Process And Mechanism Of Organic Carbon Conversion In Soil Aggregates In Long-term Conservation Tillage

Conservation tillage is the main method used in dry farming and has an important impact on soil structure and organic carbon?OC?.It is important to understand the distribution and transformation of OC in the soil.The effects of conservation tillage on the distribution of OC and aggregates in various soil layers were studied and an in-lab incubation experiment with ¹³C labeled straw was conducted to study the regulation of soil organic carbon?SOC?conversion to elucidate the mechanism of SOC fixation under conservation tillage conditions in dryland farmland.The main findings were as follows:?1?Long-term conservation tillage significantly increased surface?0—20 cm?and deep?>20 cm?soil OC content,and promoted the formation of large aggregates,increased OC content in aggregates and fractions of small macroaggregates?2000—250?m?.The positive effects of reduced tillage?RT?and no-tillage?NT?on soil organic carbon content can reach 60—80 cm and 40—60 cm soil layers,respectively.NT significantly increased the content of large macroaggregates?>2000?m?in the 0—20cm soil layer;RT and NT significantly increased the small macroaggregates content of 0—100 cm;RT improved the OC content in all aggregates,mostly concentrated in 0—20 cm and 40—60 cm soil layers,the deepest layer can be 80 cm.?2?The addition of straw had significant effects on the SOC,soil water stable aggregate distribution,OC content in aggregate and OC fractions in aggregate.The addition of straw increased the OC content of the whole soil by 3%—22%during the incubation period;the large macroaggregates increased from 12%—37%to 39%—54%;significantly increased the OC content in large macroaggregates,microaggregates?250—53?m?and silt+clay aggregates?<53?m?,while significantly reduced the OC content in small macroaggregates;significantly increased the contribution rate of OC in large and small macroaggregates and reduced the contribution rate of OC in microaggregates and silt+clay aggregates;significantly increases the proportion of fine particulate organic carbon?fPOC?and mineral-associated SOC?mSOC?and reduced the proportion of coarse particulate organic carbon?cPOC?in small macroaggregates and increased the OC content of all components.?3?The addition of straw significantly increased soil microbial biomass carbon content and soil C-related enzyme activity.The addition of straw significantly increased the soil microbial biomass carbon content,which increased higher than 124%during the period of 15 d—180 d compared with no straw.Adding straw significantly increased the activity of?-glucosidase?BG?,?-cellobioside?CBH?and?-xylosidase?BXYL?.The activity of BG was the highest in all treatments in different periods,but the largest improvement of the activity was in CBH.?4?There was significant differences in the distribution of ¹³C labeled straw carbon in soil.The value of?¹³C in whole soil OC was highest in conventional tillage+straw?CTS?,so it can be seen that straw carbon enters CTS more quickly.The value of?¹³C was highest in the large macroaggregates and silt+clay aggregates,indicating that the straw carbon was firstly incorporated into the large macroaggregates and silt+clay aggregates.The straw-derived carbon in small macroaggregates was showed mSOC>fPOC>cPOC after 90 days and that in microaggregates showed fPOC>mSOC.Combined with the high proportion of mSOC in small macroaggregates and microaggregates,it was found that the straw carbon in aggregate was finally sequestrated in mSOC.In summary,both conservation tillage and straw addition can increase the stability of soil organic carbon and aggregates,with the largest contribution rate of small macroaggregates and microaggregate;the exogenous straw carbon first incorporated into>2000?m aggregate and<53?m aggregatel and was finally sequestrated in mSOC.