Hogberg P, Johannisson C, Hog M, Nasholm T, Hallgren JE
Measurements of Abundances of N-15 and C-13 as Tools in Retrospective Studies of N Balances and Water-Stress in Forests - a Discussion of Preliminary-Results
Plant and Soil: 1995 169:125-133
Preliminary attempts to make retrospective studies of N balances and water stress in forest fertilization experiments by analyzing changes in the abundances of N-15 and C-13, respectively, are discussed. Most evidence is from the Swedish Forest Optimum Nutrition Experiments, which have been running for two decades. Annual additions of N have been given either alone or in combination with other elements, notably P and K, every third year. Processes leading to loss of N, e.g. volatilization of ammonia, nitrification followed by leaching or denitrification, and denitrification alone, discriminate against the heavy isotope N-15. A correlation was found between fractional losses of added N and the change in delta(15)N (parts per thousand) during 19 years in current needles in a Scots pine forest, irrespective of source of N. Isotope effects were larger on urea than on ammonium nitrate plots (2 as compared to 9 delta(15)N (parts per thousand)) because of ammonia volatilization and higher rates of nitrification. They developed gradually over time, which opens possibilities to analyse the development of N saturation. However, the analysis may be confounded by shifts in N-15 abundance of fertilizer N. In another trial, N isotope effects could be seen in both plants and soils 10 years after the last fertilization, they were smaller in soils because of a large pretreatment memory effect, but we expect them to persist there for decades. The enzyme RuBisCo discriminates strongly against the heavy isotope C-13 during photosynthesis, but this effect becomes less expressed as stomata close because of water stress. The supply of N may also affect the delta(13)C (parts per thousand) via effects on rates of photosynthesis, and the source of N may have an influence directly via non-RubisCo carboxylations, and indirectly via effects on water use efficiency. In a trial with Norway spruce, the effect of N fertilization on the delta(13)C (parts per thousand) of current needles was strongly correlated with production and weakly so with foliar biomass a dry year, but not a wet year. This suggested that these variations are primarily related to induced differences in the balance between supply and demand for water. Hence, studies of C-13 abundance can disentangle the role of water as such from its effects on mineralization of N and flow of N.
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