The third group of bacteria is the pathogens . Bacterial pathogens include Xymomonas and Erwinia species, and species of Agrobacterium that cause gall formation in plants. A fourth group, called lithotrophs or chemoautotrophs , obtains its energy from compounds of nitrogen, sulfur, iron or hydrogen instead of from carbon compounds. Some of these species are important to nitrogen cycling and degradation of pollutants. Bacteria from all four groups perform important services related to water dynamics, nutrient cycling, and disease suppression.
Stable aggregates improve water infiltration and the soil s water-holding ability.
In a diverse bacterial community, many organisms will compete with disease-causing organisms in roots and on aboveground surfaces of plants. When leaves or roots from the host plant decompose, soil nitrogen increases in the surrounding area. Nitrate is leached more easily from the soil, so some farmers use nitrification inhibitors to reduce the activity of one type of nitrifying bacteria. Nitrifying bacteria are suppressed in forest soils, so that most of the nitrogen remains as ammonium. Denitrifiers are anaerobic, meaning they are active where oxygen is absent, such as in saturated soils or inside soil aggregates.