Effects of nitrogen rate and harvest time on biomass yield and nutrient cycling of switchgrass and soil nitrogen balance in a semiarid sandy wasteland

Increasing demand for renewable energy has stimulated energy crop cultivation on marginal lands with low production potential. This study investigates potential biomass yield of switchgrass (Panicum virgatum L., cv. Blackwell), plant nutrient (nitrogen, N; phosphorus, P; potassium, K) concentration and uptake, N-use efficiency (NUE) and apparent soil N balance in a semiarid rainfed sandy wasteland of Inner Mongolia, China. The aim of this study was to optimize N fertilizer rate and harvest time to produce quality biomass sustainably with minimal soil nutrient removal through harvested crop biomass. Five N rates (0, 60, 120, 180, 240 kg N ha−1) and two harvest times (late summer and early fall) in one-cut system were evaluated in 2015 and 2016. Nitrogen application of 60 kg N ha‒1 significantly increased aboveground plant (AGP) biomass yield by 14.3% vs. control only during early fall harvest in 2016, with no yield response at rates >60 kg N ha−1. AGP nutrient concentration and nutrient uptake increased by increasing N rate, except for P concentration, which decreased. Partial factor productivity (PFP) and agronomic NUE (ANUE) of applied N fertilizer decreased quadratically with N rates >60 kg N ha−1. Apparent soil N budget study revealed an N deficit (0–20 cm depth) in control while N surplus increased as N rate increased which likely boosted NO3- leaching. An early fall harvest exhibited 12.5% higher biomass yield and substantially reduced biomass N and K concentration and uptake relative to late summer harvest. These findings demonstrate that an N input not exceeding 60 kg N ha−1 would be sufficient to optimize biomass yields of Blackwell switchgrass and maintain the soil N balance on a semiarid rainfed sandy wasteland of Inner Mongolia when harvested in early fall. Therefore, N fertilization and harvest time practices should be considered simultaneously in switchgrass production systems for the continuous and sustainable biomass supply with increased feedstock quality and reduced crop fertilizer requirements.