Comparison of fungal and bacterial growth after alleviating induced N-limitation in soil

The extent and type of nutrient limitation will affect soil microorganism activity and may change the balance between organism groups, like fungi and bacteria. Limiting nutrients have traditionally been measured as increased respiration after adding nutrients, but this will not differentiate between fungal or bacterial responses. We compared respiration, bacterial and fungal growth after alleviating limitation in soils originally being C-limited, but experimentally altered to N-limitation. Three soils of similar pH and organic matter but with different N availability were used. We amended the soils with C-rich substrates, starch (40 mg g−1) and straw (80 mg g−1), followed by a 4 weeks incubation at 22 °C to induce N-limitation. Starch amendment resulted in increased respiration and bacterial growth, while straw amendment increased all three variables (respiration, bacterial and fungal growth), with only minor differences between soils. Alleviating C- and N-limitation was then tested in a short-term assay after adding C (glucose) and NH4NO3 in a full factorial design. In non-amended, C-limited soils, adding C resulted in increased respiration and especially bacterial growth, while fungal growth only increased in the High N soil. Straw amendment resulted in N-limitation, since adding N increased respiration and especially fungal growth. N-limitation for bacterial growth was evident in all starch amended soils, with similar effects for respiration, although adding C also increased respiration. Fungal growth was not affected by C- or N-additions in starch-amended soils. Thus, which microbial group that responded to alleviating N-limitation depended on the C-source in the soil. Furthermore, we found no indication of growth and respiration reacting differently to alleviating N-limitation indicating altered C-use efficiency.