While there is an emerging view that roots and their associated microbes actively alter resource availability and soil organic matter (SOM) decomposition, the ecosystem consequences of such rhizosphere effects have rarely been quantified. Using a meta-analysis, we show that multiple indices of microbially mediated C and nitrogen (N) cycling, including SOM decomposition, are significantly enhanced in the rhizospheres of diverse vegetation types. Then, using a numerical model that combines rhizosphere effect sizes with fine root morphology and depth distributions, we show that root-accelerated mineralization and priming can account for up to one-third of the total C and N mineralized in temperate forest soils. Finally, using a stoichiometrically constrained microbial decomposition model, we show that these effects can be induced by relatively modest fluxes of root-derived C, on the order of 4% and 6% of gross and net primary production, respectively. Collectively, our results indicate that rhizosphere processes are a widespread, quantitatively important driver of SOM decomposition and nutrient release at the ecosystem scale, with potential consequences for global C stocks and vegetation feedbacks to climate.

http://math.bu.edu/people/mak/Finzi_Global_2015.pdf

Rhizosphere processes are quantitatively important components of terrestrial carbon and nutrient cycles.

Rhizosphere priming is the change in decomposition of soil organic matter (SOM) caused by root activity. Rhizosphere priming plays a crucial role in soil carbon (C) dynamics and their response to global climate change. Rhizosphere priming may be affected by soil nutrient availability, but rhizosphere priming itself can also affect nutrient supply to plants. These interactive effects may be of particular relevance in understanding the sustained increase in plant growth and nutrient supply in response to a rise in atmospheric CO2 concentration. We examined how these interactions were affected by elevated CO2 in two similar semiarid grassland field studies. We found that an increase in rhizosphere priming enhanced the release of nitrogen (N) through decomposition of a larger fraction of SOM in one study, but not in the other. We postulate that rhizosphere priming may enhance N supply to plants in systems that are N limited, but that rhizosphere priming may not occur in systems that are phosphorus (P) limited. Under P limitation, rhizodeposition may be used for mobilisation of P, rather than for decomposition of SOM. Therefore, with increasing atmospheric CO2 concentrations, rhizosphere priming may play a larger role in affecting C sequestration in N poor than in P poor soils.

Rhizosphere Priming: a Nutrient Perspective

Plant Functional Traits: Soil and Ecosystem Services.

The crucial biogeochemical processes such as carbon and nutrient cycling are increasingly altered at the ecosystem scale by global environmental changes. Although soil extracellular enzyme activities (EEAs) play a critical role in biogeochemical processes, the global patterns of soil EEAs in a changing world remain elusive. Here, we synthesized eight EEAs involved in carbon (C), nitrogen (N) and phosphorus (P) acquisition in response to seven global change factors based on 132 peer-reviewed papers. Our results showed that elevated CO2 concentration had no significant effects on soil EEAs. Nitrogen addition stimulated C-acquisition (9.1%) and P-acquisition (9.9%) EEAs, but suppressed oxidase activity (−6.8%). Phosphorus addition decreased P-acquisition EEA (−19.8%), while combined N and P addition increased C-acquisition EEA (30.7%). Moreover, decrease in precipitation dramatically suppressed oxidase activity (−47.2%), increase in precipitation marginally stimulated N-acquisition EEA (16.7%), while warming significantly decreased oxidase activity (−10.9%) and had minor positive effect on hydrolytic enzymes. Overall, our results provide some evidence (with exceptions) for the resource allocation theory of microbial enzyme production, and indicate that EEAs are generally more sensitive to nutrient addition than to atmospheric and climate change. We have shown that global environmental changes can alter EEAs, which have implications for soil carbon storage, nutrient cycling, and plant productivity. Further research is needed to elucidate the underlying mechanisms driving the responses of EEAs to global change and to collect data from particularly non-forest ecosystems (e.g., wetland, tundra and desert) and global-change drivers (other than N addition) that lack of EEA data. Our synthesis of the responses of soil enzyme activities to global-change drivers can be used to develop better representations of microbial processes in ecosystem and earth system models.

A meta-analysis of soil extracellular enzyme activities in response to global change

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2435.13303

A plant perspective on nitrogen cycling in the rhizosphere

Living roots and their rhizodeposits affect microbial activity and soil carbon (C) and nitrogen (N) mineralization. This so-called rhizosphere priming effect (RPE) has been increasingly recognized recently. However, the magnitude of the RPE and its driving mechanisms remain elusive. Here we investigated the RPE of two plant species (soybean and sunflower) grown in two soil types (a farm or a prairie soil) and sampled at two phenological stages (vegetative and mature stages) over an 88-day period in a greenhouse experiment. We measured soil C mineralization using a continuous 13C-labeling method, and quantified gross N mineralization with a 15N-pool dilution technique. We found that living roots significantly enhanced soil C mineralization, by 27–245%. This positive RPE on soil C mineralization did not vary between the two soils or the two phenological stages, but was significantly greater in sunflower compared to soybean. The magnitude of the RPE was positively correlated with rhizosphere respiration rate across all treatments, suggesting the variation of RPE among treatments was likely caused by variations in root activity and rhizodeposit quantity. Moreover, living roots stimulated gross N mineralization rate by 36–62% in five treatments, while they had no significant impact in the other three treatments. We also quantified soil microbial biomass and extracellular enzyme activity when plants were at the vegetative stage. Generally, living roots increased microbial biomass carbon by 0–28%, β-glucosidase activity by 19–56%, and oxidative enzyme activity by 0–46%. These results are consistent with the positive rhizosphere effect on soil C (45–79%) and N (10–52%) mineralization measured at the same period. We also found significant positive relationships between β-glucosidase activity and soil C mineralization rates and between oxidative enzyme activity and gross N mineralization rates across treatments. These relationships provide clear evidence for the microbial activation hypothesis of RPE. Our results demonstrate that root–soil–microbial interactions can stimulate soil C and N mineralization through rhizosphere effects. The relationships between the RPE and rhizosphere respiration rate and soil enzyme activity can be used for explicit representations of RPE in soil organic matter models.

/pdf/rhizosphere-priming-effects-on-soil-carbon-and-nitrogen-29woa1cyi1.pdf

Rhizosphere priming effects on soil carbon and nitrogen mineralization

Linking rhizosphere respiration rate of three grassland species with root nitrogen concentration

As commonly recognized, rhizosphere processes regulate major fluxes of energy, nutrients, and water in terrestrial ecosystems. However, mostly because of methodological limitations, our understanding of rhizosphere processes at the ecosystem level is limited. Most existing results in the literature have been obtained from individual laboratory studies using single plant species. It is widely recognized that extrapolating these existing results from laboratory-scale studies to field ecosystems can be highly unrealistic and potentially erroneous. One approach to address this issue is to search for general patterns across different experimental scales. The main objective of the project has been to address the key research question: are rhizosphere processes scalable among different plant species, different growth conditions in various soil types, and at different temporal and spatial scales? We have aimed to find the answer to this research question by testing two hypotheses: (1) rhizosphere respiration rate scales isometrically with live root N content, and this scaling relation is independent of plant taxonomic groupings or growth conditions; and (2) there exists an invariant scaling relationship between the rate of rhizosphere-primed soil organic matter decomposition and live root N content, live root biomass, and /or plant productivity when plants are grown in similar soils.

/pdf/scaling-rhizosphere-respiration-and-priming-effect-from-32y2u3xzwx.pdf

Daniel C. Keck

Papers

Rhizosphere priming effects on soil carbon and nitrogen mineralization

Linking rhizosphere respiration rate of three grassland species with root nitrogen concentration

Scaling Rhizosphere Respiration and Priming Effect from Single Plants to Field Ecosystems