H. Ferris

A framework for soil food web diagnostics : extension of the nematode faunal analysis concept

Nematodes, the earth’s most abundant metazoa, are ubiquitous in the soil environment. They are sufficiently large to be identifiable by light microscopy and sufficiently small to inhabit water films surrounding soil particles. They aggregate around and in food sources. They include component taxa of the soil food web at several trophic levels. They can be categorized into functional guilds whose members respond similarly to food web enrichment and to environmental perturbation and recovery. Indices derived through nematode faunal analysis provide bioindicators for disturbance of the soil environment and condition of the soil food web. We enhance the resolution of faunal analyses by providing a weighting system for the indicator importance of the presence and abundance of each functional guild in relation to enrichment and structure of the food web. Graphical representations of food web structure, based on nematode faunal analyses, allow diagnostic interpretation of its condition. Simple ratios of the weighted abundance of representatives of specific functional guilds provide useful indicators of food web structure, enrichment, and decomposition channels.

Nematode community structure as a bioindicator in environmental monitoring

Four of every five multicellular animals on the planet are nematodes. They occupy any niche that provides an available source of organic carbon in marine, freshwater and terrestrial environments. Nematodes vary in sensitivity to pollutants and environmental disturbance. Recent development of indices that integrate the responses of different taxa and trophic groups to perturbation provides a powerful basis for analysis of faunal assemblages in soil as in situ environmental assessment systems.

Structural and functional succession in the nematode fauna of a soil food web

Soil microplots were amended with organic materials of varying nature and complexity but providing similar amounts of carbon. Materials were either placed on the soil surface or incorporated. Unamended and mineral fertilizer control plots were established. Plots were maintained vegetation-free so that the food web activity was fueled by resident soil organic matter and the input material. Enrichment-opportunist bacterivore nematodes increased rapidly in response to low C/N plant materials and, to a lesser extent, to more complex materials. General-opportunist bacterivores increased in all plots, but at a slower rate. Fungivore nematodes also increased gradually in all plots but most rapidly in those amended with higher C/N and more complex materials. Indices derived from nematode faunal analysis suggested a constant rate of succession from enrichment-opportunist to general-opportunist bacterivore guilds across all treatments, probably mediated by bacterial abundance and differences in life course characteristics of the respective taxa. The rate of succession from bacterivore to fungivore nematodes was greatest in plots receiving high C/N materials. Succession to fungivory, presumably indicating a shift from bacterial to fungal decomposition channels, was slowest in those plots with a high level of organismal metabolic activity, as measured by soil respiration. The cumulative amounts of N mineralized in the plots were directly related to the enrichment index (EI), based on the abundance of opportunistic bacterial- and fungal-feeding nematodes. The amounts of mineralized N were inversely related to the slope of the channel index (CI), that is, the rate at which decomposition changed from bacterial to fungal. Maintenance of adequate soil fertility in systems driven by organic input may require maintenance of food web structure and function as indicated by high levels of enrichment-opportunist bacterivore nematodes. That will require frequent supply of labile organic sources. Allowing food web succession to guilds that indicate lower mineralization potential will result in lower levels of soil fertility.

Arbuscular mycorrhizas, microbial communities, nutrient availability, and soil aggregates in organic tomato production

Effects of arbuscular mycorrhzal (AM) fungi on plant growth and nutrition are well-known, but their effects on the wider soil biota are less clear. This is in part due to difficulties with establishing appropriate non-mycorrhizal controls in the field. Here we present results of a field experiment using a new approach to overcome this problem. A previously well-characterized mycorrhizal defective tomato mutant (rmc) and its mycorrhizal wildtype progenitor (76R MYC+) were grown at an organic fresh market tomato farm (Yolo County, CA). At the time of planting, root in-growth cores amended with different levels of N and P, were installed between experimental plants to study localized effects of mycorrhizal and non-mycorrhizal tomato roots on soil ecology. Whilst fruit yield and vegetative production of the two genotypes were very similar at harvest, there were large positive effects of colonization of roots by AM fungi on plant nutrient contents, especially P and Zn. The presence of roots colonized by AM fungi also resulted in improved aggregate stability by increasing the fraction of small macroaggregates, but only when N was added. Effects on the wider soil community including nematodes, fungal biomass as indicated by ergosterol, microbial biomass C, and phospholipid fatty acid (PLFA) profiles were less pronounced. Taken together, these data show that AM fungi provide important ecosystem functions in terms of plant nutrition and aggregate stability, but that a change in this one functional group had only a small effect on the wider soil biota. This indicates a high degree of stability in soil communities of this organic farm.

Nitrogen, weeds and water as yield-limiting factors in conventional, low-input, and organic tomato systems

The importance of nitrogen (N), weeds, and water as yield-limiting factors was evaluated over a 4-year period in tomato cropping systems under conventional, low-input, and organic management. The cropping systems studied were part of the Sustainable Agriculture Farming Systems (SAFS) Project at the University of California, Davis, a comparison of conventional and alternative farming systems in California’s Sacramento Valley. Water applied, soil N levels, plant N uptake, weed abundance, and tomato yield were measured and compared among treatments. Tomato yields ranged from just under 55 to over 90 t ha 1 and significant treatment differences were observed in 2 of the 4 years. Multivariate analyses, used to sort out the effects of N, weeds, and water, indicated all three factors influenced yields in this study but their relative importance was dependent upon the management system. Results indicated that N availability was most important in limiting yields in the organic system and water availability was more important under conventional management. Although weed abundance was relatively high in the organic system in 2 years of the study, weed competition for N was not evident. Instead, relative N input levels and N immobilization by soil microflora appeared to explain N uptake and tomato yield variation. The findings indicate that organic and low-input tomato systems in this region can produce yields similar to those of conventional systems but that the factors limiting yield may be more difficult to manage. ©1999 Elsevier Science B.V. All rights reserved.

Nitrogen mineralization by bacterial-feeding nematodes: verification and measurement

Bacterial feeding nematodes excrete N assimilated in excess of that required for growth. Because metabolic and developmental rates differ among nematode species, we hypothesized that their contribution to N mineralization in soil would differ. Sand-column microcosms amended with an organic substrate, bacteria, and with or without bacterial-feeding nematodes, were leached at 3-d intervals. Cumulative N, as NH4+ or NO3-, leached from columns containing nematodes was consistently greater than from columns without nematodes. Maximum N-mineralization rates for populations of rhabditid nematodes, which predominated in field soils early in the summer were at lower temperatures than those for cephalobid nematodes, which predominated later in the summer. For an organic substrate with C-to-N ratio of 11:1, rates of N mineralization among species of different body size were similar, ranging between 0.0012 and 0.0058 μg-N nematode-1 d-1, mainly as NH4+. Smaller nematodes mineralized more N per unit of body weight than larger nematodes. We hypothesized that at low C-to-N ratios of the organic substrate, bacterial growth is C-limited and N-immobilization will be minimal; at high C-to-N ratios bacterial growth will be N-limited and there may be rapid immobilization of newly-mineralized N. Consequently, net N mineralization in the presence of nematodes will be lower when the organic substrate has a high C-to-N ratio. In experiments with different nematode species, net mineralization and the nematode contribution to mineralization generally decreased with increasing C-to-N ratio, consistent with the hypothesis; however, there were exceptions.

Effects of long-term crop management on nematode trophic levels other than plant feeders disappear after 1 year of disruptive soil management

Nematode community analysis may provide a useful tool to quantify soil health. Nematode communities were monitored for 5 years during a 12-year period in the sustainable agriculture farming systems (SAFS) project at UC Davis, where conventional (CONV), low-input (LOW) and organic (ORG) management treatments were compared. After the completion of three 4-year crop rotation cycles, a uniform crop of oats was grown in 2001. The composition of the nematode genera was different from year to year, but there were significant management effects on genus composition in each year, with the CONV treatment being significantly different from the LOW and ORG treatments. Important contributors to the differences in genus composition among treatments were plant parasitic nematodes. Nematode community indices (enrichment (EI), basal (BI) and channel (CI) indices) of the CONV treatment differed from those of the ORG and LOW treatments in 1993, 1994, 1995 and 2000, but not in 2001. The difference in structure index (SI) among management treatments was significant in 1995 and 2000. EI and SI were generally lower, and BI and CI higher in CONV than in LOW and ORG treatments. There were significant crop effects on the community indices throughout the years. Even in 2001, there was a residual effect of the crop grown in 2000 on most nematode community indices. Differences in EI, BI and CI among crops were consistent, while those in SI were not. Meloidogyne javanica(Treub) Chitwood, juveniles added to various soil samples were reduced by 68% in soil where nematode trapping fungi had been added and which had low BI (12) and low CI (20) values. Soil from SAFS plots with a high BI (47) and high CI (70) after 1 year of oats and ploughing, suppressed root knot juveniles much less. There were significant negative correlations between BI and root knot nematode (RKN) suppression ( −0.72) and between CI and RKN suppression (−0.74). Thus, BI and CI appeared to be most valuable as indicators for long-term effects of management on nematode suppression. However, BI and SI may be more suitable as general indicators for the health status of a soil, since CI can be high in highly disturbed agro-ecosystems as well as in undisturbed natural ecosystems. A high BI would indicate poor ecosystem health, while a high SI would indicate a well-regulated, healthy ecosystem. For agricultural soils the presence of large populations of plant parasitic nematodes forms an additional indication of poor ecosystem health, as natural regulation is limited in this case.

Population energetics of bacterial-feeding nematodes: carbon and nitrogen budgets

Summary-Ibacterial-feeding nematodes participate in nitrogen mineralization in decomposition food webs to an extent determined by metabolic and behavioral attributes, by life history, and by the relative C-to-N ratios of the nematodes and their bacterial prey. The mean C-to-N ratio for eight nematode species cultured on Escherichia coli on agar was 5.89 (range 5.16-6.83). The mean C-to-N ratio was similar, although with greater variability among species, when nematodes were cultured in soil on a range of soil bacteria. The mean C-to-N ratio of five isolates of soil bacteria and E. coli was 4.12 (range 3.65-4.92). Where food was not limiting, production-to-assimilation ratios ranged from 0.58 to 0.86 and respiration-to-assimilation ratios from 0.14 to 0.42. The excess N assimilated during growth and egg production, and the excess N assimilated to meet the C needs of respiration, were similar across species at 20°C. The excess N from both sources provides an estimate of the amount of N excreted by each nematode during the life course. The weight of bacteria necessary to meet the growth and respiration costs at 20°C ranged from 0.87 pgpg-nematode -’ d-’ for Cephalobus persegnis to 1.99 pg pg-nematode-’ d-* for Bursilla labiata, providing estimated consumption rates between 6.61 x 10’ and 15.22 x 10’ bacterial cells pg-nematode-’ d-‘. At a field site, we estimate that the bacterial-feedin f nematode community in the top 15 cm soil mineralized N at rates increasing to 1.01 pg-N g-soil-’ din rhizospheir soil. On a monthly basis, the community contributed 0.28 kg-N ha-’ in April, 0.98 kgN ha-’ in Mly and 1.38 kg-N ha-’ in June in bulk soil. Contributions in the rhizosphere would be considerably greater depending on the ratio of rhizosphere to bulk soil. The contribution of individual species to N mineralization in the rhizosphere varied through the first 3 months of the summer growing season as a function of their abundance and their metabolic and development rates in relation to temperature. Rhabditis cucumeris was the predominant contributor in April; there were similar contributions by Acrobeloides bodenheimeri, B. labiata, Cruznema tripartitum, and R. cucumeris in May; A. bodenheimeri and B. labiata were the major contributors in June. 0 1997 Elsevier Science Ltd

Agronomic, economic, and environmental comparison of pest management in conventional and alternative tomato and corn systems in northern California

The effectiveness, economic efficiency, and environmental impact of pest management practices was compared in conventional, low-input, and organic processing tomato and field corn systems in northern California. Pests, including arthropods, weeds, pathogens, and nematodes, were monitored over an 8-year period. Although both crops responded agronomically to the production-system treatments, arthropods, pathogens, and nematodes were found to play a relatively small role in influencing yields. In contrast, weed abundance was negatively correlated with tomato and corn yields and appeared to partially account for lower yields in the alternative systems compared to the conventional systems. Lower pesticide use in the organic and low-input systems resulted in considerably less potential environmental impact but the economic feasibility of reducing pesticide use differed dramatically between the two crops. The performances of the organic and low-input systems indicate that pesticide use could be reduced by 50% or more in corn with little or no yield reduction. Furthermore, the substitution of mechanical cultivation for herbicide applications in corn could reduce pest management costs. By contrast, pesticide reductions in tomato would be economically costly due to the dependence on hand hoeing as a substitution for herbicides. Based on the performance of the low-input and organic tomato systems, a 50% pesticide reduction would increase average pest management costs by 50%.

The effects of nematode grazing on nitrogen mineralization during fungal decomposition of organic matter

Abstract Soil fungi and nematodes isolated from a long-term Sustainable Agriculture Farming Systems (SAFS) project research site at the University of California, Davis were investigated in microcosm systems. Nitrogen-free sand in the columns was amended with ground alfalfa and cellulose, with total N held constant, to create C-to-N ratios of 11:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1 and 45:1. Nitrogen mineralization by Aphelenchus avenae and Aphelenchoides composticola feeding on Rhizoctonia solani and Trichoderma sp. was determined by measuring ammonium and nitrate concentrations in the leachate from the columns at 3-d intervals. Nematode population numbers and the fatty acid 18:2 ω6c, a fungal biomass indicator, were monitored by destructive sampling on d 0, 7, 14 and 21. For R. solani, but not Trichoderma sp., there was significantly more N extracted from columns in the presence of either nematode species than in the absence of nematodes. Average N-mineralized nematode−1 d−1 was 1.8 ng for A. avenae and 3.3 ng for A. composticola when feeding on R. solani. As the C-to-N ratios of organic substrates increased, total mineral N decreased with R. solani alone, but in general remained the same in the presence of nematodes. Initial and average nematode population densities were significantly higher in columns containing R. solani than in those with Trichoderma sp. Both nematode species reduced the fungal fatty acid 18:2 ω6c in Trichoderma columns on d 21. The fatty acid 18:2 ω6c was lower in columns containing both R. solani and A. composticola on d 0 and 7 and higher on d 14 and 21 than those in the absence of nematodes.