Effects Of Long-term Fertilization On Organic Carbon And Total Nitrogen Of Bulk Soil And Aggregates In A Loess Soil

Based on a long-term experiment under winter wheat-summer fallow cropping system on a rainfed loess soil,we investigated the impact of long term fertilization on the distribution of water stable aggregates and their stability,as well as the distribution of organic carbon and nitrogen in different size of aggregates.The six treatments were selected for the separation of aggregates,including:(1)control(CK),(2)nitrogen only(N),(3)nitrogen and potassium(NK),(4)nitrogen and phosphate(NP),(5)nitrogen,phosphorus and potassium(NPK),(6)manure plus NPK(MNPK).Soil samples were collected from surface(0-10 cm)and subsurface(10-20 cm)soils,and dry combined with wet sieve methods were employed to extract water stable aggregates.The aims of this study were to:1)understand the mechanisms of organic carbon and nitrogen sequestration under long-term fertilization in a rainfed loess soil and provide a theoretical basis for improvement of soil fertility and fertilizer use efficiency in this region.The main results obtained were as follows:1)Compared with CK,application of mineral fertilizer with manure significantly decreased soil bulk density.The treatments of MNPK,NPK and NP markedly increased contents of soil aggregates by 42-48%in fraction of>2 mm,decreased 18-22%in aggregates of 2-0.25mm at 0-10 cm soil depth.The values of MWD and GMD in MNPK,NPK and NP treatments were higher than those of NK,N and CK treatmenst,and balanced application of N and P and combination of mineral fertilizers with manure could also increase the aggregates stability.2)Significant and positive correlations between soil organic carbon and total nitrogen content at both 0-10 and 10-20 cm soil depths were observed.(R~2=0.916,P<0.01,R~2=0.889,P<0.01);and soil organic carbon and total nitrogen content were higher at 0-10 cm than that at 10-20 cm soil depth.Long-term fertilization significantly increased soil organic carbon and total nitrogen content in general,and the highest value was observed in treatment receiving MNPK,then followed by NPK and NP,with the lowest values in NK,N and CK treatments.3)Long-term fertilization can increase organic carbon and total nitrogen content in water stable aggregates of all size classes.The contribution rate of organic carbon and total nitrogen in aggregates decreased with the decreasing particle size of aggregates.The organic carbon and total nitrogen were mainly distributed in larger water stable macro-aggregates of>2-mm and 2-0.25-mm,accounting for 72-81%and 81-95%,respectively.The contents of soil organic carbon and total nitrogen in bulk soils were found significantly,positively and linearly correlated to that in aggregates fractions of>2 mm?2-0.25 mm?0.25-0.053 mm,while the relationship between the content of organic carbon and total nitrogen in<0.053-mm sieve was showed a quadratic function.4)Long-term fertilization tended to decrease the C/N ratios by showed no significant effect on it.It also showed no significant effects on C/N ratios of all size soil aggregates except that the C/N ratios was higher in MNPK than other treatments in the 0.25-2 mm aggregates.In conclusion,long-term balanced fertilization of synthetic fertilizers of nitrogen and phosphorus(NP and NPK)or combination of synthetic fertilizers with organic manure(MNPK)can significantly increase the contents of organic carbon and total nitrogen,and the storage of organic carbon,improve the rate of carbon sequestration.After 25 years of fertilization,the carbon sequestration of loess soil has reached saturation in silt and clay fractions,while the other fractions of aggregates still have larger potential to sequestrate organic carbon.