Climate change scenarios forecast increased aridity in large areas worldwide with potentially important effects on nutrient availability and plant growth. Plant nitrogen and phosphorus concentrations ...(plant N and P) have been used to assess nutrient limitation, but a comprehensive understanding of drought stress on plant N and P remains elusive. We conducted a meta‐analysis to examine responses of plant N and P to drought manipulation treatments and duration of drought stress. Drought stress showed negative effects on plant N (−3.73%) and plant P (−9.18%), and a positive effect on plant N : P (+ 6.98%). Drought stress had stronger negative effects on plant N and P in the short term (< 90 d) than in the long term (> 90 d). Drought treatments that included drying–rewetting cycles showed no effect on plant N and P, while constant, prolonged, or intermittent drought stress had a negative effect on plant P. Our results suggest that negative effects on plant N and P are alleviated with extended duration of drought treatments and with drying–rewetting cycles. Availability of water, rather than of N and P, may be the main driver for reduced plant growth with increased long‐term drought stress.
Summary
From recent developments on how roots affect soil organic carbon (SOC) an apparent paradox has emerged where roots drive SOC stabilization causing SOC accrual, but also SOC destabilization ...causing SOC loss. We synthesize current results and propose the new Rhizo‐Engine framework consisting of two linked components: microbial turnover and the soil physicochemical matrix. The Rhizo‐Engine is driven by rhizodeposition, root turnover, and plant uptake of nutrients and water, thereby accelerating SOC turnover through both stabilization and destabilization mechanisms. This Rhizo‐Engine framework emphasizes the need for a more holistic approach to study root‐driven SOC dynamics. This framework would provide better understanding of plant root effects on soil carbon sequestration and the sensitivity of SOC stocks to climate and land‐use changes.
Biochar, a form of pyrogenic carbon, can contribute to agricultural and environmental sustainability by increasing soil reactivity. In soils, biochar could change its role over time through ...alterations in its surface chemistry. However, a mechanistic understanding of the aging process and its role in ionic nutrient adsorption and supply remain unclear. Here, we aged a wood biochar (550 °C) by chemical oxidation with 5–15% H2O2 and investigated the changes in surface chemistry and the adsorption behavior of ammonium and phosphate. Oxidation changed the functionality of biochar with the introduction of carboxylic and phenolic groups, a reduction of oxonium groups and the transformation of pyridine to pyridone. After oxidation, the adsorption of ammonium increased while phosphate adsorption decreased. Ammonium adsorption capacity was nonlinearly related to the biochar’s surface charge density (r 2 = 0.94) while electrostatic repulsion and loss of positive charge due to destruction of oxonium and pyridine, possibly caused the reduced phosphate adsorption. However, the oxidized biochar substantially adsorbed both ammonium and phosphate when biochar derived organic matter (BDOM) was included. Our results suggest that aging of biochar could reverse its capacity for the adsorption of cationic and anionic species but the inclusion of BDOM could increase ionic nutrient and contaminant retention.
Rhizosphere priming effects (RPEs) play a central role in modifying soil organic matter mineralization. However, effects of tree species and intraspecific competition on RPEs are poorly understood.
...We investigated RPEs of three tree species (larch, ash and Chinese fir) and the impact of intraspecific competition of these species on the RPE by growing them at two planting densities for 140 d. We determined the RPE on soil organic carbon (C) decomposition, gross and net nitrogen (N) mineralization and net plant N acquisition.
Differences in the RPE among species were associated with differences in plant biomass. Gross N mineralization and net plant N acquisition increased, but net N mineralization decreased, as the RPE on soil organic C decomposition increased. Intraspecific competition reduced the RPE on soil organic C decomposition, gross and net N mineralization, and net plant N acquisition, especially for ash and Chinese fir.
Microbial N mining may explain the overall positive RPEs across species, whereas intensified plant–microbe competition for N may have reduced the RPE with intraspecific competition. Overall, the species-specific effects of tree species play an important role in modulating the magnitude and mechanisms of RPEs and the intraspecific competition on soil C and N dynamics.
The long term effect of biochar application on soil microbial biomass is not well understood. We measured soil microbial biomass carbon (MBC) and nitrogen (MBN) in a field experiment during a winter ...wheat growing season after four consecutive years of no (CK), 4.5 (B4.5) and 9.0 t biochar ha(-1) yr(-1) (B9.0) applied. For comparison, a treatment with wheat straw residue incorporation (SR) was also included. Results showed that biochar application increased soil MBC significantly compared to the CK treatment, and that the effect size increased with biochar application rate. The B9.0 treatment showed the same effect on MBC as the SR treatment. Treatments effects on soil MBN were less strong than for MBC. The microbial biomass C∶N ratio was significantly increased by biochar. Biochar might decrease the fraction of biomass N mineralized (KN), which would make the soil MBN for biochar treatments underestimated, and microbial biomass C∶N ratios overestimated. Seasonal fluctuation in MBC was less for biochar amended soils than for CK and SR treatments, suggesting that biochar induced a less extreme environment for microorganisms throughout the season. There was a significant positive correlation between MBC and soil water content (SWC), but there was no significant correlation between MBC and soil temperature. Biochar amendments may therefore reduce temporal variability in environmental conditions for microbial growth in this system thereby reducing temporal fluctuations in C and N dynamics.
Drying and rewetting of soil can have large effects on carbon (C) and nitrogen (N) dynamics. Drying-rewetting effects have mostly been studied in the absence of plants, although it is well known that ...plant–microbe interactions can substantially alter soil C and N dynamics. We investigated for the first time how drying and rewetting affected rhizodeposition, its utilization by microbes, and its stabilization into soil (C associated with soil mineral phase). We also investigated how drying and rewetting influenced N mineralization and loss. We grew wheat (Triticum aestivum) in a controlled environment under constant moisture and under dry-rewetting cycles, and used a continuous 13C-labeling method to partition plant and soil organic matter (SOM) contribution to different soil pools. We applied a 15N label to the soil to determine N loss. We found that dry-rewetting decreased total input of plant C in microbial biomass (MB) and in the soil mineral phase, mainly due to a reduction of plant biomass. Plant derived C in MB and in the soil mineral phase were positively correlated (R2 = 0.54; P = 0.0012). N loss was reduced with dry rewetting cycles, and mineralization increased after each rewetting event. Overall drying and rewetting reduced rhizodeposition and stabilization of new C, primary through biomass reduction. However, frequency of rewetting and intensity of drought may determine the fate of C in MB and consequently into the soil mineral phase. Frequency and intensity may also be crucial in stimulating N mineralization and reducing N loss in agricultural soils.
•We assessed effects of dry-rewetting on rhizodeposition and N dynamics.•Dry-rewetting reduced the amount of rhizodeposition and its stabilization in soil.•Plant derived C in microbes was positively correlated with soil mineral associated C.•Dry-rewetting cycles decreased N loss and rewetting stimulated N mineralization.
Soil organic matter (SOM) plays a central role in the global carbon balance and in mitigating climate change. It will therefore be important to understand mechanisms of SOM decomposition and ...stabilisation. SOM stabilisation is controlled by biotic factors, such as the efficiency by which microbes use and produce organic compounds varying in chemistry, but also by abiotic factors, such as adsorption of plant- and microbially-derived organic matter onto soil minerals. Indeed, the physicochemical adsorption of organic matter onto soil minerals, forming mineral associated organic matter (MAOM), is one of the significant processes for SOM stabilisation. We integrate existing frameworks of SOM stabilisation and illustrate how microbial control over SOM stabilisation interacts with soil minerals. In our new integrated framework, we emphasise the interplay between substrate characteristics and the abundance of active clay surfaces on microbial processes such as carbon use efficiency and recycling. We postulate that microbial use and recycling of plant- and microbially-derived substrates decline with increased abundance of active clay surfaces, and that the shape of these relationships depend on the affinity of each substrate to adsorb, thereby affecting the efficiency by which organic matter remains in the soil and is stabilised into MAOM. Our framework provides avenues for novel research and ideas to incorporate interactions between clay surfaces and microbes on SOM stabilisation in biogeochemical models.
Graphical abstract
1. Drought induces changes in the nitrogen (N) and phosphorus (P) cycle but most plant species have limited flexibility to take up nutrients under such variable or unbalanced N and P availability. ...Both the degree of flexibility in plant N:P ratio and of root symbiosis with arbuscular mycorrhizal fungi might control plant resistance to drought-induced changes in nutrient availability, but this has not been directly tested. 2. Here, we examined the role of plant N:P stoichiometric status and mycorrhizal symbiosis in the drought-resistance of dominant and subordinate species in a semi-natural grassland. 3. We reduced water availability using rainout shelters (control vs. drought) and measured how plant biomass responded for the dominant and subordinate species. We then selected a dominant (Paspalum dilatatum) and a subordinate species (Cynodon dactylon), for which we investigated the N:P stoichiometric status, mycorrhizal root colonization and water-use efficiency. 4. The biomass of all dominant plant species, but not subordinate species, decreased under drought. Drought increased soil available nitrogen, and thus increased soil N:P ratio, due to decreasing plant N uptake. The dominant P. dilatatum showed a high degree of plant N:P homeostasis and a considerable reduction in biomass under drought. At the opposite, the more flexible subordinate species C. dactylon increased its N uptake and water-use efficiency, apparently due to stronger symbiosis with mycorrhizae, and maintained its biomass. 5. Synthesis. We conclude that the maintenance of N:P homeostasis in dominant species, possibly because of a large root nutrient foraging capacity, becomes inefficient when water stress limits N mobility in the soil. By contrast, we demonstrate that higher stoichiometric N:P flexibility coupled with stronger mutualistic association with mycorrhizae allow subordinate species to better withstand drought perturbations. Using a stoichiometric approach in a field experiment, our study provides for the first time clear and novel understandings of the mechanisms involved in drought-resistance within the plant-mycorrhizae-soil system.
Decomposition of soil organic carbon (SOC) is the main process governing the release of CO2 into the atmosphere from terrestrial systems. Although the importance of soil–root interactions for SOC ...decomposition has increasingly been recognized, their long‐term effect on SOC decomposition remains poorly understood. Here we provide experimental evidence for a rhizosphere priming effect, in which interactions between soil and tree roots substantially accelerate SOC decomposition. In a 395‐day greenhouse study with Ponderosa pine and Fremont cottonwood trees grown in three different soils, SOC decomposition in the planted treatments was significantly greater (up to 225%) than in soil incubations alone. This rhizosphere priming effect persisted throughout the experiment, until well after initial soil disturbance, and increased with a greater amount of root‐derived SOC formed during the experiment. Loss of old SOC was greater than the formation of new C, suggesting that increased C inputs from roots could result in net soil C loss.
Photosynthetic carbon (C) allocated below‐ground can be shared with mycorrhizal fungi in exchange for nutrients, but also added into soil as rhizodeposits that potentially increases plant nutrient ...supply by supporting microbial nutrient mineralization from organic matter. How water and nitrogen (N) availability affects plant C allocation to the rhizosphere, including both arbuscular mycorrhizal fungi (AMF) symbionts and rhizodeposits, remains largely unknown.
We used a 13CO2 pulse labelling experiment to assess the effects of drought and N addition on below‐ground allocation of C to soils and roots (quantified as excess 13C) and tested their relationships with AMF colonization in an Australian grassland. We also examined relationships between AMF and previously reported root respiration and decomposition of rhizodeposits in this study.
We found that drought decreased the absolute amount of excess 13C allocated to both soils and roots, likely due to less photosynthetic C fixation. In contrast, proportionally more excess 13C was allocated to soils but less to root biomass with drought, suggesting that relatively more C was allocated to rhizodeposits and to AMF hyphal growth and extension. However, N addition reversed drought effects on below‐ground C allocation by retaining proportionally more excess 13C in roots and less in soils, congruent with higher soil N and phosphorus availability, root biomass and number of root tips compared to drought without N addition. This suggests that the alleviation of nutrient limitation promoted plants to expend relatively more C on root growth and root trait adjustment, but less C on rhizodeposition and mycorrhizal symbiosis.
Synthesis. Mycorrhizal colonization related negatively to rhizodeposit decomposition rate but positively to both excess 13C in root biomass and root respiration, suggesting a possible trade‐off in C allocation between mycorrhizal symbiosis and rhizodeposition. We conclude that below‐ground C allocation in this grassland can be mediated by mycorrhizal colonization and is strongly affected by water and nutrient availability.
Drought proportionally increased carbon (C) deposited into soil but decreased C allocated to roots due to lower C investments on adjusting root traits for nutrient acquisition, showing decreases in specific root length (SRL) and specific root area (SRA) but an increase in root diameter (RD). Nitrogen addition counteracted drought effects on proportional C allocation between roots and soils.
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