Potential nutrient contributions of a lakeshore rice-based agroecosystem in Barangay San Antonio, Bay, Laguna to Laguna de Bay, Philippines

Data showed a large part of annual nutrient inputs to the rice fields came from chemical fertilizers which consisted of 135 kg N/ha, 21 kg P/ha and 14 kg K/ha. These amounts accounted for 55 percent of N, 64 percent of P and 5 percent of K of the total inputs. Ratio of dry weights of harvested grain and straw was 1:2.5. This ratio implied an excess supply of NPK in the soil that resulted to the production of more straws than grains to the detriment of farmer's grain yield. The amount of NPK in the drainage waters of the agroecosystem which flowed directly to the lake were the potential nutrient contributions of the agroecosystem to the lake. Based on this study, a total of 35 kg N/ha, 9 kg P/ha and 66 kg K/ha per cropping year were contributed by the paddy fields to the lake via the flow of drainage waters. High volume of water which carried the excess nutrients from the paddy fields were discharged to the lake during heavy rains. More nutrient enrichment of the lake took place during the rainy season. These nutrients which drained from the agroecosystem potentially contributed to the nutrient enrichment of the lake. The lake on the other hand, provided nutrient inputs to the agroecosystem. This was due to inundation of the paddy fields by lakewater during typhoons. In this study it was shown that flooding provided about 1 percent of the total amount of NPK inputs to the agroecosystem. The total NPK content of the grains harvested were 97 kg N/ha, 11 kg P/ha and 23 kg K/ha. About 4 percent of the harvested grains were consumed by the farmer's families and the rest were sold. Thus, the grains consumed amounted to 4 kg N/ha, 1 kg P/ha and 1 kg K/ha. Grains harvested that were sold and exported to other communities represented about 93 kg N/ha, 10 kg P/ha and 22 kg K/ha. For the whole barangay [village], these add up to about 7.3 tons of N, 0.8 tons of P and 1.7 tons of K