Long-term effects of slurry and anaerobically digested slurry on soil organic matter

Summary

Anaerobic digestion (AD) of animal manure may be beneficial to reduce fossil fuel usage and emission of greenhouse gases, but on farm level, the long-term effect of this technology on soil organic matter (SOM) and changes in concentrations over time is unknown. Of special concern is whether AD of the slurry will reduce SOM due to less input if easily decomposable C to the soil. A field experiment was established in 2011 to reveal effects on important soil and crop characteristics when slurry from a herd of organically managed dairy cows is treated by AD. Five treatments were compared, where non-digested slurry (US) and AD slurry were applied at two rates of total N (110 and 220 kg ha-1 y-1) and with a non-fertilised control (CON). The rate of manure application (30 vs. 60 t of manure per ha and year) affected soil nutrient concentrations but not SOM content. The average estimated total C input was 92 and 184 kg C ha-1 year-1 for the low and high rates of the AD slurry treatments, and 179 and 358 kg C ha-1 year-1 for the low and high rates of the US treatments. Yet, in our field study there was no difference in the long-term effect of ADS and US application and application rate in SOM content. The accumulated N and P fertilization during 2011-2021 was about -32 N ha-1 and 45 kg P ha-1 with low rate of both ADS and US and 385 kg N ha-1 and 78 kg P ha-1 with high rate of ADS and US. The CON had a cumulative deficit of -415 kg N ha-1 and -10 kg P ha-1. Addition of ADS or US did not have a significant impact on SOM storage, nevertheless the high rates of organic fertilizers increased the accumulated N and P fertilization during 2011-2021.