A Meta‐Analysis on Nitrogen Retention by Buffer Zones

Abstract

Buffer zones, established between agricultural fields and water bodies, are widely used as a measure to reduce N in surface runoff and groundwater. However, the literature indicates inconsistent results on the N removal efficiency of buffer zones between studies. We performed a weighed meta-analysis on the buffer zone effects on NO 3 -N and total N in surface runoff and groundwater by summarizing 46 studies published between 1980 and 2017. The overall effects of buffer zones were a 33 (−48 to −17%, n = 25) and 70% (−78 to −62%, n = 38) NO 3 -N reduction in surface runoff and in groundwater, respectively, compared with controls with no buffer zone. In addition, buffer zones reduced the total N in surface runoff by 57% (−68 to −43%, n = 16). The effects of buffer zones on N retention were consistent across continents and in different climates. Nitrogen retention increased with increasing initial N concentrations discharged from the source of pollution. According to a meta-regression, the N removal efficiency in surface runoff decreased in consort with increasing buffer zone age. Otherwise, the meta-analysis revealed no effects of buffer zone characteristics such as the width or species number (for grass buffer zones) on the N retention in surface runoff and groundwater. Unlike groundwater quality, which responded equally well regardless of the source of pollution, buffer zone type, or buffer zone age, surface water quality is more sensitive, and it might not be satisfactorily improved by tree buffer zones or aged buffer zones, or when the source of pollution originates from grass production fields.

Current Insights into the Effectiveness of Riparian Management, Attainment of Multiple Benefits, and Potential Technical Enhancements

“…Many studies reported here documented multiple outcomes, exemplified by the ecosystem services assessment approaches combined in the assessment of IBZ designs (Zak et al, 2019). Studies here also reported varied effectiveness for nutrients—for example, where buffers are more effective at soluble N removal; dissolved P remains an issue (Valkama et al, 2019; Vidon et al, 2019). A range of widths was considered in the studies, but the suggestion is that narrow widths are less robust to key water quality functions (Dal Ferro et al, 2019; Jaynes and Isenhart, 2019).…”

“…In their meta‐analysis across a great range in buffer widths, Valkama et al (2019) highlighted that buffer zones were more effective at reducing N in groundwater (70% reduction in NO 3 ) than in surface runoff (33% reduction). However, the meta‐analysis found that buffering effectiveness was reduced with buffer zone age and was unrelated to width.…”

“…Several studies in this collection highlight the age of the riparian zone as an important factor in relation to the delivery of ecosystems services, including biodiversity benefits (Groh et al, 2019; Jaynes and Isenhart, 2019; O'Callaghan et al, 2019; Valkama et al, 2019). Often, functions took several years to develop, associated with maturing vegetation and habitat development.…”

“…The interactions of vegetation and soil organic matter for N cycling were discussed in the primary studies by Groh et al (2019) and Jaynes and Isenhart (2019), in terms of a more established vegetated buffer facilitating stronger potential runoff NO 3 reductions. However, the metadata review by Valkama et al (2019) found limited effects of vegetation type on buffer N retention.…”

“…A range of widths was considered in the studies, but the suggestion is that narrow widths are less robust to key water quality functions (Dal Ferro et al, 2019; Jaynes and Isenhart, 2019). In general, there is great site specificity to buffers, meaning that width comes out less important than other key factors of vegetation, soil type, and biogeochemical processes, wetness, landscape context, and drainage pathways (Valkama et al, 2019). The proportionate and appropriate use of the growing “menu” of structural elements, from simple engineering principles acting on water flows to ecotechnologies incorporated in the riparian buffer space, is part of countering uncertainties in buffer functions and developing more robust solutions to landscape pressures on aquatic ecosystems.…”