| Long-term tillage and crop residue inputs are important factors that impact soil organiccarbon (SOC) pool, soil aggregation level and greenhouse gas (GHG) emissions. Anappropriate tillage method is of significant to increase regional SOC pool level and mitigateGHG emissions and to develop the conservational and sustainable agriculture in the NorthChina Plain. This study evaluated the effects of long-term tillage and crop residuemanagement on SOC pool, GHG emissions, carbon footprint, and their changes after sometillage convertions, which aimed to assess the potentials of carbon sequestration andmitigation GHG emissions under different tillage and residue systems through the carbonsequestration mechanism that including soil aggregate stability, distribution, associated carbonand labile organic carbon (LOC) pool, and the emission mechanism including the GHGemission related bacteria population (methanogens, methane-oxidizing bacteria, nitrobacteriaand denitrifying bacteria). The research was conducted in North China between2007and2012. The treatments were five tillage systems that included conventional tillage (CT),subsoiling (ST), harrow tillage (HT), rotary tillage (RT) and no-till (NT), combination withcrop residue retention (P) or residue removal (A). The results are as follows:1. Soil organic carbon pool change by a long-term tillage and residue managementsystem and its carbon sequestration mechanismSoil organic carbon (SOC) contents of the0~30cm depth layer under no tillage (NTP),subsoiling (STP), harrow tillage (HTP), rotary tillage (RTP), conventional tillage (CTP) withcrop residue input were significant higher than that of no residue input treatments (NTA、STA、HTA、RTA and CTA). The soils under NTP, STP, HTP and RTP treatments gatheredmore SOC to the0~10cm layer compared to the CTP treatment, and their SOC pool wereincreased during the experiment. The labile organic carbon (LOC) proportion in TOC was 22.38~39.14%under all treatments, and the stabled carbon proportion was60.86~77.62%.The highest LOC proportion in TOC was the RTP treatment (39.14%), and the lowest onewas STP (22.38%). The levels of SOC pool under different tillage with residue inputtreatments were higher4.6t C ha-1than those of no residue input treatments after10-yrscontinued crop residue input and tillage, becacse the annual accumulation rate under thetillage treatments with residue input (-0.27~1.34t C ha-1yr-1) were significant higher thanthose of no residue input treatments (-0.41~0.75t C ha-1yr-1), and the SOC pool level wassignificant correlated with the input rate of crop residue biomass (R2=0.78, P<0.01). In thisstudy, SOC pool under NTP treatment was significant increased and approximately13.37t Cha-1was sequestrated from2002to2012, similarly, approximately9.59t C ha-1wassequestrated in the STP treatment. However, the soil under RTP and RTA treatments wereoccured C lost and approximately0.27t C ha-1and0.41t C ha-1were lost each year, the cropresidue returned to field could mitigate this rate of C lost.The analysis of carbon sequestration mechanism showed that the differences of SOC poolunder different tillage and residue management systems were from the changes of soilaggregate proportion, stability, associated carbon and stabled SOC pool levels. Higher levelsof SOC pool were mearsured at the NTP and STA treatments, because they had the higherproportion (79.19%and81.67%), stability (1.89mm and1.82mm), associated C content(9.6g kg-1and7.36g kg-1) of soil aggregate and stabled C pool proportion (71.6%and77.6%).Therefore, the soil macro-aggregate associated C pool contribution proportion to TOC underthe NTP and STA treatments (75.9%and65.4%) were significant higher than othertreatments. The regression analysis also showed that there was a significant positivecorrelation between the soil macro-aggregate associated C pool and soil SOC level (R=0.71,P<0.05).2. Estimation emission fluxes of GHG under different tillage systems and their emissionmechanismThe soil was observed a sink of CH4uptake and emission source of N2O under differenttillage systems. The regression analysis of the CH4uptake related bacteria showed that theCH4uptake flux was correlated with the population of methane-oxidizing bacteria (MOB,R2=0.64, P<0.01) and methanogens (MPB, R2=0.66, P<0.01). Maybe the dominant bacterium in soil was MOB that leaded to the observation a sink of CH4uptake. The population ofnitrobacteria (R2=0.62, P<0.01) and denitrifying bacteria (R2=0.64, P<0.01) were drived theN2O emission in this study.Total emission flux of CH4and N2O during the experiment (2007.10~2012.10) waslargely fluctuated in different years. The highest total flux was observed at RT treatment with10.05t ha-1, the lowest was NT treatment (6.29t ha-1). The global warming potential (GWP)of CH4was ordered NT>HT>CT>ST>RT due to the difference of MOB population(RT>CT>ST>NT), the population general related with soil temperature, moisture, pH andNH4+-N contents. Meanwhile, total emission flux of N2O was NT>CT>HT>ST>RT and theGWP of N2O was NT>CT>HT>ST>RT, the reasons may was the difference of nitrobacteriapopulation (NT>CT>RT>ST) that general related with soil temperature, moisture, pH andNH4+-N contents.3. Footprints under different tillage systems in a wheat-maize cropping systemThe footprint constitute in a wheat-maize cropping system including chemical fertilizer,agricultural chemicals, diesel, seed and irrigation. The highest proportion in footprintconstitute was chemical fertilizer (73.5~77.6%), while the diesel proportion was only8.1~13.8%. In wheat period, the highest footprint was observed at ST treatment (812.8kg Ceha-1yr-1), the lowest one was NT treatment (787.2kg Ce ha-1yr-1), while the footprints inmaize period under different tillage methods were621.2kg Ce ha-1yr-1due to the equal inputof chemical fertilizer, agricultural chemicals, diesel, seed and irrigation.The highest carbon cost in a wheat-maize cropping system was observed at NT treatment(0.169kg Ce kg-1), the lowest one was observed at ST treatment (0.144kg Ce kg-1).4. Changes of soil organic carbon pool, greenhouse gas emission and crop yield by along-term single tillage and rotational tillage systemLong-term single tillage (RT, HT, NT) convertion to rotational tillage systems (RT/ST,HT/ST, NT/ST) increased the ablity of CH4uptake sink, the CH4uptake flux was increased23.6%,14.7%and26.1%after these convertions respectively, while the N2O flux was emitedlargely than the original treatments (increased10.2%,12.9%and78.3%, respectively). Thetotal emission flux of CH4and N2O under NT/ST, RT/ST and HT/ST treatments wereincreased1.08,0.06and0.17kg ha1, respectively, and the GWP of CH4and N2O was increased0.23,0.02and0.05kg CO2ha1, respectively, compared with the treatments of NT,RT and HT.In comparison to the experiment site of Longkou, higher SOC pool level of0~30cmdepth was measured at long-term single NT treatment, this tillage method at two experimentsites (Tai’an and Longkou) were locked13.37and9.51t C ha-1in10-yrs and6-yrs,respectively. However, RT treatments of two experiment sites were lost0.27and0.33t C ha-1each year. The C accumulation after HT and RT treatments convertion to HT/ST and RT/STwere increased, especially the latter, which observation of C lost converted to C accumulation.The C accumulation rate under NT/ST treatment was decreased24.4%than that of NTtreatment.The yields of wheat and maize were not improved under continued long-term singletillage system not only at Tai’an but also at Longkou, sometimes the yields were presented thedecreased trendences. After convertion to HT/ST, RT/ST and NT/ST treatments, the wheatyield was improved47.9,37.7and64.9%, respectively, and the mazie yield was improved9.4,15.5and19.4%, respectively. The total yield in a wheat-maize cropping system was observeda gradual increase trend after these rotational tillage systems. |
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