黄土丘陵区退耕地土壤可溶性氮组分季节变化与水热关系

为探究黄土丘陵区退耕地植被恢复土壤有效氮素养分累积的季节动态变化特征及水热驱动效应,以邻近坡耕地为对照,分析了植被恢复15 a的刺槐、柠条、荒草地土壤可溶性氮组分密度、分布比例在4—10月份内的动态变化状况及其与土壤温度和含水量间的关系。结果表明,整个采样期间,0—30 cm土层土壤硝态氮、可溶性有机氮和可溶性全氮动态变化显著,且各可溶性氮组分中仅硝态氮随土层变化差异显著。其中土壤硝态氮变化幅度整体为0.13—1.71 g/m~2,占可溶性全氮的5.1%—52.1%,其最大值出现在4月份,最小值出现在10月份;可溶性有机氮整体变化幅度为0.29—2.92 g/m~2,占可溶性全氮的30.9%—85.3%,在4月份和8月份分别达最小值和最大值;铵态氮动态变化不显著,4—10月份整体变化幅度为0.17—0.74 g/m~2,占6.4%—21.4%,其最小值出现在10月份,最大值出现在4月份;可溶性全氮整体变化幅度为1.25—3.52 g/m~2。可溶性氮组分中,各组分所占比例为可溶性有机氮硝态氮铵态氮,并且可溶性有机氮与硝态氮所占比例动态变化趋势相反。刺槐、柠条林和荒草地0—30 cm土壤硝态氮平均约为耕地的3.42倍、2.54倍和1.26倍,铵态氮平均为耕地的1.71倍、1.37倍和1.30倍,可溶性有机氮约为1.64倍、1.31倍和1.23倍。可溶性氮组分受土壤含水量的影响大于土壤温度,硝态氮对土壤含水量的变化最为敏感,而可溶性全氮则对土壤温度变化最为敏感。综上所述,人工林植被恢复有利于提高土壤可溶性氮组分,增加氮的有效性,同时除铵态氮外,土壤可溶性氮组分随季节变化显著。

Dynamic change in soil soluble nitrogen under farmland converted to forest in the Loess Hilly Region

Soil soluble and available nitrogen are considered to be limiting nutritional factors for the productivity of plants in terrestrial ecosystems. Land-use change has a significant impact on the physical and chemical properties of soil, particularly nitrogen pools. Furthermore, although the amounts of soil soluble nitrogen are generally very small, they can change rapidly during the plant growing season. However, the changes in soil soluble nitrogen dynamics associated with different vegetation restoration patterns are still poorly understood. Therefore, more information is essential to gain a better understanding of the changes in ecosystems dynamics that occur following the conversion of farmland to forest in the Loess Hilly Region. We studied the dynamic changes in soil soluble nitrogen from April to October, and their relation to soil moisture and temperature under three converted land types in the Loess Hilly RegionRobinia pseudoacacia(RP), Caragana korshinskii(CK), and abandoned farmland (AF)], which have undergone conversion from slope farmland (SF) for 15 years. The average densities of nitrate nitrogen (NO₃^--N), soluble organic nitrogen (SON), and soluble total nitrogen (STN) changed significantly from April to October in the 0-30 cm soil layer (P<0.05), although only NO₃^--N varied significantly with soil depth (P<0.05). During the sampling period (April to October), the average NO₃^--N density accounted for 5.1%-52.1% of STN, ranging from 0.13 g/m² (in April) to 1.71 g/m² (in October). The average density of SON varied significantly from 0.29 g/m² (in April) to 2.92 g/m² (in August), which accounted for 30.9%-85.3% of STN. Although the variation in ammonium nitrogen (NH₄⁺-N) was not significant, it ranged from 0.17 g/m² (in October) to 0.74 g/m² (in April) and accounted for 6.4%-21.4% of STN. After long-term vegetation restoration, the average densities of NO₃^--N in RP, CK, and AF were 3.42, 2.54, and 1.26 times higher, respectively, than that of SF in the 0-30 cm soil layer, whereas those of NH₄⁺-N in RP, CK, and AF were increased by 1.71, 1.37, and 1.30 times, respectively, and those of SON were increased by 1.64, 1.31, and 1.23 times, respectively, compared to SF. Correlation analyses indicated that the dynamic change in soil soluble nitrogen was affected by soil moisture and temperature, and that soluble nitrogen was more sensitive to soil moisture than to temperature. Moreover, NO₃^--N was more sensitive than SON and STN to soil moisture changes, whereas STN was the most sensitive to soil temperature. Collectively, these findings indicate that converting farm to forest can improve the density of soil soluble nitrogen, and increase the availability of soil nitrogen. It was also observed that the amounts of soil soluble nitrogen change significantly with different seasons.