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The aim of the project "Plant available boron in Swedish forest soils" is to increase the knowledge about the content of water soluble boron in our forest soils, and describe relationships between this content and site factors, and other soil chemical variables.

Based on approximately 4000 permanent sample plots, in the Swedish National Forest Inventory, where soil sampling were done by the Swedish Forest Soil Inventory (FSI) during the years 2000-2002 we chose 210 sample plots for this study. These 210 plots are located in different parts of Sweden and have varying characteristics regarding site factors and soil chemistry. From a soil sample archive, belonging to FSI, we took subsamples from stored samples of humus layer and B horizon from these 210 sample plots that were sampled in 2000-2002. These subsamples were analyzed on plant availabe boron (BHWE) after hot water extraction (modified method after Berger & Troug, 1940).

The concentrations of BHWE in samples from the humus layer were all well above the detection limit (0.10 mg kg-1 dw) and the average (1.39 mg kg-1 dw) was slightly above the BHWE-values, that have been reported previously from a few humus samples in Swedish forest soils. The average value for the humus layer from our study was well above the average for BHWE-concentration in the topsoil from Swedish agricultural soils (0.41 mg kg-1 dw).

The BHWE-concentration in the humus layer was, on average, almost an order of magnitude higher than the corresponding concentration in the B horizon.

In 30 % of the samples from the B horizon the BHWE-concentration was below the detection limit. In the statistical analyses the values below the detection limit received a value that was 50 % of the detection limit. The mean value for the BHWE-concentration in the B horizon (0.17 mg kg-1 dw) was in line with or slightly below that previously have been reported for a few samples from the B horizons of Swedish forest soils.

In our study there was no relationship between the BHWE-concentration in the humus layer and the corresponding value in the B horizon. We also studied if the BHWE-concentrations in the humus layer or the B horizon had any relationship with different site factors or soil chemical variables. For BHWE-concentrations in the humus layer we found positive correlation with stand age and average stand height, and with increased soil moisture class in the plots. The BHWE-concentrations in the humus layer also had significantly positive correlations with the content of organic carbon, nitrogen, exchangeable base cations, titratable acidity and cation exchange capacity (CEC).

The BHWE-concentrations in the B horizon had significant negative correlations with increasing latitude, longitude, soil moisture class and thickness of the B horizon. Significant positive relationships were found between BHWE-concentrations and increasing humidity, deposition, inclination within the plot, stand height and increasing soil fertility indicated by field layer and ground layer. BHWE in the B horizon also had positive relationships with the content of organic carbon, nitrogen, exchangeable potassium, sodium and aluminum, titratable acidity and CEC. A significant negative correlation also existed between BHWE and pH in the B horizon.

The high levels of BHWE in the humus layer, and the positive relationships between BHWE and primarily carbon content and cation exchange capacity and for the humus layer also the stand age and average height suggests that the supply of plant-available boron in soil is largely controlled by the internal circulation between the forest stand, the litter and the humus layer. This should be taken into account in forest management, especially after whole-tree harvesting, as much of the plant available pool of boron in the ecosystem can be exported from the system when branches and tops are removed from the whole-tree harvested area.