Dynamics Of Exogenous Carbon And Nitrogen In Mollisol And Effect Of Wetness Index On Soil Carbon And Nitrogen Distribution

Black soils(Mollisols)are most fertile and affected by many factors,such as topography and climate.The influence of these factors on the distribution and contents of carbon(C)and nitrogen(N)in Mollisol soil is important for predicting and evaluating their reserves.Returning crop residues to soil is known to strongly influence soil C and N stocks.Soil aggregates play a crucial role in the sequestration of soil organic C(SOC)and total N,but the specific contribution of crop residues to soil aggregates influenced by long-term fertilization remains largely unknown.In addition,there is still no quantitative description of the relationship between soil C,N and topography,climate in Mollisol areas under natural conditions.Therefore,a Mollisol was selected in northeast of China,the effects of long-term fertilization on the distribution of 13 C and 15N-labelled maize straw residue in different aggregate size fractions were investigated and their retention in soils over a 1-year incubation was tested.The following three fertilization treatments were selected: 1)without fertilization(CK),2)mineral fertilizer(NPK),and 3)cow manure with mineral fertilizer(NPKM).Soil samples were collected from the surface layer(0-20 cm)of a long-term field experiment of Mollisol(Luvic Phaeozem)in 2015 at Gongzhuling,Jilin Province,China(established in 1980).Soils with and without 13 C and 15N-labelled maize straw were incubated for 360 days at 25 oC.Soil for the amended treatment was mixed with 1% dried and sieved 13 C and 15N-labeled maize straw(δ13C = 578.22 ‰ and δ15N = 15289.8 ‰),and destructively collected on the days of 45,90,135,180 and 360 after treatment.Soil aggregates were separated into two fractions(macroaggregates,>250μm;microaggregate,<250μm)by wet sieving.On the other hand,the factors affecting the distribution of C and N in Mollisols had been studied in the intensively managed critical zone in United States.A Dynamic Wetness Index(DWI)has been developed for accounting the persistence of soil moisture over time at the microtopographic scale(~1 m2),and additionally included inputs of climate conditions and soil texture to build relationship between DWI and soil N and N.Six study sites (including four agricultural sites,one restored prairie and one forest site)had been selected within the Sangaman River Basin,an Intensively Managed Landscape-Critical Zone Observatory in central Illinois,USA.The main results obtained from the study were as follows:(1)Long-term no fertilization and chemical fertilizer Mollisols were more sensitive to the addition of straw C,with a faster turnover.Organic fertilizer combined with chemical fertilizer treatment could sequester more straw C to soil aggregates.The highest amount of maize straw 13 C was incorporated in macroaggregate on the days of 90,135 and 180,and in microaggregate on the days of 135,45 and 135 for CK,NPK and NPKM respectively.Our results illustrated that the soil with low organic C content promoted the newly added straw turnover.Long-term application of manure with mineral fertilizer helped to stabilize or increase the retention of exogenous C in the soil aggregates.(2)Organic fertilizer combined with chemical fertilizer treatment improved the ability of soil in Mollisol soil to retain straw 15 N.The maize straw-derived 15 N remaining in the soil aggregates(macroaggregate and microaggregate)was in the order of NPKM > NPK > CK after 360 days of incubation.The highest amount of maize straw-derived 15 N was incorporated into the macroaggregate on day of 90 in the three treatments of soils(average of 65.4%)and into the microaggregate on days of 360,360 and 45 in the NPKM(11.5%),NPK(9.5%),and CK(6.7%)treatments,respectively.The results revealed that the combined application of chemical fertilizer and organic manure had higher capacity to retain maize straw derived 15 N than that of application chemical fertilizer alone.In addition,macroaggregate has capacity to retain maize straw derived 15 N than microaggregate.(3)The dynamic wetness index(DWI)built better correlations with the surface soil C and N.Relative to topographic wetness index(TWI),DWI correlated strongly with soil surface geochemistry,specifically p H,δ13C,δ15N,and certain lignin phenols(vanillyl,cinnamyl/vanillyl,syringyl-vanillyl-cinnamyl/substituted fatty acids).Our study elucidated that besides topography in large scale,climate,micro-topography,soil texture also influenced the distribution of soil C and N.(4)The soil physical and chemical properties had the greatest impact on lignin content,while DWI had little effect on the lignin content.The proportions of soil properties influencing on Vanillyl,Syringyl and Cinnamyl were 50.93%,74.10%,65.78%,respectively.DWI and the interaction between DWI and soil properties had only less than 5% effect on lignin content.