Abstract
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.
Highlights
Nitrogen (N) is a constituent of compounds such as amino acids, proteins, RNA, DNA and several phytohormones and is thereby an essential macroelement for plants
Ammonia emissions generally occur in intensively managed agricultural ecosystems, while semi-natural ecosystems are more likely to act as NH3 sinks (Sutton et al, 1993, 1994; Schjoerring et al, 1998, 2000)
During late autumn and winter, senescent leaves and litter accounted for a large proportion of the aboveground biomass, while outside this period green leaves dominated the ryegrass canopy (Fig. 2)
Summary
Nitrogen (N) is a constituent of compounds such as amino acids, proteins, RNA, DNA and several phytohormones and is thereby an essential macroelement for plants. Nitrogen is considered to be the nutrient which most widely limits the growth of vegetation in terrestrial ecosystems (Vitousek and Howarth, 1991; Xia and Wan, 2008). Ammonia (NH3) is an important component of this increase and is becoming recognized as a reactive N pollutant in the atmosphere with impacts on a series of ecological problems such as eutrophication, acidification, alteration of biodiversity and global warming (Sutton et al, 1998; Dragosits et al, 2002; Krupa, 2003; Allen et al, 2011). NH3 exchange between vegetated surface and atmosphere is an important process in the N cycle and a key uncertainty in quantifying atmospheric NH3 and N depositions to terrestrial ecosystems (Pilegaard et al, 2009). Ammonia emissions generally occur in intensively managed agricultural ecosystems, while semi-natural ecosystems are more likely to act as NH3 sinks (Sutton et al, 1993, 1994; Schjoerring et al, 1998, 2000)
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