Effects of different plantation ages on soil aggregate stability and aggregate-associated organic carbon fractions in walnut orchards
By using the method of space-for-time replacement,we analyzed the evolution characteristics of soil organic carbon(SOC)and its fractions(active carbon and passive carbon)within soil aggregates during the walnut planting process for providing scientific basis for sustainable development of walnut orchards in China.Three walnut plantations of different ages(7,14 and 21 years)and the adjacent wasteland without walnut planting were selected as the research objects in the walnut demonstration base of Hebei Lvling Fruit Industry Co.,Ltd.By using the optimal-moisture sieving method and modified Walkley-Black method,we analyzed the differences in soil aggregate distribution and aggregate-associated SOC and its fractions across walnut orchards with different planting years.Results showed that:(1)The proportions of＞2 and＜0.25 mm aggregates in the surface soil(0-40 cm)were increased and declined with increasing walnut plantation ages,respectively,which caused the values of mean weight diameter(MWD)was increased with increasing walnut plantation ages(increased by 2.3%-18.2%).Meanwhile,the values of MWD within aggregates in the 0-20 layer were significantly higher than those in the 20-40 and 40-60 cm layers.(2)As the walnut plantation ages increased,the contents of SOC(both active carbon and passive carbon)and total nitrogen as well as SOC activity of the larger aggregates(＞2 and 2-0.25 mm)in the surface soils increased basically.However,the SOC contents,SOC activity and total nitrogen in the deep soil(40-60 cm)and＜0.25 mm aggregates were not sensitive to walnut plantation ages.(3)The SOC(i.e.,active carbon and passive carbon),total nitrogen contents,carbon nitrogen ratio(C/N)values and SOC activity showed decreasing trends with aggregate size decreased.Meanwhile,these indices decreased gradually with an increase in soil depths.In conclusion,long-term walnut plantations(＞14 years)could increase soil aggregate stability and were beneficial for SOC accumulation and the improvement of SOC activity within larger aggregates;however,these indices within＜0.25 mm aggregates and deep soils were difficultly influenced by walnut plantation ages.
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