Plant and Soil

Gifford, Roger; Barrett, Damian; Lutze, Jason

doi: 10.1023/A:1004790612630pmid: N/A

The influence of elevated CO 2 concentration ((CO 2 )) during plant growth on the carbon:nutrient ratios of tissues depends in part on the time and space scales considered. Most evidence relates to individual plants examined over weeks to just a few years. The C:N ratio of live tissues is found to increase, decrease or remain the same under elevated (CO 2 ). On average it increases by about 15% under a doubled (CO 2 ). A testable hypothesis is proposed to explain why it increases in some situations and decreases in others. It includes the notion that only in the intermediate range of N-availability will C:N of live tissues increase under elevated (CO 2 ). Five hypotheses to explain the mechanism of such increase in C:N are discussed; none of these options explains all the published results. Where elevated (CO 2 ) did increase the C:N of green leaves, that response was not necessarily expressed as a higher C:N of senesced leaves. An hypothesis is explored to explain the observed range in the degree of propogation of a CO 2 effect on live tissues through to the litter derived from them. Data on C:P ratios under elevated (CO 2 ) are sparse and also variable. They do not yet suggest a generalising-hypothesis of responses. Although, unlike for C:N, there is no theoretical expectation that C:P of plants would increase under elevated (CO 2 ), the average trend in the data is of such an increase. The processes determining the C:P response to elevated (CO 2 ) seem to be largely independent of those for C:N. Research to advance the topic should be structured to examine the components of the hypotheses to explain effects on C:N. This involves experiments in which plants are grown over the full range of N and of P availability from extreme limitation to beyond saturation. Measurements need to: distinguish structural from non-structural dry matter; organic from inorganic forms of the nutrient in the tissues; involve all parts of the plant to evaluate nutrient and C allocation changes with treatments; determine resorption factors during tissue senescence; and be made with cognisance of the temporal and spatial aspects of the phenomena involved.

Norby, Richard; Long, Tammy; Hartz-Rubin, Jennifer; O'Neill, Elizabeth

doi: 10.1023/A:1004629231766pmid: N/A

The prediction that litter quality, and hence litter decomposition rates, would be reduced when plants are grown in a CO 2 -enriched atmosphere has been based on the observation that foliar N concentrations usually are lower in elevated (CO 2 ). The implicit assumption is that the N concentration in leaf litter reflects the N concentration in green leaves. Here we evaluate that assumption by exploring whether the process of seasonal nutrient resorption is different in CO 2 -enriched plants. Nitrogen resorption was studied in two species of maple trees ( Acer rubrum L. and A. saccharum Marsh.), which were planted in unfertilized soil and grown in open-top chambers with ambient or elevated (CO 2 ) in combination with ambient or elevated temperature. In the second growing season, prior to autumn senescence, individual leaves were collected and analyzed for N and dry matter content. Other leaves at the same and an adjacent node were collected for analysis as they senesced and abscised. This data set was augmented with litter samples from the first growing season and with green leaves and leaf litter collected from white oak ( Quercus alba L.) saplings grown in ambient and elevated (CO 2 ) in open-top chambers. In chambers maintained at ambient temperature, CO 2 enrichment reduced green leaf N concentrations by 25% in A. rubrum and 19% in A. saccharum . CO 2 enrichment did not significantly reduce resorption efficiency so the N concentration also was reduced in litter. There were, however, few effects of (CO 2 ) on N dynamics in these leaves; differences in N concentration usually were the result of increased dry matter content of leaves. The effects of elevated (CO 2 ) on litter N are inherently more difficult to detect than differences in green leaves because factors that affect senescence and resorption increase variability. This is especially so when other environmental factors cause a disruption in the normal progress of resorption, such as in the first year when warming delayed senescence until leaves were killed by an early frost. The results of this experiment support the approach used in ecosystem models in which resorption efficiency is constant in ambient and elevated (CO 2 ), but the results also indicate that other factors can alter resorption efficiency.

De Angelis, Paolo; Chigwerewe, Kesari; Scarascia Mugnozza, Giuseppe

doi: 10.1023/A:1004790328560pmid: N/A

Six large open top chambers were installed to test the effect of atmospheric (CO 2 ) enrichment on clumps of natural Mediterranean vegetation starting from early spring 1992. To study the impact of (CO 2 ) enrichment on litter decomposition, leaves of three woody species ( Quercus ilex L., Phillyrea angustifolia L. and Pistacia lentiscus L.) were collected from the forest floor and subsequently incubated in situ over a two-year period. The initial slope of the exponential function, describing mass loss, indicated that there was a small negative effect of elevated (CO 2 ) on the decomposition rate of all the species. All regressions were significant. The decrease of decomposition rate is particularly notable during the initial stages of decomposition, when the differences of quality parameters, lignin/N and C/N were larger. This study points out that a decrease of decomposition rate may occur under elevated (CO 2 ) conditions; if this effect is coupled to an increase of primary production, there will be a net rise of C-storage in the soils of forest ecosystems. Forest soils may, therefore, represent a potentially increasing sink for this excess carbon.

Hartwig, Ueli; Lüscher, Andreas; Daepp, Markus; Blum, Herbert; Soussana, Jean-François; Nösberger, Josef

doi: 10.1023/A:1004601915836pmid: N/A

Experimental findings indicate that, in terrestrial ecosystems, nitrogen cycling changes under elevated partial pressure of atmospheric CO 2 (pCO 2 ). It was suggested that the concentration of N in plant litter as well as the amount of litter are responsible for these changes. However, for grassland ecosystems, there have been no relevant data available to support this hypothesis. Data from five years of the Swiss FACE experiment show that, under fertile soil conditions in a binary plant community consisting of Lolium perenne L. and Trifolium repens L., the concentration of litter N does not change under elevated atmospheric pCO 2 ; this applies to harvest losses, stubble, stolons and roots as the sources of litter. This is in strong contrast to the CO 2 response of L. perenne swards without associated legumes; in this case the above-ground concentration of biomass N decreased substantially. Increased symbiotic N 2 fixation in T. repens nodules and a greater proportion of the N-rich T. repens in the community are regarded as the main mechanisms that buffer the increased C introduction into the ecosystem under elevated atmospheric pCO 2 . Our data also suggest that elevated atmospheric pCO 2 results in greater amounts of litter, mainly due to increased root biomass production. This study indicates that, in a fertile grassland ecosystem with legumes, the concentration of N in plant litter is not affected by elevated atmospheric pCO 2 and, thus, cannot explain CO 2 -induced changes in the cycling of N.

Cotrufo, M.; Ineson, Phil

doi: 10.1023/A:1004771426605pmid: N/A

This study was conducted to test the hypothesis that wood tissues generated under elevated atmospheric (CO 2 ) have lower quality and subsequent reduced decomposition rates. Chemical composition and subsequent field decomposition rates were studied for beech ( Fagus sylvatica L.) twigs grown under ambient and elevated (CO 2 ) in open top chambers. Elevated (CO 2 ) significantly affected the chemical composition of beech twigs, which had 38% lower N and 12% lower lignin concentrations than twigs grown under ambient (CO 2 ). The strong decrease in N concentration resulted in a significant increase in the C/N and lignin/N ratios of the beech wood grown at elevated (CO 2 ). However, the elevated (CO 2 ) treatment did not reduce the decomposition rates of twigs, neither were the dynamics of N and lignin in the decomposing beech wood affected by the (CO 2 ) treatment, despite initial changes in N and lignin concentrations between the ambient and elevated (CO 2 ) beech wood.

Torbert, H.; Prior, S.; Rogers, H.; Wood, C.

doi: 10.1023/A:1004797123881pmid: N/A

A series of studies using major crops (cotton ( Gossypium hirsutum L.), wheat ( Triticum aestivum L.), grain sorghum ( Sorghum bicolor (L.) Moench.) and soybean ( Glycine max (L.) Merr.)) were reviewed to examine the impact of elevated atmospheric CO 2 on crop residue decomposition within agro-ecosystems. Experiments evaluated utilized plant and soil material collected from CO 2 study sites using Free Air CO 2 Enrichment (FACE) and open top chambers (OTC). A incubation study of FACE residue revealed that CO 2 -induced changes in cotton residue composition could alter decomposition processes, with a decrease in N mineralization observed with FACE, which was dependent on plant organ and soil series. Incubation studies utilizing plant material grown in OTC considered CO 2 -induced changes in relation to quantity and quality of crop residue for two species, soybean and grain sorghum. As with cotton, N mineralization was reduced with elevated CO 2 in both species, however, difference in both quantity and quality of residue impacted patterns of C mineralization. Over the short-term (14 d), little difference was observed for CO 2 treatments in soybean, but C mineralization was reduced with elevated CO 2 in grain sorghum. For longer incubation periods (60 d), a significant reduction in CO 2 -C mineralized per g of residue added was observed with the elevated atmospheric CO 2 treatment in both crop species. Results from incubation studies agreed with those from the OTC field observations for both measurements of short-term CO 2 efflux following spring tillage and the cumulative effect of elevated CO 2 (> 2 years) in this study. Observations from field and laboratory studies indicate that with elevated atmospheric CO 2 , the rate of plant residue decomposition may be limited by N and the release of N from decomposing plant material may be slowed. This indicates that understanding N cycling as affected by elevated CO 2 is fundamental to understanding the potential for soil C storage on a global scale.

Gorissen, A.; Cotrufo, M.F.

doi: 10.1023/A:1004744914998pmid: N/A

Leaf and root tissue of Lolium perenne L., Agrostis capillaris L. and Festuca ovina L. grown under ambient (350 μl l -1 CO 2 ) and elevated (700 μl l -1 ) CO 2 in a continuously 14 C-labelled atmosphere and at two soil N levels, were incubated at 14°C for 222 days. Decomposition of leaf and root tissue grown in the low N treatment was not affected by elevated (CO 2 ), whereas decomposition in the high N treatment was significantly reduced by 7% after 222 days. Despite the increased C/N ratio (g g -1 ) of tissue cultivated at elevated (CO 2 ) when compared with the corresponding ambient tissue, there was no significant correlation between initial C/N ratio and 14 C respired. This finding suggests that the CO 2 -induced changes in decomposition rates do not occur via CO 2 -induced changes in C/N ratios of plant materials. We combined the decomposition data with data on 14 C uptake and allocation for the same plants, and give evidence that elevated (CO 2 ) has the potential to increase soil C stores in grassland via increasing C uptake and shifting C allocation towards the roots, with an inherent slower decomposition rate than the leaves. An overall increase of 15% in 14 C remaining after 222 days was estimated for the combined tissues, i.e., the whole plants; the leaves made a much smaller contribution to the C remaining (+6%) than the roots (+26%). This shows the importance of clarifying the contribution of roots and leaves with respect to the question whether grassland soils act as a sink or source for atmospheric CO 2 .

Jastrow, J.D.; Miller, R.M.; Owensby, C.E.

doi: 10.1023/A:1004771805022pmid: N/A

We determined the effects of elevated (CO 2 ) on the quantity and quality of below-ground biomass and several soil organic matter pools at the conclusion of an eight-year CO 2 enrichment experiment on native tallgrass prairie. Plots in open-top chambers were exposed continuously to ambient and twice-ambient (CO 2 ) from early April through late October of each year. Soil was sampled to a depth of 30 cm beneath and next to the crowns of C4 grasses in these plots and in unchambered plots. Elevated (CO 2 ) increased the standing crops of rhizomes (87%), coarse roots (46%), and fibrous roots (40%) but had no effect on root litter (mostly fine root fragments and sloughed cortex material >500 μm). Soil C and N stocks also increased under elevated (CO 2 ), with accumulations in the silt/clay fraction over twice that of particulate organic matter (POM; >53 μm). The mostly root-like, light POM (density ≤1.8 Mg m -3 ) appeared to turn over more rapidly, while the more amorphous and rendered heavy POM (density >1.8 Mg m -3 ) accumulated under elevated (CO 2 ). Overall, rhizome and root C:N ratios were not greatly affected by CO 2 enrichment. However, elevated (CO 2 ) increased the C:N ratios of root litter and POM in the surface 5 cm and induced a small but significant increase in the C:N ratio of the silt/clay fraction to a depth of 15 cm. Our data suggest that 8 years of CO 2 enrichment may have affected elements of the N cycle (including mineralization, immobilization, and asymbiotic fixation) but that any changes in N dynamics were insufficient to prevent significant plant growth responses.

Johnson, D.; Cheng, W.; Ball, J.

doi: 10.1023/A:1004781810465pmid: N/A

The effects of six years treatment with elevated (CO 2 ) (350, 525, and 700 μl l -1 ) and nitrogen (N) (0, 10, and 20 g N m -2 yr -1 ) on soils, soil solution, and CO 2 efflux in an open-top chamber study with ponderosa pine ( Pinus ponderosa Laws.) are described. The clearest (CO 2 ) effect was in year 6, when a pattern of lower soil N concentration and higher C/N ratio with elevated (CO 2 ) emerged. Statistically significant effects of elevated (CO 2 ) on soil total C, extractable P, exchangeable Mg 2+ , exchangeable Ca 2+ , base saturation, and soil solution HCO 3 - and NO 3 - were also found in various treatment combinations and at various times; however, these effects were inconsistent among treatments and years, and in many cases (P, Mg 2+ , Ca 2+ , base saturation) reflected pre-treatment differences. The use of homogenized buried soil bags did not improve the power to detect changes in soil C and N or help resolve the inconsistencies in soil C patterns. Nitrogen fertilization had the expected negative effects on exchangeable Ca 2+ , K + , and Mg 2+ in year 6, presumably because of increased NO 3 - leaching, but had no consistent effect on soil C, N, or extractable P.

Johnson, D.W.; Cheng, W.; Ball, J.T.

doi: 10.1023/A:1004606901550pmid: N/A

Naturally senesced needles from ponderosa pine ( Pinus ponderosa Dougl.), grown from seed in open-top chambers under three levels of CO 2 (350, 525 and 700 μl l -1 ) and three levels of N fertilization (0, 10 and 20 g N m -2 yr -1 ), were used in a field litterbag decomposition study and in a laboratory study on potential microbial and nonmicrobial N immobilization. The litterbag studies revealed no statistically significant effects of either CO 2 or N treatment on mass loss, N concentration, or N content over a 26-month period. The laboratory study of potential 15 N immobilization revealed no statistically significant effects of CO 2 or N treatment on either total or microbial immobilization. Elevated (CO 2 ) did have a significant negative effect on nonmicrobial immobilization, however. Natural abundance of 15 N was significantly greater with elevated (CO 2 ) in both live and naturally senesced needles under all N treatments. This pattern combined with 15 N natural abundance in soils suggests that saplings grown under elevated (CO 2 ) were either taking up more N from surface horizons or from a more recalcitrant soil N pool in either horizon.