Sorghum biomass and quality and soil nitrogen balance response to nitrogen rate on semiarid marginal land

An understanding of biomass yield and quality response to N supply is essential to breeding and cultivation of sorghum (Sorghum bicolor (L.) Moench) for the production of bioethanol or forage. A two - year field experiment was conducted in 2013 and 2014 on semiarid marginal land in Inner Mongolia (39°10′N, 109°53′E) to determine the effects of N fertilizer rate (0, 60, 120, and 240 kg ha−1) on biomass yield and quality of sweet sorghum (GT-8) and biomass sorghum (GN-11), and on soil conditions. Results indicated that (1) biomass yield, leaf/stem ratio, and crude protein (CP), protein yield, theoretical ethanol yield (TEY), and soil net NO3−-N accumulation of both sorghum varieties in 2013 and 2014 significantly increased with the increasing N fertilizer rate from 0 to 240 kg ha−1, whereas dry matter ratio and relative feed value (RFV) varied inversely with the N application rates. (2) Compared to biomass sorghum (GN-11), sweet sorghum (GT-8) demonstrated significant enhancement of forage quality components including CP, dry matter digestibility (DMD), dry matter intake (DMI), total digestible nutrients (TDN), net energy for lactation (NEL), and RFV, with stems exhibiting higher forage quality than leaves. Biomass sorghum (GN-11) exhibited better performance for bioenergy production, with an average biomass yield of 7.9 t ha−1 and TEY of 3046.7 L ha−1 averaged across all N rates. (3) Annual average amount of net NO3−-N accumulation in 0-90 cm layers of two years significantly increased when fertilizer N exceeded 120 kg N ha−1. In short, N fertilizer of 60 kg ha−1 is recommended to sweet sorghum for the forage production and 120 kg N ha−1 to biomass sorghum for bioenergy feedstock sustainability, respectively, meanwhile to prevent the soil degradation on semiarid marginal lands.