Responses Of Photosynthetic Microorganism Growth And Iron Reduction To Biochar Addition For Three Paddy Soils

Biochar is a highly aromatic by-product synthesized through pyrolysis of plant- and animal-based biomass in the absence of air under a relatively "low"(<700℃) temperature. Following the widely application of biochar to soils, extensive studies were involved in its beneficial effect on soil quality improvement, pollutants detoxication, mitigation of greenhouse gas emission and crops growth. However, little research had been conducted on the response of iron oxides reduction and photosynthetic microbial growth to biochar application in paddy soils. Microbial mediated iron reduction in anaerobic submerged paddy field had gathered increasing attention due to the high abundance of Fe oxides in soil. On the other hand, paddy field environment was affected by the aquatic ecological environment which closely related to the photosynthetic microorganisms and their metabolic processes. Namely, redox state changes of paddy field which was induced by oxygen production of photosynthetic microorganisms could make more Mn(II) and Fe(II) oxidation, and further affect the availability of nutrients and the form of heavy metals. Moreover, algae toxin produced by well growing cyanobacteria under light condition could result in environmental risk of paddy fields. Therefore, studies on the response of photosynthetic microorganism growth and iron reduction process to biochar application are of significance to better understand the potential of biochar application to the inhibition of water eutrophication. In the present study, soil slurry incubation was conducted to simulate the flooded paddy soil and investigate the effect of biochar addition on the photosynthetic microorganism growth and iron reduction process. And the following conclusions were obtained:1、The addition of biochar demonstrated a great influence on the abundance and dominant species of photosynthetic microorganisms in paddy soil. The effect of biochar addition on chlorophyll synthesis was correlative with soil properties, incubation time, and the amount and particle size of biochar. In NX paddy soil under light condition, there was an inhibition of biochar addition on chlorophyll synthesis, which was increased with the increase of biochar amount; while addition of biochar particle size at 0.25-0.5 mm showed the lowest inhibitory effect to chlorophyll synthesis when amended with 4% biochar to soil. In TJ paddy soil under light condition, chlorophyll synthesis was promoted by amending 1% and 2% biochars; whereafter, there was a more obvious inhibition of biochar addition on chlorophyll synthesis with the increase of biochar amount; when added with 4% biochar, addition of biochar particle size at 1~2 mm and 2~3 mm promoted chlorophyll synthesis comparing with the rest particle sizes. In SC paddy soil, despite a slight promotion was found with the addition of 1% and 2% biochar, the inhibition was more obvious with the increase of the amount and particle size of biochar.2、Biochar addition presented a promotion on iron(III) reduction and an inhibition on photosynthesis process in paddy soil, which were both more remarkable with the increasing concentration of biochar addition. The effect of biochar addition on iron(II) oxidization varied with the concentration of biochar in different paddy soils. In TJ paddy soil, biochar concentrations of 1% showed the maximum promotion on iron oxidation, while biochar concentrations of 4% suppressed the oxidization of iron(III). In NX paddy soil, higher biochar concentrations(8% and 16%) have a positive effect on iron(II) oxidation, whereas, lower biochar concentrations(0.5 and 1 g L-1) showed a negative effect. In SC paddy soil, despite of an inhibition of biochar addition of 1%, iron(II) oxidation was simulated more significantly by biochar addition with increasing concentration from 2% to 16%.3、Under light condition, O2 produced by oxygenic photosynthesis could oxidize Fe(II) in anoxic paddy soils, while the uptake of CO2 and HCO3- through photosynthetic O2 production resulted in a pH increase of soils. There is a significant correlation among Photosynthetic microorganism growth, change of pH and the oxidization of Fe(II). Furthermore, biochar had a significant inhibitory effect on photosynthetic microbial growth, the greater the size of biochar, the more inhibition to photosynthetic microorganism growth.4、Exogenous addition of phosphate could promote the absorption and fixation of biochar to water-soluble phosphate(W-P) in paddy soils. Under the dark condition, the promotion on the one hand could be attributed to the adsorptive property of biochar. Another reason could be that the increased abundance of microorganism adsorbed more phosphate as nutrient source. On the contrary, biochar addition showed a negative effect on the absorption and fixation of water-soluble phosphate(W-P) under light condition, which may be attributed to three reasons. Firstly, pH increase by biochar addition induced P transport to negative charge. Secondly, biochar addition inhibited the growth of photosynthetic microorganisms and further suppressed Fe(II) oxidation, which reduced the absorption and fixation of W-P. At last, comparing with single iron oxides, the lower PZC of iron-biochar complex made it difficult to fix more phosphate.