NODULE CHARACTERISTICS OF THREE-YEAR-OLD CARAGANA MICROPHYLLA AND THEIR RESPONSES TO ENVIRONMENTAL CHANGES IN AN INNER MONGOLIA GRASSLAND

Abstract

Aims Caragana microphylla widely spreads in Inner Mongolia grasslands as a dominant shrubby legume. Its nodule growth has been poorly addressed. Our objective is to examine nodule characteristics and their responses to different environmental factors to provide insights into their nitro-gen fixing and nitrogen cycling processes under future changing environments. Methods We conducted a field experiment in 2008 using open-top chambers at the Inner Mongolia Grassland Ecosystem Research Station of Chinese Academy of Sciences. We examined the number,weight and length of nodules of three-year-old C. microphylla individuals and their responses to nitro-gen addition,drought,water addition and elevated atmospheric CO2 concentration. Important findings Nodules were mainly distributed in lateral roots,and most had buff or brown color. Nodule shapes were diverse,being pyriform,globular,claviform,fusiform,and Y shaped. Root growth and nodule growth were significantly inhibited under the nitrogen addition treatment,resulting in roots being whitened and most nodules dead and black-brown. Drought depressed root nodulation,with mean length of nodules decreased significantly by 50.4% compared with that under normal water supply. Water addition stimulated root growth and nodule growth. Roots nodulated best under the com-bined treatment of water addition,elevated CO2 and non-nitrogen addition,with maximum mean length and weight of individual nodules and the maximum nodule weight per plant. Nodules in fibers relatively increased with increasing water supply. Nodule growth did not significantly respond to elevated CO2. Nodule number is more sensitive to environmental changes than nodule size,there are significant nitro-gen × water interactions on nodule number and weight per plant and the effects of three environmental factors on nodule growth are different. The negative effect of nitrogen is larger than the positive effects of water and elevated CO2. It is inferred that water is the major factor affecting the infection of rhizobia and the generation of nodules in this semiarid grassland ecosystem.