Coarse particulate organic matter is the primary source of mineral N in the topsoil of three beech forests

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Inglés. Forest soils contain a variable amount of organic N roughly repartitioned among particles of different size, microbial biomass and associated with mineral compounds. All pools are alimented by annual litter fall as main input of organic N to the forest floor. Litter N is further subject to mineralization/stabilization recognized as the crucial process for the turnover of litter N. Although it is well documented that different soil types have different soil N stocks, it is presently unknown how different soil types affect the turnover of recent litter N. Here, we compared the potential mineralization of the total soil organic N with that of recent litter-released N in three beech forests varying in their soil properties. Highly (15)N-labelled beech litter was applied to stands located at Aubure, Ebrach, Collelongo, which differ in humus type, soil type and soil chemistry. After 4-5 years of litter decomposition, the upper 3 cm of the organo-mineral A horizon was sampled and the net N mineralization was measured over 112 days under controlled conditions. The origin of mineralized N (litter N versus soil organic N) was calculated using (15)N labeling. In addition, soils were fractionated according to their particle size (>2000 mu m, 200-2000 mu m, 50-200 pm, <50 mu m) and particulate organic matter (POM) was separated from the mineral fraction in size classes, except the <50 mu m fraction. Between 41 and 69% of soil organic N was recovered as POM. Litter-released (15)N was mainly to be found in the coarse POM fractions >200 mu m. On a soil mass basis. N mineralization was twofold higher at Aubure and Collelongo than at Ebrach, but, on a soil N basis, N mineralization was the lowest at Collelongo and the highest at Ebrach. On a soil N (or (15)N) basis, mineralization of litter (15)N was two to four-fold higher than mineralization of the average soil N. Furthermore, the delta(15)N of the mineral N produced was closer to that of POM than to that of the mineral-bound fraction (<50 mu m). Highest rates of (15)N mineralization happened in the soil with the lowest N content, and we found a negative relationship between accumulations of N in the upper A horizon and the mineralization of (15)N from the litter. Our results show that mineral N is preferentially mineralized from POM in the upper organo-mineral soil irrespective of the soil chemistry and that the turnover rate of litter N is faster in soils with a low N content. (C) 2010 Elsevier Ltd. All rights reserved.