Total soil C and N sequestration in a grassland following 10 years of free air CO 2 enrichment (original) (raw)
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Total Carbon and Nitrogen Budget in Pasture Soils After 10 Years of Elevated CO2
2003
Soil C sequestration may mitigate rising levels of atmospheric CO 2. However, it has yet to be determined whether net soil C sequestration occurs in N-rich grasslands exposed to long-term elevated CO 2. This study examined whether N-fertilized grasslands exposed to elevated CO 2 sequestered additional C. For 10 years, Lolium perenne, Trifolium repens, and the mixture of L. perenne/T. repens grasslands were exposed to ambient and elevated CO 2 concentrations (35 and 60 Pa pCO 2). The applied CO 2 was depleted in d 13 C and the grasslands received low (140 kg ha À1) and high (560 kg ha À1) rates of 15 N-labeled fertilizer. Annually collected soil samples from the top 10 cm of the grassland soils allowed us to follow the sequestration of new C in the surface soil layer. For the first time, we were able to collect dual-labeled soil samples to a depth of 75 cm after 10 years of elevated CO 2 and determine the total amount of new soil C and N sequestered in the whole soil profile. Elevated CO 2 , N-fertilization rate, and species had no significant effect on total soil C. On average 9.4 Mg new C ha À1 was sequestered, which corresponds to 26.5% of the total C. The mean residence time of the C present in the 0-10 cm soil depth was calculated at 4.6 AE 1.5 and 3.1 AE 1.1 years for L. perenne and T. repens soil, respectively. After 10 years, total soil N and C in the 0-75 cm soil depth was unaffected by CO 2 concentration, Nfertilization rate and plant species. The total amount of 15 N-fertilizer sequestered in the 0-75 cm soil depth was also unaffected by CO 2 concentration, but significantly more 15 N was sequestered in the L. perenne compared with the T. repens swards: 620 vs. 452 kg ha À1 at the high rate and 234 vs. 133 kg ha À1 at the low rate of N fertilization. Intermediate values of 15 N recovery were found in the mixture. The fertilizer derived N amounted to 2.8% of total N for the low rate and increased to 8.6% for the high rate of N application. On average, 13.9% of the applied 15 N-fertilizer was recovered in the 0-75 cm soil depth in soil organic matter in the L. perenne sward, whereas 8.8% was recovered under the T. repens swards, indicating that the N 2-fixing T. repens system was less effective in sequestering applied N than the non-N 2-fixing L. perenne system. Prolonged elevated CO 2 did not lead to an increase in whole soil profile C and N in these fertilized pastures. The potential use of fertilized and regular cut pastures as a net soil C sink under longterm elevated CO 2 appears to be limited and will likely not significantly contribute to the mitigation of anthropogenic C emissions.
Carbon-13 input and turn-over in a pasture soil exposed to long-term elevated atmospheric CO2
Global Change Biology, 2000
The impact of elevated CO 2 and N-fertilization on soil C-cycling in Lolium perenne and Trifolium repens pastures were investigated under Free Air Carbon dioxide Enrichment (FACE) conditions. For six years, swards were exposed to ambient or elevated CO 2 (35 and 60 Pa pCO 2) and received a low and high rate of N fertilizer. The CO 2 added in the FACE plots was depleted in 13 C compared to ambient (D ± 40½) thus the C inputs could be quanti®ed. On average, 57% of the C associated with the sand fraction of the soil was`new' C. Smaller proportions of the C associated with the silt (18%) and clay fractions (14%) were derived from FACE. Only a small fraction of the total C pool below 10 cm depth was sequestered during the FACE experiment. The annual net input of C in the FACE soil (0±10 cm) was estimated at 4.6 6 2.2 and 6.3 6 3.6 (95% con®dence interval) Mg ha ±1 for T. repens and L. perenne, respectively. The maximum amount of labile C in the T. repens sward was estimated at 8.3 6 1.6 Mg ha ±1 and 7.1 6 1.0 Mg ha ±1 in the L. perenne sward. Mean residence time (MRT) for newly sequestered soil C was estimated at 1.8 years in the T. repens plots and 1.1 years for L. perenne. An average of 18% of total soil C in the 0±10 cm depth in the T. repens sward and 24% in the L. perenne sward was derived from FACE after 6 years exposure. The majority of the change in soil d 13 C occurred in the ®rst three years of the experiment. No treatment effects on total soil C were detected. The fraction of FACE-derived C in the L. perenne sward was larger than in the T. repens sward. This suggests a priming effect in the L. perenne sward which led to increased losses of the old C. Although the rate of C cycling was affected by species and elevated CO 2 , the soil in this intensively managed grassland ecosystem did not become a sink for additional new C.
Effects of long-term exposure to enriched CO2 on the nutrient-supplying capacity of a grassland soil
Biology and Fertility of Soils, 2012
Altered soil nutrient cycling under future climate scenarios may affect pasture production and fertilizer management. We conducted a controlled-environment study to test the hypothesis that long-term exposure of pasture to enriched carbon dioxide (CO 2 ) would lower soil nutrient availability. Perennial ryegrass was grown for 9 weeks under ambient and enriched (ambient+120 ppm) CO 2 concentrations in soil collected from an 11.5-year free air CO 2 enrichment experiment in a grazed pasture in New Zealand. Nitrogen (N) and phosphorus (P) fertilizers were applied in a full factorial design at rates of 0, 12.5, 25 or 50 kg N ha −1 and 0, 17.5 or 35 kg P ha −1 . Compared to ambient CO 2 , under enriched CO 2 without P fertilizer, total plant biomass did not respond to N fertilizer, and tissue N/P ratio was increased indicating that P was co-limiting. This limitation was alleviated with the lowest rate of P fertilizer (17.5 kg P ha −1 ). Plant biomass in both CO 2 treatments increased with increasing N fertilizer when sufficient P was available. Greater inputs of P fertilizer may be required to prevent yield suppression under enriched CO 2 and to stimulate any response to N.
Global Change Biology, 2003
Reduced soil N availability under elevated CO 2 may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N 2 -fixing system. We studied the effects of Trifolium repens (an N 2 -fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO 2 under FACE conditions. 15 Nlabeled fertilizer was applied at a rate of 140 and 560 kg N ha À 1 yr À 1 and the CO 2 concentration was increased to 60 Pa pCO 2 using 13 C-depleted CO 2 . The total soil C content was unaffected by elevated CO 2 , species and rate of 15 N fertilization. However, under elevated CO 2 , the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer-N (f N ) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO 2 , had a significant effect on f N values of the total soil N pool. The fractions of newly sequestered C (f C ) differed strongly among intra-aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO 2 , the ratio of fertilizer-N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more 15 N fertilizer than T. repens: 179 vs. 101 kg N ha À 1 for the low rate of N fertilization and 393 vs. 319 kg N ha À 1 for the high N-fertilization rate. As the loss of fertilizer-15 N contributed to the 15 N-isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N 2 fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non-N 2fixing L. perenne system. This suggests that N 2 fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system.
Global Change Biology, 2004
Elevated atmospheric CO 2 may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO 2 decomposes differently from plant material produced under ambient CO 2 . Moreover, a long-term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO 2 made in coupled climate-soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO 2 at two levels of N fertilizer (140 vs. 560 kg ha À1 yr À1 ), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO 2 under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO 2 and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO 2 did not affect decomposition rates of the plant material. If under prolonged elevated CO 2 changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low-N fertilization treatment was enhanced after exposure to elevated CO 2 . This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the 13 C-CO 2 signature, reflected the turnover times of SOM-C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO 2 , as used in coupled climate-SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.
European Journal of Soil Science, 2007
The fate of carbon (C) in grassland soils is of particular interest since the vast majority in grassland ecosystems is stored below ground and respiratory C-release from soils is a major component of the global C balance. The use of 13 C-depleted CO 2 in a 10-year free-air carbon dioxide enrichment (FACE) experiment, gave a unique opportunity to study the turnover of the C sequestered during this experiment. Soil organic matter (SOM), soil air and plant material were analysed for d 13 C and C contents in the last year of the FACE experiment and in the two following growing seasons. After 10 years of exposure to CO 2 enrichment at 600 ppmv, no significant differences in SOM C content could be detected between fumigated and non-fumigated plots. A 13 C depletion of 3.4& was found in SOM (0-12 cm) of the fumigated soils in comparison with the control soils and a rapid decrease of this difference was observed after the end of fumigation. Within 2 years, 49% of the C in this SOM (0-12 cm) was exchanged with fresh C, with the limitation that this exchange cannot be further dissected into respiratory decay of old C and freshly sequestered new C. By analysing the mechanistic effects of a drought on the plant-soil system it was shown that rhizosphere respiration is the dominant factor in soil respiration. Consideration of ecophysiological factors that drive plant activity is therefore important when soil respiration is to be investigated or modelled.
Nutrient relations in calcareous grassland under elevated CO 2
Oecologia, 1998
Plant nutrient responses to 4 years of CO 2 enrichment were investigated in situ in calcareous grassland. Beginning in year 2, plant aboveground C:N ratios were increased by 9% to 22% at elevated CO 2 (P < 0.01), depending on year. Total amounts of N removed in biomass harvests during the ®rst 4 years were not aected by elevated CO 2 (19.9 1.3 and 21.1 1.3 g N m A2 at ambient and elevated CO 2), indicating that the observed plant biomass increases were solely attained by dilution of nutrients. Total aboveground P and tissue N:P ratios also were not altered by CO 2 enrichment (12.5 2 g N g A1 P in both treatments). In contrast to non-legumes (>98% of community aboveground biomass), legume C/N was not reduced at elevated CO 2 and legume N:P was slightly increased. We attribute the less reduced N concentration in legumes at elevated CO 2 to the fact that virtually all legume N originated from symbiotic N 2 ®xation (%N dfa % 90%), and thus legume growth was not limited by soil N. While total plant N was not aected by elevated CO 2 , microbial N pools increased by +18% under CO 2 enrichment (P 0.04) and plant available soil N decreased. Hence, there was a net increase in the overall biotic N pool, largely due increases in the microbial N pool. In order to assess the eects of legumes for ecosystem CO 2 responses and to estimate the degree to which plant growth was P-limited, two greenhouse experiments were conducted, using ®rstly undisturbed grassland monoliths from the ®eld site, and secondly designed`microcosm' communities on natural soil. Half the microcosms were planted with legumes and half were planted without. Both monoliths and microcosms were exposed to elevated CO 2 and P fertilization in a factored design. After two seasons, plant N pools in both unfertilized monoliths and microcosm communities were unaected by CO 2 enrichment, similar to what was found in the ®eld. However, when P was added total plant N pools increased at elevated CO 2. This community-level eect originated almost solely from legume stimulation. The results suggest a complex interaction between atmospheric CO 2 concentrations, N and P supply. Overall ecosystem productivity is N-limited, whereas CO 2 eects on legume growth and their N 2 ®xation are limited by P.
Elevated atmospheric CO2 affects the turnover of nitrogen in a European grassland
Applied Soil Ecology, 2005
The availability of nutrients in the soil is key to the potential response of a plant to elevated CO 2 and is central to correctly predicting the response of terrestrial communities to climate change. In order for a plant to fully realise the potential of increased atmospheric CO 2 , it must increase its nutrient uptake for the increased production of biomass as well as biochemical compounds. In this study the stable isotope 15 N was used to follow the fate of nitrogen contained in litter in order to determine the effect elevated atmospheric CO 2 had on the loss of nitrogen from decomposing litter and the eventual re-use of this nitrogen. During the decomposition study, on a mass basis more 15 N was transferred from the litter despite the litter grown in elevated CO 2 initially having a lower 15 N signal. This was primarily related to a higher decomposition rate of the elevated CO 2 grown litter. Despite more nitrogen entering the below-ground community under elevated atmospheric CO 2 , the additional N did not stay within the terrestrial community and was not exploited by the plants. The results confirm previous suggestions that Lolium perenne plants growing in elevated CO 2 have to derive at least a proportion of their nitrogen from a source external to either added fertiliser or decomposing litter
Free-air CO2 enrichment effects on soil carbon and nitrogen
Agricultural and Forest Meteorology, 1994
Since the onset of the industrial revolution, atmospheric CO2 concentration has increased exponentially to the current 370 #mol mo1-1 level, and continued increases are expected. Previous research has demonstrated that elevated atmospheric CO2 results in larger plants returning greater amounts of C to the soil. However, the effects of elevated CO 2 on C and N cycling and long-term storage of C in soil have not been examined. Soil samples (in 0-50, 50-100, and 100-200 mm depth increments) were collected after 3 years of cotton (Gossypium hirsutum L.) production under free-air CO 2 enrichment (FACE, at 550 #tool CO 2 mol-l), in combination with 2 years of different soil moisture regimes (wet, 100% of evapotranspiration replaced, or dry, 75% and 67% of evapotranspiration replaced in 1990 and 1991, respectively) on a Trix clay loam (fine, loamy, mixed (calcareous), hyperthermic Typic Torrifluvent) at Maricopa, Arizona. Ambient plots (370 #mol CO2 mol-I (control)), in combination with the wet and dry soil moisture regimes, were also included in the study. Soil organic C and N concentrations, potential C and N mineralization, and C turnover were measured. Increased input of cotton plant residues under FACE resulted in treatment differences and trends toward increased organic C in all three soil depths. During the first 30 days of laboratory incubation, available N apparently limited potential C mineralization and C turnover in all treatments. Between 30 and 60 days of incubation, soils from FACE plots had greater potential C mineralization with both water regimes, but C turnover increased in soils from the dry treatment and decreased in soils where cotton was not water stressed. These data indicate that in high-CO 2 environments without water stress, increased C storage in soil is likely, but it is less likely where water stress is a factor. More research is needed before the ability of soil to store additional C in a high-CO 2 world can be determined.