Reviewing the N-gap in livestock manure systems: Direct and indirect methods for measuring N losses and perspectives for quantifying N₂ emission

The main objectives of this review were to: (1) review different methods/techniques to assess gaseous N-losses from manure (2) review N-gaps, attributed to dinitrogen loss as the difference between directly measured N compounds summed as total N loss and indirectly measured N loss through a mass balance in livestock manure systems, and (3) provide approaches to close the N-gap. In literature, N-gaps run up to 80% of total N loss, this undermines N emission assessments and leaves a huge part of the emission unexplained. However, studies that measure N-gaps are scarcely available or are limited in their evaluation, hence more study is needed. Three approaches are introduced to research N-gaps: (1) measure N₂ through a suggested Gas Flow Soil Core (GFSC) technique and compare the sum of all measured N losses with the indirect method, (2) assume N₂ loss as being the N-gap and (3) include N₂ as an estimate based on ratios from literature. In a hypothetical example for poultry manure, assumed values for measurement error of 50% and variance due to physical differences between the experimental units of 50% led to a total standard deviation of 131% in the N-gap. Variance of N-gap was reduced with 80% point when assuming 16 vessels compared to single vessel. Using literature-based-ratios to estimate losses of N compounds led to variation of N-gap from 0.06% initial N overestimation to 26% of initial N underestimation. Future research should address this variance and apply methods to measure N₂ to close N-gaps.