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

Abstract

Summary 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.

Effect of seed storage temperature on fine root development and mycorrhizal colonization of young Populus nigra seedlings

“…Stadelmann et al (1998) reported that low N concentration and the high C/N ratio could be attributed to the high mortality and poor development of root system in Festuca rubra seedlings obtained from in-vitro culture. Presence of vital cortical tissue is necessary for successful mycorrhizal colonization (Peterson et al 1999;Brundrett 2002), as colonization of root tissues by arbuscular mycorrhiza is related to the amount of primary cortex, and differences in root anatomy may impact absorptive ability (Guo et al 2008). However, our study did not show such an obvious dependence, as no differences in the number of arbusculae were observed in relation to seed storage temperature.…”

Mean age of carbon in fine roots from temperate forests and grasslands with different management

“…However, measurements of isotopes in fine roots demonstrated that both the 14 C age (Gaudinski et al, 2001) and the incorporation rate of a continuous 13 C label (Matamala et al, 2003) in fine root C were inconsistent with an annual turnover. The various observations can be reconciled by assuming that fine roots are not a single homogeneous pool (Gaudinski et al, 2010;Guo et al, 2008;Strand et al, 2008;Tierney and Fahey, 2002;Trumbore, 2009). To date, published fine root radiocarbon ( 14 C) data are mostly from forest ecosystems and little is known of how these observations apply to non-forested ecosystems, or a range of forest types (Fröberg, 2012;Gaudinski et al, 2010;Riley et al, 2009).…”

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“…Second, temperatures of smoldering fires could cause significant thermal damage to roots in the adjacent patches of forest floor not directly consumed by smoldering fire. Because roots are a connected network of segments, when structural roots near the tree are killed, all roots downstream are also lost (Guo et al 2008). This indirect fire-caused root mortality may include the death of downstream roots deep into the mineral soil as well.…”