Softwood biochars work as long-term slow-release fertilizers: results from 8 year field studies

Since biochar can persist in soil for several thousands of years, the single application of biochar can be expected to produce beneficial agricultural and environmental effects for the long-term. However, there is limited information available about the long-term effects of biochar particularly from the boreal regions. We studied the effects of softwood biochar on plant nutrient uptake dynamics for the first eight years from the two ongoing long-term field experiments with contrasting soil properties: fine textured Stagnosol and coarse textured Umbrisol in Helsinki. In addition, we also investigated the effects of biochar on greenhouse gas emissions and soil physical properties in the long-term.

 

AgriChar team sampling plant samples in 2018

 

 The results were not optimistic since only limited effect of biochar was observed in terms of plant aboveground biomass yield over the eight years in these two fields. However, there were two exceptional growing seasons (barley in 2013 and peas in 2016) when biochar increased plant biomass yield in Stagnosol with higher fertilization rate. We hypothesized that the increased plant biomass yield by biochar in these two cases were the result of pre-crop effect because nitrogen fixing legume or grass were planted in the previous years in both cases. 

The most notable effect of biochar observed was increased plant K content in Umbrisol and decreased plant contents of Al and Na in Stagnosol.  This suggests that biochars can function as a K source in K poor soils and may relieve the salinity and Al toxicity stress of plants. We also studied how the effect of biochar on plant nutrient content change over the time. We found that biochar decreased the plant Mn content while increased the plant Cd and Mn contents over the time in Umbrisol. Similarly, the plant contents of several elements were increased by biochar over the time in Stagnosol. 

 

We found biochar had potential to affect the N₂O emission even after seven years of the biochar application. Whereas, the beneficial effects of biochar on soil physical properties in the initial years disappeared in the long-term.

Check out more: Kalu, S., Simojoki, A., Karhu, K., Tammeorg, P. (2021). Long-term effects of softwood biochar on soil physicalproperties, greenhouse gas emissions and crop nutrient uptake in twocontrasting boreal soils. Agriculture, Ecosystems & Environment. 316: 107454.