دانلود رایگان مقاله تعادل کربن از چراندن دائمی مرتع در جنوب بلژیک

Abstract

Grasslands are an important component of the global carbon balance, but their carbon storage potential is still highly uncertain. In particular, the impact of weather variability and management practices on grassland carbon budgets need to be assessed. This study investigated the carbon balance of an intensively managed permanent grassland and its uncertainties by drawing together 5 years of eddy covariance measurements and other organic carbon exchanges estimates. The results showed that, despite the high stocking rate and the old age of the pasture, the site acted as a relatively stable carbon sink from year to year, with a 5-year average net biome productivity of −161 [−134 −180] g C m−2 yr−1. Lateral organic carbon fluxes were found to increase the carbon sink because of high carbon imports (organic fertilization, feed complements) and low carbon exports in form of meat compared to dairy pastures. The cattle stocking density was adapted to grass production, which itself depends on weather conditions and photosynthesizing area, in order to maintain a steady meat production. This resulted in a coupling between grazing management and weather conditions. As a consequence, both weather and grazing impacts on net ecosystem exchange were difficult to distinguish. Indeed, no correlation was found between weather variables anomalies and net ecosystem exchange anomalies. This coupling could also partly explain the low C budget inter-annual variability. The findings in this study are in agreement with those reported by other studies that have shown that well-managed grasslands could act as carbon sinks.

5. Conclusion

This study established and analyzed the total C budget of grassland grazed by Belgian Blue cattle by combining data from CO2 eddy covariancemeasurements with other Cfluxes and their uncertainties. CO2 fluxes (NEE) and non CO2 fluxes in form of manure (Fmanure) and feed complements (Fimports) were the main fluxes affecting the C budget, highlighting the need to include them. The results showed that the pasture acted as a relatively stable C sink each year despite the high stocking rate and the old age of the pasture. Both management and weather conditions were found to influence C fluxes. Important C imports through organic fertilization as well as low C exports through meat production helped to maintain a carbon sink. The N fertilization also probably helped to maintain the C sink activity thanks to an improved GPP. However, fertilization could also induce N2O emissions that could affect the grassland greenhouse gas budgets. These fluxes were not measured. GPP and NEE were affected by low temperatures at the beginning of the year, before the grazing season. Indeed, these weather conditions could have caused a delay in grass growth and GPP that could not always be offset during the rest of the year. The low inter-annual variability ofthe C budget and its independence to weather variables anomalies could partially be explained by management practices that adjusted the stocking rate according to grass availability which itself responds to weather conditions. It could also been obtained partly by chance as (i) we did not experience really extreme years and (ii) in some years, compensation between events with high and low accumulation occurred. The findings in this study are in agreement with those reported by other studies that have shown that well-managed grasslands could act as C sinks. Further studies should focus oncomparing different grazing management practices in order to better quantify and understand their impact on grassland C storage. Our study also highlighted the need to evaluate the uncertainties linked to flux measurements and to assess the sensitivity of the C budget to methodological choices, such as those linked with spectral correction and the nighttime flux filtering criterion choice, in order to assess how defensible annual C budgets are.