Objective The paper is aimed to explore the variations in soil nitrogen pools along an altitude gradient and its coupling relationship with environmental factors in the Wenshan Nature Reserve, Yunnan Province of southwestern China, and provide scientific basis and references for the assessment of soil quality in the reserve.
Method Three typical vegetation types were selected to investigate the variation of soil nitrogen components along the altitude gradient in Wenshan Nature Reserve. The association of soil nitrogen components to environmental factors was analyzed through principal component analysis.
Result (1) Total nitrogen, hydrolyzable nitrogen, ammonium nitrogen and nitrate nitrogen increased significantly along an altitude gradient (P < 0.05), with the maximum occurred in mid-mountain humid evergreen broadleaved forest. In particular, total nitrogen and hydrolyzable nitrogen increased by 1.17 g/kg and 142.05 mg/kg, respectively. (2) The ratio of soil nitrogen components to total nitrogen pool varied with altitudes. Compared with the subtropical monsoon evergreen broadleaved forest at the lowest altitude, the ratio of hydrolyzable nitrogen and nitrate nitrogen decreased by 9.1% and 0.26%, while that of ammonium nitrogen increased by 0.61% in mid-mountain humid evergreen broadleaved forest at the highest altitude. (3) The environmental factors changed significantly along an altitude gradient. Along the altitude gradient, litter thickness, soil moisture and nutrients (i.e. total organic carbon, available phosphorus and available potassium) increased, while plant diversity and soil temperature decreased. (4) Principal component analysis showed that the increases in litter thickness and soil total organic carbon along the altitude gradient were the main factors determining the accumulation of total nitrogen, while the reduction of plant diversity and soil bulk density contributed mainly to increase in total nitrogen. The increase of soil moisture along the altitude gradient was the main factors for accumulating inorganic nitrogen, whereas the change of soil available phosphorus and potassium had main contribution to accumulating hydrolyzable nitrogen.
Conclusion Soil nitrogen is sensitive to the changes of environment along an altitude gradient in Wenshan Nature Reserve. Altitude gradient mainly affects litter input, microclimate (hydrothermal conditions), soil firmness (soil bulk density) and soil nutrients (e.g. total organic carbon, available phosphorus and available potassium), and then indirectly or directly regulates the changes of nitrogen composition.