Interaction Between Accumulation Of Organic Matter Fractions And Maintenance Of Aggregates Stability In Black Soils

In the context of land use change,comprehensive analysis and comparison of the characteristics of soil organic matter and agglomerates including natural and agricultural systems,and exploration of the inner laws of soil property changes,can help determine the best principles for soil quality improvement.The Northeast black soil area has been known as the"voltage regulator"of grain security in China,but recently has appeared"thinning,hardening,less"and other soil quality degradation problem.Based on the physical fractionation of SOM,stable carbon isotope analysis,testing of aggregates stability with different breakdown mechanisms,root morphology and chemical property analysis,microbial real-time quantitative PCR and high-throughput sequencing,this study comprehensively analyzed the differences in fine root,microbial characteristics,SOM fractions and aggregates characteristics among the native grassland,plantations and croplands in typical black soil area.Moreover,we investigated the relationship among SOM fractions accumulation and their influence on SOM accumulation,elucidated the role of SOM fractions on the stability maintenance of each aggregate size under different breakdown mechanisms,and tried to further explain the mechanism of aggregate counteraction on SOM conservation.The main contents are as follows:（1）Characteristics of fine root and microorganisms in black soils of different land use types The land use type change reduced fine root biomass and also had a significant negative impact on fine root geometric characteristics.The root length density in the 0～10 cm soil layer of larch plantation and soybean cropland was only 22.50 and 36.43%of that of native grassland,respectively.The fine root carbon to nitrogen ratio and non-hydrolysable index varied depending on the land use type.Soil fungal and bacterial abundance decreased after the conversion of native grassland to plantation and cropland,and the decrease was more pronounced in the cropland.Soil fungal and bacterial abundance in corn cropland was only 48.19 and 14.41%of that in native grassland,respectively.The construction of plantation had no significant effect on fungal diversity but led to an increase in bacterial diversity;the reclamation had a negative effect on both fungal and bacterial diversity.（2）Characteristics of SOM fractions accumulation and intrinsic linkage among fractions The fine particulate organic matter（fPOM）and mineral-associated organic matter（MAOM）carbon contents of the native grassland were significantly higher than those of the other land use types（p<0.05）.Its fPOM carbon content in the 0～10 cm soil layer was 2.56～16.41 times higher than the rest of the land use types,and the MAOM carbon content was 1.16～1.99 times higher than the rest of the land use types.The carbon to nitrogen ratio of MAOM was also higher in native grassland,1.04 to 1.10 times higher than the rest of the land use types.The δ¹³C values of c POM,fPOM,and MAOM decreased in turn in the corn cropland,and increased in turn in the larch plantation and native grassland.The carbon content of MAOM showed a significant positive correlation（p<0.05）with the carbon content of fPOM and non-hydrolysable carbon content of fPOM.The fPOM carbon content only showed a significant positive correlation（p<0.05）with the non-hydrolysable carbon content of c POM.In summary,it can be seen that the c POM,fPOM,and MAOM were formed sequentially in black soil.The accumulation of MAOM was mainly influenced by the content of fPOM,while the accumulation of fPOM was influenced by both the content and chemical composition of c POM.The land use type change not only caused the loss of bulk SOM by reducing the fPOM and MAOM content,but also reduced the soil carbon to nitrogen ratio by affecting the MAOM carbon to nitrogen ratio.Fine root was an important factor affecting SOM fractions and their carbon to nitrogen ratio.（3）Physicochemical characteristics of aggregate and the way SOM maintains the stability of aggregate Compared with the plantation,the physical indicators of dry and wet sieved aggregates changed more significantly（p<0.05）after the conversion of native grassland to cropland,showing an increase（decrease）in the average weight diameter and a decrease（increase）in the fractal dimension of dry sieved（wet sieved）aggregates.Compared with native grassland,the SOM carbon concentrations of soil aggregates at all sizes in larch plantation and corn cropland under dry and wet sieves decreased,and the carbon to nitrogen ratios of soil aggregates at all sizes in larch plantation and corn cropland under wet sieves also decreased significantly.Under wet sieve,the SOM carbon concentration of 2～8 mm aggregates in native grassland was 1.79 and 2.43 times higher than that in larch plantation and corn cropland,respectively,and the carbon to nitrogen ratio was 1.07 and 1.14 times higher than that in larch plantation and corn cropland,respectively.The average weight diameter of soil aggregates after slow wetting,fast wetting and wet stirring was the lowest after fast wetting for all land use types.The 2～8 mm aggregates preservation rate after fast wetting treatment ranged from 2.94 to 57.73%,and the preservation rate of native grassland was 1.76 times higher than that of larch plantation and 8.19 times higher than that of corn cropland.The preservation rate of 0.25～2 mm aggregates with fast wetting treatment reached more than 80%under all land use types.Multiple regression analysis showed that the stability of aggregates under slow wetting treatment was most closely related to the fine root length density;the destabilization of aggregates caused by fast wetting treatment was mainly dependent on the fPOM content.In general,the land use type change increased the nitrogen demand when the aggregates sequester organic matter.Compared to the differential swelling and hydro-mechanical breakdown,the black soil aggregates were significantly less capable of maintaining stability under slaking.Large size aggregates were the main component for black soil aggregates breakdown.Promoting the accumulation of fPOM in large size aggregates was the key to enhance and maintain the stability of black soil aggregates.（4）Mechanism of aggregate reaction on SOM accumulation in black soil The organic matter carbon concentration of aggregates,the mass percent of microaggregates within aggregates,and the organic matter carbon concentration of microaggregates within aggregates did not show significant differences between the 2～8 mm aggregates and the 0.25～2 mm aggregates under the same land use type.The total microaggregate mass percent of native grassland were 1.15 and 1.26 times higher than those of larch plantation and corn cropland,respectively,and the total microaggregate organic matter carbon concentration were 1.72 and 2.21 times higher than those of larch plantation and corn cropland,respectively.The correlation coefficients between the soil total microaggregate mass percent,total microaggregate organic matter carbon concentration and the SOM content were all highly significant（p<0.01）.Bacterial abundance had a significant positive effect on both mass percent and organic matter carbon concentration of total microaggregate（p<0.05）.It can be concluded that there was no significant difference between the large size aggregates and small size aggregates in terms of the protection of SOM.A larger number of more carbon-rich microaggregates was the key to promote the accumulation of SOM.The soil microbial abundance was a key factor affecting the number of microaggregates and the variation of their organic matter carbon concentration.In general,fPOM is a key fraction affecting the accumulation of SOM and the maintenance of aggregate stability in black soils.The fPOM maintains the structural stability of black soils by promoting the stability of large size aggregate,while the protection of SOM by aggregate in turn is achieved by microaggregate within aggregate.