Soil inorganic nitrogen (N) has long been recognized to play an important role in plant invasions. Whilst comparing the N use strategies of multiple invasive versus native plant congeners along an entire N gradient is key to understanding plant invasion success, there are few related studies.

We conducted a potted experiment with six invasive and native congeneric pairs, which were subjected to 11 nitrate/ammonium (NO₃−/NH₄+) ratios (i.e. 100% NO₃− at one end and 100% NH₄+ at the other end), each with low- and high-N levels. Each species–N combination was replicated eight times, and thus there were 2112 pots in total. We measured the following traits: the total biomass, growth advantage, biomass allocation, leaf chlorophyll content and low-N tolerance.

Invasive and native congeners grew well at any NO₃−/NH₄+ ratios, and their responses of growth, allocation and tolerance were approximately parallel along the 11 NO₃−/NH₄+ ratios across two N levels. Plant invaders grew larger and had greater chlorophyll contents, higher root biomass allocation and stronger low-N tolerance than their congeneric natives. These findings suggest that invasive and native plant congeners may utilize similar inorganic N forms (i.e. NO₃− and NH₄+) across an entire N composition gradient and that higher N use efficiencies could favor alien plants to invade new plant communities where congeneric natives are dominants.

Reforestation can enhance soil carbon (C) stability and promote soil C accumulation. Experimental results are, however, highly variable, and the efficacy of reforestation in enhancing soil C stability is still in debate. Consequently, it remains unclear how the different soil C pools respond to reforestation in forest ecosystems.

The response of different soil C fractions to reforestation was examined in five subtropical forests, including the plantations of Eucalyptus urophylla (EU), Acacia crassicarpa (AC), Castanopsis hystrix (CH) and 10 species mixed (MX), and a naturally recovered shrubland (NS). Soil labile C fractions (readily oxidized organic C by KMnO₄: ROC; dissolved organic C: DOC), distribution of aggregate-size classes and aggregate-associated C from different soil layers (0–10, 10–20, 20–40 and 40–60 cm) were evaluated.

We found that reforestation and forest type did not affect ROC concentration, yet the highest DOC concentration was detected in NS at four soil layers. Aggregate C concentration was the highest in all aggregate-size classes of CH at 0–10 cm depth. In addition, forest type did not alter the proportion of soil water-stable aggregates at four soil layers. However, soil depths significantly affected the distribution of soil aggregates with >0.25 mm aggregates dominating in the topsoils (0–20 cm), but 0.053–2 mm aggregates being dominant in the deep soils (20–60 cm). These results indicate that reforestation and forest type affected soil DOC (0–60 cm) and aggregate C (0–10 cm). Furthermore, soil DOC and aggregate C were more susceptive to reforestation than ROC. The findings suggest that plantations reduce soil DOC concentration, highlighting that C leaching loss may decrease compared with natural recovery. Moreover, C. hystrix plantation may enhance soil C stability by physical protection in topsoil. This study provides valuable information on tree species selection for reforestation concerning soil C sequestration in southern subtropical China.

Successful plant invaders usually exhibit three strategies: Jack-of-all-trades (more robust in stressful sites), Master-of-some (more responsive in favorable sites) and Jack-and-master (both robustness and responsiveness). To revisit these strategies, we examined how soil inorganic nitrogen (N) compositions and levels influence the success of native and invasive plant congeners in the context of plant communities.

We conducted an experiment involving three fixed factors: species origin, N composition and N level. Here, we selected 21 plant species (eight pairs of invasive and native congeners and five non-congeneric natives) to assemble plant communities, which were subject to nine N environments consisting of three N compositions (3:1, 2:2 and 1:3 NO₃−/NH₄+) and three N levels (low, medium and high N). We determined the following metrics: total biomass, relative biomass (a proxy of species success), mortality rate and mortality time.

Across nine N environments, native and invasive congeners exhibited similar total biomass, relative biomass and mortality time, but invaders had a marginally lower mortality rate than natives. Similar success between native and invasive congeners was linked to their similar growth and tolerance. N compositions influenced mortality time and N levels affected the total biomass and relative biomass. Importantly, species origin, N composition and N level interactively affected the total biomass, relative biomass and mortality time. These findings suggest that native and invasive plant congeners may be similarly successful across different N environments, and that inorganic N compositions and levels both contribute to plant invasion success.

Classical theory predicts that herbivores impact herb assemblages and soil nitrogen (N) cycling through selective plant consumption and the deposition of N-rich waste, with effects dependent upon ecosystem N availability. Herbivores are predicted to accelerate N cycling when N availability is high and decelerate cycling when availability is low. However, experimental tests of these theories in natural systems are limited and have yielded contradictory results. California’s widespread chaparral shrublands provide a tractable system in which to test these theories. They are prone to periodic crown fire, which temporarily removes living shrub cover, deposits mineral N on soils and allows diverse herbaceous assemblages to dominate the landscape for 3–5 years. Chaparral is also increasingly vulnerable to herbaceous invasion; mammalian herbivory may limit the establishment of non-native herbs in the shrub understory.

We implemented a 2-year herbivore-exclosure experiment (Hopland, CA) to assess the impact of mammalian herbivory during early post-fire chaparral succession, both on herbaceous plant assemblages and soil N and C cycling. We predicted that, in high-N post-fire conditions, mammalian herbivory would not demonstrate a strong preference for N-fixing herbs, would accelerate N cycling and would reduce the abundance of non-native herbs.

Excluding mammalian herbivores increased herb standing biomass by 54%, but changed neither the relative abundance of N-fixing vs. non-N-fixing herbs nor any measure of N or C cycling. Herbivore impacts on nutrient cycling may not be significant over the 2-year time scale of the experiment and physical effects of herbivore activity could have counteracted the influence of plant litter and animal dung/urine inputs. Mammalian herbivores concentrated their feeding on typical non-native herbs, slightly decreasing their relative abundance; however, mammalian herbivory was not sufficient to stem the invasion of chaparral by invasive herbs or alter C and N cycling over the first 2 years after fire.