Genetic variation of leaf litter quality and spatially varying soil organic matter control soil N dynamics in a silver birch population

Background/Question/Methods

Silver birch, Betula pendula, is one of the most common tree species in Europe. It can form large stands in the early successional stages of boreal forests and due to an effective pollen and seed dispersal, local populations have high genetic variation. We tested if such intrapopulation genetic variation of a dominant tree species can, through leaf litter fall, cause small-scale spatial variation in N dynamics in the forest soil. To find out whether silver birch N resorption and litter quality have a significant genetic component and whether the genetic differences remain stable across years, we collected green leaf and litter samples from 114 micropropagated trees, representing 19 genotypes, in 2008-2010 and measured their N%. We created a litter patch for each tree on the forest ground in 2008, to which we added new litter in the following autumns and from which we collected litter samples. We calculated the release of N for each litter patch using N% and mass loss data collected in the summer 2009, and to estimate the rate of ammonification and nitrification, we placed ion exchange resin bags under each litter patch for the summer 2011.

Results/Conclusions

Nitrogen resorption efficiency ranged from 46 to 63% among the genotypes, which led to significant genotypic variation in leaf litter N%. Genotypes that had low N resorption efficiency produced litter of high N%, which released N in the following summer, whereas genotypes of high resorption efficiency produced litter of low N%, which absorbed N in the summer. The genotype means of litter N% correlated positively across the years, and litter-N input, summed over the years, varied significantly among the genotypes, with the highest input being 40% higher than the lowest. NH₄-N quantities extracted from resin bags correlated positively with litter-N input, while NO₃-N quantities were positively related to soil organic matter (SOM) content. Our results show that the genetic variation in N resorption efficiency can create persistent variation in litter N loss and a trade-off between autumn N conservation and summer N availability among silver birch genotypes. This variation appears sufficiently large and stable to be able to generate spatial variation of contrasting soil N dynamics within local populations and may further interact with the spatially varying forest ground as the nitrification rate of litter N seems to be related to SOM rather than litter N%.