Data from: Nitrogen deposition cancels out exotic earthworm effects on plant-feeding nematode communities

1. The activity and spread of exotic earthworms often are spatially correlated with N deposition because both arise from human activities. Exotic earthworms, in turn, can also greatly affect soil abiotic and biotic properties, as well as related ecological processes. Previous studies showed, for example, that earthworms can counteract the detrimental effects of plant-feeding nematodes on plant growth. However, potential interactive effects of N deposition and exotic earthworms on ecosystems are poorly understood.
2. We explored the changes in density of plant-feeding nematodes in response to the presence of exotic earthworms, and whether these changes are altered by elevated N deposition in a two-factorial field mesocosm experiment at the Heshan National Field Research Station of Forest Ecosystem, in southern China.
3. Our results show that earthworm addition marginally significantly increased the density of exotic earthworms and significantly increased the mass of earthworm casts. The total density of plant-feeding nematodes was not significantly affected by exotic earthworms or N deposition. However, exotic earthworms tended to increase the density of plant-feeding nematode taxa that are less detrimental to plant growth (r-strategists), while they significantly reduced the density of more harmful plant-feeding nematodes (K-strategists). Importantly, these earthworm effects were restricted to the ambient N deposition treatment, and elevated N deposition cancelled out the earthworm effect. Although exotic earthworms and N deposition interactively altered foliar N:P ratio in the target tree species, this did not result in significant changes in shoot and root biomass in the short term.
4. Overall, our study indicates that N deposition can cancel out exotic earthworm-induced reductions in the density of harmful plant-feeding nematodes. These results suggest that anthropogenic N deposition can alter biotic interactions between exotic and native soil organisms with potential implications for ecosystem functioning.

1. 外来蚯蚓（exotic earthworms）的活动与扩散往往在空间上与氮沉降（N deposition）存在关联，二者均源于人类活动。反过来，外来蚯蚓也可显著改变土壤非生物与生物属性及相关生态过程。例如，既往研究表明，蚯蚓能够抵消植食性线虫（plant-feeding nematodes）对植物生长的不利影响。但目前学界对氮沉降与外来蚯蚓对生态系统的潜在交互效应仍知之甚少。
2. 本研究依托中国南方鹤山森林生态系统国家野外科学观测研究站开展双因素野外中型人工生态系统实验（field mesocosm experiment），探究了外来蚯蚓存在下植食性线虫密度的变化，以及氮沉降升高是否会改变这一变化。
3. 研究结果显示，蚯蚓接种仅边际显著提升了外来蚯蚓的密度，并显著增加了蚓粪质量。外来蚯蚓与氮沉降均未对植食性线虫的总密度产生显著影响。但外来蚯蚓倾向于提升对植物生长危害较弱的植食性线虫类群（r-strategists，r对策者）的密度，同时显著降低了危害性更强的植食性线虫类群（K-strategists，K对策者）的密度。值得注意的是，蚯蚓的上述效应仅在背景氮沉降处理组中显现，氮沉降升高完全抵消了蚯蚓的这一作用。尽管外来蚯蚓与氮沉降会交互改变目标树种的叶片氮磷比，但短期内并未导致地上及地下生物量出现显著变化。
4. 综合来看，本研究表明氮沉降能够抵消外来蚯蚓诱导的有害植食性线虫密度下降。上述结果提示，人为氮沉降会改变外来与本土土壤生物间的生物互作，进而可能对生态系统功能产生潜在影响。