黄土高原典型草原优势植物凋落物分解及养分释放对氮添加的响应

Influence of nitrogen addition on Stipa bungeana and Heteropappus altaicus litter decomposition and nutrient release in a steppe located on the Loess Plateau

利用原位分解袋法研究了黄土高原典型草原优势植物长芒草(Stipa bungeana)和阿尔泰狗娃花(Heteropappus altaicus)凋落物的养分释放过程对氮添加的响应,试验周期为1 a。设置6个氮添加水平,分别为N0(0)、N1(1.15 g N m^(-2)a^(-1))、N2(2.3 g N m^(-2)a^(-1))、N3(4.6 g N m^(-2)a^(-1))、N4(9.2 g N m^(-2)a^(-1))和N5(13.8 g N m^(-2)a^(-1)),氮素类型为尿素((NH_2)_2CO)。结果表明:(1)氮添加处理两年显著改变了长芒草和阿尔泰狗娃花凋落物的初始化学性质。随着氮梯度的增加,凋落物的N(氮)含量逐渐增加,木质素含量先增加后下降,C/N(碳氮比)和木质素/N降低,C(碳)、P(磷)和C/P(碳磷比)没有显著的差异。(2)氮处理对长芒草和阿尔泰狗娃花凋落物的分解速率的影响不显著。长芒草和阿尔泰狗娃花凋落物C含量随分解时间整体为降低过程,N和P含量总体上为增加过程,且整个分解过程中N含量各处理间差异显著。(3)氮处理对长芒草和阿尔泰狗娃花凋落物C和P的分解基本无影响,两种元素都呈现释放过程。氮处理对凋落物的N残留率有显著的影响,在N1—N3(1.15—4.6 g/m^2)处理下的长芒草凋落物N残留率高于其他处理,且呈现富集过程;而阿尔泰狗娃花凋落物中的N呈现富集-释放过程。在土壤养分贫瘠的黄土高原典型草原,适量的氮输入可以促进系统的固氮。

A rise in nitrogen （N） deposition can increase net primary productivity in many terrestrial ecosystems. Litter decomposition in terrestrial ecosystems is an important component of global carbon （ C ） budgets and is greatly affected by the increase in N deposition that has been recorded worldwide. Grassland covers 46% of the global terrestrial area and can have an important effect on global C, and nutrient cycling and balances. We hypothesized that litter decomposition and nutrient release may respond to N deposition differently based on the quality of the litter substrate. To investigate the effect of increased N deposition on nutrient release during litter decomposition, we conducted a one-year field experiment that simulated N deposition in a steppe located on the Loess Plateau. There were six N treatments included in the experiment. These were 0 g N m-2 a-1, 1.15 g N m-2 a-1, 2.3 g N m-2 a-1, 4.6 g N m-2 a-1, 9.2 g N m-2 a-1 and 13.8 g N m-2 a-1 After 2-years of N treatment, the N contents in the Stipa bungeana and Heteropappus altaicus litters increased and the C/N and lignin/N ratios gradually declined. Furthermore, the lignin first increased, but then decreased. None of the N treatments had a significant effect on the decomposition rates of the two litters after one year of in-situ decomposition. Our results indicated that there were significant correlations between the litter decomposition rates and the N content and C/N ratio for Stipa bungeana litter. The C content decreased over time, but the N and P contents increased as the two litters decomposed. The N treatments significantly influenced the N contents of the two litters during decomposition. The N treatments also influenced the percentage N remaining in the two litters. After in-situ decomposition for one year, the percentage N remaining in Stipa bungeana under the 1.15, 2.3, and 4.6 g N m-2 a-1 treatments were higher than for the other treatments and this percentage rose as the process progressed. The N release pattern in Heteropappus altaicus showed that N accumulated during the early stages and was released during later stages. The N treatments had no significant effects on C and P decomposition, and the C and P release patterns were similar in the two litters. Therefore, increased N deposition could contribute to the accumulation of C in a grassland ecosystem.