黄土丘陵区子午岭不同植物群落下土壤氮素及相关酶活性的特征

Soil nitrogen and enzymes involved in nitrogen metabolism under different vegetation in Ziwuling mountain in the Loess Plateau,China

以黄土丘陵区子午岭林区裸露地为对照,选择撂荒地、白羊草草地、油松、山杨和辽东栎林地五种典型植被群落下0—10cm和10—20 cm土层的土壤为研究对象,对土壤无机氮、有机氮、微生物量氮含量和脲酶、蛋白酶以及硝酸还原酶的活性进行了研究。结果表明,土壤中各种氮素基本表现为乔木林,尤其是辽东栎和油松下含量最高,而有机氮则在白羊草地富集明显。铵态氮为子午岭林区速效氮的主要形式。土壤铵态氮与微生物氮极显著正相关;有机氮和亚硝态氮、矿化氮、微生物氮均显著正相关。脲酶和硝酸还原酶活性在辽东栎群落下最高,蛋白酶在白羊草地下较高,且脲酶活性在土壤上层高于下层,而蛋白酶和硝酸还原酶并没有表现出明显规律。脲酶活性和铵态氮、有机氮含量显著正相关,与微生物量氮极显著正相关;硝酸还原酶活性与铵态氮含量显著正相关;蛋白酶活性和土壤各种氮素含量无相关性。

The Loess Plateau is an important site for many ecological restoration studies in China. Different vegetation types change the soil environment to different extents, influencing the biogeochemical cycles of materials, such as nitrogen. Recently, a growing amount of attention has been paid to the relationship between vegetation and nitrogen forms in natural ecological systems to provide a theoretical basis for ecological restoration in these areas. Determining the relationship between soil enzymes and soil nitrogen forms under different vegetation types can provide useful information for vegetation restoration in the Loess Plateau. To investigate the transformation of soil nitrogen and its relationships with enzymes involved in nitrogen metabolism under different vegetation in the Loess Plateau, we collected 0-10 cm and 10-20 cm soil profiles under four typical vegetation types （Bothriochloa ischaemun （L.） Keng, Populus davidiana Dode, Quercus liaotungensis and Pinus tabulaeformis Carr）, and from abandoned land, one of the main land use types, in the Ziwuling region. Bare fallow land, where little vegetation grows, was chosen as the control. Ziwuling （107°30＇-109°40＇ E, 33°50＇-36°50＇ N）, is situated at 1300-1700 m above the sea, in the transitional zone between forest steppe and semiarid grasslands. The climate is mild and humid. Soil basic physicochemical properties, different nitrogen form contents, urease activity, protease activity and nitric acid reductase activity were measured. The results indicated that all forms of nitrogen were greater under forest vegetation compared with control, especially, under Q. liaotungensis and P. tabulaeformis; while organic N was markedly increased under B. ischaemun, reaching 2.18 g/kg in the topsoil and 1.52 g/kg in the subsoil. Ammonium N was the dominant inorganic N in the study area. Ammonium N content correlated with microbial biomass N, while organic N content correlated with nitrous N, mineralizable N and microbial biomass N. Vegetation type and soil layer did not influence urease activity, protease activity and nitric acid reductase activity Urease activity correlated with ammonium N （r=0.66） and organic N （r=0.69）, and significantly correlated with microbial biomass N （r=0.75）. Protease activity did not affect any form of nitrogen content. Nitric acid reductase activity correlated with ammonium N. Urease activity was highest under Q. liaotungensis, and it was higher in topsoil than in subsoil. By contrast, protease activity and nitric acid reductase activity did not show any significant differences among vegetation types or soil layers. These findings suggest that forest vegetation increases soil nitrogen content. Thus, this role of forest vegetation should be considered during ecological restoration projects in the Loess Plateau region. In addition, enzymes play an important role in the nitrogen transformation process. Further research is still required to explore the processes of vegetation change and nitrogen transformation and the relationship between enzymes and nitrogen forms.