Interactions between nitrogen and phosphorus in modulating soil respiration: A meta-analysis

BACKGROUND

Economic and social development worldwide increases the input of nutrients, especially nitrogen (N) and phosphorus (P), to soils. These nutrients affect soil respiration (Rs) in terrestrial ecosystems. They may act independently or have interactive effects on Rs. The effect of N and P on Rs and its components (autotrophic respiration [Ra] and heterotrophic respiration [Rh]), however, either individually or together, is poorly understood. We performed a meta-analysis of 130 studies to examine the effects of different fertilization treatments on Rs and its components across terrestrial ecosystems.

RESULTS

Our results showed that (1) The impact of fertilizer addition on Rs varies among different fertilizer types. N addition reduced Rs and Rh significantly but did not affect Ra; P addition had no significant effect on Rs, Rh, and Ra; NP addition increased Rs significantly but did not affect Rh and Ra. (2) Ecosystem type, duration of fertilization, fertilization rate, and fertilizer form influenced the response of Rs and its components to fertilizer application. (3) Based on our study, the annual average temperature may be a driving factor of Rs response to fertilizer addition, while soil total nitrogen may be an important predictor of Rs response to fertilizer addition.

CONCLUSION

Overall, our study highlights the complex and multifaceted nature of the response of soil Rs and its components to fertilizer application, underscoring the importance of considering multiple factors when predicting and modeling future Rs and its feedback to global change.

Contextualized response of carbon-use efficiency to warming at the plant and ecosystem levels

Carbon-use efficiency (CUE) has been widely used as a constant value in many earth system models to simulate how assimilated C is partitioned in ecosystems, to estimate ecosystem C budgets, and investigate C feedbacks to climate warming. Although correlative relationships from previous studies indicated that CUE could vary with temperature, and relying on a fixed CUE value could cause large uncertainty in model projections, however, due to the lack of manipulative experiment, it remains unclear how CUE at the plant (CUEp) and ecosystem (CUEe) levels respond to warming. Based on a 7-year manipulative warming experiment in an alpine meadow ecosystem on the Qinghai-Tibet Plateau, we quantitatively distinguished various C flux components of CUE, including gross ecosystem productivity, net primary productivity, net ecosystem productivity, ecosystem respiration, plant autotrophic respiration, and microbial heterotrophic respiration and explored how CUE at different levels responded to climate warming. We found large variations in both CUEp (0.60 to 0.77) and CUEe (from 0.38 to 0.59). The warming effect on CUEp was positively correlated with ambient soil water content (SWC) and the warming effect on CUEe was negatively correlated with ambient soil temperature (ST), but was positively correlated with warming-induced changes in ST. We also found that the direction and magnitude of the warming effects on different CUE components scaled differently with changes in the background environment, which explained the variation in CUE's warming response under environmental changes. Our new insights have important implications for reducing modelling uncertainty of ecosystem C budgets and improving our ability to predict ecosystem C-climate feedbacks under climate warming.

Extreme wet precipitation and mowing stimulate soil respiration in the Eurasian meadow steppe

The imbalance of terrestrial carbon (C) inputs versus losses to extreme precipitation can have consequences for ecosystem carbon balances. However, the current understanding of how ecosystem processes will respond to predicted extreme dry and wet years is limited. The current study was conducted for three years field experiment to examine the effects of environmental variables and soil microbes on soil respiration (Rs), autotrophic respiration (Ra) and heterotrophic respiration (Rh) under extreme wet and dry conditions in mowed and unmowed grassland of Inner Mongolia. Across treatments (i.e. control, dry spring, wet spring, dry summer and wet summer), the mean of Rs was increased by 24.9 % and 24.1 % in the wet spring and wet summer precipitation treatments, respectively in mowed grassland. In other hand, the mean of Rs was decreased by -22.1 % and -3.5 % in dry spring and dry summer precipitation treatments, respectively in mowed grassland. The relative contribution of Rh and Ra to Rs showed a significant (p < 0.05) change among simulated precipitation treatments with the highest value (76.18 %) in wet summer and 26.41 % in dry summer, respectively under mowed grassland. Rs was significantly (p < 0.05) affected by the interactive effect of extreme precipitation and mowing treatments in 2020 and 2021. The effects of precipitation change via these biotic and abiotic factors explained by 52 % and 81 % in Ra and Rh, respectively in mowed grassland. The changes in microbial biomass carbon (MBC) and nitrogen (MBN) had significant (p < 0.05) direct effects on Rh in both mowed and unmowed grasslands. The influence of biotic and abiotic factors on Rs was stronger in mowed grasslands with higher standardized regression weights than in unmowed grassland (0.78 vs. 0.69). These findings highlight the importance of incorporating extreme precipitation events and mowing in regulating the responses of C cycling to global change in the semiarid Eurasian meadow steppe.

Nitrogen concentration and physical properties are key drivers of woody tissue respiration

BACKGROUND AND AIMS

Despite the critical role of woody tissues in determining net carbon exchange of terrestrial ecosystems, relatively little is known regarding the drivers of sapwood and bark respiration.

METHODS

Using one of the most comprehensive wood respiration datasets to date (82 species from Australian rainforest, savanna and temperate forest), we quantified relationships between tissue respiration rates (Rd) measured in vitro (i.e. 'respiration potential') and physical properties of bark and sapwood, and nitrogen concentration (Nmass) of leaves, sapwood and bark.

KEY RESULTS

Across all sites, tissue density and thickness explained similar, and in some cases more, variation in bark and sapwood Rd than did Nmass. Higher density bark and sapwood tissues had lower Rd for a given Nmass than lower density tissues. Rd-Nmass slopes were less steep in thicker compared with thinner-barked species and less steep in sapwood than in bark. Including the interactive effects of Nmass, density and thickness significantly increased the explanatory power for bark and sapwood respiration in branches. Among these models, Nmass contributed more to explanatory power in trunks than in branches, and in sapwood than in bark. Our findings were largely consistent across sites, which varied in their climate, soils and dominant vegetation type, suggesting generality in the observed trait relationships. Compared with a global compilation of leaf, stem and root data, Australian species showed generally lower Rd and Nmass, and less steep Rd-Nmass relationships.

CONCLUSIONS

To the best of our knowledge, this is the first study to report control of respiration-nitrogen relationships by physical properties of tissues, and one of few to report respiration-nitrogen relationships in bark and sapwood. Together, our findings indicate a potential path towards improving current estimates of autotrophic respiration by integrating variation across distinct plant tissues.

Soil acid cations induced reduction in soil respiration under nitrogen enrichment and soil acidification

Atmospheric nitrogen (N) deposition and soil acidification both can largely change soil microbial activity and root growth with a consequent impact on soil respiration (R_s). However, it remains unclear which one, N enrichment or soil acidification, plays more important role in impacting soil respiration. We conducted a manipulative experiment to simulate N enrichment (10gm^-2yr^-1 NH₄NO₃) and soil acidity (0.552molH⁺m^-2yr^-1 sulfuric acid) and compared their effects on R_s and its components in a subtropical forest. The results showed that soil pH was reduced by 0.4 similarly under N addition or acid addition after 3years' treatment. Acid addition decreased autotrophic respiration (R_a) by 22-35% and heterotrophic respiration (R_h) by 22-23%, resulting in a reduction of R_s by 22-26% in the two years. N addition reduced R_a, R_h, R_s less than acid addition did. The reductions of R_s and its components were attributed to increase of soil acid cations and reduction of cellulose degrading enzymes activity. N addition and soil acidification significantly enhanced fungal to bacterial ratio. All the cellulose degrading enzymes were reduced more by soil acidity (43-50%) than N addition (30-39%). The principal component scores of degrading enzymes activity showed significantly positive relationships with R_h. Structural equation model showed that soil acidification played more important role than N enrichment in changing R_s and its components. We therefore suggest that soil acidification is an important mechanism underlying soil respiration changes, and should be incorporated into biogeochemical models to improve the prediction of ecosystem C cycling in the future scenarios of anthropogenic N deposition and acid enrichment.

Temperature sensitivity of total soil respiration and its heterotrophic and autotrophic components in six vegetation types of subtropical China

The temperature sensitivity of soil respiration (Q₁₀) is a key parameter for estimating the feedback of soil respiration to global warming. The Q₁₀ of total soil respiration (R_t) has been reported to have high variability at both local and global scales, and vegetation type is one of the most important drivers. However, little is known about how vegetation types affect the Q₁₀ of soil heterotrophic (R_h) and autotrophic (R_a) respirations, despite their contrasting roles in soil carbon sequestration and ecosystem carbon cycles. In the present study, five typical plantation forests and a naturally developed shrub and herb land in subtropical China were selected for investigation of soil respiration. Trenching was conducted to separate R_h and R_a in each vegetation type. The results showed that both R_t and R_h were significantly correlated with soil temperature in all vegetation types, whereas R_a was significantly correlated with soil temperature in only four vegetation types. Moreover, on average, soil temperature explained only 15.0% of the variation in R_a in the six vegetation types. These results indicate that soil temperature may be not a primary factor affecting R_a. Therefore, modeling of R_a based on its temperature sensitivity may not always be valid. The Q₁₀ of R_h was significantly affected by vegetation types, which indicates that the response of the soil carbon pool to climate warming may vary with vegetation type. In contrast, differences in neither the Q₁₀ of R_t nor that of R_a among these vegetation types were significant. Additionally, variation in the Q₁₀ of R_t among vegetation types was negatively related to fine root biomass, whereas the Q₁₀ of R_h was mostly related to total soil nitrogen. However, the Q₁₀ of R_a was not correlated with any of the environmental variables monitored in this study. These results emphasize the importance of independently studying the temperature sensitivity of R_t and its heterotrophic and autotrophic components.

Strong resilience of soil respiration components to drought-induced die-off resulting in forest secondary succession

How forests cope with drought-induced perturbations and how the dependence of soil respiration on environmental and biological drivers is affected in a warming and drying context are becoming key questions. The aims of this study were to determine whether drought-induced die-off and forest succession were reflected in soil respiration and its components and to determine the influence of climate on the soil respiration components. We used the mesh exclusion method to study seasonal variations in soil respiration (R S) and its components: heterotrophic (R H) and autotrophic (R A) [further split into fine root (R R) and mycorrhizal respiration (R M)] in a mixed Mediterranean forest where Scots pine (Pinus sylvestris L.) is undergoing a drought-induced die-off and is being replaced by holm oak (Quercus ilex L.). Drought-induced pine die-off was not reflected in R S nor in its components, which denotes a high functional resilience of the plant and soil system to pine die-off. However, the succession from Scots pine to holm oak resulted in a reduction of R H and thus in an important decrease of total respiration (R S was 36 % lower in holm oaks than in non-defoliated pines). Furthermore, R S and all its components were strongly regulated by soil water content-and-temperature interaction. Since Scots pine die-off and Quercus species colonization seems to be widely occurring at the driest limit of the Scots pine distribution, the functional resilience of the soil system over die-off and the decrease of R S from Scots pine to holm oak could have direct consequences for the C balance of these ecosystems.