Effectiveness of Non-Fertilized Buffer Strips to Reduce Nutrient Loads From Lowland Agriculture to Surface Waters.

We studied the effectiveness of non-fertilized 5 m wide harvested grass buffer strips (BS) to reduce nutrient loads from agricultural lowland fields to the nearest surface water course. Buffer strip effectiveness (BSE) was defined as the relative nutrient load reduction compared with a normally managed reference strip (REF). We measured BSE for three or four years at five hydrogeologically characteristic field sites in the Netherlands by collecting nutrient loads from paired treatments strips (BS and REF) in separate reservoirs in the ditch. No statistically significant BSE was found at three out of five sites. For N we found a low statistically significant BSE of 10-15% at the peat soil site. For P we found a high, statistically significant BSE of 57‑61% at the site with pure shallow flow. BSE was low at most sites, because flow paths ran relatively deep, thus bypassing the top layers of the BS, and sometimes even bypassing the ditch with the reservoirs. Low BSE for nitrate could further be attributed to denitrification outside the BS due to relatively high groundwater levels, a process that also occurs without a BS. Based on modeling, BSE for N is expected to increase only slightly after the four years experimental period. The development of BSE for P depends on the rate of net P withdrawal from the BS and the chemical buffer capacity of the soil. Highest BSE for N is expected on fields with relatively shallow, but not pure shallow flow. The high BSE for P on the shallow flow site indicates that BS could be relevant for 'P‑leaking soils'. BSE for P increases with the amount of surface runoff or shallow flow, and with the original P status of the soil. Buffer strips are ineffective on pipe drained fields.