季节性冻融期间川西亚高山/高山森林土壤净氮矿化特征

气候变暖情景下季节性冻融格局的改变可能显著影响高寒森林土壤氮素矿化过程.本文采用原状土壤移位培养的方法,以海拔梯度形成的温度差异模拟气候变暖,研究了川西亚高山/高山森林在生长季节和季节性冻融期间土壤的净氮矿化量和净氮矿化速率.结果表明:在川西亚高山/高山森林,土壤铵态氮和硝态氮含量均表现为从生长季节至冻结初期明显下降,完全冻结期明显增加,而在融化初期明显降低的变化过程.季节性冻融期土壤的净氮矿化量和净氮矿化速率显著低于生长季节,并且出现明显的氮素固持现象.与低海拔相比,中海拔森林土壤的氮素固持作用相对较大,高海拔相对较小,可能与不同海拔梯度土壤温度变化及引起的冻融循环密切相关.在生长季节,土壤净氮矿化量和矿化速率均随海拔的降低呈明显增加趋势,尤其在低海拔处土壤的氮素矿化作用最为强烈.在气候变暖背景下,温度的增加明显促进了生长季节土壤氮素矿化,并且通过提高冻融循环频次、缩短冻结时间来影响土壤氮素矿化速率.这一过程可能受到微环境的影响.

Characteristics of soil net nitrogen mineralization in subalpine/alpine forests of west Sichuan, Southwest China during seasonal freeze-thaw period

Seasonal freeze-thaw cycle and its change pattern under the scenarios of climate warming might exert strong effects on the soil nitrogen mineralization in alpine forests. In this paper, intact soil cores were collected from the subalpine/alpine forests along an altitudinal gradient in west Sichuan, and an incubation test was conducted to study the soil net nitrogen mineralization rate and the amount of soil mineralized nitrogen in the forests during growth season and seasonal freeze-thaw period under simulated scenarios of global warming. In the test soils, the NH(4+)-N and NO(3-)-N contents both showed a clear tendency of decreased in the period from growth season to the onset stage of freezing, increased at deep freezing stage, and decreased again at the early stage of thawing. The soil net nitrogen mineralization rate and the amount of soil mineralized nitrogen were significantly lower in freeze-thaw period than in growth season, and the soil inorganic nitrogen was obviously immobilized. The soil nitrogen immobilization was stronger at middle altitudes but weaker at high altitudes, as compared with that at low altitudes, possibly due to the variation of soil temperature and its induced different freeze-thaw cycle. During growth period, the soil net nitrogen mineralization rate and the amount of soil mineralized nitrogen showed an obvious increasing trend with the decrease of altitude, and the soil nitrogen mineralization was the strongest at low altitudes, implying that under the scenarios of climate warming, the increase of soil temperature promoted the soil nitrogen mineralization during growth season, and affected the soil nitrogen mineralization rate by increasing the frequency of freeze-thaw cycle and shortening the time period of freeze-thaw. Soil micro-environment could also affect the soil nitrogen mineralization in alpine forest regions.