Nitrogen transformation rates in the Himalayan soils at different temperature and elevation conditions

Abstract

Vast expanses of climatically sensitive ecosystems, such as tropical montane, remain unstudied in terms of factors controlling nitrogen (N) availability and in turn productivity. Temperature and elevation are two important, yet understudied, factors which decide the fate of N in montane ecosystems. Therefore, the present study aimed to quantify and understand changes in rates of N transformation processes in tropical montane soils of the Himalayas under different temperature-elevation conditions. The 15N isotope dilution technique was employed to quantify the rates of gross N transformations in soils collected at two depth intervals (0–20 cm and 20–40 cm) from five elevations (3000 m, 2500 m, 2000 m, 1500 m, and 1000 m above sea level) of the Garhwal Himalayas. The rates were measured at low (10 °C) and high temperature (23 °C) conditions to decipher the effect of temperature change on N cycling in montane soils. The results indicated a significant increase in gross N mineralization rates under the high temperature condition compared to the low. Interestingly, gross nitrification rates remained unaffected of temperature change. Increase in gross N mineralization at high temperature condition was accompanied with increased ammonium (NH4+) consumption, which was largely in the form of NH4+ immobilization. In general, gross N transformation rates in the top soils showed higher response to change in temperature and elevation conditions than the bottom soils. Increase in rates of N mineralization and other soil N cycling processes at higher temperature suggests potential acceleration in N turnover due to warming in the Himalayan soils. Increased NH4+ immobilization and poor sensitivity of nitrification to temperature are likely to increase soil N conservation at higher temperature. The inconsistent variation in N transformation rates with elevation suggested higher control of edaphic factors on soil N cycling at a particular elevation.