Soil Biology

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Scanning electron micrographs of nematodes, fungi, and mites. (Photo by Mara Johnson.)

Soil biology is a major area of investigation at Russell Ranch; topics include linking ecosystem functions to microbial diversity and community composition, and investigations of how management impacts soil food webs.

Researchers have found that nematodes, fungal and bacterial populations are influenced by farming systems and tillage (Minoshima, 2007). Microbial biomass was higher under no-till, continuous cropping compared to standard tillage, fallow systems. However, nematode populations were lowest in no-till, fallow systems, compared to the continuous cropping systems. The accumulation of plant residue on the no-till, continuous cropping surface led to higher soil carbon, when compared to the standard tillage, fallow system. The lack of increase in nematode populations makes researchers suspect that higher trophic levels of nematode communities have disappeared, which reduce the efficiency of nutrient cycling. Organic management increases microbial biomass and leads to shifts in microbial community composition when compared to properties under conventional management.

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Microbial biomass carbon (MBC) for three different depths in soil and for No-Till, Continuous Cropping (NTCC), No-Till, Fallow (NTF), Standard Till, Continuous Cropping (STCC) and Standard Till, Fallow (STF). Nematode populations for the four systems are shown as well (Minoshima, 2007).


Other experiments focused on developing methods of quantifying ammonia-oxidizing bacteria. Using data from Russell Ranch, we developed a novel quantitative PCR method to determine numbers of bacteria in soil responsible for converting ammonium to nitrite (nitrifiers) (Okano et al. 2004). We found that the population density of nitrifiers was related to the amount of ammonium added to soil and that conversion of ammonium fertilizer to nitrate occurred in consort with the increase of nitrifiers.

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Ammonium, nitrate concentrations and ammonium oxiding bacteria (AOB) populations were measured after ammonium was applied to Russell Ranch field soils. The solid line represents plots with ammonium treatment and the dashed line represents plots with no ammonium (Okano et al., 2004).