Quantification of ecosystem C dynamics in a long-term FACE study on permanent grassland

Nitrous oxide effluxes from plants as a potentially important source to the atmosphere

The global budget for nitrous oxide (N₂ O), an important greenhouse gas and probably dominant ozone-depleting substance emitted in the 21^st century, is far from being fully understood. Cycling of N₂ O in terrestrial ecosystems has traditionally exclusively focused on gas exchange between the soil surface (nitrification-denitrification processes) and the atmosphere. Terrestrial vegetation has not been considered in the global budget so far, even though plants are known to release N₂ O. Here, we report the N₂ O emission rates of 32 plant species from 22 different families measured under controlled laboratory conditions. Furthermore, the first isotopocule values (δ¹⁵ N, δ¹⁸ O and δ¹⁵ N^sp ) of N₂ O emitted from plants were determined. A robust relationship established between N₂ O emission and CO₂ respiration rates, which did not alter significantly over a broad range of changing environmental conditions, was used to quantify plant-derived emissions on an ecosystem scale. Stable isotope measurements (δ¹⁵ N, δ¹⁸ O and δ¹⁵ N^sp ) of N₂ O emitted by plants clearly show that the dual isotopocule fingerprint of plant-derived N₂ O differs from that of currently known microbial or chemical processes. Our work suggests that vegetation is a natural source of N₂ O in the environment with a large fraction released by a hitherto unrecognized process.

Ecosystem responses to elevated CO2 governed by plant-soil interactions and the cost of nitrogen acquisition

Contents Summary 507 I. Introduction 507 II. The return on investment approach 508 III. CO₂ response spectrum 510 IV. Discussion 516 Acknowledgements 518 References 518 SUMMARY: Land ecosystems sequester on average about a quarter of anthropogenic CO₂ emissions. It has been proposed that nitrogen (N) availability will exert an increasingly limiting effect on plants' ability to store additional carbon (C) under rising CO₂ , but these mechanisms are not well understood. Here, we review findings from elevated CO₂ experiments using a plant economics framework, highlighting how ecosystem responses to elevated CO₂ may depend on the costs and benefits of plant interactions with mycorrhizal fungi and symbiotic N-fixing microbes. We found that N-acquisition efficiency is positively correlated with leaf-level photosynthetic capacity and plant growth, and negatively with soil C storage. Plants that associate with ectomycorrhizal fungi and N-fixers may acquire N at a lower cost than plants associated with arbuscular mycorrhizal fungi. However, the additional growth in ectomycorrhizal plants is partly offset by decreases in soil C pools via priming. Collectively, our results indicate that predictive models aimed at quantifying C cycle feedbacks to global change may be improved by treating N as a resource that can be acquired by plants in exchange for energy, with different costs depending on plant interactions with microbial symbionts.

Microbial association with the dynamics of particulate organic carbon in response to the amendment of elevated CO2-derived wheat residue into a Mollisol

As the chemical quality of crop residue is likely to be affected by elevated CO₂ (eCO₂), residue amendments may influence soil organic carbon (SOC) sequestration. However, in Mollisols, the dynamics of the SOC fractions in response to amendment with wheat residue produced under eCO₂ and the corresponding microbial community composition remain unknown. Such investigation is essential to residue management, which affects the soil quality and productivity of future farming systems. To narrow this knowledge gap, ¹³C-labeled shoot and root residue derived from ambient CO₂ (aCO₂) or eCO₂ were amended into Mollisols and incubated for 200days. The soil was sampled during the incubation period to determine the residue-C retained in the three SOC fractions, i.e., coarse intra-aggregate particulate organic C (coarse iPOC), fine iPOC and mineral-associated organic C (MOC). The soil bacterial community was assessed using a MiSeq sequencing instrument. The results showed that the increase in SOC concentrations attributable to the application of the wheat residue primarily occurred in the coarse iPOC fraction. Compared with the aCO₂-derived shoot residue, the amendment of eCO₂-derived shoot residue resulted in greater SOC concentrations, whereas no significant differences (P>0.05) were observed between the aCO₂- and eCO₂-derived roots. Principal coordinates analysis (PCoA) showed that the residue amendment significantly (P≤0.05) altered the bacterial community composition compared with the non-residue amendment. Additionally, the bacterial community in the aCO₂-derived shoot treatment differed from those in the other residue treatments until day 200 of the incubation period. The eCO₂-derived shoot treatment significantly increased (P≤0.05) the relative abundances of the genera Acidobacteriaceae_(Subgroup_1)_uncultured, Bryobacter, Candidatus_Solibacter, Gemmatimonas and Nitrosomonadaceae_uncultured, whereas the opposite trend was observed in Nonomuraea, Actinomadura, Streptomyces and Arthrobacter (P≤0.05). These results imply that the response of the microbial community to the eCO₂-derived shoot treatment is associated with its contribution to the POC fractions.

Carbon cycling in temperate grassland under elevated temperature

An increase in mean soil surface temperature has been observed over the last century, and it is predicted to further increase in the future. The effect of increased temperature on ecosystem carbon fluxes in a permanent temperate grassland was studied in a long‐term (6 years) field experiment, using multiple temperature increments induced by IR lamps. Ecosystem respiration (R‐eco) and net ecosystem exchange (NEE) were measured and modeled by a modified Lloyd and Taylor model including a soil moisture component for R‐eco (average R² of 0.78) and inclusion of a photosynthetic component based on temperature and radiation for NEE (R² = 0.65). Modeled NEE values ranged between 2.3 and 5.3 kg CO₂ m⁻² year⁻¹, depending on treatment. An increase of 2 or 3°C led to increased carbon losses, lowering the carbon storage potential by around 4 tonnes of C ha⁻¹ year⁻¹. The majority of significant NEE differences were found during night‐time compared to daytime. This suggests that during daytime the increased respiration could be offset by an increase in photosynthetic uptake. This was also supported by differences in δ¹³C and δ¹⁸O, indicating prolonged increased photosynthetic activity associated with the higher temperature treatments. However, this increase in photosynthesis was insufficient to counteract the 24 h increase in respiration, explaining the higher CO₂ emissions due to elevated temperature.