Changes in soil carbon and nitrogen during the establishment of a second crop of Pinus radiata

Abstract

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.