3 results
Soil carbon may be maintained under grazing in a St Lawrence Estuary tidal marsh
- O.T. YU, G.L. CHMURA
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- Journal:
- Environmental Conservation / Volume 36 / Issue 4 / December 2009
- Published online by Cambridge University Press:
- 26 March 2010, pp. 312-320
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Production of belowground organic matter is critical to sustainability of salt marshes. It plays a role in vertical soil accretion, a process essential for salt marshes to maintain their relative elevation and persist as sea levels rise. This paper examines belowground production and soil carbon of a high-latitude saltmarsh in the St Lawrence Estuary (Québec, Canada), which had been subjected to six years of sheep grazing. In the seventh year, without sheep, organic matter production in grazed and ungrazed sections was assessed by examining harvests of plant litter, end-of-season standing crop, and the roots and rhizomes present in in-growth cores. Excepting salinity, porewater chemistry varied little. The grazed marsh had higher soil carbon density and belowground production, yet lower aboveground biomass. Grazing reduces plant litter and increases solar exposure, soil temperature (at this latitude, soil remained frozen until April) and evapotranspiration, thus raising soil salinity and nitrogen demand, the latter a driver of root production. Grazing may not be detrimental to soil carbon storage. Permitting certain types of grazing on restored salt marshes previously drained for agriculture would provide economic incentive to restore tidal flooding, because the natural carbon sink provided in the recovered marsh would make these lands eligible for carbon payments.
Global patterns of root turnover for terrestrial ecosystems
- RICHARD A. GILL, ROBERT B. JACKSON
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- Journal:
- The New Phytologist / Volume 147 / Issue 1 / July 2000
- Published online by Cambridge University Press:
- 01 July 2000, pp. 13-31
- Print publication:
- July 2000
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Root turnover is a critical component of ecosystem nutrient dynamics and carbon sequestration and is also an important sink for plant primary productivity. We tested global controls on root turnover across climatic gradients and for plant functional groups by using a database of 190 published studies. Root turnover rates increased exponentially with mean annual temperature for fine roots of grasslands (r2 = 0.48) and forests (r2 = 0.17) and for total root biomass in shrublands (r2 = 0.55). On the basis of the best-fit exponential model, the Q10 for root turnover was 1.4 for forest small diameter roots (5 mm or less), 1.6 for grassland fine roots, and 1.9 for shrublands. Surprisingly, after accounting for temperature, there was no such global relationship between precipitation and root turnover. The slowest average turnover rates were observed for entire tree root systems (10% annually), followed by 34% for shrubland total roots, 53% for grassland fine roots, 55% for wetland fine roots, and 56% for forest fine roots. Root turnover decreased from tropical to high-latitude systems for all plant functional groups. To test whether global relationships can be used to predict interannual variability in root turnover, we evaluated 14 yr of published root turnover data from a shortgrass steppe site in northeastern Colorado, USA. At this site there was no correlation between interannual variability in mean annual temperature and root turnover. Rather, turnover was positively correlated with the ratio of growing season precipitation and maximum monthly temperature (r2 = 0.61). We conclude that there are global patterns in rates of root turnover between plant groups and across climatic gradients but that these patterns cannot always be used for the successful prediction of the relationship of root turnover to climate change at a particular site.
Dynamics of root systems in native grasslands: effects of elevated atmospheric CO2
- J. A. ARNONE, J. G. ZALLER, E. M. SPEHN, P. A. NIKLAUS, C. E. WELLS, C. KÖRNER
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- Journal:
- The New Phytologist / Volume 147 / Issue 1 / July 2000
- Published online by Cambridge University Press:
- 01 July 2000, pp. 73-85
- Print publication:
- July 2000
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The objectives of this paper were to review the literature on the responses of root systems to elevated CO2 in intact, native grassland ecosystems, and to present the results from a 2-yr study of root production and mortality in an intact calcareous grassland in Switzerland. Previous work in intact native grassland systems has revealed that significant stimulation of the size of root systems (biomass, length density or root number) is not a universal response to elevated CO2. Of the 12 studies reviewed, seven showed little or no change in root-system size under elevated CO2, while five showed marked increases (average increase 38%). Insufficient data are available on the effects of elevated CO2 on root production, mortality and life span to allow generalization about effects. The diversity of experimental techniques employed in these native grassland studies also makes generalization difficult. In the present study, root production and mortality were monitored in situ in a species-rich calcareous grassland community using minirhizotrons in order to test the hypothesis that an increase in these two measures would help explain the increase in net ecosystem CO2 uptake (net ecosystem exchange) previously observed under elevated CO2 at this site (600 vs 350 μl CO2 l−1; eight 1.2-m2 experimental plots per CO2 level using the screen-aided CO2 control method). However, results from the first 2 yr showed no difference in overall root production or mortality in the top 18 cm of soil, where 80–90% of the roots occur. Elevated CO2 was associated with an upward shift in root length density: under elevated CO2 a greater proportion of roots were found in the upper 0–6-cm soil layer, and a lower proportion of roots in the lower 12–18 cm, than under ambient CO2. Elevated CO2 was also associated with an increase in root survival probability (RSP; e.g. for roots still alive 280 d after they were produced under ambient CO2, RSP = 0.30; elevated CO2, RSP = 0.56) and an increase (48%) in median root life span in the deepest (12–18 cm) soil layer. The factors driving changes in root distribution and longevity with depth under elevated CO2 were not clear, but might have been related to increases in soil moisture under elevated CO2 interacting with vertical patterns in soil temperatures. Thus extra CO2 taken up in this grassland ecosystem during the growing season under elevated CO2 could not be explained by changes in root production and mortality. However, C and nutrient cycling might be shifted closer to the soil surface, which could potentially have a substantial effect on the activities of soil heterotrophic organisms as CO2 levels rise.