Soil Organic Carbon Accumulation And Mineralization And Their Response Mechanisms To Major Controlling Factors Under Conservation Tillage

Increasing attention has focused on the role of the carbon（C）sequestration associated with conservation tillage in the mitigation of climate change.There are uncertainties in the soil C sequestration associated with conservation tillage due to the effects induced by environmental factors,soil properties,and agronomic practices.Clarifying the accumulation and mineralization of soil organic C（SOC）and their response mechanisms to the major controls under conservation tillage is critical for the improvement in the eco-environmental benefits of conservation tillage.Based on the published studies globally and the21-year field experiment with four tillage treatments,including conventional tillage（CT）,reduced tillage（RT）,no-tillage（NT）,and subsoiling（SS）,which located in Luoyang,Henan province,this study used meta-analysis to clarify the responses of SOC accumulation and mineralization to conservation tillage and their major controls;using field sampling analysis and incubation experiments in combination with the ¹³C natural abundance method,"C quality-temperature hypothesis",and ¹³C isotope technique,this study revealed the mechanism on how conservation tillage affects SOC accumulation and mineralization,and explored the response and mechanism of SOC mineralization to the major controls under conservation tillage.The main results are as follows:（1）This study clarified the characteristics and major controls of SOC accumulation and mineralization in bulk soil and aggregate fractions response to conservation tillage.Meta-analysis revealed that NT enhanced SOC accumulation and decreased SOC mineralization;environmental factors and exogenous C input were the major controls on the changes in SOC accumulation and mineralization under NT.The meta-analysis of 225 studies globally showed that,compared with CT,NT significantly increased SOC content in bulk soil and all aggregate fractions（6–34%）and reduced cumulative SOC mineralization in bulk soil（7%）in the 0–20 cm soil layer.There was a significant negative correlation between the SOC content in bulk soil and the cumulative SOC mineralization in bulk soil,whereas a significant positive correlation existed between the SOC content in bulk soil and the SOC content in macro-aggregates（>0.25 mm）（P<0.01）.This result indicated that higher SOC accumulation under NT could be attributed to its lower SOC mineralization and higher macroaggregate-associated SOC accumulation.The changes in SOC accumulation and mineralization under NT varied with environmental factors,soil properties,and agronomic practices.The random forest model revealed that environmental factors（i.e.,mean annual temperature and precipitation）and exogenous C input were the major controls on the changes in SOC accumulation and mineralization under NT.（2）This study elucidated the mechanism on how conservation tillage promoted SOC accumulation and decreased SOC mineralization.NT and SS decreased the mineralization and loss of macroaggregate-associated SOC and thus promoted SOC accumulation by reducing the hydrolase and oxidase activities in macro-aggregates and the probability of C flows from large-to small-size aggregates.Compared with CT and RT,NT and SS significantly increased the proportions of macro-aggregates and the SOC content in bulk soil and all aggregate fractions（11.8–52.7%）in the 0–10cm soil layer.NT and SS significantly reduced the cumulative SOC mineralization in bulk soil and macro-aggregates（17.2–82.1%）compared with CT and RT,due to the significant reduction in the hydrolase and oxidase activities induced by NT and SS.The differences between the aggregate-associatedδ¹³C and the bulk-soilδ¹³C(Δ¹³C)increased with decreasing sizes of the aggregates,indicating that larger aggregates contained more new C.NT and SS significantly reduced the probability of C flows from macro-to micro-aggregates（<0.25 mm）,which helped better protect new C in macro-aggregates,ultimately reducing SOC loss and promoting SOC accumulation.（3）This study found that the responses of SOC mineralization to the changes in the temperature and moisture were controlled by the physical protection and SOC quality of macro-aggregates under conservation tillage.NT and SS increased the temperature sensitivity of SOC mineralization(Q₁₀)but to some extent suppressed Q₁₀ due to their increased soil moisture.Compared with CT and RT,NT and SS significantly reduced cumulative SOC mineralization（21.6–61.9%）under all temperature and moisture leves;however,NT and SS significantly increased Q₁₀（13.5–29.1%）at 40%and 70%water-holding capacity（WHC）.This result was attributed to the fact that NT and SS significantly promoted macro-aggregation,moreover,the cumulative SOC mineralization and its Q₁₀ of macro-aggregates were significantly lower and higher than those of micro-aggregates,respectively;meanwhile,bulk-soil Q₁₀ was significantly negatively correlated with SOC quality of macro-aggregates（P<0.05）,and NT and SS significantly reduced macroaggregate-associated SOC quality.Higher Q₁₀under NT and SS indicated that climate warming would accelerate their SOC loss.However,there was no difference in Q₁₀ among tillage practices at 100%WHC（P>0.05）,indicating that high soil moisture could effectively reduced Q₁₀.NT and SS could increase soil moisture,which in turn suppressed Q₁₀.（4）This study revealed the response mechanism of SOC mineralization to exogenous C input under conservation tillage at the aggregate scale.NT and SS decreased SOC mineralization after exogenous C input by enhancing the physical protection by macro-aggregates and increasing microbial C-use efficiency（CUE）in macro-aggregates.After exogenous C input,the cumulative mineralization of total C and glucose C in macro-aggregates were significantly lower and higher than those in micro-aggregates in the 0–10 cm soil layer,respectively.Compared with CT and RT,NT and SS significantly reduced cumulative total C mineralization（10.6–38.1%）but increased cumulative glucose C mineralization（14.5–27.9%）by promoting soil macro-aggregation.NT and SS significantly reduced positive priming effect（PE）（27.3–45.4%）but enhanced microbial CUE（20.1–77.5%）because macro-aggregates exhibited lower positive PE and higher microbial CUE than micro-aggregates.There was a significant negative correlation between positive PE and microbial CUE（P<0.001）,suggesting that higher microbial CUE could inhibit SOC mineralization through decreasing PE under NT and SS.Overall,NT increased SOC accumulation and decreased SOC mineralization,and the changes in SOC accumulation and mineralization under NT were mainly controlled by environmental factors（mean annual temperature and precipitation）and exogenous C input.NT and SS promoted soil macro-aggregation,which in turn reduced SOC mineralization and ultimately enhanced SOC accumulation.Though NT and SS enhanced Q₁₀,their increased soil moisture could suppress Q₁₀.NT and SS decreased SOC mineralization after exogenous C input by enhancing the physical protection by macro-aggregates and increasing the microbial CUE in macro-aggregates.Thus,it is concluded that SOC could be sequestered under conservation tillage（NT and SS）.