Abstract
Nitrogen availability and tree species selection play important roles in reforestation. However, long-term field studies on the effects and mechanisms of tree species composition on N transformation are very limited. Eight years after tree seedlings were planted in a field experiment, we revisited the site and tested how tree species composition affects the dynamics of N mineralization and nitrification. Both tree species composition and season significantly influenced the soil dissolved organic carbon (DOC) and nitrogen (DON). N-fixing Acacia crassicarpa monoculture had the highest DON, and 10-mixed species plantation had the highest DOC. The lowest DOC and DON concentrations were both observed in Eucalyptus urophylla monoculture. The tree species composition also significantly affected net N mineralization rates. The highest rate of net N mineralization was found in A. crassicarpa monoculture, which was over twice than that in Castanopsis hystrix monoculture. The annual net N mineralization rates of 10-mixed and 30-mixed plantations were similar as that of N-fixing monoculture. Since mixed plantations have good performance in increasing soil DOC, DON, N mineralization and plant biodiversity, we recommend that mixed species plantations should be used as a sustainable approach for the restoration of degraded land in southern China.
Highlights
Previous studies showed that tree species composition was a major factor affecting N turnover in various vegetation types globally[15,19,22]
After 8 years of plant growth, the pH, soil organic matter (SOM), total N, total P and available P in the 0-10 cm soil layer were all significantly affected by the experimental plantation types (Table 2)
The net N mineralization and nitrification rates in these experimental plantations demonstrated that tree species composition and stand age are important factors influencing soil nutrient availability
Summary
Previous studies showed that tree species composition was a major factor affecting N turnover in various vegetation types globally[15,19,22]. Wang, et al.[23] reported that the net N mineralization of 10-mixed species plantation was over two folds higher than that of Eucalyptus urophylla monoculture in a two-year restoration plantation. In two-year-old plantations, Wang, et al.[23] found that monoculture Eucalyptus and Acacia plantations had lower soil N mineralization rates and reduced N leaching loss relative to 10- and 30- species mixed stands. In another study, they investigated soil nutrients availability in 23-year-old plantations, and found that Eucalyptus and Acacia monoculture had higher, or at least equal, soil N transformations rates than the native species plantations[24]. Because of the strong effects of tree species composition and age on ecosystem development, here, we investigated soil net N mineralization and nitrification rates of six eight-year-old plantations in a controled forest experiment in southern China. We test the following three hypotheses: 1) mixed plantations would have higher soil net N mineralization and nitrification rates than monoculture; 2) Due to additional N input, net N mineralization rate of N-fixing species would be higher than that of non-N-fixing species; 3) soil net N mineralization and nitrification rates of plantations would vary with seasons and stand ages, due to the seasonal variation of temperature and rainfall and the plant growth with litter feedback
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