Long-term application of biogas digestate improves soil physical properties

Rapid recent increase in biogas plants has inspired heightened interest in agricultural digestate use. This paper investigates the long-term effect of digestate application on soil physical, chemical and mineralogical properties, and compares it with the results of mineral fertiliser and compost use. The research was initiated in 2011 as a long-term field experiment at three Czech Republic sites. The field management comprised conventional tillage with 6-year crop rotation. The following five treatments were evaluated; unfertilised control, mineral fertiliser with 27% nitrogen, digestate I comprising corn silage and cattle slurry, digestate II composed of corn silage, pig slurry, farmyard manure and hay, and finally compost. Each treatment was replicated four times in the three sites, and soil samples were collected for analysis twice a year from 2018 to 2021. Statistically significant correlations were established between the stability of 1-2 mm soil aggregates and soil characteristics. The correlation coefficient was 0.37 for total organic carbon; 0.45 for total organic nitrogen; 0.36 for hot water extractable carbon; -0.54 for bulk density; 0.57 for porosity and 0.38 for water infiltration. Although treatments did not affect mineralogical properties, the soil aggregate stability was significantly increased by compost and digestate organic fertilisers; with the highest 36.47% average on plots with digestate II and the lowest 26.22% on the control and 26.1% on mineral fertilised plots. Organic fertiliser addition also significantly decreased soil bulk density and increased porosity, with a larger proportion of capillary pores. Finally, although digestate application did not improve soil organic matter as significantly as compost, its long-term use positively affected soil physical properties and water infiltration.