拉萨河流域洪积扇不同植被类型土壤化学计量特征

洪积扇是拉萨河流域珍贵的土地资源，而明悉其土壤养分状况是对其进行科学合理开发利用的基础。为此，在拉萨河流域选取了10个洪积扇，于2020年7-8月调查了其上的植物群落组成并采集对应土壤，测定了土壤有机碳（SOC）、全氮（TN）、全磷（TP）、全钾（TK）、碱解氮（AN）、速效磷（AP）和速效钾（AK）的含量并计算其化学计量比。结果表明：10个洪积扇的60个样方中共发现植物种87种，分属于29科79属，其中禾本科和菊科的物种居多；洪积扇SOC、TN、TP、AN和AK的平均含量分别为34.38 g/kg、2.77 g/kg、0.39 g/kg、130.78 mg/kg和189.79 mg/kg，均表现为草地>灌丛>农田，其中SOC、TN、AN和AP含量在草地和农田下差异显著（P<0.05）；土壤TK的平均含量为19.68 g/kg，表现为农田>草地>灌丛；土壤AP的平均含量为3.36 mg/kg，表现为农田>灌丛>草地；土壤C：N的均值为12.75，表现为农田>灌丛>草地；土壤C：P、N：P和N：K的均值分别为102.50、8.10和0.16，均表现为草地>灌丛>农田，总体来说洪积扇土壤P元素较为稀缺；土壤N：K与SOC、TN、TP、TK、AN、AP、AK均极显著相关（P<0.01），SOC和TN与植物群落盖度极显著正相关（P<0.01）；典范对应分析（CCA）进一步表明土壤养分及其计量比对洪积扇植物群落组成影响显著（P=0.002），且TP、K：P和AP是影响洪积扇植物群落组成的主要土壤生态化学计量因子。综上所述，拉萨河流域洪积扇植物种相对丰富，但分布不均匀，组成不稳定。SOC、TN、TK含量相对较高但C：N值低，表现为有机质矿化速率高而土壤肥力低下；土壤TP和AP的含量均较低，洪积扇植物群落在生长发育过程中受到土壤P元素的限制。此外，研究还发现土壤N：K同C：N：P一样可作为评价土壤养分状况的生态化学计量指标。

Soil stoichiometry characteristics of different vegetation types in alluvial fans of the Lhasa River Basin

The alluvial fan is a precious land resource in Lhasa River Basin, and knowing its soil nutrient status is the basis for its scientific development and utilization. In this study, ten alluvial fans in the Lhasa River Basin were selected as sample plots, where the plant community composition was investigated and soil was collected from July to August, 2020. The soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), available nitrogen (AN), available phosphorus (AP) and available potassium (AK) contents were determined by the soil samples and taken to calculate measurement ratio. The results showed that 87 plant species were found in this research, belonging to 79 genus and 29 families, among which the Gramineae and Compositae plants were the majority. The average contents of SOC, TN, TP, AN and AK were 34.38 g/kg, 2.77 g/kg, 0.39 g/kg, 130.78 mg/kg and 189.79 mg/kg respectively, which all were showed grassland>shrubland>farmland, where SOC, TN, AN and AP were significantly different under grassland and farmland (P<0.05). The average content of TK was 19.68 g/kg, which was showed farmland>grassland>shrubland. The average content of AP was 3.36 mg/kg, which was showed farmland>shrubland>grassland. The average value of C:N was 12.75, which was showed farmland>shrubland>grassland. The mean values of C:P, N:P and N:K were 102.50, 8.10 and 0.16, respectively, which all were showed grassland>shrubland>farmland. Generally, P element was scarce in alluvial fan soil. There were significant correlations between soil N:K and SOC, TN, TP, TK, AN, AP, AK, and the community coverage was significantly correlated with SOC and TN (P<0.01), while the Shannon-Wiener diversity index, Margalef richness index, and Pielou evenness index showed no correlations on the SOC and TN. Additionally, Correspondence Analysis (CCA) showed that soil nutrients and their stoichiometric ratios had significant effects on species composition of plant communities in the alluvial fan (P=0.002), and TP, K:P, AP were the dominant factors of the composition of plant communities. In conclusion, the species of alluvial fan plants in Lhasa River Basin were relatively abundant, but the distribution was uneven and the composition was unstable. The contents of SOC, TN and TK were relatively high, but the C:N values were relatively low, indicating a high rate of soil organic matter mineralization and low soil fertility. Moreover, TP and AP contents were low, the plant community of alluvial fan was limited by soil P element in the process of growth and development. Furthermore, it was also found that soil N:K could be used as a soil ecological stoichiometric indicator to evaluate soil nutrient status as C:N:P.