Effects Of Additional Straw Returning On Soil Aggregate Stability And Associated Soil Organic Carbon Characteristics On An Existing Conservation Tillage Field

Studying the characteristics of soil organic carbon（SOC）and its associated components along with soil aggregate stability as influenced by additional straw returning on a typical conservation tillage field with straw incorporation and rotation practice is essential for sustaining soil quality of any agroecosystem.A conservation system,which includes straw returning,and tillage practice could distinctly change the soil organic carbon content,stock,labile organic carbon pools and soil stable aggregates.Since the return to straw management,it remains unknown the right quality needed for soil health and plant production.A sustainable management of the soil can be practiced through conservation tillage practices,including no-tillage（NT）and no till with straw incorporation（NTS）.Measurements and studies of organic carbon content with its associated components and soil aggregate stability should enable us better assess the risk of structural degradation in the soil.With the main aim of providing a further understanding on the effects of straw additions to an already existing conservation field with straw returns,this research was set up.Relying on an existing long term-term conservation tillage field experimental trial conducted by the Gansu Agricultural University,in Li Jiabu town near Dingxi city of the Gansu Province of China,this research was set up as a split plot along existing fields to study the effects and characteristics of straw additions on some major soil and crop parameters including soil physiochemical properties,aggregate stability and to determine the optimum stage of SOC along with existing relationship between studied parameters.Two main tillage types with straw incorporation（conventional tillage with straw returns-TS and no tillage with straw returns–NTS）and four different additional straw return treatment levels;no straw addition（NSA 0x）,low straw addition（LSA 1x）at a rate of 3500 kgha^-1,medium straw additions（MSA 2x）double the rate at 7000 kgha^-1and high straw addition（HSA 3x）thrice the rate at 10,500 kgha^-1were applied as main treatment factors.Treatments were applied in a systematic manner alongside the already existing field plots.The experiment was using two crops of spring wheat and field pea and conducted in two-seasons of 2019 and 2020 cropping seasons respectively.Initial and general analysis at the start of the experiment of all potential variables that could influence study outcomes in both tillage fields（NTS and TS）in-terms of soil physical and chemical properties show that,proportions of sand and silt were significant（p＜0.05）.Soils in both fields had high calcium carbonate（Ca CO₃）concentration,p H and EC values.SOC and total available N was significantly（p＜0.01）higher in NTS fields than in TS fields.C/N ratios in both fields was 8.48 and 8.28 in NTS and TS fields respectively and showing no significant difference.The mean SOC and N stocks were 13.37 mg C ha^-1and 1.50 mg N ha^-1in both fields.Initial results of straw additions on bulk density and total porosity divulged that,significant differences（p<0.05）was observed in soil bulk density across the additional straw types.The most significant effect was a decrease in soil bulk density and total porosity of MSA 2x treatment as compared to NSA 1x,LSA 2x and HSA 3x.Analysis of yield showed that,the application of straw additions to tillage fields significantly affected grain yields especially in spring wheat fields.NTS fields had higher grain yield compared to TS fields.NSA 0x and LSA 1x recorded higher yields in wheat fields whiles for pea fields LSA1x and MSA 2x had higher and significant yields.It is evident from this study that in most of the carbon related parameters,NTS showed more sensitivity to treatment effect compared to TS.This study further revealed that,NTS fields had significantly higher SOC content than TS fields.SOC content within study fields ranged from 5.56–20.77 g kg^-1.In this immediate study and on the already existing conservation fields with straw incorporation,LSA 1x treatments recorded higher contents of SOC.The observed trend in terms of SOC content among the straw addition treatments was LSA 1x>NSA 0x>MSA 2x>HSA 3x.Results of some carbon components revealed that MBC content in the upper soil surface of LSA 1x treatment was 20%higher than in NSA 0x or HSA 3x.The study subsequently found that POC fractions was most influenced by tillage type and straw additions and was highly significant at the upper soil layers on NTS fields.Similarly,this study found relevant outcomes on treatment effect on some aggregate indices.Additional straw treatment and tillage types had significant effect on LI,CPI and SCMI.The SCMI order observed was LSA1x（29）MSA 2x（29）HSA 3x（29）NSA 0x.Highest accumulated SOC stocks was observed under LSA1x while lowest SOC stocks was NSA 0x.Results obtained from analysis using FTIR spectroscopy showed less significant results for the various straw addition treatments.The relative intensities of the C-H bonds were higher in LSA 1x while the changes in vibration of the C=O bonds showed an opposite trend.Lower straw addition treatment was more effective in increasing soil hydrophobicity index and improve the activation of soil organic matter.Under TS fields,as the aggregate size decreased,the aggregate intensity value also decreased,the aromatic and aliphatic ratio increases and this made the structure more complex.Both straw addition treatments and tillage types significantly affected the distribution of soil aggregate for both dry and wet methods.Aggregate sizes of NTS were lower and significantly different across sizes of>5 mm,2-5 mm,and 1-2 mm.However,the trend changed at the smaller aggregate sizes of 0.25-0.5 mm and<0.25 mm where NTS recorded higher aggregate size values.MWD and GWD of soil aggregates significantly varied within the straw addition treatment and tillage type interaction.Significant differences were observed across NTS and TS for the various straw addition treatments in both cropping seasons.EOC was significantly higher under LSA 1x(4.82 g kg^-1)and MSA 2x(4.46 g kg^-1).In other related outcomes,highest SOC content was observed within the larger particle sizes of>5 mm and 2-5 mm in TS and NTS fields for both cropping seasons.SOC contents in aggregates was increased in the second cropping season of pea in 2020.The content of aggregates in SOC was noted to be high in macro-aggregates compared to soil micro-aggregates in both tillage types.In like manner,it was observed that a decrease in aggregate sizes,resulted in a decrease in soil organic carbon content of aggregates except for aggregate size of（29）0.25 mm.To better understand the relationship among study parameters,various relationship analysis was carried out.SOC and its associated components showed a positive and significant correlation in both tillage fields of TS and NTS.Within the tillage fields,NTS had a strong significant positive correlation between SOC and EOC,POC and MBC and MBC and SOC with correlation coefficient above 0.91**while in the TS fields,there was significant and positive correlation among EOC and SOC,MBC and DOC,and POC and EOC.SOC was both positively（p（27）0.05 and p（27）0.01）correlated with different aggregate sizes,MWD,GMD and FD.FD however showed a non-significant relation with most soil aggregate sizes except for 0.5-1 mm were a significant correlation was observed.A principal component analysis（PCA）also confirmed the earlier findings of this study that LSA 1x treatment had a significant influence on treatment effect of straw additions.This study concludes that,from the study area and on an existing conservation field with straw incorporated already,additional levels of straw has a limit and that higher levels(above7000 kgha^-1)does not positively affect or influence SOC and it associated components.Furthermore,NTS fields were noted to be more sensitive to the application of additional straw treatments and that could influence any desired outcomes.Crop type was not significantly affected by treatment effect since there was an observed similar trend on the effects of straw addition on most studies parameters on both crops studied.Low and no straw additions effect on tillage fields especially NTS significantly improved the materialization of stable macro-aggregates,aggregate fractions stability and increased SOC contents in the macro-aggregate fraction,which could represent a possible method of improving the sequestration of SOC through protecting its physical structure.This study provides further proof that higher levels of straw additions on an existing conservation field with straw incorporation does not influence or significantly affects soil or crop yields.This study also,offers a novel contribution in explaining the characteristics and effects of how straw additions relates to the very important organic carbon components and their fractions on a conservation tillage system.These findings are crucial in adding new evidence to support the positive influence of straw returning and additions to tillage fields and the fact that its application has limit.