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Nitrogen availability in the rhizosphere relies on root‐microorganism interactions, where root exudates trigger soil organic matter (SOM) decomposition through the rhizosphere priming effect (RPE). Though microbial necromass contribute significantly to organically bound soil nitrogen (N), the role of RPEs in regulating necromass recycling and plant nitrogen acquisition has received limited attention. We used 15N natural abundance as a proxy for necromass‐N since necromass is enriched in 15N compared to other soil‐N forms. We combined studies using the same experimental design for continuous 13CO2 labelling of various plant species and the same soil type, but considering top‐ and subsoil. RPE were quantified as difference in SOM‐decomposition between planted and unplanted soils. Results showed higher plant N uptake as RPEs increased. The positive relationship between 15N‐enrichment of shoots and roots and RPEs indicated an enhanced necromass‐N turnover by RPE. Moreover, our data revealed that RPEs were saturated with increasing carbon (C) input via rhizodeposition in topsoil. In subsoil, RPEs increased linearly within a small range of C input indicating a strong effect of root‐released C on decomposition rates in deeper soil horizons. Overall, this study confirmed the functional importance of rhizosphere C input for plant N acquisition through enhanced necromass turnover by RPEs. Summary statement: Interactions between roots and microorganisms in the rhizosphere enhance nitrogen availability through the rhizosphere priming effect (RPE). Our study confirmed that increasing RPEs promote higher plant nitrogen uptake and indicate enhanced turnover of necromass‐nitrogen through rhizosphere priming. [ABSTRACT FROM AUTHOR]

The study aimed to evaluate the effects of long‐term fertilisation on soil aggregation and the associated changes in soil organic carbon (SOC) and nitrogen (N) pools in aggregates. The combined application of mineral fertiliser and manure improved soil aggregation, SOC and N content in aggregates, compared to manure or mineral fertiliser alone, and thus proved to be a suitable fertilisation strategy to increase C sequestration in agroecosystems. [ABSTRACT FROM AUTHOR]

Aims: There is a trend of increasing woody biomass in tropical savannas. Here we ask what effect this increase may have on soil carbon pools and fluxes. Methods: Using a field experiment we determine the amount of soil carbon directly under grasses, a juvenile tree among grasses and a juvenile tree with no grasses. We also measure CO2 efflux at the soil surface and use gas wells to extract CO2 from several soil depths. Results: Our results show that grasses contribute substantially more than trees to both soil carbon and soil respiration. Grasses also make a disproportionate contribution to the δ13C value of SOC in the tree only treatments. The surface gas efflux data show that soil respiration increased with an increase in volumetric soil moisture and temperature and plots with both grasses and trees had higher respiration rates than plots with trees only or with grasses only. Conclusions: The highest soil respiration is in the top 20 cm of the soil with grasses the primary contributors to both δ13CSOC and δ13CR. Any increase in woody biomass will result in a decline in SOM turnover and nitrogen mineralization rates resulting in higher SOC pools. The associated increases in SOC and above ground biomass will however be associated with negative economic and biodiversity impacts. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) pools, allocated within various aggregates, are characterized by different degradability and turnover rates that depend on the spatial accessibility of organics and their recalcitrance. Hence, to understand the processes and mechanisms of SOM cycling and stability, the contribution of individual aggregate size classes to the total CO 2 efflux including extra mineralization via priming effect (PE) should be considered. In this study, we determined whether aggregate size classes and their disruption affected the mineralization of SOM and induced PE depending on the primer amount. Soil samples were separated into three aggregate size classes (>2 mm, 2–0.25 mm macroaggregates and <0.25 mm microaggregates). Half of the samples within each class were left intact, whereas half were crushed. After the addition of two levels of 14 C-labeled glucose, the amount of 14 C in CO 2 efflux and microbial biomass were measured several times during the 49-day incubation. Cumulative SOM-derived CO 2 production from the macroaggregates was 16–21% greater than the CO 2 production from the microaggregates after 49 days. The percentage of glucose mineralized to CO 2 increased with the level of glucose addition, but 14 C incorporation into microbial biomass decreased, indicating lower carbon (C) use efficiency at high substrate availability. Aggregate disruption had no effect on the cumulative total and SOM-derived CO 2 production, but it increased glucose mineralization up to 11.2% while the percentage of added glucose incorporated into microbial biomass in macroaggregates decreased. The PE increased with an increased glucose level for the intact aggregates. Aggregate disruption increased the PE in all aggregates sizes under low glucose level. In summary, our findings demonstrate that the aggregate size class has clear effects on C mineralization while their disruption affects the added labile C decomposition and transformation, indicating the relevance of soil structure for SOM cycling in terms of priming and C sequestration. [ABSTRACT FROM AUTHOR]

Abstract: Living roots and their rhizodeposits can stimulate microbial activity and soil organic matter (SOM) decomposition up to several folds. This so-called rhizosphere priming effect (RPE) varies widely among plant species possibly due to species-specific differences in the quality and quantity of rhizodeposits and other root functions. However, whether the RPE is influenced by plant inter-species interactions remains largely unexplored, even though these interactions can fundamentally shape plant functions such as carbon allocation and nutrient uptake. In a 60-day greenhouse experiment, we continuously labeled monocultures and mixtures of sunflower, soybean and wheat with 13C-depleted CO2 and partitioned total CO2 efflux released from soil at two stages of plant development for SOM- and root-derived CO2. The RPE was calculated as the difference in SOM-derived CO2 between the planted and the unplanted soil, and was compared among the monocultures and mixtures. We found that the RPE was positive under all plants, ranging from 43% to 136% increase above the unplanted control. There were no significant differences in RPE at the vegetative stage. At the flowering stage however, the RPE in the soybean–wheat mixture was significantly higher than those in the sunflower monoculture, the sunflower–wheat mixture, and the sunflower–soybean mixture. These results indicated that the influence of plant inter-specific interactions on the RPE is case-specific and phenology-dependent. To evaluate the intensity of inter-specific effects on priming, we calculated an expected RPE for the mixtures based on the RPE of the monocultures weighted by their root biomass and compared it to the measured RPE under mixtures. At flowering, the measured RPE was significantly lower for the sunflower–wheat mixture than what can be expected from their monocultures, suggesting that RPE was significantly reduced by the inter-species effects of sunflower and wheat. In summary, our results clearly demonstrated that inter-species interactions can significantly modify rhizosphere priming on SOM decomposition. [Copyright &y& Elsevier]