Effects of Land-Use Changes on Sources, Sinks and Fluxes of Carbon in European Mountain Grasslands

Response of ecosystem CO2 fluxes to grazing intensities - a five-year experiment in the Hulunber meadow steppe of China

Grazing is the primary land use in the Hulunber meadow steppe. However, the quantitative effects of grazing on ecosystem carbon dioxide (CO₂) fluxes in this zone remain unclear. A controlled experiment was conducted from 2010 to 2014 to study the effects of six stocking rates on CO₂ flux, and the results showed that there were significant differences in CO₂ fluxes by year, treatment, and month. The effects of light and intermediate grazing remained relatively constant with grazing year, whereas the effects of heavy grazing increased substantially with grazing duration. CO₂ flux significantly decreased with increasing grazing intensity and duration, and it was significantly positively correlated with rainfall, soil moisture (SM), the carbon to nitrogen ratio (C/N ratio), soil available phosphorus (SAP), soil NH₄⁺-N, soil NO₃⁻N, aboveground biomass (AGB), coverage, height, and litter and negatively correlated with air temperature, total soil N (TN) and microbial biomass N (MBN). A correspondence analysis showed that the main factors influencing changes in CO₂ emissions under grazing were AGB, height, coverage, SM, NH₄⁺-N and NO₃⁻N. Increased rainfall and reduced grazing resulted in greater CO₂ emissions. Our study provides important information to improve our understanding of the role of livestock grazing in GHG emissions.

Carbon storage potential in size-density fractions from semi-natural grassland ecosystems with different productivities over varying soil depths

Researchers have increasingly recognised a profound need for more information on SOC stocks in the soil and the factors governing their stability and dynamics. Many questions still remain unanswered about the interplay between changes in plant communities and the extent to which changes in aboveground productivity affect the carbon dynamics in soils through changes in its quantity and quality. Therefore, the main aim of this research was to examine the SOC accumulation potential of semi-natural grasslands of different productivities and determine the distribution of SOM fractions over varying soil depth intervals (0-10, 10-20, 20-30 30-50 50-80 and 80+cm). SOM fractionation was considered as a relative measure of stability to separate SOM associated with clay minerals from SOM of specific light densities less than 2 g cm(-3) (size-density fractionation). Two clay-associated fractions (CF1, <1 μm; and CF2, 1-2 μm) and two light fractions (LF1, <1.8 g cm(-3); and LF2, 1.8-2.0 g cm(-3)) were separated. The stability of these fractions was characterised by their carbon hot water extractability (CHWE) and stable carbon isotope composition. In the semi-natural grasslands studied, most OC was stored in the top 30 cm, where turnover is rapid. Effects of low productivity grasslands became only significantly apparent when fractional OC contributions of total SOM was considered (CF1 and LF1). In deeper soil depths OC was largely attributed to the CF1 fraction of low productivity grasslands. We suggest that the majority of OM in deeper soil depth intervals is microbially-derived, as evidenced by decreasing C/N ratios and decreasing δ(13)C values. The hot water extraction and natural δ(13)C abundance, employed here allowed the characterisation of SOM stabilisation properties, however how climatic changes affect the fate of OM within different soil depth intervals is still unknown.

Land use affects the net ecosystem CO(2) exchange and its components in mountain grasslands

Changes in land use and management have been strongly affecting mountain grassland, however, their effects on the net ecosystem exchange of CO(2) (NEE) and its components have not yet been well documented. We analysed chamber-based estimates of NEE, gross primary productivity (GPP), ecosystem respiration (R) and light use efficiency (LUE) of six mountain grasslands differing in land use and management, and thus site fertility, for the growing seasons of 2002 to 2008. The main findings of the study are that: (1) land use and management affected seasonal NEE, GPP and R, which all decreased from managed to unmanaged grasslands; (2) these changes were explained by differences in leaf area index (LAI), biomass and leaf-area-independent changes that were likely related to photosynthetic physiology; (3) diurnal variations of NEE were primarily controlled by photosynthetically active photon flux density and soil and air temperature; seasonal variations were associated with changes in LAI; (4) parameters of light response curves were generally closely related to each other, and the ratio of R at a reference temperature/ maximum GPP was nearly constant across the sites; (5) similarly to our study, maximum GPP and R for other grasslands on the globe decreased with decreasing land use intensity, while their ratio remained remarkably constant. We conclude that decreasing intensity of management and, in particular, abandonment of mountain grassland lead to a decrease in NEE and its component processes. While GPP and R are generally closely coupled during most of the growing season, GPP is more immediately and strongly affected by land management (mowing, grazing) and season. This suggests that management and growing season length, as well as their possible future changes, may play an important role for the annual C balance of mountain grassland.

Soil Respiration in European Grasslands in Relation to Climate and Assimilate Supply

Soil respiration constitutes the second largest flux of carbon (C) between terrestrial ecosystems and the atmosphere. This study provides a synthesis of soil respiration (R(s)) in 20 European grasslands across a climatic transect, including ten meadows, eight pastures and two unmanaged grasslands. Maximum rates of R(s) (R(s(max) )), R(s) at a reference soil temperature (10°C; R(s(10) )) and annual R(s) (estimated for 13 sites) ranged from 1.9 to 15.9 μmol CO(2) m(-2) s(-1), 0.3 to 5.5 μmol CO(2) m(-2) s(-1) and 58 to 1988 g C m(-2) y(-1), respectively. Values obtained for Central European mountain meadows are amongst the highest so far reported for any type of ecosystem. Across all sites R(s(max) ) was closely related to R(s(10) ).Assimilate supply affected R(s) at timescales from daily (but not necessarily diurnal) to annual. Reductions of assimilate supply by removal of aboveground biomass through grazing and cutting resulted in a rapid and a significant decrease of R(s). Temperature-independent seasonal fluctuations of R(s) of an intensively managed pasture were closely related to changes in leaf area index (LAI). Across sites R(s(10) ) increased with mean annual soil temperature (MAT), LAI and gross primary productivity (GPP), indicating that assimilate supply overrides potential acclimation to prevailing temperatures. Also annual R(s) was closely related to LAI and GPP. Because the latter two parameters were coupled to MAT, temperature was a suitable surrogate for deriving estimates of annual R(s) across the grasslands studied. These findings contribute to our understanding of regional patterns of soil C fluxes and highlight the importance of assimilate supply for soil CO(2) emissions at various timescales.