Effects Of Tillage Management With Residue Return On Soil Organic Carbon Fractions And Stability

The dynamics of soil organic carbon?SOC?has important effects on soil quality and CO₂ cycle.Reduction of SOC storage can reduce the soil fertility and thus affects yield.Hence,the increasing of SOC storage became a standard to promote soil productivity.Residue returned become the most popular management to increase SOC storage due to high carbon inputs.However,what is the SOC storage increasing rate after residue returned combine with different tillage management?What is the distribution of SOC associated with fractions and the how is SOC stability among treatments?How about the SOC distribution in fractions within the carbon inputs only from residue returned?In order to answer those questions,we took the black soil as the research object in Northeast China based on a long-term conservation experiment.Residue was returned in all treatments except conventional tillage?CTMM?,five treatments were studied in our research:two tillage practices?no tillage,NT;mouldborad plowing,MP?combined with two cropping systems?maize-soybean,MS;maize-maize,MM?and CTMM was the control.The turnover of SOC in physical and chemical fractions was studied to assess the effects of tillage and cropping system.The stability of SOC was also evaluated in all treatments.A filed trace(¹³C)experiment was set to calculate the exactly amount of new carbon in different fractions which originated from residue returned and compared the difference in fractions between different NT and MP.Our object was to reveal the effects of tillage and cropping system on SOC fractions and stability.The main conclusions were as follows:1.Compared with the SOC storage in 2001?initial?,all the treatments based on residue returned had enhanced the SOC storage in 0-30cm layer.The SOC storage in all treatments followed the order:NTMM>MPMM>MPMS?NTMS.NTMM had the highest SOC storage change rate(0.80 Mg C ha^-1 yr^-1)and the rate of residue carbon retential?16%?among treatments.2.The NTMM had the highest amount and SOC content of occluded micro-aggregate which was good for the long-term sequestration of SOC in the view of aggregate protection.The increasing of SOC storage mostly stored in clay.The results showed that tillage affected the light fraction and sand fraction?NT>MP?while cropping system significantly influenced the silt and clay fraction?silt:MM>MS,clay:MS>MM?.3.Tillage only affected the labile carbon pool I?primarily composed of polysaccharides derived from plant hemicellulose,starch and microorganisms?and II?mainly derived from cellulose?on surface layer?0-5cm?but cropping system influenced all the layers.MM had more labile carbon pool 1and MS had more labile carbon pool 2.Both tillage and cropping system affected the recalcitrant carbon pool?lignin?with MS was higher than MM.Among the amino sugars,the muramic acid which was originated from bacteria was the most sensitive with the change of tillage and cropping systems in 0-20cm layer.The glucosimine which can represent fungal was almost 60%higher in NT than MP in surface layer?0-5cm?.The total amino sugars was highly related with SOC,and they had linear relationship before SOC reached a certainly content but the total amino sugars become steady after SOC reached that content.4.The bio-stability of SOC could be assessed by the potential SOC mineralization and SOC biodegradability.The potential SOC mineralization was affected by the tillage and cropping system which depended on the SOC proportion in light fraction.However,the SOC biodegradability was only affected by cropping system and did not change with the amount and way of residue returned which illustrated that the ability of microbes to decompose SOC was driven by the quality of carbon inputs?cropping system?and not affected by tillage practice.5.The results of Rock-Eval analysis demonstrated that NT increased the hydrogen-rich compounds but did not decline the thermal stability of SOC.The thermal stability in NT was higher than MP due to the microbes decomposed the residue and derived to microbial residue which was stable.Meantime,there was a good relationship between potential SOC mineralization and Rock-Eval indices indicated that Rock-Eval analysis can represent the bio-stability.6.The new carbon from residue returned can accumulate in all layers from the field trace experiment and NT had more new carbon than MP in 0-20cm layer.Tillage only affected the residue derived carbon in>250?m aggregate.In both tillage practices,the residue derived new carbon in different aggregates follow the order:?>250?m?>?53-250?m?>?<53?m?.From the results in NT,we firmly demonstrated that macro-aggregate can protect the occluded micro-aggregate from decomposition.Based on the particle size fractionation,it showed that sand had the highest new carbon content followed by clay and silt had the lowest new carbon content.This result was not consistent with the results in long-term experiment which showed that clay had the highest SOC storage among all particles.This result indicated that it has a difference between the short-term and long-term scales in evaluating the SOC in clay dynamics.Residue derived new carbon was found in both labile and recalcitrant carbon pool from the chemical fractionation results.Tillage only affected the new carbon content in labile fraction and had no influences on recalcitrant carbon pool.