Hogberg P, Johannisson C, Hallgren JE
Studies of C-13 in the Foliage Reveal Interactions between Nutrients and Water in Forest Fertilization Experiments
Plant and Soil: 1993 152:207-214
Addition of N to an initially N-limited forest increases foliage biomass, demand for water and the probability of water stress. Effects of water and N on tree growth are thus compounded. The C-13 abundance of plant tissues is directly correlated with water-use efficiency (WUE), and could be used to disentangle the effect of water alone on carbon fixation. However, the C-13 abundance may also be directly influenced by changes in rates of photosynthesis related to variations in N status, and by variations in N metabolism via non-RuBisCo carboxylations, and indirectly by effects of N source on WUE. We studied the C-13 abundance of current needles from top whorls in two long-term fertilization experiments, one in Norway spruce (Picea abies Karst.) and one in Scots pine (Pinus sylvestris L.). As predicted, N fertilization increased foliage biomass and deltaC-13 (parts per thousand). In the experiment with spruce this effect on C-13 abundance was correlated with volume production and foliage biomass in a dry year, but was not seen in a wet year after 19 years of continuous annual N fertilization, which rules out the possible influences of N metabolism and changes in rates of photosynthesis. In the experiment with pine, which was at a drier site, needles from N-fertilized plots had a higher C-13 abundance in three dry years, but not significantly so in a wet year. We suggest that effects of N source (NH4+ or NO3-) on C-13 abundance are unlikely to be important under these experimental conditions. The balance between demand and supply of water should thus be the major determinant of the C-13 abundance of current needles on top whorls. This opens possibilities to conduct retrospective studies of the role of water supply in fertilization experiments.
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