Nitrogen loss through runoff and leaching

As a result of comparing the amount of leachate collected on the day of rainfall precipitation (0 day) and the amount of leachate collected two days later, the amount of N released from the 0 day was the highest in the control (Fig 4.16).The least amount of N loss was occurred in HTSS+LAS treatment (63.6 mg L^-1). The N contents of leachate collected two days after the rainfall showed the opposite result in the AS treatment and control compared to the 0 day. The AS treatment showed more N content than the control and increased N content than 0 day. The main reason is that the control has already lost a considerable amount of N at 0 day. In the case of AS treatment, it was confirmed that N loss occurred continuously (rather increasingly) through leaching two days after the rainfall. Therefore, it is considered that the use of TSS in the saturated paddy soils in case of rainfall is a suitable N fertilization practice which can reduce the environmental load due to N loss via leaching.

Considering the crop production, the use of chemical fertilizer and TSS together will be an appropriate compromise compared to chemical fertilizer only. The characteristics of N loss through runoff was shown in Fig 4.16.The runoff was collected 5 times over a total of 110 minutes of rain. The N tended to decrease over time, but the values were slightly unstable. The amount of N lost through runoff during the rainfall was highest in the HTSS+LAS treatment (24.4 mg L

-1). Especially, the rate of N loss of each treatments was increased up to 50 minutes after the start of rainfall and maintained the highest level. In the case of HTSS + LAS and C, it gradually decreased until 110 min. The AS and LTSS + HAS exhibited a slightly unstable N loss characteristic such that the N loss

rate increased again in about 90 minutes and decreased to 110 minutes. About 67 to 77% of the annual water used in rice fields (irrigation water + rainfall) was discharged outside the paddy field, and this discharge contained 18% to 35%

of the N used in the rice farming (Jang, 2013). However, the amount of N lost in the runoff of this study was smaller than leaching. This is the result of disagreement with the amount N loss via runoff in paddy soils reported by Jang (2013). Paddy soils used in this study were sandy loam or loam. Therefore, it had relatively low runoff and relatively high leaching characteristics (Fig 4.17).

In sandy soils where leaching can occur relatively easily, it is recommended to reduce TSS use in order to prevent leaching non-point source pollution. In the case of rice farming, it is considered that the use of AS with TSS method can reduce the environmental load due to non-point pollution of runoff N. However, if there is rainfall after two days of fertilizer application, a certain amount of runoff N loss is unavoidable. Therefore, when using TSS, it is necessary to maintain rice paddies to prevent non-point source pollution such as a management of ridge between rice fields, irrigation management, and soil management can be suggested as an additional solution.

Fig 4.16. Cumulative Total N loss from runoff and leaching. Three rainfall events (June 03, August 10 and September 21) with moderate intensity (9.0

mm hr^-1) were applied during the experimental period. Error bars represent standard deviations (n=3) of the means of total N in the water sample

Fig 4.17. Proportion of total N loss (%) in runoff and leaching processes

During the rainfall, the amount of N loss through the leaching seems to be determined by the characteristics of the soil. When TSS was used, the amount of N lost due to leaching was smaller than that of the control. Therefore, it can be interpreted that TSS application plays a role in assisting N adsorption in the soil. Taking the cation exchange capacity (CEC) as an example, the smaller the amount of CEC, the less capacity to hold cations, and in the case of rain, the soil is more likely to flow N ions. In contrast, the CEC of the control was not effective in the adsorption of cations such as NH4+ in the soil. CEC has a positive relationship with the amount of organic matter. In other words, the more organic soil, the larger the CEC can be regarded as the loam soil that can provide enough nutrients to the plant. When fertilizers containing cations such as ammonium, potassium, sodium, and copper are used in low CEC soil, they cannot be utilized by plants and are lost in large quantities, adversely affecting

the environment. CEC of each treatments showed a relatively low value in chemical fertilizer treatment compared to other treatments (Fig 4.18). Thus, it is considered that chemical fertilizer is not suitable for soils with insufficient organic matter. This is supported by the high leachate N content of paddy soils treated with chemical fertilizer only. On the other hand, TSS used treatments (HTSS+LAS and LTSS+HAS) showed higher CEC in the soil due to the organic matter content of the TSS. As is generally known, livestock manure (liquid form in this study) can be interpreted to play a role in making the loam soil. These results are consistent with the EC results (Fig 4.19). The EC is generally related to soil salts. The EC value is determined by the amount of water soluble organic components, that is, the amount of cations and anions.

Fig 4.18. Cation exchange capacity (CEC) of each N fertilization practice

16.9 16.3 16.1 15.6

0.0 5.0 10.0 15.0 20.0

HTSS+LAS LTSS+HAS AS C

CEC, cmol/kg

Fig 4.19. The electrical conductivity (EC) of each N fertilization practice