Soil physico-chemical properties are critical for predicting carbon storage and nutrient availability across Australia

Soil carbon and nutrient availability play crucial roles in ecosystem sustainability, and they are controlled by the interaction of climatic, biotic, and soil physico-chemical variables. Although soil physico-chemical properties have been recognized as vital variables for predicting soil organic carbon (SOC) and nutrients, their relative influence across broad geographical scales has yet to be evaluated when simultaneously considering many other drivers. Using boosted regression tree and structural equation modelling analyses of observations from topsoil (0-10 cm) and subsoil (20-30 cm) at 628 sites across Australia, we investigated the effects and relative influence of climate (mean annual temperature and aridity index), plant productivity, soil biodiversity (bacterial and fungal richness), and soil physical (clay and silt) and chemical (pH and iron) properties on SOC content and nutrient availability (i.e. nitrogen, phosphorus, and potassium). Among these variables, we found that soil physico-chemical properties primarily predicted the continent-scale SOC storage and nutrient availability. In contrast, climate, plant productivity, and soil biodiversity played relatively small roles. The importance of physico-chemical properties was evident across soil depths and ecosystem types (i.e. tropical, temperate, arid, and cropland). Our findings point to the need to better understand the role of soil physico-chemical properties in soil carbon and nutrient cycling and including these variables in predictions of SOC and nutrient dynamics at the ecosystem to continental scale

An incubation study of temperature sensitivity of greenhouse gas fluxes in three land-cover types near Sydney, Australia

Greenhouse gas (GHG) fluxes play crucial roles in regulating the Earth surface temperature. However, our understanding of the effect of land-cover and soil depth on the potential GHG fluxes and their temperature sensitivities (Q10) is limited, which consequently increases the uncertainty to predict GHG exchange between soils and the atmosphere. In the present study, we sampled soils with contrasting characteristics from three land-cover types (wetland, grassland, and forest) and soil depths (0–10, 10–20, and 20–30 cm) from the Cumberland Plain near Sydney, Australia, and incubated at optimal (60%) water holding capacity at three temperatures (15, 25, and 35 °C). Overall, GHG fluxes and Q10 values differed significantly among land-cover types and soil depths. CO2 and N2O emissions were highest in wetland followed by grassland and forest soils, and they decreased with soil depth. In contrast, CH4 uptake was highest in grassland followed by forest and wetland soils, and it increased with soil depth. Combining the three major GHGs, the global warming potential in soil from wetland was higher than that from grassland and forest. Moreover, Q10 values of CO2 and N2O emissions were: wetland > grassland > forest, while Q10 value of CH4 uptake showed the opposite pattern. Q10 values of CO2 and N2O emissions and CH4 uptake all increased with soil depth, demonstrating that subsoil has a higher potential for CO2 and N2O emissions and CH4 uptake in a warming climate. While these experiments were conducted under ideally controlled laboratory conditions, results suggest that the large carbon stocks in wetland soils are vulnerable to loss and thus may amplify climate warming; upland soils are crucial CH4 sinks and thus potentially mitigate climate change. In addition, the greater temperature sensitivities of CO2 and N2O emissions and CH4 uptake in subsoil should be accounted for in carbon and nitrogen cycling models

Soil physico-chemical properties are more important than microbial diversity and enzyme activity in controlling carbon and nitrogen stocks near Sydney, Australia

Soil organic carbon (SOC) and available nitrogen (N) stocks are controlled by the complex interplay of soil physical, chemical, and biological conditions. However, the interrelations of SOC or available N with these drivers as well as their relative importance are rarely evaluated quantitatively. Using investigations of SOC density (SOCD) and available N density (ND) with other detailed soil properties of topsoil (0–10 cm) and subsoil (20–30 cm) from 33 sites under different ecosystems in the vicinity of an eddy flux tower near Sydney, Australia, we investigated the controls of soil physical, chemical, and biological properties (a total of 19 variables) on SOCD and available ND. Structural equation models showed that only physical and chemical properties significantly and directly affected SOCD and available ND. Among these variables, physical and chemical properties were the most influential factors, while the relative influences of microbial biodiversity and enzyme activity were small based on boosted regression tree analysis. In addition, the effects of variables on SOCD and available ND differed between the topsoil and subsoil. In the topsoil, soil physical properties had the highest relative influence followed by chemical properties, enzyme activities, and microbial biodiversity; in the subsoil, however, soil chemical properties had the highest relative importance followed by physical properties, enzyme activities, and microbial biodiversity. This comprehensive soil characterization provides the biogeochemical context for ecosystem carbon cycling being monitored at a nearby eddy flux tower, and demonstrates the importance of including accurate measurements of soil physical and chemical properties to reduce uncertainty in soil C and N predictions in process-based models. However, this is a local-scale study, and large-scale studies are warranted to gain further understanding on this issue

Root effects on the temperature sensitivity of soil respiration depend on climatic condition and ecosystem type

Root activity may alter the temperature sensitivity (Q10) of soil respiration. However, we lack a comprehensive understanding of root effects on Q10 across different climatic regions and ecosystem types. Here, we conducted a global synthesis of 87 observations of Q10 values of soil respiration and its components from 40 published studies. We found that roots significantly enhanced Q10 of soil respiration because root and rhizosphere respiration was more temperature-sensitive than the respiration of root-free soil, especially in cold regions (mean annual temperature < 10 °C). Moreover, roots significantly enhanced Q10 of soil respiration in grassland and cropland but not in forest ecosystems. Overall, our results suggest that the positive effects of roots on Q10 largely depend on climatic condition and ecosystem type, and are potentially useful for parameterizing and benchmarking biogeochemical and Earth system models

Spatial heterogeneity of temperature sensitivity of soil respiration : a global analysis of field observations

The temperature sensitivity of soil respiration (Q10) plays a key role in benchmarking the intensity of terrestrial soil carbon-climate feedbacks. However, there is large spatial heterogeneity of Q10 that remains unexplained at the global scale. Here, we collected 480 estimates of Q10 values from field studies to explore the spatial heterogeneity of Q10 values and their controls at both global and regional scales. We used structural equation modeling to explore the direct and indirect factors and their relative importance predicting Q10 values at the global scale, and in different ecosystem types (i.e. forests and grasslands) and climatic zones (i.e. tropical, subtropical, temperate, and boreal). We found that mean annual temperature (MAT) was the most important factor in predicting field Q10 at the global scale, rather than mean annual precipitation (MAP) or soil properties (e.g. soil organic carbon (SOC) content). However, different dominant factors controlled Q10 in different ecosystems and climatic zones. Across forests, MAT was the dominant factor except in the tropics, where Q10 was mainly regulated by clay and SOC content. For grasslands, MAP, pH, and SOC were the most important factors in predicting Q10. These findings indicate that global field Q10 is mainly controlled by MAT, and this is inconsistent with most previous incubation experiments showing that soil properties are more important than climatic factors in predicting Q10 values. Moreover, recognizing different dominant factors of Q10 in different ecosystems and climatic zones improves our understanding of soil carbon-climate feedbacks in a warming climate

Microbial carbon use efficiency, biomass residence time and temperature sensitivity across ecosystems and soil depths

Decomposition of soil organic matter by microorganisms is a fundamental mechanism driving the terrestrial carbon (C) cycle. Microbial C use efficiency (CUE), microbial biomass residence time (MRT), and soil C temperature sensitivity (Q10) co-determine the fate of soil C in a changing climate. In order to reveal the effect of soil depth and varying hydrologic properties on CUE, MRT, and Q10 of microbial respiration, we incubated soils from three ecosystems (wetland, grassland, and forest) and soil depths (0–10, 20–30, and 50–60 cm) at two temperatures (10 and 30 °C). Microbial CUE was estimated using a substrate-independent method by incorporating 18O from labeled water into microbial DNA with the simultaneous measurement of microbial respiration. CUE ranged from about 0.2 to 0.7 with a mean value of 0.5 ± 0.1, MRT ranged from 4 to 73 days with a mean value of 26 ± 19 days, and Q10 ranged from 1.8 to 2.9, averaging 2.3 ± 0.3 across all samples. We found that CUE increased but MRT and Q10 decreased along the wetland-grassland-forest hydrologic gradient; and they all increased with soil depth. Moreover, CUE and MRT were lower at 30 °C than that at 10 °C. Although there were some differences in factors regulating the variation in CUE, MRT or Q10 among soil depths and ecosystem types, both within individual ecosystems and depths, CUE, MRT, and Q10 were strongly correlated to available C:N ratios, clay content, and C quality, respectively. In conclusion, our findings emphasize the importance of stoichiometry and C quality of available substrates in predicting the variation in microbial C use efficiency and soil C temperature sensitivity in different soil depths and along a hydrologic gradient

Depth dependence of soil carbon temperature sensitivity across Tibetan permafrost regions

Permafrost regions with high soil organic carbon (SOC) storage are extremely vulnerable to global warming. However, our understanding of the temperature sensitivity of SOC decomposition in permafrost regions remains limited, leading to considerable uncertainties in predicting carbon-climate feedback magnitude and direction in these regions. Here, we investigate general patterns and underlying mechanisms of SOC decomposition rate and its temperature sensitivity (Q10) at different soil depths across Tibetan permafrost regions. Soils were collected at two depths (0–10 and 20–30 cm) from 91 sites across Tibetan permafrost regions. SOC decomposition rate and Q10 value were estimated using a continuous-flow incubation system. We found that the SOC decomposition rate in the upper layer (0–10 cm) was significantly greater than that in the lower layer (20–30 cm). The SOC content governed spatial variations in decomposition rates in both soil layers. However, the Q10 value in the upper layer was significantly lower than that in the lower layer. Soil pH and SOC decomposability had the greatest predictive power for spatial variations in Q10 value within the upper and lower layers, respectively. Owing to the greater temperature sensitivity in the lower layer, our results imply that subsurface soil carbon is at high risk of loss, and that soil carbon sequestration potential might decrease in these regions in a warming world

Biogeographic variation in temperature sensitivity of decomposition in forest soils

Determining soil carbon (C) responses to rising temperature is critical for projections of the feedbacks between terrestrial ecosystems, C cycle, and climate change. However, the direction and magnitude of this feedback remain highly uncertain due largely to our limited understanding of the spatial heterogeneity of soil C decomposition and its temperature sensitivity (Q10). Here, we quantified C decomposition and its response to temperature change with an incubation study of soils from 203 sites across tropical to boreal forests in China spanning a wide range of latitudes (18o16' to 50o26'N) and longitudes (81o01' to 129o28'E). Mean annual temperature (MAT) and mean annual precipitation primarily explained the biogeographic variation in the decomposition rate and temperature sensitivity of soils: soil C decomposition rate decreased from warm and wet forests to cold and dry forests, while Q10‐MAT (standardized to the MAT of each site) values displayed the opposite pattern. In contrast, biological factors (i.e. plant productivity and soil bacterial diversity) and soil factors (e.g. clay, pH, and C availability of microbial biomass C and dissolved organic C) played relatively small roles in the biogeographic patterns. Moreover, no significant relationship was found between Q10‐MAT and soil C quality, challenging the current C quality‐temperature hypothesis. Using a single, fixed Q10‐MAT value (the mean across all forests), as is usually done in model predictions, would bias the estimated soil CO2 emissions at a temperature increase of 3.0 °C. This would lead to overestimation of emissions in warm biomes, underestimation in cold biomes, and likely significant overestimation of overall C release from soil to the atmosphere. Our results highlight that climate‐related biogeographic variation in soil C responses to temperature needs to be included in next‐generation C cycle models to improve predictions of C‐climate feedback

Elevated CO2 alters the temperature sensitivity of stem CO2 efflux in a mature eucalypt woodland

The CO2 efflux from tree stem surfaces to atmosphere (RS) is an important component in the carbon (C) balance of forest ecosystems. Rising atmospheric carbon dioxide concentrations [CO2] are expected to stimulate RS, because of greater C assimilation and carbohydrate supply to stems under rising [CO2]. Growth respiration (Rg) and maintenance respiration (Rm) during the warm growing season may respond differently to rising [CO2] due to different metabolic demands. To test the effect of elevated [CO2] (eCO2, ambient +150 ppm) on RS, we examined RS in mature Eucalyptus trees on a monthly basis for an entire year during the seventh year of exposure to eCO2. RS varied seasonally and mirrored seasonal variation in temperature. While RS was not significantly increased under eCO2 compared to ambient CO2 (aCO2), its temperature sensitivity was significantly decreased (Q10 of 1.92 for aCO2 and 1.56 for eCO2). The estimated annual Rg accounted for approximately 7–8% of annual total RS, 419 ± 103 g C m− 2 yr-1, indicating that Rm contributes substantially to total RS in this mature woodland. Monthly mean RS was correlated with monthly mean soil temperature, soil moisture and monthly stem growth rate in this dry year, but soil moisture levels may have been insufficient to observe the impacts of eCO2 on stem growth in this droughted and phosphorous limited site. Our results highlight that eCO2 tends to increase Rm at low temperatures during the non-growing season, thus decreasing the temperature sensitivity of RS, despite a neutral effect of eCO2 on RS rates on a yearly basis

Does root respiration in Australian rainforest tree seedlings acclimate to experimental warming?

Plant respiration can acclimate to changing environmental conditions and vary between species as well as biome-types, though belowground respiration responses to on-going climate warming is not well-understood. Understanding the thermal acclimation capacity of root respiration (Rroot) in relation to increasing temperatures is therefore critical in elucidating a key uncertainty in plant function in response to warming. However, the degree of temperature acclimation of Rroot in rainforest trees and how root chemical and morphological traits are related to acclimation is unknown. Here we investigated the extent to which respiration of fine roots (≤ 2 mm) of four tropical and four warm-temperate rainforest tree seedlings differed in response to warmer growth temperatures (control and + 6°C), including temperature sensitivity (Q10) and the degree of acclimation of Rroot. Regardless of biome-type, we found no consistent pattern in the short-term temperature responses of Rroot to elevated growth temperature: a significant reduction in the temperature response of Rroot to +6°C treatment was only observed for a tropical species, Cryptocarya mackinnoniana, whereas the other seven species had either some stimulation or no alteration. Across species, Rroot was positively correlated with root tissue nitrogen concentration (mg g−1), while Q10 was positively correlated with root tissue density (g cm−3). Warming increased root tissue density by 20.8%, but did not alter root nitrogen across species. We conclude that thermal acclimation capacity of Rroot to warming is species-specific, and suggest that root tissue density is a useful predictor of Rroot and its thermal responses of rainforest tree seedlings