Nutrient Limitation of Bacterial Growth in Soil

Lack of carbon has been assumed to be the most common limiting factor for bacterial growth in soil, although there are reports of limitation by nitrogen and phosphorus. I have studied nutrient limitation of bacterial growth in soil using a novel technique, where thymidine or leucine incorporation is used to detect changes in growth rates after adding nutrients. I have studied which nutrient(s) limited growth rates of bacteria in 28 Swedish soils. I found that bacterial growth in most of the soils was limited by a lack of carbon, and in some soils adding both carbon and nitrogen increased the bacterial growth rates even further. In three soils bacterial growth was concurrently limited by a lack of carbon as well as nitrogen. The effect of nitrogen fertilization on microorganisms was studied in three coniferous forest soils. Most indicators of microbial biomass and activity decreased in fertilized soils. Bacterial growth was limited by a lack of carbon, irrespective of fertilization treatment. However, nitrogen fertilization made the carbon limitation more pronounced. Long-term manipulations simulating different aspects of global climate change were studied in a subarctic tundra heath. Fertilization changed the nutrient that limited bacterial growth from concurrent limitations to carbon limitation. In warmed soils no significant change in bacterial growth limitation was observed. This indicated that warming did not increase mineralization sufficiently to change the limiting factors for bacterial growth. Effect of adding easily available and more complex carbon sources, with and without nitrogen, on fungal and bacterial growth and activity in soil was studied in the laboratory. There were indications of synergistic effects in that bacterial growth only increased after the fungi had started to grow, which occurred after adding cellulose. Treatments resulting in high bacterial growth rates led to decreased fungal growth rates compared with the control, indicating antagonistic effects of bacteria. Adding nitrogen did not affect the bacteria's limitation by a lack of carbon. Adding carbon resulted in the bacteria being nitrogen limited in the short-term, although in the long-term neither carbon nor nitrogen added singly affected bacterial growth rates. My results have shown that the determination of limiting nutrients is a valuable indicator of the microbial status in soil. This easy and rapid technique makes it suitable to include in soil microbial characterisation along with standard biomass and activity measurements