Abstract

Introduction

The United Nations summit on Sustainable Development in Johannesburg in September 2002 called for the protection of the marine environment from land-based activities to reduce, prevent and control waste and pollution, and minimize their health-related impacts in small island developing states. It also set the target that by 2020 the use and production of chemicals in these states would not lead to significant adverse effects on human health and the environment (United Nations 2002).

The management of water resources on small islands is affected by complex and dynamic interactions of actors with different demands and interests. Agricultural activity is often one of the main actors influencing a society’s water resources and environmental integrity.

On Tongatapu, the main island of the Kingdom of Tonga, an increased export of squash (Cucurbita maxima) to Japan has led to an increased import of agrichemicals. Since the start of the squash industry in 1987, the export of squash has increased from less than 1 tonne, to about 15 tonnes: the importation of agrichemicals increased from 0.25 to 1.7 million T$ over the period 1999 – 2001. These agrichemicals include fertilizers, as well as pesticides. Here we just discuss our findings in relation to the use of nitrogen fertilizer.

There are community concerns about the pollution of groundwater and the connected lagoon (Figure 1) due to the increased agricultural intensity. These concerns led to the EU and NZAID sponsored project called ‘CROPPRO’ (www.croppro.alterra.nl). This project focuses on the sustainable development of agriculture in small resource-constrained Pacific islands.

Figure 1. Tongatapu is a raised atoll with an internal lagoon (top left). The underlying freshwater lenses (bottom left), which are connected to the lagoon and the fringing reef, provide the potable water supply. Squash pumpkin production (top right) relies on the use of fertilizers, and pesticides (bottom right).

Freshwater on these islands is present as lenses floating on denser salt water underneath. Our research sought to develop alternative production practices to avoid losses of agrichemicals from the rootzone to the fresh-water lenses.

Water Flux Meters

In this tropical environment, robust and easily operated tools are needed to quantify the drainage of water and solutes towards the freshwater lenses. To evaluate the leaching of agrichemicals on Tongatapu, we used three types of newly developed water flux meters (WFM), four suction types (Gee et al. 2003), and two non-suction models. Here we report our results for drainage and nitrogen leaching using the non-suction WFMs.

Figure 2. The buried flux meter showing the tipping-bucket flow-recording device, and the reservoir from which solution can be collected using a syringe (upper right).

Figure 3. The flux meter about to be buried in a pit where it can record drainage and allow sampling of the leachate.

The WFMs were installed in an agricultural field cropped with squash during the 2002 (van der Velde et al. 2003), and 2003 (Figure 3) growing seasons. The soil is a heavy, structured clay derived from volcanic ash. The grass was ploughed in during June, and several diskings of the field ensured a good seed-bed. Total soil depth is about 2.8 m, after which there is, to depth, a permeable coral limestone base. The freshwater at this site occurs at a depth of 23 m. In contrast to the 2002 season, the WFMs in 2003 had a 200 mm ring on top to minimize flow divergence (Figure 2). The soil above the WFMs was repacked to ensure good contact with the collector. The structure of the repacked soil above the WFMs was similar to that of the ploughed and disked field. The WFMs were connected to a Campbell CR10X datalogger for continuous measurement of the drainage. Also connected to the logger were two Campbell Scientific CS615 soil moisture probes. The leachate from the WFMs was sampled manually after drainage had occurred. Cumulative rain, drainage and plant water use is shown in Figure 4 and the drainage flux and soil moisture changes in Figure 5.

Results and Discussion

During the growing season of 2002, our WFMs only measured, in total, some 16% of the cumulative rain. Then, there was no confining ring above the WFM. The data now presented for 2003 shows a capture of drainage that accounts for 58 to 82 % of the cumulative rain. This demonstrates the effectiveness of the flow-confining rings above the WFMs for ensuring representative sampling of the drainage.

During the period between DOY 220 and 260, there were three large rainfall events (Figure 5). Large volumes of drainage were recorded after these rains. Saturated flow, as measured by these non-suction WFMs, accounts for a large fraction of the deep drainage under these tropical conditions (Figure 4). As the crop cover developed beyond DOY 240, only then did transpiration increase. After full leaf-cover developed, and transpiration became higher, no further saturated drainage was measured by these WFMs. In Figure 4 is shown the modeled water use by the squash pumpkins. The modeled transpiration was corroborated using measurements of plant-water use. The transpiration was measured using heat-pulse equipment to monitor sap flow in the stems of the plants (Figure 6).

Here WFM 1 and WFM 2 refer to the two flux meters, and SM 90 1 and SM 90 2 are the two Campbell soil moisture probes. Both measured rain and modeled transpiration are shown along with drainage in Figure 4.

Figure 6. The water-use of the squash pumpkin was measured using heat-pulse monitoring of the sap flow in the stems of the plant. The white wires in the stem on the right of the photo are connected to a heater needle (centre), and two temperature sensors either side.

The standard fertilizer practice is that the soil of the mounds (1.5 x 1.5 m) receives an application of 120 g NPK-S fertiliser (62 kg of N/ha) fertilizer at planting. In 2003, the NPK-S was applied to the mounds on DOY 204. Planting took place on the 25^th of July (DOY 207). The drainage water collected from the WFMs was analyzed for NO₃-N (Figure 7) and NH₄-N. From Figure 4 it can be seen that a significant amount of drainage (Figure 5) and leaching (Figure 7) can occur before the leaf-area of the plants had grown to any extent. Given the low levels of plant water use, the majority of the rainfall is lost as drainage, and it carries with it a lot of nitrogen.

Figure 7. The measured concentration of N-NO₃ (ppm) in the drainage water of the WFMs.

Both WFMs record elevated levels of NO₃-N., and the first peak in NO₃-N can be observed in the leachate shortly after the N fertilization on DOY 204. There appears to be a gradual rise in the level of NO₃-N as further rainfall seems effective at leaching the applied N from the soil. The concentration in the drainage is well above WHO drinking water standards of 11.3 ppm NO₃-N for drinking water. The total amount of N lost over this period is about 125 kg-N/ha, which is nearly twice that applied as fertilizer. It would seem that the remainder has been supplied by N mineralized from the composting of the ploughed-in grass. Irrespective, it is clear that the N from the initial fertilization has been lost. Between DOY 200 and 250, the majority of the 350 mm of rain was lost as drainage because plant transpiration was insignificant as the plants had yet to develop a full canopy. Because the root system would not have been extensive, the plants would be incapable of taking up either water, or nitrogen.

The initial application of N would not only seem wasteful, but also prejudicial to the quality of the receiving waters.

Conclusion

A mix of social, economic and environmental factors drives conjunctive management of Tongatapu’s agricultural practices and water resources. It is important that economically sustainable squash-production practices are developed, and that these protect the island’s susceptible water resources and fragile environment

Our measurements and modeling of the water balance and nitrate fluxes is a first step in an integrated effort to find balanced solutions to develop economic production systems for squash farmers, and to ensure that these protect Tongatapu’s freshwater resources and aquatic environments. Our next step will be to prescribe sustainable pesticide practices that are effective for disease control, yet do not result in the leaching of the active ingredients to groundwater.

References

Gee, G.W., Z. F. Zhang and A.L. Ward., 2003. A modified vadose zone fluxmeter with solution collection capability, Vadose Zone Journal 2, 627 – 632.

van der Velde, M., Vanclooster, M., Snow, V., Green, S., Manu, V., Clothier, B., Minoneti, V., Pochet, G. and van den Dijssel, C., 2003. Fertilisers and the contamination risk to groundwater and the lagoon of Tongatapu island, Kingdom of Tonga. In: Environmental Management Using Soil-Plant Systems. Eds. L.D. Curie, R.B. Stewart and C.W.N. Anderson. Occasional Report No. 16. Fertiliser and Lime Research Centre, Massey University, Palmerston North. 262 -270 pp.

United Nations. Johannesburg Summit 2002. Key Commitments, Targets and Timetables from the Johannesburg Plan of Implementation. 7 pp.