| Soil organic carbon(SOC)is an important component of soil quality and productivity.But its measurement alone does not adequately reflect changes in soil quality and nutrient status.This is because SOC has large pool size and inherent spatial variability.But soil active organic carbon fractions:microbial biomass carbon(MBC)potential mineralized carbon(PCM)change rapidly with different tillage methods which could better reflect soil organic matter.They can indicate the comprehensive vitality of soil,and then we could put forward theoretical basis for the sustainable development of agriculture based on that.Nitrogen is the main limiting factor of grain output on Loess Plateau.As the increase of nitrogen fertilizer use and unreasonable application,nitrogen use efficiency is getting lower and lower.The redundant nitrogen in the soil pollutes the groundwater and ecological environment seriously.Therefore,putting forward the rational and effective measures according to soil nitrogen supply capacity is a fundamental way to protect the environment.In this study,the effects of fertilization and rotation on soil nitrogen pool were studied by measuring soil total nitrogen(STN),microbial biomass nitrogen(MBN),potential mineralized nitrogen(PNM)and particulate organic nitrogen(PON).Our study based on the long-term experiment in Changwu of Shanxi Province which was started in 1984,examined the effects of long-term fertilization and crop rotation on soil carbon and nitrogen pools in dryland winter wheat field.Nine treatments were included as control treatment(CK),nitrogen and phosphorus combined fertilization(NP),nitrogen,phosphorus and manure combined fertilization(NPM)and bare land without fertilization and cropping(BL),3-yr rotations of corn-winter wheat-winter wheat-millet(C/W/W+M),3-yr rotations of pea-winter wheat-winter wheat-millet(Pe/W/W+M),sainfoin-winter wheat-winter wheat-sainfoin(S/W/W+S),4-yr rotation of pea-winter wheat-winter wheat-corn(Pe/W/W/C)and 8-yr rotation of alfalfa(4 yr)-potato(1 yr)-winter wheat(3 yr)(A/A/A/A/Po/W/W/W+A).The main conclusions were:(1)The results showed that the contents of SOC,MBC,PCM and carbon management index(CMI)at 0-30 cm soil layer were higher by 42.2%?55.9%?40.9%and 40.0%(P<0.05)in NPM than in CK after 30-yr continuous fertilization,respectively.No difference in SOC and MBC was found between NP and CK,however,the PCM content and CMI were significantly higher in NP than in CK.The effect of long term fertilization on soil active organic carbon fractions was more significant at 15-30 cm soil layers than 0-15 cm.No cropping reduced the contents of SOC and its fractions.SOC and PCM was lower by 23.5%and 20.5%in BL than in CK,respectively.Correlation analysis showed that SOC,MBC,PCM and CMI correlated with each other significantly.SOC changed in agreement with its fractions for all treatments except for NP,and the two active organic carbon ratio of soil organic carbon and PCM/MBC were all not significant among treatments.In general,continuous long-term fertilization can effectively increase soil organic carbon and its active fractions which contributing to soil fertility and carbon sequestration under dryland farming systems.(2)In the rotation system,the content of SOC at 0-30 cm soil layer was the highest in A/A/A/A/Po/W/W/W+A which was higher by 20.5%and 29.3%(P<0.05)than in Pe/W/W+M and Pe/W/W/C,respectively.The content of MBC in S/W/W+S was signific-antly higher than in other treatments.The content was higher by 44.4%,47.8%,27.8%and 28.7%(P<0.05)in S/W/W+S than in C/W/W+M,Pe/W/W+M,Pe/W/W/C and A/A/A/A/Po/W/W/W+A,respectively.The PCM content in S/W/W+S and A/A/A/A/Po/W/W/W+A were higher which was similar to the sequencing of SOC content.The content of CMI in S/W/W+S was significantly higher than in other treatments.The content was higher by 22.5%,47.0%,27.8%and 21.5%(P<0.05)in S/W/W+S than in C/W/W+M,Pe/W/W/W+M,Pe/W/W/C and A/A/A/A/Po/W/W/W+A,respectively.The correlation analysis showed that there was a significant positive correlation between SOC,MBC,PCM and CMI,In general,crop rotation contribute to increasing the content of SOC and its active fractions.In this study,the effect of S/W/W+S and A/A/A/A/Po/W/W/w+A was better than other treatments.(3)The results showed that the contents of STN,MBN and PNM at 0-30 cm soil layer increased by 35.0%,27.0%and 48.9%(P<0.05)in NPM,respectively,compared with those in CK after 30-yr continuous fertilization.The effect of long term fertilization on soil nitrogen fractions was more significant at 15-30 cm soil layer.The contents of STN at 15-30 cm soil layer increased by 41.3%(P<0.05)in NPM,compared with those in CK,but no difference was found in PON.In addition,MBN and PCN content in NP increased by 35.2%and 307.5%(P<0.05)respectively,compared with those in CK.The content of PNM was the only one which has big difference between NP and CK.No crop growing treatment reduced the contents of STN,MBN,PNM and PON by 15.0%,1.8%,31.3%and 33.6%compared with CK,but the reductions were not significant.Correlation analysis showed that the content of STN,MBN,PNM,PON showed significant positive correlations with each other.In general,continuous long-term fertilization can effectively increase the content of STN and its active fractions,and is helpful for maintaining soil health and nitrogen supply capacity.(4)The content of STN in the 0-30 cm soil layer was increased by 17.2%,7.9%and 15.4%(P<0.05)in A/A/A/A/Po/W/W/W+A than in Pe/W/W+M,S/W/W+S and Pe/W/W/C,respectively,after rotation.The content of MBN in Pe/W/W/C was significantly lower than in other treatments.The content of PNM was higher by 34.5%,45.8%,24.2%and 42.3%(P<0.05)in A/A/A/A/Po/W/W/W+A than in C/W/W+M,Pe/W/W+M,S/W/W+S and Pe/W/W/C.The content of PON and PON/SON in C/W/W+M was the highest in all treatments.In addition,STN changed in agreement with its fractions for all treatments.There was a positive correlation between STN and its active fractions except PON and PNM.In general,rotation measurements are helpful to increase soil nitrogen input,improve soil structure and increase nitrogen supply capacity. |
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