1. Introduction
Agricultural fields may be regarded as nonpoint pollution sources, because their runoff may contain residual fertilizers and pesticides that negatively affect the quality of downstream water bodies. Fertilizers are necessary to ensure crop yields, but are often applied excessively. The excessive fertilizers remain on top of the soils and enter the groundwater through deep percolation, which results in environmental pollution. Alternative methods of fertilization and cultivation have been discussed to reduce the amount of residual fertilizers that are transported into waterbodies [
1,
2,
3,
4,
5]. For example, Liu
et al. [
4] found that applied organic and low-release fertilizers can reduce the loss of nitrogen (N) and phosphorous (P) and maintain tea yields. Sainju
et al. [
2] suggested that cultivating a combination of legumes and non-legumes and reducing the use of N fertilizers can decrease the loss of soil N and benefit crop yields. Kim
et al. [
6] confirmed through model calculations that the application of excess fertilizers produces high levels of N and P in the runoff water.
The application of excess fertilizer can result in high nutrient loss. For this reason, numerous best management practices (BMPs) have been implemented to mitigate the detrimental effects of nutrient loss on soil and water. Traditional BMPs treat collected runoff pollutants as a whole, and specific BMPs for N or P have not been designed. However, some water bodies are affected by particular pollutants; therefore, a specific BMP might be more effective. The Feitsui Reservoir is located in northern Taiwan and supplies drinking water to five million people in Taipei City and New Taipei City, Taiwan. P concentrations are a major cause of concern for the water quality in the Feitsui Reservoir [
7], and tea plantations are the one of main pollution sources. Zehetner
et al. [
8] analyzed sediments from the Feitsui Reservoir and tea soils and confirmed that tea plantations are the sources of P in the reservoir. Chang [
9] sampled runoff from tea plantations and concluded that the P concentration in the runoff after fertilization could be up to three times greater than the concentration before fertilization. Wu [
10] evaluated long-term water quality monitoring data and found that the total phosphorous (TP) and NH
3-N concentrations were increased in areas proximal to intensive tea cultivation. Tea cultivation has been the traditional economic activity in the Feitsui watershed, even before construction of the reservoir. Many BMPs that include structural and nonstructural measures have been implemented to reduce nonpoint pollution from tea plantations. In this study, we implemented a novel nonstructural BMP that was designed specifically for the control of P.
Excessive P fertilization causes P to accumulate in the soil, and its residues then flow into bodies of water along with stormwater runoff [
1]. Thus, the appropriate application of P fertilizer is an ideal BMP that may reduce the threat of excessive P concentrations and provide the required nutrients for tea cultivation. However, the process by which applied P is transformed and made available to the plants is complex, which makes it difficult to determine the optimal quantity of applied P. The available P for plants is derived from liquid and solid phases and involves biological and non-biological transformations [
1,
11]. Applied P changes the P stock in soils and influences the amount of P that is available for plants. Applied organic P helps to increase the available P in the topsoil and deeper soil layers. However, the excessive organic P might change soil texture and the adsorption capacity of soils, and reduced adsorption capacity and increased porosity may result in higher soil erosion and P loss, which increases the risk of water pollution.
Previous studies have suggested that the reduction or elimination of cultivation would help to fix P in the soil and reduce its loss [
12,
13]. Rational fertilizer application is also regarded as a BMP for nonpoint source pollution control. In this study, we introduced low-P fertilizers as a new BMP that is similar to the rational fertilizer method, but is specifically focused on the problem of excessive P. Traditional fertilizers were modified to include less than half of the usual amount of P and were then applied to the low-P field. The Taipei Feitsui Reservoir Administration and the Tea Research and Extension Station cooperated with the Taiwan Fertilizer Company to manufacture low-P fertilizers for tea cultivation. This study implemented low-P fertilizer and regular-P fertilizer treatments and monitored and compared the growth, yield and quality of tea plants, as well as the impact to soil and runoff.
3. Results and Discussion
3.1. Soil Quality
The soil fertility in the spring and winter seasons is summarized in
Table 2 and
Table 3, respectively. The soil was sampled during the harvest seasons, and the samples demonstrate the effects of the different fertilizers on the soil. We also tested the soil fertility before fertilization to characterize the original soil status. The soil quality was similar in both test fields before fertilization, because the two test fields were located in the same area and subject to the same environmental conditions. Therefore, we will only discuss the difference in soil quality during the harvest seasons.
Table 2.
Soil fertility in the spring harvest season.
Table 2.
Soil fertility in the spring harvest season.
Treatment | pH (1:1) | Organic matter (OM) (g/kg) | Bray-1 P (mg/kg) | Extractable K (mg/kg) | Extractable Ca (mg/kg) | Extractable Mg (mg/kg) |
---|
Regular-P | | | | | | |
Topsoil | 4.01 ± 0.16 | 91.2 ± 8.2 | 776.3 ± 178.2 | 269.8 ± 27.2 | 504.1 ± 116.0 | 32.4 ± 7.6 |
Bottom soil | 4.26 ± 0.10 | 65.7 ± 5.0 | 400.2 ± 133.2 | 108.4 ± 11.5 | 414.8 ± 44.7 | 36.7 ± 3.5 |
Low-P | | | | | | |
Topsoil | 4.35 ± 0.07 | 99.9 ± 4.4 | 920.6 ± 78.5 | 221.1 ± 12.7 | 465.9 ± 86.9 | 41.2 ± 9.2 |
Bottom soil | 4.39 ± 0.16 | 59.4 ± 1.7 | 681.4 ± 65.0 | 157.5 ± 9.4 | 373.3 ± 53.3 | 39.7 ± 5.3 |
Table 3.
Soil fertility in the winter harvest season.
Table 3.
Soil fertility in the winter harvest season.
Treatment | pH (1:1) | OM (g/kg) | Bray-1 P (mg/kg) | Extractable K (mg/kg) | Extractable Ca (mg/kg) | Extractable Mg (mg/kg) |
---|
Regular-P | | | | | | |
Topsoil | 3.64 ± 0.14 | 65.0 ± 6.2 | 526.8 ± 195.3 | 201.7 ± 13.1 | 525.9 ± 150.1 | 32.0 ± 7.3 |
Bottom soil | 3.66 ± 0.08 | 34.5 ± 1.4 | 192.5 ± 156.9 | 130.0 ± 11.8 | 203.6 ± 10.4 | 18.0 ± 2.5 |
Low-P | | | | | | |
Topsoil | 3.55 ± 0.04 | 76.7 ± 9.9 | 454.3 ± 164.4 | 216.3 ± 3.78 | 373.3 ± 52.3 | 28.9 ± 3.6 |
Bottom soil | 3.73 ± 0.03 | 32.7 ± 3.4 | 231.0 ± 96.1 | 139.2 ± 3.3 | 259.4 ± 26.3 | 24.3 ± 2.4 |
The spring season soil samples were collected in April, and they showed that the soil acidity was moderate and the organic matter was increasing, especially in the topsoil. The increase in organic matter resulted from plowing the topsoil and incorporating the pruned leaves. The available P in the soil had increased in both fields, and the concentration was higher in the low-P field than in the regular-P field. Both sites had already been rich in available P before fertilization, with the average available P in the topsoil of the low-P site at 669 mg/kg and the regular-P site at 579 mg/kg. Even the application of the low-P fertilizer resulted in excess P, because of the high original P values. Mineral elements, such as K, Ca and Mg were moderate and similar in both fields. The spring test results were influenced by the precondition of the soil and masked the effect of low-P fertilization in reducing the residual P concentrations.
The second soil sampling was conducted in October during the winter tea harvest. The soil was more acidic than in the spring, and there were less organic matter and available P. The available P in the soil of both sites was still high, but there was a significant reduction in available P in the low-P site. There was a 50.6% reduction in available P in the low-P site from spring (920 mg/kg) to winter (454 mg/kg), which was greater than the reduction at the regular-P site (32.2%). K and Ca were present in moderate amounts, but Mg was less than the recommended concentrations at both sites.
3.2. Effluent Water Quality
The results of the effluent water quality analyses for the regular-P and low-P sites are summarized in
Table 4. The regular-P site had high conductivity, TDS, COD and total nitrogen (TN), and the low-P site had high average SS and TP concentrations. The SS concentration was affected by site disturbances and had a high variance. The average SS concentrations of the regular-P site and the low-P site were 50.5 mg/L and 68.0 mg/L, with standard deviations of 63.7 and 24.9, respectively. The results showed high variability on the SS concentration. The test fields were flat and well maintained for tea production, and they were cultivated and fertilized in the same manner. It was assumed that the similar field environment and soil conditions should result in analogous soil erosion and SS concentration. The difference in SS concentration might have been influenced by the transportation processes that occurred in underground drainage instead of from the application of the different fertilizers. The TP concentration also showed high variation. Although the data showed a higher average TP concentration at the low-P site, there was a decreasing trend of TP concentrations (
Figure 4). The TP concentration might have been affected by soil preconditions, and the soil quality analysis confirmed this hypothesis. There was an increased amount of available P in the soil of the low-P site compared to that of the regular-P site at the start of the experiment. In the winter season, however, the available P in the soil of the low-P site then decreased and became lower than that of the regular-P site. The effluent quality from the low-P site followed the same trend, with a higher initial concentration and lower concentration as the season progressed. The TP concentration in the soil of the regular-P site was unstable and showed no clear trend.
The effluent from the regular-P field had very high TN (22.5 mg/L). The major constituents of TN are NO2-N and NO3-N. The NO3-N concentration was 11.75 mg/L in the effluent from the regular-P site, which was almost 10-times greater than that from the low-P site. High NO2-N and NO3-N present threats to the groundwater and the receiving water bodies. In contrast, the major constituent of TN in the effluent from the low-P site was TKN, which consists of ammonia N and organic N, and was available for biological utilization. However, the mechanism by which the P fertilizers influenced the N contents in the soil or effluents was unclear. Additional research and discussions are required to resolve this issue.
Table 4.
The quality of effluents from tea plantations with different experimental fertilizer treatments. DO, dissolved oxygen; TDS, total dissolved solids; COD, chemical oxygen demand; SS, suspended solids; TP, total phosphorus; TKN, total Kjeldahl nitrogen.
Table 4.
The quality of effluents from tea plantations with different experimental fertilizer treatments. DO, dissolved oxygen; TDS, total dissolved solids; COD, chemical oxygen demand; SS, suspended solids; TP, total phosphorus; TKN, total Kjeldahl nitrogen.
Treatment | | DO (mg/L) | pH | Conductivity | TDS (mg/L) | COD (mg/L) | SS (mg/L) | TP (mg/L) | TKN (mg/L) | NO2-N (mg/L) | NO3-N (mg/L) | TN (mg/L) |
---|
Regular-P | Average | 9.84 | 7.45 | 198.98 | 143.22 | 78.20 | 50.50 | 0.45 | 5.40 | 7.16 | 11.75 | 22.52 |
SD | 4.17 | 0.72 | 139.56 | 74.70 | 88.10 | 63.75 | 0.40 | 8.49 | 11.63 | 10.56 | 20.27 |
Low-P | Average | 13.47 | 6.98 | 176.88 | 128.14 | 54.73 | 68.00 | 0.88 | 6.53 | 0.54 | 1.40 | 8.47 |
SD | 10.60 | 0.49 | 126.88 | 84.47 | 27.23 | 24.92 | 0.70 | 4.75 | 0.79 | 1.02 | 5.50 |
Figure 4.
The TP concentration of effluents in each sample. There is a decrease in TP concentration in the effluent from the low-P site and an unstable distribution of TP concentration in the effluent from the regular-P site.
Figure 4.
The TP concentration of effluents in each sample. There is a decrease in TP concentration in the effluent from the low-P site and an unstable distribution of TP concentration in the effluent from the regular-P site.
3.3. Tea Quality
The appearance and yield of harvested tea leaves were not significantly different in the two fields, but the thickness of the tea leaves and the number of tea buds were significantly different in the winter season (
Table 5). The thickness of the tea leaves and the amount of harvested tea from a fixed sampling area in the low-P field were lower than those in the regular-P field, which was most likely a result of insect damage to the low-P field in the winter. There was no difference in the general appearance of the harvested tea leaves from the two fields.
Table 5.
Agricultural characteristics of the tea buds and leaves with different fertilizer treatments.
Table 5.
Agricultural characteristics of the tea buds and leaves with different fertilizer treatments.
Treatment | Leaf length (mm) | Leaf width (mm) | Internode length (mm) | Leaf thickness (μm) | Number of tea buds from a standard area (buds) | Weight of 100 tea buds (g) |
---|
Spring |
Regular-P | 69.4 ± 2.1 a | 33.1 ± 0.8 a | 32.2 ± 1.5 a | 27.8 ± 0.4 a | 45.6 ± 1.4 a | 52.2 ± 2.7 a |
Low-P | 66.8 ± 1.9 a | 32.6 ± 0.8 a | 29.0 ± 1.0 a | 27.5 ± 0.4 a | 42.0 ± 3.9 a | 53.7 ± 2.6 a |
Winter |
Regular-P | 61.2 ± 1.1 a | 31.2 ± 0.5 a | 24.7 ± 0.8 a | 26.7 ± 0.3 a | 43.0 ± 2.8 a | 54.4 ± 1.5 a |
Low-P | 58.4 ± 1.1 a | 31.2 ± 0.5 a | 25.4 ± 0.8 a | 25.3 ± 0.2 b | 27.2 ± 2.9 b | 53.8 ± 1.9 a |
Table 6 shows the characteristics of the harvested tea leaves and buds from the two fields. There were few significant differences between the leaves and buds from the two fields in either the spring or winter season. The elements contained in the tea leaves were approximately the same, which indicates that excessive fertilization does not help to increase plant nutrient uptake. The P concentration in the tea leaves was 3.1 g/kg in spring and 3.6 g/kg in winter.
Table 6.
Element contents of tea leaves with different fertilizer treatments.
Table 6.
Element contents of tea leaves with different fertilizer treatments.
Treatment | N (g/kg) | P (g/kg) | K (g/kg) | Ca (g/kg) | Mg (g/kg) |
---|
Spring |
Regular-P | 28.7 ± 0.8 b | 3.1 ± 0.1 a | 11.6 ± 0.1 a | 2.9 ± 0.1 a | 1.4 ± 0.0 a |
Low-P | 36.9 ± 0.7 a | 3.1 ± 0.1 a | 11.8 ± 0.1 a | 3.1 ± 0.0 a | 1.4 ± 0.0 a |
Winter |
Regular-P | 43.0 ± 0.3 a | 3.6 ± 0.0 a | 26.1 ± 1.2 a | 2.4 ± 0.0 b | 1.6 ± 0.0 a |
Low-P | 42.2 ± 0.2 a | 3.6 ± 0.0 a | 25.6 ± 2.4 a | 2.7 ± 0.0 a | 1.6 ± 0.0 a |
Treatment | Fe (mg/kg) | Mn (mg/kg) | Cu (mg/kg) | Al (mg/kg) | Zn (mg/kg) |
Spring |
Regular-P | 218.6 ± 6.8 a | 720.0 ± 5.5 b | 13.1 ± 1.5 a | 720.3 ± 5.7 a | 29.4 ± 1.7 a |
Low-P | 118.4 ± 1.5 b | 796.0 ± 3.8 a | 11.3 ± 0.2 a | 544.0 ± 3.7 b | 24.2 ± 0.6 a |
Winter |
Regular-P | 112.8 ± 1.7 a | 787.3 ± 14.3 a | 7.0 ± 0.1 a | - | 20.3 ± 0.3 a |
Low-P | 106.9 ± 1.9 a | 767.5 ± 9.6 a | 6.7 ± 0.2 a | - | 17.3 ± 0.4 b |
We invited three local tea sensory evaluation experts to judge the quality of the tea, including the appearance of the treated tea leaves and drinking quality. The total quality scores of the tea from the low-P field were higher than those from the regular-P field in both the spring and winter seasons (
Table 7). The smell and taste of the spring tea were judged to be better, whereas almost all of the drinking quality factors of the winter tea from the low-P field received higher scores. The score for appearance was lower for the low-P tea, but this may have been influenced by the insect infestation. Therefore, the application of low-P fertilizers can improve tea quality on tea plantations that already have sufficient available P in the soil, and excessive fertilization did not improve the tea quality.
Figure 5 is a photograph of the tea sensory evaluation setting.
We analyzed the quality of brewed tea based on the theanine, caffeine, catechin and total catechin contents to determine why the tea from the low-P field received higher taste scores than the tea from the regular-P field. These four compounds affect tea quality, with theanine helping to sweeten the tea and catechin contributing to bitterness. The results of the analysis are shown in
Table 8. For spring tea, only the theanine content of the tea that was grown in the low-P and regular-P fields differed significantly. None of the compounds differed significantly in the winter tea. However, the higher theanine content of the tea from the low-P field might emphasize the sweetness, which resulted in a higher tea quality score for tea from the low-P field.
Table 7.
The results of sensory evaluations of the manufactured tea (graded by three tea experts, with a total score of 100).
Table 7.
The results of sensory evaluations of the manufactured tea (graded by three tea experts, with a total score of 100).
Treatment | Appearance (20%) | Color of liquid (20%) | Aroma (30%) | Taste (30%) | Total score (100%) |
---|
Spring |
Regular-P | 11.0 | 14.0 | 21.0 | 21.0 | 67.0 |
Low-P | 11.0 | 14.0 | 25.5 | 25.5 | 76.0 |
Winter |
Regular-P | 15.3 | 13.3 | 23.3 | 22.3 | 74.3 |
Low-P | 14.7 | 14.7 | 24.0 | 23.0 | 76.3 |
Figure 5.
Photograph of the tea sensory evaluation setting. Evaluations included the appearance of the manufactured tea, the color of the liquid and the aroma and taste of the drinking tea.
Figure 5.
Photograph of the tea sensory evaluation setting. Evaluations included the appearance of the manufactured tea, the color of the liquid and the aroma and taste of the drinking tea.
Table 8.
The results of a chemical analysis of the manufactured tea.
Table 8.
The results of a chemical analysis of the manufactured tea.
Treatment | Theanine (mg/g) | Caffeine (mg/g) | Catechin (mg/g) | Total catechin (mg/g) |
---|
Spring |
Regular-P | 9.26 ± 0.09 b | 15.55 ± 0.44 a | 23.34 ± 0.53 a | 96.44 ± 1.59 a |
Low-P | 10.76 ± 0.00 a | 17.01 ± 0.42 a | 24.75 ± 1.22 a | 95.16 ± 2.51 a |
Winter |
Regular-P | 6.55 ± 0.31 a | 14.98 ± 0.67 a | 21.54 ± 1.44 a | 46.00 ± 2.26 a |
Low-P | 7.49 ± 0.23 a | 15.13 ± 0.24 a | 19.49 ± 0.42 a | 43.15 ± 0.50 a |
4. Conclusions
Excessive fertilization does not improve plant growth, but threatens soil and water quality. Taipei Feitsui Reservoir Administration in Taiwan attempts to preserve good water quality by promoting low-P fertilizers as a new BMP. This study demonstrated the results of applying low-P and regular-P fertilizers and concluded that low-P fertilizers help to improve the quality of the effluents and brewed tea at the study sites. There were no significant differences in the agricultural characteristics and yield between the low-P and regular-P fields. The taste of the manufactured tea from the low-P field was judged to be better, because the brewed tea contained more theanine. Although this study did not focus on N, the extremely high TN concentration of effluents from the regular-P field should be noted for future studies.
The test fields were located on existing tea plantations and have been cultivated and harvested for many years. Therefore, the precondition of the soil influenced the study results. The available P in the original soil of the low-P test field was already enriched, so the low-P fertilizers were sufficient to maintain the tea yield and even improved the tea quality. Low-P fertilizers should be considered as an alternative BMP for specific situations, but they should not be used as a universal BMP for all croplands.