A meta-analysis of field experiments on the effect of organic matter inputs on N2O emissions in European arable land

The focus of previous research regarding the effects of applying organic matter inputs on carbon sequestration has been mostly restricted to a simple quantification of soil organic carbon stock or CO₂ balance. However, all a more comprehensive assessment of net carbon balance should also consider non-CO₂ GHG sources and sinks across the soil-plant system. The objective of this study is to synthesize results from field experiments, which explore the effects of adding different organic matter inputs to soil on N₂O emissions in Europe.

A comprehensive literature search was conducted using keywords in Web of Science, Agricola, Scopus, ScienceDirect, Google Scholar, and in the reference list of published reviews and meta-analyses. We included European experiments with diverse arable crops cultivated in monoculture or in crop rotations on mineral soils. Cumulative N₂O emissions were monitored in the fields that received either solely organic matter inputs (crop residues, green manure, livestock manure, slurry, digestate, compost, biochar or sludge) or in combination with inorganic fertilization, and inorganic fertilization served as a control. Laboratory, greenhouse studies, or field studies solely on organic soils were excluded from the database, as well as studies on permanent crops, pasture, rice fields, forest and semi-natural areas and wetlands.  

The entire database consists of 51 peer-reviewed scientific articles published between 1993 and 2022, and three unpublished items. Nitrous oxide emissions were measured at each site over a period of one month to three years in about 50 sites, in 16 European countries covering all European climate zones, from Alpine North to Mediterranean South. Annual rainfall and average annual temperature varied between 250 mm and 1300 mm, and between 4.5 ^⸰C and 19.6 ^⸰C, respectively. The impact of organic matter input in terms of quantity, nature, as well as quality was captured by C/N ratio (1 - 390) and amount of N (20 - 420 kg ha^-1).  The list of potential moderators consists of soil physical and chemical properties, such as the percentage of soil organic C, clay, silt, sand, soil texture, C/N ratio and pH for each site, and agricultural management practices, such as type and the amount of inorganic fertilization, tillage system, crop residue management and irrigation type.

The mean values of cumulative N₂O emissions for a duration, standard deviations, and sample sizes (equal to the number of replicates) for organic matter inputs and inorganic fertilization were extracted from tables and figures in articles. To perform a meta-analysis, logarithm response ratio as an index of effect size will be calculated for each study, which will then be summarized across the studies by using weighing procedure. The impact of pedo-climatic characteristics, agricultural management practices, the nature and quality of organic matter inputs on N₂O emissions will be studied. Findings of the current meta-analysis will provide a robust conclusion on which type of organic matter inputs under which pedo-climatic conditions and management practices emit negligible or significant amount of N₂O compared to inorganic fertilizers.