A meta-analysis on nitrogen retention by buffer zones

Buffer zones, established between agricultural fields and water bodies, are widely used as a measure to reduce nitrogen (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 nitrate-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 a 70% (–78% to –62%, n=38) nitrate-N reduction in surface runoff and in groundwater, respectively, compared to 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 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.