Photosynthesis controls of CO2 efflux from maize rhizosphere

The effects of different shading periods of maize plants on rhizosphere respiration and soil organic matter decomposition were investigated by using a ¹³C natural abundance and ¹⁴C pulse labeling simultaneously. ¹³C was a tracer for total C assimilated by maize during the whole growth period, and ¹⁴C was a tracer for recently assimilated C. CO₂ efflux from bare soil was 4 times less than the total CO₂ efflux from planted soil under normal lighting. Comparing to the normal lighting control (12/12 h day/night), eight days with reduced photosynthesis (12/36 h day/night period) and strongly reduced photosynthesis (12/84 h day/night period) resulted in 39% and 68% decrease of the total CO₂ efflux from soil, respectively. The analysis of ¹³C natural abundance showed that root-derived CO₂ efflux accounted for 82%, 68% and 56% of total CO₂ efflux from the planted soil with normal, prolonged and strongly prolonged night periods, respectively. Clear diurnal dynamics of the total CO₂ efflux from soil with normal day-night period as well as its strong reduction by prolonged night period indicated tight coupling with plant photosynthetic activity. The light-on events after prolonged dark periods led to increases of root-derived and therefore of total CO₂ efflux from soil. Any factor affecting photosynthesis, or substrate supply to roots and rhizosphere microorganisms, is an important determinant of root-derived CO₂ efflux, and thereby, total CO₂ efflux from soils. ¹⁴C labeling of plants before the first light treatment did not show any significant differences in the ¹⁴CO₂ respired in the rhizosphere between different dark periods because the assimilate level in the plants was high. Second labeling, conducted after prolonged night phases, showed higher contribution of recently assimilated C (¹⁴C) to the root-derived CO₂ efflux by shaded plants. Results from ¹³C natural abundance showed that the cultivation of maize on Chromic Luvisol decreased soil organic matter (SOM) mineralization compared to unplanted soil (negative priming effect). A more important finding is the observed tight coupling of the negative rhizosphere effect on SOM decomposition with photosynthesis.     

The input and fate of new C in two forest soils under elevated CO2

The aim of this study was to estimate (i) the influence of different soil types on the net input of new C into soils under CO₂ enrichment and (ii) the stability and fate of these new C inputs in soils. We exposed young beech–spruce model ecosystems on an acidic loam and calcareous sand for 4 years to elevated CO₂. The added CO₂ was depleted in ¹³C, allowing to trace new C inputs in the plant–soil system. We measured CO₂‐derived new C in soil C pools fractionated into particle sizes and monitored respiration as well as leaching of this new C during incubation for 1 year. Soil type played a crucial role in the partitioning of C. The net input of new C into soils under elevated CO₂ was about 75% greater in the acidic loam than in the calcareous sand, despite a 100% and a 45% greater above‐ and below‐ground biomass on the calcareous sand. This was most likely caused by a higher turnover of C in the calcareous sand as indicated by 30% higher losses of new C from the calcareous sand than from the acidic loam during incubation. Therefore, soil properties determining stabilization of soil C were apparently more important for the accumulation of C in soils than tree productivity. Soil fractionation revealed that about 60% of the CO₂‐derived new soil C was incorporated into sand fractions. Low natural ¹³C abundance and wide C/N ratios show that sand fractions comprise little decomposed organic matter. Consistently, incubation indicated that new soil C was preferentially respired as CO₂. During the first month, evolved CO₂ consisted to 40–55% of new C, whereas the fraction of new C in bulk soil C was 15–23% only. Leaching of DOC accounted for 8–23% of the total losses of new soil C. The overall effects of CO₂ enrichment on soil C were small in both soils, although tree growth increased significantly on the calcareous sand. Our results suggest that the potential of soils for C sequestration is limited, because only a small fraction of new C inputs into soils will become long‐term soil C.     

Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter

The mechanisms behind the ¹³C enrichment of organic matter with increasing soil depth in forests are unclear. To determine if ¹³C discrimination during respiration could contribute to this pattern, we compared δ¹³C signatures of respired CO₂ from sieved mineral soil, litter layer and litterfall with measurements of δ¹³C and δ¹⁵N of mineral soil, litter layer, litterfall, roots and fungal mycelia sampled from a 68-year-old Norway spruce forest stand planted on previously cultivated land. Because the land was subjected to ploughing before establishment of the forest stand, shifts in δ¹³C in the top 20 cm reflect processes that have been active since the beginning of the reforestation process. As ¹³C-depleted organic matter accumulated in the upper soil, a 1.0‰ δ¹³C gradient from −28.5‰ in the litter layer to −27.6‰ at a depth of 2–6 cm was formed. This can be explained by the 1‰ drop in δ¹³C of atmospheric CO₂ since the beginning of reforestation together with the mixing of new C (forest) and old C (farmland). However, the isotopic change of the atmospheric CO₂ explains only a portion of the additional 1.0‰ increase in δ¹³C below a depth of 20 cm. The δ¹³C of the respired CO₂ was similar to that of the organic matter in the upper soil layers but became increasingly ¹³C enriched with depth, up to 2.5‰ relative to the organic matter. We hypothesise that this ¹³C enrichment of the CO₂ as well as the residual increase in δ¹³C of the organic matter below a soil depth of 20 cm results from the increased contribution of ¹³C-enriched microbially derived C with depth. Our results suggest that ¹³C discrimination during microbial respiration does not contribute to the ¹³C enrichment of organic matter in soils. We therefore recommend that these results should be taken into consideration when natural variations in δ¹³C of respired CO₂ are used to separate different components of soil respiration or ecosystem respiration.     

Effects of six years atmospheric CO2 enrichment on plant,soil, and soil microbial C of a calcareous grassland

Stimulated plant production and often even larger stimulation of photosynthesis at elevated CO₂ raise the possibility of increased C storage in plants and soils. We analysed ecosystem C partitioning and soil C fluxes in calcareous grassland exposed to elevated CO₂ for 6 years. At elevated CO₂, C pools increased in plants (+23%) and surface litter (+24%), but were not altered in microbes and soil organic matter. Soils were fractionated into particle size and density separates. The amount of low-density macroorganic C, an indicator of particulate soil C inputs from root litter, was not affected by elevated CO₂. Incorporation of C fixed during the experiment (C_new) was tracked by C isotopic analysis of soil fractions which were labelled due to ¹³C depletion of the commercial CO₂ used for atmospheric enrichment. This data constrains estimates of C sequestration (absolute upper bound) and indicates where in soils potentially sequestered C is stored. C_new entered soils at an initial rate of 210±42 g C m^–2 year^–1, but only 554±39 g C_new m^–2 were recovered after 6 years due to the low mean residence time of 1.8 years. Previous process-oriented measurements did not indicate increased plant–soil C fluxes at elevated CO₂ in the same system (¹³C kinetics in soil microbes and fine roots after pulse labelling, and minirhizotron observations). Overall experimental evidence suggests that C storage under elevated CO₂ occurred only in rapidly turned-over fractions such as plants and detritus, and that potential extra soil C inputs were rapidly re-mineralised. We argue that this inference does not conflict with the observed increases in photosynthetic fixation at elevated CO₂, because these are not good predictors of plant growth and soil C fluxes for allometric reasons. C sequestration in this natural system may also be lower than suggested by plant biomass responses to elevated CO₂ because C storage may be limited by stabilisation of C_new in slowly turned-over soil fractions (a prerequisite for long-term storage) rather than by the magnitude of C inputs per se.     

Assimilate partitioning affects 13C fractionation of recently assimilated carbon in maize

Coupling ¹³C natural abundance and ¹⁴C pulse labelling enabled us to investigate the dependence of ¹³C fractionation on assimilate partitioning between shoots, roots, exudates, and CO₂ respired by maize roots. The amount of recently assimilated C in these four pools was controlled by three levels of nutrient supply: full nutrient supply (NS), 10 times diluted nutrient supply (DNS), and deionised water (DW). After pulse labelling of maize shoots in a ¹⁴CO₂ atmosphere, ¹⁴C was traced to determine the amounts of recently assimilated C in the four pools and the δ¹³C values of the four pools were measured. Increasing amounts of recently assimilated C in the roots (from 8% to 10% of recovered ¹⁴C in NS and DNS treatments) led to a 0.3‰ ¹³C enrichment from NS to DNS treatments. A further increase of C allocation in the roots (from 10% to 13% of recovered ¹⁴C in DNS and DW treatments) resulted in an additional enrichment of the roots from DNS to DW treatments by 0.3‰. These findings support the hypothesis that ¹³C enrichment in a pool increases with an increasing amount of C transferred into that pool. δ¹³C of CO₂ evolved by root respiration was similar to that of the roots in DNS and DW treatments. However, if the amount of recently assimilated C in root respiration was reduced (NS treatment), the respired CO₂ became 0.7‰ ¹³C depleted compared to roots. Increasing amounts of recently assimilated C in the CO₂ from NS via DNS to DW treatments resulted in a 1.6‰ δ¹³C increase of root respired CO₂ from NS to DW treatments. Thus, for both pools, i.e. roots and root respiration, increasing amounts of recently assimilated C in the pool led to a δ¹³C increase. In DW and DNS plants there was no ¹³C fractionation between roots and exudates. However, high nutrient supply decreased the amount of recently assimilated C in exudates compared to the other two treatments and led to a 5.3‰ ¹³C enrichment in exudates compared to roots. We conclude that ¹³C discrimination between plant pools and within processes such as exudation and root respiration is not constant but strongly depends on the amount of C in the respective pool and on partitioning of recently assimilated C between plant pools. Section Editor: H. Lambers     

Is growth of soil microorganisms in boreal forests limited by carbon or nitrogen availability?

To study whether the biomass of soil microorganisms in a boreal Pinus sylvestris-Vaccinium vitis-idaea forest was limited by the availability of carbon or nitrogen, we applied sucrose from sugar cane, a C₄ plant, to the organic mor-layer of the C₃–C dominated soil. We can distinguish between microbial mineralization of the added sucrose and respiration of endogenous carbon (root and microbial) by using the C₄-sucrose as a tracer, exploiting the difference in natural abundance of ¹³C between the added C₄-sucrose (¹³C –10.8) and the endogenous C₃–carbon (¹³C –26.6 ). In addition to sucrose, NH₄Cl (340 kg N ha^–1) was added factorially to the mor-layer. We followed the microbial activity for nine days after the treatments, by in situ sampling of CO₂ evolved from the soil and mass spectrometric analyses of ¹³C in the CO₂. We found that microbial biomass was limited by the availability of carbon, rather than nitrogen availability, since there was a 50% increase in soil respiration in situ between 1 h and 5 days after adding the sucrose. However, no further increase was observed unless nitrogen was also added. Analyses of the ¹³C ratios of the evolved CO₂ showed that increases in respiration observed between 1 h and 9 days after the additions could be accounted for by an increase in mineralization of the added C₄–C.     

Rhizosphere priming effects on the decomposition of soil organic matter in C4 and C3 grassland soils

Using a natural abundance ¹³C method, soil organic matter (SOM) decomposition was studied in a C₃ plant – `C<sub>4</sub> soil' (C<sub>3</sub> plant grown in a soil obtained from a grassland dominated by C<sub>4</sub> grasses) system and a C<sub>4</sub> plant – `C₃ soil' (C₄ plant grown in a soil taken from a pasture dominated by C₃ grasses) system. In C₃ plant – `C<sub>4</sub> soil' system, cumulative soil-derived CO<sub>2</sub>–C were higher in the soils planted with soybean (5499 mg pot<sup>–1</sup>) and sunflower (4484 mg pot<sup>–1</sup>) than that in `C₄ soil' control (3237 mg pot^–1) without plants. In other words, the decomposition of SOM in soils planted with soybean and sunflower were 69.9% and 38.5% faster than `C<sub>4</sub> soil' control. In C<sub>4</sub> plant – `C₃ soil' system, there was an overall negative priming effect of live roots on the decomposition of SOM. The cumulative soil-derived CO₂–C were lower in the soils planted with sorghum (2308 mg pot^–1) and amaranthus (2413 mg pot^–1) than that in `C<sub>3</sub> soil' control (2541 mg pot<sup>–1</sup>). The decomposition of SOM in soils planted with sorghum and amaranthus were 9.2% and 5.1% slower than `C₃ soil' control. Our results also showed that rhizosphere priming effects on SOM decomposition were positive at all developmental stages in C₃ plant – `C<sub>4</sub> soil' system, but the direction of the rhizosphere priming effect changed at different developmental stages in the C<sub>4</sub> plant – `C₃ soil' system. Implications of rhizosphere priming effects on SOM decomposition were discussed.     

Respiration from C3 plant green manure added to a C4 plant carbon dominated soil

Application of tree leaves (C₃ plants) on maize (Zea mays L.) (C₄ plant) fields is an agroforestry management technology to restore or maintain soil fertility. The rate at which the tree leaves decompose is crucial for the nutrient supply to the crop. We studied the in situ decomposition of Sesbania sesban (L.) Merr. leaves or C₃ sugar for 4 – 8 days after application to a maize field in Kenya. By using the difference of around 10‰ in natural abundance of ¹³C between the endogenous soil C (mainly C₄) and the applied C (C₃), we could calculate the contributions of the two C sources to soil respiration. The δ¹³C value of the basal respiration was from –15.9 to –16.7‰. The microbial response to the additions of leaves and sugar to this tropical soil was immediate. Application of sesbania leaves gave an initial peak in respiration rates that lasted from one to less than 6 days, after which it levelled off and remained about 2 – 3 times higher (230–270 mg C m^-2 h^-1) than the control respiration rates throughout the rest of the experiment (5 – 8 days). In the sugar treatment, there was no initial peak in respiration rate. The respiration rate was 170 mg C m^-2 h^-1 after 4 days. At the end of the experiments, after 4–8 days, as much as 14–17% of the added C had been respired and about 60% of the total respiration was from the added sesbania leaves or C₃ sugar. This non-destructive method allows repeated measurements of the actual rate of C mineralisation and facilitates decomposition studies with high temporal resolution in the field. This revised version was published online in August 2006 with corrections to the Cover Date.     

The fate of carbon in pulse-labelled crops of barley and wheat

Wheat (cv. Gutha) and barley (cv. O'Connor) were grown as field crops on a shallow duplex soil (sand over clay) in Western Australia with their root systems contained within pvc columns. At four stages during growth, the shoots were pulse-labelled for 1.5h with¹⁴CO₂; immediately prior to labelling, the soil was isolated from the shoot atmosphere by pvc sheets. After labelling, the soil atmosphere was pumped through NaOH to trap respired CO₂ and after 2.5, 5, 7.5 and 24 h from the start of labelling, columns were destructively sampled to recover¹⁴C from the roots, soil and shoot.Both species showed similar patterns of¹⁴C distribution and changes in distribution through the growing season. During early tillering, 15–25% of the¹⁴C recovered after 24 h had been respired by the roots and rhizosphere, 17–27% was retained in the roots, 0.4–1.8% was recovered as water-soluble¹⁴C in the soil and the remainder (45–67%) was present in the shoot. These percentages changed during growth so that during grain filling only 2–3% of the¹⁴C recovered after 24 h was as respired CO₂, 2–6% was in the roots, 0.2% was in the soil and over 90% was in the shoot.The distribution of¹⁴C in components of the soil-plant system changed during the 24 h after labelling with the most rapid changes occurring generally during the first 7.5 h after labelling.Using growth measurements from adjacent plots, the amounts of C added to the soil were estimated for the whole season. Carbon input to the soil was about 48 gC m^–2 for wheat and 58 gC m^–2 for barley; the crops produced total shoot dry matter of 494 (wheat) and 735 g m^–2 (barley). Of the C input to the soil, 27.8% (wheat) and 40.3% (barley) was as respired C and only 3.3 (wheat) and 4.1% (barley) was collected as exudate (water-soluble material).     

Altered patterns of soil carbon substrate usage and heterotrophic respiration in a pine forest with elevated CO2 and N fertilization

To assess how heterotrophic microorganisms may alter their activities and thus their CO₂‐C return to the atmosphere with elevated CO₂ and changing N availability, we examined soil organic matter (SOM) dynamics at the Duke Free Air Carbon Enrichment (FACE) site, after N fertilizer was applied. We measured heterotrophic respiration during early and late stages of SOM mineralization in soil incubations to capture activity on relatively labile and refractory SOM pools. We also measured δ¹³C of respired CO₂‐C and phospholipid fatty acids (PLFAs) during early mineralization stages to track the microbial groups involved in substrate use. We calculated , a measure of δ¹³C_PLFA normalized by respired δ¹³CO₂, to assess microbial function with C substrates formed with elevated CO₂ and altered N availability, via the distinct δ¹³C of the supplemental CO₂. We also quantified extracellular enzyme activity (EEA) during labile and recalcitrant SOM mineralization. Early in the incubations, increased N availability reduced heterotrophic CO₂‐C release. By the later stages of SOM mineralization, elevated CO₂ soils with fertilization had respired 72% of the CO₂‐C respired by all other soils. values suggest that fungi in elevated CO₂ plots took up C substrates possessing the δ¹³C signature of recently formed SOM, and added N promoted the activity of Gram‐negative bacteria and reduced that of Gram‐positive bacteria, particularly actinomycetes. Consistent with this, the enzyme responsible for the degradation of peptidoglycan and chitin, compounds produced by Gram‐positive bacteria and fungi, respectively, experienced a decline in activity with N fertilization. If patterns observed in this study with N additions are reversed with progressive N limitation at this site, actinomycetes and other Gram‐positive bacteria responsible for mineralizing relatively recalcitrant substrates may experience increases in their activity. Such shifts in microbial functioning may result in increased turnover of, and C release from, relatively decay‐resistant material.     

Measurement of rhizosphere respiration and organic matter decomposition using natural 13C

Due to the limitations in methodology it has been a difficult task to measure rhizosphere respiration and original soil carbon decomposition under the influence of living roots. ¹⁴C-labeling has been widely used for this purpose in spite of numerous problems associated with the labeling method. In this paper, a natural ¹³C method was used to measure rhizosphere respiration and original soil carbon decomposition in a short-term growth chamber experiment. The main objective of the experiment was to validate a key assumption of this method: the ¹³C value of the roots represents the ¹³C value of the rhizosphere respired CO₂. Results from plants grown in inoculated carbon-free medium indicated that this assumption was valid. This natural ¹³C method was demonstrated to be advantageous for studying rhizosphere respiration and the effects of living roots on original soil carbon decomposition.     

Immobilization, stabilization and remobilization of nitrogen in forest soils at elevated CO2: a 15N and 13C tracer study

The fate of immobilized N in soils is one of the great uncertainties in predicting C sequestration at increased CO₂ and N deposition. In a dual isotope tracer experiment (¹³C, ¹⁵N) within a 4‐year CO₂ enrichment (+200 ppm_v) study with forest model ecosystems, we (i) quantified the effects of elevated CO₂ on the partitioning of N; (ii) traced immobilized N into physically separated pools of soil organic matter (SOM) with turnover rates known from their ¹³C signals; and (iii) estimated the remobilization and thus, the bio‐availability of newly sequestered C and N. (1) CO₂ enrichment significantly decreased NO₃^? concentrations in soil waters and export from 1.5 m deep lysimeters by 30–80%. Consequently, elevated CO₂ increased the overall retention of N in the model ecosystems. (2) About 60–80% of added ¹⁵NH₄¹⁵NO₃ were retained in soils. The clay fraction was the greatest sink for the immobilized ¹⁵N sequestering 50–60% of the total new soil N. SOM associated with clay contained only 25% of the total new soil C pool and had small C/N ratios (<13), indicating that it consists of humified organic matter with a relatively slow turn over rate. This implies that added ¹⁵N was mainly immobilized in stable mineral‐bound SOM pools. (3) Incubation of soils for 1 year showed that the remobilization of newly sequestered N was three to nine times smaller than that of newly sequestered C. Thus, inorganic inputs of N were stabilized more effectively in soils than C. Significantly less newly sequestered N was remobilized from soils previously exposed to elevated CO₂. In summary, our results show firstly that a large fraction of inorganic N inputs becomes effectively immobilized in relative stable SOM pools and secondly that elevated CO₂ can increase N retention in soils and hence it may tighten N cycling and diminish the risk of nitrate leaching to groundwater.     

Quantification of canola root morphological traits under heat and drought stresses with electrical measurements

Background and aims

Crop residues and soil types play an important role in soil C and N storage. The objectives of this study were to quantify the effects of crop residue quality and interactions with soil type on soil C and N, in the short- and medium-term, and to determine the responses related to the priming effect (PE).

Methods

Residues of vetch (Vicia sativa L.), pea (Pisum sativum L.) and wheat (Triticum aestivum L.) crops with different chemical compositions and labelled with ¹³C and ¹⁵N were left to decompose on the surface of either a sandy-loam soil or a clay soil incubated under laboratory conditions at 25 °C for 360 days. We measured the total CO₂-C and CO₂-¹³C emitted during decomposition, the soil mineral N content and the amounts of ¹³C and ¹⁵N remaining in both the surface residue particles and the bulk soil.

Results

Over the short-term, the vetch residues decomposed faster than those of wheat and pea on the soil surface due to their more favourable chemical composition for biodegradation; after one year, however, this difference disappeared. We observed extra soil C mineralization in all cases, i.e., the PE was positive for all treatments and was directly related to the water-soluble (vetch > pea > wheat) and soil C contents (clay soil > sandy-loam soil). Conversely, the fate of the added ¹⁵N and net N mineralization differed considerably between the three residues and was strongly related to the initial N content of the residue.

Conclusions

Crop residue quality and soil type affected the soil PE and soil C balance but not the fate of crop residue-C after one year. Net soil N mineralization was observed in all crop residues, with large early differences (vetch > pea > wheat), which were maintained on a medium-term basis. Our results emphasize the need to jointly consider C and N dynamics as well as short- and medium-term effects to manage agricultural and environmental services provided by the recycling of crop residues to agricultural soils.

     

A pulse-labelling method to generate 13C- enriched plant materials

Plant materials labelled with ¹³C can be used to trace litter decomposition and root carbon flow, but only if the isotope is uniformly distributed in the plant. We postulated that if ¹³CO₂ were applied at regular intervals, in direct proportion to the rate of photosynthesis, then the abundance of ¹³C would be uniform among plant parts. To test this hypothesis, wheat plants were grown in the greenhouse, and exposed weekly to ¹³CO₂ for six hours in a closed chamber. A constant dose of ¹³CO₂ (about 33 atom%) was injected whenever CO₂ concentration fell below a prescribed limit, so that ¹³CO₂ was added in proportion to photosynthetic rate. Wheat exposed for 13 weeks (starting 11 days after seeding) had reasonably consistent ¹³C abundance among plant parts: grain = 3.41, chaff = 3.41, stem = 3.65, and root = 3.50 atom%. The `leaf' fraction had slightly higher abundance (3.99 atom%), perhaps because recently-fixed ¹³C was not translocated from senescing tissue. Exposing plants only during early stages of the growing season increased differences among plant parts. The approach offers a practical way to label plants with ¹³C.     

Interpretation of soil δ13C as an indicator of vegetation change in African savannas

Question: The relationship between carbon‐13 in soil organic matter and C₃ and C₄ plant abundance is complicated because of differential productivity, litter fall and decomposition. As a result, applying a mass balance equation to δ¹³C data from soils cannot be used to infer past C₃ and C₄ plant abundance; only the proportion of carbon derived from C₃ and C₄ plants can be estimated. In this paper, we compare δ¹³C of surface soil samples with vegetation data, in order to establish whether the ratio of C₃:C₄ plants (rather than the proportion of carbon from C₃ and C₄ plants) can be inferred from soil δ¹³C. Location: The Tsavo National Park, in southeastern Kenya. Methods: We compare vegetation data with δ¹³C of organic matter in surface soil samples and derive regression equations relating the δ¹³C of soil organic matter to C₃:C₄ plant abundance. We use these equations to interpret δ¹³C data from soil profiles in terms of changes in inferred C₃:C₄ plant ratio. We compare our method of interpretation with that derived from a mass balance approach. Results: There was a statistically significant, linear relationship between the δ¹³C of organic matter in surface soil samples and the natural logarithm of the ratio of C₃:C₄ plants in the 100m² surrounding the soil sample. Conclusions: We suggest that interpretation of δ¹³C data from organic matter in soil profiles can be improved by comparing vegetation surveys with δ¹³C of organic matter in surface soil samples. Our results suggest that past C₃ plant abundance might be under‐estimated if a mass balance approach is used.     

Indirect effects of soil moisture reverse soil C sequestration responses of a spring wheat agroecosystem to elevated CO2

Increased plant productivity under elevated atmospheric CO₂ concentrations might increase soil carbon (C) inputs and storage, which would constitute an important negative feedback on the ongoing atmospheric CO₂ rise. However, elevated CO₂ often also leads to increased soil moisture, which could accelerate the decomposition of soil organic matter, thus counteracting the positive effects via C cycling. We investigated soil C sequestration responses to 5 years of elevated CO₂ treatment in a temperate spring wheat agroecosystem. The application of ¹³C‐depleted CO₂ to the elevated CO₂ plots enabled us to partition soil C into recently fixed C (C_new) and pre‐experimental C (C_old) by ¹³C/¹²C mass balance. Gross C inputs to soils associated with C_new accumulation and the decomposition of C_old were then simulated using the Rothamsted C model ‘RothC.’ We also ran simulations with a modified RothC version that was driven directly by measured soil moisture and temperature data instead of the original water balance equation that required potential evaporation and precipitation as input. The model accurately reproduced the measured C_new in bulk soil and microbial biomass C. Assuming equal soil moisture in both ambient and elevated CO₂, simulation results indicated that elevated CO₂ soils accumulated an extra ～40–50 g C m^?2 relative to ambient CO₂ soils over the 5 year treatment period. However, when accounting for the increased soil moisture under elevated CO₂ that we observed, a faster decomposition of C_old resulted; this extra C loss under elevated CO₂ resulted in a negative net effect on total soil C of ～30 g C m^?2 relative to ambient conditions. The present study therefore demonstrates that positive effects of elevated CO₂ on soil C due to extra soil C inputs can be more than compensated by negative effects of elevated CO₂ via the hydrological cycle.     

Effects of pasture introduction on soil CO2 emissions during the dry season in the state of Rondônia,Brazil

Soil CO₂ evolution rates, soil temperatures and moisture were measured during the dry season in two forest-to-pasture chronosequences in Rondônia, Brazil. The study included pastures ranging from 3 to 80 years-old. Mean dry-season CO₂ evolution from the forest in chronosequence 1, 88.8 mg CO₂-C m^–2h^–1 was lower than from the pastures which ranged from 111 to 158 mg CO₂-C m^–2h^–1. We found that temperature was not a good predictor of CO₂ emissions from pasture but that there was a significant relationship (r = 0.72,p < 0.05) between soil moisture and pasture emissions. The ¹³C of the soil CO₂ emissions also was measured on chronosequence I; ¹³C of the CO₂ emitted from the C3 forest was –29.43%. Pasture¹³CO₂ values increased from –17.91%. in the 3 year-old pasture to –12.86% in the 80 year-old, reflecting the increasing C₄ inputs with pasture age. Even in the youngest (3 year-old) pasture, 70 percent of the CO₂ evolved originated from C₄ pasture-derived carbon.     

Effects of land use on 15N natural abundance of soils in Ethiopian highlands

We used the natural abundance of ¹⁵N in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in ¹³C natural abundance, patterns of δ¹⁵N and δ¹³C values were compared to determine if shifts of ¹⁵N correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found δ¹⁵N values from −8.8 to +3.5‰ in litter, from −3.5 to +4.5‰ in 0–10 cm soil layer, and from −1.5 to +6.8‰ at >20 cm soil depth. The low δ¹⁵N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low δ¹³C of the surface horizon and the high δ¹³C of the lower soil horizons is clear evidence of a long phase of C₄ grass dominance or cultivation of C₄ crops before the establishment of the forest >500 years ago. In contrast, at Wendo-Genet, high δ¹³C of soils reveals that most of the land has been uncovered by forests until recently. Soil δ¹⁵N was high throughout (3.4–9.8‰), and there were no major differences between forested, cultivated and pasture soils in δ¹⁵N values of surface mineral soils. The high δ¹⁵N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management, it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes at Menagesha after a long period of grass vegetation cover or cultivation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Feedback from plant species change amplifies CO2 enhancement of grassland productivity

Dynamic global vegetation models simulate feedbacks of vegetation change on ecosystem processes, but direct, experimental evidence for feedbacks that result from atmospheric CO₂ enrichment is rare. We hypothesized that feedbacks from species change would amplify the initial CO₂ stimulation of aboveground net primary productivity (ANPP) of tallgrass prairie communities. Communities of perennial forb and C₄ grass species were grown for 5 years along a field CO₂ gradient (250–500 μL L^?1) in central Texas USA on each of three soil types, including upland and lowland clay soils and a sandy soil. CO₂ enrichment increased community ANPP by 0–117% among years and soils and increased the contribution of the tallgrass species Sorghastrum nutans (Indian grass) to community ANPP on each of the three soil types. CO₂‐induced changes in ANPP and Sorghastrum abundance were linked. The slope of ANPP‐CO₂ regressions increased between initial and final years on the two clay soils because of a positive feedback from the increase in Sorghastrum fraction. This feedback accounted for 30–60% of the CO₂‐mediated increase in ANPP on the upland and lowland clay soils during the final 3 years and 1 year of the experiment, respectively. By contrast, species change had little influence on the ANPP‐CO₂ response on the sandy soil, possibly because Sorghastrum increased largely at the expense of a functionally similar C₄ grass species. By favoring a mesic C₄ tall grass, CO₂ enrichment approximately doubled the initial enhancement of community ANPP on two clay soils. The CO₂‐stimulation of grassland productivity may be significantly underestimated if feedbacks from plant community change are not considered.     

Influence of maize root mucilage on soil aggregate stability

This study was undertaken to determine the effects of root exudates on soil aggregate stability. Root mucilage was collected from two-month old maize plants (Zea mays L.) Mucilage and glucose solutions were added at a rate of 2.45 g C kg⁻¹ dry soil to silty clay and silt loam soils. Amended soils, placed in serum flasks, were incubated for 42 d with a drying-wetting cycle after 21 d. Evolved CO₂ was measured periodically as well as the water-stable aggregates and soluble sugar and polysaccharide content of the soil. In mucilage-amended soils CO₂ evolution started with a lag phase of 2–3 days, which was not observed in glucose-amended soils. There was then a sharp increase in evolved CO₂ up to day 7. During the second incubation period there were only small differences in evolved C between treatments. Incorporation of mucilage in both soils resulted in a spectacular and immediate increase in soil aggregate stability. Thereafter, the percent of water-stable aggregates quickly decreased parallel to microbial degradation. On completion of the incubation, aggregate stability in the silty clay soil was still significantly higher in the presence of mucilage than in the control. This work supports the assumption that freshly released mucilage is able to stick very rapidly to soil particles and may protect the newly formed aggregates against water destruction. On the silty clay, microbial activity contributes to a stabilization of these established organo-mineral bounds.