The Effects Of Environmental Factors And Leaf Mineral Elements On Plant Or Soil Nitrogen Isotope Composition

Nitrogen(N)cycling is a fundamental biogeochemical process.At present,the global changes such as climate wariming,rainfall redistribution,nutrient imbalance and the increase of atmospheric nitrogen deposition were accelerated by human activities.These changes would have strong influences on the processes of the terrestrial N cycling.Thus,studying the effects of global changes on N cycling and revealing the response mechnisms of N cycling to global changes could provide the theory basis for forecasting the N cycling patterns in the future.Nitrogen isotope ratios(δ15N)in plants and soil could act as an integrator of terrestrial N cycling,and could be used to revealing the response patterns of N cycling to global changes.Thus,in order to illustrate the response of terrestrial N cycling to global changes,the present study explored the effects of temperature,precipitation,edaphic traits,leaf mineral elements contents and atmospheric N deposition on leaf δ15N or soil δ15N.The main findings were as follows:(1)On Mount Tianshan,leaf δ15N,soil δ15N and △δ15Nleaf-soil(△δ15Nleaf-soil=leaf δ15N-soil δ15N)on the south slope were greater than that on the north slope,this suggested that the south slope has higher soil N transformation rates and soil N availability than the north slope and indicated that the N cycling was different between the south slope and north slope on which the climate and environment conditions were obviously different.Besides,this study observed that the significant influential factors of leaf 815N were temperature,precipitation,leaf N,leaf C/N,soil moisture and silt/clay ratio on the north slope,whereas on the south slope only leaf C/N was related to leaf δ15N.The significant influential factors of soil 815N were temperature,precipitation,soil moisture and silt/clay ratio on the north slope,whereas on the south slope,precipitation and soil moisture exerted significant effects.Precipitation exerted contrary effects on soil δ15N between the two slopes.These results suggested that the relationships between leaf and soil δ15N and environmental factors are localized.(2)In areas along the 400 mm isopleth of mean annual precipitation in China,leaf δ15N and soilδ15N increased with MAT in areas with MAT<0.6 ℃;while leaf 815N and soil 815N decreased with MAT in areas with MAT>0.6 ℃.Furthermore,the variations in leaf 815N and soil 815N with MAT were independent on vegetation types and soil types.Leaf δ15N correlated positively with soil δ15N.After accounting for the effects of soil δ15N on leaf 815N,the residual leaf 815N showed no trend with MAT both for sites with MAT<0.6 ℃ or MAT>0.6 ℃.This suggested that the mechanisms of variations in leaf 815N with MAT were consistent with that of vairaitions in soil δ15N with MAT.In areas with MAT<0.6 ℃,15N-enrichment in soil with temperature was associated with soil N losses induced by the decrease in soil C:N ratio and N concentrations and the increase in soil pH with temperature.In areas with MAT>0.6 ℃,the mechanisms for the 15N-depletion in soil with temperature needed further research.(3)Leaf δ15N was observed to correlate highly with the concentrations of N,K,Ca,Mg,P,Na,Zn,Fe and A1 in leaves,while not correlate with Mn,B and Cu.All of these mineral elements accounted for 52.0%variations in leaf δ15N except B,Cu and Na,and the influences of leaf N,K and Mg on leaf δ15N were bigger than other elements.Leaf mineral elements concentrations were related with leaf N contents except leaf Mn and Na.In addition,after accounting for the effects of leaf N concentrations on leaf δ15N,the correlations between the residual leaf δ15N and leaf mineral elements concentrations became weaker relative to the correlations between leaf δ15N and leaf mineral elements concentrations.(4)The atmospheric nitrogen deposition in Baiwangshan is higher than that in Mount Dongling in Beijing,but leaf N contents and soil N contents showed no differences between Baiwangshan and Mount Dongling,this suggested that the N sources of plants grown on the two regions were mainly derived from soil N pool.The original leaf δ15N was significantly lower in Baiwangshan than Mount Dongling.After accounting for a predicted value assumed to vary with temperature,the corrected leafδ15N was obviously more negative in Baiwangshan than Mount Dongling.Thus,this suggested the necessity of temperature correction in revealing the effect of N deposition on leaf δ15N.Temperature,soil N sources and mycorrhizal fungi types could not explain the difference in leaf 815N between the two regions;therefore,atmospheric N deposition should be responsible for this difference and had a negative effect on plant δ15N.