Bamboo (Phyllostachys pubescens) expansion into adjacent forests is a widespread phenomenon in subtropical regions, and it has greatly changed the dominance hierarchy from trees to bamboos. This process may be accompanied by changes in productivity, nutrients accumulation and biogeochemical cycles. We compared the net primary production (NPP) and major pools and fluxes of nitrogen (N) in bamboo-dominant forest (BDF) and neighboring secondary evergreen broadleaved forest (EBF) in South China using the space-for-time substitution method. We found that the mean NPP of the BDF was 30.0 t ha⁻¹yr⁻¹, which was 51.5% greater than that of the EBF (19.8 t ha⁻¹yr⁻¹). The plant N pool for the BDF was 37.5% larger than that of the EBF, whereas the soil inorganic N pool significantly decreased by 31.2% with conversion of the EBF to BDF. Additionally, the ratio of N return to N uptake was 0.69 in the BDF and 0.88 in the EBF because of the lower litter N return of the BDF compared with that of the EBF. These results indicated that the expansion ofP. pubescenssignificantly increased the NPP and plant N accumulation but reduced the soil N available pool and slowed the N cycling rate, which could lead to soil degradation.

The unbridled expansion of bamboo has imposed serious threats on ecosystem processes and functions. Considerable evidence indicates that bamboo invasions can alter plant-available soil nitrogen (N) pools and rates of N cycling, but the consequences of altered N availability for plant growth and community structure have this far been poorly characterized. The primary soil-accessible N forms for most plants are ammonium (NH₄⁺) and nitrate (NO₃⁻), but plants differ in their ability to use the different N forms, and these differences can be related to their ecological characteristics and drive community structure. In this context, we evaluated the growth response, N uptake, and interspecific competition in two subtropical species, Phyllostachys edulis and Castanopsis fargesii, dominant species of bamboo and secondary evergreen broad-leaved forests, respectively, under changing N availability in seedlings supplied with different N concentrations and NH₄⁺/NO₃⁻ proportions, in vermiculite culture, in a controlled environment. The results show that: (1) Both species display an NH₄⁺ preference at elevated N concentrations. Growth of P. edulis seedlings was strongly enhanced at increased ratios of NH₄⁺ to NO₃⁻ especially at higher N concentrations, but to a much lesser extent in C. fargesii; (2) NH₄⁺ preference at the level of N uptake in both species was confirmed by the Non-invasive Micro-test Technology (NMT)and by examining ¹⁵N signatures. P. edulis had higher NH₄⁺ net fluxes and N concentration under NH₄⁺ treatments than C. fargesii; (3) NH₄⁺ at higher concentrations caused toxicity in both species as it inhibited root growth and even caused seedling death, but P. edulis had a higher NH₄⁺-tolerance threshold (24 mM) than C. fargesii (16 mM); (4) When mixed-species cultures were examined in an NH₄⁺-rich medium, growth of C. fargesii, but not P. edulis, was significantly inhibited compared to growth in monoculture. Therefore, P. edulis exhibited stronger plasticity and adaptation to changing N availability, while C. fargesii had low responsiveness and capacity to acclimate to soil N changes. P. edulis displayed a significant competitive growth advantage compared toC. fargesii on NH₄⁺-dominated substrates.