The effects of elevated [CO2] on plant-soil carbon below-ground: A summary and synthesis

Abstract

We undertake a synthesis of the most relevant results from the presentations at the meeting "Plant-Soil Carbon Below-Ground: The Effects of Elevated CO2" (Oxford-UK, September 1995), many of which are published in this Special Issue. Below-ground responses to elevated [CO2] are important because the capacity of soils for long-term carbon sequestration. We draw the following conclusions: (i) several ecosystems exposed to elevated [CO2] showed sustained increased CO2 uptake at the plot level for many years. A few systems, however, showed complete down-regulation of net CO2 uptake after several years of elevated [CO2] exposure; (ii) under elevated [CO2], a greater proportion of fixed carbon is generally allocated below-ground, potentially increasing the capacity of below-ground sinks; and (iii) some of the increased capacity of these sinks may lead to increased long-term soil carbon sequestration, although strong evidence is still lacking. We highlight the need for more soil studies to be undertaken within ongoing ecosystem-level experiments, and suggest that while some key experiments already established should be maintained to allow long term effects and feedbacks to take place, more research effort should be directed to mechanisms of soil organic matter stabilization.

Spatial and temporal deployment of crop roots in CO2‐enriched environments

“…In natural systems, finite resource availability might constrain the long-term flux of C into plant and soil systems, but for agricultural systems in which nutrients and water are often less growth-limiting, constraints will be less stringent (Berntson & Bazzaz, 1996 ;Canadell et al, 1996).…”

“…For several crop species, up to 40% of net C fixed is allocated below ground (Gorissen, 1996 ;Gregory et al, 1996), and the proportion of total plant photosynthate allocated to roots tends to increase in plants growing in elevated [CO # ] (Canadell et al, 1996). For example, Rattray et al (1995) reported that the proportion of "%C translocated below ground almost doubled, from 18% at ambient [CO # ] to 34% under elevated [CO # ].…”

“…Uncertainties regarding the transfer of C to soil via root exudation notwithstanding, still less is known about the transfer of C to soil via fine-root turnover, especially for crops. The periodic replacement of finer root elements might represent an REVIEW Crop root growth in CO # -enriched environments 59 even greater energy demand than maintenance respiration, exudation, C transfer to symbionts, and belowground predation (Caldwell, 1977 ;Canadell et al, 1996). Recently, the use of minirhizotrons to measure the dynamics of root initiation, growth, and death, and the C tracers "%C and "$C have proved to be powerful tools for determining not only how much C is transported below ground, but how C is being partitioned between different belowground C pools (Meharg, 1994 ;Cheng & Johnson, 1998).…”

“…However, indirect effects, mediated by shifts in plant water and nutrient relations, are likely. Experimental evidence shows that root turnover increases more often than not (Pregitzer et al, 1995 ;Canadell et al, 1996). Canadell et al (1996) summarized several studies in which root turnover was measured.…”

“…Experimental evidence shows that root turnover increases more often than not (Pregitzer et al, 1995 ;Canadell et al, 1996). Canadell et al (1996) summarized several studies in which root turnover was measured. In four experiments with grass systems, increased root turnover occurred in three, and in one did not change.…”