Bacterial-feeding Nematodes and Soil Fertility

Four of every five
multicellular animals on the planet are nematodes. Many species are well known
as important and devastating parasites of humans, domestic animals and plants.
However, most species are not pests; they occupy any niche that provides an
available source of organic carbon in marine, freshwater and terrestrial
environments. There may be 50 different species of nematodes in a handful of
soil and millions of individuals can occupy 1 m2. Of the nematodes in
soil that do not feed on higher plants, some feed on fungi or bacteria; others
are carnivores or omnivores.
Nematodes respond rapidly
to disturbance and enrichment of their environment; increased microbial activity
in soil leads to changes in the proportion of opportunistic bacterial feeders in
a community. Over time the enrichment opportunists are followed by more general
opportunists which include fungal feeders and different genera of bacterial
feeders (Bongers and Ferris, 1999). This succession of nematode species plays a
significant role in decomposition of soil organic matter, mineralization of
plant nutrients and nutrient cycling (Ingham et al, 1985; Hunt et al, 1987;
Griffiths, 1990).
Bacterial-feeding
nematodes have a higher carbon:nitrogen (C:N) ratio (± 5.9) than their
substrate (± 4.1) 12 (Ferris et al., 1997), so that in consuming
bacteria they take in more N than necessary for their body structure. The excess
nitrogen is excreted as ammonia (Lee and Atkinson, 1977; Rogers, 1989). The C:N
ratio of fungal-feeding nematodes is closer to that of their food source (Chen
and Ferris, in press). However, for nematodes of both feeding habits, a
considerable proportion of the C consumed is used in respiration (perhaps 40 %
of the food intake (Ingham et al, 1985; Marchant and Nicholas, 1974)). The N
associated with respired C that is in excess of structural needs is also
excreted. The excreted N is available in the soil solution for uptake by plants
and by microbes. Because 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.
By developing C and N
budgets for individual nematode species, we calculated that the
bacterial-feeding nematode community in the top 15 cm of a field soil
mineralized N at rates increasing to 1.01 µg-N g-soil-1 d-1 in the rhizosphere
(Ferris et al., 1995, 1996, 1997). In microcosm experiments we determined
that rates of N mineralization by bacterial-feeding nematode species of
different body size ranged between 0.0012 and 0.0058 m g-N nematode-1
d-1, mainly as NH4+ (Ferris et al, 1998). The
amount of N as NH4+ released from microcosms in which
barley straw was colonized by the fungus Rhizoctonia sp. was always
greater in the presence of the fungal-feeding nematode Aphelenchus avenae
(Chen and Ferris, 1998).
In three years of field
experiments we have demonstrated that N-availability in organic and low-input
tomato production is enhanced by an abundance of microbial-feeding nematodes.
The challenge is to manage the system so that those nematodes are in abundance
at the start of the growing season. In our agricultural system we have shown
that the constraining factor is usually soil moisture availability in the late
summer and early fall of the previous year. By irrigating the soil at that time
we can increase microbial activity and decomposition of crop residues. That
provides substrate for the bacterial-feeding nematodes so that their population
levels increase and remain at high levels until the next spring. In some cases
the amount of C available to fuel the microbial community is also limiting and
it may be necessary to incorporate organic material. Necessary approaches will
vary with cropping system and climatic zone, but management of soil to enhance
activity of bacterial- and fungal-feeding nematodes can increase soil fertility.
Soil nematode communities
may also provide useful indicators of soil condition. 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. Application of nematode
faunal composition analysis provides information on succession and changes in
decomposition pathways in the soil food-web, nutrient status and soil fertility,
acidity, and the effects of soil contaminants (Bongers and Ferris, 1999).
References
Bongers T and Ferris, H. 1999. Nematode community structure as a bioindicator
in environmental monitoring. Trends in Evolution and Ecology (in press).
Soil fertility is a key determinant of the success of a farming system. Management systems that rely on organic inputs as sources of fertilizer
have different dynamics of soil nutrient availability than systems receiving mineral sources. These dynamics are, as of yet, not well understood,
especially in the Mediterranean irrigated systems found in the Central Valley of California.
Though biological processes control the availability of many soil nutrients, interest in the soil biological community has been overlooked
because high levels of mineral fertilizer overshadow or inhibit biological activities. Heavy applications of pesticides also may alter the soil
community and decrease rates of certain key processes, for example N mineralization. An understanding of soil biology is especially important in
cover crop systems where one relies on below-ground biota to liberate plant nutrients. Cover crops provide organic forms of C and N which are
quickly metabolized to inorganic nitrogen and other nutrients, primarily by bacteria and fungi. N is also mineralized as predators of bacteria and
fungi, such as protozoa and microbivorous ("microorganism eating") nematodes, graze on prey which contain more N than required by the
predators. Although more research attention is given to the plant parasitic nematodes, microbivorous organisms make up a large portion of the
nematode community. Excess N generated by grazing is released to the soil and becomes available for plant uptake. For optimal management of
cropping systems receiving organic inputs, it is important to understand what factors control the timing and release of nutrients by microbial
processes and how this relates to crop uptake requirements.
During the transition from a conventional to low input or organic management systems, there are many changes in the chemical and
biological properties of soil. The changes result in part from significantly higher inputs of carbon associated with organic forms matter and the
reduction or termination of pesticide use. Studies from other regions of the U.S. indicate that it takes several years of organic management for
previously conventional soils to begin to function successfully in terms of nutrient cycling. Protocols of the California Certified Organic Farmers
(CCOF) Association require a three year transition period of organic management before a farm can be certified as organic. Although this
requirement is driven primarily by the need to eliminate pesticide residues, this period may also allow time for the soil microbial community to adapt
to the higher rates of nutrient cycling required in an organic system. Limitation of crop growth due to nitrogen deficiency is a commonly
recognized problem during the transition period, but less is known about other nutrient limitations.
The transition period from conventional to organic farming can be problematic with regard to soil fertility. A new organic farmer cannot use
mineral fertilizers and pesticides, thus he or she may be more vulnerable to crop loss than the conventional farmer. This is especially true in the
transition period when the soil biota may not be able to support the rates of cover crop decomposition needed to provide sufficient soil fertility
in the early growing season. The low-input provides an alternative system, though lacking the economic incentives of an organic system, where some
mineral fertilizer amendments and pesticides can be used as needed. On-farm comparisons of organic and conventional tomato fields have shown
that organically-managed soils typically have lower inorganic N pools, higher microbial activity, and greater abundance an biomass of bacterial-
feeding nematodes than conventional.