Effects Of Root Growth On The Litter Decomposition Of A Semi-arid Grassland In The Loess Plateau Under The Conditions Of Warming,precipitation And Nitrogen Addition

The Loess Plateau in Northwest China is the largest loess distribution region in the world.It has a total area of about 620,000 square kilometers,of which grassland area accounts for about 200,000 square kilometers.Due to the harsh environmental conditions and long-term intensive exploitation by humans,the grassland ecosystems in this region has become very fragile and sensitive to future climate and environmental changes.Since the last century,the rapid development of industry and agriculture has been leading to global warming,changes in rainfall patterns and atmospheric nitrogen(N)deposition.Changes in soil environmental factors caused by these global change factors can affect decomposition by directly impacting microbial growth and activities,and indirectly by modifying the relationship between plant roots and soil microorganisms.However,the direction and magnitude of these effects,as well as the underlying mechanisms,remain largely unclear.Taking advantage of a multi-factor manipulation experiment in a Loess semi-arid grassland(average annual rainfall 400 mm),we examined root litter decomposition in the absence or presence of alive roots under different simulated global change conditions.The field global change experiment manipulated three climate-related factors,namely temperature(the control vs.warming),precipitation(precipitation reduction,the control and precipitation increase)and nitrogen level(the control vs.nitrogen addition),leading to a total of 12 treatment combinations(2×3×2).Litterbags made from nylon fabric with different mesh size(1 mm to allow the penetration of fine roots or 30μm to prevent root penetration)were used to investigate the effects of plant roots on litter decomposition.In May 2017,litterbags were placed into the soil in each field plot,and litter and soil were collected in September 2017.Litter mass remaining,soil enzyme activity and soil microbial respiration were determined.The soil microbial biomass carbon(MBC)was determined by chloroform fumigation.The soil microbial community was characterized through analyzing the composition of phospholipid fatty acid(PLFA).The main results of this study are as follows:1.Warming significantly inhibited litter decomposition and soil microbial respiration rate,and reduced phenoloxidase and peroxidase activities,but did not affect the structure of the soil microbial community;1.Rainfall increase promoted litter decomposition,and increased soil microbial respiration rate,and phenoloxidase and peroxidase activities;however,changes in rainfall had no significant effect on the soil microbial community;2.Nitrogen addition significantly altered the structure of the soil microbial community,significantly reduced fungal and bacterial abundances and microbial respiration,but did not significantly affect litter decomposition;3.Presence of alive plant roots increased soil MBC,but reduced the activities of C-cycling related enzymes(i.e.,α-glucosidase,β-glucosidase,cellobiohydrolase andβ-xylosidase),suggesting that microbes preferentially used labile carbon released from roots;4.Roots suppressed the decomposition of root litter materials,likely exacerbating water limitation on microbial activities.Rainfall increase amplified the inhibitory effect of plant roots on the litter decomposition.