Effects of increasing fire frequency on black carbon and organic matter in Podzols of Siberian Scots pine forests

Fires in boreal forests frequently convert organic matter in the organic layer to black carbon, but we know little of how changing fire frequency alters the amount, composition and distribution of black carbon and organic matter within soils, or affects podzolization. We compared black carbon and organic matter (organic carbon and nitrogen) in soils of three Siberian Scots pine forests with frequent, moderately frequent and infrequent fires. Black carbon did not significantly contribute to the storage of organic matter, most likely because it is consumed by intense fires. We found 99% of black carbon in the organic layer; maximum stocks were 72 g m(-2). Less intense fires consumed only parts of the organic layer and converted some organic matter to black carbon (> 5 g m(-2)), whereas more intense fires consumed almost the entire organic layer. In the upper 0.25 m of the mineral soil, black carbon stocks were 0.1 g m(-2) in the infrequent fire regime. After fire, organic carbon and nitrogen in the organic layer accumulated with an estimated rate of 14.4 g C m(-2) year(-1) or 0.241 g N m(-2) year(-1). Maximum stocks 140 years after fire were 2190 g organic C m(-2) and 40 g N m(-2), with no differences among fire regimes. With increasing fire frequency, stocks of organic carbon increased from 600 to 1100 g m(-2) (0-0.25 m). Stocks of nitrogen in the mineral soil were similar among the regimes (0.04 g m(-2)). We found that greater intensities of fire reduce amounts of organic matter in the organic layer but that the greater frequencies may slightly increase amounts in the mineral soil.