Sheep grazing increases the forage yield and reduces the yield-scaled soil CO2 emissions of sown pastures in an inland arid region

Context

Forage species are widely planted in arid and semi-arid agro-pastoral regions to increase livestock carrying capacity and thereby relieve excessive grazing pressure. The effect of grazing on forage yield and relevant soil CO₂ emissions in sown pastures converted from cropland remains unclear.

Objective

The main objective of the study was to investigate the effect of utilization methods (grazing vs. haying) on annual and perennial forage yields and soil CO₂ emissions during the growing seasons in saline cropland soils and to derive the optimum number of continuous utilization years based on the combined consideration of forage productivity and soil CO₂ emission.

Methods

Stands of annual and perennial forage species were established in a saline cropland area of northwest China in a 4-year experiment to investigate the effect of sheep grazing and haying on soil CO₂ emissions during the growing seasons. The relationships between soil CO₂ fluxes and soil properties were fitted. In addition, yield-scaled soil CO₂ emissions were used as an index to evaluate forage productivity.

Results

Grazing significantly increased the mean forage yield by 44 % and 14 % over that of haying, and significantly decreased the mean yield-scaled soil CO₂ emissions (CO₂ emission intensity, CO₂EI, kg of CO₂ kg⁻¹ of dry forage yield) by 36 % and 23 % over that of haying in the annual and perennial stands, respectively, from 2014 to 2017. Grazing did not differ from haying for cumulative soil CO₂ flux during the growing seasons in the annual forages but had 17 % lesser (P < 0.05) cumulative CO₂ flux in the perennial forages in 2015. A negative correlation (r = –0.55, P < 0.05) between soil CO₂ flux and soil water content was found in the perennial forages but not in the annual forages. Multiple linear regression results indicated that soil temperature accounted for ≥ 72 % of the variation in soil CO₂ flux. Results of the structural equation model indicated, whether annual or perennial sown pastures, that grazing had the greatest positive effect on forage yield and the greatest negative effect on soil CO₂EI.

Conclusion

Grazing mainly reduced the soil CO₂EI by increasing forage yield in annual sown pastures and by reducing soil respiration in perennial pastures. Grazing is the optimal approach to improving forage production while mitigating soil CO₂ emissions in sown pasture in continental arid regions.