Study Of Organic Carbon On Red Soil Aggregate Formation And Its Molecular Mechanism During The Organic Fertilization

Organic fertilization is the most important way for increasing the soil fertility and improving soil structure,however,the relationship between the molecular structure of organic carbon and the red soil aggregate formation is unclear during the organic fertilization.Therefore,the objectives of the present research were to examine the effects of different organic carbon fractions and its chemical structures on the soil aggregates formation and the relationship between organic functional groups and aggregate formation under different fertilization patterns(chemical fertilizer,reduced organic fertilizer,and organic fertilizer)and during the decomposing processes of organic amendments(manure fertilizer,maize straw,and rape straw)with different chemical structures,depending on Fourier-transform infrared spectroscopy,¹³C nuclear magnetic resource NMR spectroscopy,phospholipid fatty acid,X-ray photoelectron spectroscopy technologies,at Yueyang Agricultural Environmental Science Experimental Station.The major results of the research are the following:1)The effect of organic carbon on aggregate formation under the continuous organic fertilizer.The aggregate stability,the>0.25 mm aggregate proportion,organic carbon content within different-sized aggregates increased by 26.9%,38.4%,and 59.7%-28.8%respectively(P<0.05),after continuous application of organic fertilizer for 4 years,compared to no fertilizer treatment.The OC accumulation of macroaggregate in M was attributed to OC content increase in silt plus clay subfraction(P<0.05),oppositely,in microaggregate it was located in the relatively stable fine i POM(P<0.05).The macroaggregate formation and stability was controlled by the fine i POM within macroaggregates(P<0.01),whose abundant polysaccharide-C and aliphatic-C after manure fertilization advanced the microbial growth except for gram-positive bacteria,which further promoted macroaggregate formation and stability.The free silt plus clay fraction also affected macroaggregate formation and stability(P<0.05),and its polysaccharide-C derived from microorganisms or decomposing SOM was positively associated with MWD and macroaggregate proportion.Because polysaccharide-C can benefit mineral particles aggregation to forming micro-or macroaggregate.2)To study the effect of the chemical compositions of organic amendments on aggregate formation,the three most common organic amendments in red soil area,pig manure,rape straw,and maize straw were selected for experiment.Manure,having a lower carbon(C):nitrogen(N)ratio(9.25)than the other amendments and abundant carbonyl C(2.26%),N-alkyl C(10.82%),and alkyl C(27.14%),induced strong but nonpersistent increases in MWD and the formation of>2 mm aggregates.Rape straw and maize straw had initial slower and more persistent effect than the manure fertilizer on MWD and>2 mm aggregate formation due to its higher C:N ratio(51.45-89.63)and higher abundances of O-alkyl C(70.5%-80.4%)and lignin(23.3%-39.8%),which degraded slowly.The 1-0.5 mm aggregate formation was affected by both soil type and organic amendment type,and can be improved by carbonyl C,N-alkyl C,and alkyl C.3)The relationship of dynamics of organic carbon contents and chemical compositions with the formation of different sized aggregates needed further study.As the organic amendments decomposed,the free particulate organic matter(FPOM)showed a descending trend gradually and the mineral-associated organic matter(MOM)showed an opposite trend gradually.The occluded particulate organic matter(OPOM)first increased and then decreased in the manure and rape straw treatments,but increasing gradually in the maize treatment.FPOM,OPOM,MOM,and Fe oxides had combined effects on aggregate formation.The FPOM(P<0.001)and OPOM(P<0.001)mainly affected the formations of>2 mm and 2-1 mm aggregates,and the>2 mm aggregate formation was also significantly and positively correlated with the alkyl C,N-alkyl C,and carbonyl C in the OPOM(r>0.6,P<0.01),with the O-alkyl C and di-O-alkyl C negatively(0.67<r<-0.55,P<0.01).The 1-0.5 mm aggregate formation was affected by both Fe oxides and MOM.4)The study of organic carbon compositions in the MOM was beneficial to analyze the contributing factors of the 1-0.5 mm aggregate formation.The MOM was divided into three fractions,i.e.<0.2,0.2-2,and 2-53?m,based on soil particle-size.Compared to 2-53?m fraction,the<0.2 and 0.2-2?m clay fractions contained more Fe and Al oxides,and retained a large quantity of C and N,which were affected by organic amendments types(P<0.05).The OC content of the 0.2?m clay fraction rich in Fe and Al oxides was higher in the manure and rape straw treatments than that in the maize treatment(P<0.05),but the OC content of the 0.2?2?m clay fractions rich in Si showed manure>rape straw>maize straw(P<0.05).The manure treatments also had more N content in both<0.2 and 0.2-2?m clay fractions compared to rape straw and maize straw(P<0.05).Furthermore,the structure character of OC in<0.2 and 0.2-2?m clay fractions had obvious differences.The OC in the<0.2?m clay fraction was mainly polysaccharides and had a higher C:N ratio.But the 0.2?2?m clay fractions preferentially retained to the OC with relatively high degrees of humification and lower C:N ratio.The rape straw and maize straw promoted the accumulation of C-H and C-C organic groups in the<0.2?m clay fraction,nevertheless,manure facilitated the C-O-C and C=O/C(O)N accumulation in the 0.2-2?m clay fraction.In a word,organic fertilizer application can promote the transformation of different organic carbon components,and facilitate the soil aggregate formation and stability.Moreover,the organic amendments rich alkyl C,N-alkyl C,and carbonyl C,such as manure,can rapidly improve the red soil structure.