Effects of nitrogen fertilizer sources and temperature on soil CO2 efflux in Italian ryegrass crop under Mediterranean conditions

We report the results of a study that aimed to assess the dynamics of total and heterotrophic soil respiration and its relationships with soil temperature or soil moisture of an Italian ryegrass haycrop managed with different nitrogen (N) fertilizer sources. The field experiment was carried out in the Nitrate Vulnerable Zone of the dairy district of Arborea, a reclaimed wetland in central-western Sardinia, Italy. This is an area characterized by sandy soils, shallow water table and intensive dairy cattle farming systems. Italian ryegrass is grown for hay production in the context of a double cropping rotation with silage maize. We analyzed the effects of N fertilizer treatments on soil carbon dioxide (CO₂) efflux, soil water content and soil temperature: i) farmyard manure; ii) cattle slurry; iii) mineral fertilizer; iv) 70 kg ha^-1 from slurry and 60 kg ha^-1 from mineral fertilizer that corresponds to the prescriptions of the vulnerable zone management plan. During the monitoring period, soil water content never fell below 8.6% vol., corresponding to approximately -33 kPa matric potential. Total and heterotrophic soil respiration dynamics were both influenced by soil temperature over winter and early spring, reaching a maximum in the first ten days of April in manure and slurry treatments. In the last 30 days of the Italian ryegrass crop cycle, total soil respiration decreased and seemed not to be affected by temperature. The analysis of covariance with soil temperature as covariate showed that average respiration rates were significantly higher under the manure treatment and lower with mineral fertilizer than the slurry and slurry+mineral treatments, but with similar rates of respiration per unit increase of soil temperature for all treatments. The average soil respiration rates were significantly and positively related to the soil carbon (C) inputs derived from fertilizers and preceding crop residuals. We concluded that: i) the fertilizer source influenced soil CO₂ efflux of the winter haycrop according to the amount of C input; and ii) that the temporal dynamics of soil respiration can be explained by soil temperature regime only in winter and early spring. These findings suggest that further studies are needed to analyze the role of soil biological factors controlling soil respiration dynamics of intensive forage cropping systems under Mediterranean conditions.