The National Natural Science Foundation of China (General Program, Key Program, Major Research Plan)
凋落物是植物在其生长发育过程中新陈代谢的产物,是土壤有机质输入的重要途径,凋落物分解是生态系统养分循环的关键过程之一。在全球气候变化背景下,热带地区干旱事件发生的频率和强度均在增加,同时,普遍认为热带地区受磷(P)限制,所以探讨干旱胁迫和土壤磷可用性对热带地区叶凋落物分解的影响及两者是否存在交互效应十分必要,有助于了解干旱对该区叶凋落物分解的影响机制以及是否受土壤磷调控。依据植物多度、碳固持类型、叶质地,以海南三亚甘什岭热带低地雨林的4个树种叶凋落物(铁凌 Hopea exalata、白茶树 Koilodepas bainanense、黑叶谷木 Memecylon nigrescens、山油柑 Acronychia pedunculata)为实验材料,依托2019年在该区建成的热带低地雨林模拟穿透雨减少、磷(P)添加双因素交互控制实验平台,包括干旱(D -50%穿透雨)、P添加(P +50Kg P hm-2a-1)、模拟干旱×P添加(DP -50%穿透雨×+50Kg P hm-2a-1)、对照(CK)4个处理,且4种处理随机分布于3个区组,即设置了3个重复。使用常规的凋落物分解袋法探究实验处理对4个树种叶凋落物的分解系数、碳(C)、氮(N)元素动态变化的影响。结果表明:不同树种的叶凋落物因基质质量不同分解存在差异。模拟干旱处理对叶凋落物C、N损失产生抑制作用,但是对不同树种叶凋落物的抑制作用不同,原因是干旱处理通过抑制土壤分解者活动、减弱凋落物的物理破碎作用,间接抑制凋落物分解,并且由于高质量(含N量高)凋落物受微生物分解者影响较大,所以该凋落物分解受干旱抑制程度较大;P添加处理对叶凋落物C损失存在促进作用、N损失存在抑制作用,原因是土壤中P含量的升高,提高了微生物分解高C物质的能力,以及当土壤中P含量较高时,间接抑制微生物通过分解凋落物获取养分或者促进微生物优先完成自身生长代谢需要而不是合成分解凋落物所需要的酶,导致叶凋落物N损失下降;模拟干旱与P添加处理存在显著交互效应,P添加处理缓解或反转了干旱胁迫对叶凋落物分解的抑制作用。以上结果表明,不同基质质量的凋落物分解存在差异,对干旱胁迫的响应不同;在叶凋落物分解过程中,P添加促进C损失、抑制N损失;此外,在热带低地雨林,土壤中P可用性变化可调节干旱对凋落物分解的影响。
Litter is the products of plant metabolism during its growth and development, and is an important way of soil organic matter inputting. Litter decomposition is one of the key processes of ecosystem nutrient cycles. Under the background of global climate change, the frequency and intensity of drought events in tropical areas are both increasing. Meanwhile, it is generally believed that the tropical systems are soil phosphorus (P) limited. Therefore, it is necessary to explore the effects of drought stress and soil P availability on leaf litter decomposition and figure out whether there is an interaction between drought stress and soil P availability in tropical forests, which would be helpful to understand the way that drought stress affects leaf litter decomposition and reveal whether it is regulated by soil P availability or not. Here we conducted a leaf litter decomposition experiment on the floor of a lowland tropical rainforest in Ganzha Ridge, Sanya, Hainan Province. The forest was treated by a two-factor interactive control experiment of throughfall reduction and P addition which was established in 2019. We selected four type of tree species leaf litter, i.e., Hopea exalata, Koilodepas bainanense, Memecylon nigrescens and Acronychia pedunculata, based on dominance, carbon allocation type and leaf texture. The experiment included four treatments, i.e., the simulated drought (D, 50% throughfall reduction), P addition (P, 50Kg P hm-2a-1), simulated drought×P addition (DP, 50% throughfall reduction×50Kg P hm-2a-1) and control (CK). All treatments were randomly distributed in each one of the three blocks which were made three replicates. By employing litterbag method, we investigated the variations of decomposition constants, litter carbon (C) and nitrogen (N) losses of four-tree species under the four treatments. Our results showed that the four-tree species litter with different litter qualities decomposed differently, and the simulated drought inhibited litter C and N losses of the four-tree species differently, by weakening litter physical fragmentation indirectly and the activities of soil decomposers. Also, the simulated drought decreased four-tree species litter decomposition constants differently, because leaf litter with high quality was much more affected by simulated drought which is more easily decomposed by microorganisms. P addition treatment promoted litter C loss and restrained litter N loss, because increasing soil P availability improved soil microorganisms' ability to decompose high-carbon materials and made microorganisms preferring to take use of the nutrient to grow and metabolize rather than use them to produce enzyme related to decomposing litter. There was a significant interaction between the simulated drought and P addition on decomposition that P addition alleviated or reversed the inhibition of simulated drought on litter decomposition. The results suggest that four-tree species leaf litter with different qualities decompose and response to simulated drought differently; P addition would accelerate litter C loss and inhibit litter N loss; Soil P availability could regulate the effects of simulated drought on leaf litter decomposition in lowland tropical rainforest.