Effect Of Land Use And Fertilization On Aggregate-associated Carbon In Mollisols

Improving soil organic carbon （SOC） levels helps to maintain nutrientavailability and agricultural sustainability as well as to improve C sequestration tomitigate CO2emissions to the atmosphere. Improved soil aggregation has beenrecognized as a strategy to increase SOC. Natural Mollisols have good-qualityaggregates. However, the aggregate quality degraded when it has been intensivelycultivated. The study was conducted based on the long-term field experiment toinvestigate the effects of land use and fertilization on bulk soil C and aggregateassociated C pools and to elucidate C sequestration mechanisms in a Mollisol inNortheast China. The main results and conclusions are as follows:（1） The conversion from cropland （CL） to naturally restored grassland （GL） ledto increases in SOC by7.6%, and the accumulation rate in the surface soil （020cm）were estimated to be0.17MgCha^-1yr^-1；1over a period of27years; In contrast,converting cropland to bare land （BL） resulted in a decrease in SOC by14.1%, andthe accumulation rate in the surface soil （020cm） was about0.22MgCha^-1yr^-1；.Compared with the parent materials of Mollisols （P）, the8-year naturallydeveloped soil （PNat） and cropped-soils under no fertilizers （PCS F）, mineralfertilizers only （PCS+F）, mineral fertilizer plus crop biomass returned to the field（PCS+F+BM） and mineral fertilizer plus modeled organic manure （PCS+F+OM）promoted C sequestration. Compared with the P treatment, PNat, PCS F and PCS+Fincreased SOC by41%54%, with no significant difference in SOC between them,while soils under PCS+F+BM and PCS+F+OM contained higher organic C than othertreatments and increased SOC by114%122%. No significant improvements inorganic C levels of mature Mollisols were found when mineral fertilizers were applied alone compared with the unfertilized control. In contrast,19-year continuousapplication of organic manure along with mineral fertilizer promoted SOCaccumulation. The SOC increases but the C sequestration efficiency decreases withmanure application rate. However, no significant differences were found between thetwo greatest manure application rates.（2） There was an increase in the weight proportion of macroaggregates （>250μm）, especially large macroaggregates （>2000μm）, but a decrease in microaggregatesand silt plus clay fraction in GL relative to CL. As a result, the mean weight diameters（MWD） of water-stable aggregates under GL （1.74mm） was higher than that underCL （0.55mm）. The proportion of microaggregates was decreased in BL relative to CL,thus leading to a small but non-significant reduction in the MWD of aggregates.Compared with the P treatment, the PNat, PCS F and PCS+F did not affect theMWD of aggregates, while the PCS+F+BM and PCS+F+OM greatly improvedmacroaggregation, thus leading to higher MWD of aggregates than other treatments.Mere mineral fertilizer amendment had no significant effect on aggregation of matureMollisols compared with the unfertilized control. The combined use of organicmanure with mineral fertilizer increased the proportion of macroaggregates. Theproportion of small macroaggregate （250-2000μm） tended to increase, and theproportion of the silt plus clay fraction tended to decrease with manure additions.However, the MWD of the aggregates did not significantly differ between the twogreatest manure application rates.（3） At the aggregate level, the total coarse particulate organic matter （total cPOM）fraction contributed300%to the increases in SOC in GL. The total fine particulateorganic matter （total fPOM） fraction was a major contributor to the C loss in BL,which accounted for60%of the total C decreases. These results indicated that the Caccumulation or loss in mature Mollisols was mainly happened in the relatively labileC pools. Compared with the P treatment, the C in the total cPOM, total fPOM and total silt plus clay （total s+c） were all increased in the naturally developed soils andcropped-soils under different fertilizations. There was no significance in C content inthese fractions between PNat and PCS F. Soil under PCS+F had higher C in the totalfPOM and total s+c than soil under PCS F. The C of these fractions in soils underPCS+F+BM and PCS+F+OM was all significantly higher than other soils.In contrast, mere chemical fertilizer additions did not affect aggregate-associatedC in mature Mollisols. The increment of organic C following organic manure plusinorganic fertilizer amendment was located in the total cPOM and total fPOMfractions, which contributed55%and45%to the total increases in bulk SOC inmanure-added soil compared with the control, respectively.Carbon saturation occurs in a hierarchical fashion in a Mollisol. When the SOCis rather low, the additional C was allocated to all aggregate fractions. As the C inputincreases, the smaller SOC pools saturate before the larger pools and consequentlyadditional C input will only accumulate in labile SOC pools （cPOM） inmacroaggregates that have a relatively faster turnover.