There is an urgent need for increased resource efficiency in farming systems to make the transition to more circular agro-ecosystems, which depend less on external inputs and conserve natural resources (soil, water, biodiversity), especially in the context of climate change. FAB refers to the application of farm management practices that enhance and exploit elements of biodiversity for their role in providing ecosystem services (e.g., pollination, biological pest control, soil erosion control, water retention) and ecosystem functions, and in supporting sustainable agricultural production and human well-being [160,161]. The researchers found that the outcomes of implementing FAB measures were largely positive, with a number of mixed effects. Positive outcomes include improvements in above- and below-ground biodiversity (e.g., [43,47,58,63,64,88,89,127]), improvements in soil structure [57], and the diversity of root architecture that can reduce soil compaction [48,49], improve water conservation and reduce risks of flooding [46]. Many of the measures lead to reductions in fertiliser application [60], which has direct impacts, e.g., on water quality, and indirect impacts, e.g., reductions in GHG from their production. Many of the measures that improve soil erosion also positively benefit water quality. Mixed effects include impacts on GHG emissions, with reductions in some GHGs offset by increased N₂O emissions [39,40,52,53,77] or mixed impacts on soil organic carbon (SOC) [25,26]. There are also large uncertainties in some areas (Figure 1), particularly surrounding the impact of FAB measures on yield, with a range of both positive and negative results being reported. This is predominantly due to the metrics, location and timeframe reviewed in each study and complex interactions between management and context. Some effects on yield could be short-term, and the longer-term benefits of a more sustainable system may exceed yield loss with time [35,103,107,119,120,136]. There may be issues with the methods used to calculate yield, e.g., not using a systems approach to calculate productivity in agroforestry [15]. Evidence is also currently limited in other areas (Figure 1); for example, organic matter input and reduced tillage are often implemented to enhance soil quality [35], yet effects on biodiversity are less well studied. There may be negative effects on biodiversity from fertilisation [24], regardless of the source (i.e., organic vs. non-organic), and mixed effects on soil microbial diversity from biochar and biosolids [83].

Although the possibilities of FAB measures are extensive and many of the core evidence gaps are closing, the impact size, timescale and socio-economic barriers that exist before broad-scale uptake could occur are still significant and need further investigation and testing [7].

The evidence of the magnitude of impact that many of the measures can have upon yield, soil health and water quality over a broad spectrum of space and time is often still lacking (Figure 1). Quantitative evidence for how much of a given intervention is needed to deliver a given benefit is lacking and is also likely to be context-specific [24]. There are knowledge gaps on biodiversity, and studies often focus on selected taxa (e.g., [63,64,119]), perhaps those more easily studied. Often, when an intervention has been implemented for its effect on a different Ecosystem Service, biodiversity impacts may not be as well documented. Cases of success, in particular in farming environments, provide hope for the circular system applicability, yet the evidence in some instances produces conflicting results, possibly due to variance in FAB intervention suitability to an environment (e.g., soil type) or even differences in FAB application or impact monitoring and assessment. It could be that the agricultural matrix is already so degraded (e.g., species pools, soil quality) that it does not have the capacity to recover in the short term or without remedial action [163]. Furthermore, the potential for publication bias in the literature, which tends to report mostly successful trials rather than the negligible or negative results that often never make it to publication, must be noted [15,21,28,95]. This may have an impact when meta-analysis is used to develop the evidence base.

A key step in the successful implementation of FAB measures is the understanding that one intervention alone will not solve all challenges faced, yet a combination of measures implemented in a strategic way can enhance the output success, especially when adopted on a whole farm basis (Table S2). When implementing several different interventions as a package at farm or field level, the effect of the whole is greater than the sum of the parts [164].

Understanding past land management practices and having access to expertise for measuring impacts is vital to the successful application and effective management of the co-benefits and trade-offs at farm and regional scales (e.g., [22,24,35,41,74,108,119]).

However, beyond the closing of evidence gaps, FAB intervention uptake will continue to be limited unless social and economic barriers are removed [7,49]. For many of the FAB measures investigated here, the financial implications of uptake limit the viability of such options, with the business case to invest in long-term nature-based measures currently not adequately supported. Farmers are often proud of their yields, but it is important to convince farmers to look at profit as the difference between income and expenses. Not only yield is taken in consideration in the economic balance, but also the reduction of costs, like expenses on PPPs, chemical fertilisers and fuel.

Furthermore, education and an improved understanding of the benefits changes in the agricultural system could have at a farm and regional scale is lacking in most cases, limited to small networks rather than incentivised at regional or national governmental level. These and other barriers to implementing FAB measures must be addressed by future policy and transdisciplinary research programs, for which FABulous Farmers is an example [12], and those that will be developed in the future under the European Green Deal.