Effects Of Tillage And Straw Returning On Soil Carbon Source And Crop Photosynthetic Carbon Intercept In Farmland

This experiment was carried out in the Huang-Huai-Hai Plain,where the grain yield accounts for one third of the total national output,thus playing a very important role in ensuring food security.There were many factors contributing to the instability of grain yield,among which long-term soil tillage will not only cause soil hardening and shallowing,but also reduce the ability of soil to store water and preserve soil moisture,thus causing adverse consequences to the high and stable yield of national grain.In addition,due to more farming procedures and excessive fertilization,farmland non-point source pollution is serious,which makes soil erosion worse,thus easily affecting crop yield and farmers'enthusiasm,and also hindering the sustainable development of agriculture.Therefore,it is very important to find a reasonable land use pattern to maintain soil fertility and increase grain yield.This study was carried out on a long-term conservation tillage experimental site?began in 2002?in the Agronomy Experimental Station of Shandong Agricultural University,using a combination of field and microzone test of winter wheat?Jimai 22?-summer maize?Zhengdan958?.The objective of this study was to evaluate the effects of different tillage and straw returning methods on soil organic carbon?SOC?and crop productivity,and distinguish the SOC sources from wheat?C3?and maize?C4?plants using the two-source mixing model from natural abundance of¹³C.In the experiment,four tillage practices,including conventional tillage?CT?,subsoiling?ST?,rotary tillage?RT?and no-tillage?NT?and three straw returning amounts,including no straw returning?0?,1 m maize straw returning?1?and all the maize straw returning to the field?a?.The interannual variation and mechanism of SOC and crop yield in wheat-maize double cropping system under the condition of long-term tillage methods and straw returning from 2005 to 2017 were systematically discussed.The contribution of wheat and maize to SOC from 2002 to 2017 was evaluated by using the two-source mixed model based on ¹³C natural abundance method.Through the study of soil carbon sequestration,to find the best way of tillage practices and straw returning methods.In order to compare the changes of maize photosynthesis and evaluate the contribution of soil emission to maize photosynthesis under the condition of controlling carbon emission.In the present work,stable isotope and Keeling plot methods were used to separate the sources of CO₂ involved in photosynthesis.The contribution ratio of soil respiration to photosynthesis was calculated.The main research results were as follows:1 Effects of different tillage practices and straw returning methods on SOC and its fractionsAfter 15 years of long-term conservation tillage,the results showed that at a depth of0-10 cm,the average SOC concentrations of ST_a and RT₀ were the highest and lowest,14.29g/kg and 10.49 g/kg respectively,which were 41.41%higher and 0.58%lower than that of CT₀ respectively.At 10-20 cm depth,the average SOC concentrations of ST_a and RT₀ were the highest and lowest,18.41%higher and 16.79%lower than that of CT₀,respectively.In20-30 cm soil layer,CT_a treatment and RT₀ had the highest and lowest average SOC concentrations,9.79 g/kg and 5.95 g/kg,respectively,which were 14.76%higher and 30.26%lower than that of CT₀,respectively.The results indicated that the highest mean SOC concentration in the 0–30 cm soil was found under ST_a?11.80 g/kg?,which increased by 2.29g/kg more than that under CT₀,while RT₀ had the lowest mean SOC concentration?8.10 g/kg?.In the 0-10 cm,10-20 cm and 20-30 cm soil layers,there were significant differences in SOC stability index between some treatments.In the 0-30 cm soil layer,ST_a and RT₀treatments had the highest and lowest carbon pool management indexes?CPMA?respectively.In comparison to those of CT₀,the SOC stock?SOCs?and carbon sequestration rate?CSR?of ST_a were the highest and increased by 15.64 t/ha and 1.05 t/ha/yr,respectively,in the 0–30cm soil.Therefore,ST_a treatment has the most obvious effect on improving SOC content and soil fertility.In 2016 and 2017,SOC of ST₁ treatment was significantly increased by 51.33%compared with CT₀ treatment in 0-20 cm soil layer,and NT₀ treatment was the lowest.Compared with CT₀,POC,POxC,LFOC and MBC contents in ST₁ treatment increased by45.68%,34.55%,51.85%and 51.02%respectively,while RT₀ treatment was the lowest.In20-40 cm soil layer,SOC showed different differences among different treatments.Compared with CT₀,POC,POxC and LFOC contents of ST₁ treatment increased by 143.62%,17.57%and 161.20%,respectively,and MBC concentration ranged from 212.83?RT₀?to315.58?ST_a?mg/kg.In 40-100 cm soil layer,the SOC concentration of ST₁ in 2016 was significantly higher than that of other treatments?P<0.05?,and the SOC concentration of NT₀ treatment in 2017 was the lowest.Compared with CT₀,POC increased by 46.94%?RT_a?to 75.85%?NT₀?.POxC showed different results in 2016 and 2017.LFOC in ST₁ decreased by 21.43%,MBC concentration in ST₁ and NT₀ treatments increased by 50.25%and decreased by 14.28%,respectively.In addition,POC and MBC were more sensitive to soil management measures than POxC,and there was a significant positive correlation between SOC and labile carbon fractions.2 Effects of different tillage practices and straw returning methods on SOC sourcesST_a treatment had the highest carbon input in wheat and maize,and RT₀ treatment had the lowest carbon input.Carbon input from wheat in ST_a and RT₀ treatments was 167.56%higher and 2.55%lower than that in CT₀ treatment,while carbon input from maize in ST_a and RT₀ treatments was 198.13%higher and 6.81%lower than that in CT₀ treatment,respectively.The contribution of maize to SOC in ST_a and RT₀ treatments was 154.07%higher and48.89%lower than that in CT₀ treatments respectively.Although the amount of carbon input of wheat was lower than that of maize,the contribution of wheat to SOC was higher than that of maize.The ratio of wheat transformed to carbon was less than 50%,while the contribution of wheat to SOC storage was higher than 50%,which indicated that the conversion rate of wheat to SOC was faster and has stronger transformation ability.Thus,when comparing NT and STS,especially for C3 residues,straw incorporation was more suitable for restoring degraded land and increasing productivity.3 Effects of different tillage practices and straw returning methods on crop yieldAfter long-term conservation tillage,the average wheat yield of ST_a and RT₀ treatments was the highest and lowest during 2005-2017,8.08 t/ha and 6.91 t/ha respectively,which increased by 9.60%and decreased by 6.33%,compared with CT₀ treatments,respectively.The average maize yield of ST_a and RT₀ treatments was the highest and lowest,11.98 t/ha and9.54 t/ha respectively,which increased 14.36%and decreased 8.91%respectively compared with CT₀ treatment.The increases in annual yield ranged from 0.58?NT₀?to 4.93?ST₀?t/ha during 2005–2017.In comparison to the annual yield of CT₀,that of ST_a increased by 2 t/ha and was significantly higher than the annual yields of the other treatments?P<0.05??except for those of the NT_a and CT_a treatments?.For the wheat yield stability index,ST₀ and RT₀treatments were the highest and lowest respectively,but there was no significant difference among the treatments.For the maize yield stability index,the opposite results were presented.RT₀ treatment was significantly higher than ST₀ treatment?P<0.05?.The annual yield stability index showed the same results as maize.Moreover,it can be seen that the yield stability index of wheat was higher than that of maize.In summary,the results show that ST_a treatment had maintained a high level for a long time.In 2016 and 2017,ST_a and ST₁ treatments showed higher wheat yield,while CT_a,CT₁,ST_a and ST₁ treatments showed significantly higher maize yield than other treatments.Compared with CT₀ treatment,the annual yield of wheat and maize in ST₁ treatment increased by 17.96%,while that in RT₀ treatment was significantly lower than that in other treatments?P<0.05?.In short,compared with other treatments,ST₁ treatment was a more long-term and effective land management measure in increasing SOC and crop yield.4.Analysis of contribution of soil carbon emission to crop photosynthesisAccording to the determination of photosynthesis and water use efficiency of maize through the soil emission control test,the A,g_s,T_r and WUE_i of maize without any control measures increased by 13.98%,12.81%,16.76%and 1.51%respectively,compared with those with soil carbon emission control measures,however,?¹³C,C_i/C_a and WUE_t decreased by 5.51%,8.57%and 3.56%respectively.The range of CO₂ recycling index was 0.82 to 0.90,and the contribution range of soil carbon emissions to photosynthesis was 20.37%to 29.03%.Therefore,soil emission had a certain effect on the photosynthesis of maize.Under the condition of high soil emission,the photosynthetic rate of maize had increased.How to regulate soil emission to improve the photosynthesis of crops still needed further study.