Bacterial growth and growth-limiting nutrients following chronic nitrogen additions to a hardwood forest soil

Increasing nitrogen (N) deposition due to anthropogenic activities has become a significant global change threat to N-poor terrestrial ecosystems. We compared bacterial growth and nutrients limiting bacterial growth in one of the longest running experiments on increasing N-deposition to a temperate forest, the Chronic Nitrogen Amendment Study at Harvard Forest, USA. Soil samples were collected in fall 2009 from the organic and mineral horizons of plots treated annually since 1988 with 0 (unfertilized), 50 (low N) or 150 (high N) kg N ha−1 as NH4NO3. In the organic horizon, bacterial growth (leucine incorporation) decreased by 5 times in the high N plots compared to the unfertilized treatment, while no decrease was observed in the mineral horizon. Bacterial growth in all soils was primarily limited by lack of carbon (C), although adding only C (as glucose) resulted in only a minor increase in bacterial growth in the unfertilized soil compared to adding C in combination with N. The bacterial growth induced by adding only C increased with higher level of N fertilization, up to 7–8 times the level without any C addition in the high N treatment, suggesting increased availability of N for the bacteria with increasing N addition.