4:15 PM - 4:30 PM
[MIS22-10] CO2 fixation by soil algae in agricultural lands
Keywords:Soil algae, Photosynthesis, Terrestrial carbon cycle, CO2 flux
In the soil carbon cycle, microorganisms are mainly positioned as decomposers and converters of plant-derived organic matter, and the significance of CO2 fixation by soil microorganisms has not been fully considered. In this study, we evaluated CO2 fixation by soil algae in agricultural lands and analyzed the relationship with its spatio-temporal variation and environmental factors.
Soil CO2 fluxes were measured under dark and light conditions (light intensity: 800 µmol m-2 s-1) in rice paddies and field plots at the Togo Field, Graduate School of Bioagricultural Sciences, Nagoya University, by placing a chamber with LED lights on the soil, avoiding vegetation. Of a total of 347 measurements, 263 (75%) showed lower CO2 flux in the light condition than in the dark condition, suggesting that a part of the CO2 released to the atmosphere by soil respiration was reabsorbed at the soil surface by algal photosynthesis at many sites. The rate of CO2 uptake by soil algae (NPP) was calculated to account for 0-100% (median: 24%) of the soil respiration rate (D flux); in addition to CO2 derived from soil respiration, additional atmospheric CO2 uptake was observed. NPP was positively correlated with D flux, temperature (soil temperature and air temperature), and moisture (soil moisture content, humidity, and precipitation). CO2 uptake in response to light irradiation (200 µmol m-2 s-1) was also observed in indoor culture (25 °C) using soil cores (0-5 cm, n=68) collected from multiple sites. NPP calculated in the indoor culture was positively correlated with D flux, soil water content, and Chlorophyll-a content. In paddy soil cores, it was also positively correlated with water-extractable NH4+-N. Random forest regression using data from field observations and soil core incubation experiments showed the importance of D flux and soil moisture content as factors affecting NPP.
The effects of a temporal shift in soil moisture content on NPP were tested in a laboratory experiment. Soil cores (0-5 cm) were collected from an upland field and rice paddy in the Togo field and incubated for 10 days (upland field) or 19 days (rice paddies) at 25°C, 12 h dark / 12 h light. D flux and NPP were measured over time in cores prepared under wet and dry conditions, respectively, in which water lost during incubation was replenished as needed. Both paddy and field soil cores under wet conditions (33% and 20% of water content, respectively) showed D flux and NPP equal to or higher than D flux throughout the period. Under dry conditions, NPP remained equal to or higher than that on the first day of incubation until moisture content dropped to 20% in the paddy soil cores and 12% in the field soil cores, and NPP became lower when moisture content dropped to 7.4%. After rewetting, NPP increased within a few hours in both paddy and field cores, reaching a maximum in 4 days in paddy and 2 days in field cores. NPP at this point was equivalent to that before soil drying proceeded. When the soil moisture content decreased again, a decrease in NPP was observed, as observed in the initial drying conditions.
The results of this study indicate that photosynthesis by soil algae plays an essential role in the carbon cycle of agricultural soils. The photosynthetic activity is closely related to soil respiration and moisture. Photosynthesis of soil algae is tolerant to soil desiccation to a certain degree, and it quickly recovers when the soil is rewetted after desiccation, suggesting that it is also adaptive to changes in soil moisture.
Soil CO2 fluxes were measured under dark and light conditions (light intensity: 800 µmol m-2 s-1) in rice paddies and field plots at the Togo Field, Graduate School of Bioagricultural Sciences, Nagoya University, by placing a chamber with LED lights on the soil, avoiding vegetation. Of a total of 347 measurements, 263 (75%) showed lower CO2 flux in the light condition than in the dark condition, suggesting that a part of the CO2 released to the atmosphere by soil respiration was reabsorbed at the soil surface by algal photosynthesis at many sites. The rate of CO2 uptake by soil algae (NPP) was calculated to account for 0-100% (median: 24%) of the soil respiration rate (D flux); in addition to CO2 derived from soil respiration, additional atmospheric CO2 uptake was observed. NPP was positively correlated with D flux, temperature (soil temperature and air temperature), and moisture (soil moisture content, humidity, and precipitation). CO2 uptake in response to light irradiation (200 µmol m-2 s-1) was also observed in indoor culture (25 °C) using soil cores (0-5 cm, n=68) collected from multiple sites. NPP calculated in the indoor culture was positively correlated with D flux, soil water content, and Chlorophyll-a content. In paddy soil cores, it was also positively correlated with water-extractable NH4+-N. Random forest regression using data from field observations and soil core incubation experiments showed the importance of D flux and soil moisture content as factors affecting NPP.
The effects of a temporal shift in soil moisture content on NPP were tested in a laboratory experiment. Soil cores (0-5 cm) were collected from an upland field and rice paddy in the Togo field and incubated for 10 days (upland field) or 19 days (rice paddies) at 25°C, 12 h dark / 12 h light. D flux and NPP were measured over time in cores prepared under wet and dry conditions, respectively, in which water lost during incubation was replenished as needed. Both paddy and field soil cores under wet conditions (33% and 20% of water content, respectively) showed D flux and NPP equal to or higher than D flux throughout the period. Under dry conditions, NPP remained equal to or higher than that on the first day of incubation until moisture content dropped to 20% in the paddy soil cores and 12% in the field soil cores, and NPP became lower when moisture content dropped to 7.4%. After rewetting, NPP increased within a few hours in both paddy and field cores, reaching a maximum in 4 days in paddy and 2 days in field cores. NPP at this point was equivalent to that before soil drying proceeded. When the soil moisture content decreased again, a decrease in NPP was observed, as observed in the initial drying conditions.
The results of this study indicate that photosynthesis by soil algae plays an essential role in the carbon cycle of agricultural soils. The photosynthetic activity is closely related to soil respiration and moisture. Photosynthesis of soil algae is tolerant to soil desiccation to a certain degree, and it quickly recovers when the soil is rewetted after desiccation, suggesting that it is also adaptive to changes in soil moisture.