Effect And Mechanism Of Ca/Mg Modified Biochar On Organic Carbon Sequestration In Citrus Orchard Soils

Guangxi is the largest citrus producing area in China and citrus orchard soils commonly suffer from soil acidification and calcium and magnesium deficiency.Therefore,in this study,biochar was prepared from citrus peel residue,a common material in citrus orchards,in order to achieve local utilisation of resources,and the biochar was modified with Ca/Mg and returned to the field.To investigate its carbon sequestration potential and soil amendment effect on citrus orchard soil,indoor constant temperature soil incubation tests and soil organic carbon mineralisation tests were carried out on a 5-year-old citrus orchard soil,with virgin biochar（OBC）,calcium-modified biochar（OBC-Ca）and magnesium-modified biochar（OBC-Mg）as additives.OBC,OBC-Ca and OBC-Mg were applied to citrus orchard soil at four application rates of 0%,1%,2%and 4%,respectively,and incubated for 100 days.The experimental results showed that:（1）Citrus trees are a weakly acid-loving plant and the application of Ca/Mg modified biochar improved the physicochemical properties of citrus orchard soil soil acidification was improved and the amount of cation exchange and available fraction content increased.Soil pH increased with the proportion of Ca/Mg modified biochar applied,with soil pH increasing by 0.36～2.78 units compared to the control,with OBC-Mg having the greatest effect on soil pH,with all proportions of OBC,OBC-Ca and 1%OBC-Mg applied to make soil pH suitable for the growth of citrus plants;CEC increased by 12.33%～207.80%compared to the control,with OBC-Mg having the greatest effect on soil pH.The CEC increased by 12.33%～207.8%compared to the control,and the application of OBC-Mg could better increase the soil cation exchange;the soil available phosphorus content increased by 28.62%～114.47%compared to the control.Compared with OBC,Ca/Mg modification could better increase soil available phosphorus content;OBC application increased soil available potassium content by 80.37%～290.79%compared with the control,while Ca/Mg modified biochar reduced soil available potassium content by19.12%～67.38%compared with the control.（2）The application of Ca/Mg modified biochar had a significant effect on soil organic carbon mineralization（p<0.05）.The changes in soil organic carbon mineralization rate had a similar pattern under different treatments.The mineralization rate decreased with increasing incubation time,was at a high level at the beginning of the incubation period and gradually levelled off at the end of the incubation period.At the end of incubation,the mineralization rate of OBC and OBC-Mg was higher than that of the control,and the mineralization rate of OBC-Ca was lower than that of the control.The cumulative mineralization of soil organic carbon was reduced by 2.14%～17.00%compared to the control,where OBC-Ca could better mitigate CO₂emissions.The first order kinetic equation could better fit the variation pattern of cumulative soil organic carbon mineralization after Ca/Mg modified biochar application versus incubation time.There was a significant negative correlation between soil organic carbon mineralization rate and available phosphorus content.There was a significant negative correlation between the cumulative mineralization of soil organic carbon and CEC and mineralization rate.（3）Mg-modified biochar had a greater effect on soil organic carbon and its active fraction when applied to soil compared to Ca-modified biochar.Compared with the control,the soil organic carbon（SOC）content increased by 8.26%～114.23%,the SOC content increased with the proportion of biochar applied,and the effect on soil SOC in citrus orchards showed that OBC-Mg>OBC>OBC-Ca;the soil microbial quantity carbon（MBC）content increased by 5.12%～18.95%,and OBC-Mg could better Soil soluble organic carbon（DOC）content increased by 0.48%～25.67%,the effect on soil DOC content was OBC>OBC-Mg>OBC-Ca,and there was a significant positive correlation between soil DOC and pH;soil oxidizable organic carbon（ROC）content increased by OBC-Mg could better increase the soil ROC content.（4）The application of Ca/Mg modified biochar increased the enzyme activity in the soil.Compared with the control,soil catalase activity increased by 26.67%～116.67%,and catalase activity increased with increasing percentage of biochar application;urease activity increased by 60.82%～279.91%;and sucrase activity increased by 8.21%～417.60%.The effect of Mg-modified biochar on soil enzyme activity was even greater.soil urease and sucrase activities were significantly and positively correlated with MBC.（5）The application of Ca/Mg modified biochar significantly altered the Ca/Mg fraction in the soil（p<0.05）.Compared to the control,soil water-soluble calcium content increased by 5.02%～1895.31%,and soil water-soluble calcium content was significantly and positively correlated with cumulative soil organic carbon mineralisation.Soil water-soluble magnesium content varied from-19.86%～649.97%.Soil exchangeable calcium content increased by 71.69%～624.04%.Soil exchangeable magnesium content increased by 143.45%～1237.65%.Ca-modified biochar leaches more of the active form of soil calcium.In summary,the application of calcium/magnesium modified biochar significantly increased the content of soil organic carbon and its active fraction,and to a certain extent slowed down the emission of carbon dioxide,indicating that calcium/magnesium modified biochar has a carbon sequestration effect on the soil.It can also improve soil pH,increase enzyme activity and soil Ca/Mg content,and improve the current situation of soil Ca/Mg deficiency.This study can provide data to support the theory of soil carbon sink in citrus orchards.