Carbon sequestration within millet phytoliths from dry-farming of crops in China

Phytoliths are noncrystalline minerals that form inside cells and cell walls of different parts of plants. Organic carbon in living cells can be occluded in phytoliths during plant growth. It has been documented that the occluded carbon within phytoliths is an important long-term terrestrial carbon reservoir that has a major role in the global carbon cycle. Common millet and foxtail millet have become typical dry-farming crops in China since the Neolithic Age. The study of carbon conservation within phytoliths in these crops could provide insights into anthropogenic influences on the carbon cycle. In this study, we analyzed the carbon content in phytoliths of common millet and foxtail millet. The results indicated that (1) common millet and foxtail millet contained 0.136% ± 0.070% and 0.129%± 0.085% phytolith-occluded carbon (PhytOC) on a dry mass basis, respectively; (2) based on the mean annual production of common millet and foxtail millet in the last 10 years, the phytolith occluded carbon accumulation rate of common millet and foxtail millet was approximately 0.023 ± 0.015 and 0.020± 0.010 t CO₂ ha⁻¹ a⁻¹, respectively; (3) assuming a similar phytolith occluded carbon accumulation rate as for common millet (the highest accumulation rate was 0.038 t CO₂ ha⁻¹ a⁻¹), this could result in the sequestration of 2.37 × 10⁶ t CO₂ per year for the 62.4 × 10⁶ ha dry-farming crops in China. Although there was a decline in the annual production rate and planting area of foxtail millet during 1949 to 2008, the total phytolith carbon sequestration rate was 7×10⁶ t CO₂ within the 60-year period. However, phytolith occluded carbon has not yet been fully considered as a global carbon sink. Also, this carbon fraction is probably one of the best candidates for the missing carbon sink.