Biogas production on dairy farms is promoted as a climate change measure since it captures methane, a greenhouse gas emitted by manure, and produces renewable energy. Digestate is a byproduct of biogas production and is often used for nutrient recycling in a similar way as traditional manure. Despite having similar functions, manure and digestate have different behaviors related to nitrogen recycling and nitrogen emissions which are significant agricultural and environmental concerns of manure. This paper provides an insight into the impact of biogas production on nitrogen emissions and nitrogen recycling issues of the current dairy farming practice. Using the Substance Flow Analysis (SFA) approach, we analyzed the changes on three levels: manure handling, dairy farm, and the whole chain. Four biogas production options on a Dutch dairy farm related to types and sources of feedstocks were considered. We quantified biogas output, nitrogen fertilizer replacement percentage (%) and consequential nitrogen emissions (kgN/year; kgN/m 3 biogas produced) of these productions in comparison with the baseline of current dairy farming without biogas. We conclude that biogas production options with additional feedstocks will cause profound changes in the nitrogen recycling on dairy farms and the nitrogen emissions at the chain level. Besides, the results show that determining the optimal biogas production option can be challenging as the evaluation is highly dependent on the used nitrogen indicator and the included level of analysis. Our findings show how SFA and a multilevel perspective can give a broader understanding of environmental trade-offs. K E Y W O R D Scodigestion, dairy biogas, multiple level assessment, nitrogen emissions, nitrogen recycling, substance flow analysis (SFA)

Biogas is expected to contribute 10% of the total renewable energy use in Europe in 2030. This expectation largely depends on the use of several biomass byproducts and wastes as feedstocks. However, the current development of a biobased economy requires biomass sources for multiple purposes. If alternative applications also use biogas feedstocks, it becomes doubtful whether they will be available for biogas production. To explore this issue, this paper aims to provide an overview of potential alternative uses of different biogas feedstocks being researched in literature. We conducted a literature review using the machine learning technique “co-occurrence analysis of terms”. This technique reads thousands of abstracts from literature and records when pairs of biogas feedstock-application are co-mentioned. These pairs are assumed to represent the use of a feedstock for an application. We reviewed 109 biogas feedstocks and 217 biomass applications, revealing 1053 connections between them in nearly 55,000 scientific articles. Our results provide two insights. First, a large share of the biomass streams presently considered in the biogas estimates have many alternative uses, which likely limit their contribution to future biogas production. Second, there are streams not being considered in present estimates for biogas production although they have the proper characteristics.

The use of Dutch dairy manure for biogas production is expected to increase from 10% in 2020 to 60% in 2030. Traditionally, manure is returned to fields as a source of nutrients and organic carbon. Since a share of manure carbon is converted into biogas, this practice impacts the organic carbon input to soil (OCIS) of the dairy farms. The magnitude of the impact depends on the magnitude of the other sources of organic carbon. This impact is not considered by current advocates for large‐scale use of dairy manure for biogas while understanding it is essential because of the risk of decreasing carbon soil input. Therefore, a study of carbon flows of dairy farms that eventually contribute to the OCIS is required. In this paper, we use substance flow analysis to quantify the carbon flows on different Dutch dairy farms and investigate the impact of using manure for biogas production to their OCIS (kgC/year/ha). The farms differ in farming practices such as whether cows are grazed outside or not. The results show that about 40% of OCIS of a Dutch dairy farm comes from manure and the rest comes from its crop production. The organic carbon from manure to the soil is also limited by the need to export manure due to the Dutch nutrient regulations. The overall reduction in OCIS caused by biogas production is 10%–20%. The impact is largest in farms with no grazing. These findings provide insights into the possible trade‐offs of using manure for biogas production.