Soil restoration under pasture after topsoil removal: Some factors influencing C and N mineralization and measurements of microbial biomass

Factors influencing rates of C and N mineralization of soil and plant materials, and the reliability of different procedures for estimating microbial biomass, were measured in a soil (Typic Dystrochrept) that had been restored under grazed pasture in a temperate environment for 10–11 years after 20 cm of the original topsoil had been removed by stripping. Rates of net N mineralization were appreciably lower, but CO₂-C production higher, in the stripped than in the unstripped soil. These activities were not influenced directly by levels of soil mineral-N, but they were influenced by differences in plant composition. Herbage and litter, and roots, from the stripped plots were generally mineralized more readily to CO₂-C, but more slowly to net mineral-N, than were the corresponding materials from the unstripped plots. Rates of mineralization of herbage and litter, or roots, were mainly indistinguishable in stripped and unstripped soil, whereas rates of mineralization of all standing dead material were lower in stripped soil. Measurements of extractable-C flush, and of CO₂-C flush (using a fumigated soil control) and mineral-N flush by fumigation-incubation procedures, indicated that microbial biomass in stripped soil had recovered to at least 88 percent of the levels in unstripped soil. Substrate-induced respiration also generally indicated high levels of recovery of microbial biomass. The fumigation-incubation procedure appeared to under-estimate microbial biomass markedly in stripped soil when unfumigated soil controls were used; the used of a large soil inoculum (20 percent w/w) only sometimes overcame this problem. Possible reasons for apparent anomalies in estimation of microbial C are discussed.     

Elevated CO2 reduces field decomposition rates of Betula pendula (Roth.) leaf litter

The effect of elevated atmospheric CO₂ and nutrient supply on elemental composition and decomposition rates of tree leaf litter was studied using litters derived from birch (Betula pendula Roth.) plants grown under two levels of atmospheric CO₂ (ambient and ambient +250 ppm) and two nutrient regimes in solar domes. CO₂ and nutrient treatments affected the chemical composition of leaves, both independently and interactively. The elevated CO₂ and unfertilized soil regime significantly enhanced lignin/N and C/N ratios of birch leaves. Decomposition was studied using field litter-bags, and marked differences were observed in the decomposition rates of litters derived from the two treatments, with the highest weight remaining being associated with litter derived from the enhanced CO₂ and unfertilized regime. Highly significant correlations were shown between birch litter decomposition rates and lignin/N and C/N ratios. It can be concluded, from this study, that at levels of atmospheric CO₂ predicted for the middle of the next century a deterioration of litter quality will result in decreased decomposition rates, leading to reduction of nutrient mineralization and increased C storage in forest ecosystems. However, such conclusions are difficult to generalize, since tree responses to elevated CO₂ depend on soil nutritional status.     

Chemistry and decomposition of litter from Populus tremuloides Michaux grown at elevated atmospheric CO2 and varying N availability

It has been hypothesized that greater production of total nonstructural carbohydrates (TNC) in foliage grown under elevated atmospheric carbon dioxide (CO₂) will result in higher concentrations of defensive compounds in tree leaf litter, possibly leading to reduced rates of decomposition and nutrient cycling in forest ecosystems of the future. To evaluate the effects of elevated atmospheric CO₂ on litter chemistry and decomposition, we performed a 111 day laboratory incubation with leaf litter of trembling aspen (Populus tremuloides Michaux) produced at 36 Pa and 56 Pa CO₂ and two levels of soil nitrogen (N) availability. Decomposition was quantified as microbially respired CO₂ and dissolved organic carbon (DOC) in soil solution, and concentrations of nonstructural carbohydrates, N, carbon (C), and condensed tannins were monitored throughout the incubation. Growth under elevated atmospheric CO₂ did not significantly affect initial litter concentrations of TNC, N, or condensed tannins. Rates of decomposition, measured as both microbially respired CO₂ and DOC did not differ between litter produced under ambient and elevated CO₂. Total C lost from the samples was 38 mg g^?1 litter as respired CO₂ and 138 mg g^?1 litter as DOC, suggesting short‐term pulses of dissolved C in soil solution are important components of the terrestrial C cycle. We conclude that litter chemistry and decomposition in trembling aspen are minimally affected by growth under higher concentrations of CO₂.     

Effects of enhanced atmospheric CO2 and nutrient supply on the quality and subsequent decomposition of fine roots of Betula pendula Roth. and Picea sitchensis (Bong.) Carr.

Fine root litter derived from birch (Betula pendula Roth.) and Sitka spruce (Picea sitchensis (Bong.) Carr.) plants grown under two CO₂ atmospheric concentrations (350 ppm and 600 ppm) and two nutrient regimes was used for decomposition studies in laboratory microcosms. Although there were interactions between litter type, CO₂/fertiliser treatments and decomposition rates, in general, an increase in the C/N ratio of the root tissue was observed for roots of both species grown under elevated CO₂ in unfertilized soil. Both weight loss and respiration of decomposing birch roots were significantly reduced in materials derived from enriched CO₂, whilst the decomposition of spruce roots showed no such effect. A parallel experiment was performed using Betula pendula root litter grown under different N regimes, in order to test the relationship between C/N ratio of litter and root decomposition rate. A highly significant (p<0.001) negative correlation between C/N ratio and root litter respiration was found, with an r²=0.97. The results suggest that the increased C/N ratio of plant tissues induced by elevated CO₂ can result in a reduction of decomposition rate, with a resulting increase in forest soil C stores.     

Elevated CO2 increases the long-term decomposition rate of Quercus myrtifolia leaf litter

Decomposition of Quercus myrtifolia leaf litter in a Florida scrub oak community was followed for 3 years in two separate experiments. In the first experiment, we examined the effects CO₂ and herbivore damage on litter quality and subsequent decomposition. Undamaged, chewed and mined litter generated under ambient and elevated (ambient+350 ppm V) CO₂ was allowed to decompose under ambient conditions for 3 years. Initial litter chemistry indicated that CO₂ levels had minor effects on litter quality. Litter damaged by leaf miners had higher initial concentrations of condensed tannins and nitrogen (N) and lower concentrations of hemicellulose and C : N ratios compared with undamaged and chewed litter. Despite variation in litter quality associated with CO₂, herbivory, and their interaction, there was no subsequent effect on rates of decomposition under ambient atmospheric conditions. In the second experiment, we examined the effects of source (ambient and elevated) of litter and decomposition site (ambient and elevated) on litter decomposition and N dynamics. Litter was not separated by damage type. The litter from both elevated and ambient CO₂ was then decomposed in both elevated and ambient CO₂ chambers. Initial litter chemistry indicated that concentrations of carbon (C), hemicellulose, and lignin were higher in litter from elevated than ambient CO₂ chambers. Despite differences in C and fiber concentrations, litter from ambient and elevated CO₂ decomposed at comparable rates. However, the atmosphere in which the decomposition took place resulted in significant differences in rates of decomposition. Litter decomposing under elevated CO₂ decomposed more rapidly than litter under ambient CO₂, and exhibited higher rates of mineral N accumulation. The results suggest that the atmospheric conditions during the decomposition process have a greater impact on rates of decomposition and N cycling than do the atmospheric conditions under which the foliage was produced.     

Leaf litter production and decomposition in a poplar short-rotation coppice exposed to free air CO2 enrichment (POPFACE)

The capacity of forest ecosystems to sequester C in the soil relies on the net balance between litter production above, as well as, below ground, and decomposition processes. Nitrogen mineralization and its availability for plant growth and microbial activity often control the speed of both processes. Litter production, decomposition and N mineralization are strongly interdependent. Thus, their responses to global environmental changes (i.e. elevated CO₂, climate, N deposition, etc.) cannot be fully understood if they are studied in isolation. In the present experiment, we investigated litter fall, litter decomposition and N dynamics in decomposing litter of three Populus spp., in the second and third growing season of a short rotation coppice under FACE. Elevated CO₂ did not affect annual litter production but slightly retarded litter fall in the third growing season. In all species, elevated CO₂ lowered N concentration, resulting in a reduction of N input to the soil via litter fall, but did not affect lignin concentrations. Litter decomposition was studied in bags incubated in situ both in control and FACE plots. Litter lost between 15% and 18% of the original mass during the eight months of field incubation. On average, litter produced under elevated CO₂ attained higher residual mass than control litter. On the other end, when litter was incubated in FACE plots it exhibited higher decay rates. These responses were strongly species‐specific. All litter increased their N content during decomposition, indicating immobilization of N from external sources. Independent of the initial quality, litter incubated on FACE soils immobilized less N, possibly as a result of lower N availability in the soil. Indeed, our results refer to a short‐term decomposition experiment. However, according to a longer‐term model extrapolation of our results, we anticipate that in Mediterranean climate, under elevated atmospheric CO₂, soil organic C pool of forest ecosystems may initially display faster turnover, but soil N availability will eventually limit the process.     

Plant species and nutritional-mediated control over rhizodeposition and root decomposition

This study focuses on the influence of nitrogen (N) availability and species on rhizodeposition and on decomposition of rhizodeposits, roots and soil organic matter. Four perennial grass species were studied that are characteristic of grassland habitats that differ in nutrient availability. These perennial grass species, Holcus lanatus L., Festuca rubra L., Anthoxanthum odoratum L. and Festuca ovina L., were homogeneously labeled with ¹⁴CO₂. Plants were grown on soil without N addition and with N addition (14 g N m^–2). After 8 weeks, plants were harvested and root production and the remaining amount of rhizodeposits in the soil were measured. ¹⁴C-labelled roots were incubated in fresh soil. Decomposition was measured of 1) the labeled rhizodeposits in the soil in which the plants had been growing and 2) the labeled dead roots incubated in fresh soil, by trapping the evolved ¹⁴CO₂, over 69 days.In general, decomposability of both roots and rhizodeposits increased when nitrogen availability increased. Moreover, the species differed in their response to N. Higher N supply increased total rhizodeposition of H. lanatus and the decomposability of rhizodeposited carbon compounds of this high fertility species was greater than of the low fertility species F. ovina, but lower than of A. odoratum. The presented study gives no evidence for a relation between root decomposition rate and the nutrient availability of the habitat of the four species. Overall, we suggest on the basis of the results that species can affect nutrient cycling by differences in rates of rhizodeposition and litter production. This offers a mechanism whereby species can influence species replacement during succession.     

Isopod effects on decomposition of litter produced under elevated CO2, N deposition and different soil types

The performance of Oniscus asellus (Isopoda) and its influence on litter mass loss and mineralization was assessed in a microcosm experiment, using beech (Fagus sylvatica) leaf litter that was produced on different soil types, contrasting atmospheric CO₂ concentrations, and different nitrogen deposition rates. Litter quality was significantly altered by these treatments, and many of the CO₂ and N effects differed between soil types. Litter quality affected subsequent litter mass loss rates, microbial respiration, and leaching of dissolved organic carbon (DOC) and nitrate. These effects were largely independent of the presence of isopods, even though isopods highly accelerated litter mass loss, stimulated microbial respiration by 37%, and increased nitrate leaching by 50%. Isopods did not change their relative rates of litter consumption and growth in response to litter quality. Isopod mortality, however, increased with increasing litter lignin/N ratios, and was significantly different between soil types, which may indicate long‐term effects on litter decomposition through altered isopod densities. Having the choice among the litter of three different species [maple (Acer pseudoplatanus), beech (Fagus sylvatica) and oak (Quercus robur)] grown at either ambient or elevated CO₂, isopods preferred maple to beech when all the litter was produced under elevated CO₂. This suggests that beyond changes in consumption, an altered food selection by isopods in a CO₂‐enriched atmosphere may affect the temporal and spatial composition of the litter layer in temperate forests. In contrast to previous findings, we conclude that isopods do not always increase their consumption rates, and hence do not differentially affect microbial decomposition in litter of poorer quality. Nevertheless changes in animal densities and/or shifts in their food preferences, could result in altered decomposition and carbon and nutrient turnover rates.     

Decomposition of tree leaf litters grown under elevated CO2: Effect of litter quality

Ash (Fraxinus excelsior L.), birch (Betula pubescens Ehrh.), sycamore (Acer pseudoplatanus L.) and Sitka spruce (Picea sitchensis (Bong.) Carr.) leaf litters were monitored for decomposition rates and nutrient release in a laboratory microcosm experiment. Litters were derived from solar domes where plants had been exposed to two different CO₂ regimes: ambient (350 L L^-1 CO₂) and enriched (600 L L^-1 CO₂).Elevated CO₂ significantly affected some of the major litter quality parameters, with lower N, higher lignin concentrations and higher ratios of C/N and lignin/N for litters derived from enriched CO₂. Respiration rates of the deciduous species were significantly decreased for litters grown under elevated CO₂, and reductions in mass loss at the end of the experiment were generally observed in litters derived from the 600 ppm CO₂ treatment. Nutrient mineralization, dissolved organic carbon, and pH in microcosm leachates did not differ significantly between the two CO₂ treatments for any of the species studied. Litter quality parameters were examined for correlations with cumulative respiration and decomposition rates: N concentration, C/N and lignin/N ratios showed the highest correlations, with differences between litter types. The results indicate that higher C storage will occur in soil as a consequence of litter quality changes resulting from higher atmospheric concentrations of CO₂.Abbreviations CHO soluble carbohydrates - DOC dissolved organic carbon - HCel holocellulose - WTREM weight remaining     

Decomposition of litter produced under elevated CO2: Dependence on plant species and nutrient supply

We investigated the effect of CO₂ concentration and soilnutrient availability during growth on the subsequent decomposition andnitrogen (N) release from litter of four annual grasses that differ inresource requirements and native habitat. Vulpia microstachys isa native grass found on California serpentine soils, whereas Avenafatua, Bromus hordaceus, and Lolium multiflorum areintroduced grasses restricted to more fertile sandstone soils (Hobbs & Mooney 1991). Growth in elevated CO₂ altered litter C:N ratio,decomposition, and N release, but the direction and magnitude of thechanges differed among plant species and nutrient treatments. ElevatedCO₂ had relatively modest effects on C:N ratio of litter,increasing this ratio in Lolium roots (and shoots at high nutrients),but decreasing C:N ratio in Avena shoots. Growth of plants underelevated CO₂ decreased the decomposition rate of Vulpialitter, but increased decomposition of Avena litter from the high-nutrient treatment. The impact of elevated CO₂ on N loss fromlitter also differed among species, with Vulpia litter from high-CO₂ plants releasing N more slowly than ambient-CO₂litter, whereas growth under elevated CO₂ caused increased Nloss from Avena litter. CO₂ effects on N release in Lolium and Bromus depended on the nutrient regime in whichplants were grown. There was no overall relationship between litter C:Nratio and decomposition rate or N release across species and treatments.Based on our study and the literature, we conclude that the effects ofelevated CO₂ on decomposition and N release from litter arehighly species-specific. These results do not support the hypothesis thatCO₂ effects on litter quality consistently lead to decreasednutrient availability in nutrient-limited ecosystems exposed to elevatedCO₂.     

Effects of elevated [CO2] on forest growth and carbon storage: a modelling analysis of the consequences of changes in litter quality/quantity and root exudation

Many researchers have proposed that the stimulus of plant growth under elevated [CO₂] observed in short-term experiments will be moderated in the longer term by a reduction in soil nitrogen (N) availability linked to decreased litter quality and/or increased litter production. However, these negative feedbacks may be offset to some extent by a stimulus in N fixation linked to increased root exudation. The aim of this modelling study is to examine how changes in litter quality/quantity and root exudation –- if they occur –- will affect the CO₂ responses of net primary productivity and ecosystem carbon (C) storage on different timescales. We apply a model of C and N cycling in forest ecosystems (G’DAY) to stands of Norway spruce (Picea abies, L. Cast) growing at a N-limited experimental site at Flakaliden, Sweden, and draw the following conclusions: (1) in the absence of changes in litter quality and root exudation, the short-term CO₂ stimulus of litter quantity leads to only a minimal CO₂ stimulus of productivity or C storage in the medium term (≈ 20 years) and long term (≈ 200 years), because of constraints on soil N availability; (2) increasing plant nitrogen use efficiency (via a decrease in the N:C ratio of new litter) makes little impact on these results; (3) a significant CO₂ response in the medium term requires a substantial decrease in the N:C ratio of older litter, when it is approaching stabilisation as soil organic matter, although the long-term CO₂ response remains small; and (4) an increase in N fixation leads to a small effect on productivity in the short term, but a very large effect on both productivity and C storage in the long term. These results suggest that soil N constraints on the long-term CO₂-fertilisation effect can be overcome to a significant extent only by increases in N acquisition, although only modest increases may be required. This revised version was published online in June 2006 with corrections to the Cover Date.     

Laboratory incubations reveal potential responses of soil nitrogen cycling to changes in soil C and N availability in Mojave Desert soils exposed to elevated atmospheric CO2

Elevated atmospheric carbon dioxide (CO₂) has the potential to alter soil carbon (C) and nitrogen (N) cycling in arid ecosystems through changes in net primary productivity. However, an associated feedback exists because any sustained increases in plant productivity will depend upon the continued availability of soil N. We took soils from under the canopies of major shrubs, grasses, and plant interspaces in a Mojave Desert ecosystem exposed to elevated atmospheric CO₂ and incubated them in the laboratory with amendments of labile C and N to determine if elevated CO₂ altered the mechanistic controls of soil C and N on microbial N cycling. Net ammonification increased under shrubs exposed to elevated CO₂, while net nitrification decreased. Elevated CO₂ treatments exhibited greater fluxes of N₂O–N under Lycium spp., but not other microsites. The proportion of microbial/extractable organic N increased under shrubs exposed to elevated CO₂. Heterotrophic N₂‐fixation and C mineralization increased with C addition, while denitrification enzyme activity and N₂O–N fluxes increased when C and N were added in combination. Laboratory results demonstrated the potential for elevated CO₂ to affect soil N cycling under shrubs and supports the hypothesis that energy limited microbes may increase net inorganic N cycling rates as the amount of soil‐available C increases under elevated CO₂. The effect of CO₂ enrichment on N‐cycling processes is mediated by its effect on the plants, particularly shrubs. The potential for elevated atmospheric CO₂ to lead to accumulation of NH₄⁺ under shrubs and the subsequent volatilization of NH₃ may result in greater losses of N from this system, leading to changes in the form and amount of plant‐available inorganic N. This introduces the potential for a negative feedback mechanism that could act to constrain the degree to which plants can increase productivity in the face of elevated atmospheric CO₂.     

Short-term decomposition of litter produced by plants grown in ambient and elevated atmospheric CO2 concentrations

The effects of elevated atmospheric CO₂ (ambient + 340 μmol mol^–1) on above-ground litter decomposition were investigated over a 6-week period using a field-based mesocosm system. Soil respiratory activity in mesocosms incubated in ambient and elevated atmospheric CO₂ concentrations were not significantly different (t-test, P > 0.05) indicating that there were no direct effects of elevated atmospheric CO₂ on litter decomposition. A study of the indirect effects of CO₂ on soil respiration showed that soil mesocosms to which naturally senescent plant litter had been added (0.5% w/w) from the C₃ sedge Scirpus olneyi grown in elevated atmospheric CO₂ was reduced by an average of 17% throughout the study when compared to soil mesocosms to which litter from Scirpus olneyi grown in ambient conditions had been added. In contrast, similar experiments using senescent material from the C₄ grass Spartina patens showed no difference in soil respiration rates between mesocosms to which litter from plants grown in elevated or ambient CO₂ conditions had been added. Analysis of the C:N ratio and lignin content of the senescent material showed that, while the C:N ratio and lignin content of the Spartina patens litter did not vary with atmospheric CO₂ conditions, the C:N ratio (but not the lignin content) of the litter from Scirpus olneyi was significantly greater (t-test;P < 0.05) when derived from plants grown under elevated CO₂ (105:1 compared to 86:1 for litter derived from Scirpus olneyi grown under ambient conditions). The results suggest that the increased C:N ratio of the litter from the C₃ plant Scirpus olneyi grown under elevated CO₂ led to the lower rates of biodegradation observed as reduced soil respiration in the mesocosms. Further long-term experiments are now required to determine the effects of elevated CO₂ on C partitioning in terrestrial ecosystems.     

Litter decomposition and nitrogen mineralization of soils in subtropical plantation forests of southern China,with special attention to comparisons between legumes and non-legumes

Litter decomposition and nitrogen mineralization were investigated in subtropical plantation forests in southern China. The CO₂ –C release from incubated litter and the forest floor of Acacia mangium, Acacia auriculaeformis, Eucalyptus citriodora, Pinus elliotii and Schima superba stands were used to estimate relative rates of litter decomposition. Decomposition was not positively correlated with litter nitrogen. E. citridora litter decomposed most rapidly and A. mangium litter most slowly, both with and without the addition of exotic nitrogen. Aerobic incubation and intact soil core incubation at 30 °C over a period of 30 days were used to assess nitrogen mineralization of six forest soils. Although there were differences in results obtained using the two methods, patterns between legume and non-legume species were the same regardless of method. All soils had pH values below 4.5, but this did not prevent nitrification. The dominant form of mineral nitrogen was nitrate for legume species and ammonium for non-legume species. The nitrogen mineralization potential was highest for soils in which legumes were growing.     

Impacts of elevated atmospheric CO2 on litter quality, litter decomposability and nitrogen turnover rate of two oak species in a Mediterranean forest ecosystem

Elevated CO₂ may affect litter quality of plants, and subsequently C and N cycling in terrestrial ecosystems, but changes in litter quality associated with elevated CO₂ are poorly known. Abscised leaf litter of two oak species (Quercus cerris L. and Q. pubescens Willd.) exposed to long-term elevated CO₂ around a natural CO₂ spring in Tuscany (Italy) was used to study the impact of increasing concentration of atmospheric CO₂ on litter quality and C and N turnover rates in a Mediterranean-type ecosystem. Litter samples were collected in an area with elevated CO₂ (>500 ppm) and in an area with ambient CO₂ concentration (360 ppm). Leaf samples were analysed for concentrations of total C, N, lignin, cellulose, acid detergent residue (ADR) and polyphenol. The decomposition rate of litter was studied using a litter bag experiment (12 months) and laboratory incubations (3 months). In the laboratory incubations, N mineralization in litter samples was measured as well (125 days). Litter quality was expressed in terms of chemical composition and element ratios. None of the litter quality parameters was affected by elevated CO₂ for the two Quercus species. Remaining mass in Q. cerris and Q. pubescens litter from elevated CO₂ was similar to that from ambient conditions. C mineralization in Q. pubescens litter from elevated CO₂ was lower than that from ambient CO₂, but the difference was insignificant. This effect was not observed for Q. cerris. N mineralization was higher from litter grown at elevated CO₂, but this difference disappeared at the end of the incubation. Litter of Q. pubescens had a higher quality than Q. cerris, and indeed mineralized more rapidly in the laboratory, but not under field conditions.     

In situ litter decomposition and litter quality in a Mojave Desert ecosystem: effects of elevated atmospheric CO2 and interannual climate variability

Rising atmospheric CO₂ has been predicted to reduce litter decomposition as a result of CO₂‐induced reductions in litter quality. However, available data have not supported this hypothesis in mesic ecosystems, and no data are available for desert or semi‐arid ecosystems, which account for more than 35% of the Earth's land area. The objective of our study was to explore controls on litter decomposition in the Mojave Desert using elevated CO₂ and interannual climate variability as driving environmental factors. In particular, we sought to evaluate the extent to which decomposition is modulated by litter chemistry (C:N) and litter species and tissue composition. Naturally senesced litter was collected from each of nine 25 m diameter experimental plots, with six plots exposed to ambient [CO₂] or 367 μL CO₂ L^?1 and three plots continuously fumigated with elevated [CO₂] (550 μL CO₂ L^?1) using FACE technology beginning in April 1997. All litter collected in 1998 (a wet, or El Niño year; 306 mm precipitation) was pooled as was litter collected in 1999 (a dry year; 94 mm). Samples were allowed to decompose for 4 and 12 months starting in May 2001 in mesh litterbags in the locations from which litter was collected. Decomposition of litter produced under elevated CO₂ and ambient CO₂ did not differ. Litter produced in the wetter year showed more rapid initial decomposition (over the first 4 months) than that produced in the drier year (27±2% yr^?1 or 7.8±0.7 g m^?2 yr^?1 for 1998 litter; 18±3% yr^?1 or 2.2±0.4 g m^?2 yr^?1 for 1999 litter). C:N ratios of litter produced under elevated CO₂ (wet year: 37±0.5; dry year: 42±2.5) were higher than those of litter produced under ambient CO₂ (wet year: 34±1.1; dry year: 35±1.4). Litter production in the wet year (amb. CO₂: 25.1±1.1 g m^?2 yr^?1; elev. CO₂: 35.0±1.1 g m^?2 yr^?1) was more than twice as high as that in the dry year (amb. CO₂: 11.6±1.7 g m^?2, elev. CO₂: 13.3±3.4 g m^?2), and contained a greater proportion of Lycium pallidum and a lower proportion of Larrea tridentata than litter produced in the dry year. Decomposition, viewed across all treatments, decreased with increasing C:N ratios, decreased with increasing proportions of Larrea tridentata and increased with increasing proportions of Lycium pallidum and Lycium andersonii. Because litter C:N did not vary by litter production year, and CO₂ did not alter decomposition or litter species/tissue composition, it is likely that the impact of year‐to‐year variation in precipitation on the proportion of key plant species in the litter may be the most important way in which litter decomposition will be modulated in the Mojave Desert under future rising atmospheric CO₂.     

Chemical Composition and Decomposition of Silver Birch Leaf Litter Produced under Elevated CO2 and O3

Two field-growing silver birch (Betula pendula Roth) clones (clone 4 and 80) were exposed to elevated CO₂ and O₃ over three growing seasons (1999–2001). In each year, the nutrients and cell wall chemistry of naturally abscised leaf litter were analyzed in order to determine the possible CO₂- and O₃-induced changes in the litter quality. Also CO₂ and O₃ effects on the early leaf litter decomposition dynamics (i.e. decomposition before the lignin decay has started) were studied with litter-bag experiments (Incubation 1 with 1999 leaf litter, Incubation 2 with 2000 leaf litter, and Incubation 3 with 2001 leaf litter) in a nearby silver birch forest. Elevated CO₂ decreased N, S, C:P and α-cellulose concentrations, but increased P, hemicellulose and lignin+polyphenolic concentrations, C:N and lignin+polyphenolic:N in both clones. CO₂ enrichment decreased the subsequent decomposition of leaves of clone 4 transiently (in Incubations 1 and 2), whereas elevated CO₂ effects on the subsequent leaf decomposition of clone 80 were inconsistent. In contrast to CO₂, O₃ decreased P concentrations and increased C:P, but both of these trends were visible in elevated O₃ treatment only. O₃-induced decreases in Mn, Zn and B concentrations were observed also, but O₃ effects on the cell wall chemistry of leaf litter were minor. Some O₃-induced changes either became more consistent in leaf litter collected during 2001 (decrease in B concentrations) or appeared only in this litter lot (decrease in N concentrations, decrease in decomposition at the end of Incubation 3). In conclusion, in northern birch forests elevated CO₂ and O₃ levels have the potential to affect leaf litter quality, but consistent CO₂ and O₃ effects on the decomposition process remain to be validated.     

Elevated CO2 effects on decomposition processes in a grazed grassland

The effects of elevated atmospheric CO₂ (475 μL L^?1) on in situ decomposition of plant litter and animal faecal material were studied over 2 years in a free air CO₂ enrichment (FACE) facility. The pasture was grazed by sheep and contained a mixture of C₃ and C₄ grasses, legumes and forbs. There was no effect of elevated CO₂ on decomposition within plant species but marked differences between species with faster decomposition in dicots; a group that increased in abundance at elevated CO₂. Decomposition of mixed herbage root material occurred at a similar rate to that of leaf litter suggesting that any CO₂‐induced increase in carbon allocation to roots would not reduce rates of decomposition. Sheep faeces resulting from a ‘high‐CO₂ diet’ decomposed significantly slower during summer but not during winter. The overall outcome of these experiments were explored using scenarios that took account of changes in botanical composition, allocation to roots and the presence of herbivores. In the absence of herbivores, elevated CO₂ led to a 15% increase in the rate of mass loss and an 18% increase in the rate of nitrogen (N) release. In the presence of herbivores, these effects were partially removed (11% increase in rate of mass loss and 9% decrease in N release rate) because of the recycling occurring through the animals in the form of faeces.     

Differential Controls of Water Input on Litter Decomposition and Nitrogen Dynamics in the Patagonian Steppe

Studies of the effects of precipitation on litter decomposition and nitrogen mineralization in arid and semiarid environments have demonstrated contradictory results. We conducted a manipulative experiment with rainout shelters in the semiarid Patagonian steppe, aimed at assessing the direct effects of water availability on litter decomposition and net nitrogen mineralization while isolating the indirect effects. We created four levels of precipitation input: control and three levels (30, 55 and 80%) of precipitation interception and we examined litter decomposition and nutrient release of a dominant grass species, Stipa speciosa, inorganic soil nitrogen, and in situ net nitrogen mineralization over two consecutive years. Litter decomposition rates (k, year⁻¹) varied significantly (P < 0.001) among precipitation interception treatments and were positively correlated with incoming annual precipitation (APPT, mm/year) (k = 0.0007 × APPT + 0.137). In contrast, net N mineralization was not correlated with incoming precipitation. Soil NO₃⁻ significantly decreased with increasing precipitation input, whereas soil NH₄⁺ concentration did not differ among precipitation interception treatments. Controls of water input on litter decomposition appear to be different from controls on N mineralization in the semiarid Patagonian steppe. We suggest that although water availability affects both the mineralization of C and N, it differentially affects the movement and fate of the inorganic products. A consequence of the accumulation of inorganic N during dry episodes is that periods of maximum water and soil nutrient availability may occur at different times. This asynchrony in the availability of N and water in the soil may explain the observed lags in the response of primary production to increases in water availability.     

Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities?

Though field data for naturally senesced leaf litter are rare, it is commonly assumed that rising atmospheric CO₂ concentrations will reduce leaf litter quality and decomposition rates in terrestrial ecosystems and that this will lead to decreased rates of nutrient cycling and increased carbon sequestration in native ecosystems. We generally found that the quality of␣naturally senesced leaf litter (i.e. concentrations of C, N and lignin; C:N, lignin:N) of a variety of native plant species produced in alpine, temperate and tropical communities maintained at elevated CO₂ (600–680 μl l⁻¹) was not significantly different from that produced in similar communities maintained at current ambient CO₂ concentrations (340–355 μl l⁻¹). When this litter was allowed to decompose in situ in a humid tropical forest in Panama (Cecropia peltata, Elettaria cardamomum, and Ficus benjamina, 130 days exposure) and in a lowland temperate calcareous grassland in Switzerland (Carex flacca and a graminoid species mixture; 261 days exposure), decomposition rates of litter produced under ambient and elevated CO₂ did not differ significantly. The one exception to this pattern occurred in the high alpine sedge, Carex curvula, growing in the Swiss Alps. Decomposition of litter produced in situ under elevated CO₂ was significantly slower than that of litter produced under ambient CO₂ (14% vs. 21% of the initial litter mass had decomposed over a 61-day exposure period, respectively). Overall, our results indicate that relatively little or no change in leaf litter quality can be expected in plant communities growing under soil fertilities common in many native ecosystems as atmospheric CO₂ concentrations continue to rise. Even in situations where small reductions in litter quality do occur, these may not necessarily lead to significantly slower rates of decomposition. Hence in many native species in situ litter decomposition rates, and the time course of decomposition, may remain relatively unaffected by rising CO₂. Received: 12 September 1996 / Accepted: 30 November 1996