Cumulative cellulolytic enzyme activities and initial litter quality in prediction of cellulose degradation in an alpine meadow of the eastern Tibetan Plateau

Aims Plant litter decomposition is a key ecosystem process that determines carbon and nutrient cycling in terrestrial ecosystems.As a main component of litter,cellulose is a vital energy source for the microbes associated with litter decomposition.The important role of cellulolytic enzymes in litter cellulose degradation is well understood,but seasonal patterns of cellulose degradation and whether cumulative enzyme activities and litter quality forecast cellulose degradation in an alpine meadow remain elusive,which limits our understanding of cellulose degradation in herbaceous plant litter.Methods A two-year field litterbag experiment involving three dominant species(Ajuga ovalifolia,Festuca wallichanica,and Pedicularis roylei)was conducted in an alpine meadow of the eastern Tibetan Plateau to explore the seasonal patterns of cellulose degradation and how cumulative cellulolytic enzyme activities and initial litter quality impact cellulose degradation.Important findings Our study demonstrates that cellulose degraded rapidly and exceeded 50%during the first year,which mainly occurred in the first growing season(31.9%–43.3%).At two years of decomposition,cellulose degradation was driven by cumulative endoglucanase(R^(2)=0.70),cumulative cellobiohydrolase(R^(2)=0.59)and cumulative 1,4-β-glucosidase(R^(2)=0.57).In addition,the concentrations of cellulose,dissolved organic carbon,total phenol,lignin and lignin/N accounted for 52%–78%of the variation in cellulose degradation during the two years of decomposition.The best model for predicting cellulose degradation was the initial cellulose concentration(R^(2)=0.78).The enzymatic efficiencies and the allocation of cellulolytic enzyme activities were different among species.The cellulolytic enzyme efficiencies were higher in the litter of F.wallichanica with relatively lower quality.For the complete cellulose degradation of the leaf litter,A.ovalifolia and F.wallichanica required 4-fold and 6.7-fold more endoglucanase activity,3-fold and 4.5-fold more cellobiohydrolase activity and 1.2-fold and 1.4-fold more 1,4-β-glucosidase activity,respectively,than those required by P.roylei.Our results demonstrated that although microbial activity and litter quality both have significant impacts on cellulose degradation in an alpine meadow,using cellulose concentration to predict cellulose degradation is a good way to simplify the model of cellulose degradation and C cycling during litter decomposition.

植物凋落物分解是决定陆地生态系统碳和养分循环的关键生态系统过程。作为凋落物的主要组成部分,纤维素是与凋落物分解相关的微生物的重要能量来源。纤维素酶在凋落物纤维素降解过程中的重要作用已为人们所熟知,然而纤维素降解的季节模式、累积酶活性和凋落物质量是否能预测高寒草甸的纤维素降解仍是一个未解之谜,这限制了我们对草本植物凋落物纤维素降解的认识。为了探究纤维素降解的季节性模式以及累积纤维素分解酶活性和凋落叶初始质量对纤维素降解的影响,我们在青藏高原东部的高山草甸选取了三种优势种[圆叶筋骨草(Ajuga ovalifolia)、藏羊茅(Festuca wallichanica)和草甸马先蒿(Pedicularis roylei)],进行了为期两年的凋落物网袋分解实验。我们的研究发现,纤维素在第一年中迅速降解且降解率超过50%,而且主要发生在第一个生长季(31.9%-43.3%)在两年的分解过程中,纤维素降解由累积内切葡聚糖酶(R^(2)=0.70),累积纤维二糖水解酶(R^(2)=0.59)和累积1,4-β-葡糖苷酶(R^(2)=0.57)共同驱动。此外,在这两年的分解过程中,纤维素、可溶性有机碳、总酚、木质素的浓度和木质素/N可以解释纤维素降解变异的(52%-78%)。用初始纤维素浓度模型预测纤维素降解效果最佳(R^(2)=0.78)。在凋落物的分解过程中,酶效率和微生物对纤维素分解酶的分配因物种而异。藏羊茅凋落物中纤维素酶效率较高,但质量相对较低。与草甸马先蒿相比,完全降解圆叶筋骨草和藏羊茅的纤维素需要4倍和6.7倍的内切葡聚糖酶、3倍和4.5倍的纤维二糖水解酶、1.2倍和1.4倍的1,4-β-葡糖苷酶。我们的研究结果表明,虽然微生物酶活性和凋落物初始质量都对高山草甸纤维素降解有显著影响,但使用纤维素浓度来预测纤维素降解是简化凋落物分解过程中纤维素降解和C循环模型的好方法。