4.1. Fertilizer Application and Fate of P in Soils
We have identified fertilizer use as the major P input in the Haean study watershed. Specifically, organic compost was the biggest contributor to the total P input for most of the common crops except rice. For the case of rice, chemical fertilizer was found to be the largest P input. Although the RDA has not recommended any organic compost application in rice fields, most farmers in the Haean watershed do apply organic compost in their rice fields. The estimated fertilizer consumption in Korea is 3.6 times higher than the global average [22
]. Furthermore, application rates in the study watershed for chemical fertilizers and organic compost application rates have been 2.5 to 4 and 4.5 to 8 times higher than the recommended application rates (kg/ha/year) by the RDA, respectively.
As part of this study, our interviews with local farmers showed that fertilizer application rates for rice, cabbage, potato, radish, and soybean surpassed the RDA recommended P fertilizer application rates by 5.7, 3.4, 6, 4, and 4.6 times, respectively. Farmers in the Haean watershed generally applied all the P fertilizer in a single dose at the beginning of the growing season and did not use additional applications of P fertilizer later in the year. At the beginning of the growing season, plant growth is thought to be slow with relatively low P uptake. Thus, excess P fertilizer application early in the growing season combined with substantial rainfall could result in enhanced loss of P from soils. Therefore, splitting P fertilizer applications into two or three separate fertilizer applications based on crop P requirement could be a useful measure to reduce harmful P losses. While annual chemical fertilizer consumption in the Haean watershed has been decreasing in recent years [14
], there are still considerable opportunities to reduce P fertilizer inputs and eventually decrease P surpluses in the watershed.
The detailed inventory of P inputs and outputs in this study has shown that P inputs are higher than the outputs (Table 8
). Approximately 150,177 kg P/year (Table 8
) is retained in soils in the Haean watershed, signifying the potential for contributing to eutrophication and leaching into ground water. This finding is further supported by the increase in soil P concentration in the watershed. An increase in average soil test P concentrations in the dry fields from 76.3 mg/kg (estimated in 2009) [23
] to 95.9 mg/kg (this study) suggests that the excess P is being held in the soil. Based on the calculation method used by Bennet et al. [1
], if farmers in the watershed immediately stop applying excess P fertilizer and apply the same amount of P that is exported from the watershed (assuming that agricultural production and P export will not decline with reduced fertilizer application), more than 22 years would be required for P in the soil to reach equilibrium P concentrations. Therefore, achieving reductions in soil P concentrations in the Haean watershed is a goal with long-term benefits.
P sequestered in soil is often considered a ‘chemical time bomb’ [24
]. Excessive P stored in soils can result in increased water-soluble P concentrations in soils, thus amplifying the potential for hydrologic export [25
]. The general soil texture of soils in the Haean watershed is sandy loam; these soils are susceptible to erosion during the summer monsoon season [26
]. Related to this, observations from other monitoring studies show that most (>83%) of the P exported from the Haean watershed in surface water is in particulate form [12
]. Even if P fertilizer applications are reduced, soil P will continue to be available for hydrologic export to Lake Soyang. Soil P can be desorbed by changes in soil chemistry which might be related to changing inputs of acidic precipitation [5
]. With continued climate change, the intensity of tropical and subtropical cyclones is likely to increase in summer monsoon regions [27
]. Future efforts focused on the design and implementation of revised P management programs should consider the importance of P retained in highland agricultural soils in the study area.
In this study, we measured P inputs in wet deposition of 3719 kg P/year. This estimate is similar to P inputs in wet deposition in other studies [29
]. Although forested land covers more than 61% of the total watershed area, only a small amount of P was lost from forests in the Haean watershed (274 kg P/year). This estimate is within the range of measurements of P lost in surface runoff from forested land in other regions [31
]. Forested-agricultural watersheds can be found throughout Korea and future research directed at understanding P retention in mixed forested-agricultural systems would provide insight on how to better manage P losses at long-term scales.
Our study was not focused on identifying the biogeochemical processes controlling specific pathways of P sequestration or export. Generally, P losses from agricultural land are related to soil erosion [33
]. In the Haean watershed, hydrologic export of P is influenced by meteorological factors including rainfall amount, rainfall intensity, and the number of antecedent dry days [4