Managing the soil food web in legume-vegetable rotations
by Howard Ferris, Louise E. Jackson, Hideomi Minoshima, Jeffrey P. Mitchell, Sara Sanchez Moreno, Kate M. Scow, Steven R. Temple

These SAFS project researchers study the changes in soil biology that occur as a result of farming practices. Achieving a functioning soil community following a history of conventional agricultural practices may require a prolonged transition. Here researchers describe the importance of soil food webs in alternative farming systems and explore some approaches to enhancing their activity.

Introduction – Structure, Functions and Importance of Soil Food Webs

The soil food web is that community of organisms that utilize one another, either by predation or consumption of dead bodies, as sources of carbon and energy. The activities of soil organisms result in ecological functions essential to crop production and soil fertility (see box). By consuming, digesting, assimilating, and metabolizing the bodies of their food sources, organisms convert complex organic molecules into forms suitable for their own structural and metabolic needs. Materials indigestible to the consumer are eliminated in simpler forms that are more accessible to other organisms. Some of the molecules that are digested may be in excess of the consumer’s needs and are excreted in mineral forms that are readily available to plants and to other soil organisms. Molecules taken up by bacteria and passed on to their consumers are considered to be in the “bacterial decomposition channel” (Fig. 1). Many of the organisms in this channel are metabolically very active and molecules pass through the bacterial channel rapidly. Materials decomposed and digested by fungi are often more complex and their flow through the “fungal decomposition channel (Fig. 2)” may be slower.

Carbon and energy obtained by consumers at the entry level of the food web are utilized for growth, reproduction and respiration. Carbon dioxide lost from the soil due to respiration of organisms represents a net loss in resources to the consumers of those organisms, that is, the next trophic (feeding) level. The loss of carbon at each trophic level limits the abundance of predators (Fig. 3) that can be supported by any group of prey. Predators, which may regulate or even suppress pest species, are usually larger organisms. Their environment is easily destroyed by physical disturbance of the soil; they are slow to recover from toxic or environmental perturbations, their life cycles are longer and their reproductive potential lower than opportunistic organisms at the entry level of the food web.

Tillage mixes organic matter into the soil so that products of its decomposition are available to plant roots. However, the disturbance disrupts the higher trophic levels of the food web. Managing the delivery of resources into the food web without disturbing its structure in the process, and at the same time optimizing crop growth, may be the greatest challenge of conservation tillage systems.

A Food Web Management Experiment: Rationale and Approach

We hypothesized that continuous inputs of plant-derived carbon and nitrogen, combined with conservation tillage (CT), should promote soil communities that decompose residue and result in a more complex multi-layered food web than in systems with periodic fallow and standard tillage (ST). The potential for lower yields in crops grown with CT than ST must be evaluated against the benefits of storing carbon in the soil and the functions of a complex food web. At the Long-Term Research for Agricultural Systems (LTRAS) facility at UC Davis, we compared four cropping systems:

• Conservation tillage and continuous crop rotation (CTCC);
• Conservation tillage and fallow rotation (CTF);
• Standard tillage and continuous crop rotation (STCC);
• Standard tillage and fallow rotation (STF).

The continuous crop (CC) rotation had greater plant biomass, more crop cycles, more continuous plant cover, and more crop diversity than the fallow rotation (F) which included a fall and summer fallow (Table 1).

Although this was a very successful experiment, we experienced some farming problems with the CTCC systems; the sudan/sorghum cover crop had an inhibitory effect on stand establishment of the garbanzos where there was a thick residue on the soil. Herbicides used immediately after planting may have impacted garbanzo growth since a herbicide-free control plot had higher biomass. Garbanzos in the CT plots were difficult to harvest because the high density of weeds interfered with cutting the dried stalks. Cowpeas were originally intended as a cash crop, but since planting was delayed due to late harvest of garbanzos, the cowpeas became a cover crop. Water infiltration during the summer irrigation of cowpeas was uneven in the CT treatments due to accumulation of crop residues.

Soil Carbon, Nitrogen, Soil Food Webs: Though total soil C did not increase after one year of CTCC cropping, increases in total microbial biomass, fungi, and total nematodes were evident in the surface layer, compared to ST, or CTF. Total soil C (g/m2 at 0-30 cm) was similar in CTCC, CTF and STCC treatments, and higher than in STF (Fig. 4).

Nitrate did not differ significantly among treatments but was much higher in June 2004 than on other sampling dates, and higher at 0-5 cm than deeper in the soil, possibly due to the dry summer conditions that minimized leaching.

The soil microbial biomass (MBC), a good indicator of soil C availability, was significantly higher at 0-5 cm than below 5 cm. MBC was greatest in the CT plots in which microbes accumulated in the surface layer presumably due to easy access to residue on the soil surface (Fig. 5).

Nematodes accumulated in the soil surface layer in all treatments, especially in CT plots (Fig. 6). The Enrichment Index (EI), which indicates the biomass of opportunistic fungal- and bacterialfeeding nematodes that respond rapidly to increases in food resources, was higher in ST plots, especially STCC plots. The Channel Index (CI), an indicator of the decomposition pathway by bacteria or fungi, was higher in CT than ST plots, and much lower in STCC than either CT plot, indicating greater activity in fungal decomposition pathways in CT plots. The EI and CI levels suggest that lack of disturbance by tillage leads to favorable habitats for fungi and that disturbance by tillage, with readily available food due to continuous cropping, leads to more bacteria.

In summary, we explored options for replacing the typical tomato/wheat fallow rotation of the Sacramento Valley of California with alternative crops, lower inputs of non-renewable resources, and increased C sequestration. Conservation tillage with continuous crop rotations of tomato and legumes resulted in lower yields and similar C storage at 0-30 cm compared to standard tillage with continuous cropping, or conservation tillage with periodic fallow. Conservation tillage with continuous C input at the soil surface led to a habitat favorable for microbial biomass, fungi, and nematodes. However, conservation tillage will require innovative management solutions to reduce problems such as high weed biomass and uneven water availability during furrow irrigation.

This research was supported by grants from the Kearney Foundation of Soil Science and CalFed Ecosystem Restoration Program.

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