Seasonal variation in nitrogen pools

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Seasonal changes in nitrogen (N) pools, carbon (C) content and natural abundance of 13 C and 15 N in different tissues of ryegrass plants were investigated in two intensively managed grassland fields in order to address their ammonia (NH 3) exchange potential. Green leaves generally had the largest total N concentration followed by stems 5 and inflorescences. Senescent leaves had the lowest N concentration, indicating N reallocation. The seasonal pattern of the Γ value, i.e. the ratio between NH + 4 and H + concentrations, was similar for the various tissues of the ryegrass plants but the magnitude of Γ differed considerably among the different tissues. Green leaves and stems generally had substantially lower Γ values than senescent leaves and litter. Substantial 10 peaks in Γ were observed during spring and summer in response to fertilization and grazing. These peaks were associated with high NH + 4 rather than with low H + concentrations. Peaks in Γ also appeared during the winter, coinciding with increasing δ 15 N values, indicating absorption of N derived from mineralization of soil organic matter. At the same time, δ 13 C values were declining, suggesting reduced photosynthesis and 15 capacity for N assimilation. δ 15 N and δ 13 C values were more influenced by mean monthly temperature than by the accumulated monthly precipitation. In conclusion, ryegrass plants showed a clear seasonal pattern in N pools. Green leaves and stems of ryegrass plants generally seem to constitute a sink for NH 3 , while senescent leaves have a large potential for NH 3 emission. However, management events such as fer-20 tilisation and grazing may create a high NH 3 emission potential even in green plant parts. The obtained results provide input for future modelling of plant-atmosphere NH 3 exchange.