Effects Of Sulfur Addition On Plant Litter Decomposition In A Meadow Steppe

Litter decomposition is an important ecological process for the assessment of the carbon sequestration potential and nutrient cycling of soil-plant system in grassland ecosystem.Litter decomposition is regulated by climate,litter quality,soil biological and abiotic properties.Sulfur(S)deposition and its effects on terrestrial ecosystems is an important environmental issue affecting soil p H and nutrient availability and thus changing litter mass,decomposition rate and nutrient dynamics.Based on S addition experiment in the meadow steppe,we systematically studied the effects of S addition on litter decomposition characteristics of four common species including Leymus chinensis(grass),Stipa krylovii(grass),Carex duriuscula(sedge)and Pulsatilla turczaninovii(forb).By measuring litter initial chemical properties including(non-structural carbohydrates,total carbon(C),nitrogen(N),phosphorus(P),calcium(Ca),magnesium(Mg),iron(Fe)and manganese(Mn)),litter mass,decomposition rate and nutrient dynamic,the effects of soil abiotic and microbiological properties on litter decomposition were analyzed.The research results were as follows:1)Sulfur addition significantly decreased soil p H values and increased soil electric conductivity.Sulfur addition resulted in the accumulation of soil ammonium and the decrease of soil nitrate,and led to the loss of exchangeable calcium and magnesium ions.Soil available phosphorus,DTPA-Fe and DTPA-Mn were increased along with increasing S addition.Soil microbial biomass carbon and nitrogen decreased significantly with the increase of S addition,which significantly inhibited the activities of carbon,nitrogen and phosphorus acquiring enzymes.Soil acidification decreased carbon: nitrogen acquiring enzymatic ratios and increased the ratio of nitrogen: phosphorus acquisition enzyme,indicating that microbial N requirement increased.Sulfur addition significantly increased fungi: bacteria ratio which indicated fungi could adapt to low p H soil.2)After three years of decomposition,S addition significantly inhibited litter mass loss in the late-stage of decomposition.S addition reduced litter decomposition rate of different species which were faster for the forb and sedge species than for the two grasses.The litter decomposition rates of different species were positively correlated with soil p H,microbial biomass carbon and nitrogen,acid phosphomonesterase,but negatively correlated with soil available phosphorus,DTPA-Fe,soil nitrogen and phosphorus acquiring enzyme and fungi/bacteria(F/B).The litter decomposition rate was regulated by litter initial chemical properties,and is positively correlated with the litter initial N,P and non-structural carbohydrates(soluble sugars and starches)contents,and negatively correlated with litter initial carbon and stoichiometric ratio.3)During the three-year decomposition,litter C,soluble sugars and starches of different species were continuously released.Litter N,P and S were first released and then immobilized of three species,but were continuously released for Pulsatilla turczaninovii litter.Litter Ca、Mg were released over time,but litter Fe was immobilized,litter Mn was first immobilized and then released.Sulfur addition significantly inhibited the release of C,N and S,which was mainly affected by soil p H and nutrient availability.Sulfur addition promoted the release of litter P,Ca,Mg,Fe and Mn,which were mainly regulated by the decrease of soil microbial biomass carbon and nitrogen and soil enzymes and the imbalance of enzyme stoichiometric ratio caused by S deposition and consequent soil acidification.The results can provide a necessary basic data set for nutrient cycling in the semiarid meadow steppe of northern China under the background of sulfur deposition,and provide a theoretical basis for the systematic study of the response of plant litter to atmospheric sulfur deposition in the grassland ecosystem.