SOIL GROSS NITROGEN MINERALISATION AND FOREST GROWTH IN FOUR HEMIBOREAL FOREST STANDS IN SOUTHWEST SWEDEN

Abstract: Boreal and temperate forests together make up the largest terrestrial net C sink in the world. They take up carbon dioxide (CO2) from the atmosphere and store it in plant biomass and soil as they grow, making them crucial in mitigating global climate change. In addition, as climate is warming, the boreal C sink is expected to increase, provided that tree growth is not restricted by e.g. nutrient, especially nitrogen (N), supply. N is an important macronutrient for plants, and is made bioavailable through microbial N mineralisation during decomposition of soil organic matter (SOM). How well microorganisms are able to decompose SOM depends on its elemental ratio between C and N (C:N ratio), making this ratio a measure for site fertility. As a result, both gross N mineralisation and tree growth have been linked to the C:N ratio of soil and litter. Despite this, research on the link between gross N mineralisation rates and forest tree growth is lacking, while methods for estimating gross N mineralisation are more inefficient regarding time and resources compared to measuring soil C:N. Thus, this study investigates the possibility of using soil C:N as a proxy for gross N mineralisation, as well as gross N mineralisation as a driver of tree biomass production, using the 15N pool dilution technique combined with a circular plot forest inventory method commonly used in forest management practices. Field work was conducted in four Norway Spruce dominated forest stands in southwest Sweden, representing a soil fertility gradient, with mean soil C:N ratios ranging between 17-30. Across three of the four forest stands, there were clear relationships between the three parameters. Low soil C:N corresponded with high gross N mineralisation rates, which in turn correlated positively to the estimated biomass production rates. However, these correlations were only significant after accounting for soil C concentrations when calculating the rates of mineralisation. This suggests that the observed positive relationship between gross N mineralisation and tree growth could be applied both ways; that increased concentrations of bioavailable N promotes forest biomass production, while increased tree growth enables higher rates of SOM decomposition and N mineralisation following the addition of labile C to the soil though root exudation (‘priming’). Contrasting this, at the fourth site, the relationship between soil C:N and N mineralisation was close to parallel to that of the other three sites, but elevated, while the connection between gross N mineralisation and biomass growth was inverted; increasing rates of mineralisation were followed by a decrease in biomass production, with a possible explanation for this lying outside the scope of this study. Therefore, based solely on the results found in this study, soil C:N ratio cannot be used as a proxy for gross N mineralisation, nor is it possible to declare gross N mineralisation as the main driver of biomass production. Instead, to close the knowledge gap of how forest ecosystem C sequestration and the forest soil N cycle is connected, this study highlights that more research is required.