Seasonal variation in nitrogen pools and 15N/13C natural abundances in different tissues of grassland plants

[摘要] Seasonal changes in nitrogen (N) pools, carbon (C) content and natural abundance of¹³C and ¹⁵N in different tissues of ryegrass plants wereinvestigated in two intensively managed grassland fields in order to addresstheir ammonia (NH₃) exchange potential. Green leaves generally had thelargest total N concentration followed by stems and inflorescences.Senescent leaves had the lowest N concentration, indicating N re-allocation.The seasonal pattern of the Γ value, i.e. the ratio betweenNH₄⁺ and H⁺ concentrations, was similar for the varioustissues of the ryegrass plants but the magnitude of Γ differedconsiderably among the different tissues. Green leaves and stems generallyhad substantially lower Γ values than senescent leaves and litter.Substantial peaks in Γ were observed during spring and summer inresponse to fertilization and grazing. These peaks were associated with highNH₄⁺ rather than with low H⁺ concentrations. Peaks in Γalso appeared during the winter, coinciding with increasing δ¹⁵Nvalues, indicating absorption of N derived from mineralization ofsoil organic matter. At the same time, δ¹³C values weredeclining, suggesting reduced photosynthesis and capacity for Nassimilation. δ¹⁵N and δ¹³C values were moreinfluenced by mean monthly temperature than by the accumulated monthlyprecipitation. In conclusion, ryegrass plants showed a clear seasonalpattern in N pools. Green leaves and stems of ryegrass plants generally seemto constitute a sink for NH₃, while senescent leaves have a largepotential for NH₃ emission. However, management events such asfertilisation and grazing may create a high NH₃ emission potential evenin green plant parts. The obtained results provide input for futuremodelling of plant-atmosphere NH₃ exchange.

[学科分类] 地球化学与岩石