Warming and Labile Substrate Addition Alter Enzyme Activities and Composition of Soil Organic Carbon

Warming can increase the efflux of carbon dioxide (CO2) from soils and can potentially feedback to climate change. In ajout to warming, the input of labile carbon can enhance the microbial activity by stimulating the co-metabolism of recalcitrant soil organic matter (SOM). This is particularly true with SOM under invaded ecosystems where elevated CO2 and warming may increase the biomass of invasive species resulting in higher addition of labile substrates. We hypothesized that the input of labile carbon would instigate a greater soil organic carbon (SOC) loss with warming compared to the ambient température. We investigated this by incubating soils collected from a native pine (Pinus taeda) forest to which labile carbon from the invasive species kudzu (Pueraria lobata) was added. We evaluated the microbial extracellular enzyme activité, molecular composition of SOC and the temperature sensitivity of soil CO2 efflux under warming and labile carbon addition. After 14 months of soil incubation, the addition of labile C through kudzu extract increased the activity of β-1,4-glucosidase compared with the control. However, the activity of N-acetyl-β-D-glucosaminidase and fungal biomasse (ergosterol) decreased with labile carbon addition. The activity of peroxidase increased with warming after 14 months of soil incubation. Although the carbon content of incubated soils did not vary with substrate and temperature treatments, the molecular composition of SOC indicated a general decrease in biopolymers such as cutin, suberin, long-chain fatty acids, and phytosterol with warming and an increasing trend of microbial-derived compounds with labile substrate addition. In soils that received an addition of labile C, the macro-aggregate stability was higher while the temperature sensitivity of soil C efflux was lower compared with the control. The increase in aggregate stability could enhance the physical protection of SOC from microbial decomposition potentially contributing to the observed pattern of temperature sensitivity. Our results suggest that warming could preferentially accelerate the decomposition of recalcitrant compounds while the addition of labile substrates could enhance microbial-derived compounds that are relatively resistant to further decomposition. Our study further emphasizes that global change factors such as plant invasion and climate change can differentially alter soil microbial activity and the composition of SOC

The Responses of Soil and Rhizosphere Respiration to Simulated Climatic Changes Vary by Season.

Responses of soil respiration (Rs) to anthropogenic climate change will affect terrestrial carbon storage and, thus, feed back to warming. To provide insight into how warming and changes in precipitation regimes affect the rate and temperature sensitivity of Rs and rhizosphere respiration (Rr) across the year, we subjected a New England old-field ecosystem to four levels of warming and three levels of precipitation (ambient, drought, and wet treatments). We measured Rs and heterotrophic respiration (Rh) monthly (in areas of the plots with and without plants, respectively) and estimated Rr by calculating the difference in respiration between Rs and Rh. Even in this mesic ecosystem, Rs and Rr responded strongly to the precipitation treatments. Drought reduced Rs and Rr, both annually and during the growing season. Annual cumulative Rs responded nonlinearly to precipitation treatments; both drought and supplemental precipitation suppressed Rs compared to the ambient treatment. Warming increased Rs and Rr in spring and winter when soil moisture was optimal but decreased these rates in summer when moisture was limiting. Cumulative winter Rr increased by about 200% in the high warming (approximately 3.5 degrees C) treatment. The effect of climate treatments on the temperature sensitivity of Rs depended on the season. In the fall, the drought treatment decreased apparent Q10 relative to the other precipitation treatments. The responses of Rs to warming and altered precipitation were largely driven by changes in Rr. We emphasize the importance of incorporating realistic soil moisture responses into simulations of soil carbon fluxes; the long-term effects of warming on carbon--climate feedback will depend on future precipitation regimes. Our results highlight the nonlinear responses of soil respiration to soil moisture and, to our knowledge, quantify for the first time the loss of carbon through winter rhizosphere respiration due to warming. While this additional loss is small relative to the cumulative annual flux in this system, such increases in rhizosphere respiration during the non-growing season could have greater consequences in ecosystems where they offset or reduce subsequent warming-induced gains in plant growth

Warming and elevated CO\u3csub\u3e2\u3c/sub\u3e alter the suberin chemistry in roots of photosynthetically divergent grass species

A majority of soil carbon (C) is either directly or indirectly derived from fine roots, yet roots remain the least understood component of the terrestrial carbon cycle. The decomposability of fine roots and their potential to contribute to soil C is partly regulated by their tissue chemical composition. Roots rely heavily on heteropolymers such as suberins, lignins and tannins to adapt to various environmental pressures and to maximize their resource uptake functions. Since the chemical construction of roots is partly shaped by their immediate biotic/abiotic soil environments, global changes that perturb soil resource availability and plant growth could potentially alter root chemistry, and hence the decomposability of roots. However, the effect of global change on the quantity and composition of root heteropolymers are seldom investigated. We examined the effects of elevated CO2 and warming on the quantity and composition of suberin in roots of Bouteloua gracilis (C4) and Hesperostipa comata (C3) grass species at the Prairie Heating and CO2 Enrichment (PHACE) experiment at Wyoming, USA. Roots of B. gracilis exposed to elevated CO2 and warming had higher abundances of suberin and lignin than those exposed to ambient climate treatments. In addition to changes in their abundance, roots exposed to warming and elevated CO2 had higher ω-hydroxy acids compared to plants grown under ambient conditions. The suberin content and composition in roots of H. comata was less responsive to climate treatments. In H. comata, α,ω-dioic acids increased with the main effect of elevated CO2, whereas the total quantity of suberin exhibited an increasing trend with the main effect of warming and elevated CO2. The increase in suberin content and altered composition could lower root decomposition rates with implications for root-derived soil carbon under global change. Our study also suggests that the climate change induced alterations in species composition will further mediate potential suberin contributions to soil carbon pools

Litters of photosynthetically divergent grasses exhibit differential metabolic responses to warming and elevated CO\u3csub\u3e2\u3c/sub\u3e

Climatic stress such as warming would alter physiological pathways in plants leading to changes in tissue chemistry. Elevated CO2 could partly mitigate warming induced moisture stress, and the degree of this mitigation may vary with plant functional types. We studied the composition of structural and non-structural metabolites in senesced tissues of Bouteloua gracilis (C4) and Pascopyrum smithii (C3) at the Prairie Heating and CO2 Enrichment experiment, Wyoming, USA. We hypothesized that P. smithii and B. gracilis would respond to unfavorable global change factors by producing structural metabolites and osmoregulatory compounds that are necessary to combat stress. However, due to the inherent variation in the tolerance of their photosynthetic pathways to warming and CO2, we hypothesized that these species will exhibit differential response under different combinations of warming and CO2 conditions. Due to a lower thermo-tolerance of the C4 photosynthesis we expected B. gracilis to exhibit a greater metabolic response under warming with ambient CO2 (cT) and P. smithii to exhibit a similar response under warming combined with elevated CO2 (CT). Our hypothesis was supported by the differential response of structural compounds in these two species, where cT increased the content of lignin and cuticular-matrix in B. gracilis. In P. smithii a similar response was observed in plants exposed to CT, possibly due to the partial alleviation of moisture stress.With warming, the total cell-wall bound phenolic acids that cross link polysaccharides to lignins increased in B. gracilis and decreased in P. smithii, indicating a potentially adaptive response of C4 pathway to warming alone. Similarly, in B. gracilis, extractable polar metabolites such as sugars and phenolic acids increased with the main effect of warming. Conversely, in P. smithii, only sugars showed a higher abundance in plants exposed to warming treatments indicating that warming alone might be metabolically too disruptive for the C3 photosynthetic pathway. Here we show for the first time, that along with traditionally probed extractable metabolites, warming and elevated CO2 differentially influence the structural metabolites in litters of photosynthetically divergent grass species. If these unique metabolite responses occur in other species of similar functional types, this could potentially alter carbon cycling in grasslands due to the varying degradability of these litters

Litters of photosynthetically divergent grasses exhibit differential metabolic responses to warming and elevated CO\u3csub\u3e2\u3c/sub\u3e

Climatic stress such as warming would alter physiological pathways in plants leading to changes in tissue chemistry. Elevated CO2 could partly mitigate warming induced moisture stress, and the degree of this mitigation may vary with plant functional types. We studied the composition of structural and non-structural metabolites in senesced tissues of Bouteloua gracilis (C4) and Pascopyrum smithii (C3) at the Prairie Heating and CO2 Enrichment experiment, Wyoming, USA. We hypothesized that P. smithii and B. gracilis would respond to unfavorable global change factors by producing structural metabolites and osmoregulatory compounds that are necessary to combat stress. However, due to the inherent variation in the tolerance of their photosynthetic pathways to warming and CO2, we hypothesized that these species will exhibit differential response under different combinations of warming and CO2 conditions. Due to a lower thermo-tolerance of the C4 photosynthesis we expected B. gracilis to exhibit a greater metabolic response under warming with ambient CO2 (cT) and P. smithii to exhibit a similar response under warming combined with elevated CO2 (CT). Our hypothesis was supported by the differential response of structural compounds in these two species, where cT increased the content of lignin and cuticular-matrix in B. gracilis. In P. smithii a similar response was observed in plants exposed to CT, possibly due to the partial alleviation of moisture stress.With warming, the total cell-wall bound phenolic acids that cross link polysaccharides to lignins increased in B. gracilis and decreased in P. smithii, indicating a potentially adaptive response of C4 pathway to warming alone. Similarly, in B. gracilis, extractable polar metabolites such as sugars and phenolic acids increased with the main effect of warming. Conversely, in P. smithii, only sugars showed a higher abundance in plants exposed to warming treatments indicating that warming alone might be metabolically too disruptive for the C3 photosynthetic pathway. Here we show for the first time, that along with traditionally probed extractable metabolites, warming and elevated CO2 differentially influence the structural metabolites in litters of photosynthetically divergent grass species. If these unique metabolite responses occur in other species of similar functional types, this could potentially alter carbon cycling in grasslands due to the varying degradability of these litters

Linking the carbon cycle to climate change: Effects of warming and altered precipitation on organic matter decomposition

Untangling the effects of multiple factors of climate change on terrestrial carbon stocks is complex due to the differential responses of heterotrophic (Rh) and autotrophic (rhizosphere; Rr) respiration. Our lack of understanding of the relative sensitivities of these responses limits our ability to predict soil carbon loss in future climate scenarios. I measured soil and heterotrophic respiration monthly, and used these values to estimate Rr. The heterotrophic respiration in this mesic ecosystem strongly responded to precipitation. During the summer, when Rh was highest, there was threshold, hysteretic responses to soil moisture: R h decreased sharply when volumetric soil moisture dropped below ∼15% or exceeded ∼26%, but Rh increased more gradually when soil moisture rose from the lower threshold. The effect of climate treatments on the temperature sensitivity (Q10) of Rh depended on the season where high warming decreased Q10 in spring and fall and drought decreased Q10 in fall alone. To my knowledge this was the first study that identified the seasonal variation in the temperature sensitivity of microbial respiration in the field. Currently, most biogeochemical models represent the relationship between soil organic matter decomposition and warming using a temperature function with a fixed Q10 and my research supports the argument that that models with seasonally varying parameters may be more accurate than those with constant parameters. The Q10 values of Rh from this study and from future work in other biomes could be used to develop a temporally variable Q10 function that responds to abiotic conditions. In this mesic ecosystem, both Rs and Rr responded strongly to precipitation. Drought reduced Rs and Rr, both annually and during the growing season. Annual cumulative Rs responded non-linearly to precipitation treatments; both drought and supplemental precipitation suppressed Rs compared to the ambient treatment. Cumulative winter Rr increased by about 200% in the high warming (∼3.5oC) treatment. This carbon loss, presumably to maintenance respiration, should reduce net primary production (NPP) in the subsequent season, thereby affecting the longer-term carbon balance. The effect of climate treatments on the temperature sensitivity of Rs depended on the season. Drought decreased apparent Q10 in fall compared to the other precipitation treatments. These results highlight the non-linear responses of soil respiration to soil moisture, and to my knowledge quantify for the first time the loss of carbon through winter rhizosphere respiration due to warming. Since plants form the substrate for decomposition process, the quality of plant tissue is an important determinant of the stability of carbon under future climate. The effect of climate change on the preferential decomposition of labile (easily degradable) and recalcitrant compounds in organic matter is still a matter of debate. I studied how warming and altered precipitation affected the decomposition of recalcitrant matrix in litter and the associated changes in microbial extracellular enzyme activity using three litter types (the shrub-like Polygonum cuspidatum, at two stages—newly senesced litter (referred hereafter as NEW litter) and standing litter that has been decomposing for a year (OLD litter) and the grass Poa trivialis) the BACE. The OLD litter was enriched in recalcitrant compounds compared to NEW as indicated by the initial 13C-NMR spectral analysis and C:N ratios. After three years of decomposition in the BACE plots, using DRIFT (Diffuse Reflectance Infrared Fourier Transform) spectroscopy, I found that OLD litter with a high proportion of recalcitrant compounds responded faster to precipitation compared to NEW litter with relatively higher proportion of labile compounds. Supplemental precipitation along with high warming conditions accelerated the degradation of lignins and phenolic bands. This study also revealed a non-linear response of microbial enzymes and change in fungal biomass (ergosterol content) due to warming and altered precipitation. I conclude that warming along with changes in precipitation would alter the decomposition of recalcitrant compounds in plant litter, thus changing the amount and quality of carbon available for sequestration. (Abstract shortened by UMI.

Appendix A. Seven figures depicting seasonal variation in soil temperature and moisture and response to precipitation and temperature treatments, as well as nine tables showing results from mixed-model REML analysis of response to warming and precipitation treatments.

Seven figures depicting seasonal variation in soil temperature and moisture and response to precipitation and temperature treatments, as well as nine tables showing results from mixed-model REML analysis of response to warming and precipitation treatments