Effects of wood-ash application on potential carbon and nitrogen mineralisation at two forest sites with different tree species, climate and N status

In the future it may become common practice to return wood-ash to forest ecosystems in order to replenish nutrients removed when brash has been extracted as a source of bioenergy. Wood-ash contains most of the nutrients that are present in the brash before its removal and burning, with the important exception of nitrogen (N). In the present paper we report measurements of CO2 emissions and net N mineralisation in the humus layer and the upper 5 cm of mineral soil 12 years after the application of wood-ash to two study sites, representing different tree species, climatic conditions and N deposition histories. We hypothesized that application of wood-ash would increase both carbon (C) and net N mineralisation rates at Torup, an N-rich site with Norway spruce (Picea abies (L.) Karst.) in the south, whereas the net N mineralisation rates would not be affected at Vindeln, an N-poor site with Scots pine (Pinus sylvestris L.) in the north, where a possible N-limitation would restrict any N mineralisation. The treatments, comprising additions of 0, 1, 3 or 6 Mg of granulated wood-ash ha−1, were applied in a randomised block design, replicated three times. Wood-ash from the same batch was used for all treatments at both sites. All factors were measured under laboratory conditions with controlled temperature and moisture levels. The potential CO2 emissions (kg ha−1 year−1 of CO2–C) at Torup were significantly higher in the 3 and 6 Mg ha−1 treatments than in the control treatment, and the highest application resulted in an extra loss of 0.5 Mg ha−1 of soil C annually as compared to the control. No such differences were detected at Vindeln. The results suggest that wood-ash application can deplete soil organic C at locations with similar characteristics (N-rich soil, spruce dominated, warm climate) as at Torup in this study.