Articles by Mats Varik

The main aim of the current study was to estimate the annual net nitrogen mineralization (NNM) flux in stands of different tree species growing on drained peatlands, as well as to clarify the effect of tree species, soil properties and litter on annual NNM dynamics. Three study sites were set up in May 2014: a downy birch (Betula pubescens Ehrh.) stand and a Norway spruce (Picea abies (L.) Karst.) stand in Oxalis full-drained swamp (ODS) and a Scots pine (Pinus sylvestris L.) stand in Myrtillus full-drained swamp (MDS). The NNM flux was estimated using the in situ method with incubated polyethylene bags. The highest value of NNM was found in stands that were growing on fertile ODS: 127.5 kg N ha^–1 yr^–1 and 87.7 kg N ha^–1 yr^–1, in the downy birch stand and in the Norway spruce stand, respectively. A significantly lower annual NNM flux (11.8 kg N ha^–1 yr^–1) occurred in the Scots pine stand growing in MDS. Nitrification was highest at fertile ODS sites and ammonification was the highest at the low fertility MDS site. For all study sites, positive correlation was found between soil temperature and NNM intensity. The difference in annual NNM between the downy birch stand and the Norway spruce stand growing on similar drained fertile peatlands was due to litter quality. The annual N input into the soil through leaf litter was the highest at the downy birch site where also the C/N ratio of litter was the lowest. The second highest N input into the soil was found in the spruce stand and the lowest in the pine stand.

• Becker, Chair of Silviculture and Forest Ecology, Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Friedrich Reinhold Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: hardo.becker@emu.ee
• Aosaar, Chair of Silviculture and Forest Ecology, Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Friedrich Reinhold Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: jyrgen.aosaar@emu.ee
• Varik, Chair of Silviculture and Forest Ecology, Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Friedrich Reinhold Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: mats.varik@emu.ee
• Morozov, Chair of Silviculture and Forest Ecology, Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Friedrich Reinhold Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: gunnar.morozov@emu.ee
• Aun, Chair of Silviculture and Forest Ecology, Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Friedrich Reinhold Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: kristiina.aun@emu.ee
• Mander, Department of Geography, Institute of Ecology & Earth Sciences, University of Tartu, Ülikooli 18, 50090 Tartu, Estonia E-mail: ulo.mander@ut.ee
• Soosaar, Department of Geography, Institute of Ecology & Earth Sciences, University of Tartu, Ülikooli 18, 50090 Tartu, Estonia E-mail: kaido.soosaar@ut.ee
• Uri, Chair of Silviculture and Forest Ecology, Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Friedrich Reinhold Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: veiko.uri@emu.ee

Silver birch (Betula pendula Roth.) is one of the main pioneer tree species occupying large areas of abandoned agricultural lands under natural succession in Estonia. We estimated aboveground biomass (AGB) dynamics during 17 growing seasons, and analysed soil nitrogen (N) and carbon (C) dynamics for 10 year period in a silver birch stand growing on former arable land. Main N fluxes were estimated and nitrogen budget for 10-year-old stand was compiled. The leafless AGB and stem mass of the stand at the age of 17-years were 94 and 76 Mg ha^–1 respectively. The current annual increment (CAI) of stemwood fluctuated, peaking at 10 Mg ha^–1 yr^–1 at the age of 15 years; the mean annual increment (MAI) fluctuated at around 4–5 Mg ha^–1. The annual leaf mass of the stand stabilised at around 3 Mg ha^–1 yr^–1. The stand density decreased from 11600 to 2700 trees ha^–1 in the 8- and 17-year-old stand, respectively. The largest fluxes in N budget were net nitrogen mineralization and gaseous N₂-N emission. The estimated fluxes of N₂O and N₂ were 0.12 and 83 kg ha^–1 yr^–1, respectively; N leaching was negligible. Nitrogen retranslocation from senescing leaves was approximately 45 kg ha^–1, N was mainly retranslocated into stembark. The N content in the upper 0–10 cm soil layer increased significantly (145 kg ha^–1) from 2004 to 2014; soil C content remained stable. Both the woody biomass dynamics and the N cycling of the stand witness the potential for bioenergetics of such ecosystems.

• Aosaar, Estonian University of Life Sciences, Institute of Forestry and Rural Engineering, Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: jyrgen.aosaar@emu.ee
• Mander, University of Tartu, Institute of Ecology and Earth Sciences, Ülikooli 18, 50090 Tartu, Estonia E-mail: ulo.mander@ut.ee
• Varik, Estonian University of Life Sciences, Institute of Forestry and Rural Engineering, Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: mats.varik@emu.ee
• Becker, Estonian University of Life Sciences, Institute of Forestry and Rural Engineering, Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: hardo.becker@emu.ee
• Morozov, Estonian University of Life Sciences, Institute of Forestry and Rural Engineering, Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: gunnar.morozov@emu.ee
• Maddison, University of Tartu, Institute of Ecology and Earth Sciences, Ülikooli 18, 50090 Tartu, Estonia E-mail: martin.maddison@ut.ee
• Uri, Estonian University of Life Sciences, Institute of Forestry and Rural Engineering, Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: veiko.uri@emu.ee