Non-point source pollution control in farmland by source-reduction strategies coupled with wetland treatment

In recent years, intensive agricultural practices have been adopted to enhance crop yields for food production. Therefore, the issue of agricultural non-point source pollution attracts attention due to the increasing use of fertilizer. The non-point source pollution control is diverse and difficult to manage because of the physical landscape and agricultural practices (e.g., topography, soil texture, farming, and irrigation). In addition, excessive fertilizer used to enhance crop yields would lead to land degradation. During a rainfall event, nitrogen and phosphorus from fertilizers would be washed into surface water and infiltrated into groundwater, resulting in the degradation of the aquatic environment.

In this study, the use of slow-release fertilizer, compared to the reference of conventional chemical fertilizer, and a constructed wetland were adopted at a pilot-scale study. During a 40-day growing cycle, Brassica Chinensis L.(Pak-Choi) was chosen as the model crop, and two types (i.e., chemical fertilizer and slow-release fertilizer) of fertilizers were applied under pre-designed conditions. Two simulated intense rainfalls were operated on the 26^th and 33^rd days, and the surface runoff was introduced to the constructed wetland for further nutrient removal. The soil and water samples from the soil and wetland were analyzed for nutrient concentration variation, and the nutrient distribution and removal efficiency were assessed. The results showed that the chemical fertilizer has a higher nutrient loss rate. The nitrogen (N), phosphorus (P), and potassium (K) contents in the soil increased rapidly after top dressing and decreased significantly after the simulated rainfall. In contrast, slow-release fertilizer has a relatively steady nutrient loss rate during the growing cycle. Meanwhile, the chemical fertilizer has a higher total N, P, and K loss via infiltration and runoff than slow-release fertilizer. For the wetland treatment, the N removal for chemical fertilizer and slow-release fertilizer after 15 days was 24.4 % (i.e., from 19.99 ppm to 15.11 ppm) and 29.5 % (i.e., from 20.81 ppm to 14.68 ppm), the P removal amount was 38.1 % (i.e., from 0.21 ppm to 0.13 ppm) and 87.5 % (i.e., from 0.08 ppm to 0.01 ppm), respectively. Opposing to the conventional chemical fertilizers, the use of slow-release fertilizers could reduce nutrient loss. The constructed wetland demonstrated a positive effect on removing the nutrients in neighboring water, which reduced the impact of agricultural non-point source pollution.

Keywords: Conventional farming, Best management practices, Slow-release fertilizer, Constructed wetlands