Quantifying the impact of grazing on ecosystem CO2 exchange in intensively managed pastures

<p>Long term flux measurements are needed to improve our understanding of the carbon balance of arable lands. The objective of our study was to determine the seasonal dynamics of carbon cycling in a Hungarian cropland and to examine the effect of crop rotation on net ecosystem exchange of CO<sub>2</sub> (NEE), furthermore to assess the influences of C outputs and inputs derived from lateral fluxes on soil organic carbon (SOC) stock. In this study we update the results presented in our poster of last year’s conference (EGU21-10977).</p><p>The experiment began in 2017 and crop rotation of the measured field consisted of winter wheat (2017-2018 and 2019-2020), rapeseed (2018), sorghum (2019) and sunflower (2021). CO<sub>2</sub> fluxes and annual net ecosystem exchange (NEE) of CO<sub>2</sub> were measured by a field-scale eddy covariance (EC) station at a Central Hungarian cropland site. Both vertical and lateral C fluxes were taken into account when calculating the net ecosystem carbon budget (NECB).</p><p>As presented in our previous study the largest sink activity was observed in the sorghum season (-277 g C m<sup>-2</sup> from sowing to harvest). The cropland acted as a source of CO<sub>2</sub> during the rapeseed season (140 g C m<sup>-2</sup>) due to incomplete germination caused by extreme autumnal drought.</p><p>We found that during the study period both meteorological variables and lateral carbon fluxes such as C inputs derived from seed and crop residues and outputs (harvest) had significant influence on the C dynamics. The higher temperatures and precipitation amount that characterised the fall of 2019 caused large differences in NEE dynamics for winter wheat when compared to 2017. The impact of climatic factors could be seen in the sunflower period since lack of precipitation in 2021 led to remarkably low carbon uptake.</p><p>Fallow periods in total covered a relatively long period of time (approximately 1 year out of the 4 year long study period). These fallow periods had a significant effect on NECB values due to immense C loss. During the four years of our experiment cumulative NEE was -222 g C m<sup>-2</sup> and NECB was 726 g C m<sup>-2</sup> as carbon loss during fallow periods (437 g C m<sup>-2</sup> in total) and carbon export through harvest (964 g C m<sup>-2</sup> in total) counterbalanced the crop’s CO<sub>2</sub> uptake.</p><p>We can conclude that while this Hungarian cropland was a sink of carbon it could not maintain the soil organic carbon content as it was not able to sequester enough carbon to do so. Cover plants and crop residue retention could be a solution to reduce the risk of soil carbon stock depletion but further studies are needed in the field of soil management practices.</p>

The continuous field measurements of net ecosystem exchange (NEE) of CO 2 were provided at ridge-hollow oligotrophic bog in the Middle Taiga zone of West Siberia, Russia in 2017-2018. The model of net ecosystem exchange of CO 2 was suggested to describe the influence of different environmental factors on NEE and to estimate the total carbon budget of the bog over the growing season. The model uses air and soil temperature, incoming photosynthetically active radiation (PAR) and water table depth, as the key factors influencing gross primary production (GPP) and ecosystem respiration (ER). The model coefficients were calibrated using the data collected by automated soil CO 2 flux system with two transparent long-term chambers placed at large hollow and small ridge sites. Experimental and modeling results showed that the Mukhrino bog acted over the study period as a carbon sink, with an average NEE of –87.7 gC m -2 at the hollow site and –50.2 gC m -2 at the ridge site. GPP was – 344.8 and –228.5 gC m -2 whereas ER was 287.6 and 140.9 gC m -2 at ridge and hollow sites, respectively. Despite of a large difference in NEE estimates between 2017 and 2018 the growing season variability of NEE were quite similar.

We analyzed 13 years (1992−2004) of CO2 flux data, biometry, and meteorology from a mixed deciduous forest in central Massachusetts. Annual net uptake of CO2 ranged from 1.0 to 4.7 Mg‐C ha−1yr−1, with an average of 2.5 Mg‐C ha−1yr−1. Uptake rates increased systematically, nearly doubling over the period despite forest age of 75–110 years; there were parallel increases in midsummer photosynthetic capacity at high light level (21.5−31.5 μmole m−2s−1), woody biomass (101−115 Mg‐C ha−1 from 1993−2005, mostly due to growth of one species, red oak), and peak leaf area index (4.5−5.5 from 1998–2005). The long‐term trends were interrupted in 1998 by sharp declines in photosynthetic capacity, net ecosystem exchange (NEE) of CO2, and other parameters, with recovery over the next 3 years. The observations were compared to empirical functions giving the mean responses to temperature and light, and to a terrestrial ecosystem model (IBIS2). Variations in gross ecosystem exchange of CO2 (GEE) and NEE on hourly to monthly timescales were represented well as prompt responses to the environment, but interannual variations and long‐term trends were not. IBIS2 simulated mean annual NEE, but greatly overpredicted the amplitude of the seasonal cycle and did not predict the decadal trend. The drivers of interannual and decadal changes in NEE are long‐term increases in tree biomass, successional change in forest composition, and disturbance events, processes not well represented in current models.

Changes in vegetation structure and composition, particularly due to the invasion of exotic species, are predicted to influence biosphere-atmosphere exchanges of mass and energy. Invasion of Cynara cardunculus (cardoon or artichoke thistle), a perennial, non-native thistle in coastal California grasslands presently dominated by non-native annual grasses, may alter rates of ecosystem CO2 exchange and evapotranspiration (ET). During spring and summer 2006, we compared midday maximum net ecosystem CO2 exchange (NEE) and ET among adjacent grassland plots where Cynara was present and where it was absent. Measurements of NEE supported the prediction that deeply-rooted Cynara increase midday ecosystem C-assimilation. Cynara-mediated shifts in NEE were associated with increases in ecosystem photosynthesis rather than changes in ecosystem respiration. Furthermore, the presence of Cynara was associated with increased ET during the growing season. An increase in aboveground live biomass (a proxy for leaf area) associated with Cynara invasion may underlie shifts in ecosystem CO2 and water vapor exchange. Following mid-growing season sampling during April, we removed Cynara from half of the Cynara-containing plots with spot applications of herbicide. Three weeks later, midday fluxes in removal plots were indistinguishable from those in plots where Cynara was never present suggesting a lack of biogeochemical legacy effects. Similar to woody-encroachment in some semi-arid ecosystems, Cynara invasion increases midday ecosystem CO2 assimilation and evapotranspiration rates and has the potential to increase C-storage in California coastal grasslands.

Both water and nitrogen (N) availability have significant effects on ecosystem CO2 exchange (ECE), which includes net ecosystem productivity (NEP), ecosystem respiration (ER) and gross ecosystem photosynthesis (GEP). How water and N availability influence ECE in arid and semiarid grasslands is still uncertain. A manipulative experiment with additions of rainfall, snow and N was conducted to test their effects on ECE in a semiarid temperate steppe of northern China for three consecutive years with contrasting natural precipitation. ECE increased with annual precipitation but approached peak values at different precipitation amount. Water addition, especially summer water addition, had significantly positive effects on ECE in years when the natural precipitation was normal or below normal, but showed trivial effect on GEP when the natural precipitation was above normal as effects on ER and NEP offset one another. Nitrogen addition exerted non-significant or negative effects on ECE when precipitation was low but switched to a positive effect when precipitation was high, indicating N effect triggered by water availability. Our results indicate that both water and N availability control ECE and the effects of future precipitation changes and increasing N deposition will depend on how they can change collaboratively in this semiarid steppe ecosystem.

The intensification of the global hydrological cycle is anticipated to increase the variability of precipitation patterns. Brackish wetlands respond to changes in precipitation patterns by regulating the absorption and release of CO2 and H2O to maintain the stability of ecosystem functions. However, there is limited understanding of how the inter‐seasonal precipitation distribution (SPD) affects ecosystem CO2 and H2O exchange compared with annual precipitation totals. Here, we conducted four consecutive years of field experiments in a brackish wetland, manipulating the proportion of precipitation across different seasons while maintaining a constant annual precipitation total. We utilized five inter‐SPD proportions (+73%, +56%, control (CK), −56%, −73%) to examine the effects of SPD on ecosystem CO2 and H2O exchange. Our findings revealed that the annual ecosystem CO2 and H2O fluxes showed a trend of decreasing with the decrease in spring precipitation distribution. Among them, the annual net ecosystem CO2 exchange, evapotranspiration, carbon use efficiency and water use efficiency were shown to be more sensitive to decrease in spring precipitation distribution and increase in summer and autumn precipitation distribution. This negative asymmetric response pattern suggests that annual ecosystem CO2 and H2O exchange is primarily governed by seasonal precipitation variability, with spring soil water–salt dynamics identified as the key driver. Therefore, this association can be explained by the fact that drought of the early growth stage exacerbates soil salinization and inhibits vegetation colonization and growth, thereby greatly impairing the annual CO2–H2O exchange capacity of brackish wetlands. Our results emphasized that the spring extreme precipitation‐induced soil water–salt conditions will greatly influence CO2 and H2O exchange in brackish wetlands in the future. These findings are crucial for improving predictions of the carbon sequestration and water‐holding capacity of brackish wetlands. Read the free Plain Language Summary for this article on the Journal blog.

Agricultural reclamation effects on ecosystem CO2 exchange of a coastal wetland in the Yellow River Delta

The ecological consequences of precipitation change and increased atmospheric nitrogen (N) deposition have profound impacts on ecosystem CO2 exchange in grassland ecosystems. Water and N can largely influence grassland productivity, community composition and ecosystem functions. However, the influences of water and N addition on the ecosystem CO2 exchange of alpine grassland ecosystems remain unclear. A field manipulative experiment with water and N additions was conducted in an alpine meadow on the Tibetan Plateau over 4 years with contrasting precipitation patterns. There were four treatments: control (Ctrl), N addition (N), water addition (W) and N and water addition (NW), each replicated three times. N addition, but not water addition, increased gross ecosystem productivity (GEP), plant biomass, community cover and community-weighted mean height. The responses of ecosystem CO2 exchange to water and N addition varied between the wet and dry years. Water addition had a positive effect on net ecosystem carbon exchange (NEE) due to a larger increase in GEP than in ecosystem respiration (ER) only in the dry year. On the contrary, N addition significantly enhanced ecosystem CO2 exchange only in the wet year. The increased GEP in N addition was attributed to the larger increase in NEE than ER. Moreover, N addition stimulated NEE mainly through increasing the cover of dominant species. Our observations highlight the important roles of precipitation and dominant species in regulating ecosystem CO2 exchange response to global environmental change in alpine grasslands.

Resolving systematic errors in estimates of net ecosystem exchange of CO 2 and ecosystem respiration in a tropical forest biome

Aims Humid savannas, as a result of high precipitation amounts, are highly productive. They are also hotspots for land use change and potential sources of carbon dioxide (CO 2) due to the large soil carbon (C) stocks. Understanding how ecosystem CO 2 exchange is influenced by changes arising from agricultural land use is vital in future management of these ecosystems and in responding to the ongoing shifts in management and climate. The aim of this study was to identify how ecosystem CO2 exchange and biomass productivity of the herbaceous layer of a humid savanna in Kenya respond to current management practices. Methods We used flux chambers to quantify CO2 fluxes, while monthly harvests were undertaken to determine biomass development of the herbaceous layer of three sites that were (i) fenced to exclude livestock grazing, (ii) subjected to grazing by livestock and (iii) abandoned after being cultivated for maize production and also open to grazing by livestock. Important findings The peak aboveground biomass ranged between 380 and 1449 g m −2 and biomass production was significantly (P < 0.05) lower in the grazed and abandoned plots. The maximum gross primary production (GPP) and net ecosystem CO 2 exchange (NEE) ranged between 21.8 ± 1.3 to 32.5 ± 2.7 and −9.6 ± 0.7 to −17.9 ± 4.8 µmol m −2 s −1 , respectively. Seasonal NEE fluctuations ranged between 10 and 21 µmol m −2 s −1 , while spatial (among sites) differences ranged between 2 and 10 µmol m −2 s −1 . Ecosystem respiration (Reco) fluctuated between 5 and 10 µmol m −2 s −1 during the growing season. Reco was, however, not significantly different among the sites. Unlike in other similar ecosystems where ecosystem respiration is determined by the ambient temperature, we did not find any relationship between Reco and temperature in this savanna. Instead, soil moisture accounted for 38–88% of the spatial and seasonal fluctuations in ecosystem CO2 fluxes and aboveground biomass production. Management influenced the maximum GPP and NEE rates through modification of soil moisture, plant species composition and aboveground biomass. We concluded that soil moisture is the key determinant of ecosystem CO2 exchange and productivity in this tropical savanna. Management, however, significantly modifies C fluxes and productivity through its influence on soil moisture, plant species composition and aboveground green biomass and should be taken into consideration in future similar studies.

Significant changes in ecosystem CO2 exchange and vegetation characteristics were observed following multiple additions of nitrogen (N) and factorial additions of N and phosphorus (P) to prostrate dwarf‐shrub, herb tundra in Northwest Greenland. Ecosystem CO2 exchange and vegetation cover and composition were very sensitive to low rates of N inputs (0.5 g m−2 y−1), indicating that even low rates of atmospheric N deposition may alter high arctic ecosystem structure and function. Increasing N addition from 1 to 5 g N m−2 y−1 did not alter CO2 exchange or vegetation characteristics, suggesting the ecosystem had become N saturated. Factorial additions of both N and P released the ecosystem from N saturation and dramatically increased gross ecosystem photosynthesis (+500%) and ecosystem respiration (+250%), such that the ecosystem switched from a small source of CO2 to a small sink for CO2 at midday during the 2005 growing season. Changes in the component fluxes of CO2 exchange were largely explained by a doubling of the normalized difference vegetation index, a 100% increase in vascular plant cover and dramatic increases in the abundance of several previously rare grass species. Our results clearly demonstrate that high arctic prostrate dwarf‐shrub, herb tundra is highly sensitive to low levels of N addition and that future increases in N deposition or N mineralization will likely lead to change in carbon cycling and vegetation characteristics, but the magnitude of the response will be constrained by P availability.

We used the eddy—correlation technique to investigate the exchange of CO2 between an undisturbed old—growth forest and the atmosphere at a remote Southern Hemisphere site on 15 d between 1989 and 1990. Our goal was to determine how environmental factors regulate ecosystem CO2 exchange, and to test whether present knowledge of leaf—level processes was sufficient to understand ecosystem—level exchange. On clear summer days the maximum rate of net ecosystem CO2 uptake exceeded 15 μmol°m—2°s—1, about an order of magnitude greater than the maximum values observed on sunny days in the winter. Mean nighttime respiration rates varied between °—2 and —7 μmol°m—2°s—1. Nighttime CO2 efflux rate roughly doubled with a 10°C increase in temperature. Daytime variation in net ecosystem CO2 exchange rate was primarily associated with changes in total photosynthetically active photon flux density (PPFD). Air temperature, saturation deficit, and the diffuse PPFD were of lesser, but still significant, influence. These results are in broad agreement with expectations based on the biochemistry of leaf gas exchange and penetration of radiation through a canopy. However, at night, the short—term exchange of CO2 between the forest and the atmosphere appeared to be regulated principally by atmospheric transport processes. There was a positive linear relationship between nocturnal CO2 exchange rate and downward sensible heat flux density. This new result has implications for the development of models for diurnal ecosystem CO2 exchange. The daytime light—use efficiency of the ecosystem (CO2 uptake/incident PPFD) was between 1.6 and 7.1 mmol/mol on clear days in the summer but decreased to 0.8 mmol/mol after frosts on clear winter days. Ecosystem CO2 uptake was enhanced by diffuse PPFD, a result of potentially global significance given recent increases in Northern Hemisphere haze.

Tidal effects on ecosystem CO2 exchange in a Phragmites salt marsh of an intertidal shoal

Post-fire co-stimulation of gross primary production and ecosystem respiration in a meadow grassland on the Tibetan Plateau

To study long-term impacts of nutrient addition on carbon sequestration capacity, we investigated changes in vegetation and ecosystem CO2 exchange at Mer Bleue Bog, Canada in plots that had been fertilized with nitrogen (N) or with N plus phosphorus (P) and potassium (K) and in non-fertilized control plots for 13-18 years. The vegetation structure and species composition were measured in all treatments mid July 2001-2018 (14 measurement years) using a point intercept method. Gross photosynthesis, ecosystem respiration, and net CO2 exchange were measured weekly during June&amp;#8211;August 2001-2016 (7 measurement years, usually every two years) using climate-controlled chambers. Using Bayesian approach, we analyzed whether there were changes over time in vegetation and ecosystem CO2 exchange and whether those trends differed between treatments. We found that shrubs had become taller and more abundant at the unfertilized plots during the 18 study years likely owing to warmer summers and a drying trend that favor shrubs. At the fertilized plots, the increase in shrub height was greater and faster than in unfertilized plots, and the addition of PK with N further accelerated growth of the shrub canopy. Among the dwarf shrubs, only Chamaedaphne calyculata benefitted from the fertilization. No change towards more gramineous vegetation was observed. Because the plants at the bog are N-P co-limited rather than N-limited, PK addition alleviated growth limitation. Sphagnum cover decreased with the increasing nutrient load. Ecosystem respiration increased in all treatments, but it increased faster and more in fertilized plots than in unfertilized plots. In all treatments, increases in ecosystem respiration resulted in less net CO2 uptake during the recent ten years (since 2008), because gross photosynthesis rates did not compensate for increases in ecosystem respiration. In general, the magnitude of this trend of reduced net C sink potential did not differ markedly in unfertilized from fertilized plots. These CO2 flux trends could be explained by changes in nutrient availability, a larger proportion of nongreen biomass in dense stands and enhanced peat decomposition. Our long-term field experiment revealed that ecosystem responses to the combination of nutrient addition and drying must be considered when evaluating the impact of climate change on the carbon sink potential of peatlands.