Spatial and temporal effects of nitrogen addition on root life span of Leymus chinensis in a typical steppe of Inner Mongolia

1. To understand root dynamics of the native, perennial clonal grass Leymus chinensis in Inner Mongolia steppe, we examined spatial and temporal effects of N addition on root survivorship and longevity traits of L. chinensis by studying responses of survivorship of roots distributed in three soil layers (0-10, 10-20 and 20-30 cm) and born in three seasons (spring, summer and autumn) to N addition with modified rhizotron technique during the growing seasons of 2004-05. Kaplan-Meier analysis was used to generate survival functions and to estimate mean root life spans. 2. Roots in the three soil layers displayed similar life spans with mean overall value of 81 days in the control plots. N addition reduced life span of roots in the soil layers of 0-10 cm and 20-30 cm, but it had no effect on life span of roots in the soil layer of 10-20 cm. 3. Roots born in the three seasons differed in their life spans with the life span values of 21-58, 60-153 and 101-151 days for those roots born in spring, summer and autumn, respectively. N addition markedly reduced life span of roots born in summer in the three soil layers with the greatest reduction occurring in roots present in soil layer of 20-30 cm, but it had marginal effect on life span of the spring-born and autumn-born roots. 4. The differential sensitivity of survivorship and longevity of L. chinensis roots occurred in different soil depths and born in different seasons to N addition highlights the importance of spatial and temporal characteristics of roots in response to soil N availability in Inner Mongolia steppe.