Appropriate N fertilizer addition mitigates NO emissions from forage crop fields.

Forage crops are widely cultivated as livestock feed to relieve grazing pressure in agro-pastoral regions with arid climates. However, gaseous losses of soil nitrogen (N) following N fertilizer application have been considerable in response to the pursuit of increased crop yield. A two-year experiment was carried out in a typical saline field under a temperate continental arid climate to investigate the effect of N application rate on NO emissions from barley (Hordeum vulgare L.), corngrass (Zea mays × Zea Mexicana), rye (Secale cereale L.), and sorghum-sudangrass hybrid (Sorghum bicolor × Sorghum sudanense). The dynamics of NO emissions, hay yield, and crude protein (CP) yield were measured under four N application rates (0, 150, 200, and 250 kg ha) in 2016 and 2017. An NO emission peak was observed for all crop species five days after each N application. Cumulative NO fluxes in the growing season ranged from 0.66 to 2.40 kg ha and responded exponentially to N application rate. Emission factors of NO showed a linear increase with N application rate for all crop species, but the linear slopes significantly differed between barley or rye and corngrass and sorghum-sudangrass hybrid. The hay and CP yields of all forage grasses significantly increased with the increase of N application rate from 0 to 200 kg ha. Barley and rye with lower hay and CP yields showed higher NO emission intensities. The increased level of NO emission intensity was higher from 200 to 250 kg ha than from 150 to 200 kg ha. At N application rates of 200 and 250 kg ha, CP yield had a significantly negative correlation with cumulative NO emission and explained 50.5% and 62.9% of the variation, respectively. In conclusion, ~200 kg ha is the optimal N rate for forage crops to minimize NO emission while maintaining yield in continental arid regions.