Comparison of dairy manure versus compost effects on short-term nitrogen mineralization and microbial biomass in organic annual forage production system

Since the adoption of national rules for organic agriculture in the United States, there has been a continued interest in meeting crop nitrogen (N) needs using animal manure. However, a lack of consistent information on the N supplying potential of manure creates uncertainty for farmers and often leads to overapplication, which can negatively impact both crop productivity and environmental sustainability. We investigated short-term N mineralization and microbial biomass carbon (MBC) and nitrogen (MBN) following dairy manure (DM) and its compost (DMC) application to organic annual forage production system. N mineralization was determined based on the change in mineral N during a ≤75-day in-field soil core-resin bag incubation. DM and DMC application rates were targeted to supply 123 and 56 kg potentially plant-available nitrogen (PAN) ha⁻¹ in the first and second year of application, respectively. Net N mineralization exhibited a range of 42–277 kg N ha⁻¹ in Year 1 and 31–54 kg N ha⁻¹ in Year 2 across amendment treatments and increased over the course of incubation duration in both years. The proportion of total N added that was mineralized in Year 1 was greater from DM than DMC (≤35% vs. ≤7%, respectively), suggesting the inability of DMC to supply optimal levels of N to annual forages in the first crop season. In Year 2, net N mineralization did not differ between DM and DMC, but was significantly less in the unamended control than both amendments. MBC and MBN were more influenced by seasonality and soil sampling depth than by organic amendments.