Sydney Institute of Agriculture, School of Life and Environmental Sciences, The University of Sydney, Camden, NSW 2570, Australia.
Sydney Institute of Agriculture, School of Life and Environmental Sciences, The University of Sydney, Camden, NSW 2570, Australia.
Sci Total Environ. 2022 Nov 10;846:157430. doi: 10.1016/j.scitotenv.2022.157430. Epub 2022 Jul 18.
Grassland plants allocate photosynthetically fixed carbon (C) belowground to root biomass and rhizodeposition, but also to support arbuscular mycorrhizal fungi (AMF). These C allocation pathways could increase nutrient scavenging, but also mining of nutrients through enhanced organic matter decomposition. While important for grassland ecosystem functioning, methodological constraints have limited our ability to measure these processes under field conditions. We used CO and N pulse labelling methods to examine belowground C allocation to root biomass production, rhizodeposition and AMF colonisation during peak plant growth in a grassland field experiment after three years of N fertilisation (0 and 40 kg N ha year) and defoliation frequency treatments ("low" and "high", with 3-4 and 6-8 simulated grazing events per year, mimicking moderate and intense grazing, respectively). Moreover, we quantified the consequences for plant nitrogen (N) uptake and decomposition of soil organic C (SOC). Nitrogen fertilisation increased rhizodeposition and AMF colonisation (by 63 % and 54 %), but reduced root biomass (by 25 %). With high defoliation frequency, AMF colonisation increased (by 60 %), but both root biomass and rhizodeposition declined (by 35 % and 58 %). Plant N uptake was highest without N fertilisation and low defoliation frequency, and positively related to root biomass and the number of root tips. Therefore, when N supply is low and the capacity to produce C through photosynthesis is high, belowground C allocation to root production and associated root tips was important to scavenge for N in the soil. In contrast, the strong positive relationship between the rate of rhizodeposition and SOC decomposition, suggests that rhizodeposition may help plants to mine for nutrients locked in SOC. Taken together, the results of this study suggest that belowground C allocation pathways affected by N fertilisation and defoliation frequency affect plant N scavenging and mining with important consequences for long-term grassland C dynamics.
草原植物将光合作用固定的碳（C）分配到地下，用于根系生物量和根分泌物，但也用于支持丛枝菌根真菌（AMF）。这些 C 分配途径可以增加养分的获取，但也可以通过增强有机质分解来挖掘养分。虽然这对草原生态系统的功能很重要，但方法上的限制限制了我们在野外条件下测量这些过程的能力。我们使用 CO 和 N 脉冲标记方法，在经过三年 N 施肥（0 和 40 kg N ha 年）和刈割频率处理（“低”和“高”，每年模拟 3-4 和 6-8 次放牧事件，分别模拟中度和重度放牧）后，在草原田间试验中，在植物生长高峰期，研究了地下 C 分配到根系生物量生产、根分泌物和 AMF 定殖的情况。此外，我们还量化了对植物氮（N）吸收和土壤有机碳（SOC）分解的影响。N 施肥增加了根分泌物和 AMF 定殖（增加了 63%和 54%），但减少了根系生物量（减少了 25%）。在高刈割频率下，AMF 定殖增加（增加了 60%），但根系生物量和根分泌物均下降（减少了 35%和 58%）。在没有 N 施肥和低刈割频率的情况下，植物 N 吸收量最高，与根系生物量和根根尖数量呈正相关。因此，当 N 供应较低且通过光合作用产生 C 的能力较高时，地下 C 分配到根系生产和相关的根根尖对于在土壤中获取 N 很重要。相比之下，根分泌物与 SOC 分解之间的强正相关关系表明，根分泌物可能有助于植物挖掘 SOC 中锁定的养分。总的来说，这项研究的结果表明，受 N 施肥和刈割频率影响的地下 C 分配途径会影响植物的 N 获取和挖掘，对长期草原 C 动态具有重要意义。
