Effect Of Different Tillage Practices On Soil Organic Carbon Transformation And Water Use In Dryland Winter Wheat

Dryland agriculture played an important role in the national economy and food security, occupyingan important position in Chinese agricultural production. It is heavy droughty and has concentratedrainfall in Northern arid regions, resulting in the soils subjected drying and wetting frequently. Andtillage practices and less straw returning resulted in poor soil. So the conservation tillage was taken toimprove soil quality, increase soil production, and thus achieve water use efficiency of crops. However,under different tillage practices soil moisture and soil organic carbon usually have a lot of changes.Most studies focused on soil moisture content and soil organic carbon in the soil profile, and generallyfor the studies of soil moisture and organic carbon were separated. The cycling of soil water and thecarbon in farmland ecosystem is not isolated from each other, with interdependent relationship. Theinteraction between soil organic carbon and soil moisture must be synthetically consideration, andcomprehensive and systematic study should be done. Therefore, the studies on character of soil moistureand soil organic carbon and their interdependencies in northern arid regions may plays an important rolein improving soil quality and high-efficient use of soil water.This study was based on long-term (20years) experiment in Linfen, using combined method ofincubation and field experiment on soil organic carbon and soil moisture. The long-term experiment wasconducted from1992in Lifen City, Shanxi Province, China. The long-term experimental plots wereperformed using two different treatments: conventional-tillage (CT) and no-tillage (NT) treatments. TheCT treatment included moldboard plowing without returning wheat straws to the field at the end ofAugust, but retaining10-15cm stubbles after harvest; the tillage depth was bout20cm. For the NTsystem, all of the residues from the prior winter wheat straw were flattened and mulched in the field.The winter wheat (Triticum aestivum L.) was sown at September end and harvested in middle Juneevery year. The fallow period lasted from the harvest to September end, during which, herbicides wereapplied to control weeds. In June,2012, after harvesting of wheat, straws were returned to the field inproportion on the NT plots,100%straw mulch (NT3,4500kg·hm-2),66%straw mulch (NT2),33%straw mulch (NT1) and0%straw mulch. Twelve DW cycles were implemented during the experimentalperiod (120d). Each cycle contained two periods, four days of wet conditions and six days of dryconditions at25°C according to the daily precipitation and air temperature records of the area. Thetreatments and their combinations were as follows (2straw treatments×2tillage-treated soils×2watercontents):(i) with or without straw input,(ii) soil from the CT field or NT field and (iii) with or withoutexposure to DW cycles, which was characterised by wetting at field capacity (0.186g H2O g-1soil) andair drying (0.022g H-2O g1soil) at25℃in an incubator with controlled temperature and humidity.Through the incubation and field experiment we explored the effect of fluctuation of water on soilorganic carbon mineralization and the effects of different amounts of organic carbon on soil watercapacity and crop water structures, and then revealed the mechanism of interaction between carbon andwater, and further improved the research on the carbon sequestration and water retention under conservation tillage practices.1. Soil organic C, POC, MBC and DOC were all significantly higher under a NT system whencompared with a CT system, at the surface layer. Compared to CT treatment, NT treatment markedlyreduced the fluctuation of DOC and MBC in0-30cm depths and fluctuation of POC in0-60cm depths.While the SOC increased slowly in the surface soils during crop growth with the NT treatment, itshowed less fluctuation with the CT treatment. MBC and POC increased after sowing and reached afirst peak before winter, then dropped to a minimum during the jointed stage, reaching their maximumduring the anthesis stage and then decreasing until maturity. DOC was highest before sowing, decreasedbefore the winter and jointed stages, and increased again during the anthesis stage. Long-term tillagepractices could cause differences in the SOC soil profile and in its fraction distribution. On average, theNT treatment had82.1%and52.9%higher SOC at0-5cm and5-10cm depths than did the CTtreatment, respectively. POC, MBC and DOC were all significantly higher with NT than with CT in theupper10cm. While SOC and POC were slightly higher under CT than when using the NT practice at10-50cm depths, the MBC and DOC began to increase after the jointed stage at20-50cm and30-50cmdepths. The POC, MBC and DOC were highly correlated with the SOC. In our study, we found thatlabile SOC in NT and CT soils had different season fluctuations. Compared to CT treatment, NTtreatment markedly reduced the fluctuation of DOC and MBC in0-20cm depths and fluctuation ofPOC in0-60cm depths. This study demonstrated that measuring the effect of tillage practices on SOCbased on soil organic C fractions must consider both seasonal changes and profile distribution.2. The differences of CO2emissons under different treatments were attributed to wheat straw mulching.The CO2emissions were positively correlated with soil temperatures and slightly positively with soilwater content.The CO2emission rate immediately after plowing treatment was significantly higher than CTtreatment (P＜0.001),2.7times the CT treatment, but not significantly after48hours. Tillage practiceshad a significant influence on the CO2emissions during whole years. Higher CO2emissions weredetected in NT plots than in CT and NT0treatments during fallow and late growing period. The CO2emissions were positively correlated with soil temperatures in the top20cm (R2=0.71, P＜0.001) andslightly positively with soil water content (R2=0.05, P＞0.05). While using a regression analysisprocedure, a significant multiple regression equation was developed between soil CO2emissions andwater content, temperatures (P＜0.001), and the determination coefficients (R2) of different treatmentswere raised, by the highest amplitude under the NT treatment. Annual CO2emissions were significantlyhigher under NT (7218kg CO12-C ha-1·yr-) than under CT (5870kg CO-12-C ha·yr-1) and NT0(5585kgCO-12-C ha·yr-1), but not significantly (P＞0.05) between CT and NT0. The soil CO2emission duringthe wheat growing period was significantly greater than during the fallow period (P＜0.001). Theresults indicated that although NT increased soil CO2emissions compared to CT and NT0, thesedifferences under different treatments were attributed to wheat straw mulching and there was0.3t Cha-1left in the NT soil annually which increased soil carbon sequestration.3. Water use efficiency (WUE) was increased under NT practice when compared with CT practice. From20years of statistical data the water storage under NT capacity was average111.1mm,12.5%higher than NT. The storage capacity was highly correlated with precipitation during fallowperiod, with a significant difference. The difference between them gradually decreased as the increase ofrainfall during the fallow period. So the water retention of NT during the low rainfall fallow periodhighlighted. The water consumption of NT practice was7%less than the CT practices during thewheat-growing period. Soil evaporation of NT in the whole year was47mm less than CT and the rate ofevaporation reduction can reach24%, with a significant difference. In the key period of wheat floweringand filling stage the rate of transpiration in CT was0.3mm·d-1, only33%of NT. The proportion ofevaporation to evaportransportation under CT during wheat-growing period was41%, while NT was39%, and the difference between them is mainly reflected in the wheat sowing to tillering and lategrowing period. With the increase of the accumulation of soil organic matter and moisture content, theyield of NT practices was average37%higher than CT since2000. Studies have shown that the yielddependence on rainfall under CT practice was greater than NT, and the increment rate of WUE underNT practices relative to CT decreased as the precipitation.4. Water use efficiency (WUE) was highest in100%straw mulch under no-tillage practice, but had norelationship to the different rates of straw mulch.In no-till conditions, soil unsaturated hydraulic conductivity under NT0treatment was greater thanNT3treatment, but the rate of straw mulch and unsaturated hydraulic conductivity had no proportionalrelationship. Straw returning had greater effect on soil surface moisture than the bottom and the NT3treatment improved soil surface moisture, reducing soil subsurface moisture at the harvest time. Inno-till conditions, soil water storage under straw mulch treatments basic basically performed: NT3＞NT2＞NT1and storage capacity increased with the rate of straw mulch, but during drought periods thecrusts under NT0surface soil affected soil water storage. The annual amount of soil evaporation underNT3treatment was lowest, and the rate of evaporation reduction was14%. During wheat-growingperiod the amount transpiration under NT3treatment was24%higher than NT0treatment. The ratio ofevaporation to evapotranspiration under NT3treatment was39%, and the NT3treatment could reduceunproductive water consumption with the WUE of13.5kg·hm-2·mm-1, but there is no proportionalrelationship with the rate of straw mulch.5. DW cycles have a significant effect on C mineralization rate as well as on PE.The SOC mineralisation rate in rewetted soils was greater than that in soils that were kept at aconstant water content, with an average of3.90mg C-CO12kg-soil day-1. Following its input, wheatstraw significantly (207%) increased the rate of total C mineralisation, which was sharply (373%)reduced from2d to11d under the continuously wet treatment. The proportion of CO2from strawdeclined dramatically (78%) during the first10days, and this proportion under the continuously wettreatment (44%to34%) was greater than that under the DW cycles (33%to13%) from the rewettingday. The intensity of the priming effect (PE) that was induced by wheat straw decreased over time; thepriming direction was first positive, and then negative under the same water treatment. This intensity ofthe no-tillage soil under DW cycles ranged from15.32to﹣0.62mg C kg-1soil day-1, which was higher than that of the soils that were kept at a constant water content. There was no significant difference inthe SOC mineralisation rate between the top-20-cm samples from the NT and CT soils. The dataindicate that the DW cycles had a significant effect on the SOC mineralisation rate as well as on the PE,demonstrating a positive interaction between wheat straw and moisture fluctuations. Our results suggestthat under the simulated DW cycle conditions, the CO2release rate in the rewetted native soils underthe NT system (with straw) was greater than in the soils under the CT system (without straw), and thisdifference decreased with the incubation time.. Therefore, we consider our results to be the first todemonstrate the dynamics of priming effects in a long-term no-tillage soil of wheat straw under DWcycles as in the field.