CHANGES IN SOIL CARBON AND NITROGEN DURING THE ESTABLISHMENT OF A 2ND CROP OF PINUS-RADIATA

Mineralization, leaching and uptake of N, and changes in concentrations of soil C were measured in the top 15 cm of soil during the first 3 years after planting Pinus radiata under three levels of weed control: nil, strip and total. The soil was a podzolised sand. In the nil weed control treatment, the N content of the above-ground weed biomass remained constant at 21 kg N ha(-1) between 15 and 26 months after planting, but decreased to 9 kg N ha(-1) by 37 months. By 37 months, the above-ground N content of trees increased to 17 kg, 30 kg and 50 kg N ha(-1) in nil, strip and total weed control treatments, respectively. In weed-free areas, rates of N mineralization in the surface soil decreased from 76 to 38 kg ha(-1) year(-1) from planting to year 3. The total amounts of N mineralized over 3 years were the same in weedy and weed-free areas ( 171 kg N ha(-1)). Although differences in annual rates of N mineralization also were insignificant, rates in weedy areas were 6-10% lower than in weed-free areas during the first 2 years, and 37% higher during the third year. The amounts of N leached between 8 and 36 months from weed-free and weedy areas were 79 kg and 55 kg ha(-1), respectively. In weed-free areas during approximately the same period, there were decreases in the concentrations of total C (2.7-1.9%), C:N ratios (38-31), specific rates of N mineralization (207-90 g N month(-1) t(-1) C) and estimated rates of soil respiration (0.59-0.26 t C ha(-1) month(-1)). These results suggest that changes in the amounts and forms of organic matter during the first few years after planting led to reduced rates of N mineralization. Although weeds reduced leaching of N, by accumulating N they reduced also the amount of N available for trees. After weeds senesced, more N was mineralized in the weedy compared with weed-free plots, but most of this N was leached below 15 cm depth.