Partitioning Soil Respiration Research Articles

Interactive effects of climate warming and management on grassland soil respiration partitioning

AbstractGrassland ecosystems are important for the provision of food, fuel and fibre. They represent globally important carbon (C) reservoirs that are under pressure from intensive management and ongoing climate change. How these drivers of change will interact to affect grassland soil C and nitrogen (N) cycling and heterotrophic and autotrophic respiration remains uncertain. Roots and mycelia in grassland soil are important regulators of ecosystem functioning and likely to be an influential determinant of CO2 fluxes responses to global change. The aim of this study was to investigate the interactive effect of climate warming and grassland management on soil respiration originating from roots rhizosphere, mycelia and free‐living microbes. The experiment used a block design to measure the interactive effects of warming, nitrogen addition, aboveground biomass (AGB) removal on belowground respiration in a temperate grassland ecosystem. An in‐growth core method using cores with different mesh sizes was used to partition belowground respiration due to its simplicity of design and efficacy. We found that basal respiration (free‐living microorganisms) was the highest (58.5% of the total emissions), followed by that from roots (22.8%) and mycelia (18.7%) across all treatments. Warming reduced basal respiration whilst AGB removal increased it. An antagonistic interaction between warming and nitrogen addition reduced root respiration, and a three‐way interaction between warming, nitrogen addition and AGB removal affected mycelial respiration. The results show different contributions of belowground biota to soil respiration, and how interactions between climate change and grassland management may influence effects on soil respiration.

Root exclusion methods for partitioning of soil respiration: Review and methodological considerations

Soil respiration is a vital process in all terrestrial ecosystems, through which the soil releases carbon dioxide (CO2) into the atmosphere at an estimated annual rate of 68–101 Pg carbon, making it the second highest terrestrial contributor to carbon fluxes. Since soil respiration consists of autotrophic and heterotrophic constituents, methods for accurately determining the contribution of each constituent to the total soil respiration are critical for understanding their differential responses to environmental factors and aiding the reduction of CO2 emissions. Owing to its low cost and simplicity, the root exclusion (RE) technique, combined with manual chamber measurements, is frequently used in field studies of soil respiration partitioning. Nevertheless, RE treatments alter the soil environment, leading to potential bias in respiration measurements. This review aims to elucidate the current understanding of RE, i.e., trenching (Tr) and deep collar (DC) insertion techniques, by examining soil respiration partitioning studies performed in several ecosystems. Additionally, we discuss methodological considerations when using RE and the combinations of RE with stable isotopic and modeling approaches. Finally, future research directions for improving the Tr and DC insertion methods in RE are suggested.

Arbuscular mycorrhizal hyphal respiration makes a large contribution to soil respiration in a subtropical forest under various N input rates

Arbuscular mycorrhizal fungi (AMF) are widespread in subtropical forests and play a crucial role in belowground carbon (C) dynamics. Nitrogen (N) deposition or fertilization may affect AMF and thus the flux of plant-derived C back to the atmosphere via AMF hyphae. However, the contribution of AMF hyphal respiration to soil respiration and the response AMF hyphal respiration to increased soil N availability remain unknown. We studied the effect of N fertilization (0, 50, 100 and 200 kg N ha−1 yr−1) on AMF hyphal respiration, root respiration and heterotrophic (microbial) respiration in a subtropical Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantation. We found that short-term N addition did not affect root, AMF hyphal and soil microbial respiration, because soil N availability and extraradical hyphae were not affected by N addition. The AMF hyphal respiration contributed 12 % of total soil respiration and 25 % of the autotrophic respiration. Root, AMF hyphal and soil microbial respiration were positively correlated with soil moisture content but not with soil temperature. Our results indicate that AMF hyphal respiration is a large source of soil respiration, and should be considered in partitioning soil respiration into different components in future studies to better understand the response of soil respiration to N addition.

Environmental and Plant-Derived Controls on the Seasonality and Partitioning of Soil Respiration in an American Sycamore (Platanus occidentalis) Bioenergy Plantation Grown at Different Planting Densities

Bioenergy is one of the most considered alternatives to fossil fuels. Short-rotation woody crops (SRWCs) as bioenergy sources are capable of alleviating energy constraints and sequestering atmospheric CO2. However, studies investigating soil carbon (C) dynamics at SWRC plantations are scarce. We studied American sycamore (Platanus occidentalis) as a model tree species for SRWC at different planting densities ((1) 0.5 × 2.0 m (10,000 trees·ha−1 or tph), (2) 1.0 × 2.0 m (5000 tph), and (3) 2.0 × 2.0 m (2500 tph)) to examine seasonal variation in total soil respiration (Rtotal), partitioned into heterotrophic (Rh) and autotrophic (Ra) respiration, and we evaluated climatic and biological controls on soil respiration. Rtotal and Rh exhibited larger seasonal variation than Ra (p < 0.05). During the nongrowing seasons, the average Rtotal was 0.60 ± 0.21 g·C·m−2·day−1 in winter and 1.41 ± 0.73 g·C·m−2·day−1 in fall. During the growing season, Rtotal was 2–7 times higher in spring (3.49 ± 1.44 g·C·m−2·day−1) and summer (4.01 ± 1.17 g·C·m−2·day−1) than winter. Average Rtotal was 2.30 ± 0.63 g·C·m−2·day−1 in 2500 tph, 2.43 ± 0.64 g·C·m−2·day−1 in 5000 tph, and 2.41 ± 0.75 g·C·m−2·day−1 in 10,000 tph treatments. Average Rh was 1.72 ± 0.40 g·C·m−2·day−1 in 2500 tph, 1.57 ± 0.39 g·C·m−2·day−1 in 5000 tph, and 1.93 ± 0.64 g·C·m−2·day−1 in 10,000 tph, whereas Ra had the lowest rates, with 0.59 ± 0.53 g·C·m−2·day−1 in 2500 tph, 0.86 ± 0.51 g·C·m−2·d−1 in 5000 tph, and 0.48 ± 0.34 g·C·m−2·day−1 in 10,000 tph treatments. Rh had a greater contribution to Rtotal (63%–80%) compared to Ra (20%–37%). Soil temperature was highly correlated to Rtotal (R2 = 0.92) and Rh (R2 = 0.77), while the correlation to Ra was weak (R2 = 0.21). Rtotal, Rh, and Ra significantly declined with soil water content extremes (e.g., <20% or >50%). Total root biomass in winter (469 ± 127 g·C·m−2) was smaller than in summer (616 ± 161 g·C·m−2), and the relationship of total root biomass to Rtotal, Rh, and Ra was only significant during the growing seasons (R2 = 0.12 to 0.50). The litterfall in 5000 tph (121 ± 16 g DW·m−2) did not differ (p > 0.05) from the 2500 tph (108 ± 16 g DW·m−2) or 10,000 tph (132 ± 16 g DW·m−2) treatments. In no circumstances were Rtotal, Rh, and Ra significantly correlated with litterfall amount across planting densities and seasons (p > 0.05). Overall, our results show that Rtotal in American sycamore SRWC is dominated by the heterotrophic component (Rh), is strongly correlated to soil environmental conditions, and can be minimized by planting at a certain tree density (5000 tph).

Spatio-Temporal Variability of Peat CH4 and N2O Fluxes and Their Contribution to Peat GHG Budgets in Indonesian Forests and Oil Palm Plantations

Land-use change in tropical peatlands substantially impacts peat emissions of methane (CH4) and nitrous oxide (N2O) in addition to emissions of carbon dioxide (CO2). However, assessments of full peat greenhouse gas (GHG) budgets are scarce and CH4 and N2O contributions remain highly uncertain. The objective of our research was to assess changes in peat GHG flux and budget associated with peat swamp forest disturbance and conversion to oil palm plantation and to evaluate drivers of variation in trace gas fluxes. Over a period of one and a half year, we monitored monthly CH4 and N2O fluxes together with environmental variables in three undrained peat swamp forests and three oil palm plantations on peat in Central Kalimantan. The forests included two primary forests and one 30-year-old secondary forest. We calculated the peat GHG budget in both ecosystems using soil respiration and litterfall rates measured concurrently with CH4 and N2O fluxes, site-specific soil respiration partitioning ratios, and literature-based values of root inputs and dissolved organic carbon export. Peat CH4 fluxes (kg CH4 ha−1 year−1) were insignificant in oil palm (0.3 ± 0.4) while emissions in forest were high (14.0 ± 2.8), and larger in wet than in dry months. N2O emissions (kg N2O ha−1 year−1) were highly variable spatially and temporally and similar across land-uses (5.0 ± 3.9 and 5.2 ± 3.7 in oil palm and forest). Temporal variation of CH4 was controlled by water table level and soil water-filled pore space in forest and oil palm, respectively. Monthly fluctuations of N2O were linked to water table level in forest. The peat GHG budget (Mg CO2 equivalent ha−1 year−1) in oil palm (31.7 ± 8.6) was nearly eight times the budget in forest (4.0 ± 4.8) owing mainly to decreased peat C inputs and increased peat C outputs. The GHG budget was also ten times higher in the secondary forest (10.2 ± 4.5) than in the primary forests (0.9 ± 3.9) on the account of a larger peat C budget and N2O emission rate. In oil palm 96% of emissions were released as CO2 whereas in forest CH4 and N2O together contributed 65% to the budget. Our study highlights the disastrous atmospheric impact associated with forest degradation and conversion to oil palm in tropical peatlands and stresses the need to investigate GHG fluxes in disturbed undrained lands.

A simple model for partitioning forest soil respiration based on root allometry

Partitioning soil respiration (Rs) into heterotrophic (Rh) and autotrophic (Ra) compartments is an important first step in understanding the response of soil carbon flux to changes in climate. This is because Rh and Ra respond differently to changing environmental conditions. However, the methods currently used to partition Rs have high costs and large uncertainties. In this study, we developed a simple model to partition forest Rs based on root biomass, Rs=Rh+∑i=1n[ci×DBHidi×f(Di)], which the latter is modelled as the sum of root biomass density (MT) derived from an existing root allometric equation (MT=c×DBHd, where DBH is tree diameter at breast height) multiplied by a horizonal distance function (f(D)) based on the relationship between in situ Rs and distance (D) from the tree base. We applied this model to a complex tropical natural forest in Southeast Asia by inputting a site-specific root allometric equation and in situ Rs–D function to obtain: Rs=Rh+c′∑i=1n(DBHi2.59×Di−0.452). Compared to studies that used the classical root biomass regression, our results show a stronger linear correlation between Rs and simulated root biomass density (r2 = 0.797). By using the y-intercept (i.e. no root biomass at all), Rh was estimated to contribute 66.9 ± 5.7% to total Rs. We validated the root-allometry-based model by using the trenching method. In our study area, it took about 1 week (6.5 days) for tree roots to die after trenching, and Rh/Rs ratios estimated from our model were about 2.7% higher than those estimated with the trenching method. With the increasing number of allometry equations for global woody plants, the model has the potential of being used to partition Rs in various forests at regional or global scales.

Soil Microbial Respiration in Subtaiga and Forest-Steppe Ecosystems of European Russia: Field and Laboratory Approaches

Our study focuses on testing laboratory measurements of soil microbial respiration as a proxy that in the field conditions. The soil microbial respiration was measured in field (MRfield) and laboratory (MRlab) conditions monthly (from May to October) in subtaiga (mixed forest, meadow) and forest-steppe (broad-leaved forest, virgin steppe) ecosystems of the European Russia. The MRfield was determined through soil respiration partitioning by the conventional substrate-induced respiration method. The MRlab was measured as basal respiration of 10 cm topsoil at 22°C and 60% water holding capacity. The contribution of MRfield to total soil respiration varied during the growing season from 25 to 82% for subtaiga and from 41 to 88% for forest-steppe. The MRfield for studied ecosystems varied from 2.2 to 21.7 g СО2/(m2 d), while MRlab was from 3.5 to 18.6 g СО2/(m2 d). Similar results obtained by field and laboratory approaches were in 50% of measurements in the subtaiga ecosystems and in almost 20% of cases on the forest-steppe. The average MRfield and MRlab for growing season did not significantly differ for all studied ecosystems. These findings demonstrate possible prospects of using laboratory measurement of soil microbial respiration during the growing season to approximate and predict average MRfield for various ecosystems.

Stand age‐related effects on soil respiration in rubber plantations (Hevea brasiliensis) in southwest China

AbstractThe land use change from tropical forests to rubber plantations has had a great influence on ecosystem‐level carbon (C) exchange and soil C dynamics. This study aimed to assess the variation of soil respiration and its components in rubber plantations of different stand age. A trenching method was used to partition soil respiration (RS) into autotrophic respiration (RA) and heterotrophic respiration (RH) for 3 years in 12, 24, 32 and 49‐year‐old rubber plantations. Our results showed that RS changed significantly among rubber plantations, with the highest RS in the old‐growth rubber plantation (49‐year plantation, 147.30 ± 6.91 mg CO2‐C m−2 hr−1) followed by the mature (137.16 ± 7.68 and 108.10 ± 4.83 in 24‐year and 32‐year plantations, respectively) and the young (12‐year plantation, 78.43 ± 3.84 mg CO2‐C m−2 hr−1) rubber plantations. RS was significantly and exponentially related to soil temperature (p < .0001) rather than soil water content, which led to higher RS in the rainy season than in the dry season. The contribution of RA to RS was higher in the mature rubber plantation than in the young (27%) and the old‐growth (20%) rubber plantations. The sensitivity of RS to increasing temperature (Q10) was higher in 32‐ and 49‐year plantations than in 12‐ and 24‐year rubber plantations. The largest Q10 value of RH and RA was found in the old‐growth (49‐year plantation, 2.95) and the mature (32‐year plantation, 8.01) rubber plantations, respectively. Our results highlight the importance of environmental factors in determining the variation in RS in rubber plantations with different stand age. However, further studies are still required to elucidate the mechanisms impacting stand age‐related effects on soil respiration, such as the differences in photosynthesis in rubber plantations.Highlights Soil respiration (RS) in four rubber plantations of different stand age was examined. RS tended to increase with the increasing stand age. The contribution of autotrophic respiration (RA) to RS was higher in the mature stands. Q10 values of heterotrophic respiration (RH) and RA were higher in old‐growth and mature stands, respectively.

Soil respiration in a tropical montane grassland ecosystem is largely heterotroph-driven and increases under simulated warming

Soil respiration, a major source of atmospheric carbon (C), can feed into climate warming, which in turn can amplify soil CO2 efflux by affecting respiration by plant roots, arbuscular mycorrhizal fungi (AMF) and other heterotrophic organisms. Although tropical ecosystems contribute >60% of the global soil CO2 efflux, there is currently a dearth of data on tropical soil respiration responses to increasing temperature. Here we report a simulated warming and soil respiration partitioning experiment in tropical montane grasslands in the Western Ghats in southern India. The study aimed to (a) evaluate soil respiration responses to warming, (b) assess the relative contributions of autotrophic and heterotrophic components to soil respiration, and (c) assess the roles of soil temperature and soil moisture in influencing soil respiration in this system. Soil respiration was tightly coupled with instantaneous soil moisture availability in both the warmed and control plots, with CO2 efflux levels peaking during the wet season. Soil warming by ˜1.4 °C nearly doubled soil respiration from 0.62 g CO2 m−2 hr−1 under ambient conditions to 1.16 g CO2 m−2 hr−1 under warmed conditions. Warming effects on soil CO2 efflux were dependent on water availability, with greater relative increases in soil respiration observed under conditions of low (with a minimum of 2.6%), compared to high (with a maximum of 64.3%), soil moisture. Heterotrophs contributed to the majority of soil CO2 efflux, with respiration remaining unchanged when roots and/or AMF hyphae were excluded as the partitioning treatments were statistically indistinguishable. Overall, our results indicate that future warming is likely to substantially increase the largely heterotroph-driven soil C fluxes in this tropical montane grassland ecosystem.

Will heterotrophic soil respiration be more sensitive to warming than autotrophic respiration in subtropical forests?

Understanding the responses of heterotrophic (Rh) and autotrophic (Ra) components of soil respiration (Rs) to warming is important in evaluating and modelling the effects of changes in climate on soil carbon (C) cycling in terrestrial ecosystems. We used a mesocosm system with buried heating cables (5°C warming) to investigate the responses of Rs, Rh and Ra to warming in a subtropical forest in southern China. Soil CO2 effluxes were measured with a portable automatic soil CO2 flux system from March 2014 to July 2015. We found that warming increased mean Rs and Rh from 788 to 1036 g C m−2 year−1 (+31%) and from 512 to 707 g C m−2 year−1 (+38%), respectively. There was no difference in Ra between the warming treatment and the control. The lack of response of Ra to warming was probably because the fine root biomass did not change with warming treatment. Soil warming also increased available dissolved organic carbon, microbial biomass carbon, actinomycetal biomass and arbuscular mycorrhizal biomass. Our results suggest that Rh might be more sensitive to climate warming than Ra, and future climate warming could increase soil C loss from increased Rh in subtropical forest ecosystems.Highlights A field warming experiment with partitioning of soil respiration in a humid subtropical forest. Warming increased Rs and Rh without significantly altering soil microbial substrate availability. Heterotrophic respiration appeared more sensitive to warming than autotrophic respiration. Warming increased Actinomycetes bacteria and Arbuscular mycorrhizal fungi.

Soil respiration partitioning in afforested temperate peatlands

Understanding and quantifying soil respiration and its component fluxes are necessary to model global carbon cycling in a changing climate as small changes in soil CO2 fluxes could have important implications for future climatic conditions. A soil respiration partitioning study was conducted in eight afforested peatland sites in south-west Ireland. Using trenched points, annual soil CO2 emissions, and the contributions of root and heterotrophic respiration as components of total soil respiration, were estimated. Nonlinear regression models were evaluated to determine the best predictive soil respiration model for each component flux, using soil temperature and water table level as explanatory variables. Temporal variation in soil CO2 efflux was driven by soil temperature at 10 cm depth, with all treatment points also affected by water table level fluctuations. The effect of water table level on soil respiration was best accounted for by incorporating a water level Gaussian function into the soil-temperature–soil-respiration model. Mean root respiration was 44% of mean total soil respiration, varying between 1100 and 2049 g CO2 m−2 year−1. Heterotrophic respiration was divided between peat respiration and litter respiration, which accounted for 35 and 21% of total soil respiration, respectively. While peat respiration varied between 774 and 1492 g CO2 m−2 year−1, litter respiration varied between 514 and 1013 g CO2 m−2 year−1. Although the extrapolation of these results to other sites should be done with caution, the empirical models developed for the entire dataset in this study are a useful tool to predict and simulate CO2 emissions in similar afforested peatlands (e.g. pine and spruce plantations) in temperate maritime climate conditions.

Partitioning soil respiration: quantifying the artifacts of the trenching method

Total soil respiration (Rt) is a combination of autotrophic (Ra) and heterotrophic respiration (Rh). Root exclusion methods, such as soil trenching, are often utilized to separate these components. This method involves severing the rooting system surrounding a plot to remove the Ra component. However, soil trenching has potential limitations including (1) reduced water uptake in trenched plots that increases soil water content, which is one of the environmental controllers of Rt in many ecosystems, and (2) increased available carbon substrate for Rh caused by recently severed dead roots. We present a methodology that utilizes a bayesian modeling framework to quantify the magnitude of artifacts from a large trenching manipulation experiment. Thus methodology corrects Rh and Ra observations at daily to seasonal time scales. This study finds that the artifacts, due to recently severed roots, persist over a 2 years study period and the artifacts due to altered soil moisture had the greatest impact during drought conditions.

Separation of soil respiration: a site-specific comparison of partition methods

Abstract. Without accurate data on soil heterotrophic respiration (Rh), assessments of soil carbon (C) sequestration rate and C balance are challenging to produce. Accordingly, it is essential to determine the contribution of the different sources of the total soil CO2 efflux (Rs) in different ecosystems, but to date, there are still many uncertainties and unknowns regarding the soil respiration partitioning procedures currently available. This study compared the suitability and relative accuracy of five different Rs partitioning methods in a subtropical forest: (1) regression between root biomass and CO2 efflux, (2) lab incubations with minimally disturbed soil microcosm cores, (3) root exclusion bags with hand-sorted roots, (4) root exclusion bags with intact soil blocks and (5) soil δ13C–CO2 natural abundance. The relationship between Rh and soil moisture and temperature was also investigated. A qualitative evaluation table of the partition methods with five performance parameters was produced. The Rs was measured weekly from 3 February to 19 April 2017 and found to average 6.1 ± 0.3 MgCha-1yr-1. During this period, the Rh measured with the in situ mesh bags with intact soil blocks and hand-sorted roots was estimated to contribute 49 ± 7 and 79 ± 3 % of Rs, respectively. The Rh percentages estimated with the root biomass regression, microcosm incubation and δ13C–CO2 natural abundance were 54 ± 41, 8–17 and 61 ± 39 %, respectively. Overall, no systematically superior or inferior Rs partition method was found. The paper discusses the strengths and weaknesses of each technique with the conclusion that combining two or more methods optimizes Rh assessment reliability.

Soil autotrophic and heterotrophic respiration in response to different N fertilization and environmental conditions from a cropland in Northeast China

Partitioning soil respiration (Rs) into its heterotrophic (Rh) and autotrophic (Ra) components is crucial to evaluate the effects of inorganic and organic nitrogen (N) fertilization on carbon (C) cycling in agricultural ecosystems. We carried out a field experiment in a maize cropland in Northeast China using the root exclusion method to separate Rh and Ra, and investigate their responses to different fertilization regimes. These included no N fertilization (CK), inorganic N fertilizer (NPK), 75% urea N plus 25% pig (PM1) or chicken (CM1) manure N, and 50% urea N plus 50% pig (PM2) or chicken (CM2) manure N. Annual Rs was significantly increased from 314 g C m−2 in CK to 389, 366, and 371 g C m−2 in NPK, CM1, and PM2, respectively, and further to 420 g C m−2 in PM1, whereas a similar value to CK was observed in CM2 (327 g C m−2). N-induced increases in Rs were largely attributable to the response of Ra (except CM2), which increased by 18–54% due to higher nitrate supply. Rh increased from 183 to 192–209 g C m−2 in plots receiving N fertilizer, with significant increases observed in PM1 and PM2, likely due to the high ammonium and labile organic C concentrations in these treatments. Manure type and application rate had significant effects on Rs and Ra, but not Rh. Compared with CM, PM was more effective in stimulating Ra due to its greater decomposability. Rs and Ra decreased in the order of PM1 > PM2 and CM1 ≥ CM2, presumably because of the lower inorganic N supply with increasing manure application rate. The estimated C sequestration rate shifted from negative in CK and NPK to positive in the manure treatments, especially in PM2 and CM2 that gained 0.44 and 0.49 Mg C ha−1 yr−1, respectively. These results suggested that combined application of half inorganic N plus half organic N might have potential to enhance soil C sequestration in cropland of Northeast China.

Partitioning of soil respiration in a first rotation beech plantation

Partitioning of soil respiration in a first rotation beech plantation

Partitioning of soil respiration in a first rotation beech plantation

Partitioning of soil respiration in a first rotation beech plantation

三源区分土壤呼吸组分研究

三源区分土壤呼吸组分研究

Partitioning soil respiration in two typical forests in semi-arid regions, North China

This investigation examines the contributions of autotrophic respiration (RA) and heterotrophic respiration (RH) to total soil respiration (RT) in two typical forests (Armeniaca sibirica Lam. (AS) and Vitex negundo Linn. var. Heterophylla (VN)) in semi-arid region of North China. The soil respiration components' responses to changing temperatures were also examined. A similar pattern was identified in the diurnal variation of RT and RH; RA exhibited a different diurnal pattern with nighttime values being greater than daytime values. On the seasonal scale, the variations of RT, RH and RA exhibited a similar and strong single-peak pattern with values peaking in early August for both forest sites. The seasonal variations of RT, RA and RH were strongly affected by soil temperature and moisture, with soil temperature accounting for more variations in soil respiration components at both sites. The contributions of RA to RT (RA/RT ratio) exhibited remarkable diurnal and seasonal variations. Due to the lag time between photosynthesis and root respiration, diurnal variation of RA/RT was lower during the daytime than at night in both AS and VN sites. Meanwhile, under the influence of plant physiology, the seasonal variation of RA/RT presented a bimodal curve, with ratios peaking in April and August and bottoming out in October. In addition, due to having higher fine root biomass, VN had a significantly higher annual mean RA/RT ratio (24.44%) than AS (17.46%). Regardless of vegetation type, the responses of RT, RA and RH to soil temperature were more sensitive during the dormant season than during the growing season, with their Q10 ranked as RA>RT>RH. Our results indicate that RA is more sensitive to temperature variation than RH, and that the dormant season may have greater soil respiration potential than the growing season in our study areas in the context of increasing global temperatures. The response of RT, RA and RH to soil temperature showed greater sensitivity for VN than for AS during the annual time scale. We can infer that soil respiration under VN may be more sensitive to temperature variations under global warming scenarios.

Partitioning soil respiration across four age classes of loblolly pine (Pinus taeda L.) on the Virginia Piedmont

Quantification of the heterotrophic component of total soil respiration is important for estimating forest carbon pools and fluxes, and for understanding carbon dynamics associated with stand development and silvicultural management. We measured the proportion of heterotrophic respiration (RH) to total soil respiration (RS) in extensively managed loblolly pine (Pinus taeda L.) stands of four age classes in the Piedmont physiographic province of Virginia. Our objectives were to evaluate the influence of stand age and seasonality on the proportion of RH to RS (RH:RS). RH was partitioned using root exclusion cores, and both RS and RH were measured 90days following installation of cores for five seasons. Repeated measures analysis revealed that stand age and measurement season each had a significant effect on RH:RS (P<0.001), but that there were no interactive effects (P=0.202). Mean RH:RS during the 12-month study declined with stand age, and were 0.82, 0.73, 0.59, and 0.50 for 3-year-old, 9-year-old, 18- year-old, and 25-year-old stands, respectively. Across all age classes, the winter season had the highest mean RH:RS of 0.85 while summer had the lowest of 0.55. This study provides estimates of RH:RS in managed loblolly pine systems, and demonstrates the need to consider the impact of stand age and seasonal patterns when estimating net annual carbon (C) budgets of forest ecosystems and to identify the point at which plantations switch from functioning as C sources to a C sinks.

Partitioning of ecosystem respiration in winter wheat and silage maize—modeling seasonal temperature effects

The response of agroecosystem carbon (C) respiration fluxes to environmental changes needs to be better understood as respiration subcomponents may respond differently to management and seasonal weather dynamics, which is important for soil organic matter (SOM) modeling. Respiration measurements at two different spatial and temporal scales (eddy covariance (EC) and soil chambers) were used to ascertain the relationship between temperature and CO2 flux of different ecosystem respiration components (ecosystem (Reco), soil and root combined, and soil). Further, different model approaches (static versus dynamic reference CO2 rate (rb) and activation energy type parameter (E0) with an Arrhenius-like function) in order to partition Reco into above- and belowground autotrophic (RA_above, RA_below) and heterotrophic respiration (RH_SOM) were tested. Canopy level CO2 fluxes in winter wheat and silage maize were measured by EC stations and soil surface CO2 flux by a handheld chamber analyzer in arable fields in Southwest Germany over a period of three growing seasons (2009, 2010, and 2012). Additionally, successive bare fallow plots were installed at the beginning of each growing season to partition soil respiration between autotrophic and heterotrophic sources (including “labile” soil C (newest bare fallow) as the difference to the oldest bare fallow). Stepwise model building was tested with keeping rb and E0 constant (static method) and then by varying rb and E0 each individually or together by time period (dynamic method) over the whole growing season (15, 10 or 7days for Reco, measurement periods for soil chamber measurements). The dynamic models were superior as measured by Aaike Information Criteria (AIC) and coefficient of determination (average R2, 0.15 for the static model and 0.50 for the dynamic model). In the best fitting model for each crop-year (lowest AIC), rb was successfully estimated in each time period (relative standard error<50%), while seasonally variable E0 estimates were found in half of the crop years. Estimated Q10 values were between 1 to 6.01 between different components and seasons. Estimated Reco components during 2012, autotrophic above ground respiration accounted for the largest component during the intense measurement periods under both winter wheat (50%) and maize (60%), with root respiration accounting for 19% and 21%, respectively. Additionally under winter wheat 31% of Reco was estimated as heterotrophic respiration, with 15% from labile soil C. The results highlight the need to apply individual temperature response functions when using temperature as a driving force for ecosystem respiration components (autotrophic and soil heterotrophic).