Nitrate concentrations and seasonality of nitrate fluxes

In the growing season of 2013 and 2014 nitrate concentrations in rainfall ranged between 0.6 or 0.2 and 16.9 or 13.6 mg L^-1 respectively. These most extreme concentrations were always found in the monsoon season (Table 2). Mean nitrate concentrations in rainfall were quite similar for pre-monsoon and pre-monsoon season and for the years 2013 and 2014 and ranged between 2.4 and 3.8 mg L^-1 (Table 2). However due to different rainfall, nitrogen input from nitrate in rainfall over the sampling periods from May to August was on an area basis in 2013 much higher (16.4 kg N_NO3- ha

-1) than in 2014 (2.1 kg NNO3- ha^-1). The forest area received in both years more NNO3- ha^-1 and higher

55

proportional N_NO3- than the agricultural areas (Table 3). Figure 2 provides an overview over the temporal distribution of rainfall and NNO3-input per area unit. No significant correlations between rainfall and NO3- concentrations in precipitation were found in any of the years (p>0.050). The N_NO3^-input by rainfall during the monsoon season in 2013 was 9% of the total river N_NO3^- discharge.

In 2014 the N_NO3^- river discharge was lower than the total N_NO3^-rain input (Tables 3 and 5). Related to the mineral nitrogen fertilizer application rate common in Haean (313 kg N ha^-1 yr^-1) nitrogen input from nitrate rain deposition was only 5.2% in 2013 and 0.7% in 2014.

Table 2. Maximum (Max), minimum (Min) and average (Ave) nitrate concentrations (mg L^-1) in rainfall in the investigated Haean agricultural sub-catchment during the sampling campaigns in 2013 and 2014. Nitrate concentrations are separated for the usually dry pre-monsoon season (May and June) and the usually rainy monsoon season (July and August).

Haean agricultural

sub-catchment 2013 2014

Nitrate concentrations Pre-monsoon season

Monsoon season

Pre-monsoon season

Monsoon season Max (mg L^-1) 16.9 13.2 13.6 9.2 Rain Ave (mg L^-1) 2.8±2.5 3.8±2.1 2.4±2.5 2.8±2.2

Min (mg L^-1) 0.6 1.0 0.2 0.9

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Table 3. Total nitrogen input from nitrate deposition by rainfall separated for three sampling areas, forest, dry land and rice paddies and for the campaigns in 2013 and 2014 and calculated on a hectare basis (kg NNO3- ha ^-1), on a percentage basis or on the basis of the area covered by the three respective sampling regions (kg NNO3- area ^-1) in the investigated Haean agricultural sub-catchment.

Haean agricultural sub-

catchment 2013 2014

Sampling plot

land use ha N_NO3- kg ha^-1 N_NO3- kg Area^-1 Percentage

(%) N_NO3- kg ha^-1 N_NO3- kg Area^-1 Percentage (%)

Forest area 13.2 6.4 85.9 39.3 1.4 18.4 63.5

Dry land area 144.8 5.0 725.7 30.4 0.6 160.2 30.0

Rice paddies area 42.6 4.9 212.4 30.2 0.1 37.2 6.4

Total 200.7 16.4 1024.1 100 2.1 162 100

57 3.2.2 Stream runoff

Nitrate concentrations in river discharge ranged between 0.4 mg L^-1 and 64.1 mg L^-1 in 2013 and between 1.2 and 27.6 mg L^-1 in 2014. Extreme nitrate concentration values of 34.5 and 64.1 mg L^-1 were found during the dry pre-monsoon season in 2013 at the rice paddies patch and during the monsoon season at the dry land patch respectively (Fig. 3 and Table 4). In 2013 mean nitrate concentrations in stream runoff from the forested site (S1) ranged in the order of about 10 mg L^-1 and thus, was about half of the mean nitrate concentrations measured in the stream runoff from the dry land (S2, S3 and S4) and rice paddy (S5 and S6) patches (Table 4). In the monsoon season of the wet year 2013 mean nitrate concentrations in stream runoff from the forest, dry land and rice paddy patches were always slightly lower than in the pre-monsoon season (Table 4). Maximum, minimum and average nitrate concentrations in stream runoff in the dry year 2014 were almost consistently lower than in 2013 and no systematic differences between pre-monsoon season and monsoon season were found (Table 4, Fig. 3). In 2014 again, the average nitrate concentrations in the stream runoff from the forest were considerably lower than in the stream runoff after passage of the dry land and rice paddies. No significant relationship between discharge and nitrate concentrations in river water was found in any of the years (p>0.050).

Fig.3. Temporal distribution of nitrate concentrations (mg L^-1) in stream runoff measured at the three sampling locations forest, dry land and rice paddies within the investigated Haean agricultural sub-catchment during the sampling campaigns (May to August) in 2013 and 2014. During sampling campaign 2013 values from S2, S3 and S4 within dry land were considered for this graph, while during sampling campaign 2014 only the values from S2 and S4 were considered due to the lack of differences between S2 and S3 in the year before.

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Table 4. Maximum (Max), minimum (Min) and average (Ave) nitrate concentrations (mg L^-1) in stream runoff measured at the three sampling areas forest, dry land and rice paddies within the investigated Haean agricultural sub-catchment during the pre-monsoon (May and June) and monsoon season (July and August) of the sampling campaigns in 2013 and 2014. Dry land data was calculated from three sampling points (S2, S3 and S4) in 2013 and from two sampling points (S2 and S4) in 2014.

Nitrate concentrations Pre-monsoon season 2013 Monsoon season 2013 Pre-monsoon season 2014 Monsoon season 2014

Sampling point Forest

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Total NNO3- river discharge fluxes measured at four sampling locations are represented in Fig. 4 in a multiple line and scatter flux diagram over time and in Table 5 calculated as cumulative values.

Rather striking is the difference in total NNO3- river discharge between the sampling periods in 2013 and 2014. While total NNO3- river discharge in the growing season of 2013 was 10.3 x 10³ kg for the entire sub-catchment area, it was 43.4 kg for the entire sub-catchment area in 2014, which represents just 0.4% of the total NNO3- river discharge in 2013. This huge difference was mainly driven by the pronounced NNO3- river discharge fluxes during the heavy monsoon season in 2013 (Fig. 4). The N_NO3^- river discharge increased with the frequency of rainfall during the monsoon season in 2013. This period contributed 90% to the total N_NO3^-discharge during the whole sampling campaign in 2013. The NNO3- river discharge in the pre-monsoon season and the weak monsoon season in 2014 showed no such difference. Dry land field patches contributed more than 60% to the entire total N_NO3^- river discharge in the growing season of 2013, while the rice paddy patch contributed about 25% and the forest patch only 13% (Table 5). In 2014 the proportional contribution to the total N_NO3^- river discharge was highest for the rice paddy patches.

Fig. 4. Temporal distribution of total N_NO3- fluxes (kg ha^-1 d^-1) in stream runoff measured at three sampling locations forest, dry land and rice paddies within the investigated Haean agricultural sub-catchment during the sampling campaigns (May to August) in 2013 and 2014. One single value for rice paddies is given in this graph because the areas of S5 and S6 were measured together within a total rice paddies area. The value of NNO3- fluxes (kg ha^-1 d^-1) is calculated with the mean nitrate concentration values between S5 and S6 in both years.

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Table 5. Total nitrogen discharge runoff separated for three sampling areas, forest, dry land and rice paddies and for the campaign in 2013 and 2014 and calculated on a hectare basis (kg NNO3- ha ^-1) on a percentage basis or on the basis of the area covered by the three respective sampling regions (kg NNO3- area ^-1) in the investigated Haean agricultural sub-catchment. One single value for rice paddies is given in this graph because the areas of S5 and S6 were measured together within a total rice paddies area. The value of NNO3- fluxes (kg ha^-1 d^-1) is calculated with the mean nitrate concentration values between S5 and S6 in both years.

Haean agricultural sub-

catchment 2013 2014

Sampling plot land use ha N_NO3- kg ha^-1 N_NO3- kg area^-1

Percentage

(%) N_NO3- kg ha^-1 N_NO3- kg area^-1

Percentage (%)

Forest 13.2 23.5 312.6 13.0 0.08 1.05 9.0

Dry land S2 61.3 56.2 3.4x10³ 31.1 0.19 11.4 21.2

Dry land S4 83.5 56.0 4.7x10³ 31.0 0.12 9.9 13.5

Rice paddies 42.6 44.7 1.9x10³ 24.7 0.49 21.0 56.1

Total 200.72 10.3x10³ 100 43.49 100

61 3.2.3 Groundwater

Nitrate concentration in groundwater ranged between 0.2 and 67.0 mg L^-1. Extreme values of 67 and 65.7 mg L^-1 were recorded at the rice paddies in the beginning of the pre-monsoon season and the end of the monsoon season (Fig. 5 and Table 6). A pronounced difference between nitrate concentration in groundwater at the dry land field and rice paddy patches was found (Fig. 5). In dry land field groundwater nitrate concentrations ranged from 0.2 to 39.4 mg L^-1 while groundwater nitrate concentrations at the rice paddies ranged from 18.0 to 67.0 mg L^-1 (Table 6). Pronounced differences in groundwater nitrate concentrations between dry pre-monsoon season and monsoon season were not found in any of the sites. We found significant differences (p<0.001) between the wells sampled within the dry land (Fig 5 well 1 and well 2). Significant differences in nitrate concentrations were found between groundwater at dry land field patches and rice paddy patches (p< 0.001). The high groundwater nitrate concentrations specifically in the rice paddy area but also in some parts of the dry land area suggested an impact of nitrogen fertilizer application on groundwater quality.

Fig. 5. Temporal distribution of nitrate concentrations (mg L^-1) in groundwater measured at two sampling locations dry land (two wells) and rice paddies (three wells) within the investigated Haean agricultural sub-catchment during the sampling campaigns (May to August) in 2013.

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Table 6. Maximum (Max), minimum (Min) and average (Ave) nitrate concentrations (mg L-1) in groundwater measured at two sampling locations dry land (two wells) and rice paddies (three wells) within the investigated Haean agricultural sub-catchment during the sampling campaigns (May to August) in 2013.