Effect of long-term irrigation with wastewater on growth, biomass production and water use by Eucalyptus (Eucalyptus tereticornis Sm.) planted at variable stocking density

Abstract

Irrigation of high transpiring forest species has been put forward for recycling and reuse of wastewater and conservation of nutrient energy into biomass and thereby bringing multiple benefits such as fuel wood production, environmental sanitation and eco-restoration. But loading rates, the tree plantations can carry, continue to be contradictory. Therefore, the growth patterns, biomass production, water use and changes in soil properties were evaluated for a 10-year rotation of Eucalyptus (Eucalyptus tereticornis Sm.) plantations of variable stocking density and irrigated with either sewage (SW) or a good quality groundwater (GW). The irrigated trees grew rapidly and the stock volumes attained after 10 years were 164.0 and 127.1 m3 ha−1 with SW and GW, respectively. The tree growth improved with stocking density and the maximum shoot biomass (262 Mg ha−1) was produced under high (HD, 1993 stems ha−1), followed by the recommended (RD, 517 stems ha−1; 178 Mg ha−1), very high (VHD, 6530 stems ha−1; 127 Mg ha−1) and low stocking density (LD, 163 stems ha−1; 55 Mg ha−1). Sap flow values almost coincided with growth rates and increased until sixth year of planting and stabilised thereafter. The annual sap flow values ranged between 418–473, 1373–1417 and 1567–1628 mm during 7–10 year of planting under LD, RD and HD, respectively. The daily sap flow values were 0.56*PAN-E (USWB Class A Open Pan Evaporation) during summer months of April–June, 1.24*PAN-E during August–October, i.e. active growth period and 1.12*PAN-E during winter months of December–February. Reference evapotranspiration (ETref) computed using Penman–Monteith method could better describe water use; the sap flow being 0.87–1.23*ETref with an average 1.03*ETref. The water productivity for timber was 1.54, 1.71 and 1.99 kg m−3 for LD, RD and HD, respectively. Similarly, the water use efficiency increased by about 40% with HD and also with SW (11%) under RD. The soil quality improved considerably with sewage irrigation and the plant absorbed carbon was also greater. The annual carbon absorption was 3.5, 12.0, 13.9 and 7.0 Mg ha−1 under LD, RD, HD and VHD, respectively. It is concluded that Eucalyptus plantations can act as potential sites for year round and about 1.5 fold recycling of sewage than the annual crops. However, cautions, rather regulatory mechanism should be devised to control loading rates since these are not as profligate consumers of water as has been claimed.