Efeitos de diferentes niveis de irrigação sobre o crescimiento de mudas de Pinus (Pinus elliottii var. elliottii) e de Eucalipto (Eucalyptus urograndis) cultivados em substrato contendo lodo de esgoto e sobre a lixiviação de metais pesados presentes no composto

Abstract

In Brazil, approximately 70% of all water consumed is used in agriculture. Expectations of increased demand, by different segments, require actions that guarantee everyone the right of access to this resource. The treatment of sewage in addition to demanding water produces a residue (sewage sludge), which discarded in landfills reduces the useful life of the same, however is widely used in improving soil fertility, although there are risks associated with the presence of heavy metals . Its use as a substrate in the production of forest seedlings is an interesting alternative. Considering the importance of pine and eucalyptus for the timber industries in Brazil and Argentina, the objective of this study was to evaluate the effects of different levels of irrigation (100%, 200%, 300% and 400% of crop evapotranspiration) on the growth of pine (Pinus elliottii var. elliottii) and eucalyptus (Eucalyptus urograndis) seedlings, by means of the following variables: height (H), collar diameter (CD), height:collar diameter ratio (HCDR), total dry biomass (TDM), ratio of shoot and root biomass (SMRMR), Dickson Quality Index (DQI) and Water Use Efficiency (WUE), grown on substrate containing sewage sludge (60% + 40% vermiculite) and on the leaching of heavy metals (zinc, cadmium, lead, copper, nickel and chrome) present in the compound. The experiment was conducted at the Botanical Garden of Rio de Janeiro. The mini-lysimeter constructed to measure the evapotranspiration of the seedlings had its performance considered satisfactory for measurements in scales lower than the daily, evaluated by precision (r), accuracy (d) and confidence (c) indicators and with errors less than 1 gram per carton. The evapotranspiration (ETc) of the pine and eucalyptus seedlings, observed in the minilismeters presented averages of 2.30 and 1.66 mm d-1, respectively, obtaining average values of Kc equal to 0.95 and 1.48, respectively. No effects of the different applied slides (irrigation levels) on the studied growth variables (H, CD, HCDR, TDM, SMRMR and DQI) were verified. However, the depth corresponding to 100% ETc showed to be more efficient (greater WUE), so that the excess water applied by the superior treatments did not result in effective benefits (mass gain) for the pine and eucalyptus seedlings, representing waste of water. The use of sewage sludge in the substrate composition was feasible, maintaining values similar to those observed in the literature. Regarding heavy metals, the most worrying element was zinc, since, initially, its content in the substrate was 27% above the limit established by resolution No. 375/2006 of CONAMA; and in the effluent generated by the excess irrigation was in the blades equivalent to 200% and 300% of evapotranspiration above 39.4% and 15.6%, respectively, of the limits of resolution No. 430/2011 of CONAMA. The other elements studied were always below the limits defined by the two resolutions. The use of sewage sludge as a substrate for the production of forest seedlings is an interesting alternative but requires careful management of irrigation.