[Four hypotheses about the effects of soil nitrogen availability on fine root production and turnover]

With global changes such as increasing temperature and enhanced N deposition, soil nitrogen (N) availability is predicted to increase substantially, and how fine root dynamics responds to the altered soil N has become one of the key questions in terrestrial ecology. As such, a number of hypotheses have been proposed to explain the relationship between increasing soil N availability and fine root production, mortality, and turnover. This article considered four major hypotheses: with increasing soil N availability, 1) both fine root production and turnover rate would increase, 2) both fine root production and turnover rate would decrease, 3) fine root production would decrease while fine root turnover rate would increase, and 4) fine root production would increase while fine root turnover rate would decrease. Current evidence suggests that the patterns depicted in hypothesis 1) and 2) could both occur in nature and may reflect characteristics of different species. Hypotheses 3) and 4) were thought to characterize only a transient stage of the responses of fine root dynamics to increasing N availability. To better understand the response of root dynamics to increasing soil N, future studies should consider: 1) the definition of fine roots and heterogeneity in fine root structure and function; 2) methods used in estimating fine root production and turnover rate; 3) changes of soil N availability both in space and time. More attention should also be paid to the influences of mycorrhizal infection on root dynamic responses to soil N availability.