Managing Microbial Mineralization/Immobilization Dynamics to Moderate Legume Residue N Availability.

Legume cover crop residues can be an important source of on-farm fertility, but poor synchrony between net N mineralization and crop demand can lead to substantial early-season losses.  Promoting microbial immobilization through the addition of readily decomposable carbon sources can conserve legume N, but remineralization of immobilized N at appropriate timings and in sufficient amounts to benefit following crops is a challenge.  Using packed columns of loamy sand field soil, we evaluated how the incorporation of hairy vetch (Vicia villosa Roth) residue (600 g/m² equivalent) alone and in combination with two additive rates of cereal straw (1:1 and 1:2 vetch:straw) influenced the dynamics of net N mineralization during a 100 d incubation, the relative storage of N in microbial biomass at incubation end, and the re-mineralization potential of microbial N following soil disturbance, all relative to an unamended control.  Net N mineralization/immobilization was evaluated through weekly column leaching, microbial biomass C and N were measured through chloroform-fumigation incubation, and N remineralization potential was evaluated with a 28 d aerobic incubation after simulated tillage.  Total residue C:N was 12.4, 22.5, and 30.8 for 1:0, 1:1, and 1:2 vetch:straw treatments, respectively. Both levels of straw addition immobilized vetch N during the incubation, but after 48 d, the 1:1 treatments exhibited net N mineralization relative to the unamended control. While cumulative inorganic N recovery in leachate was lower for straw additions than for vetch alone, soil microbial biomass N was greater.  Correspondingly, N re-mineralization potentials following simulated tillage were on the order 1:2 > 1:1 > vetch alone > unamended control.  Microbial immobilization may hold promise for improving N synchrony within a single crop cycle, but success will depend on residue management and strategies to promote timely remineralization from microbial biomass.