Fertile temperate grassland under elevated atmospheric CO 2—role of feed-back mechanisms and availability of growth resources

Abstract

Human activities are responsible for the strong and very fast increase in the concentration of atmospheric CO 2 over the past 100 years; this trend will continue. An increase in CO 2 is expected to affect significantly grassland vegetation, which covers approximately 70% of agricultural land world-wide and undoubtedly plays an important role in the global C cycle. This review summarises the information about the response of temperate, intensively managed grassland to elevated atmospheric CO 2 under field conditions. 1. The yield response of grassland to elevated CO 2 under field conditions was weaker than expected from short-term laboratory experiments. 2. Different functional types of plant species responded differently to elevated CO 2, leading to changes in the proportion of plant species in grassland. 3. Adapted management of grassland was able to counteract—at least partly—CO 2-induced changes in species proportion. 4. The availability of growth resources other than CO 2 (e.g., N and P) strongly affected the plant’s response to elevated CO 2. 5. Symbiotic N 2 fixation and a strong sink for additionally fixed C (high growth rate, tillering, storage organs) were the functional traits that played a key role in the strong increase in yield under elevated CO 2. 6. An increased input of N through symbiotic N 2 fixation under elevated CO 2 played an important role in maintaining the C/N balance of the legume plant and of entire fertile grassland ecosystems. 7. The yield response to elevated CO 2 changed in the long-term indicating that processes in the soil that responded slowly (e.g., nutrient cycling and sequestration, activity and mass of micro-organisms) gradually adapted to the new environmental conditions. These results demonstrate that the response of ecosystems to elevated CO 2 cannot be deduced from measurements based on individual plants in controlled environments. Long-term experiments in the field where the availability of growth resources is varied, the plants interact with each other and the soil interacts with the plants (feed-back mechanisms) are necessary for predicting the effect of elevated CO 2.