Measurements of abundances of ¹⁵N and ¹³C as tools in retrospective studies of N balances and water stress in forests: A discussion of preliminary results

• Published in: Plant and soil 1995-01, Vol.168/169 (1), p.125-133
• Main Authors: Högberg, Peter, Johannisson, Christian, Högberg, Mona, Högbom, Lars, Näsholm, Torgny, Hällgren, Jan-Erik
• Format: Article
• Language: English
• Subjects: Acid soils; Dehydration; Fertilization; Fertilizers; Forest soils; Nitrates; Nitrogen; NUTRIENT AND WATER UPTAKE BY ROOTS; Pine trees; Plants; Soil pollution
• ISSN: 0032-079X; 1573-5036
• DOI: 10.1007/BF00029321

Preliminary attempts to make retrospective studies of N balances and water stress in forest fertilization experiments by analyzing changes in the abundances of ¹⁵N and ¹³C, 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 ¹⁵N. A correlation was found between fractional losses of added N and the change in δ¹⁵N (°/oo) 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 δ¹⁵N (°/oo)) 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 ¹⁵N 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 ¹³C during photosynthesis, but this effect becomes less expressed as stomata close because of water stress. The supply of N may also affect the δ¹³C (°/oo) 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 δ¹³C (°/oo) 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 ¹³C abundance can disentangle the role of water as such from its effects on mineralization of N and flow of N.