Comparison of root mean residence times estimated from steady-state turnover modeling (x axis) and root age as estimated from curve reading of the atmospheric radiocarbon bomb peak (y axis) for depth 0–10 cm.

<p>Comparison of root mean residence times estimated from steady-state turnover modeling (x axis) and root age as estimated from curve reading of the atmospheric radiocarbon bomb peak (y axis) for depth 0–10 cm.</p

Overview of apparent temperature sensitivities for root turnover rates (y^-1)in i) the current data set, ii) the global data set for grassland encompassing all climatic zones (Gill and Jackson [7]) and iii) a reduced data set extracted from Gill and Jackson [7] for grassland spanning a range in MAT of -0.7–12°C which corresponds to the range in MAT in the current data set. Exponents are given (± 1 SE).

<p>Overview of apparent temperature sensitivities for root turnover rates (y^-1)in i) the current data set, ii) the global data set for grassland encompassing all climatic zones (Gill and Jackson [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119184#pone.0119184.ref007" target="_blank">7</a>]) and iii) a reduced data set extracted from Gill and Jackson [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119184#pone.0119184.ref007" target="_blank">7</a>] for grassland spanning a range in MAT of -0.7–12°C which corresponds to the range in MAT in the current data set. Exponents are given (± 1 SE).</p

Mean annual temperature and root carbon mean residence time (0–10 cm) of all sampling plots.

<p>The curve displays the exponential relationship mean residence time (y) = 13.38 [1.94] * exp(-0.24 [0.05] * x), R2 = 0.53,P < 0.001. Values in square brackets are 1 SE.</p

Comparison of root turnover rates (y^-1) between the current data set (dashed lines) and the global data set from grassland soils in Gill and Jackson [7] (for explanation, please see text; triangles and solid lines).

<p>Data from the current data set are displayed as letters which refer to the management intensity of the plots (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119184#pone.0119184.t001" target="_blank">Table 1</a>). Only the temperature range relevant for this study is shown. Envelopes are 95% confidence intervals of the regression lines.</p

Depth distribution of root mean residence times along a gradient in mean annual temperature from + 7.8°C to 0.0°C (corresponding to plots “Wallis” and “Furka” in Table 1. Lines are a guide for the eyes only.

<p>Depth distribution of root mean residence times along a gradient in mean annual temperature from + 7.8°C to 0.0°C (corresponding to plots “Wallis” and “Furka” in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119184#pone.0119184.t001" target="_blank">Table 1</a>. Lines are a guide for the eyes only.</p

Temperature sensitivity of soil respiration rates enhanced by microbial community response

Soils store about four times as much carbon as plant biomass, and soil microbial respiration releases about 60 petagrams of carbon per year to the atmosphere as carbon dioxide. Short-term experiments have shown that soil microbial respiration increases exponentially with temperature. This information has been incorporated into soil carbon and Earth-system models, which suggest that warming-induced increases in carbon dioxide release from soils represent an important positive feedback loop that could influence twenty-first-century climate change. The magnitude of this feedback remains uncertain, however, not least because the response of soil microbial communities to changing temperatures has the potential to either decrease or increase warming-induced carbon losses substantially. Here we collect soils from different ecosystems along a climate gradient from the Arctic to the Amazon and investigate how microbial community-level responses control the temperature sensitivity of soil respiration. We find that the microbial community-level response more often enhances than reduces the mid- to long-term (90 days) temperature sensitivity of respiration. Furthermore, the strongest enhancing responses were observed in soils with high carbon-to-nitrogen ratios and in soils from cold climatic regions. After 90 days, microbial community responses increased the temperature sensitivity of respiration in high-latitude soils by a factor of 1.4 compared to the instantaneous temperature response. This suggests that the substantial carbon stores in Arctic and boreal soils could be more vulnerable to climate warming than currently predicted

Carbon-nitrogen interactions in European forests and semi-natural vegetation - Part 1 : Fluxes and budgets of carbon, nitrogen and greenhouse gases from ecosystem monitoring and modelling

The impact of atmospheric reactive nitrogen (N-r) deposition on carbon (C) sequestration in soils and biomass of unfertilized, natural, semi-natural and forest ecosystems has been much debated. Many previous results of this dC/dN response were based on changes in carbon stocks from periodical soil and ecosystem inventories, associated with estimates of N-r deposition obtained from large-scale chemical transport models. This study and a companion paper (Flechard et al., 2020) strive to reduce uncertainties of N effects on C sequestration by linking multi-annual gross and net ecosystem productivity estimates from 40 eddy covariance flux towers across Europe to local measurement-based estimates of dry and wet N-r deposition from a dedicated collocated monitoring network. To identify possible ecological drivers and processes affecting the interplay between C and N-r inputs and losses, these data were also combined with in situ flux measurements of NO, N2O and CH4 fluxes; soil NO3- leaching sampling; and results of soil incubation experiments for N and greenhouse gas (GHG) emissions, as well as surveys of available data from online databases and from the literature, together with forest ecosystem (BAS-FOR) modelling. Multi-year averages of net ecosystem productivity (NEP) in forests ranged from -70 to 826 gCm(-2) yr(-1) at total wet + dry inorganic N-r deposition rates (N-dep) of 0.3 to 4.3 gNm(-2) yr(-1) and from -4 to 361 g Cm-2 yr(-1) at N-dep rates of 0.1 to 3.1 gNm(-2) yr(-1) in short semi-natural vegetation (moorlands, wetlands and unfertilized extensively managed grasslands). The GHG budgets of the forests were strongly dominated by CO2 exchange, while CH4 and N2O exchange comprised a larger proportion of the GHG balance in short semi-natural vegetation. Uncertainties in elemental budgets were much larger for nitrogen than carbon, especially at sites with elevated N-dep where N-r leaching losses were also very large, and compounded by the lack of reliable data on organic nitrogen and N-2 losses by denitrification. Nitrogen losses in the form of NO, N2O and especially NO3- were on average 27%(range 6 %-54 %) of N-dep at sites with N-dep 3 gNm(-2) yr(-1). Such large levels of N-r loss likely indicate that different stages of N saturation occurred at a number of sites. The joint analysis of the C and N budgets provided further hints that N saturation could be detected in altered patterns of forest growth. Net ecosystem productivity increased with N-r deposition up to 2-2.5 gNm(-2) yr(-1), with large scatter associated with a wide range in carbon sequestration efficiency (CSE, defined as the NEP/GPP ratio). At elevated N-dep levels (> 2.5 gNm(-2) yr(-1)), where inorganic N-r losses were also increasingly large, NEP levelled off and then decreased. The apparent increase in NEP at low to intermediate N-dep levels was partly the result of geographical cross-correlations between N-dep and climate, indicating that the actual mean dC/dN response at individual sites was significantly lower than would be suggested by a simple, straightforward regression of NEP vs. N-dep.Peer reviewe