Leaching Losses: Result

[addw2p name=”leachingLosses”]

Ammonium nitrogen: The mean NH_4-N concentration of N1P2K1 at 33.69 mg L^-1 was significantly higher than N1P2K0 at 8.15 mg L^-1 (Fig. 1). Both treatments had higher NH_4-N concentrations than treatments without N (N0P0K0 and N0P2K1). In the presence of K, NH_4-N concentrations increased 4.1 fold when N fertilizer was applied and 3.5 times in the absence of N application.

Most of the NH_4-N was found in the top 60 cm of the soil profile where majority of the palm roots may be located[7]. However, the mean NH_4-N concentrations decreased significantly with soil depth (Fig. 2). It was 17.89 mg L^-1 at 30 cm depth declining to 12.19 and 6.52 mg L^-1 at soil depths of 60 and 120 cm, respectively. The decline in NH_4-N concentration was more rapid between 30 and 60 cm compared with the lower soil depths.

The changes in NH_4-N concentrations among fertilizer treatments and between soil depths varied significantly across time (Fig. 3 and 4). At day 15, which was the first sampling date after treatments, the NH_4-N concentrations in the soil solution of N0P0K0 and N0P2K1 were 1.99 and 5.48 mg L^-1, respectively (Fig. 3). This indicated that even without N fertilizer, there were NH_4-N ions present in the soil solution probably from the native soil N and decaying palm biomass. These values provided the baseline NH_4-N concentrations in the soils in the experimental site implying poor N fertility. With N treatments, N1P2K0 and N1P2K1, the NH_4-N concentrations at the 15th day were 20.82 and 121.35 mg L^1-, respectively. The NH_4- N concentrations of N1P2K1 treatment decreased sharply between day 15 and day 30 and then more gradually until it reached 1.86 mg L^-1 at day 150. However, the NH_4-N concentrations were statistically similar to the baseline values from day 75 after treatment. The NH_4-N concentrations of N1P2K0 also declined rapidly and reached the baseline value 30 days after treatment. It continued to decrease to 1.60 mg L^-1 at day 150.

Fig. 1: Concentration of NH_4-N in soil solution for each fertilizer treatment

Fig. 2: Concentration of NH_4-N in soil solution across soil vertical profile

Fig. 3: Fertilizer × time effect on concentration of NH_4- N in soil solution (a) for comparing two times at difference level of treatments (b) for comparing two times at the same level of treatments

Fifteen days after treatments, the NH_4-N concentrations in the soil solution were similar at soil depths of 30 and 60 cm (Fig. 4). They were 40.45 and 50.55 mg L^-1, respectively and both concentrations were higher than at 120 cm depth of 21.23 mg L^-1.

Fig. 4: Depth × time effect on concentration of NH_4-N in soil solution (c) for comparing two times at difference level of depths (d) for comparing two times at the same level of depths

The NH_4-N concentrations in all soil depths decreased rapidly and reached similar values after 90 days from treatments. The NH_4-N concentration at day 150 at 30 cm depth was 1.1 mg L^-1, 60 cm depth 0.73 mg L^-1 and 120 cm depth 1.32 mg L^-1 (Fig. 4).

Nitrate nitrogen: The average NO_3-N concentration at 15 days after treatment was low at 0.214 mg L^-1 (Fig. 5). It gradually rose to 0.485 mg L^-1at day 60. A period of relatively dry weather between day 45 and 60 seemed to enhance nitrification resulting in the NO,_3- N concentration increasing significantly to 1.37 mg L^-1 at day 75. However, it declined continuously to 0.141 mg L^-1 at day 150 indicating that the transformation of NH_4-N to NO_3-N was not a major process during the monsoon period. The mean NO_3-N concentrations between 30 and 60 cm soil depth were 0.599 and 0.732 mg L^-1, which were significantly higher than at 120 cm soil depth of 0.266 mg L^-1 (Fig. 6). The proportion of NO_3-N to total inorganic N was in the range of 0.03-0.06 only indicating relatively low nitrification rate.

Total inorganic nitrogen: The total inorganic N was mainly composed of NH_4-N and thus the effects of fertilizer treatments on its concentrations were similar to NH_4-N concentrations as discussed earlier. Briefly, the total inorganic N concentration of N1P2K1 at 35.03 mg L^-1was significantly higher than the other three fertilizer treatments (Fig. 7). Although the total inorganic N concentrations in the first three treatments were statistically insignificant, there was a clear trend showing higher N concentrations in the presence of K (6.09 mg L^-1) and N (8.45 mg L^-1) compared with control (1.69 mg L^-1) as shown in Fig. 7.

Fig. 5: Concentration of NO_3-N in soil solution over time

Fig. 6: Concentration of NO_3-N in soil solution across soil vertical profile

The mean total inorganic N concentration at 30 cm soil depth was 18.71 mg L^-1 decreasing to 12.94 mg L^-1 at 60 cm depth and 6.79 mg L^-1 at 120 cm. Thus, 150 days after fertilizer treatments, a large proportion of inorganic N was still found in the top 60 cm soil depth where most palm roots were present despite the high rainfall during the monsoonal period.

The three factor interaction, fertilizer x depth x time, was significant for total inorganic N concentration. This significant interaction was mainly due to the sharp increase in inorganic N concentration upon the applications of N and K fertilizers, N1P2K1, compared with those without N input. For example, the inorganic N concentrations of N0P0K0 were very low and fluctuated between 0.37-12.11 mg L^-1 at 30 cm soil depth over the 150 days of measurements after treatment (Fig. 9). They were even lower with inorganic N concentrations ranging from 0.39-1.75 and 0.32-1.10 mg L^-1 at 60 and 120 cm depth, respectively. In contrast, the inorganic N concentration of N1P2K1 at 30 cm depth was about 10 fold higher at 127 mg L^-1 just 15 days after application.

Fig. 7: Concentration of total inorganic N in soil solution for each fertilizer treatment

Fig. 8: Concentration of total inorganic N in soil solution across soil vertical profile

In fact, the inorganic N seemed to have move downwards to at least 60 cm depth where its N concentration was even higher at 179 mg L^-1. However, at all soil depths, the inorganic N concentrations of N1P2K1 decreased rapidly where by 105 days after treatments, they were similar to the other treatments except at 120 cm depth. The inorganic N concentrations appeared to hover around 13.5 mg L^-1 between 90 and 135 days after treatment (Fig. 9) before declining to 4.3 mg L^-1 at day 150.

Potassium: Similar to N, the K concentrations in the soil solution decreased with soil depth where the concentrations were 105, 75 and 48 mg L^-1at 30, 60 and 120 cm soil depth respectively (Fig. 10). However, unlike N, the decline in K concentration was much smaller and between 30 and 120 cm, it was only about 2 fold.

Fig. 9: Effect of fertilizer x time x depth on concentration of total inorganic N in soil solution (a) 30 cm depth (b) 60 cm depth (c) 120 cm depth

In the absence of K application such as treatments, N0P0K0 and N1P2K0, the K concentrations were very low ranging from 1.3-10.7 mg L^-1 (Fig. 11). Upon K application, the K concentrations were increased to 237 mg L^-1 after just 15 days in N0P2K1 and 295 mg L^-1 in N1P2K1. The K concentrations for both treatments then decreased rapidly in the next 15 days before a more gradual decline was observed. At day 150, the K concentration of N0P2K1 was 67 mg L^-1 and that of N1P2K1 was 126 mg L^-1. The difference in K concentration at day 150 was about 59 mg L^-1which was similar to the differential at day 15 suggesting that the trend lines over time were parallel for both treatments. It also implied that this differential K concentration was due to the displacement of exchangeable K to the soil solution by NH₄₊ and the“disappearance” of K from the soil solution across time was not affected by N application.

Fig. 10: Concentration of K in soil solution across soil vertical profile

Fig. 11: Fertilizer x time effect on concentration of K in soil solution (k) for comparing two times at different level of treatments (l) for comparing two times at the same level of treatments

Leaching losses at 120 cm soil depth: The amount of leaching losses measured at 120 cm soil depth over 150 days was based on the volume of soil solution and nutrient concentrations for two replicates only to avoid missing values (Table 1). The leaching losses of inorganic N ranged from 0.043-0.589 g m^-2. For treatment, N1P2K1, it translated into a leaching loss of N of only 1.6% of the applied N fertilizer. This was agreeable with the results of Foong[10] for mature palms. Most of the N loss was in the form of NH_4-N since nitrification rate seemed low in the soils. Without the application of K fertilizer, the leaching losses of K were only 0.09 g m^-2 (Table 1). Applications of K fertilizer increased the leaching
losses of K over 150 days to 6.35 and 2.92 g m^-2 for N0P2K1 and N1P2K1, respectively (Table 1). These were equivalent to 5.3 and 2.4% of the applied K fertilizer for the above treatments, respectively. The higher K losses from the applied fertilizer in the absence of N (N0P2K1) might be attributed to the poorer yield compared with N1P2K1 and thus, lower K uptake from the soils by the palms (Table 1).

Table 1:Cumulative leaching losses of N and K fertilizers under mature oil palms as influenced by fertilizer treatments

Groundwater quality: The NH_4-N concentrations in the groundwater of the monitoring wells were similar for N treatments at N0 or N1 rate regardless of K application rate (Fig. 12). They were mainly below the WHO’s maximum admissible limit of drinking water of 0.5 mg L^-1. However, when excessive N rate, which was about twice the optimal N rate for oil palm, was applied, the NH_4-N concentration in the groundwater was increased to 2.7 mg L^-1. This indicated that contamination of the groundwater of monitoring well might occur if large amount of unabsorbed N from soluble N fertilizer was present in the soils during the monsoon period.

The NO_3-N concentrations in the groundwater were also raised by the applications of N fertilizer ranging from 0.07-0.25 mg L^-1 (Fig. 12). However, they were all below the WHO’s maximum admissible limit of drinking water of 10 mg L^-1.

Without K application, the K concentrations in the groundwater were very low at less than 1 mg L^-1 (Fig. 12). The applications of K fertilizer increased the K concentrations of groundwater to between 4.28 and 9.54 mg L^-1. The higher concentration of K in the groundwater in the absence of N (N0P2K1) compared with N1P2K1 might be contributed by the higher leaching losses due to poorer K uptake by the palm as reflected the poorer yield (Table 1). However, only when excessive rate of N was applied (N2P2K1), the concentration of K in the groundwater was increased
which was higher than at N0P2K1 and N1P2K1.

Fig. 12: Effect of fertilizer treatments on groundwater quality

Fig. 13: Rainfall pattern and N and K concentrations in groundwater during the monsoon period (Oct 08-Feb 09) in Sabah at 15 days interval for a period of 150 days

Rainfall pattern and nutrient concentrations in groundwater in monitoring well: The NH_4-N concentrations were relatively similar between the fertilizer treatments in the first 30 days after application (Fig. 13). However, the continuous heavy rains over the next 15 days resulted in a sharp rise in NH_4-N concentration in the groundwater of N2P2K1. It then showed a declining trend in NH_4-N concentration although high rainfall tended to increased it temporary. The NH_4-N concentration in the groundwater of N1P2K1 started to decrease 45 days after fertilizer application whereas in N0P2K1 treatment, it declined to its baseline value after only 30 days. The NH_4-N concentrations of both treatments were similar from day 75 onwards.

The NO_3-N concentrations in the groundwater of all fertilizer treatments were low but despite this, the fluctuations in NO_3-N concentration seemed to correspond with the rainfall pattern particularly for N2P2K1 (Fig. 13). The temporal variation in total inorganic N concentrations was similar to NH_4-N concentrations since the latter ion dominated the fraction of inorganic N (Fig. 13).

The K concentrations in the groundwater of the fertilizer treatments, N2P2K1, N1P2K1 and N0P2K1, were similar 15 days after applications. However, the heavy rains between day 15 and 45 which caused an increase in NH_4-N concentration as discussed above probably also displaced soil K with consequent higher K concentrations in the groundwater of treatments, N2P2K1 and N1P2K1. In treatment N2P2K1, the K concentration declined during the period of 45-60 day when rainfall was low before increasing again probably due to the excess applied K reaching the groundwater. However, the same effect was not seen in treatment, N1P2K1, because there was no excess K⁺ reaching the groundwater. Without N, the K concentration in the groundwater declined continuously until 90 day before it increased again. This implied that it took about 90
days before the applied K which was not absorbed by the palms reached the groundwater during the monsoon period.