Effects Of Ammonium Sulfate Addition On Chemical Properties And Enzyme Activities Of Soil Aggregates In A Meadow Grassland Of Northern China

Previous studies have demonstrated positive net primary production effects with increased nitrogen(N)and mowing in Inner Mongolia meadow grasslands.However,the responses of elements and enzyme activities within bulk soil and soil aggregates to N and mowing are still unclear.A manipulative experiment with 10 treatments and five replicates,including five addition rates(0,5,10,20,50 g N m-2 yr-1),and mown or unmown,was conducted in a meadow grassland by field manipulation in Erguna ForestSteppe Ecotone Research Station since 2014.We tested the effects of N addition and mowing on soil properties and enzyme activities within bulk soil,large macroaggregates(>2000 pm),small macroaggregates(250-2000 ?m)and microaggregates(<250 ?m).The results showed that:(1)Soil pH,soil exchangeable base cations(SEB),soil acid neutralizing capacity(ANCpH4.0)decreased with ammonium sulfate addition of bulk soil and soil aggregates.Soil Electrical Conductivity(EC)and Soil acid buffering capacity(ABC)increased with ammonium sulfate addition.Mowing increased soil pH and ANCpH4.0(by up to 113.2%and 177%in bulk soil,respectively)without ammonium sulfate addition.(2)The total carbon(TC)concentration of bulk soil and soil aggregates showed no response to ammonium sulfate addition.The total nitrogen(TN),total sulfur(TS),dissolved organic carbon(DOC),total dissolved nitrogen(TDN),available sulfur(AS),Olsen-P concentration were increased with ammonium sulfate addition of bulk soil and soil aggregates.Ammonium sulfate addition and mowing alters the distribution patterns of biogenic elements within soil aggregates.The concentration of biogenic elements were significantly higher in small macroaggregates than in large macroaggregates and microaggregates.(3)The exchangeable Ca,Mg concentration were decreased with ammonium sulfate addition of bulk soil and soil aggregates.Available Fe,Mn concentration of bulk soil and soil aggregates and available Cu of soil aggregates were increased with ammonium sulfate addition.The exchangeable K,Na and available Zn concentration showed no response to ammonium sulfate addition.Mowing had no effect on soil available micronutrients,but there were significant differences among soil aggregate available micronutrients.(4)The enzymes activities of ?-glucosidase(BG),Leucine-aminopeptidase(LAP),acid phosphomonoesterase(Acid PME),aryl-sulfatase(ArS)decreased with ammonium sulfate addition of bulk soil and soil aggregates.However,the unit biomass enzyme activity of BG,LAP,N-acetyl-?-D-glucosaminidase(NAG),Acid PME increased with ammonium sulfate addition of bulk soil and soil aggregates.The main reason is that soil acidification caused by ammonium sulfate addition induced a decrease of microbial biomass,which had a positive correlation with soil enzymes activities.