Gross rates of nitrogen transformation in soils of a global biodiversity hotspot (Western Ghats, India)

Background:Being one of the important nutrients required by plants during photosynthesis, nitrogen (N) exerts a significant control on primary productivity and carbon dioxide sequestration. Therefore, it is essential to quantify the rates of N cycling processes, such as gross N mineralization and nitrification, to understand the flow and availability of N in different ecosystems. Lack of such data in tropical regions, particularly the Indian subcontinent, has constrained our understanding of N dynamics in soils of the subcontinent. Aims:Here, we report the results of gross N transformation rates from the hot and humid tropical forest soils of the Western Ghats, one of the global biodiversity hotspots located in India. Results:The experiments conducted using the(15)N isotope dilution technique showed higher average gross N mineralization (2.6 +/- 1.1 mg N kg(-1)d(-1)) and gross nitrification (1.7 +/- 1.1 mg N kg(-1)d(-1)) rates in the top soil layers (0-20 cm), which decreased significantly with depth (20-40 cm) indicating potential for higher microbial activity in the top soils. The overall consumption rate of mineral N exceeded its production rate with higher consumption of ammonium (NH4+) than nitrate (NO3-) suggestingNH4+as a preferred N source for microbes. The consumption ofNH4+was higher throughNH4+immobilization than nitrification indicating active N conservation mechanism and an efficient microbial adaptation. Conclusion:Despite high rates of consumption, accumulation ofNH4+andNO3-in soils of the Western Ghats suggested additional sources of mineral N other than internal recycling of N. Long residence time ofNH4+andNO3-in soils implied longer availability of unutilized mineral N leading to high soil acidity and potential for loss of N through leaching and denitrification.