Soil Organic Carbon Dynamics in Tropical and Subtropical Grassland Ecosystem

Abstract

Grassland ecosystems occupy a vast area on the Earth’s land surface and play a significant role in mitigating the climate change and global warming by sequestering atmospheric CO₂. As much as 20% of the total terrestrial C is stored in their root zone as soil organic carbon. However, through anthropogenic activities, these grasslands can become a source of CO₂ emissions to the atmosphere. CO₂ flux from grasslands is highly influenced by factors such as soil moisture, soil temperature and amount of organic carbon in the soil. A third of total C captured annually by the aboveground vegetation may be lost though CO₂ emissions as observed in Imperata grasslands of northeast India, which, otherwise exhibits a significantly high capacity to store SOC stocks in the absence of intensified grazing and burning events. Southern grasslands of China, on the other hand, have been reported to be a weak C sink as examined on the basis of spatiotemporal C cycle. These grasslands act as a C sink during the wet season but as a source of CO₂ during the dry season. Net preservation and stabilization of C, however, depends on the impact of type of land management, which can be judged from the changes in the labile or free C fractions. These labile C pools of SOC are the first to get affected by disturbances of the grasslands through different management practices. Grazing and burning together can significantly increase CO₂ fluxes as observed in Andean grasslands. However, under undisturbed native conditions, temperature and moisture are the major drivers of SOM decomposition. With the introduction of high-yielding grass species and with liberal use of chemical fertilizers, grazing land intensification has been found to rather promote SOC sequestrations in Andean grassland ecosystems. Much of the C added to the soil under such conditions is in the form of labile C fractions, which are highly prone to decomposition with release of CO₂. A rapid transfer of plant inputs through active and intermediate C pools into mineral-dominated pools is the ultimate outcome required for building and stabilizing the SOC stocks. Such results have been observed with high-yielding tropical perennial C4 grass species in the least soil disturbance production systems. Grazer effects have been reported to shift from negative to positive with decreasing precipitation, increasing fineness of soil texture, changing dominating grass species from C3 to C4 and, of course, decreasing grazing intensity.