Nitrous oxide and methane emissions affected by grazing and nitrogen fertilization in an integrated crop-livestock system

Abstract

Attending the global demands for primary resources without jeopardizing the environment’s sustainability has become a challenge in recent decades. The adoption of conservationist systems, such as integrated crop-livestock systems (ICLs), has been widely discussed as a promising strategy. However, there is still scarce knowledge concerning pasture management, optimal fertilizer rates, and effects of livestock on soil processes, especially in the greenhouse gas emissions in subtropical climate regions. We aimed to assess soil nitrous oxide (N2O) and methane (CH4) emissions, crop yield, and nitrogen use efficiency (NUE) in an ICLs under grazing intensities and nitrogen (N) fertilization rates. A field experiment was conducted during the winter and summer crop seasons. In winter, black oat (Avena strigosa Schreb.) pasture was subject to grazing intensities: ungrazed, moderately grazed, and intensively grazed by heifers; and fertilized with different N rates: 0, 75, and 150 kg N ha−1. In the summer, soybean (Glycine max (L.) Merr.) was grown over those winter treatments. Soil N2O and CH4emissions were measured by the static chamber method, and the fluxes and cumulative emissions were calculated. Simultaneously, we measured the crops’ productivity and the NUE. Moderate grazing did not affect N2O and CH4 emissions relative to ungrazed but reduced the NUE in black oat. Conversely, intensive grazing combined with 75 and 150 kg N ha−1 increased soil N2O emissions, mainly during the winter. Grazing and N application did not change soil CH4 fluxes, characterized by CH4 uptake most of the time. Nitrogen fertilization, regardless of the rate, increased soil N2O emissions, but this fertilization and its residual effects were crucial to achieving high black oat and soybean yields. Results indicate that black oat can reach high yields in ICLs under moderate grazing and N fertilization rates of 75 and 150 kg N ha−1, with lower N2O emissions than under intensive grazing. Our study provided a quantitative assessment of an ICLs production efficiency in the Brazilian subtropics, regarding crop yield, NUE, and the emissions of important greenhouse gases to the atmosphere. The findings suggest that the use of moderate grazing with N fertilization on black oats may determine the trade-offs between food production and the harmful effects induced by land use.