Effects of Conservation Tillage on Soil Quality,Crop Productivity,and Greenhouse Gas Emissions in Rainfed Agroecosystems on the Semi-arid Loess Plateau of Northwestern China

Author:AbdulRauf Malimanga Alhassan

Supervisor:jiang zhi rong li guang


Degree Year:2019





The Loess Plateau region is characterized by inadequate rainfall,poor soil fertility and severe erosion which results in very low crop productivity.Yet,conventional tillage practice of tilling the soil without residue cover is widely practiced by majority of farmers.Conservation tillage has been reported to be environmentally friendly and may result in higher productivity.However there is variability of the reports of the impacts of conservation tillage on crop productivity,soil and environment,arising from differences in climatic conditions,type of conservation tillage practiced,duration of practice,and type of soil and crop.This study was conducted to analyze the efficacy of soil conservation measures on crop productivity and greenhouse gas mitigation capacities of conservation tillage practices on the Loess Plateau.In order to make comprehensive recommendations for sustainable spring wheat production in this study area,field experiments with regression models were employed in this study between 2016 and 2019 to examine the impact of conservation tillage on soil properties,spring wheat productivity and greenhouse gas emissions in the cold semi-arid climate of the Western Loess Plateau of China and to analyze the response of GHG fluxes to changes in environmental variables due to tillage practices.Four treatments which were laid in a randomized complete block design[conventional tillage(CT),conventional tillage with straw mulch(CTS),no-till(NT)and no-till with straw mulch(NTS)]were also tested to assess the impact of straw mulch and no-till on soil properties,wheat grain and biomass yield as well as greenhouse gas emissions.All treatments were replicated three times.The study also employed the use of 3 parameter(3p)Logistic,Gompertz,Richards and Weibull models to assess biomass accumulation under straw and no straw treatments using the following treatments[early sowing of spring wheat with tillage(ESW-T);early sowing of spring wheat with tillage and straw mulch(ESW-TS);normal sowing of spring wheat with tillage(NSW-T);normal sowing of spring wheat with tillage and straw mulch(NSW-TS);late sowing of spring wheat with tillage(LSW-T)and late sowing of spring wheat with tillage and straw mulch(LSW-TS)].The main conclusions are shown below:(1)All straw amended plots gave higher root and aboveground biomass,grain yield,and soil water content than no-straw treatments.Compared with no-straw treatments,grain yields in straw treatments increased by 7.53,0.78 and 45.18%respectively at early,normal and late sowing times respectively while aboveground biomass yield also increased by 37.41,48.09 and28.97%respectively.The 3 parameter logistic model was the best model based on Akaike information criterion(AIC)and R2 values.In all treatments,the 3 parameter Logistic had the lowest change in AIC.On the basis of R2 value,the 3 parameter Logistic model had the highest value for almost all the treatments while the Gompertz model was next by being higher than the other 2 models in 4 out of 6 treatments.Using root means square error(RMSE),the 3p Logistic model was better than the Gompertz model but not better the Richards and Weibull models.The AIC which is the criteria for selecting the best model among candidate models showed that the Logistic model was the best based on our data,followed by the Gompertz model.The models,according to their performance may be arranged as follows:Logistic>Gompertz>Weibull>Richards.The highest biomass yield was obtained under ESW-TS from simulated results indicating that early sowing and straw amendment could improve wheat yields in dryland areas.(2)Soil properties,especially within the 10 cm depth were highly influenced by tillage and straw mulch.NTS showed significant improvement of soil water content(SWC)at p<0.05in two years.In 2018,at sowing time,NTS increased SWC by 49.07%within the 0-10 cm soil depth.In 2017,at harvest CTS significantly reduced soil bulk density at the 20-40 cm depth(p<0.05),while by harvest in 2018 NTS showed the lowest Bulk density(BD)within the 0-10 and10-20 cm depths.At harvest in 2018,at the 0-10 cm,CTS and NTS significantly increased water–filled pore space(WFPS)than CT and NT.Generally soil physical properties improved under conservation tillage,especially under NTS and CTS.Conservation tillage significantly increased soil organic carbon(SOC)in this study,however,SOC decreased along soil depth irrespective of treatment.In 2017,NTS increased SOC within the 0-10 cm depth and was greater than CT by 14.93%,while SOC in CTS and NT were greater than CT by 7.87 and 5.89%respectively.Total nitrogen(TN)also increased under CTS,NT and NTS than in CT at all depths.In 2018,SOC was significantly greater in CTS,NT and NTS than in CT at 0-10 and 10-20 cm depths while TN in CTS,NT and NTS were also greater than in CT within the same depths.By the end of 2-year experiment within the 0-10 cm depth,SOC followed the order:NTS>CTS>NT>CT while TN followed the order:NTS>NT>CTS>CT.(3)Grain yields were also impacted by tillage and straw mulch.The highest grain yields were found in NTS and CTS in 2017 and 2018 respectively.Over the 2 years,average values showed that NTS gave the highest yield following the order:NTS>CTS>NT>CT.Higher SOC in NTS and CTS than in CT contributed to higher yields.In 2017,NTS had the greatest aboveground biomass which was 23.44%greater than that of CT while in 2018,the greatest root and aboveground biomass were found in CTS.Less soil disturbance and straw mulching resulted in lower carbon loss and higher carbon input,resulting in higher crop productivity in conservation tillage plots.There were significant positive correlation of grain yield in 2017 and SOC(p<0.01),TN(p<0.01),WFPS(p<0.05)and water storage(p<0.05).(4)Tillage and straw mulch also significantly affected greenhouse gas fluxes.The results indicated that conservation tillage significantly affected ecosystem respiration(Reco)and showed strong seasonal variability.Cumulative Reco rates followed the order:CTS>CT>NT>NTS.Compared with CT,cumulative Reco decreased by 28.25 and 32.94%under NT and NTS respectively.Cumulative Reco was significantly higher in the growing season than in the non-growing season.Emissions in the non-growing season contributed 10.71,15.56,15.06 and 16.90%to total Recoco in CT,CTS,NT and NTS respectively.Carbon(C)sequestration followed the order:NTS>CTS>NT>CT.All tillage treatments resulted in net absorption of CH4 but at varying rates.NTS absorbed significantly greater CH4 than CT.Fluxes of N2O were small in this study but conservation tillage did not reduce fluxes.There were also lower fluxes of CH4 and N2O in the non-growing season compared with the growing season in all treatments.Due to higher carbon input as result of relatively greater crop productivity,NTS showed the least global warming potential and greenhouse gas intensity.(5)Grain yield significantly correlated positively with SWC,soil water storage,WFPS,SOC and TN.Ecosystem respiration(Reco)significantly correlated positively with soil temperature(ST)and SWC;CH4 fluxes significantly correlated negatively with ST but did not show significant correlation with SWC.An exponential function best described the relationship between Reco and ST while power function gave the best description of relationship between Recoco and SWC.Apart from CH4-ST relationship,linear function characterized the relations of ST and SWC with CH4 and N2O.Adoption of Conservation tillage such as straw mulching and no-till on the Loess Plateau could enhance crop productivity by enhancing soil physical and chemical properties.Furthermore,the combination of conservation tillage with straw mulching could enhance crop productivity and ensure environmental sustainability on the semi-arid Loess Plateau due to higher soil C sequestration and reduced greenhouse gas emissions.Conservation tillage could be used as strategy to mitigate greenhouse gases and increase crop productivity.However,integrated crop management through the incorporation of nutrient management with conservation tillage is needed to further increase the benefits of conservation tillage in water limited areas such as the Western Loess Plateau.