Influence of interactions between litter decomposition and rhizosphere activity on soil respiration and on the temperature sensitivity in a subtropical montane forest in SW China

Aims

The aims were to identify the effects of interactions between litter decomposition and rhizosphere activity on soil respiration and on the temperature sensitivity of soil respiration in a subtropical forest in SW China.

Methods

Four treatments were established: control (CK), litter removal (NL), trenching (NR) and trenching together with litter removal (NRNL). Soil CO₂ efflux, soil temperature, and soil water content were measured once a month over two years. Soil respiration was divided into four components: the decomposition of basic soil organic matter (SOM), litter respiration, root respiration, and the interaction effect between litter decomposition and rhizosphere activity. A two-factor regression equation was used to correct the value of soil CO₂ efflux.

Results

We found a significant effect of the interaction between litter decomposition and rhizosphere activity (R _INT) on total soil respiration, and R _INT exhibited significant seasonal variation, accounting for 26 and 31 % of total soil respiration in the dry and rainy seasons, respectively. However, we found no significant interaction effect on the temperature sensitivity of soil respiration. The temperature sensitivity was significantly increased by trenching compared with the control, but was unchanged by litter removal.

Conclusions

Though the interaction between litter decomposition and rhizosphere activity had no effects on temperature sensitivity, it had a significant positive effect on soil respiration. Our results not only showed strong influence of rhizosphere activity on temperature sensitivity, but provided a viable way to identify the contribution of SOM to soil respiration, which could help researchers gain insights on the carbon cycle.     

Effects of irrigation on the soil CO2 efflux from different poplar clone plantations in arid northwest China

Aims

Soil respiration in forest plantations can be greatly affected by management practices such as irrigation. In northwest China, soil water is usually a limiting factor for the development of forest plantations. This study aims to examine the effects of irrigation intensity on soil respiration from three poplar clone plantations in this arid area.

Methods

The experiment included three poplar clones subjected to three irrigation intensities (without, low and high). Soil respiration was measured using a Li-6400-09 chamber during the growing season in 2007.

Results

Mean soil respiration rates were 2.92, 4.74 and 3.49 μmol m^?2 s^?1 for control, low and high irrigation treatments, respectively. Soil respiration decreased once soil water content was below a lower (14.8 %) or above an upper (26.2 %) threshold. When soil water content ranged from 14.8 % to 26.2 %, soil respiration increased and correlated with soil temperature. Fine root also played a role in the significant differences in soil CO₂ efflux among the three treatments. Furthermore, the three poplar hybrid clones responded differently to irrigation regarding fine root production and soil CO₂ efflux.

Conclusions

Irrigation intensity had a strong impact on soil respiration of the three poplar clone plantations, which was mainly because fine root biomass and microbial activities were greatly influenced by soil water conditions. Our results suggest that irrigation management is a main factor controlling soil carbon dynamics in forest plantation in arid regions.     

Carbon allocation in Larrea tridentata plant-soil systems as affected by elevated soil moisture and N availability

Background and Aims

Global change will likely express itself in southwestern United States arid lands through changes in amounts and timing of precipitation in response to elevated CO₂ concentrations. In addition, increased nitrogen (N) deposition may occur due to increased urban development. This study addressed the effects of water and N availability on C allocation in arid land soil-plant systems.

Methods

Columns filled with Mojave Desert topsoil containing Larrea tridentata seedlings with two treatment levels each of N and soil moisture were labeled by exposure to ¹³C-enriched CO₂.

Results

Increased soil moisture increased plant biomass, total ¹³C uptake, ¹³C levels in leaves, soil organic matter, and soil respiration, decreased relative C allocation to stems but increased allocation to soil organic matter. Increased soil N availability increased N uptake but decreased C allocation to soil respiration presumably due to decreased substrate supply for microbes. There was no detectable label in carbonate C, suggesting that this pool does not significantly contribute to ecosystem C fluxes.

Conclusions

Our study indicates that increased water availability causes increased C uptake with increased C allocation to soil organic matter in Larrea tridentata-dominated communities while increased N deposition will have a minimal impact on C sequestration.     

Annual variation in soil respiration and its component parts in two structurally contrasting woody savannas in Central Brazil

Background and aims

Due to the high spatial and temporal variation in soil CO₂ efflux, terrestrial carbon budgets rely on a detailed understanding of the drivers of soil respiration from a diverse range of ecosystems and climate zones. In this study we aim to evaluate the independent influence of vegetation structure and climate on soil CO₂ efflux within cerrado ecosystems.

Methods

We examine the seasonal and diel variation of soil CO₂ efflux, including its autotrophic and heterotrophic components, within two adjacent and structurally contrasting woody savannas in central Brazil.

Principle results

We found no significant difference in the annual soil CO₂ efflux between the two stands (p?=?0.53) despite a clear disparity in both LAI (p?<?0.01) and leaf litterfall (p?<?0.01). The mean annual loss of carbon from the soil was 17.32(±1.48) Mg C?ha^?1 of which approximately 63% was accounted for by autotrophic respiration. The relative contribution of autotrophic respiration varied seasonally between 55% in the wet season to 79% of the total soil CO₂ efflux in the dry season. Furthermore, seasonal fluctuations of all the soil respiration components were strongly correlated with soil moisture (R ²?=?0.79–0.90, p?<?0.01).

Conclusions

Across these two structurally distinct cerrado stands, seasonal variations in rainfall, was the main driver of soil CO₂ efflux and its components. Consequently, soil respiration within these ecosystems is likely to be highly sensitive to any changes in seasonal precipitation patterns.     

Components of forest soil CO2 efflux estimated from Δ14C values of soil organic matter

Aims

The partitioning of the total soil CO₂ efflux into its two main components: respiration from roots (and root-associated organisms) and microbial respiration (by means of soil organic matter (SOM) and litter decomposition), is a major need in soil carbon dynamics studies in order to understand if a soil is a net sink or source of carbon.

Methods

The heterotrophic component of the CO₂ efflux was estimated for 11 forest sites as the ratio between the carbon stocks of different SOM pools and previously published (Δ¹⁴C derived) turnover times. The autotrophic component, including root and root-associated respiration, was calculated by subtracting the heterotrophic component from total soil chamber measured CO₂ efflux.

Results

Results suggested that, on average, 50.4 % of total soil CO₂ efflux was derived from the respiration of the living roots, 42.4 % from decomposition of the litter layers and less than 10 % from decomposition of belowground SOM.

Conclusions

The Δ¹⁴C method proved to be an efficient tool by which to partition soil CO₂ efflux and quantify the contribution of the different components of soil respiration. However the average calculated heterotrophic respiration was statistically lower compared with two previous studies dealing with soil CO₂ efflux partitioning (one performed in the same study area; the other a meta-analysis of soil respiration partitioning). These differences were probably due to the heterogeneity of the SOM fraction and to a sub-optimal choice of the litter sampling period.     

Soil warming accelerates decomposition of fine woody debris

Aims

There is evidence that increased N inputs to boreal forests, via atmospheric deposition or intentional fertilization, may impact negatively on ectomycorrhizal (ECM) fungi leading to a reduced flux of plant-derived carbon (C) back to the atmosphere via ECM. Our aim was to investigate the impact of N fertilization of a Pinus sylvestris (L.) forest stand on the return of recently photoassimilated C via the ECM component of soil respiration.

Methods

We used an in situ, large-scale, ¹³C-CO₂ isotopic pulse labelling approach and monitored the ¹³C label return using soil gas efflux chambers placed over three different types of soil collar to distinguish between heterotrophic (R_H), autotrophic (R_A; partitioned further into contributions from ECM hyphae and total R_A) and total (R_S) soil respiration.

Results

The impact of N fertilization was to significantly reduce R_A, particularly respiration via extramatrical ECM hyphae. ECM hyphal flux in control plots showed substantial spatial variability, resulting in mean flux estimates exceeding estimates of total R_A, while ECM contributions to R_A in N treated plots were estimated at around 30%.

Conclusion

Significant impacts on soil C cycling may be caused by reduced plant C allocation to ECM fungi in response to increased N inputs to boreal forests; ecosystem models so far lack this detail.     

Root respiration of barley in a semiarid Mediterranean agroecosystem: field and modelling approaches

Aims

Root respiration is a major contributor to soil CO₂ flux, and its response to management practices needs to be evaluated. The aim was to determine the effect of management practices (tillage systems and nitrogen fertilization levels) on root respiration and to develop a model able to simulate root respiration and its components.

Methods

The study was carried out during two contrasting growing seasons (2007–2008 and 2008–2009). Root respiration, including root tissue respiration (R _ts ) and rhizomicrobial respiration of exudates (R _rz ), was estimated as the difference between the soil CO₂ flux of cropped and bare soil (the so-called root exclusion technique). Additionally a novel sub-model of R _ts , was used to simulate root respiration based on root growth and specific root respiration rates.

Results

Root respiration was reduced under no-tillage. The model agreed well with the patterns and the amounts of the observed values of root respiration, although prior calibration was needed.

Conclusions

Root respiration was reduced by the long-term adoption of no-tillage, but was increased by N fertilizer. The root exclusion technique and the model were useful means to estimate root respiration on cropland under semiarid Mediterranean conditions. Additionally the model successfully separated out the theoretical contributions of R _ts and R _rz to root respiration.     

Effects of elevated CO2 and nitrogen addition on soil organic carbon fractions in a subtropical forest

Aims

The aim of this study was to investigate the effects of elevated CO₂ concentration and nitrogen addition on soil organic carbon fractions in subtropical forests where the ambient N deposition was high.

Methods

Seedlings of typical subtropical forest ecosystems were transplanted in ten open-top chambers and grown under CO₂ and nitrogen treatments. The treatments included: 1) elevated CO₂ (700?μmol?mol^-1); 2) N addition of 100?kg NH₄NO₃ ha^-1?yr^-1; 3) combined elevated CO₂ and N addition; and 4) control. We measured soil total organic carbon (TOC), particulate organic carbon (POC), readily oxidizable organic carbon (ROC), and microbial biomass carbon (MBC).

Results

Results showed that elevated CO₂ alone did not significantly affect soil TOC, POC and ROC after 4?years of treatment, but increased soil MBC and soil respiration compared to the control. N addition alone had no significant effect neither on soil TOC, POC and ROC, but decreased MBC and soil respiration over time. However, the elevated CO₂ and N addition together significantly increased soil POC and ROC, and had no significant effect on soil MBC.

Conclusions

This study indicated that even in N-rich subtropical forest ecosystems, inputs of N are still needed in order to sustain soil C accumulation under elevated CO₂.     

Responses of CO2 efflux from an alpine meadow soil on the Qinghai Tibetan Plateau to multi-form and low-level N addition

Aims

To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai–Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO₂) exchange to N addition.

Methods

Based on a multi-form, low-level N addition experiment, soil CO₂ effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO₂ efflux.

Results

The results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH ₄ ⁺ -N pool. N applications significantly increased aboveground biomass and soil CO₂ efflux; moreover, this effect was more significant from NH ₄ ⁺ -N than from NO ₃ ^? -N fertilizer. In addition, the soil CO₂ efflux was mainly driven by soil temperature, followed by aboveground biomass and NH ₄ ⁺ -N pool.

Conclusions

These results suggest that chronic atmospheric N deposition will stimulate soil CO₂ efflux in the alpine meadow on the Qinghai–Tibetan Plateau by increasing available N content and promoting plant growth.     

Precipitation variability does not affect soil respiration and nitrogen dynamics in the understorey of a Mediterranean oak woodland

Background and aims

Future climate scenarios for the Mediterranean imply increasing precipitation variability. This study presents a large-scale water manipulation experiment simulating changes in precipitation variability, aiming at a better understanding of the effects of rainfall patterns on soil C and N cycling and understorey productivity in a Mediterranean oak woodland.

Methods

We used rain-out shelters to achieve (1) a normal dry period (7 days), and (2) a dry period increased three-fold (21 days), without altering total annual precipitation inputs.

Results

The temporal patterns of soil respiration (R _s) and soil inorganic N were not affected by treatment. However, water infiltration and N leaching increased with large infrequent watering events. R _s and soil NH₄ ⁺-N correlated with soil temperature, with soil NO₃ ^?-N being influenced by leaching.

Conclusions

The lack of significant treatment effects on either R _s or soil inorganic N can be explained by (1) minor differences in plant productivity between the treatments, suggesting equal plant N demand, and (2) the absence of moisture dependence of R _s and soil NH₄ ⁺-N. Increased N leaching with large infrequent precipitation events may have longer-term consequences for ecosystem functioning. Our results contribute to an improved understanding of possible climate change effects on key ecosystem processes in Mediterranean ecosystems.     

Exposure to warming and CO2 enrichment promotes greater above-ground biomass, nitrogen, phosphorus and arbuscular mycorrhizal colonization in newly established grasslands

Aims

In view of the projected increase in global air temperature and CO₂ concentration, the effects of climatic changes on biomass production, CO₂ fluxes and arbuscular mycorrhizal fungi (AMF) colonization in newly established grassland communities were investigated. We hypothesized that above- and below-ground biomass, gross primary productivity (GPP), AMF root colonization and nutrient acquisition would increase in response to the future climate conditions. Furthermore, we expected that increased below-ground C allocation would enhance soil respiration (R_soil).

Methods

Grassland communities were grown either at ambient temperatures with 375?ppm CO₂ (Amb) or at ambient temperatures +3°C with 620?ppm CO₂ (T+CO₂).

Results

Total biomass production and GPP were stimulated under T+CO₂. Above-ground biomass was increased under T+CO₂ while belowground biomass was similar under both climates. The significant increase in root colonization intensity under T+CO₂, and therefore the better contact between roots and AMF, probably determined the higher above-ground P and N content. R_soil was not significantly affected by the future climate conditions, only showing a tendency to increase under future climate at the end of the season.

Conclusions

Newly established grasslands benefited from the exposure to elevated CO₂ and temperature in terms of total biomass production; higher root AMF colonization may partly provide the nutrients required to sustain this growth response.     

On the relative roles of hydrology, salinity, temperature, and root productivity in controlling soil respiration from coastal swamps (freshwater)

Background and aims

Soil CO₂ emissions can dominate gaseous carbon losses from forested wetlands (swamps), especially those positioned in coastal environments. Understanding the varied roles of hydroperiod, salinity, temperature, and root productivity on soil respiration is important in discerning how carbon balances may shift as freshwater swamps retreat inland with sea-level rise and salinity incursion, and convert to mixed communities with marsh plants.

Methods

We exposed soil mesocosms to combinations of permanent flooding, tide, and salinity, and tracked soil respiration over 2½ growing seasons. We also related these measurements to rates from field sites along the lower Savannah River, Georgia, USA. Soil temperature and root productivity were assessed simultaneously for both experiments.

Results

Soil respiration from mesocosms (22.7–1678.2 mg CO₂ m^?2 h^?1) differed significantly among treatments during four of the seven sampling intervals, where permanently flooded treatments contributed to low rates of soil respiration and tidally flooded treatments sometimes contributed to higher rates. Permanent flooding reduced the overall capacity for soil respiration as soils warmed. Salinity did reduce soil respiration at times in tidal treatments, indicating that salinity may affect the amount of CO₂ respired with tide more strongly than under permanent flooding. However, soil respiration related greatest to root biomass (mesocosm) and standing root length (field); any stress reducing root productivity (incl. salinity and permanent flooding) therefore reduces soil respiration.

Conclusions

Overall, we hypothesized a stronger, direct role for salinity on soil respiration, and found that salinity effects were being masked by varied capacities for increases in respiration with soil warming as dictated by hydrology, and the indirect influence that salinity can have on plant productivity.     

Response of microbial activity and biomass in rhizosphere and bulk soils to increasing salinity

Background and aims

The impact of salinity on microbes has been studied extensively but little is known about the response of soil microbial activity and biomass to increasing salinity in rhizosphere compared to bulk (non-rhizosphere) soil.

Methods

Barley was grown for 5 weeks in non-saline loamy sand to which salt (NaCl) was added. The electrical conductivity in the saturated extract (ECe) was 1, 13 and 19 dS m^?1 for non-saline and two saline soils. Pots without plants were prepared in the same manner and placed next to those with plants. The water content in all pots was maintained at 75 % of water-holding capacity by weight. After 5 weeks the planted and unplanted pots were harvested to collect rhizosphere and bulk soil, respectively. The collected soil was then used for an incubation experiment. The EC levels in the pot experiment (EC1, EC13 and EC19, referred to as original) were either maintained or increased by adding NaCl to adjust the EC to 13, 19, 31 and 44 dS m^?1. CO₂ release was measured continuously for 20 days, microbial biomass C (MBC) was measured at the start and the end of the incubation experiment.

Results

In general, cumulative respiration and microbial biomass C concentration in rhizosphere and bulk soil decreased to a similar extent with increasing adjusted EC. However, compared to the treatments where the EC was maintained, the percentage decrease in cumulative respiration when the EC was increased to EC44 was smaller in rhizosphere than in bulk soil.

Conclusion

Overall, the reduction of cumulative respiration with increasing salinity did not differ between rhizophere and bulk soil. But microbes in rhizosphere soil were more tolerant to high EC than those in bulk soil which could be due to the greater substrate availability in the rhizosphere even after the soil was removed from the roots.     

Fertilizer N uptake of paddy rice in two soils with different fertility under experimental warming with elevated CO2

Background and aims

Only limited information is available in the research area on the effect of elevated CO₂ concentration ([CO₂]) and air temperature (T_air) on the fertilizer N uptake by rice. This study was conducted to investigate changes in rice uptake of N derived from fertilizer (NDFF) and soil (NDFS) as well as fertilizer N uptake efficiency (FUE) with elevated [CO₂] and T_air in two soils with different fertility.

Methods

Rice (Oryza sativa L.) plants were grown with ¹⁵N-urea for two growing seasons (2007 in the less fertile and 2008 in the more fertile soil) in temperature gradient chambers under two (ambient and elevated) levels of [CO₂] and T_air regimes. At harvest, dry matter (DM) and N uptake amount of rice compartments (root, shoot, and grain) were determined.

Results

The DM of whole rice increased (P?<?0.01) with co-elevation of [CO₂] and T_air in both years (by 28.0 % in 2007 and by 27.4 % in 2008). The DM in 2008 was greater than that in 2007 by 48.1 to 63.1 % probably due to better soil fertility as well as longer sunshine hours (456 h vs. 568 h). Co-elevation of [CO₂] and T_air increased total N uptake, NDFF, and NDFS by 19.4 to 29.1 % in general compared to the ambient conditions. The FUE increased with co-elevation of [CO₂] and T_air from 46.5 to 59.5 % in 2007 and from 36.7 to 43.8 % in 2008.

Conclusions

The projected global warming with elevated [CO₂] is expected to increase FUE via enhanced DM accumulation with less increments in the soils that have higher indigenous soil N availabilities.     

Contribution of newly grown extramatrical ectomycorrhizal mycelium and fine roots to soil respiration in a young Norway spruce site

Background and aims

Partitioning of soil respiration is a challenging task when resolving the C cycling in forest ecosystems. Our aim was to partition the respiration of newly grown extramatrical ectomycorrhizal mycelium (ECM) and fine roots (and their associated microorganisms) in a young Norway spruce forest.

Methods

Ingrowth mesh bags of 16 cm diameter and 12 cm height were placed in the upper soil and left for 12–16 months in 2010 and 2011. The 2 mm mesh size allowed the ingrowth of ECM and fine roots whereas a 45 μm mesh size allowed only the ingrowth of ECM. The mesh bags were filled with either homogenized EA horizon soil, pure quartz sand (QS) or crushed granite (CG, only 2011), each with five replicates. Controls without any ingrowth were established for each substrate by solid plastic tubes (2010) and by 1 μm mesh bags (2011). Fluxes of CO₂ from the mesh bags and controls were measured biweekly during the growing season by the closed chamber method.

Results

The contribution of ECM to soil respiration was largest in the QS treatments, reaching cumulatively 1.2 and 2.2 Mg C ha^?1 6 months^?1 in 2010 and 2011, respectively. For EA and CG treatments, the cumulative respiration from ECM was larger than from controls, however the differences being not statistically significant. The respiration of newly grown fine roots in QS amounted to 1.0 Mg C ha^?1 in 2010, but could not be identified in 2011 since fluxes from 2 mm and 45 μm mesh bags were similar. The correlation of total root length in single QS mesh bags to CO₂ fluxes was poor. The contribution of fine root respiration was also not detectable in the EA and CG treatment. No correlation was found between the autumnal biomass of newly grown ECM and its cumulative respiration.

Conclusion

Our results suggest a substantial contribution of newly grown ECM to soil respiration. Respiration of ECM might be larger than respiration of fine roots.     

Changes in water content of two agricultural soils does not alter labile P and C pools

Aims

An incubation study was conducted to investigate how changes in soil water content affect labile phosphorus and carbon pools, mineralisation patterns and microbial community composition.

Methods

Two soils from different climatic histories were subjected to four long-term (15 weeks) soil water regimes (constant field capacity (m); 3 dry-rewet (DRW) cycles evenly spaced (intermittent, int); 3 DRW cycles with a shorter interval after a long dry period (false break, fb); constantly air-dry (d)) (incubation period 1). In the subsequent incubation period 2, a set of cores from each treatment were subjected to one DRW cycle (air-dry for 7 day; field capacity for 14 day) or maintained at field capacity.

Results

Long-term soil water regime altered soil respiration with the largest CO₂ pulse occurring in soil with the longest dry period. However, changing the distribution of the 3 DRW events within incubation period 1 (int/fb) did not alter cumulative CO₂. In addition, DRW during incubation period 2 did not affect cumulative CO₂ in either treatment (m, int, fb, d) (except for Hamilton int). Our results show that carbon and phosphorus availability and the size and community composition of the microbial biomass were largely unaffected by fluctuating soil water content.

Conclusions

Changes in soil water content altered respiration, phosphatase activity and microbial C:P ratio and indicate physiological and/or functional changes in the microbial community. However, it appeared that these would have little impact on plant P availability.     

Drivers of increased soil respiration in a poplar coppice exposed to elevated CO2

Background and aims

The response of soil respiration (SR) to elevated CO₂ is driven by a number of processes and feedbacks. This work aims to i) detect the effect of elevated CO₂ on soil respiration during the second rotation of a short rotation forest, at two levels of N availability; and ii) identify the main drivers behind any changes in soil respiration.

Methods

A poplar plantation (POP-EUROFACE) was grown for two rotations of 3 years under elevated CO₂ maintained by a FACE (Free Air CO₂ Enrichment) technique. Root biomass, litter production and soil respiration were followed for two consecutive years after coppice.

Results

In the plantation, the stimulation of fine root and litter production under elevated CO₂ observed at the beginning of the rotation declined over time. Soil respiration (SR) was continuously stimulated by elevated CO₂, with a much larger enhancement during the growing (up to 111 %) than in the dormant season (40 %). The SR increase at first appeared to be due to the increase in fine root biomass, but at the end of the 2nd rotation was supported by litter decomposition and the availability of labile C. Soil respiration increase under elevated CO₂ was not affected by N availability.

Conclusions

The stimulation of SR by elevated CO₂ was sustained by the decomposition of above and belowground litter and by the greater availability of easily decomposable substrates into the soil. In the final year as elevated CO₂ did not increase C allocation to roots, the higher SR suggests greater C losses from the soil, thus reducing the potential for C accumulation.     

Respiration costs associated with nitrate reduction as estimated by 14CO2 pulse labeling of corn at various growth stages

Utilization of nitrogen in the form of either nitrate (NO ₃ ^? ) or ammonium (NH ₄ ⁺ ) ions may affect the carbohydrate metabolism and energy budget of plants. Recent studies showed that greater expenses of NO ₃ ^? to NH ₄ ⁺ reduction mostly occur in the roots and during darkness. Fertilization of corn with ¹⁵N-labeled nitrate and ammonium, combined with pulse labeling of plants in a ¹⁴CO₂ atmosphere at the V6 and V8 growth stages, allowed us to evaluate the effect of N form on the CO₂ efflux from soil. NH ₄ ⁺ oxidation was inhibited by adding dicyandiamide. In respect to ammonium, nitrate addition increased root-derived CO₂ efflux from corn by 2.6 times at stage V6 and by 1.8 times at stage V8. The time of peak ¹⁴CO₂ efflux from soil also differed between two growing stages: at V6, efflux peaked only on the second day after pulse labeling, while at V8 this occurred within the first 6 h. The strong effect of NO ₃ ^? and NH ₄ ⁺ on root respiration requires considering the N form in the soil and the nitrate reduction site location in a plant when modeling soil respiration changes and when separately estimating individual CO₂ sources that contribute to the total soil CO₂ efflux.     

Environmental controls and the influence of tree species on temporal variation in soil respiration in subtropical China

Background and aims

The knowledge of individual tree species impacts on soil respiration based on rigorous experimental designs is limited, but is crucial to help guide selection of species for reforestation and carbon (C) management purposes.

Methods

We assessed monthly soil respiration and its components, litterfall input, fine root production and mortality under 19-year-old native coniferous Cunninghamia lanceolata and broadleaved Mytilaria laosensis plantations in sub-tropical China.

Results

Total soil respiration from October 2011 to March 2013 was significantly lower under the C. lanceolata than the M. laosensis plantation. The difference in respiration rates derived from fine roots and the litter layer explained much of the variation of total soil respiration between the two tree species. We used an exponential equation and base temperature (10 °C) to normalize soil respiration rate and its components (R₁₀) and determined the correlation between R₁₀ and soil moisture. Although soil moisture had a positive relationship with R₁₀ derived from roots or litter under both C. lanceolata and M. laosensis forests, these positive correlations were masked by negative relationships between soil moisture and R₁₀ derived from root-free soil, which resulted in a neutral correlation between total R₁₀ and soil moisture under C. lanceolata forests. Monthly litterfall input was associated with variation in concurrent total soil respiration rate under the M. laosensis plantation and respiration rate lagging 3 months behind under the C. lanceolata plantation, which may suggest that litterfall input from M. laosensis can more rapidly produce C substrates for microbial respiration than litterfall from C. lanceolata.

Conclusions

This study highlighted that tree species-induced variation in the quality and quantity of fine roots and litterfall can impact not only the soil respiration rate but also the seasonal variation model of forest soil respiration.     

Nitrogen deposition and soil carbon content affect nitrogen mineralization during primary succession in acid inland drift sand vegetation

Background and aims

Two inland dunes in the Netherlands receiving low (24) and high (41 kg N ha^?1 yr^?1) nitrogen (N) deposition were compared for N dynamics and microbial activity to investigate the potential effect of N on succession rate of the vegetation and loss of pioneer habitats.

Methods

Primary succession stages were sampled, including bare sand, and vegetation dominated by Polytrichum piliferum, Campylopus introflexus, lichens and grasses respectively, representing a series of vegetation types in undisturbed drift sand sites with succession starting on bare sand containing virtually no organic matter. Microbial characteristics and potential N mineralization were analysed in a laboratory experiment.

Results

Organic matter accumulated during succession, resulting in a lower pH and in higher microbial biomass (bacteria and fungi), respiration and net N mineralization. The increase in respiration and N mineralization was largely due to the development of an ectorganic layer in the middle stages of succession. The observed effects of N deposition were (1) decrease of microbial biomass, (2) higher net N mineralization per m², (3) higher levels of free nitrogen in the soil, and (4) a higher microbial N:P ratio.

Conclusions

Elevated N deposition leads to higher N availability which may cause accelerated succession.