Organic carbon leaching from effluent irrigated lysimeters as affected by residence time

Under irrigation with secondary effluent, it is often assumed that reduced leaching would delay the transport of effluent organic matter (OM) from within the root zone for long enough for biodegradation to practically eliminate it. This hypothesis was tested in a lysimeter study which compared tap water with low-quality secondary effluent (total organic C [TOC] 190 mg C L-1) for Eucalyptus irrigation. Lysimeters (200-L drums) were packed with dune sand or one of two clayey soils (A horizons of a Calcic Haploxeroll and a Typic Palexeralf) and were either not planted or planted. Leaching treatments were leaching fraction 1 (LF 1) (not planted) and LF 0.2, intermittent leaching and prolonged deficit irrigation. These provided residence times of the water in the soil that ranged from 0.8 to over 200 d. While root organic C (OC) made a negligible net contribution to leachate OC, the effluent-derived OC was rather recalcitrant as was indicated by: (i) the concentrations of TOC in the leachate increased with decreasing LF to over 250 mg C L-1; (ii) the recovery of the applied TOC in the leachate was significant (10-20%) even at RTs over 20 d (LF congruent to 0.06 in the sand-packed lysimeters). Most all the dissolved and colloidal organic C (DOC) was smaller than 30 kDa. We expect these concentrations of low-molecular-weight mobile OC to govern many aspects of the biological and chemical behavior in solutions of effluent-irrigated soils, especially under reduced-leaching regimes.