Stability of pyrochar and hydrochar in agricultural soil - a new field incubation method

The application of chars on soil offers an option to sequester carbon (C) and to improve soil fertility. Different types of chars are available as soil amendments, produced with mainly two different processes: pyrochar produced with pyrolysis and hydrochar produced with hydrothermal carbonization. However, there are few studies to assess the stability of these two char types in soils in situ at field sites. A 19-month in-situ field incubation was conducted in northern Germany. With a mini-plot method we were able to assess the decomposition dynamic of chars with few operational costs. Zinc was used as inert tracer mixed with the char in order to account mixing so that char losses could be accurately quantified. We used chars from Miscanthus (C4-plant) as feedstock with a higher d13C value than the C3 plant derived soil C. Changes in d13C value allowed the calculations of char-derived C in the soil at three sampling dates. While C derived from pyrochar did not change significantly over time, 23–30% of initial added hydrochar-C was mineralized after 19 months in-situ field incubation. There was no difference in the decomposition dynamics of the chars among the three field sites with different soil types. Moreover, we did not observe a decline in decomposition rates with time. For hydrochar the data were well fitted with a linear one-pool decay model. The average model derived mean residence times were 4 (95% confidence interval: 3–14) years for hydrochar and 60 (95% confidence interval: 16–224) years for pyrochar. Thus, while pyrochar has a higher potential for C-sequestration, faster mineralization of hydrochar compared to pyrochar showed their potential to act as a mid-term fertilizer through slow nutrient release to soils. Advantages of the Zinc method were the low price for application and analysis as well as the ability for farmers to manage their field in the course of their normal activities. However, variability in results gained from the Zinc method is not insignificant which mostly affect the calculated MRTs.