| In recent years,the global climate change caused by the greenhouse effect has become the focus of worldwide attention: both carbon fixation and carbon emission play an important role in agricultural ecosystems,therefore,the evaluation of soil carbon emissions,soil organic carbon dynamics,and agricultural greenhouse gas emission reduction potential under different agricultural measures have attracted much research effort.In addition,it is important to mitigate the contribution of agriculture to the global greenhouse effect by strengthening farmland soil conservation tillage management,scientifically understanding and regulating the energy flow of farmland farming systems,and improving the carbon sequestration and abatement capacity of agricultural ecosystems;however,comprehensive evaluations of the carbon balance,energy balance,and economic benefit under different tillage measures in a winter wheat-summer maize ecosystem in northwest China are relatively rare.Thus,the study of a soil conservation tillage experiment involving a winter wheat-summer maize rotation ecosystem was conducted in northwest China over a two year continuous period(2013 to 2015).The field observation data,carbon/energy input parameters,and agro-ecosystem carbon/energy balance analysis methods were applied.The effects of different tillage treatments(CT: conventional mouldboard plough tillage without crop straw;RTS: rotary tillage with the incorporation of straw;STS: chisel plough tillage with the incorporation of straw,and NTS: no-tillage with straw mulching)on a winter wheat-summer maize ecosystem carbon/energy balance and economic parameters were evaluated.The main conclusions were as follows:1.Analysis of the crop yields and economic benefits of a winter wheat and summer maize ecosystem under conservation tillage treatmentsDuring the two-year trial period,the chisel plough tillage with straw-returning field could increase crop annual productivity and total output value compared with other tillage treatments.In the years 2013-14,and 2014-15,the annual productivities under STS were 16.2 %,11.9 %,12.7 %,and 18.3 %,13.5 %,and 14.6 % higher as compared with CT,NTS and RTS treatments,respectively.The three types of conservation tillage measure reduce the input cost compared with traditional tillage treatment.The reduction of mechanical input with no-tillage,chisel plough tillage,and rotary tillage is conducive to increasing the economic benefit accruing from such a wheat-maize farmland ecosystem.The economic benefit accruing from this wheat-maize farmland ecosystem was improved by using no tillage,chisel plough tillage,and rotary tillage,and the benefit of increasing yield therefrom was higher under STS treatment than those of NTS and RTS treatments.2.Soil respiration and its components under conservation tillage measuresDuring the two-year field experiment period(2013.10-2015.10),the trend of soil respiration during the winter wheat growing season was similar across the application of different treatments.Soil respiration decreased after sowing,reached its lowest level in the wintering stage,then increased gradually only to slightly decrease by harvest time.The trend in cumulative soil respiration was,in descending order: STS,RTS,CT,and NTS.The contribution of root respiration to soil respiration under different tillage measures fluctuated between 20.4% and 25.7% during the whole winter wheat growth season.Soil respiration showed a unimodal trend during the summer maize growth season under the four different tillage systems.The maximum soil respiration was recorded at the booting stage of the summer maize,and thereafter it decreased slowly.The total soil respiration,in descending order,was: STS,RTS,CT,and NTS through the summer maize growth season.Under different cultivation measures the percentage of root respiration rate of soil respiration rate varied from 45.3% to 62.2%.3.The relationship between soil respiration and soil moisture,temperatureThe soil temperature during the winter wheat growth seasons in 2013-14 and 2014-15 could explain 50% to 86 % of the seasonal variation in soil respiration under four tillage treatments,respectively.During the two summer maize growth seasons,soil temperature can explain 1% to 41% of the seasonal variation in soil respiration.During the winter wheat growth season,there was no correlation between soil moisture and soil respiration in 2013-14.Soil moisture was positively correlated with soil respiration under RTS and STS treatments,while soil moisture was not correlated with soil respiration under CT and NTS treatments.During the summer maize growth season,soil moisture was positively correlated with soil respiration in 2013-14 under CT and RTS treatments,while soil moisture was not correlated with soil respiration under these four treatments in 2014-15.4.The net carbon flux from a winter wheat-summer maize field under conservation tillage treatmentsThe results showed that fertiliser,and in particular,N-fertiliser,is the largest source of carbon emissions from this winter wheat-summer maize ecosystem,regardless of tillage treatment.The carbon emissions from agricultural production of winter wheat-summer maize under NTS treatment was 22.4% less than that of CT,which indicated that the NTS technique is an effective low-carbon production technology.The carbon emissions from farmland input were,in descending order: CT,RTS,STS,and NTS.The NTS treatment reduced carbon emissions by 168.78,75.01,and 59.50 kg C hm-1 as compared with CT,RTS,and STS treatments,respectively.The no-tillage straw mulching system promoted carbon sequestration in this winter wheat-summer maize ecosystem.The soil carbon fixation in this winter wheat-summer maize farmland was,in descending order,as follows: NTS,STS,RTS,and CT.5.The carbon balance and energy use efficiency of winter wheat-summer maize ecosystems under different tillage treatmentsThe results showed that the winter wheat-summer maize ecosystem acted as a carbon sink under the application of conservation tillage treatments,but as a carbon source under the application of traditional tillage treatments.In addition,the difference in net carbon flux under different tillage treatments was significant: the NTS,RTS,and STS treatments could increase the net carbon flux by 2.53,1.05,and 1.24 t C ha-1 yr-1,and 6.14,4.66,and 4.85 t C ha-1 yr-1 when compared to CT treatment.The three types of conservation tillage treatments can reduce the energy input of this winter wheat-summer maize ecosystem as compared with traditional tillage,and is an effective measure when used to maintain and improve the energy efficiency of farming.6.The mechanism of soil carbon sequestration in wheat-maize field in Northwest China under conservation tillage measuresIn order to explore the mechanism of carbon sequestration under conservation tillage from the perspective of microorganism.The 18 S rRNA method was applied to analyze the diversity of soil arbuscular mycorrhizal(AM)fungi in the field.The results showed that NTS could increase the percentage of septoglomus and glomus as compared with CT treatment,but NTS treatment significantly(P<0.05)decreased the percentage of Glomerales.When compared to CT treatment,NTS significantly increased the percentages of large macroaggregates and small macroaggregates.Compared with CT treatment,NTS treatment increased the soil organic carbon content in all aggregates by 9.2-91.1%.In addition,NTS increased the mean weight diameter of soil aggregates by 9.8% as compared with CT treatment.There was a significant positive correlation relationship between soil organic carbon content and the percentage of Septoglomus and Glomus,and the percentage of large macroaggregates was the main factor affecting the organic carbon content in this area.The soil respiration rate therefore showed seasonal variability,and the total respiration under different tillage treatments,in descending order,was as follows: STS,RTS,CT,and NTS,during the trial period.In addition,the proportion of root respiration to soil respiration was different during the different stages.According to an analysis of the energy/carbon balance of this ecosystem,it can be concluded that,under conservation tillage treatments,farmers could increase soil organic carbon,the economic benefit accruing,and their energy production efficiency by increasing energy consumption and reducing soil disturbance.The effect of no-tillage treatment was more favourable to carbon fixation in this winter wheat-summer maize cropland ecosystem in China's northwest drylands,while the yield was much higher than that when using no-tillage,rotary tillage,or tillage systems.NTS treatment is benefit for the protection of AM fungal community composition and thus conducive to the sustainable development of agriculture in the region.At present,northwest dryland croplands are mainly dependent on chemical fertilisers to increase production: from the perspective of sustainable agricultural development,improving fertiliser utilisation and selecting appropriate agricultural management measures(to rely on straw and other resources)is fundamental to greenhouse gas emission reductions.In addition,this study was a short-term field experiment: future research into the long-term effects of different tillage measures on soil organic carbon flux is recommended. |
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