Microbial growth, biomass, community structure and nutrient limitation in high pH and salinity soils from Pravaranagar (India)

pH, salinity and nutrient conditions are major determinants of microbial biomass, activity and community composition; all being hypothesized to favour bacterial over fungal activity. Soils from Pravaranagar (India), having high nutrient content and high pH (pHw 7.4-8.8), with sometimes increased salinity, were thus expected to have high bacterial/fungal ratios. Twelve soils were characterized for microbial growth, biomass, community structure and nutrient limitation. The phospholipid fatty acid (PLFA) pattern was typical for high pH soils, with relative high amounts of several unsaturated PLFAs, like 18:ω7, and relatively low in e.g. cy19:0. The adaptation to high pH was also seen in the bacterial community tolerance to pH, with optimum pH for growth around pH 7.5. The high pH had resulted in soils with high bacterial but low fungal growth. However, adding substrate conducive for fungal growth, like straw, could induce fungal growth. Some soils had high electric conductivity, indicating salinization. The bacterial community had developed increased tolerance to NaCl in these soils. These soils also differed in the PLFA pattern, suggesting that saline soils had more fungal biomass. In all soils bacterial growth was limited by lack of carbon, but secondary limitation due to nitrogen was also found, while phosphorus addition did not affect growth. The high nutrient condition was evident in more than 5-fold increases in bacterial growth in some soils when adding only C. We could thus show that in these soils, high pH, salinity and nutrient conditions all had affected soil microbial activity and community structure. Although the high pH favoured bacteria, this was, however, not found for high salinity soils.