A major management concern in low-input
and organic farming systems is the maintenance of soil fertility. Nitrogen
is supplied through crop residues, incorporated cover crops and
manures in low-input and organic farming systems; it is applied to the soil in the conventional farming system as mineral N. Large amounts of carbon are included in the material required to achieve adequate N-levels in organic farming systems, so that they are carbon-rich while conventional systems are carbon-starved as interpreted on the basis of microbial biomass-C, microbial biomass-N, and measures of microbial activity.
Nematodes may play a significant role in decomposition of soil organic matter and mineralization of plant nutrients. Many of the nematodes in biologically active and productive soils are not herbivores but are bacterial- and fungal-feeding species. Primary decomposition is accomplished by bacteria and fungi, which are grazed upon by Protista and microbivorous nematodes. There is substantial evidence of the importance of the nematodes in mineral and nutrient cycling (Griffiths, 1990; Hunt et al, 1987; Ingham et al, 1985). Since bacterial-feeding nematodes are purported to have a higher C:N ratio (* 10:1) than their substrate (* 5:1), considerable mineralization of N is associated with their metabolic activity (Anderson et al, 1981). In eating sufficient bacteria to provide the carbon necessary for their body structure and respiration, nematodes will take in more nitrogen than necessary. The excess nitrogen is excreted as ammonia (Lee and Atkinson, 1977; Rogers, 1989). Even if the reported C:N ratios of nematodes are incorrect and their body composition is closer to that of bacteria, as suggested by analyses of Persson (1983), the nitrogen consumed with carbon that is used in respiration (perhaps 40 % of the food intake (Marchant and Nicholas, 1974)), will be in excess of structural needs and will be excreted. However, microbivorous nematodes exhibit a wide range of metabolic rates and behavioral attributes. The contribution of individual species to nitrogen cycling and soil fertility may vary considerably.
Soil fertility, microbial biomass and bacterial-feeding nematodes In a field experiment at UC Davis in 1993, the available nitrate in the soil solution differed during the growing season in the conventional and organic tomato plots. In the conventional system there was abundant inorganic N detected in soil samples between April and mid-June, resulting from preplant and sidedress applications of mineral fertilizer. Those levels were probably in excess of plant needs, but available during key growth periods for the tomato plants. In the organic plots, the level of inorganic N was very low at tomato planting and remained low throughout the growing season. It was below the norms for conventional agriculture and, under those criteria, the soil would be diagnosed as nitrogen deficient for tomato production. Plants exhibited nitrogen stress in the organic tomatoes during the early part of the growing season. However, crop yields in the two systems were comparable in 1993.
Microbial biomass, measured as MBC, was significantly higher in the organic farming system than the conventional system throughout the tomato growing season. Following cover crop incorporation in spring in the organic farming system, MBC increased until the end of June. After mid-summer MBC decreased and declined to levels comparable with those in the conventional plots by the end of the growing season. In the conventional plots, MBC remained relatively constant throughout the growing season.
Total numbers of bacterial-feeding nematodes were similar in conventional and organic farming systems at the time of tomato planting. It was more than 30 days after cover crop incorporation before the bacterial-feeding nematodes in the organic plots became more abundant than those in the conventional system. Numbers in the conventional system remained low throughout the growing season, while those in the organic system declined from their high levels following decrease in microbial biomass at mid-summer. By the end of the growing season the numbers in the two farming systems were not significantly different.
Eleven taxa of bacterial-feeding nematodes, all in the order Rhabditida, were monitored during the course of this intensive sampling study. Those identified routinely to species were Acrobeloides bodenheimeri Thorne, A. buetschlii Steiner and Buhrer, A. tricornis Thorne, Acrobeles sp., Cephalobus persegnis Bastian and Chiloplacus sp. (all family Cephalobidae); Bursilla labiata Andrássy, Cruznema tripartitum Zullini, Diploscapter sp. and Rhabditis cucumeris Andrássy (all family Rhabditidae); and Panagrolaimus detritophagus Fuchs (family Panagrolaimidae).
Different taxa of bacterial-feeding nematodes predominated at different times during the growing season. In the organic plots, Panagrolaimus sp. and Rhabditis sp. were most abundant in the top 15 cm of soil early in the tomato growing season and within about 20 days of organic matter incorporation. Bursilla sp. became numerically predominant by the end of June in the organic plots, and remained at high population levels until the end of the tomato growing season. Chiloplacus sp. were more numerous in the conventional than the organic plots until near the end of the growing season.
The bacterial-feeding nematode community in the conventionally-farmed plots was more diverse than that in the organic plots through most of the growing season. That reflected the temporal predominance of individual opportunistic species in the organic plots in response to the incorporation of organic matter and the subsequent flush of bacteria.
After incorporation of vetch at the beginning of the growing season, Bongers' Maturity Index suggested a higher level of maturity, that is a trend towards persister (K-selected) species, across all nematode species and trophic groups in the conventional than in the organic farming system. Conversely, there was a preponderance of opportunistic colonizer species in the organic system. As with other indices, the data are strongly influenced by the diversity among the bacterial-feeding nematode species in the two farming systems. Actually, the maturity indices for the bacterial-feeding nematodes are based on only two maturity groups.
Nematodes in the families Rhabditidae (Bursilla sp., Cruznema sp. and Rhabditis sp.) and Panagrolaimidae (Panagrolaimus sp.) predominated in the organic system (Fig. 4A). These nematodes are all in maturity group 1, the most opportunistic colonizers (Bongers, 1990). In the conventional system nematodes in the family Cephalobidae (Acrobeloides spp., Acrobeles sp., Cephalobus sp. and Chiloplacus sp.) were more abundant. These nematodes are all in maturity group 2 and slightly less dynamic in their colonizing capabilities (Bongers, 1990).
Bursilla labiata, a small nematode abundant for much of the growing season in the organic farming system, had a maximum calculated biomass of 13.9 mg/l soil on June 21 at mid-season, many times larger than that of any other bacterial-feeding nematode in either farming system. Maximum biomass for all bacterial-feeding nematode species in both farming systems occurred at the June 21 sampling when it was 3.25 times greater in the organic (17.4 mg/l soil) than in the conventional (5.4 mg/l soil) system. At that date, the total number of bacterial-feeding nematodes counted in the organic system was only 2.25 times greater than in the conventional system. Although nematode numbers and biomass were similar at the time of tomato planting in both farming systems, both measures had become significantly greater in the organic system by May 10 and remained that way throughout the growing season.
Bacterial-feeding nematode biomass per
unit of MBC during the first half of the growing season increased in the organic
farming system, indicating that food availability was not a limiting constraint
during that period. After the mid-summer decline in MBC there was a
decline in nematode biomass per unit of MBC. That suggested that the
bacterial-feeding nematode community was then greater than the carrying capacity
of the environment and
declined correspondingly. In the conventional farming system the biomass
per unit of MBC remained relatively constant throughout the growing season,
suggesting that the two measures were in dynamic equilibrium and that food
supply was a constraint in
nematode population and biomass increase in that system.
Although population densities of
bacterial-feeding nematodes at the beginning of April (day 95) were similar in
the conventional and organic plots, individual species did not increase
opportunistically in the conventional system as in the carbon-fed
organic system. In the conventional systems, the organic matter incorporated into the soil is crop residue high in carbon content. The manure and leguminous cover crops incorporated into the low-input and organic plots have lower C:N ratios than the crop residues. It is possible that complex, high C:N ratio organic materials may select for fungal- rather than bacterial-dominated decomposition pathways. This may explain the greater numbers of fungal-feeding nematodes in the conventional plots early in the growing season. Similar observation have been made in the spring in other conventional and organic farming system comparisons (Freckman and Ettema, 1993) and in differentiating between perennial and annual cropping systems (Neher and Campbell, 1994).
Although the concept is the subject of ecological debate, the notion that diversity is an indicator of stability may provide some insights into the condition and resilience of soil systems. Diversity indices within a trophic group are of greatest interest since they reflect the level of redundancy of common function in the system. They are more likely to be true indicators of stability within a feeding category than indices constructed from unrelated organisms across trophic and functional levels. At any point in time there was less redundancy in common function among the bacterial-feeding nematodes of the organic farming system, perhaps suggesting that this component of the system would be readily perturbed by small disturbances.
Bongers' Maturity Index applied to
bacterial-feeding nematodes provided an intriguing hint that the abundance of
individuals in maturity group 1 (families
Diplogasteridae and others) may be indicators of a biologically active soil with
enhanced rates of N-mineralization. These nematodes may have similar
metabolic rates as species in the Cephalobidae (Ferris et al, 1994), but the
dynamics of their population increase allow them to become more abundant in a
shorter period of time. We infer that a large biomass of bacterial-feeding
nematodes feeding on bacteria, there will be more N mineralized than by a
smaller biomass with similar metabolic rates.
We further infer that at this field site, the 13.9 mg/l soil of Bursilla labiata biomass, of a total bacterial-feeding nematode biomass of 17.4 mg/l soil in the organic system, indicate that this nematode is the most significant contributor to N-mineralization. We conclude that correction for extraction efficiency and calculation of biomass are important components of the determination of relative contribution of various species.
Clearly bacterial-feeding nematodes are more abundant, represent greater biomass, and consume greater numbers of bacteria in the organic farming system than in the conventional farming system by about one month after tomatoes are planted. Soil nitrate levels throughout the growing season, and especially early, were low. They were never as great in the organic system as in the conventional system, particularly during key periods of nitrogen demand during vegetative growth and fruit set. Plants in the organic system displayed symptoms of nitrogen deficiency early in the growing season (Scow et al, 1994). If bacterial-feeding nematodes are important contributors to N-mineralization, that contribution is minimal during the first month of the growing season but may be more important during fruit set. It seems desirable to increase their abundance and activity in N-mineralization early in the growing season in organic plots to meet the needs of young seedlings. That may require altering community structure by introducing large-bodied, actively-feeding nematodes, or changing cultural operations to influence the abundance, biomass and activity of bacterial-feeding nematodes, early in the growing season.
Since metabolic rates of
bacterial-feeding nematodes from this field site are at extremely low levels at
the soil temperatures experienced during the winter months (Ferris et al, 1994),
and cumulative physiological time increases slowly early in the growing
season, it appears that an the appropriate strategy may be to maximize nematode
abundance at the end of the growing season. The most appropriate strategy
for increasing their springtime abundance is to ensure that there are large
population levels in the
soil the previous fall. At that time their food abundance may be a
limiting factor, although decline in numbers may also be influenced by abiotic
factors such as cessation of irrigation prior to harvest. A manure
application at termination of the previous summer crop in
the organic farming system, followed by irrigation, may enhance bacterial
abundance in the fall and so maximize nematode abundance at that time. If
overwinter survival is primarily mediated by abiotic (non density-dependent
factors), a greater abundance of
nematodes in the fall should result in a greater abundance in the spring. That would enhance the potential contribution of bacterial-feeding nematodes to N-mineralization during a critical plant growth period in the first month of the next summer crop.
The dynamics are such that spring incorporation of a winter cover crop to increase MBC is followed by too great a lag period in increase of the bacterial-feeding nematode community for effective impact on N-mineralization. The bacterial-feeding nematodes do not increase in the rhizospheres of the winter cover-crop because the cover-crop is planted too late and /or sown and left to germinate with winter rain. Soil temperatures by that time are below a level conducive to rapid reproduction of the nematodes. Early (mid-September) germination of the cover crop would promote biological activity and nematode increase through about mid-October. Since activity in the rhizosphere will be proportional to root size, which is initially zero, it will be enhanced by manure incorporation in September (as early as possible) and by following incorporated crop residue or incorporated manure with an irrigation to stimulate biological activity.
The abundance and biomass of individual
species of bacterial-feeding nematodes varied through the tomato growing
season. We infer that the contribution of the individual species to soil
fertility through N-mineralization differed with their temporal
dynamics. Species in maturity group 1 (Bongers, 1990) were most responsive to increase in their food source and, in particular, Bursilla labiata (family Rhabditidae) dominated the bacterial-feeding nematode community, both in abundance and biomass, for much of the growing season. We hypothesize that increasing the abundance, biomass and activity of maturity group 1 bacterial-feeding nematodes in the spring by organic matter incorporation at the end of the previous crop would reduce the observed nitrogen stress in organic tomatoes early in the growing season. We suggest that an abundance of maturity group 1 bacterial-feeding nematodes at the time of planting indicates a biologically-active soil in which the nitrogen provided by incorporation of organic material will not remain bound in the microbial biomass but will be mineralized through the grazing activities of the nematodes. Differences in abundance of fungal-feeding nematodes between conventional and organic plots suggest the decomposition of higher C:N ratio organic sources through fungal-dominated pathways in the conventional plots. The dynamics of the plant-parasitic nematode species reflected the crop sequences in rotations used in each system.
Predaceous nematode populations were low in both farming systems and may have had little impact on other nematode populations in the top 15 cm of soil.
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