Response of Soil Respiration and Its Components to Precipitation Exclusion in Vitex negundo Var. Heterophylla Shrubland of the Middle Taihang Mountain in North China

Assessing the response of soil heterotrophic and autotrophic respiration to climate change is critical for forecasting terrestrial carbon cycle behavior in the future. In the present study, we conducted a drought experiment in Vitexnegundo var. heterophylla shrub ecosystem of the Middle Taihang Mountain. Three precipitation manipulation treatments (natural conditions/ambient precipitation (CK), reduced precipitation by 30% (PE30), and reduced precipitation by 60% (PE60)) were used to study the impact of different levels of precipitation exclusion on total soil respiration (Rs) and its heterotrophic (Rh) and autotrophic (Ra) components. Our results showed that the rates of Rs and its components were significantly decreased under the precipitation exclusion treatments. The proportion of Rh in Rs reduced from 72.6% for CK to 71.9% under PE60. The annual cumulative C fluxes of Rs decreased by 47.8 g C m−2 in PE30 and 106.0 g C m−2 in PE60, respectively. An exponential relationship was observed between the rate of each soil respiration component and soil temperature in all treatments ( p < 0.01). Moreover, each soil respiration component rate was better represented by a quadratic model which included soil moisture ( p < 0.01). However, including both of soil temperature and soil moisture did not explain more variation in soil respiration components compared than the regression model with soil moisture only. In addition, excluding precipitation increased the temperature sensitivity (Q10 values) of Rs and its Ra and Rh components compared to the control. Collectively, our findings suggest that increased drought will inhibit the release of carbon from the soil to the atmosphere, and will likely decrease the contribution of Rh to Rs in this semiarid shrubland ecosystem.

Separating autotrophic and heterotrophic soil CO₂ effluxes in afforested peatlands

Peatlands are a significant global carbon (C) store, which can be compromised by drainage and afforestation. Quantifying the rate of C loss from peat soils under forestry is challenging, as soil CO2 efflux includes both CO2 produced from heterotrophic peat decomposition and CO2 produced by tree roots and associated fungal networks (autotrophic respiration). We experimentally terminated autotrophic belowground respiration in replicated forest plots by cutting through all living tree roots (trenching), and measured soil surface CO2 flux, litter input, litter decay rate and soil temperature and moisture over two years. Annual peat decomposition (heterotrophic CO2 flux) was 115 ± 16 g C m−2 y−1, representing c. 40 % of total soil respiration. Decomposition of needle litter is accelerated in the presence of an active rhizosphere, indicating a priming effects by labile C inputs from roots. This suggests that our estimates of peat mineralization in our trenched plots are conservative, and underestimate overall rates of peat C loss. Considering also input of litter from trees, our results indicate that the soils in these 30 year-old drained and afforested peatlands are a net sink for C, since substantially more C enters the soil as organic matter, than is decomposed heterotrophically. However, the C balance for these soils should be taken over the lifespan of the trees, in order to determine if the soils under these drained and afforested peatlands are a sustained sink of C, or become a net source over longer periods of forestry.

Hot or not? connecting rhizosphere hotspots to total soil respiration

Aims Soil organic carbon (C) efflux is tightly linked to the rhizosphere, where soil microorganisms rapidly decompose organic compounds released from roots. Recently, imaging approaches have greatly improved our understanding of small-scale C-turnover heterogeneity and promoted the term ‘rhizosphere hotspots’ for highly active areas. However, despite often assumed, the effect of these hotspots on total soil C balances is still unknown. We aim to bridge this gap by correlating rhizosphere imaging data to soil respiration on individual plant scale. Methods We grew 17 maize (Zea mays L.) plants in rhizoboxes filled with sandy arable soil. After four weeks, the plants were labelled with 14CO2 and root exudation was visualized and quantified by 14C-imaging one day after labeling. The evolved CO2 was trapped in NaOH and 14CO2 as well as total CO2 was quantified before and after labelling. Enzyme activity (β-glucosidase) was quantified by soil zymography. Results Bulk soil β-glucosidase activitiy negatively correlated to total CO2 efflux, and was the most important predictor (R2 = 0.55). Total and rhizosphere specific 14C-activity were solely correlated to 14CO2 efflux (r = 0.51, r = 0.58). A combination of bulk soil β-glucosidase activity, rhizosphere-14C activity and root biomass, explained about 75% of variance in CO2 efflux. Conclusions This indicates that root exudation and enzyme-activity hotspots are suitable predictors for total soil respiration, particularly when combined with root biomass to account for three-dimensional variation, and that hotspots on the rhizosphere scale are directly linked to larger scale C balances.

Comparative mycorrhizal fungal production and respiration of a neotropical rainforest versus a California mixed forest

<p>Mycorrhizae are a symbiosis between fungi and plants. We have learned about the complexity of mechanisms of interaction and interactions between the mycorrhizae and the local environment from over a century of laboratory observations experiments. Point observations and laboratory studies identify processes, but cannot delineate activity. Our goal is to use an in situ system to study mycorrhizal roots and fungi during hot moments, daily shifts, and seasonal change.</p><p>We integrated continuous in situ observation-sensor measurements using our Soil Ecosystem Observatories. As turnover rate estimates are related to sample frequency, individual scans using manual minirhizotrons (Bartz and Rhizosystems) and Rhizosystems Automated Minirhizotrons (32,000-3.01mm x 2.26mm 307,200 pixel images). Automated scans were collected up to 4x daily. Manual scans across multiple tubes in campaigns provided spatial variation. Images were organized into mosaics using RootView software, and roots and hyphae identified and length, width and biovolume determined using RootDetector <http://www.rhizosystems.com/>. Individual roots and hyphae were tracked using RootFly <https://cecas.clemson.edu/~stb/rootfly/>. Lifespans were determined using Mark-Recapture modeling and turnover calculated. With each minirhizotron tube, sensors were placed at 3 or 4 depths for temperature, moisture, CO<sub>2</sub> and O<sub>2</sub> at 5minute intervals.</p><p>Mycorrhizal fungi (MF) explore soil for nutrients and requiring C. Most C to the hyphae is respired (with a <sup>14</sup>C signal of autotrophic respiration), with the remaining divided into decomposing (heterotrophic respiration) and sequestered C pools.</p><p>Our first site is a mature neotropical rainforest, the La Selva Biological Station, Costa Rica. Trees predominantly form arbuscular mycorrhizae (AM). AMF fungi comprise 50% of total fungal mass (PLFA). Aboveground NPP-C was 750g/m<sup>2</sup>. Root standing crop C was 120g/m<sup>2</sup>, average lifespan 60days, =6 generations/y, = root NPP of 720g/m<sup>2</sup>/y. The AMF hyphal standing crop C was 12.5g/m<sup>2</sup>, average lifespan of 25 days, =14.7 generations/y, = AMF NPP of 183g/m<sup>2</sup>/y. With an NPP of 1,650g/m<sup>2</sup>/y, then AMF comprises 11% of NPP.</p><p>Soil respiration provides CO<sub>2</sub>, converting in water to HCO<sub>3</sub><sup>-</sup>, altering soil pH (Henry's Law). AMF respiration thereby increases P availability. If 10% of the AM fungal hyphae are live, then the hyphal respiration is 438g/m<sup>2</sup>/y of C, =38% of total soil respiration and 16% of site respiration.</p><p>Our second site is a mature California mixed forest, USA. Ectomycorrhizal (EM) trees predominate. Annual NPP-C was 200g/m<sup>2</sup>, and root NPP was 200g/m<sup>2</sup>. EMF NPP was 162.6g/m<sup>2</sup>, or 27% of the NPP. N, water, and temperature limit NPP. The seasonal signal was very high in this ecosystem. Peak standing crop of extramatrical EM hyphae was 19gC/m<sup>2</sup> in April. Total soil respiration in April was 0.26g/h, and extramatrical hyphae 0.029g/h, or 11% of the total soil respiration. Since P is less limiting, but N and water are, hyphae likely play a greater role in enzymatic activity and exploratory surface area.</p><p>In summary, different mycorrhizal fungi play different roles depending on ecosystem limiting factors. With global change, our challenge is to determine how an ecosystem will change and the extent and rapidity of mycorrhizal fungal change.</p>

Autotrophic and heterotrophic contributions to soil respiration in a subtropical camphor tree forest

Understanding the contributions of autotrophic respiration (Ra) and heterotrophic respiration (Rh) to total soil respiration (Rs) is necessary for accurate prediction of global carbon balance and net ecosystem production under environmental change. In this research, annual Rs and Rh and estimated were investigated by using a root trenching experiment in a Camphor tree (Cinnamomum camphora) forest in subtropical China for two years to qualify the relative contribution of Ra and Rh components to Rs, and to determine the environmental factors that control the seasonal changes in Ra, Rh and Rs. The results showed that annual mean Rs was 405 ± 219 gC m-2 year-1 in the studied forests, of which Rh and Ra were 240 ± 120 gC m-2 year-1 and 164 ±102 gC m-2 year-1, respectively. The contribution of Rh to Rs averaged 58.1%, ranging from 45 to 81%. The seasonal changes in Rs and Rh were highly correlated with soil temperature, but not to soil water content. Our results suggest microbial community and activity make a primary contribution to carbon flux released from soil to atmosphere in the studied forest ecosystems.

Contribution of root respiration to total soil respiration during non-growing season in mine reclaimed soil with different covering-soil thicknesses

An accurate assessment of root respiration in mine reclaimed soil is important for effectively evaluating mining area ecosystems. This study investigated dynamic changes in root respiration and the contribution of root respiration to total soil respiration (Rr/Rt ratio) during the non-growing season in mine reclaimed soil, with different covering-soil thicknesses. According to the covering-soil thicknesses, the study area was divided into four sites: 10–25 cm (site A), 25–45 cm (site B), 45–55 cm (site C), and 55–65 cm (site D). From November 2017 to April 2018 (except February in 2018), the soil respiration, root respiration, temperature at 5 cm, water content, and root biomass were measured. The results show that soil temperature and root respiration exhibited similar diurnal and monthly variations. The root respiration is strongly influenced by soil temperature during the non-growing season, with an exponential and positive relationship (P < 0.001). Root respiration varies with the covering-soil thickness and is greatest with a covering-soil thickness of 25–45 cm. The Rr/Rt ratio also exhibits monthly variations. During the non-growing season, the mean value of the Rr/Rt ratio is 51.15% in mine reclaimed soil. The study indicates that root respiration is the primary source of soil respiration and is an important factor for estimating the potential emission of soil CO2 from mine reclaimed soil at the regional scale.

Vertical partitioning of CO₂ production in a forest soil

Large amounts of total organic carbon are temporarily stored in soils, which makes soil respiration one of the major sources of terrestrial CO2 fluxes within the global carbon cycle. More than half of global soil organic carbon (SOC) is stored in subsoils (below 30 cm), which represent a significant carbon (C) pool. Although several studies and models have investigated soil respiration, little is known about the quantitative contribution of subsoils to total soil respiration or about the sources of CO2 production in subsoils. In a 2-year field study in a European beech forest in northern Germany, vertical CO2 concentration profiles were continuously measured at three locations, and CO2 production was quantified in the topsoil and the subsoil. To determine the contribution of fresh litter-derived C to CO2 production in the three soil profiles, an isotopic labelling experiment, using 13C-enriched leaf litter, was performed. Additionally, radiocarbon measurements of CO2 in the soil atmosphere were used to obtain information about the age of the C source in the CO2 production. At the study site, it was found that 90 % of total soil respiration was produced in the first 30 cm of the soil profile, where 53 % of the SOC stock is stored. Freshly labelled litter inputs in the form of dissolved organic matter were only a minor source for CO2 production below a depth of 10 cm. In the first 2 months after litter application, fresh litter-derived C contributed, on average, 1 % at 10 cm depth and 0.1 % at 150 cm depth to CO2 in the soil profile. Thereafter, its contribution was less than 0.3 % and 0.05 % at 10 and 150 cm depths, respectively. Furthermore CO2 in the soil profile had the same modern radiocarbon signature at all depths, indicating that CO2 in the subsoil originated from young C sources despite a radiocarbon age bulk SOC in the subsoil. This suggests that fresh C inputs in subsoils, in the form of roots and root exudates, are rapidly respired, and that other subsoil SOC seems to be relatively stable. The field labelling experiment also revealed a downward diffusion of 13CO2 in the soil profile against the total CO2 gradient. This isotopic dependency should be taken into account when using labelled 13C and 14C isotope data as an age proxy for CO2 sources in the soil.

Contribution of root respiration to total soil respiration during non-growing season in mine reclaimed soil with different covering soil thicknesses

An accurate assessment of root respiration in mine reclaimed soil is important for effectively evaluating mining area ecosystem. This study investigated dynamic changes in root respiration and contribution of root respiration to total soil respiration (Rr/Rt ratio) during the non-growth season in mine reclaimed soil with different covering soil thicknesses. According to covering soil thicknesses, the study area was divided into four sites: 10-25 cm (site A), 25-45 cm (site B), 45-55 cm (site C) and 55-65 cm (site D). From November 2017 to April 2018 (except February in 2018), the soil respiration, root respiration, temperature at 5 cm, water content and root biomass were measured. The results showed that soil temperature and root respiration exhibited similar diurnal and monthly variations. The root respiration was strongly influenced by soil temperature during the non-growing season, which showed an exponential and positive relationship with soil temperature (P<0.001). The root respiration varied with the covering soil thickness and was the greatest with the covering soil thickness at 25–45 cm. The Rr/Rt ratio also exhibited monthly variations. During the non-growth season, the mean value of the Rr/Rt ratio were 51.15% in mine reclaimed soil. The study indicated that root respiration was the primary source of soil respiration and important to estimate the potential of emission of soil CO 2 at regional scale in mine reclaimed soil.

Effects of Experimental Throughfall Exclusion on Soil Respiration in a Continental Coniferous Stand, South Korea

Severe droughts and changing precipitation patterns could alter the biogeochemical properties of the soil, affecting soil carbon cycles in forest ecosystems. A throughfall exclusion (TFE) experiment was conducted in a continental climate coniferous stand in Gangwon Province, Korea, to examine the effects of excluding rainfall on total soil respiration (SR), heterotrophic soil respiration (HR), autotrophic soil respiration (AR), sapling diameter growth, and soil bacterial communities from July 2016 to October 2017. The soil water content (SWC) was significantly decreased by the exclusion of the throughfall, resulting in changes in the bacterial communities, and subsequently a decrease in HR. Although AR did not present significant differences between the control and TFE plots, the rate of sapling growth was significantly lower in the TFE plots compared with that in the control plots. An exponential function relating SR to soil temperature accounted for 0.61% and 0.82% of the variance in SR in the control and TFE plots, respectively (Q10 = 2.48 and 2.86, respectively). Furthermore, a multivariate nonlinear model based on soil temperature and SWC explained 0.89% and 0.88% of the variance in SR in the control and TFE plots, respectively. When soil temperature was high, SR showed high fluctuations due to SWC variation. However, when SWC was low, we detected relatively small fluctuations in SR due to soil temperature. The results of this study show that the activity of soil microbial and root respiration during the growing season may be lower under future drought conditions.

Contribution of root respiration to total soil respiration during non-growing season in mine reclaimed soil with different covering soil thicknesses

An accurate assessment of root respiration in mine reclaimed soil is important for effectively evaluating mining area ecosystem. This study investigated dynamic changes in root respiration and contribution of root respiration to total soil respiration (Rr/Rt ratio) during the non-growing season in mine reclaimed soil with different covering soil thicknesses. According to covering soil thicknesses, the study area was divided into four sites: 10-25 cm (site A), 25-45 cm (site B), 45-55 cm (site C) and 55-65 cm (site D). From November 2017 to April 2018 (except February in 2018), the soil respiration, root respiration, temperature at 5 cm, water content and root biomass were measured. The results showed that soil temperature and root respiration exhibited similar diurnal and monthly variations. The root respiration was strongly influenced by soil temperature during the non-growing season, which showed an exponential and positive relationship with soil temperature (P<0.001). The root respiration varied with the covering soil thickness and was the greatest with the covering soil thickness at 25–45 cm. The Rr/Rt ratio also exhibited monthly variations. During the non-growing season, the mean value of the Rr/Rt ratio was 51.15% in mine reclaimed soil. The study indicated that root respiration was the primary source of soil respiration and important to estimate the potential emission of soil CO2 at regional scale in mine reclaimed soil.