Diet management to effectively abate N2O emissions from surface applied pig slurry

ABSTRACT Application of manure (urine and/or feces) to agricultural soils enhances emissions of gases such as nitrous oxide (N 2 O) and carbon dioxide (CO 2 ). Some minor N compounds such as hippuric acid and benzoic acid present in urine can be controlled through diet manipulation to mitigate these emissions. The aim of this study was to evaluate how the inclusion of fibrous by-products in the diet of pigs affects hippuric and benzoic acid concentrations in the excreted urine/slurry, and their possible effect on N 2 O emissions following application of these manures to soil. Slurries were obtained from growing-finishing pigs fed five contrasting diets: a conventional diet (pig slurry control, PSC); and orange pulp and carob meal at a dietary fiber level of 75 or 150 g kg −1 (OP-75; OP-150; CM-75; CM-150) and were then applied to mesocosms containing young ryegrass plants. A control treatment without slurry was also included. The N 2 O and CO 2 emissions were measured using static chambers following slurry application, alongside measurements of soil ammonium (NH 4 + ), nitrate (NO 3 − ), and dissolved organic carbon. Soils amended with slurries obtained from fibre by-products, OP and CM, decreased N 2 O emissions by 65 and 47%, respectively, compared with slurries obtained through a conventional pig diet. Benzoic acid was negatively correlated with N 2 O emission for slurries from OP diets, which had over double the hippuric acid content, and more than 1.8 times the benzoic acid content than the CM. However, this effect only occurred during the first week due to rapid degradation of this compound within soil. The possible toxic effect of benzoic acid did not appear to affect soil respiration, since a positive correlation was found. Results of a benzoic acid balance (considering both intake through feed and release through urine) indicated that the source of both acids were phenolic compounds (polyphenolic or lignin) present in the fibrous fraction. These results show that N 2 O emissions are more affected than CO 2 by to compounds within urine/faeces that can be manipulated indirectly through the diet.