P availability and P leaching after reducing the mineral P fertilization and the use of digestate products as new organic fertilizers in a 4-year field trial with high P status

Historically high fertilization rates have resulted in accumulation of soil P and increased P leaching losses in several regions of northwestern Europe. Implementation of the EU Nitrates and Water Framework Directives has restricted P fertilizer use and have indirectly given rise to new organic fertilizer products from manure processing and anaerobic digestion. In a 4-year field trial with an arable crop rotation we investigated the impact of (i) zero to moderate mineral P fertilization levels (0–41 kg P ha⁻¹) and (ii) several organic fertilizer types (yearly dose of 37 kg P ha⁻¹) on the evolution of 0.01 M CaCl₂ and hot water extractable P, ammonium lactate extractable P (P–AL), P leaching, crop yield and P balances. The organic fertilizers were several processed digestate products, biothermically dried organic waste, dairy cattle slurry, and vegetable, fruit and garden compost. The results of our study indicate that a significant decrease in soil P availability (0.01 M CaCl₂ and hot water extractable P) and P leaching can already be achieved by zero-P fertilizer application during this 4-year period without any crop yield losses and with equal P export. The experimental period was too short to detect clear effects of reduced P fertilization levels (from 7 to 33 kg P ha⁻¹ year⁻¹) on P–AL. Our results showed that both the solid fraction and effluent of digestate separation, biothermically dried organic waste and two types of dried digestates may replace mineral fertilizer and cattle slurry without crop yield losses. The use of solid fraction of mechanically separated digestate leads to enhanced P availability and consequently to an increased risk for P leaching, without any benefit in terms of either crop yields or crop P uptake.