Ecological Stoichiometry Characteristics Under Long-term Grass In Apple Orchards On The Loess Plateau

In order to ascertain the soil resources and nutrient limitation under the long-term growing grass in the orchard,this paper relies on the 14-year positioning experiment of the apple orchard on the Loess Plateau,taking white clover,crown vetch and orchard grass as examples.The research object explores the correlation between the stoichiometry of each component by analyzing the carbon,nitrogen and phosphorus in the leaves and roots of grass seeds,soil,microbial biomass,extracellular enzyme activities and their stoichiometric characteristics.Aiming to provide strong scientific data and theoretical support for orchard grass management.The main results are as follows:(1)Long-term growing grass significantly increased the nutrient content of the orchard soil: the treatment of orchard grass with the highest soil organic carbon content,and the leguminous white clover soil total nitrogen,dissolved organic nitrogen,dissolved organic carbon,total phosphorus,and quick-acting phosphorus content was significantly higher than Gramineae orchard grass;The carbon,nitrogen and phosphorus contents of different grass species were different: the C content of the leaves of the orchard grass was significantly higher than other treatments(p<0.05),the N and P contents of white clover roots and leaves are significantly higher than other treatments(p<0.05);The correlation analysis showed that the carbon,nitrogen,and phosphorus contents of grass seeds’ leaves and roots have a strong correlation with soil nutrients within 0-60 cm.This is the main reason for the difference in soil nutrients between different grass treatments.(2)The effects of different grass species on the soil stoichiometry are significantly different: Leguminosae white clover and crown vetch significantly reduce soil C:N and increase soil N:P at 0-60 cm,but the opposite is true for orchard grass.The stoichiometric characteristics of grass species are different: the C:N of the leaves and roots of orchard grass is the largest,and the N:P is the lowest and less than 14.In contrast,the C:N of the legume white clover and crown vetch is lower,N:P is relatively high;Correlation analysis shows that grass species stoichiometry and shallow soil stoichiometry have a certain degree of covariance,and the relationship with soil total nutrient stoichiometry is closer than active nutrient stoichiometry.(3)Long-term growing grass significantly increases the soil microbial biomass and microbial entropy,but has no effect on the microbial biomass stoichiometry: 0-40 cm microbial biomass is by white clover > crown vetch> orchard grass> conventional tillage,There was no significant difference among the treatments in 40-80 cm.The ranges of q MBC,q MBN and q MBP were 1.04%～2.07%,1.65%～4.03%,and 0.46%～2.16%,respectively.Redundant analysis showed that the main factors driving the change of microbial biomass were SOC and DOC;The homeostasis model analysis shows that the microbial community maintains a steady state or a strict steady state.(4)Long-term growing grass significantly improves the activities of multiple enzymes,reduces the enzyme N:P,and eases the microbial N restriction: the white clover treatment has the highest carbon,nitrogen,and phosphorus enzyme activities,followed by the crown vetch,in addition,both of them increase Enzyme C:N and Enzyme C:P,orchard grass are the opposite.The activity ratio of enzyme is ln(BG+CBH): ln(NAG+LAP):ln(AKP)=1: 1.4: 1.4,which is deviated from the global average of 1: 1: 1;Vector analysis of enzyme stoichiometry showed that the N restriction of long-term grasses for crown vetch and white clover was converted to P restriction,and there was no N or P restriction for orchard grass;Redundancy analysis showed that the main factors affecting enzyme activity and enzyme stoichiometric changes are SWC,q MBP,and leaf C:P.In summary,Long-term growing grass has a positive effect on soil nutrients,microbial biomass and enzyme activity,and has a significant impact on soil stoichiometry and enzyme stoichiometry,and has almost no effect on microbial stoichiometry.Leguminous white clover and crown vetch promote soil nitrogen accumulation and soil nutrient cycling,and Gramineae orchard grass contribute to occumulation of soil organic carbon.In addition,growing grass for a long time alleviates the nitrogen limitation of microorganisms,and orchard grass has the best effect.