Agroforestry systems for municipal effluent disposal

Seven agroforestry (AF) systems namely Eucalyptus grandis Hill ex Maiden (flooded gum, E), Dendrocalamus latiflorus Munro (Ma bamboo, B) and Digitaria decumbens Stent (Pangola, P), and their mixed plantations (BE, BP, EP, BEP) were monitored during the initial two years of growth for their efficiency in effluent removal. The municipal effluent irrigated plantation trial was established on a 1.6 ha site in the Capricornia coastal region at Yeppoon, Queensland in June 2002. This site was monitored until June 2004 for changes in site hydrology, physical, chemical and microbial properties of soil, and for plant growth, biomass production and nutrient uptake. The residual maximum likelihood method (REML) and analysis of variance (ANOVA) were extensively used to statistically compare the changes.

The plantations were irrigated at a modelled conservative irrigation rate of 1.42 ML ha^-1yr^-1. The total water use of all the AF systems was four to six times of that provided via irrigation. The mixed AF systems used more water than monoculture AF systems. During the initial two years, all the AF systems were effective in utilising effluent (both water and nutrients) due to the presence of groundflora. Over time, the efficiency and total water use may decline with reduction in groundflora and photosynthetically active canopy.

The vegetation of these AF systems accumulated six to ten times the major nutrients supplied via effluent irrigation. However, the AF systems with the exception of the E system failed to take up the supplied sodium and sulphur. Progressive build up of sodium in the soil would introduce sodicity in the soil and this may restrict the long term use of effluent disposal site. The electrical conductivity (EC_1.5) increased significantly in upper two soil layers whereas exchangeable sodium, CEC and ESP significantly increased over time in all the soil layers. Although soil compaction, reduction in infiltration rate, and increase in electrical conductivity and exchangeable sodium were observed at the site, the current changes did not indicate any ecological degradation. A long term study may help determine the optional rotation age of the AF crops to avoid adverse impact to the site.

Although the build up of microbiota in the effluent irrigated site increased considerably in all the AF systems, it did not reveal any health hazard. A study in mortality rates of different enteric bacteria indicated that the pathogens died within 15 days in winter and in less than 27 days in summer upon cessation of irrigation. Restricting access to the effluent irrigated site for up to 30 days would reduce the probability of a human health hazard due to the effluent irrigation.

At the conservative rate of irrigation (1.42 ML ha^-1yr^-1), the plantations produced up to 72 tonne biomass ha^-1 (dry biomass) within 24 months. Largely, the biomass produced in the AF systems was non-woody, thus it would need non-conventional markets. Leaves were the major site of nutrient storage. Senescence of leaves and twigs may accelerate nutrient recycling and this may defeat the objective of effluent irrigation. Moreover, the plants were selective in nutrient sequestering. Not more than one kg ha^-1 of any micro-nutrients was accumulated by bamboo and groundflora. For sustainability of effluent irrigation, the plantations may have to be either harvested in short rotation or irrigation has to be regulated according to plant growth. The commercial viability of the biomass produced in the AF systems was assessed to determine local suitability of the plantation models.

Canonical variate analysis (CVA), a type of multiple variate analysis, based on the combined data of various parameters showed differences between AF systems. These differences are likely to diverge with age of the plantations. The CVA based on more than four parameters showed differences amongst the AF systems. Moreover, the CVA for the datasets of some parameters collected at different time also revealed divergence amongst the AF systems. The analysis showed that the AF systems may develop contrasting differences with maturity. Monoculture E plantation produced more biomass than other AF systems, and it also accumulated more nutrients, compared to mixed AF systems. In contrast mixed AF systems (e.g., BP and BE) used more water than the monoculture systems. Overall, at 24 months, the mixed AF systems prove to offer improved benefits for effluent irrigation in terms of technical feasibility, ecological safety, and commercial viability of the effluent irrigated plantations.