Effect of repeated applications of sewage sludge on the fate of N in soils under Monterey pine stands

An experiment was conducted to evaluate the influence of repeated application of sewage sludge, on N dynamics in soils under Monterey pine (Pinus radiata D. Don.) stands. Sludge with a total N concentration of 4.2% was applied at four different loading rates (0, 2.4, 17, and 60 Mg ha−1 year−1, DW equivalent), in two consecutive autumns. Soils were sampled once a year to a depth of 53 cm, and leachates were collected at 25 and 50 cm depth, after heavy rain. The soils under study have low pH (4.1-4.4), low CEC, and low base saturation (<15% below the first 4 cm depth), typical of soils located in a high leaching environment. Addition of municipal sewage sludge at a rate of 2.4 Mg ha−1 in two consecutive years did not result in significant NO3−-N and NH4+-N leaching, with concentrations in leachates at 50 cm below 10 and 0.3 mg L−1, respectively, although nitrification was enhanced after the second application. The higher doses of sludge resulted in contamination of groundwater by NO3−-N and NH4+-N, even after only one application, with peaks of 104.2 and 48.7 mg L−1, respectively, for the 60 Mg ha−1 treatment. Soil acidification at depth was observed with all treatments (pH ranging from 3.6 to 4.0). Plant analysis indicated that mean foliar N concentration was significantly greater (P < 0.01) in the control (22.0 g kg−1) than in the sludge amended pine trees (all <17.5 g kg−1), whereas the inverse pattern was observed for mean foliar P (3.0 mg kg−1 versus >4.0 mg kg−1). Pine growth, as indicated by the trunk diameter, was significantly greater (P < 0.05) in the sludge-amended stands than in the control stands, whereas there were no significant differences (P < 0.05) between the different sludge treatments. The concentration of NO3−-N in leachates from the control plots was high (mean at 50 cm depth: 19.6 mg L−1 NO3−-N) due to subsurface lateral flow, therefore differences in plant growth were attributed to P deficiency in the non-amended plots. The results obtained indicated that in this weakly acid-buffered forest soil, sewage sludge application at a low rate (2.4 Mg ha−1) in two consecutive years is a feasible method of fertilisation - if lime is applied to counterbalance the acidifying effect of N oxidation - with minimal risk of groundwater contamination by NO3−, although the risk of heavy metal contamination has yet to be clarified.