Appropriate N fertilizer addition mitigates N2O emissions from forage crop fields

Abstract

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 N2O 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 N2O emissions, hay yield, and crude protein (CP) yield were measured under four N application rates (0, 150, 200, and 250 kg ha−1) in 2016 and 2017. An N2O emission peak was observed for all crop species five days after each N application. Cumulative N2O fluxes in the growing season ranged from 0.66 to 2.40 kg ha−1 and responded exponentially to N application rate. Emission factors of N2O 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−1. Barley and rye with lower hay and CP yields showed higher N2O emission intensities. The increased level of N2O emission intensity was higher from 200 to 250 kg ha−1 than from 150 to 200 kg ha−1. At N application rates of 200 and 250 kg ha−1, CP yield had a significantly negative correlation with cumulative N2O emission and explained 50.5% and 62.9% of the variation, respectively. In conclusion, ~200 kg ha−1 is the optimal N rate for forage crops to minimize N2O emission while maintaining yield in continental arid regions.