Reduction of nitrogen, phosphorus, and suspended solids effluent loads from paddy fields by transplanting into retained ponding water using a GNSS-controlled rice transplanter

We measured nutrient inflows and outflows on reclaimed land (157 km2, including 100 km2 of paddy fields) in Lake Hachiro. The net effluent loads (NELs) in 2017 were 3.5 × 105 kg nitrogen (N), 6.1 × 104 kg phosphorous (P), and 2.4 × 107 kg suspended solids (SS). The largest monthly NEL was 7.4 × 104 kg N, 1.6 × 104 kg P, and 8.1 × 106 kg SS, in May. If rice can be transplanted without field drainage, the discharge of muddy water at the puddling and transplanting stages, which carries the largest loads, can be reduced. While farmers used an autonomous rice transplanter guided by the global navigation satellite system to transplant rice into retained ponding water, we measured the volume of water and the loads of N, P, and SS released from the fields. The reductions of effluent loads by retention of ponding water were 0.9–2.0 (average 1.6) kg N ha−1, 0.13–0.37 (average 0.30) kg P ha−1, and 45–233 (average 147) kg SS ha−1. The NELs in May were 7.4 kg N ha−1, 1.6 kg P ha−1, and 810 kg SS ha−1. Therefore, transplanting into the retained ponding water reduced the pollutant loads on Lake Hachiro by 22% of N, 19% of P, and 18% of SS in May.