Root length, biomass, tissue chemistry and mycorrhizal colonization following 14 years of CO2 enrichment and 6 years of N fertilization in a warm temperate forest

Abstract

Root systems serve important roles in carbon (C) storage and resource acquisition required for the increased photosynthesis expected in CO2-enriched atmospheres. For these reasons, understanding the changes in size, distribution and tissue chemistry of roots is central to predicting the ability of forests to capture anthropogenic CO2. We sampled 8000 cm(3) soil monoliths in a pine forest exposed to 14 years of free-air-CO2-enrichment and 6 years of nitrogen (N) fertilization to determine changes in root length, biomass, tissue C : N and mycorrhizal colonization. CO2 fumigation led to greater root length (98%) in unfertilized plots, but root biomass increases under elevated CO2 were only found for roots <1 mm in diameter in unfertilized plots (59%). Neither fine root [C] nor [N] was significantly affected by increased CO2. There was significantly less root biomass in N-fertilized plots (19%), but fine root [N] and [C] both increased under N fertilization (29 and 2%, respectively). Mycorrhizal root tip biomass responded positively to CO2 fumigation in unfertilized plots, but was unaffected by CO2 under N fertilization. Changes in fine root [N] and [C] call for further study of the effects of N fertilization on fine root function. Here, we show that the stimulation of pine roots by elevated CO2 persisted after 14 years of fumigation, and that trees did not rely exclusively on increased mycorrhizal associations to acquire greater amounts of required N in CO2-enriched plots. Stimulation of root systems by CO2 enrichment was seen primarily for fine root length rather than biomass. This observation indicates that studies measuring only biomass might overlook shifts in root systems that better reflect treatment effects on the potential for soil resource uptake. These results suggest an increase in fine root exploration as a primary means for acquiring additional soil resources under elevated CO2.

Contrasting effects of ectomycorrhizal and arbuscular mycorrhizal tropical tree species on soil nitrogen cycling: the potential mechanisms and corresponding adaptive strategies

“…6b). These results indicate that root length density rather than fine root biomass may be the primary metric representing root function in N uptake, as suggested by other studies (Eissenstat 1992, Taylor et al 2014.…”

“…In addition to mining N from SOM via rhizosphere effects, plants are also suggested to scavenge N via increasing investment in root construction to expand exploited soil volume under N limitation (Jackson et al 2008, Taylor et al 2014. In this study, ECM trees were found to have higher soil exploration relative to AM trees in terms of root length density (Table 2), which was negatively correlated with soil N availability (Fig.…”

Production dynamics of Cenococcum geophilum ectomycorrhizas in response to long-term elevated CO2 and N fertilization

“…These changes may also have been associated with reductions in belowground allocation to fine roots and mycorrhizal fungi in general. The magnitude of the fertilization effect observed for C. geophilum ectomycorrhizas was substantially larger than that observed for fine-root biomass (~12% reduction) and for all other ectomycorrhizas (~20% reduction) but was similar in magnitude for the reduction in EM extramatrical mycelia (57% reduction) previously reported at our study site (Jackson et al, 2009;Pritchard et al, 2014;Taylor et al, 2014;Ekblad et al, 2016). This suggests that production of C. geophilum ectomycorrhizas was more sensitive to changes in site fertility than other aspects of belowground production ostensibly associated with soil resource acquisition.…”

Short‐term carbon cycling responses of a mature eucalypt woodland to gradual stepwise enrichment of atmospheric CO2 concentration

“…Atmospheric CO 2 enrichment is likely to increase ecosystem C inputs from photosynthesis, at least in the short term, but whether this leads to an increase in ecosystem C storage is dependent on many processes. In some temperate forest experiments, the additional C fixed under eCO 2 was used to stimulate additional soil exploration by fine roots and nutrient extraction from soil organic matter (SOM) decomposition, such that soil nitrogen (N) uptake increased under eCO 2 , providing a positive feedback to growth Finzi et al, 2007;Drake et al, 2011;Talhelm et al, 2014;Taylor et al, 2014). In contrast, a whole-tree chamber experiment in a strongly N-limited boreal forest showed no response of tree growth to eCO 2 in the absence of N-fertilization despite sustained increases in canopy C uptake; this additional C was returned to the atmosphere through increased respiration, particularly in soils (Comstedt et al, 2011;Hall et al, 2013;Sigurdsson et al, 2013).…”