INTERACTIVE EFFECTS OF ATMOSPHERIC CO₂AND SOIL-N AVAILABILITY ON FINE ROOTS OFPOPULUS TREMULOIDES

Abstract

The objective of this experiment was to understand how atmospheric carbon dioxide (CO 2) and soil-nitrogen (N) availability influence Populus tremuloides fine-root growth and morphology. Soil-N availability may limit the growth response of forests to elevated CO 2 and interact with atmospheric CO 2 to alter litter quality and ecosystem carbon (C) and N cycling. We established a CO 2 N factorial field experiment and grew six genotypes of P. tremuloides for 2.5 growing seasons in 20 large open-top chamber/root-box experimental units at the University of Michigan Biological Station in northern lower Michigan (USA). In this paper we describe an integrated examination of how atmospheric CO 2 and soil-N availability influence fine-root morphology, growth, mortality, and biomass. We also studied the relationship between root biomass and total soil respiration. Over 80% of the absorbing root length of P. tremuloides was accounted for by roots 0.4 mm in diameter, and specific root length (100-250 m/g) was much greater than reports for other temperate and boreal deciduous trees. Elevated atmospheric CO 2 increased the diameter and length of individual roots. In contrast, soil N had no effect on root morphology. Fine-root length production and mortality, measured with minirhizotrons, was controlled by the interaction between atmospheric CO 2 and soil N. Rates of root production and mortality were significantly greater at elevated CO 2 when trees grew in high-N soil, but there were no CO 2 effects at low soil N. Fine-root biomass increased 137-194% in high-N compared to low-N soil, and elevated atmospheric CO 2 increased fine-root biomass (52%) in high soil N, but differences in low soil N were not significant. Across all treatments, dynamic estimates of net fine-root production were highly correlated with fine-root biomass (soil cores; r 0.975). Mean rates of soil respiration were more than double in high-N compared to low-N soil, and elevated atmospheric CO 2 , when compared to ambient atmospheric CO 2 , increased mean rates of soil respiration 19% in 1995 and 25% in 1996. Across all treatments, total root biomass was linearly related to mean rates of soil respiration (r 2 0.96). Our results indicate that atmospheric CO 2 and soil-N availability strongly interact to influence P. tremuloides fine-root morphology, growth, and C turnover. Aspen-dominated ecosystems of the future are likely to have greater productivity fueled by greater nutrient uptake due to greater root length production. Further, it appears that elevated atmospheric CO 2 will result in greater C inputs to soil through greater rates of fine-root production and turnover, especially in high-fertility soils. Increased C inputs to soil result in greater rates of soil respiration. At this time, it is not clear what effects increased rates of root turnover will have on C storage in the soil.

Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO 2

“…The most common explanation is that significant increases in fine root production under elevated CO 2 allow trees to explore more of the soil volume for available N [''root exploration'' (22,(31)(32)(33)(34)(35)]. The underlying assumption for this model is that N is being mineralized in excess of microbial demand in the soil and that this supply of N is only available to trees growing under elevated CO 2 because of a more extensive fine root network.…”

Aboveground sink strength in forests controls the allocation of carbon below ground and its [CO 2 ]-induced enhancement

“…Fine root increment was measured by using minirhizotrons and in-growth cores at ORNL-FACE (49) and EUROFACE (50), estimated by using a flow compartment model at Duke FACE (51) and from standing fine root biomass at AspenFACE (45) combined with rates of aspen root turnover (52). ⌬(C litter ϩ C soil ) was estimated from published data [Duke FACE (30,31), ORNL-FACE (53), EUROFACE (29), and AspenFACE (54)].…”

Increase in fine root biomass enhances root exudation by long-term soil warming in a temperate forest

“…The responses of root exudation to long-term soil warming have not yet been studied in detail. In contrast to direct short-term effects of warming, long-term effects on root exudation may be more strongly related to indirect effects, e.g., by changing nutrient availability or soil moisture, which in turn can affect root morphology and physiology (Pregitzer et al, 2000;Li et al, 2019). Trees were shown to invest more C into root exudation under nutrient deficiency, such as of nitrogen (N) or phosphorus (P) (Ward et al, 2011;Yin et al, 2014;Meier et al, 2020).…”