[Gradient distribution of soil nitrogen and its response to climate change along the Northeast China Transect].
Terrestrial transect is an important and effective method for global change study. The Northeast China Transect (NECT), which is assigned along the latitude 43 degrees 30'N in the mid-latitude of temperate zone and located at 112 degrees-130 degrees 30'E and 42-46 degrees N, is one of the fifteen global transects recognized by IGBP. It is about 1600 km in length and 300 km in width. The NECT is mainly driven by precipitation, and becomes an effective platform of global change study in China. Based on the field survey in 2001 and a simulated experiment, this paper analyzed the gradient distribution of soil nitrogen and its response to climate change along the Transect. The results indicated that soil total and available nitrogen in NECT were significantly related to longitude, with a correlation coefficient being 0.695 (P < 0.001) and 0. 636 (P < 0.001), respectively, and had a similar horizontal distribution with soil organic carbon. Soil available nitrogen content in the NECT was decreased from east to west, and could be one of factors restricting plant growth. The decreasing rate of soil total and available nitrogen from topsoil to subsoil was different with ecosystems along the NECT. Soil total and available nitrogen contents had a close linear relationship with soil pH, total and labile carbon, total and available phosphorus, total sulphur, total and available zinc, available potassium, available manganese, bulk density, water holding capacity, and total porosity. They also had a significant linear relationship with precipitation, the correlation coefficient being 0.682 (P < 0.001) and 0.688 (P < 0.001), respectively. The short-term simulated experiment showed that doubled ambient CO2 concentration and soil moisture regime had no significant effects on soil total and available nitrogen, the variation coefficients being 5.55% and 3.84%, respectively.