Effects of long-term CO2 enrichment on forage quality of extensively managed temperate grassland

Different studies revealed an increased biomass production in grasslands due to elevated CO2 (eCO(2)). Thus, the question arises whether forage quality of grassland biomass is also influenced. In the present study, we assessed the long-term effects of eCO(2) in context of the accompanied site conditions soil moisture, air temperature and precipitation on different forage quality and energy parameters of grassland biomass in an extensively managed temperate grassland. The study took place at the Free-Air Carbon dioxide Enrichment site near Giessen [GiFACE], which was set up in 1998. Our main objective was to assess the differences in the quality and energy content of grassland forage, exposed to elevated CO2 [+ 20%; eCO(2)] and ambient CO2 (aCO(2)) concentrations. Crude protein, C/N-ratio, crude fat, total non-structural carbohydrates, ash, crude fibre, metabolisable energy [ME] and net energy for lactation [NEL] were determined to assess the forage quality and energy content of grassland biomass separated in grasses and forbs, which was harvested twice per year (end of May, beginning of September), over the period of 2006-2015. Forbs showed a significant decrease for most tested parameters under eCO(2) at both harvests, except for crude fibre, which showed a significant increase at the second harvest of the year under eCO(2). Grasses showed a significant negative CO2 effect for ash content at the first harvest of the year. In addition to eCO(2), site and climatic conditions, e.g. soil moisture, affected the investigated parameters. Based on the last 10 observation years (2006-2015) within the 18 year long experiment, eCO(2) is likely to cause a decline in forage quality, especially of forbs caused by reduced crude protein and fat contents and increased crude fibre content, in extensively managed grassland ecosystems. However, site and climatic conditions should also be considered showing the importance of long-term observations that contain the full range of climatic conditions. Periods of heat, drought and precipitation, which are expected to increase in the future, lead to a decrease of the CO2 fertilization effect on grassland biomass and also to a negative impact on forage quality.

Automated summary

The study found that the forage quality of grassland biomass exposed to elevated CO2 for a long period is influenced by various factors, with forbs being particularly vulnerable. While increased CO2 concentrations lead to a decline in forage quality, the experiment also highlighted the significant role of site and climatic conditions in this process.