Endogenous and exogenous controls of root life span, mortality and nitrogen flux in a longleaf pine forest: Root branch order predominates

Abstract

1. Root life span regulates the quantity and quality of root-derived organic matter transferred to the soil organic matter pool. However, poor understanding of the rates and controls of root life span has hindered the prediction of carbon (C) flow and nutrient cycling dynamics at the ecosystem scale. 2. We examined the effects of root branch order, root diameter, mycorrhizal colonization, season of birth, depth in the soil, nitrogen (N) fertilization and foliage removal on root life span in a longleaf pine (Pinus palustris Mill.) forest from 2001 to 2004 using minirhizotron and soil monolith sampling. 3. Among all factors, root branch order had the strongest and most consistent effect on life span, with higher order roots having a 46% longer life span than roots one order lower. 4. Within first order roots, mycorrhizal colonization significantly increased root life span by > 45% in 2 of 3 years. 5. Roots born in winter and spring generally lived longer than roots born in summer and autumn. Root life span was positively correlated with depth in the soil and root diameter, but the correlations were weaker than with order, year and season. Neither N fertilization nor foliage removal had a significant impact on root life span. 6. When biomass mortality and associated N flux were estimated based on order-specific mean life span, N concentration and ecosystem-scale biomass estimates, first order roots constituted approximately 50% of the total biomass mortality and > 60% of the N flux for the first three root orders combined. 7. Synthesis. Our results show that (i) root branch order was the strongest predictor of life span among all covariates and can effectively partition the distal longleaf pine root systems into three or more populations with different turnover rates; (ii) only a fraction of fine roots turns over annually, whereas models of C cycles assume an annual turnover for the entire fine root system. We conclude that an order-based approach holds greater promise than the traditional diameter class approach for evaluating the role of different fine root populations in C flow and nutrient cycling. © 2008 The Authors.