Influences of Alternative Vegetable Systems on Arthropods/Soil Biological Dynamics and Soil Quality Trajectory

This is an in-depth study of soil biology and above ground arthropods as well as soil chemical and physical properties under conventional or alternative vegetable systems in western Oregon. This included a parallel effort to identify sensitive early indicators of soil quality.

The approach, in brief, has been to have a multidisciplinary team that covered entomology, soil fauna, soil microbiology, soil physics, and soil biochemistry investigating two types of management systems. One system was conventional which had winter fallow and traditional tillage. The other improved system had winter cover crops and when possible reduced disturbance via tillage. The work was conducted at two experiment station research sites (fully replicated with statistically valid experiments initiated in 1989 and 1993) and on five farmers’ fields who split their field into two management systems (each farmer has designed there own improved management system and the design is RCB where each farm is a block).

The first year of 1996 was a baseline year, which involved detailed soil descriptions (done by Steve Campbell of NRCS), of each field/experimental site and identification of sampling sites on farmers’ fields. We are collecting data at three levels of resolution – (1) subjective questionnaire, (2) on-farm field kit, and (3) researcher managed field and lab methods.

A major accomplishment of the project was the development of a soil quality card and an accompanying guide that were developed at farmer focus sessions. Besides being now available as a tool for farmers to assess their own soil quality, the participating farmers in our study are using the card to evaluate the two systems they are testing. This is providing a rich data set that can be compared to the more rigorous lab based measurements.

Objective 1 - Early Indicators of Soil Quality

Enzyme activity (ß-glucosidase and arylsulfatase), microbial biomass carbon (MBC) and cotton cloth strip decomposition were tested to detect early changes in soil quality. Results indicated ß-glucosidase activity was the most sensitive biological assay in detecting cover crop effects on both on-farm research and experiment station sites. Greater microbial activity was observed on the no-till system compared to conventional tillage. Microbial biomass carbon was generally higher on the cover crop and no-tilled soils but showed high seasonal and site-to-site variability. The cotton strip assay (buried in soil from 14 to 21 days) had significantly greater decomposition on soil cover cropped than winter fallowed after three years of cropping at the Vegetable Farm Research Site. At shorter time periods of two or less years at on-farm sites, cotton cloth decomposition was generally not significant due to cover cropping. However, it was sensitive to no-till management after only one year. A lab incubation study indicated use of cotton strips must be used with caution and under the same conditions to make comparisons from year-to-year or site-to-site.

Of the integrative physical measurements (bulk density, compaction by penetrometer, and water infiltration), water infiltration was the most sensitive in showing cover crop treatment of these physical measurements. Furthermore, it is a simple test that land managers can adopt readily. It has an advantage over the other methods in that it integrates over a larger volume of soil. The drawback is that it is somewhat time consuming and clearly wheel track vs. nonwheel track area must be considered when using this method. The high CVs suggest that infiltration has a high degree of spatial variability and that more subsampling may be needed to adequately characterize a field, which further increases time requirements of this method. It can only be done on a comparative basis, as rates of infiltration are strongly influenced by soil type.

The statistical significance and trends for water infiltration indicate cover cropping was improving soil structure over winter fallowing. Consequently, it follows that bulk density and compaction were also changed. We had little direct evidence for this, but a significant negative correlation between infiltration and bulk density provides indirect evidence for this conclusion.

From a practical perspective, the penetrometer was the easiest to use but was not effective in picking up treatment effects, even after 4 years of cover cropping. Although this measurement is affected by soil moisture, it is unlikely in our study that this affected the outcome as there were no significant treatment effects on soil moisture.

The density gauge provides very accurate readings of bulk density, but represents a very small part of the soil for each reading. This suggests spatial variability is too high to pick up treatment effectively.

The results of this study indicate that these soil physical measurements for bulk density and penetrometer resistances are best suited to identify major differences between soil types but would be relatively ineffective as early indicators of changes in soil management within a soil type. Infiltration was the most effective in showing treatment effects and suggested that cover cropping was having a positive effect on soil structure. This indicates cover crops should improve water relations for summer row crops. Aggregate size distribution data for 1997 and 1998 showed significantly more aggregates in the 1-2 mm size for the winter cover crop treatment compared to the winter fallow. Water stable aggregates appear to increase with cover cropping but is less evident than the increase in the 1-2 mm fraction that occurred with cover cropping.

Particulate organic carbon (POC) which is partially decomposed plant residues and is a fast cycling part of soil organic matter appeared to increase with cover crop treatment in 2 of the 5 sites on farm.

These results are encouraging in that certain microbial and physical properties are sensitive to changes after only one year of a change in management. The significant increase in the large aggregates is important relative to soil quality and health. An increase in the larger aggregates facilitates water infiltration and retention; provides biological habitat, and a better rooting environment.

The sensitivity of some of these physical and biological measures to management within 1 to 2 years, gives hope for the potential use of soil quality indicators to assist farmers to manage soil for long-term sustainability. In turn, this could lead to higher, more stable crop yields. Furthermore, use of cover crop systems is improving soil quality that is quantifiable.

Objective 2 - Effects of Cropping on Soil Community Structure

Microorganisms

The composition of soil microbial communities were monitored by identifying the types and relative abundance of microbial fatty acids extracted directly from soils. The extracted fatty acids were separated and analyzed by gas chromatography which provided signature fingerprinting of the functional microbial groups and in some cases specific species. This was used to determine the microbial diversity of the soils with and without cover cropping.
Analyses of commercial grower fields and research stations over the course of two growing seasons revealed underlying factors that influence the composition of the microbial community. Shifts in community composition occurred throughout the growing season in response to environmental and habitat changes. Within each growing season, the community composition was dependent on the soil properties and the type of vegetation present. Microbial community composition was also affected by the addition of winter cover crops residues to soil. Cover crop residues often increased the diversity of fatty acids extracted from soil, suggesting an increase in microbial diversity in response to cover cropping. Shifts in community composition occurred in response to the residues, but the members of the community affected by residues differed according to each field. However, there is evidence from fatty acid analyses that there is an increase in the relative abundance of eukaryotic organisms, including fungi and protozoa, in soils with incorporated residue.

Studies were also conducted to determine the ability of the soil microbial community to decompose residues as an indicator of the functional diversity of soils with or with out cover crops. This was done by measuring carbon substrate utilization potential of soil microbial communities which was done by challenging the soil with 96 different substrates that range from simple organic compounds such as glucose to complex polymers. Substrate utilization potential analysis showed that in some fields, the addition of winter cover crop residues to soil increased the ability of the community to utilize a diverse range of carbon substrates. Additional studies conducted in the laboratory suggested that cover crop residues provide a diverse range of substrates for soil microorganisms and selects for communities that are able to metabolize a wider range of food resources.

Conclusions from this research are that the residues of winter cover crops appear to have a greater impact on microbial diversity than the rhizosphere effect of the living cover crop in Oregon. Enhanced soil food web interactions are suggested by the increase in relative abundance of protozoan fatty acid markers in cover-cropped soils. In addition, it appears that cover crops also stimulate fungal populations, which could be important in stabilizing soil structure through aggregate formation.

Soil Fauna

Soil microfauna analysis conclusively demonstrated that cover-cropping increases the abundance and diversity of the soil fauna. Direct linkages between the number of resident fungivores and macropredators can be made on theoretical grounds to “soil health”. Soil-dwelling fungivores have been repeatedly shown to regulate the rate of nutrient mobilization while macropredator density has been correlated with pest control.

The effectiveness of cover-cropping at increasing faunal density and complexity is due to characteristics of the cover crop and the length of time cover-cropping has been practiced. Cover-cropping, as opposed to bare fallow, increases arthropod density primarily because it provides a food source and foodweb energy base for the winter months. In the absence of either a fungal resource or prey upon which to feed, arthropods will leave the soil system. Though many of them are capable of suspended animation, most clearly choose to leave. The smaller species, less capable of movement over hundreds of meters, may remain cryptobiotic, but unless food in sufficient quantity is there when they emerge in the spring they will perish as well. Under bare-fallow conditions sufficient fungal food resources are unlikely to be present until canopy closure. These results show clearly elevated population densities after one year of cover-cropping at the time of spring planting. However, only after several years of cover-cropping is the population of soil microarthropods stable enough to survive cover-crop removal and the disturbances of planting to be detectable at canopy closure as well. Detection of a cover-crop effect at harvest is generally swamped by differences in the actual crops being raised; we have no doubt that the cover-crop effect exists until harvest, but an experimental design utilizing only the same crop would be necessary to demonstrate it.

The effectiveness of cover-cropping also depends upon two other variables: firstly, the density and structural diversity of the cover crop; and secondly, the management protocol for the cover-crop in the spring. The denser the above-ground cover-crop is (i.e., rye is better than oats), the more insulation is provided for arthropod activity during the winter (high activity in the climate of the Willamette Valley). The greater the root biomass of the cover-crop, more labile carbohydrates are released which promote rhizosphere activity of bacteria and fungi. The more species rich the cover-crop (intentionally or unintentionally) the greater the diversity of microhabitats for refuging; in particular the number of horizontal leaves in the vicinity of the ground surface is critical.

Mixed cover crops utilizing legumes are more effective than pure cereal crops; very weedy incidental cover-crops are often very effective as well. The springtime choice of green manuring versus tillage is also extremely significant. In a related SARE investigation we showed that the refuges provided by green manuring were absolutely critical in maintaining populations of the biocontrol species, Pergamasus quisquiliarum. Other studies have shown the effectiveness of green manuring in elevating the ratio of fungi to bacteria in the soil, especially critical at this time for soil microarthropod density.

One purpose of these investigations was to develop utilitarian indices of soil health involving arthropods, independent of the crop type employed. We found that the best index of overall microarthropod density and diversity was the population density of springtails. Depending on crop, time of year, and management type these densities range from several dozen to several dozen thousand per square meter. In the cover-cropping system there is generally a peak of perhaps 10,000/m² in the spring, which drops catastrophically during cover-crop removal and planting to several hundred/m², and then rises to 10-40,000/m² by year’s end (depending on crop). Under fallow situations, the spring population is at best several hundred to several thousand/m²; this difference can not be made up during the course of the subsequent growing period.

The best indicator of overall soil predator performance is the ratio of total micro-predators (mites, microspiders, etc) to total springtails. Since the diversity of micropredators in agricultural fields is only a small fraction of the diversity under natural conditions, a small change in diversity can have a major effect on interpretation, hence we have utilized only the total population density of micropredators.
Total density of all earthworm species was extremely low throughout. It seemed to be affected positively by cover-cropping, but more extensive sampling is necessary. An effort needs to be made in the Willamette Valley to increase the number of earthworms, especially in the southern regions where soil compaction is a much more serious threat. Much of the difference in soil aggregation in cover-cropped versus fallow soils is likely to be the indirect effect of earthworm and enchytraeid activity.

In conclusion, earthworms appear to be stimulated by cover cropping even under conventional tillage, but more in-depth studies are needed to confirm this. Nutrient-mobilizing species such as fungus-feeding springtails and mites are conserved by cover crops and reduced tillage. Deposition of green manure cover crops on the soil surface supports high populations of a mite (Pergamasus) which is currently being developed as a bio-control agent of symphylans.

Above-ground Arthropods

Soil management practices including the use of cover crops and reduced tillage planting systems have an impact on the above ground arthropods in farming systems. We chose to focus our attention in this study on the impact of soil management on ground beetles (Coleoptera: Carabidae) and spiders (Araneae).

Many ground beetle species are residential; they overwinter in or near the agricultural fields that they colonize during the growing season. Ground beetles have intermediate levels of mobility compared to other arthropods; their limited mobility allowed us to assess the impact of soil management practices on a local-field spatial scale. Ground beetles spend a portion of their life cycle (egg and larval stages) in the soil and, thus, are particularly sensitive to soil management. We hypothesized that changes in the abundance and diversity of the ground beetle assemblage would be a useful early indicator of changes in the agroecosystem management associated with alternative management practices.
Highly mobile spiders in the family Linyphiidae dominate most agricultural landscapes in northern temperate climates. These spiders balloon on silken webs for long distances and colonize agricultural fields following tillage and pesticide operations. Lycosidae are sensitive to tillage, habitat structure, soil applied pesticides because these spiders spend most of their time hunting on the soil surface. We hypothesized that a change in the relative proportion of Lycosidae to Linyphiidae would be an early indicator of change associated with alternative agricultural practices.

We sampled ground beetles and spiders during the growing season within vegetable crop fields at ten sites for three years at two levels of intensity using pitfall traps (10 cm dia./4.5 cm deep with soapy water). At eight on-farm sites, sets of ten pitfall samples (five in each treatment) were taken three times per year: prior to cover crop plow down, at crop canopy closure, and shortly after harvest. At the OSU Vegetable research farm, we sampled intensively for longer periods of time and with large numbers of pitfall traps.

At the end of the project, no changes in spider fauna were observed. Regardless of the cropping systems, the Linyphiidae remained the dominant spider taxa. Lycosidae remained absent or rare at all ten sites. The remainder of this discussion, therefore, will be devoted to an analysis of the carabid assemblage.

The ground beetle assemblages at eight of the ten sites were similar and dominated by Pterostichus melanarius (F.), a crop-adapted European species that was introduced into the Pacific Northwest from Europe during the 1800s. Other common carabid species at the ten sites were: Anisodactylus binotatus F., Clivina fossor L., Harpalus pennsylvanicus (DeGeer), Harpalus affinis Schrank, Agonum suturale Herbst, Bradycellus congener LeConte, and several species of Amara.

In 1996, the native Pterostichus species, P. algidus, was the most common ground beetle in our samples at the Grover site which had previously been a remnant forest of mature Douglas fir trees; P. melanarius was absent. P. melanarius was present on the Kenagy farm, but the carabid assemblage was dominated by Agonum and Amara species. Agonum and Amara species are strong flyers; they tend to dominant riparian areas where winter flooding occurs. Significant flooding occurred on the Kenagy site during 1996 and 1997. By 1998 P. melanarius had become the dominant beetle species at the Grower farm; P. algidus was no longer detected.

On the farms where P. melanarius was dominant, cover crops may have enhanced the survival of beetle larvae and adults during the winter; however, beetle densities during the growing season were similar in portions of the fields that had cover crops versus winter fallow. The lack of apparent treatment effect was probably due to dispersal of the beetles across the treatments. P. melanarius migrates by walking and will disperse up to 200 meters from a tillage refuge during the course of the growing season.

At the Lucht farm no difference in P. melanarius density was observed between cover crop versus winter fallow portions of the field. However, P. melanarius density was much higher in cauliflower near a planting of rhubarb. At the OSU Vegetable Research Farm, a mark, release, and recapture experiment verified that P. melanarius dispersed from undisturbed strips of cover crop into developing broccoli plantings.

In summary, the Carabidae may be useful indicators of changes in tillage, vegetation management, and habitat fragmentation. The assemblage may be more useful as indicator species if it is divided into three groups based on mobility and abundance: 1) mid sized carabid beetles that disperse primarily by walking (including P. melanarius, A. binotatus, H. pensylvanicus, and H. affinis) dominate disturbed agricultural settings that are well-drained, upland sites with some semi natural habitat, 2) medium to small-sized beetles that disperse primarily by flying (including B. congener, A. suturale, and Amara species) were more common and in some cases dominant in highly disturbed or regularly flooded sites, and 3) medium to large sized native beetle species that disperse by walking (P. algidus) that were rare except where significant amount of undisturbed native or semi native habitat was present. These groupings and assumptions need to be challenged in future studies.

Objective 3 - Participatory research and education programs

We completed the Willamette Valley Soil Quality Card and the accompanying Soil Quality Scorecard Guide, which was developed in collaboration with farmers. This was done with a series of focus sessions that included about 40 farmers. This work was done in cooperation with NRCS who used our experience in Oregon to develop a national program for developing local/farmer based scorecards (at least 10 other states have developed scorecards that used the focus session methodology we helped develop here at Oregon State University).

The Scorecard has been very popular and is nearly into it second printing after one year. This has been distributed widely and used extensively at farmer meetings and at courses at Oregon State University. Master gardeners are also using it to work with home gardeners.

On July 22, 1997 we held a field day entitled “Soil Quality and Cover Crops”, at the OSU Vegetable Research Farm, Corvallis OR (~95 participants). At this meeting we provided project results, demonstrated the soil quality kit and had participants use the soil quality card. We participated with a lecture and workshop in the “Soil Biology Workshop: An Introductory Mini-course for Growers, Ranchers, and Agricultural Professionals”, OSU Campus, Corvallis, OR on November 12-13, 1998 (~110 participants).

Over the life of the project we have had close contact with our cooperating farmers both in the field and with meetings. They have given informal input under field conditions and have guided the research. Each winter we held a dinner and meeting with the cooperating farmers where we discussed the results and got their interpretations of the data. We had 3 graduate students in the project and they learned a lot from farmers on the practical side of farming and how to present results that are understandable to farmers. This has been a learning experience for both scientists and farmers as we worked to set up the experimental splits, and began sampling and measuring soil properties and insect levels.

We have had extremely good cooperation with our cooperating farmers, despite their busy schedules. They have been a valuable asset of the project.