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

Abstract. Seasonal changes in nitrogen (N) pools, carbon (C) content and natural abundance of ¹³C and ¹⁵N in different tissues of ryegrass plants were investigated in two intensively managed grassland fields in order to address their ammonia (NH₃) exchange potential. Green leaves generally had the largest 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 between NH₄⁺ 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 peaks in Γ were observed during spring and summer in response to fertilization and grazing. These peaks were associated with high NH₄⁺ rather than with low H⁺ concentrations. Peaks in Γ also appeared during the winter, coinciding with increasing δ¹⁵N values, indicating absorption of N derived from mineralization of soil organic matter. At the same time, δ¹³C values were declining, suggesting reduced photosynthesis and capacity for N assimilation. δ¹⁵N and δ¹³C 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 NH₃, while senescent leaves have a large potential for NH₃ emission. However, management events such as fertilisation and grazing may create a high NH₃ emission potential even in green plant parts. The obtained results provide input for future modelling of plant-atmosphere NH₃ exchange.