Allocation of fine root biomass and its response to nitrogen deposition in poplar plantations with different stand ages

Nitrogen deposition is a hot topic issue of global change. Understanding the response patterns and feedback mechanisms of forest ecosystems associated with this continued,increased nitrogen deposition and rapid nitrogen cycle is vital for the maintenance of forest ecosystem health and productivity. Increasing nitrogen deposition may alter the distribution structure of fine roots, such as vertical distribution and diameter size distribution formed in the long-term evolution process. We conducted an experiment to examine the allocation of fine root biomass and its response to nitrogen deposition in poplar plantations with different stand ages in the coastal plain of Dongtai,Jiangsu. The randomized block design was used with five nitrogen deposition concentration treatments,i. e. N0( 0 g N·m- 2·a- 1),N1( 5 g N·m- 2·a- 1),N2( 10 g N·m- 2· a- 1),N3( 15 g N·m- 2·a- 1),and N4( 30 g N·m- 2·a- 1) in the field. The results showed that:( 1) The fine root biomass was obviously enriched in the surface soil,with 70%- 80% of fine root biomass allocated to the 0- 20 cm soil layer. With the increase of exogenous nitrogen, the proportion of fine root biomass at the 0- 10 cm soil layer increased in the young plantation( 4 years old),while reduced in the mid-aged and mature plantations( 8 and 15 years old,respectively) to some extent.( 2) Fine root biomass was mainly distributed in the diameter class of 0-0. 5 and 0. 5- 1. 0 mm,and about 50% of the fine root biomass( 2. 0 mm) was allocated in the diameter class of 0- 0. 5 mm. Exogenous nitrogen increased the proportion of very fine root biomass( 0- 0. 5 mm),especially in the young plantation.( 3) In the 30- 40 cm soil layer, fine root biomass was allocated with a greater proportion in 0- 0. 5 mm diameter class in the mature plantation than in the young and mid-aged plantations,which showed that small fine roots had a decreasing trend with the increase age.( 4) Four factors( plantation age,soil layer,diameter class,and nitrogen concentration) in combination explained 66. 3% of variation of fine root biomass. Plantation age,soil layer,and diameter class respectively explained 17. 6%,16. 1%,and 10. 4% of variation,with significant effects( P 0. 01),and the increase of nitrogen deposition only explained 0. 24% of variation of fine root biomass,with no significant effect( P 0. 05).