Louise C. Andresen

Nitrogen cycle responses to elevated CO 2 depend on ecosystem nutrient status

Nitrogen (N) limitation of terrestrial ecosystems is a crucial factor for predicting how these ecosystems respond and feedback to climate change. Nitrogen availability for plants in terrestrial ecosystems depends on the internal soil N cycle and inputs to the ecosystem via biological N2 fixation. We reviewed the effect of elevated atmospheric CO2 concentrations (eCO2) on gross soil N transformations to advance our understanding of ecosystem responses to eCO2. Overall, neither gross mineralization nor gross nitrification was altered by eCO2. However, emerging from ecosystem specific analysis, we propose a new conceptual model for eCO2 effects on gross mineralization based on ecosystem nutrient status: gross mineralization is only stimulated in N limited ecosystems, but unaffected in phosphorus limited ecosystems. Moreover, the ratio of ammonium oxidation to immobilization is decreased under eCO2, indicating a tighter N cycle with reduced ecosystem N losses. This new conceptual model on N cycle responses to eCO2 should be tested in the future in independent experiments and it provides a new concept for refining mechanistic models of ecosystem responses to climate change.

Predicting soil carbon loss with warming

Crowther et al. reported that the best predictor of surface soil carbon (top 10u2009cm) losses in response to warming is the size of the surface carbon stock in the soil (that is, carbon stocks in plots that have not been warmed), finding that soils that are high in soil carbon also lose more carbon under warming conditions. This relationship was based on a linear regression of soil carbon losses and soil carbon stocks in field warming studies, which was then used to project carbon losses over time and to generate a map of soil carbon vulnerability. However, a few extreme data points (high-leverage points) can strongly influence the slope of a regression line. Only 5 of the 49 sites analysed by Crowther et al.1 are in the upper half of the carbon stock range, which raises the possibility that the relationship they observed could be substantially altered by introducing data from sites with relatively high surface soil carbon stocks. There is a Reply to this Comment by Crowther, T. W. et al. Nature 554, 10.1038/nature25746 (2018).

Biomass responses in a temperate European grassland through 17 years of elevated CO2

Future increase in atmospheric CO2 concentrations will potentially enhance grassland biomass production and shift the functional group composition with consequences for ecosystem functioning. In the GiFACE experiment (Giessen Free Air Carbon dioxide Enrichment), fertilized grassland plots were fumigated with elevated CO2 (eCO2 ) year-round during daylight hours since 1998, at a level of +20% relative to ambient concentrations (in 1998, aCO2 was 364xa0ppm and eCO2 399xa0ppm; in 2014, aCO2 was 397xa0ppm and eCO2 518xa0ppm). Harvests were conducted twice annually through 23xa0years including 17xa0years with eCO2 (1998 to 2014). Biomass consisted of C3 grasses and forbs, with a small proportion of legumes. The total aboveground biomass (TAB) was significantly increased under eCO2 (pxa0=xa0.045 and .025, at first and second harvest). The dominant plant functional group grasses responded positively at the start, but for forbs, the effect of eCO2 started out as a negative response. The increase in TAB in response to eCO2 was approximately 15% during the period from 2006 to 2014, suggesting that there was no attenuation of eCO2 effects over time, tentatively a consequence of the fertilization management. Biomass and soil moisture responses were closely linked. The soil moisture surplus (c. 3%) in eCO2 manifested in the latter years was associated with a positive biomass response of both functional groups. The direction of the biomass response of the functional group forbs changed over the experimental duration, intensified by extreme weather conditions, pointing to the need of long-term field studies for obtaining reliable responses of perennial ecosystems to eCO2 and as a basis for model development.

Long-term and realistic global change manipulations had low impact on diversity of soil biota in temperate heathland

In a dry heathland ecosystem we manipulated temperature (warming), precipitation (drought) and atmospheric concentration of CO2 in a full-factorial experiment in order to investigate changes in below-ground biodiversity as a result of future climate change. We investigated the responses in community diversity of nematodes, enchytraeids, collembolans and oribatid mites at two and eight years of manipulations. We used a structural equation modelling (SEM) approach analyzing the three manipulations, soil moisture and temperature, and seven soil biological and chemical variables. The analysis revealed a persistent and positive effect of elevated CO2 on litter C:N ratio. After two years of treatment, the fungi to bacteria ratio was increased by warming, and the diversities within oribatid mites, collembolans and nematode groups were all affected by elevated CO2 mediated through increased litter C:N ratio. After eight years of treatment, however, the CO2-increased litter C:N ratio did not influence the diversity in any of the four fauna groups. The number of significant correlations between treatments, food source quality, and soil biota diversities was reduced from six to three after two and eight years, respectively. These results suggest a remarkable resilience within the soil biota against global climate change treatments in the long term.

Isotopic methods for non‐destructive assessment of carbon dynamics in shrublands under long‐term climate change manipulation

1.Long-term climate change experiments are extremely valuable for studying ecosystem responses to environmental change. Examination of the vegetation and the soil should be non-destructive to guarantee long-term research. In this paper, we review field methods using isotope techniques for assessing carbon dynamics in the plant-soil-air continuum, based on recent field experience and examples from a European climate change manipulation network. 2.Eight European semi-natural shrubland ecosystems were exposed to warming and drought manipulations. One field site was additionally exposed to elevated atmospheric CO2. We evaluate the isotope methods that were used across the network to evaluate carbon fluxes and ecosystem responses, including: 1) analysis of the naturally rare isotopes of carbon (13C and 14C) and nitrogen (15N); 2) use of in-situ pulse labelling with 13CO2, soil injections of 13C- and 15N-enriched substrates, or continuous labelling by Free Air Carbon dioxide Enrichment (FACE) and 3) manipulation of isotopic composition of soil substrates (14C) in lab-based studies. 3.The natural 14C signature of soil respiration gave insight into a possible long-term shift in the partitioning between the decomposition of young and old soil carbon sources. Contrastingly, the stable isotopes 13C and 15N were used for shorter-term processes, as the residence time in a certain compartment of the stable isotope label signal is limited. The use of labelled carbon-compounds to study carbon mineralization by soil microorganisms enabled to determine the long-term effect of climate change on microbial carbon uptake kinetics and turnover. 4.Based on the experience with the experimental work, we provide recommendations for the application of the reviewed methods to study carbon fluxes in the plant-soil-air continuum in climate change experiments. 13C-labelling techniques exert minimal physical disturbances, however, the dilution of the applied isotopic signal can be challenging. In addition, the contamination of the field site with excess 13C or 14C can be a problem for subsequent natural abundance (14C and 13C) or label studies. The use of slight changes in carbon and nitrogen natural abundance does not present problems related to potential dilution or contamination risks, but the usefulness depends on the fractionation rate of the studied processes. This article is protected by copyright. All rights reserved.