A Bioassay of Nitrogen Availability in Soils Amended with Solid Digestate from Anaerobically Digested Beef Cattle Feedlot Manure

Abstract

Anaerobically digested manure (ADM) from biogas plants is often applied to cropland at rates equivalent to those for raw manure. However, the digestion process may alter nutrient release patterns, necessitating adjustment of application rates. We tested this hypothesis using five cycles of forage barley (Hordeum vulgare L.) grown in pots containing a Dark Brown Chernozem (mixed Typic Haploboroll) or a Black Chernozem (Typic Haplocryoll) amended with beef cattle (Bos taurus) feedlot manure, solid digestate (SD, that is, the separated solids fraction of ADM), pelletized SD (PSD), and urea + monoammonium phosphate (UMAP) at rates equivalent to 400 and 800 mg total N kg⁻¹ soil. Biomass yields did not differ significantly among manure, SD, and UMAP (mean = 32 g kg⁻¹ soil) and were lowest for PSD (18 g kg⁻¹ soil). Overall, apparent N recovery (ANR) from amendments over the five crop cycles decreased in the order: UMAP (53%) > manure (35%) ≈ SD (20%) > PSD (−7%) in the Dark Brown Chernozem; and UMAP (45%) ≈ manure (42%) > SD (26%) > PSD (−13%) in the Black Chernozem. Apparent net mineralization of organic N was higher in the Black Chernozem than the Dark Brown Chernozem and, in both soils, higher for manure (33% of initial total organic N) than SD (24%) and PSD (−16%). Although biomass yields were similar for manure and SD, slower organic N mineralization and lower apparent N recovery in SD-amended soils may limit the N supply capacity of SD. It may be prudent to apply SD at higher rates than manure or to supplement the SD with mineral N fertilizer to ensure adequate available N for plant uptake.