赣中亚热带森林转换对土壤氮素矿化及有效性的影响

The effects of forest conversion on soil N mineralization and its availability in central jiangxi subtropical region

采用原位培养法和时空替代法,对江西中部亚热带常绿阔叶林、天然马尾松林、人工杉木林、人工马褂木林的土壤氮素矿化速率及其有效性进行了比较研究,以探讨森林转换对土壤氮素矿化作用的影响。结果表明:转换前后各森林土壤无机氮库(NH+4-N、NO-3-N)及氮素矿化速率(氨化速率、硝化速率)均呈现明显的季节动态,NH+4-N库冬春较大,NO-3-N库夏秋较大,氨化速率与硝化速率均以夏秋强烈。森林转换改变了土壤氮素矿化格局,常绿阔叶林转变成马尾松林、杉木林、马褂木林后,土壤年均氨化速率分别降低了110.67%、100.76%、96.20%,而硝化速率提高了54.92%、24.19%、24.46%;马尾松林年均总净矿化速率与常绿阔叶林相近,杉木林、马褂木林分别降低了24.68%、26.01%。另外,除常绿阔叶林外,马尾松林、杉木林、马褂木林的土壤氮素矿化量都小于植被吸收量。这些研究结果说明亚热带地区常绿阔叶林转换成其它次生林会增加氮素流失的危险性,氮素缺乏会成为这些森林生长的限制因子。

Subtropical region has dramatically experienced land practice has persistent effects on many ecosystem processes such as forest conversion from zonal forest to secondary forest can provide on soil. So far, however, few studies have conducted to investigate subtropical region of China. use and coverage change. Historical forest management soil nitrogen （N） mineralization. Soil N dynamics during substantial insight into the impacts of forest management the effects of forest conversion on soil N transformation in Based on a space-time substitution method, we selected four vegetation types including evergreen broad-leaved forest （EBF, zonal forest）, Pinus massoniana forest （PMF, natural regeneration forest）, Cunninghamia （CLP, artificial forest ）, Liriodendron chinense plantation （LCP, artificial forest ） in Dagangshan lanceolata plantation National Ecological Station, Jiangxi province. For each forest type, we randomly set two 20 m×20 m plots separated by at least 30 m buffer zones. The concentration of NH4＋-N and NO3--N, and the rate of N ammonification and nitrification were measured through in situ incubations using the PVC method. Soil properties （total carbon, total N, C ：N ratio, and pH value, 0--15 cm in depth） , forest fine root （living root biomass, dead root storage） and the rate of N absorption of vegetation were alsoquantified. Least significant difference （LSD） in One-way ANOVA was used to determine the effects of forest type on the soil variables. The results showed as follows ： （ 1 ） Inorganic N pools and N mineralization rates in all forests performed seasonal dynamics, NH4＋-N being higher in winter and spring （ 11.64--15.55 kg/hm2） than in summer and autumn （3.89--8.36 kg/hm2）, NO3--N being higher in summer and autumn （0.42--.60 kg,/hm2） than in winter and spring （0.24--6.10 kg/hm2）. The pool of NH4＋-N was considerably larger than NO3--N pool, and thus NH4＋-N was the main component of inorganic N in these forests. The rates of ammonification and nitrification were faster in summer and autumn than in spring and winter. （2） The forest conversion from EBF to PMF, CLP and LCP, soil N mineralization patterns had changed, and annual ammonification rates decreased by 110.67%, 100.76%, 96.20%, however annual nitrification rates increased by 54. 92%, 24.19%, 24.46% respectively. （3） Total N mineralization rates were lower 24.68%, 26.01% in CLP and LCP, respectively than in EBF, and insignificant difference between PMF and EBF. Meanwhile, the N supply amount with soil net N mineralization rate in PMF, CLP and LCP （111.85, 89.58, 88.00 kg. hm-2. a-1） were lower than their needs for plant N uptake （ 137.83, 92.76, 99.28 kg. hm-2.a-1, respectively）. （4） The size of inorganic N pool depended on mineralization rate and vegetation N uptake rate. Ammonification rate positively correlated with storage of dead root, N uptake positively correlated with biomass of live root, and nitrification rate positively correlated with NH4＋-N concentration and negatively correlated with NH4＋-N uptake rate by vegetation. These findings indicated that the forest conversion from EBF to other secondary forests would lead to N leaching, thus N deficiency might become a growth-limiting factor for these forests.