Contribution of the postharvest period to soil N2O emissions from arable Mediterranean crops

Soil nitrous oxide (N2O) emissions are a relevant contributor to the global warming potential of Mediterranean agro-ecosystems and therefore should be accurately quantified. Here, we investigated the quantitative relevance of considering postharvest N2O measurements when calculating annual soil N2O emissions from herbaceous crops (excluding rice). Data from 25 studies conducted in Mediterranean climates were compiled from a variety of crops and cropping systems, soil types, tillage practices, and nitrogen (N) fertilizer management practices. The differences between cumulative N2O emissions in the cropping period (preharvest emissions) and in the postharvest period were evaluated through a meta-analysis, as were the maximum N2O peaks in both periods and the resultant N2O emission factors (EFs). The relative contribution of the postharvest period to total cumulative N2O emissions was a mean 26%, but showed high variability. The average N2O EFs in the field experiments included in this study were 0.21% and 0.27% when not considering and when considering the postharvest period, respectively. The relative and absolute contribution of postharvest emissions was significantly greater for winter cereals than for either horticultural crops or maize. The maximum preharvest fluxes were higher than the maximum postharvest fluxes in 72% of the observations, but notable postharvest N2O peaks after soil rewetting were obtained in some studies. On average, postharvest emissions in nonfertilized plots were similar to those in fertilized winter crops. Other factors such as N source (greater relevance for organic fertilizers, particularly in summer crops), N rate, tillage intensity and soil texture significantly affected the amount of postharvest emissions. Taking measurements during the postharvest period in Mediterranean cropping systems are encouraged in studies that include winter crops, use of organic fertilizers or have unbalanced N rates. Our results may contribute to improving N2O measurement protocols and to re-evaluating N2O EFs in Mediterranean and semiarid areas.