Changes in lowland paddy soil fertility in the Philippines after 50 years of the Green Revolution

The Green Revolution (GR) made a major contribution to increasing the rice yield in the Philippines from 1.3 t ha−1 in 1966 to 4.0 t ha−1 in 2018, but the changes in paddy soil fertility that occurred during this 50-year period remain unknown. We collected 37 soil samples from the plow layers of paddy fields in Luzon, Leyte, Panay, and Mindanao in the Philippines in 2016 and 2017 (the ‘2010s’) and compared their fertility-related soil properties with published data for soil samples collected from the same or nearby sampling sites in 1969 (the ‘1960s’). Paddy soils in the Philippines had relatively high exchangeable calcium (Ca) and magnesium (Mg) content, cation exchange capacity (CEC), available silicon (Si) content, and total carbon (C) and nitrogen (N) content, reflecting their indigenous pedological background. We found that the 2010s soils had a lower available N content, particularly in wetter regions, despite the high input of N fertilizers, whereas the available phosphorus (P) showed a six-fold increase from the 1960s to the 2010s. The total potassium (K) content had significantly decreased during this period, possibly due to the mining effect of intensive farming with K-deficient fertilization. The 2010s soils contained a greater silt content and higher proportion of smectite in the clay fraction, which may have been transported from upstream via irrigation water. The increase in smectite corresponded with a significant increase in the cation exchange capacity, despite the organic C and clay contents tending to decrease, and these increases in smectite and soil pH may have contributed to the increase in the available fraction of Si that had not been applied as fertilizers. These findings demonstrate that paddy soil fertility in the Philippines changed in many ways over this 50-year period, largely owing to the direct impact of GR implementation in paddy fields but also potentially due to offsite effects from upland fields. The environmental impact of the nutrient loss and soil organic matter degradation behind the fertility improvement should be assessed holistically for the sustainable development of rice paddy ecosystems. (337 words)

绿色革命（Green Revolution, GR）对菲律宾水稻单产提升贡献显著，使其从1966年的1.3 t·ha⁻¹升至2018年的4.0 t·ha⁻¹，但这50年间稻田土壤肥力的变化仍未明确。本研究于2016至2017年（即“2010年代”）从菲律宾吕宋岛、莱特岛、班乃岛和棉兰老岛的稻田耕作层采集37份土壤样品，并将其与1969年（“1960年代”）在相同或邻近采样点采集的已发表土壤样品的肥力相关性质进行对比。菲律宾稻田土壤原本具有较高的交换性钙（Ca）、镁（Mg）含量，阳离子交换量（CEC）、有效硅（Si）含量，以及总碳（C）和总氮（N）含量，这反映了其原生土壤本底特征。研究发现，尽管氮肥投入量较高，但2010年代采集的土壤有效氮含量有所降低，在湿润区域尤为显著；而有效磷（P）含量则从1960年代至2010年代增长了六倍。此期间总钾（K）含量显著下降，这可能与低钾施肥下的集约化耕作引发的钾素耗竭效应有关。2010年代的土壤粉粒含量更高，黏粒组分中蒙脱石占比也更高，这些物质可能通过灌溉水从上游输送而来。尽管有机碳和黏粒含量呈下降趋势，但蒙脱石占比的提升与阳离子交换量的显著升高相契合；同时蒙脱石含量增加与土壤pH值提升，可能共同推动了非肥料来源的有效硅组分增加。本研究结果表明，菲律宾稻田土壤肥力在这50年间发生了多方面变化，这在很大程度上源于绿色革命在稻田实施的直接影响，同时也可能受到旱地农田的异地效应影响。为实现稻田生态系统的可持续发展，需全面评估肥力提升背后养分流失与土壤有机质降解所带来的环境影响。