Seasonal variation in soil nitrogen availability under Mongolian pine plantations at the Keerqin Sand Lands, China

The objectives of our research were to describe the seasonal patterns in soil inorganic N and microbial biomass N and to assess the rates of net N mineralization and net nitrification in 29-year-old Mongolian pine (Pinus sylvestris var. mongolica) stands in the south-eastern area of Keerqin Sand Lands, China. The concentrations of soil ammonium-N (NH4+-N), nitrate-N (NO3−-N), and inorganic N as well as the rates of net N mineralization and net nitrification clearly showed seasonal patterns of a bell shape from April to October. However, the seasonal patterns were less obvious for microbial biomass N. Higher NO3−-N concentrations were found in the unfenced, grazed stand and higher NH4+-N concentrations were found in the fenced, ungrazed stand. The rate of cumulative net N mineralization was 55.4 kg ha−1 year−1 in the fenced stand and 33.0 kg ha−1 year−1 in the unfenced stand. Of the annual net N mineralization in the fenced stand, only 33.2% was nitrified to NO3−-N, whereas in the unfenced stand, 62.2% was nitrified. Apparently, N availability components in Mongolian pine stands were significantly affected by the two different management options. Our results suggest that a deficiency in N availability to these Mongolian pine plantations happens in the early part of the annual growing season.