塔里木沙漠公路防护林地表凋落物分解对施肥的响应

Effects of fertilizer addition on surface litter decomposition in the Tarim Desert Highway shelterbelt

为探讨沙漠公路防护林地表凋落物的分解速率和养分释放动态对施肥的响应,采用凋落物分解袋法,对塔里木沙漠公路防护林地乔木状沙拐枣(Calligonum arborescens)同化枝、梭梭(Haloxylon ammodendron)同化枝和多枝柽柳(Tamarix ramosissima)枝凋落物在施肥处理下的分解及养分释放特征进行研究。结果表明:经过420d的分解,3种凋落物质量残留率在对照(不施肥)、施用氮肥、施用磷钾复合肥处理间存在显著性差异(P<0.05)。乔木状沙拐枣同化枝、梭梭同化枝和多枝柽柳枝在对照处理下的质量残留率分别为56.95%、31.32%和50.24%。施肥处理下3种凋落物均呈现出梭梭同化枝分解速率最快,多枝柽柳枝次之,乔木状沙拐枣同化枝分解最慢。施用磷钾复合肥极显著提高了3种凋落物的分解速率(P<0.01);施用氮肥则促进多枝柽柳枝的分解,抑制乔木状沙拐枣和梭梭同化枝的分解。凋落物分解过程中,对照组3种植物凋落物的C、N、P和K元素均呈现净释放状态;施肥后凋落物的N、P和K元素呈现出富集-释放的模式。凋落物初始P含量和C/N、C/P比值是分解初期的主导因素,初始K、木质素、纤维素含量和C/N、木质素/N比值是分解后期的主要控制因素。研究表明,施肥显著影响沙漠公路防护林地表凋落物的分解,增加防护林地表凋落物的养分归还量,延后养分释放的时间,改善塔里木沙漠公路防护林地的土壤肥力。凋落物初始C/N比值是预测塔里木沙漠凋落物分解的重要因素,且不同分解时期影响凋落物分解的初始化学组成有所差异。

Litter decomposition determines the nutrient and carbon cycling processes and regulates nutrient return to the soil in many terrestrial ecosystems. Wood plant litter decomposition is a crucial biogeochemical process for carbon and nutrient cycling, especially in arid and nutrient-constrained ecosystems. However, the role of litter decomposition in extreme drought deserts and the determining factors remain debatable and poorly understood. The Tarim Desert Highway Shelterbelt consists of three artificial plant species that account for the majority of net primary productivity, and their litters are the major contributors to carbon and nutrient recycling. In such artificial desert shelter forests, the addition of fertilizer causes contradictory effects on litter decomposition and nutrient dynamics. This study aims to （1） examine how the addition of fertilizer affects surface litter decomposition and nutrient dynamics, and （2） determine the major factors influencing litter decomposition in arid deserts. The litterbag method was used to investigate the assimilative branches decomposition of Calligonum arborescens and Haloxylon ammodendron, and the branches decomposition of Tamarix ramosissima in the Taklimakan Desert Research Station. Filled litterbags were placed on soil surfaces at different plots in March 2013. Fertilizer addition and irrigation were implemented from March to November during the experiment. Litterbags were collected in May, July, September, and November of 2013, and in March and May of 2014. The mass remaining, elemental content （carbon, nitrogen, phosphorus, and potassium）, and decomposition rates of litter were analyzed at each decomposition stage. Results showed that after 420 days of decomposition, significant differences were found among control （without fertilizer）, addition of nitrogen fertilizer, and phosphorus and potassium compound fertilizer treatments. The mass remaining for the assimilative branches of C. arborescens, assimilative branches of H. ammodendron, and branches of T. ramosissima without fertilizer addition were 56.95%, 31.32%, and 50.24%, respectively. The decomposition rate of fertilizer treatments among different litters was the highest for the assimilative branches of H. ammodendron, followed by the branches of T. ramosissima, and then the assimilative branches of C. arborescens. The addition of phosphorus and potassium compound fertilizer significantly accelerated the litter decomposition rate of the three plant species, whereas nitrogen fertilizer increased the decomposition rate of the branches of T. ramosissima and restrained the decomposition rate of the assimilative branches of C. arborescens and H. ammodendron. During the decomposition process, litter nutrients presented net release patterns in the treatments without fertilizer. In contrast, the nitrogen, phosphorus, and potassium contents of the fertilizer treatments showed an accumulation-release pattern. The litter decomposition rate was determined by the initial phosphorus content and C/N and C/P ratios at the initial decomposition stage. However, the decomposition rate was controlled by the initial potassium, lignin, and cellulose contents and C/N, and lignin/N ratios at the final stage. Overall, the results suggest that the addition of fertilizer significantly changed the decomposition rate of surface litters, increased the amount of returned nutrients, and postponed the time of nutrient release. Improving soil fertility in the Tarim Desert Highway shelterbelt is essential in enhancing the quality of this valuable ecosystem. The initial litter C/N ratio is critical to surface litter decomposition in the Tarim Desert, and the initial chemical composition to surface litter decomposition rate varies at different decomposition stages.