| Based on the long-term positioning experiment in Rain-fed Agricultural Area of the Loess Plateau,the diversity of soil microbial functions in alfalfa and crop-grass rotation system with different growth years was studied.The experiment of different growth years included 6a,13 a and 15 a alfalfa.The experiment of crop-grass system included continuous cropping of Alfalfa(LC),alfalfa-leisure(LF),alfalfa-leisure-wheat(LFW),alfalfa-leisure-maize(LFM).Alfalfa-potato(LP),alfalfa-millet(LM).By discussing the soil organic carbon,microbial biomass carbon,total nitrogen,soil microbial biomass nitrogen,and activities related to carbon-nitrogen transformation in alfalfa and its crop-grass rotation systems with different growth years,the microbial mechanism of soil quality changes and the characteristics of microbial functional diversity in alfalfa fields with different growth years and under different crop-grass rotation modes were revealed in order to reveal the differences of microbial functional diversity in different growth years.In order to enrich the basic theory of grassland farming system and provide scientific basis for the management and sustainable utilization of alfalfa grassland and the stable yield of dry land crops in semi-arid rain-fed area of the Loess Plateau,the evolution law of soil fertility under the limited alfalfa and Grain-grass rotation mode was studied.The results are as follows:(1)There were significant differences in soil organic carbon and microbial biomass carbon among alfalfa growth years.Soil organic carbon was 13a>6a>15a,the growth years of13 was 0.84% higher than the years of 6,the growth years of 13 was 82.44% higher than the years of 15.And microbial biomass carbon was 6a>13a>15a,the growth years of 6 was37.41% higher than the years of 13,the growth years of 6 was 54.44% higher than the years of15.Under different crop-grass rotation systems,the soil total organic carbon content showed significant difference between treatments in the whole 0~60 cm soil profile.Compared with LC treatments,the organic carbon decreased by 1.67%~12.77% and microbial biomass carbon decreased by 0.11%~23.09%.(2)With the increase of soil depth,the contents of total nitrogen(TN)and microbial biomass nitrogen(MBN)in alfalfa soils showed a downward trend.There were significant differences between total nitrogen and microbial biomass nitrogen in alfalfa soils under different growth years.It showed that 13a>15a>6a in the range of 0~60cm of total nitrogen,and it showed that 6a>13a>15a in the range of 0~60cm.(3)Alkaline phosphatase,urease and invertase activities decreased with the prolongation of alfalfa in diffeent soil depth,and there were significant differences among different growth years.(4)It showed that microbial metabolism decreased significantly with the deepening of soil layer in biolog analysis.In the 0~60 cm soil depth,Average well color development(AWCD)showed that growth years of 6 was significantly higher than in 13 and 15 years treatments,while AWCD decreased significantly with the increase of soil depth.The treatment of LC and LF in the Grain-grass system were significantly higher than those of other treatments,reflecting a strong ability to utilize carbon sources.In the 30~60 cm soil depth,the treatment of LFM were the highest,while those of LC treatment were higher than those of LP and LM treatment,and lower than those of LFM,LF and LFW treatments.(5)There were no significant differences in soil microbial diversity index(H),evenness index(U)and carbon source utilization richness index(S)between different growth years in0~30cm of alfalfa soil;under 30~60cm of alfalfa soil depth,in soil microbial diversity index(H)and carbon source utilization richness index(S),the growth years of 6 were the highest,with significant differences to the growth years of 15.In crop-grass rotation system,it showed that high AWCD of soil microbial community,Shannon index(H),evenness index(U)and richness index(S)of microbial community in LC and LF treatments,which reflected strong carbon utilization ability.(6)Principal component analysis showed that the utilization characteristics of microbial carbon sources in alfalfa soils varied in different growth years.Alfalfa cultivation reduces the metabolic intensity of soil microorganisms to carbohydrates and amino acids,and improves the utilization of carboxylic acids,polymers,phenolic acids and amines.Soil microorganisms in alfalfa fields can also make good use of amino acids.Compared with alfalfa continuous cropping,different cropping systems improved the utilization of carboxylic carbon sources by microorganisms and reduced the metabolic intensity of amino acid carbon sources.(7)Carbohydrates and carboxylic acids can be used as sensitive carbon sources to distinguish microbial utilization in alfalfa soils of different growth years.Carbohydrates and amino acid can be used as sensitive carbon sources to distinguish the differences of different planting systems in rain-fed agricultural areas on the loess plateau. |
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