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Rhizosphere priming is crucial for regulating soil carbon and nitrogen biogeochemical cycles. An appreciable number of studies have been conducted to quantify the rhizosphere priming effect (RPE), and have shown that the RPE is sensitive to changes of plant and soil conditions. These diverse results across individual studies offer us an opportunity to explore for potential general patterns and variability. In this study, we conducted a meta-analysis of RPE values taken from 31 publications. Our results showed that, on average, the RPE enhanced soil organic carbon mineralization rate by 59% across all studies. The magnitudes of the RPE significantly varied among plant types and soil texture. Within plant types, woody species showed the highest RPE followed by grasses while crops had the lowest level of the RPE, indicating that plant traits and physiology may exert important controls on the RPE. Soils with finer texture tended to produce stronger RPEs than soils with coarser texture, suggesting that interactions between the rhizosphere and the soil matrix may modulate the RPE. Furthermore, the level of the RPE is positively correlated with aboveground plant biomass, but surprisingly not with root biomass which is the commonly believed key variable for influencing the RPE. In addition, the RPE increased with the length of experimental duration, which implies that the RPE may persist much longer than previously believed because it impacts stabilized soil carbon more than labile carbon as the length of experimental duration increases. Overall, the results from this meta-analysis further illustrate several complex features of the RPE and call for future attentions to decipher this complexity.

Root turnover is a key process contributing to soil carbon storage, nutrient cycling, and other ecosystem functions. However, quantifying root turnover rates remains highly uncertain and methodologically challenging. Field rhizotrons were employed to quantify root turnover times using median longevities of five branching orders in a Larix gmelinii plantation. Root images were recorded by scanning the rhizotron windows at a monthly interval during four growing seasons. Root demographic data and branching orders were obtained by analyzing these images using Rootfly software coupled with manual mouse‐tracing of individual roots. Root longevities and turnover estimates were calculated using these data. Roots of different branching orders showed significantly different turnover times. The mean turnover times of the first‐order roots and second‐order roots were 284 and 994 d, respectively. Roots of higher branching orders (third to fifth orders) remained alive at the end of the 4‐yr experimental period, indicating much longer turnover times than the duration of the experiment. Root turnover times increased exponentially with branching orders. Further analysis of these data suggested that root branching orders combined with sampling biases, timing of root cohorts, and longevity distribution patterns crucially influenced root turnover times. The method of combining field glass rhizotrons with electronic scanning permits quantification of root turnover for five branching orders in a temperate forest. The overall result empirically demonstrates the crucial role of branching orders for accurately quantifying root turnover times.

The architecture and morphology of absorptive roots show substantial plasticity in response to forest management practices. These traits are known to play important roles in the acquisition of soil resources by trees. However, the effects of nutrient addition, thinning and pruning on absorptive root traits and their feedback to root foraging strategies remain unclear. We investigated the values and plasticity of traits related to nutrient foraging (root architecture, growth and morphology) for first- and second-order roots (absorptive roots) following nitrogen (N) addition, phosphorous (P) addition, thinning and pruning treatments in a young Chinese fir ( Cunninghamia lanceolata ) plantation. We measured twelve traits of absorptive roots under the five treatments (control, N addition, P addition, thinning and pruning) and determined relationships between the values and plasticity of root traits and stem growth rate. We demonstrated clear patterns of root traits and their plasticity in response to the treatments. N and P addition increased root biomass (B) and root tissue density (RTD). Thinning and pruning led to larger specific root length (SRL) and root nitrogen concentrations (N), but resulted in lower root length (L) and root length density (RLD). Principal component analysis of the measured traits and plasticity provided evidence for two suites of traits related to resource acquisition and conservation strategies among treatments. The trait syndromes exhibiting resource acquisition strategy (SRL and N) is arrayed well along the positive part of the first axis, whereas the opposite trait syndromes with resource conservation strategy (root diameter and RTD) is along the negative part of the first axis. The first axis also separates nutrient-induced treatments (N and P addition) from light-mediated treatments (pruning and thinning). Furthermore, first-order root exhibited higher foraging sensitivity and precision (expressed as relative fine root length difference) in response to P addition than to the other treatments. The foraging sensitivity and precision, plus B, L and RLD showed that first-order roots are more responsive to environment than second-order roots across the treatments. Stem growth rate was correlated positively with absorptive root traits (biomass, root surface area index, root length density, and root tip number) after thinning, but negatively with these traits after N and P addition. These findings demonstrate that C . lanceolata finely tuned root foraging strategies between first- and second-order root traits and their plasticity at the intraspecific level in response to forest management practices. Further studies may explore nutrient-induced and light-mediated foraging strategies between absorptive roots across root branch orders in mature Chinese fir plantation.