Role of Soil Microbial Biomass and Microbivorous Nematodes in Functioning of Sustainable Agriculture Systems

1992 Annual Report for AW92-007

Project Type: Research and Education
Funds awarded in 1992: $0.00
Projected End Date: 12/31/1994
Matching Non-Federal Funds: $118,319.00
ACE Funds: $156,731.00
Region: Western
State: California
Principal Investigator:
Kate Scow
University of California, Dept. of Land, Air, and Water Resources

Role of Soil Microbial Biomass and Microbivorous Nematodes in Functioning of Sustainable Agriculture Systems


1. Compare microbial biomass/activity and nematode numbers and diversity in organic, low-input and conventional farming systems.
2. Determine the relationship between microbial and nematode parameters over the growing season, particularly at times of organic inputs;
3. Determine the relationship between the rate of cover crop decomposition and nitrogen availability and microbial and nematode measurements;
4. Compare the ability of soils from low-input and organic versus conventional farming systems to withstand stresses associated with agronomic practices.

The premise of this study is that sustainable farming practices need to address the below-ground, as well as above-ground, components of an agroecosystem. Thus objectives were to develop an understanding of the structure and function of microbial and nematode communities associated with different farming management systems, to determine whether differences in soil biology are manifested in farming system properties, and to explore whether soil biology can be managed to improve farming practices. The study was carried out at the Sustainable Farming Systems (SAFS) project at UC Davis comparing two- and four-year rotations (including tomatoes, safflower, corn, wheat/beans), managed using conventional, low-input or organic practices.

Major findings include the discoveries that:

Microbial populations are higher in size and activity in organic and low-input in comparison to conventional farming systems. Bacterial and fungal populations are greater in organic than conventional systems; however, fungi constitute a minor portion of the total microbial community in all systems. Bacterial-feeding nematodes are present at higher densities in organic than conventional soils, while plant parasitic nematodes are at a lower numbers in organic systems. Bacterial-feeding nematodes are believed to play major roles in the fertility of farming systems relying on soil biological processes to provide soil fertility. A few species of bacterial-feeding nematodes dominate following incorporation of organic matter and thus overall diversity is lower in organic and low-input than conventional systems.

In organic amended systems, microbial populations rapidly respond to the addition of organic inputs, steadily increase until the mid growing season and then decline. Associated with increases in microbial populations are usually small increases in soil inorganic nitrogen. The dynamics of bacterial-feeding nematodes closely mirror microbial population dynamics. Those nematode species most successful at exploiting the microbial food sources in field plots are also important in stimulating nitrogen mineralization in laboratory studies. In organic corn and tomatoes, composted poultry manure appears unable to support crop demands for nitrogen for most years. Fertility from a combination of cover crops with mineral fertilizer in the low-input plots result s in corn and tomato yields that are equivalent to or higher than in conventional treatments. The cover crop amendments appear to have prevented the degeneration of soil structure and reduction in water infiltration rates observed in the conventional soils.

Some of the difference between farming systems in soil fertility and structure are related to difference in soil biology. Microbial biomass is positively related to significantly higher water stable aggregation in organic compared to conventional soils. Soil nitrate levels are significantly lower in organic than conventional tomato soils and are inversely correlated with microbial biomass levels in the organic systems. High microbial biomass and activity, and high numbers of bacterial-feeding nematodes grazing on and releasing nitrogen from the biomass, may provide sufficient nitrogen to crops without excessive accumulation of nitrate in soil.

Certain biological parameters are substantially enhanced in organic compared to conventional soils; however, the conventional soil is still very active biologically. Cover crops added to conventional soil decomposed at rates equivalent to those added to organic soils, suggesting that a long transition period is not necessary before soils can rapidly break down cover crops. Also, there was no difference in the carbon source utilization pattern of conventional and organic tomato soils.

Several approaches were developed to measure the impact of pollutant and environmental stresses on soil biology. Respiration of soil communities was severely impacted by the fumigant metam sodium at concentrations lower than typical usage rates, and recovery from exposure was slow. An aromatic pollutant associated with petroleum (toluene), however had little impact on respiration, nitrogen mineralization, denitrification, and bacterial population density. The pollutant, however, strongly reduced nitrification rates and nitrifier populations, as well as impacted carbon source utilization patterns.

Site Information
The experiment was conducted on Yolo Silt Loam, a medium to heavy soil. The climate is Mediterranean with average summer day temperatures of 90 F. The majority of rainfall is between December and March with a yearly average of 25 inches. Irrigation water is above-average in quality and low in nitrate and minerals. The soils are fairly representative of the Sacramento valley, as are the crops grown in the rotation.

Potential Contributions
One of the most evident benefits of this project is the development of new information on the management of farming systems receiving organic forms of fertilizer, particularly with respect to the importance of soil biota in crucial processes. Our research suggests that higher carbon to nitrogen ratios than previously thought acceptable may be beneficial in providing sufficient plant fertility while at the same time reducing soil nitrate levels. The outcome of this three-year study will help pin the development of practical guidelines for organic fertilizer amendments.

Another important finding is that conventionally managed agricultural soils respond quickly to organic inputs and thus may rapidly develop large and metabolically active microbial populations in the presence of such inputs. We are continuing to assess the role of bacterial-feeding nematodes in nutrient cycling and identifying the most important species. We are currently evaluating whether stimulation of microbial and nematode activity in the fall can lead to better soil fertility in the early part of the growing season in organic farming systems. The results of these studies may support the development of guidelines on effectively managing soil communities to promote soil fertility.

Reported in 1996.