Genetic × environmental effects on N resorption in perennial grasses for bioenergy
Nutrient recycling is an important mechanism for nitrogen (N) retention in plants and to reduce N losses and fertilizer inputs in perennial bioenergy cropping systems. N resorption in perennial plants varies greatly and is attributed to both genetic and environmental factors. Soil N availability is of particular interest, as crops will likely be fertilized, and the effects of N availability on resorption remain unclear. The timing of N resorption also has significant agronomic implications, since delaying harvest reduces biomass yields. Therefore, we set out to answer 1) how variable is the timing and magnitude of N resorption and 2) do genetic and environmental factors affect N resorption? We collected tissue samples of switchgrass (Panicum virgatum) from naturally-occurring populations across Wisconsin, with sites ranging in soil type and productivity. We also collected samples from cultivated plants, where known genotypes of both upland and lowland ecotypes were replicated in common gardens located on prime and marginal soils under fertilized or unfertilized conditions. Plant samples were used to calculate metrics of resorption efficiency (proportional reduction in plant N content relative to N content at anthesis) and resorption proficiency (plant N concentration [N] after senescence, indicating how completely the plant resorbed N).
Senesced leaf [N] of switchgrass growing in the native prairie sites ranged from 0.25% to 1.01%; resorption efficiency ranged from 52% to 79%. Soil N and senesced leaf [N] were significantly and positively correlated (R2 = 0.37, p < 0.01), but soil N and resorption efficiency were not. In the common garden experiment, upland genotypes began resorbing approximately 35 days earlier than lowlands and ultimately achieved higher resorption efficiencies, despite the remarkably high resorption rates of lowland switchgrass late in the season. N fertilization significantly reduced resorption efficiency, but had no notable effect on timing. Plants grown on the marginal site had significantly greater resorption proficiency and efficiency, but ecotype and N fertilizer had no effect on N proficiency. To summarize, it appears that soil N status may be more important than fertilizer level in determining N proficiency of switchgrass. Resorption efficiency responses to soil N were inconsistent between native populations and cultivated switchgrass. While resorption is highly variable, plants growing in more N-limited conditions generally had higher N-resorbing capabilities, indicating that fertilizing perennial bioenergy crops may reduce an important N-conserving strategy.