Quantification of nitrous oxide emissions and emission factors from beef and dairy cattle excreta deposited on grazed pastoral hill lands

Abstract

Excreta deposition by grazing livestock is the single largest source of agricultural nitrous oxide (N2O) emissions in New Zealand (NZ). N2O emissions and emission factors (EF3; percentage of excreta nitrogen emitted as N2O) from sheep, beef- and dairy-cattle excreta deposited can vary with differences in the hill land slope and disaggregation of EF3 based on slope may therefore result in a more accurate and improved inventory of national N2O emission. However, research data on N2O emissions from dairy cattle grazing on low and medium slopes of hill country or for beef cattle on low slopes is scarce. Field trials were therefore conducted in four regions of NZ on dairy-grazed (three regions) or sheep & beef cattle-grazed hill land (one region). N2O emissions were measured following dairy and beef cattle urine and dung application on low (<12°) and medium (12–24°) sloping land in late autumn/early winter. Application of either cattle urine or dung increased N2O emissions which mostly varied with excreta type (urine vs dung), slope class (low vs medium) or region. However, there was no significant (P > 0.05) slope-effect for EF3 values for beef and dairy cattle excreta (urine or dung) from each individual region, or combined from the three dairy-grazed sites. These findings confirm our hypothesis that N2O EF3 values for excretal inputs on established dairy hill land on low and medium slopes are similar for dairy-grazed hill land. However, the sheep & beef cattle grazed sites had lower (P < 0.05) EF3 for excreta deposited on medium slope land than for excreted deposited on low slope land. Our results help the refinement of NZ’s country-specific variations in EF3 to improve the accuracy and transparency of farm-, regional- and national-scale N2O inventories.