Effects of elevated CO2 and temperature on plant litter decomposition in freshwaters – a meta-analysis. / Efeitos do aumento da cocnetração de CO2 e da temperatura na decomposição de detritos vegetais em sistemas aquáticos de água doce – uma meta-análise

Abstract

Elevated atmospheric [CO2] can affect litter decomposition by (1) decreasing litter quality and (2) increasing temperature. At elevated [CO2], plants tend to over-invest in secondary and structural compounds, resulting in low-quality litter (tougher litter, higher lignin concentration, and lower N:C ratio). These compounds decrease litter decomposition rates because of their bitter taste, toxicity and interference with digestion of litter decomposers (microbes and macroinvertebrates). On the other hand, elevated temperature can increase litter decomposition rates by increasing leaching of the recalcitrant compounds and metabolic rates of litter decomposers. However, it is not understood how litter decomposition responds to increases in both [CO2] and temperature. This study tested the hypothesis that the overall effect size of elevated [CO2] and temperature is significant and positive. The findings of 43 published and unpublished studies conducted worldwide, between 1993 and 2017, on the effects of elevated [CO2], elevated temperature or both on litter decomposition in freshwaters are synthesized by meta-analysis. After estimating the standardized mean difference between litter decomposition rates reported in impacted (increases in CO2, temperature or both) and control conditions, the overall effect size of elevated atmospheric [CO2] and temperature on litter decomposition in freshwaters was positive. However, elevated atmospheric [CO2] decreased litter decomposition, temperature+CO2 did not affect litter decomposition, while elevated temperature increased litter decomposition. The effect of elevated temperature did not depend on the type of study (laboratory vs. field). Elevated [CO2] inhibited litter decomposition in lentic, but not lotic systems. The effects of elevated atmospheric [CO2], temperature and temperature+CO2 on litter decomposition were species-specific. The type of community did not affect the response of litter decomposition to elevated atmospheric [CO2] and temperature. Faster decomposition rates might reduce food availability for higher trophic consumers under future global warming scenarios. However, conclusions are geographically limited since most of the primary studies were conducted in Europe, suggesting a need for studies in other parts of the world.

Elevated atmospheric [CO2] can affect litter decomposition by (1) decreasing litter quality and (2) increasing temperature. At elevated [CO2], plants tend to over-invest in secondary and structural compounds, resulting in low-quality litter (tougher litter, higher lignin concentration, and lower N:C ratio). These compounds decrease litter decomposition rates because of their bitter taste, toxicity and interference with digestion of litter decomposers (microbes and macroinvertebrates). On the other hand, elevated temperature can increase litter decomposition rates by increasing leaching of the recalcitrant compounds and metabolic rates of litter decomposers. However, it is not understood how litter decomposition responds to increases in both [CO2] and temperature. This study tested the hypothesis that the overall effect size of elevated [CO2] and temperature is significant and positive. The findings of 43 published and unpublished studies conducted worldwide, between 1993 and 2017, on the effects of elevated [CO2], elevated temperature or both on litter decomposition in freshwaters are synthesized by meta-analysis. After estimating the standardized mean difference between litter decomposition rates reported in impacted (increases in CO2, temperature or both) and control conditions, the overall effect size of elevated atmospheric [CO2] and temperature on litter decomposition in freshwaters was positive. However, elevated atmospheric [CO2] decreased litter decomposition, temperature+CO2 did not affect litter decomposition, while elevated temperature increased litter decomposition. The effect of elevated temperature did not depend on the type of study (laboratory vs. field). Elevated [CO2] inhibited litter decomposition in lentic, but not lotic systems. The effects of elevated atmospheric [CO2], temperature and temperature+CO2 on litter decomposition were species-specific. The type of community did not affect the response of litter decomposition to elevated atmospheric [CO2] and temperature. Faster decomposition rates might reduce food availability for higher trophic consumers under future global warming scenarios. However, conclusions are geographically limited since most of the primary studies were conducted in Europe, suggesting a need for studies in other parts of the world.