The effects of land use on soil N mineralization during the growing season on the northern Loess Plateau of China

The mineralization and availability of soil nitrogen (N) are vital processes in the sustainable management of soil resources and ecosystems. We studied the mineralization and availability of soil N on the northern Loess Plateau of China during the growing season under three different land use conditions: Chinese Pine woods, grassland, and Korshinsk Peashrub land. Mineral N varied significantly during the growing season and was always significantly higher at soil depths of 0-10 cm soils than at depths of 10-20 cm. NO(3)(-)-N predominated shrublands, while NH(4)(+)-N was dominant in grassland in both the 0-10 cm and 10-20 cm soil layers. In Pine woods, NO(3)(-)-N dominated the 0-10 cm soil layer, while NH(4)(+)-N dominated in the 10-20 cm soil layers. In shrublands, the 0-10 cm and 10-20 cm soil layers showed nitrification rates of 1.6-3.6 and 1.0-3.5 times higher than those in the corresponding soil layers of grassland, respectively. However, the ammonification rate in shrublands did not differ from that in grasslands. Ammonification and nitrification dominated the N mineralization process at the beginning and middle of the growing season, respectively, while both processes were balanced at the end of the growing period. The cumulative nitrified, ammonified and total mineralized N in shrublands were 178%, 27%, and 116% and 161%, 29%, and 93% higher in the 0-10 cm and 10-20 cm soil layers than those in grasslands and 203%, 97%, and 165% and 235%, 69%, and 151% higher than those in pine woods, respectively. The conversion of grasslands to pine woods reduced N availability at soil depths of 0-20 cm, but the conversion of grasslands to shrublands increased N availability. The uptake of soil N by vegetation was close to (in pine woods) or less than (in grasslands and shrublands) the mineralized N in the 0-20 cm soil layer during the period of vegetation growth. Overall, the results indicated that there was abundant N availability; therefore we conclude that N availability is not likely to limit ecosystem productivity in either of these three land use patterns on the northern Loess Plateau. (C) 2010 Elsevier B.V. All rights reserved