Climate change is associated with a change in soil organic carbon (SOC) stocks, implying a feedback mechanism on global warming. Grassland soils represent 28 % of the global soil C sink and are therefore important for the atmospheric greenhouse gas concentration.
In a field experiment in the Swiss Alps we recorded changes in the ecosystem organic carbon stock under climate change conditions, while quantifying the ecosystem C fluxes at the same time (ecosystem respiration, gross primary productivity, C export in plant material and leachate water). We exposed 216 grassland monoliths to six different climate scenarios (CSs) in an altitudinal transplantation experiment. In addition, we applied an
irrigation treatment (
In 5 years the ecosystem C stock, consisting of plant C and SOC, dropped
dramatically by about
The organic C stock contained in soils has long been recognized both as a substantial sink for anthropogenic CO
Storage of organic C (OC) is positively related to plant growth. Thus, increased plant growth may be expected to have a similarly positive effect on ecosystem C sequestration (Vitousek et al., 1997). For example, Ammann et al. (2009) found higher C sequestration in an intensively managed compared to an extensively managed grassland. In forests productivity increases following atmospheric N deposition, revealing a strong positive correlation with C sequestration (Magnani et al., 2007). Beyond edaphic factors, the grassland OC turnover is driven to a large degree by temperature, so that warmer soils have lower SOC contents. This effect can be observed along latitudinal gradients (Jones et al., 2005), as well as along altitudinal gradients.
This leads to the apparently paradox situation that less productive
ecosystems support larger soil C sinks. In Swiss grasslands for example,
more than 58 % of SOC is stored at 1000–2000 m a.s.l. (37 % of the total area), and despite the very shallow and cold soils 24 % of SOC is found above 2000 m altitude (21 % of the total area; Leifeld et al., 2005, 2009). As a result the 1000–2000 m a.s.l. region stores 3.6 times more SOC per unit land area compared to the
Under current global warming, the cold regions of high altitude and high
latitude are most strongly affected (Core writing team, IPCC, 2014), and
predicting the fate of the large biological CO
In addition, air pollution in the form of atmospheric N deposition may
constitute a fertilization effect. The N deposition rate is commonly very
low at sites far away from agriculture and fossil fuel burning (
However, the highly complex interactions of climate parameters (e.g., water
availability and temperature) and pollution factors (e.g., N) have led to
assume that the C sink of terrestrial ecosystems may also turn into a
substantial source of atmospheric CO
In this paper, we quantify the response of a subalpine grassland ecosystem C
budget in the face of multiple climate change factors that may favor plant
productivity. We present a comprehensive set of data related to relevant C
flux pathways to illuminate mechanisms controlling the ecosystem C sink and source properties. In a 5-year field experiment in the central Swiss
Alps, a climate scenario treatment was established that resulted in warming.
In addition, to uncouple potential temperature effects from
temperature-driven soil moisture effects and to consider effects of
atmospheric N deposition, a two-level irrigation treatment and a three-level
N treatment were set up in a factorial design. Using a transplantation
approach along an altitudinal gradient to accomplish the climate scenario
treatment, we affected not only temperatures, but also the length of snow
cover and the growing period. The long duration of the experiment provided a
large between-year weather variability. Because the investigated grasslands
had developed under a low-intensity management that was unaltered for
decades if not centuries, we considered the SOC stock to be in a steady
state on a mid- and long-term perspective. We hypothesized the following:
Under a climate scenario (CS) similar to the present climate, changes in productivity and decomposition will compensate each other and result in small or no changes in the SOC stock over 5 years. CS with strong temperature increases significantly alter the SOC stock towards a sink or a source, depending on whether plant productivity or SOC decomposition is affected more by climate change effects. Irrigation mitigates effects of water shortage due to warming, and N deposition reduces possible N limitation of microbial activity, both factors thus exhibiting a favorable effect on decomposition and reducing the SOC stock.
This study on ecosystem C fluxes is part of the AlpGrass experiment, and the Materials section refers only to those aspects relevant to the study of the C fluxes. We refer to Volk et al. (2021) for more details on the experimental design. Details on the gas exchange measurement and parameterization are provided in Appendix A.
The experiment used grassland monoliths to investigate climate change
effects on the soil carbon stock of subalpine grassland ecosystems in the
central Alps. At six sites with summer livestock grazing (within
Monoliths of 0.1 m
The AlpGrass experiment is located close to Ardez in the Lower Engadine
Valley (Graubünden, Switzerland). The site covers a 680 m altitudinal
gradient on the south slope of Piz Cotschen (3029 m), ranging from montane
forest (WGS 84 46.77818
Climate parameters at the climate scenario sites (CSs) between 2012 and 2017. Precipitation sums for climate scenario sites, aggregated from April to October and annually. Mean air temperature from April to October and for the whole year. Air temperature difference (
At each of the six CSs, 36 monoliths (six from each of six sites of origin) were installed in the ground within their drained plastic boxes, level with the surrounding soil surface, resulting in a total of 216 transplanted monoliths. Monoliths in their containers were set side by side without a gap. To prevent the invasion of new species or genotypes, the surroundings of the monolith array were frequently mown.
In addition to the climate scenario treatment, an irrigation and an N deposition treatment were set up in a full-factorial design at each CS. One-half of the 36 monoliths received only ambient precipitation, and the other half received additional water during the growing season. Within both irrigation treatment levels monoliths were subjected to three levels of N deposition. At the CS sites, irrigation and N treatments were set up in a randomized complete block design (six blocks each containing all six irrigation and N treatment combinations).
The climate scenario treatment was induced by the different altitudes of the
CSs at the AlpGrass site, where monoliths from the sites of origin were
installed. As a result, the transplanted monoliths experienced distinctly
different climatic conditions (Table 1). To describe the climate scenarios,
we focused on the mean growing period temperature from April to October,
instead of the annual mean temperature. The temperature under the snow cover
was ca. 0
A two-level irrigation treatment was set up to distinguish the warming effect from the soil moisture effect, driven by warming. Precipitation equivalents to 20 mm were applied to the monoliths under the irrigation treatment in four to six applications throughout the growing period. Depending on the year, this treatment amounted to 80–120 mm or 12 %–21 % of the recorded precipitation sum during the growing periods.
The N deposition treatment simulated an atmospheric N deposition from air
pollution, equivalent of
At all CSs, air temperature, relative humidity (Hygroclip 2, Rotronic,
Switzerland) and precipitation were measured (ARG100, Campbell Scientific,
UK). Global radiation (GR) as W m
Ambient wet N deposition was 3.3 kg N ha
Aboveground plant material, including mosses and lichens, was cut annually
at 2 cm above the soil at canopy maturity. Accordingly, mean harvest dates
for CS1 to CS6 were 12 August, 26 July, 22 July, 14 July, 9 July and 5 July, respectively. Plant productivity responses to the climate scenario, N
deposition and irrigation treatments were presented in Volk et al. (2021).
In addition at the end of the experiment in the fall of 2017, total
aboveground plant material was harvested including all stubble, and root
mass was assessed using two 5 cm diameter soil cores to 10 cm depth per
monolith. For the above- and belowground fraction, C content was measured
with a
Net ecosystem CO
For the 5-year parameterization of climate scenario effects on NEE, we
focused on a subset of monoliths from the control treatment group (no N
deposition, no irrigation) that provided the highest measurement frequency
(six control monoliths from each CS2
Lacking NEE data during the snow-covered period, a potential ER substrate limitation during the winter was not accounted for, since respiration rates were on an extremely low level due to low temperatures. Accordingly, temperature-normalized ER during the snow-covered period was modeled to remain constant between the last fall measurement and the first measurement of the new growing period, just after snowmelt.
In October 2012, 0–10 cm soil cores (5 cm diameter) were obtained in the grassland immediately beside the monolith's excavation site. Again in October 2017, two soil cores within each monolith were sampled to 10 cm depth to study the change of SOC stock and belowground biomass during the 5-year experimental phase. All samples were dried and sieved (2 mm).
We measured soil organic C and N contents by elemental analysis (oxidation
of C-CO
Monolith containers at CS2
Data were modeled for C stocks and C fluxes. SOC stock data were available
for 2012 and 2017, to calculate the SOC stock
Temperature effects on SOC stock change and root and shoot C stock data were also modeled directly as a function of temperature change, induced by the climate change treatment using generalized additive models (GAMs). Generalized additive models had to be used because simple linear models could not appropriately handle this relationship. The GAMs included a fixed intercept and a smooth term for temperature change. In the case of root and shoot C stock, the gamma function with log-link was chosen as the underlying distribution; following this amendment, model validation revealed that the assumptions of GAMs were met. The GAMs for the three response variables were modeled twice: first using all monoliths and second using only the control monoliths that received neither irrigation nor additional N. The latter was done to receive a direct comparison to the C flux data, which were measured only on control monoliths.
Regarding C fluxes, GPP, ER and NEP of CS2
Finally, we calculated the net ecosystem C balance to estimate the climate
change effect by comparing the ecosystem C budget of CS4 (
We detected significant effects of the climate scenario (CS) treatment on
soil organic C (SOC) stock (Table 2, Appendix Table B1). Across all
monoliths, the cooling associated with CS1 left the SOC stock largely
unchanged. At CS2
SOC stock change (Delta C soil) of subalpine grassland between
2012 and 2017 at six climate scenario sites (CS) as a function of the
temperature change (Delta temperature of the April–October mean) induced by
the climate change treatment.
SOC stock (g C m
No significant effects on SOC stock changes were associated with the irrigation and the N deposition treatments (Appendix Fig. A1, Table B1). Considering only the control monoliths, which received neither irrigation nor additional N, the same patterns appeared, although with larger standard errors due to smaller sample size (Fig. 1b, Table 2b).
In the final 2017 harvest, across all monoliths moderate warming at CS3
resulted in an increased root C stock of
Root and shoot carbon stock of subalpine grassland at five climate
scenario sites (CSs) as a function of the temperature change (Delta
temperature of the April–October mean) induced by the climate scenario
treatment. Data are from 2017, after 5 years of experimental duration.
Regarding the control monoliths group, the CS treatments revealed similar effects on each root and shoot C compared to all monoliths, although the reduction of root C stock at CS6 was somewhat less pronounced (Fig. 2b).
Seasonal temperature, soil moisture and canopy development determined the
magnitude of gross primary productivity (GPP) and ecosystem respiration (ER)
during 5 years at the three climate scenario sites CS2
Cumulative trajectory of gross primary productivity (GPP, solid
lines), ecosystem respiration (ER, dashed lines) and net ecosystem
productivity (NEP, colored lines) at three climate scenario sites from
October 2012 to September 2017. Displayed are means
Cumulative shoot C harvested over the 5 experimental years and cumulative
losses of leachate C were small relative to the cumulative ER losses:
cumulative shoot C was about
Net ecosystem C balance for CS2
cum.: cumulative.
The net ecosystem C balance largely agreed between the two approaches (Table 3). Compared to CS2
Physical and chemical soil properties limit the potential maximum size of
the SOC stock. While belowground biomass turnover rate, root exudates and
aboveground litter production rate determine the major C input rate, the C
output rate is determined by decomposition of OC through soil microbiota.
Both C input and output strongly depend on temperature and water
availability. As a consequence of the altitudinal transplantation, the
climatic conditions at the climate change CS sites were radically different
compared to CS2
It is important to note that our description of the C balance temperature
response is not based on soil temperature, but based on air temperature
change, because it is the reference to describe climate change effects on
ecosystems. Also, under field conditions there is no single soil
temperature, but an extremely dynamic, diurnal soil depth temperature
gradient that drives the CO
By integrating the C stock changes of a grassland ecosystem with intact C input pathways, our study avoids many of the shortfalls that impair the prediction of the fate of the terrestrial soil C sink, such as monitoring the temperature sensitivity of SOC decomposition in incubated soils (Crowther et al., 2016).
At the moderately warmer CS3 and CS4 and at the colder CS1, SOC stocks were
not significantly different from CS2
At extreme warming climate scenarios, the dynamics of root OC stock were
strikingly similar to SOC stock change, and both were substantially reduced
at CS5 and CS6 (compare Figs. 1 and 2). This indicates that under these
climatic conditions a reduced supply of organic material from belowground
plant fractions is one likely reason for the shrinking SOC stock at CS5 and
CS6. Importantly though, because SOC derives from dead plant material, OC
supply to the soil does not depend directly on the plant standing C stock,
but on the turnover rate of this C stock. We suggest that in our study the
allocation pattern at the control site CS2
The N resource is of great importance for plant productivity and microbial
decomposition of SOC. For example in a similar subalpine grassland (Alp Flix
experiment), a 10 and 50 kg N ha
Water availability is an essential factor for the ecosystem response to warming (compare below), but the irrigation treatment in our experiment yielded no effect. We assume that the applied amount was insufficient to make a difference, in particular at the warmer CSs, because we deem it likely that water was a limiting factor there. For details on water availability at the climate scenarios and the effect of irrigation on aboveground plant productivity, please refer to Volk et al. (2021; Table 2). Thus, results from the current experiment must leave it open whether mitigation of water shortage due to warming would change SOC stocks.
Warming, nitrogen and water must also be expected to affect plant species
composition, which in turn may affect ecosystem C fluxes. In a very similar
environment Bassin et al. (2009) studied 11 key plant species of a
subalpine pasture and found only very small responses of growth to N
deposition, except for the cyperaceous
Lacking other pathways of OC input, such as manure applications for
fertilization, the single source for all OC contained in our grassland
ecosystem is photosynthetic assimilates (GPP). Despite a positive effect of
warming on aboveground plant productivity (Volk et al., 2021), the 5-year GPP flux – quantifying the total amount of assimilated C – was not
significantly different between climate scenario treatments
CS2
C flux balance of 5-year totals of gross primary productivity
(GPP), ecosystem respiration (ER) and net ecosystem productivity (NEP) at
three climate scenario sites. Displayed are means
The annual mean ER observed at CS4 was very similar (656 g C m
The asymmetric response of GPP and ER to warming in our experiment resulted
in a substantially negative CO
Because the C balance for CS2
In contrast, other mountain grassland studies that present annual C balances often report C sinks. These studies mostly use eddy covariance measurements and have no multi-level treatments or replications that would allow to test a mechanistic hypothesis against the ecosystem response or assign a between-subject error to the reported fluxes.
For example, a recent analysis of net greenhouse gas (GHG) balances (including N
In comparison, in an overview of the all-European CARBOMONT project
Berninger et al. (2015) did not identify substantial C sink properties, but
they find that “especially the natural mountain grasslands in our study were
quite close to carbon neutrality”. In our 5-year experiment, the
equivalent value based on C flux measurements at CS2
Taken together, we suspect that substantial C sequestration situations cannot be considered typical in permanent mountain grasslands, but that a deviation from a zero balance indicates either a weather-driven year-to-year variability or an unaccounted for agricultural management effect. This implies that annual C budgets often represent a spotlight on a highly dynamic transition phase of the ecosystem OC stock.
Short-term grassland warming studies like our experiment must be regarded with caution when used to make long-term predictions, but analyses from the Icelandic ForHot experiment rated the parameter “SOC stock” to be a stable and consequently a useful predictor for the future state of the ecosystem already after 5–8 years of warming treatment (Walker et al., 2020). Because temperature sensitivity does not increase with soil depth (Pries et al., 2017) or varying recalcitrance of organic matter (Conen et al., 2006), topsoil temperature responses are also representative for subsoil responses. Thus, we assume that we missed no pathway of additional C input to supply the substrate consumed by increased ER and present a valid balance here.
Consequently, with respect to stocks and fluxes, we expect three alternative
developments under sustained warming.
The remaining SOC stock is sufficiently protected to resist further decomposition at high rates, and ER will soon decrease. Despite a very recalcitrant remaining SOC stock, the positive biomass response at intermediate climate scenarios not covered in this three-level comparison may supply sufficient new, labile OC from plants, and ER may remain high, with no further decline of the SOC stock. The more active microbial community succeeds in accessing even more of the previously protected SOC stock for decomposition, and ER will remain high, leading to a further decline of the SOC stock.
The small change in the SOC stock at the CS2
Throughout this study we adopt an ecosystem perspective when stating gas
fluxes. This implies that gross primary productivity (GPP) has a positive
value, while ecosystem respiration (ER) has a negative value. Net ecosystem
exchange (NEE) is positive when GPP
At 7–12 d yr
To measure NEE, we used a dynamic CO
After placement of the chamber we waited a few moments for a continuous
CO
Soil carbon stock change (Delta C) of subalpine grassland between
2012 and 2017 as a function of the altitude of climate scenario sites (CSs)
and
Daily flux sums (mean) of CO
Measured NEE
NEE
From clear-sky NEE
NEP was used in the sense of describing the balance between GPP and ER,
equivalent to an hourly, daily or annual CO
Summary of analyses for the effects of climate scenario (CS),
irrigation and N deposition on SOC stock change of subalpine grassland between 2012 and
2017.
df
Summary of analyses for the effects of climate scenario (CS),
irrigation and N deposition on root carbon stock of subalpine grassland in 2017, after 5 years of experimental treatment.
df
Summary of analyses for the effects of climate scenario (CS),
irrigation and N deposition on shoot carbon stock of subalpine grassland in 2017, after 5 years of experimental treatment.
df
A general note to the generalized additive models: in all models, the
default from the mgcv package has been used with the exception that the
“gamma” statement of the gam() function was sometimes changed to adapt the
degree of smoothing of the fitted line. This, however, did not or only
marginally influence the inference drawn from the model, i.e., the
Summary of analyses for the effects of temperature change (delta
temperature) induced by the climate change treatments on soil carbon stock change (Delta C soil) of subalpine grassland between 2012 and 2017.
df: degrees of freedom. edf: effective degrees of freedom (which can be fractional in smooth terms of generalized additive models).
Summary of analyses for the effects of temperature change (delta
temperature) induced by the climate change treatments root carbon stock (Root C) at 2017
after 5 years of experimental treatment.
Summary of analyses for the effects of temperature change (delta
temperature) induced by the climate change treatments shoot carbon stock (Shoot C) at 2017 after 5 years of experimental treatment.
The data are available at the CERN Zenodo data repository (
MV and SB designed the experiment. MV, ALW and SB conducted fieldwork. MV and MS analyzed the data. MV led the writing of the manuscript, with significant contribution from MS. All authors contributed critically to the drafts and gave final approval for publication.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We received essential financial support through the Federal Office for the Environment (contract no. 00.5100.PZ/R442-1499). The Gemeinde Ardez and Alpmeister Claudio Franziscus generously allowed us to work on the Allmend. We are grateful to Robin Giger for his untiring support in the field and the lab and to the scientific site manager Andreas Gauer, who was in charge of the field sites.
This research has been supported by the Bundesamt für Umwelt (grant no. 00.5100.PZ/R442-1499).
This paper was edited by Ben Bond-Lamberty and reviewed by two anonymous referees.