activity to levels above the statistically derived suppressive threshold. While there is only 1 yr of data, other researchers have reported variable impacts on disease incidence after only 1 yr of relatively low rate amendment (Lumsden et al., 1983; Lewis et al., 1992).
Interestingly, 12 mo after amendment, there was an increase in corn root rot severity in amended treatments and a significant positive relationship between severity of root rot of corn and soil fPOM and microbial biomass-C contents. This relationship was not observed 2 mo after the first or second amendment. This effect may be due to the higher water holding capacity of the amended soils accompanied by their low microbial activity and disease suppressive potential. Severity of root rot of sweet corn is more severe in fields that receive high levels of irrigation before silking (R.E. Peachey, personal communication, 2004). Interestingly, increased severity of DO of cucumber and root rot of bean was not observed in amended treatments 12 mo after amendment. Subsequent work in this amended system will investigate the soil factors contributing to the increase in corn root rot severity at 12 mo after amendment.
Suppression in treatment soils was lost sometime between two and 6 mo after amendment, regardless of amendment type or rate. The loss of suppression over a several month period as an organic substrate decomposes has also been observed in container systems. For example, Pythium DO was suppressed in peat-based container mixes for days to a few months, composted pine bark mixes for up to 9 mo, and composted hardwood bark mixes for 2 yr (Hoitink and Boehm, 1999). Damping-off of cucumber was suppressed for 1 yr in a sand amended with dairy manure compost incubated in containers (Stone et al., 2001). It is not clear why soil suppressiveness was lost after several months in this amended soil, while suppression was maintained for a year in the amended sand container system. In this experiment, amended field soils may have lost suppression more quickly than the amended sand because of an interaction with soil water holding capacity or differences in initial substrate quality. Similarly, Widmer et al. (1998) reported a loss of suppression to Phytophthora root rot of citrus (P. nicotianae) 6 mo after a sandy soil was amended with municipal solid waste compost. Future work on the composition of the fPOM (and its indirect measure, FDA activity) in this soil system should help elucidate the relationships between fPOM quantity and quality and the duration of disease suppression.
Soil Biological Characteristics
Organic amendment increased microbial biomass and microbial activity (FDA activity) as has been reported previously (Craft and Nelson, 1996; Albiach et al., 2000; Aryantha et al., 2000). Fluorescein diacetate is hydrolyzed by a wide variety of soil microbes and is considered a measure of general microbial extracellular hydrolase activity (Dick et al., 1996). Arylsulfatase activity was not higher in amended treatments compared with the non-amended control 2 mo after the first amendment, but was higher at all subsequent sampling dates. Arylsulfatase hydrolyzes ester sulfate, which is found primarily in fungi, and is therefore considered to be an indirect measure of fungal biomass (Saggar et al., 1981). It may take longer than 2 mo to significantly increase fungal biomass or activity in amended soil systems. Longer-term soil amendment studies have reported significant increases in arylsulfatase activity (Bandick and Dick, 1999; Albiach et al., 2000).
Soil Physical Characteristics
Soil fPOM contents were significantly increased after amendment regardless of type or rate of amendment. Manure amendments consistently increase and maintain SOM and POM contents over the short and long term (Wander et al., 1994; Paustian et al., 1997). Higher fPOM contents in soils amended with composts, relative to those amended with manure, were most likely due to the higher rate of amendment for the compost treatments in the first year. In addition, manure amendments decompose more rapidly than compost during the first few months of decomposition, as compost undergoes the early stages of decomposition during the composting process.
In general, fresh manure amendments increased the percentage of WSA to a greater degree than the composted manure treatments. Similarly, Sela and Goldrat (1998) reported that organic residues composted for 10 d increased aggregate stability and infiltration rates to a greater degree than soils amended with the same organic material composted for 60 d. Manure is less decomposed than composted manure, as described previously, and bacteria produce polysaccharides which enhance the formation and stabilization of aggregates as they decompose the most labile constituents of organic residues (Tisdall, 1995).
Relationships between Soil Properties and Disease Severity
All soil properties except arylsulfatase activity and oPOM content were negatively correlated to disease severity. The relationships were strong for DO of cucumber and root rot of snap bean. Relationships between soil properties and severity of root rot of sweet corn were much weaker, due to the positive relationship between amendment rate and root rot severity 12 mo after amendment. Previous reports on OM-mediated suppression of Pythium DO in container systems have documented negative relationships between severity of Pythium DO and fPOM, FDA activity, and microbial biomass (Chen et al., 1988; Boehm et al., 1997; Stone et al., 2001). This is the first report relating these factors to DO of cucumber in a field soil, and the first report relating these factors to severity of root rots of sweet corn and snap bean. To our knowledge, no other studies have demonstrated a relationship between arylsulfatase activity and oPOM contents and severity of any disease. Occluded POM is typically an older pool of C that has been sequestered within aggregates (Golchin et al., 1994). Since this fraction is sequestered and more decomposed, we would not expect it to play a role in biological suppression.
Boehm and Hoitink (1992) reported that suppression of Pythium DO (causal agent Pythium ultimum) was supported as long as the peat mix sustained a rate of hydrolysis of FDA 3.2 µg min–1 g–1 dry wt peat-based potting mix; this level is used as a predictive "disease suppressive threshold" for sphagnum peats (Boehm et al., 1997). In our soil system, suppression of all diseases was sustained in all treatments of FDA activities 2.88 µg min–1 g–1 dry wt soil. It is not surprising that these thresholds might be different, as soil-less container mixes and field soils have very different properties, the procedures to measure FDA activity were different, and more years of data must be collected in this soil system to generate a predictive disease suppressive threshold. Workneh et al. (1993) observed higher levels (1.3 µg min–1 g–1 dry wt soil) of FDA activity on organic farms where compost or green manures had been applied annually for four or more years than on conventional farms (0.70 µg min–1 g–1 dry wt soil) that had not used those practices; incidence of corky root (Pyrenochaeta lycopersici) of tomato was, overall, lower on the organic than on the conventional farms. Dissanayake and Hoy (1999) observed that a rate of FDA hydrolysis above 5.2 µg min–1 g–1 dry wt soil was required to suppress root rot of sugarcane (causal agent, Pythium arrhenomanes). From the data generated in this study and these few literature reports it does not appear that a universal "disease suppressive threshold" exists for FDA activity across soil, cropping, and disease systems. Standardizing the procedure for measuring FDA activity and sampling soils at a specified time (e.g., 1 or 2 mo after amendment) should reduce the variability of FDA measurements so that over time researchers and farmers will be able to develop some general guidelines for using FDA activity as an indicator of organic amendment-mediated soil disease suppressive potential.
Lightly decomposed fPOM (2 mo after amendment) was suppressive to all three diseases, but after 12 mo of decomposition it was no longer suppressive, suggesting that fPOM composition may be related to disease suppression. At 12 mo after amendment, the fPOM data point representing the high rate of MSC fell above the fPOM threshold for suppression but the soil was not suppressive. We hypothesize that although the fPOM content was at a level high enough to generate suppression it had decomposed to the point where its quality was too poor to support suppression. This phenomenon was first documented by Boehm et al. (1997), who showed that as light peat decomposed and lost suppressiveness there was a reduction in the carbohydrate concentration of the mid-sized peat particles in the mix (as determined by 13C CP-MAS NMR spectroscopy). Similarly, a difference in composition of Pythium DO-suppressive and -conducive POM was observed in a sand amended with composted dairy manure solids incubated in containers (Stone et al., 2001). The relationship between fPOM composition and disease suppression in this field soil system is currently under investigation.
Microbial biomass C may not be as reliable a measure of disease suppressive potential as FDA activity. At 2 mo after amendment in both 2001 and 2002, disease suppression was observed in soils when levels of microbial biomass C were above 92 µg C g–1 dry wt. However, as with fPOM (as described above), the high MSC treatment soils had biomass-C levels above this threshold but were not suppressive at 12 mo after amendment. In agreement, Boehm et al. (1997) reported that microbial biomass remained at high levels after disease suppression was lost and FDA activity declined. Microbial biomass appears to respond more slowly to changes in substrate quality than FDA activity.
Organic amendments, including manures, typically increase soil aggregate stability as was observed with this study (Mbagwu and Bazzoffi, 1988; Aoyama et al., 1999). Interestingly, in this study WSA was also linearly related to disease severity. However, the relationship between disease severity and WSA is likely not causal, as MS-amended soils had higher percentage of WSA as well as higher disease severity relative to the MSC-amended soils.
It is not surprising that the amount of fPOM in the soil was related to FDA activity, microbial biomass, and percentage of WSA. There is considerable evidence that this labile organic matter fraction is closely linked to soil biological characteristics (Wander et al., 1994; Scow, 1997). However, FDA activity was not as closely related as microbial biomass to fPOM content. This provides further support that FDA activity is related to OM quality. Free POM content was strongly related to FDA activity at 2 mo after amendment but the relationship was weaker 10 mo later. As organic residues decompose over time, the relationship between fPOM content and FDA activity likely becomes increasingly weaker.
In conclusion, fresh and composted dairy manure amendments suppressed DO of cucumber and root rots of snap bean and sweet corn in both the first and second years of serial application. Suppression was relatively short in duration, lasting less than 6 mo after amendment. At 12 mo after amendment, there was no treatment effect on severity of DO of cucumber and snap bean, while severity of root rot of sweet corn was positively related to amendment rate. Suppression was related to soil quality indicators such as fPOM, FDA activity, microbial biomass C, and percentage of WSA, but the relationships were weaker for root rot of corn than for DO of cucumber and root rot of snap bean. Free POM content was strongly positively related to disease suppression and FDA activity 2 mo after amendment but these relationships weakened as the fPOM decomposed over time. Interestingly, fPOM content and microbial biomass C were positively related to corn radicle rot severity at 12 mo after amendment; this phenomenon is currently under investigation. When suppressive thresholds were tested for each of these soil factors, only the threshold for FDA activity held up across all treatments and sample times. Fluorescein diacetate activity was the most reliable indicator of the disease suppressive potential of this field soil, as it appears to be a good indirect measure of both organic matter quantity and quality relative to disease suppression. However, more work is required to further develop predictive "disease suppressive thresholds" for these soil/disease systems.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Received for publication August 5, 2004.
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Within each disease and sampling time, treatment bars followed by the same letter are not significantly different (P < 0.05).
), 12 mo after the first amendment (•), and 2 mo after the second amendment (
). Each solid line represents a statistically derived "suppressive threshold": the level of disease severity significantly different from the control (P < 0.05).
