Grassland degradation significantly enhances soil CO2 emission

Grassland degradation reduces net primary production and, subsequently, soil fertility and soil organic carbon stocks (SOCs); however, little is known about its impact on soil CO2 emissions, particularly the emissions relative to SOCs and biomass produced. The main objective of this study, performed in KwaZulu-Natal province of South Africa, was to quantify the impact of grass basal cover, as main indicator of grassland degradation, on soil CO2 emissions. The soil CO2 emissions were measured from three grass cover levels (non-degraded, with 100% grass cover; moderately degraded: 25 < grass cover < 50%; and highly degraded: 0 < grass cover < 5%) using a LI-COR 6400XT. The measurements were done at three randomly selected positions in each grass cover level, from January 2013 to April 2015. At each position, measurements were done once during winter months and twice during summer months, resulting in a total of 1053 measurements for the entire study period. The measured average gross soil CO2 emission was significantly higher (1.78 ± 0.013 g CO2-C m−2 day−1) in non-degraded than moderately (1.60 ± 0.12 g CO2-C m−2 day−1) and highly degraded grasslands (0.68 ± 0.10 g CO2-C m−2 day−1). However, when expressed relative to SOCs and aboveground biomass produced, the trends were opposite. Average soil CO2 emission relative to SOCs was lowest in the non-degraded grassland (0.034 ± 0.01 g CO2-C g−1C day−1) and highest in the moderately degraded grassland (0.058 ± 0.02 g CO2-C g−1C day−1) with the highly degraded grassland being intermediate (0.04 ± 0.00 g CO2 g−1C day−1). Similarly, soil CO2 emission relative to aboveground biomass produced was lowest in the non-degraded grassland at 0.15 ± 0.02 kg CO2-C kg−1 biomass year−1, which was almost 5 fold lower than 0.73 ± 0.01 kg CO2-C kg−1 biomass year−1 in the highly degraded grassland. Gross soil CO2 emission correlated significantly and positively with SOC (r = 0.83 and 0.82 for SOC content and stocks, respectively), SON (0.67 and 0.53 for content and stocks, respectively), C:N ration (0.62), and soil water content (0.75) but negatively with clay content (−0.89). Soil CO2 emission relative to SOCs correlated significantly and negatively with both SOC (−0.50 and −0.51 for content and stocks, respectively) and SON (−0.45 and −0.42 for content and stocks, respectively). While gross CO2 emissions decreased with grassland degradation, CO2 emission relative to both SOCs and aboveground biomass increased with grassland degradation. These results point to direct links between grassland degradation and global warming because CO2 is one of the key greenhouse gases. Therefore, strategies for rehabilitating degraded grasslands need to aim at reducing soil CO2 emission in order to mitigate climate change.