Publications

Rhizodeposition of plants is the most uncertain component of the carbon (C) cycle. By existing approaches the amount of rhizodeposition can only roughly be estimated since its persistence in soil is very short compared to other organic C pools. We suggest an approach to quantify rhizodeposition at the field scale by assuming a constant ratio between rhizodeposited-C to root-C. Maize plants were pulse-labeled with 14CO2 under controlled conditions and the soil 14CO2 efflux was separated into root and rhizomicrobial respiration. The latter and the 14C activity remaining in the soil corresponded to total rhizodeposition. By relating rhizodeposited-14C to root-14C a rhizodeposition-to-root ratio of 0.56 was calculated. This ratio was applied to the root biomass C measured in the field to estimate rhizodeposition under field conditions. Maize allocated 298 kg C ha−1 as root-C and 166 kg C ha−1 as rhizodeposited-C belowground, 50 % of which were recovered in the upper 10 cm. The fate of rhizodeposits was estimated based on the 14C data, which showed that 62 % of total rhizodeposition was mineralized within 16 days, 7 % and 0.3 % was incorporated into microbial biomass and DOC, respectively, and 31 % was recovered in the soil. We conclude that the present approach allows for an improved estimation of total rhizodeposition, since it accounts not only for the fraction of rhizodeposits remaining in soil, but also for that decomposed by microorganisms and released from the soil as CO2.

The formation of microrelief forms in peatlands - elevated and dry hummocks, depressed wet hollows and intermediate lawns - is controlled by the interaction of water table, nutrient availability and dominant plant communities. This affects the composition and activity of various functional groups of microorganisms. With depth, the change in peat quality from less to more highly processed organic material additionally regulates microbial activity. We hypothesized that microbial biomass and enzyme activities are driven by aeration and by peat quality and therefore (i) they increase from hollows (water saturated/anaerobic) through lawns (intermediate) to hummocks (aerobic) in the top peat and ii) they decrease with depth due to increasing distance from fresh plant-derived inputs and lower oxygen availability. These hypotheses were tested for enzymes catalysing the decomposition of C-, N-, P- and S-containing organic compounds in peat of the three microform types at three depths (15, 50 and 200 cm). Microbial biomass and peat chemical characteristics were compared with enzyme kinetic parameters, i.e. maximal potential activity (Vmax) and the Michaelis constant (Km). Microbial biomass carbon (MBC) and Vmax of β-glucosidase and N-acetyl glucosaminidase increased by 30–70% from hummocks and lawns to hollows in the top 15 cm, contradicting the hypothesis. Similarly, Km and the catalytic efficiency of enzymes (Ka = Vmax/Km) were best related to MBC distribution and not to the aeration gradient. With depth, Vmax of β-glucosidase, xylosidase and leucine aminopeptidase followed the hypothesized pattern in hollows. In contrast, MBC was 1.3–4 times higher at 50 cm, followed by successively lower contents at 15 and 200 cm in all microforms. The same depth pattern characterized the Vmax distribution of 6 out of 8 enzymes. Phosphatase activity decreased from drier hummock to wetter hollows and the higher activity throughout the peat profile suggested a high microbial demand for P. Enzyme activities and catalytic efficiency in peat were closely linked to the distribution of microbial biomass with depth, which in turn was best explained by P content. From the ecological perspective, these results clearly show that peat decomposition will be accelerated when microbial activity is stimulated e.g. by increased P availability.

Desert ecosystems with a high soil pH are considered as a sink for atmospheric CO2. Although high CO2-uptakevia chemical processes is the major CO2-uptake process in saline and alkaline soil, microorganisms may affect the exchange of inorganic carbon between the soil solution and the solid phase. Some studies related to microbial processes on carbonate formation have been published, but seldom focus on saline and alkaline soils. To determine the microbial effects on the inorganic C flux and CaCO3 recrystallization, sterile and non-sterile Solonchak samples were incubated in a 14CO2-labelled atmosphere at two CO2 concentrations (0.2% and 2%). The 14C activity was measured in the soil solution, air, and in CaCO3 after 2, 7, 21, 92 and 197 days. The 14C in dissolved organic carbon in the sterile and non-sterile soils was below 5% of the 14C input. 14C trapping in CaCO3 increased with the decrease of the 14C remaining in the gaseous CO2. Carbonate recrystallization increased logarithmically with time (R 2 > 0.96). A two percent initial CO2 concentration lead to higher CaCO3 recrystallization (5–50 × 10−3% of initial CaCO3) compared to a 0.2% initial CO2 concentration (0.8–7 × 10−3% of initial CaCO3) from 2 to 197 days. CaCO3 recrystallization (% of initial CaCO3) in the sterile Solonchak was higher by 10% to 190% than the non-sterile Solonchak, indicating that the microorganisms decreased the 14C trapping of CaCO3 by 4–23% from 2 to 197 days. We assume that microorganisms grew on the carbonate surface like a coating. This must have retarded the carbonate recrystallization. We concluded that the soil CO2 concentration is the most important factor for CaCO3 recrystallization. The presence of microorganisms strongly decreased the CaCO3 recrystallization. CaCO3 recrystallization process including CO2 in soil water could be the abiotic CO2 uptake from the atmosphere by saline and alkaline soil.

Plant interactions and feedbacks with soil microorganisms play an important role in sustaining the functions and stability of terrestrial ecosystems, yet the effects of tree species diversity on soil microbial community in forest ecosystems are still not well understood. Here, we examined the effects of tree species richness (1–12 species) and the presence of certain influential tree species (sampling effect) on soil bacterial and fungal communities in Chinese subtropical forests, using high-throughput Illumina sequencing for microbial identification. We observed that beta rather than alpha diversities of tree species and soil microorganisms were strong coupled. Multivariate regression and redundancy analyses revealed that the effects of tree species identity dominated over tree species richness on the diversity and composition of bacterial and fungal communities in both organic and top mineral soil horizons. Soil pH, nutrients and topography were always identified as significant predictors in the best multivariate models. Tree species have stronger effect on fungi than bacteria in organic soil, and on ectomycorrhizal fungi than saprotrophic fungi in mineral topsoil. Concluding, tree species identity, along with abiotic soil and topographical conditions, were more important factors determining the soil microbial communities in subtropical forests than tree diversity per se.

The priming effect (PE) is a globally important regulator of soil organic matter (SOM) turnover that closely correlates with N availability. PE intensity is usually higher under N-limited conditions and lower under N inputs, but this relationship may depend on temperature. To investigate whether temperature influences the role of N availability in the PE, we conducted a 200-d incubation of soil samples amended with 13C-labeled rice litter at 15 °C, 25 °C, and 35 °C, adding NH4 + and NO3 − to examine impacts of increased N availability on PE. Litter amendment caused net N immobilization and exacerbated soil N limitation, thereby reducing SOM mineralization (which was negative PE) at 25 °C and 35 °C. N addition mitigated the negative PE intensity, with a marked decrease at 25 °C (from −21 % to −5 %) and a marginal impact at 35 °C. Microbial biomass C and N contents peaked at 25 °C but declined by ∼20 % at 35 °C. The ratio of C- to N-acquiring enzyme activities, as well as that for gene copies, was lower at 25 °C than at 35 °C, suggesting N limitation at 25 °C. In contrast, rapid labile C depletion and high microbial metabolic quotient caused severe microbial C limitation at 35 °C, which forced microorganisms to increase abundance of genes involved in recalcitrant compound decomposition. Warming to 35 °C also shifted microbial communities from the dominance of r-strategists (25 °C) to that of K-strategists (35 °C), as indicated by increases in the ratio of K- to r-related functional genes. The dominance of K-strategists, which generally have low N demands, rendered PE less sensitive to N inputs at 35 °C. Our findings underscore the temperature-dependent nature of soil N versus C limitations, which may lead to lower microbial N limitation and decreased impact of N availability on PE under a warming climate.

Root-derived resources are receiving increased attention as basal resources for soil animal food webs. They predominantly function as carbon and energy resources for microbial metabolism in the rhizosphere, however, root-derived nitrogen may also be important. We explored both the role of root-derived carbon (C) and nitrogen (N) for the nutrition of soil animal species. Using 13C and 15N pulse labeling we followed in situ the flux of shoot-derived C and N into the soil animal food web of young beech (Fagus sylvatica) and ash (Fraxinus excelsior) trees. For labeling with 13C, trees were exposed to increased atmospheric concentrations of 13CO2 and for labeling with 15N leaves were immersed in a solution of Ca15NO3. Twenty days after labeling root-derived N was detected in each of the studied soil animal species whereas incorporation of root-derived C was only detected in the ash rhizosphere. More root-derived N was incorporated into soil animals from the beech as compared to the ash rhizosphere, in spite of the higher 15N signatures in fine roots of ash as compared to beech. The results suggest that soil animal food webs not only rely on root C but also on root N with the contribution of root N to soil animal nutrition varying with tree species. This novel pathway of plant N highlights the importance of root-derived resources for soil animal food webs.

Date palm ( Phoenix dactylifera L.) fruit is one of the ancient crops that have been cultivated for thousands of years in the arid regions of the Middle East and North Africa. Its nutritional and economic importance has extended the cultivation to other countries with suitable environmental conditions. Despite the large expansion in date palm cultivation and the associated improvement in its yield and quality over the last few decades, challenges remain in its cultivation and sustainability. Here, we review: (1) the current status of date palm cultivation, (2) the key environmental and soil‐related factors influencing its cultivation and productivity, and (3) the challenges under global change in addition to assessing the future potentials for improvements and sustainability. The global production of date palm has increased by 18% during the last ten years, reaching 8.53 million tons, on a total area of 1.11 million ha. Water scarcity and salinity stresses are the major constraints affecting date palm productivity, which is a serious issue in arid regions, where water resources are becoming depleted and soil is becoming increasingly salinized due to limited rainfall, high evapotranspiration, and incorrect irrigation. Although date palm can tolerate salinity up to 12 dS m −1 , accumulation of salts in soil strongly reduces the yield. The main cultivation areas are dominated by soils of sandy texture with high pH and a low capacity for water and nutrient retention. Organic fertilizers increase water and nutrient retention. There is a lack of long‐term studies (e.g., ≥10 years) designed to monitor date palm cultivar responses to abiotic stresses as well as to nutrition and management practices. Future challenges facing date palm cultivation mainly include water shortages, extremely high temperatures, salinity, and soil degradation, as well as diseases and pests. The implementation of efficient management practices, including modern irrigation systems, selecting cultivars most suitable for the local environmental conditions, and increasing soil water and nutrient retention, help to sustain date palm cultivation and productivity.

Increase of phosphatase activity and rhizosphere acidification are two main physiological mechanisms of phosphorus mobilization by plants. To understand plant P-acquisition strategies, the spatial distribution of phosphatase activity and pH in the rhizosphere in response to P availability between legumes and cereals and along individual roots were investigated. Zymography of acid and alkaline phosphatase activities visualizing P hydrolysis potential was combined with planar optodes to localize pH changes in maize and lupine rhizosphere. Lupine was more adapted to P-deficiency than maize by increasing phosphatase activity (up to 30%), decreasing pH for 0.8 units in the rhizosphere and having larger hotspot areas compared to P addition. The P deficiency-induced increases of phosphatase activities and acidification were position-specific along taproot of lupine: acid phosphatase was highest while alkaline phosphatase was least at apical parts; pH was 1.7 units lower at the apical than at the basal part. The phosphatase activities and pH along lateral roots of lupine were independent on P availability but had specific distribution patterns: acid phosphatase activity declined from the root tips toward basal part, while the alkaline phosphatase activity was slightly increased from root tips; the pH was highest at the root tips (first 5 mm) and declined thereafter until stabilized after 20 mm. We conclude that 1) lupine is better adapted to P deficiency than maize by a stronger increase of phosphatase activities and facilitation of rhizosphere acidification; 2) The organic P mobilization by phosphatases and mineral P solubilization by acidification along individual roots were dependent on the root zone.

A reliable and precise estimate of the temperature sensitivity (Q 10) of soil organic matter (SOM) decomposition is critical to predict feedbacks between the global carbon (C) cycle and climate change. In this study, we first summarize two commonly used approaches for estimating Q 10 ( Approach A : constant temperature incubation and discontinuous measurements, CDM model; Approach B : varying temperature incubation and discontinuous measurements, VDM model). We then introduced a newly developed approach ( Approach C, VCM model) that combines rapidly varying temperature incubations and continuous measurements of SOM decomposition rates (R s) that may be more realistic and suitable for Q 10 estimation, especially for large scale estimation. Then, we conducted a 26-day incubation experiment using three different soils to compare the performance of these three approaches for estimating Q 10 using R 2 and P-values as indicators. Our results demonstrate that the fitting goodness of the exponential model was consistently higher for Approach C , with higher R 2 values, lower confidence intervals, and lower P-values in almost all cases compared with Approaches A and B . Furthermore, results showed that Approaches A and B underestimated the Q 10 value by 9.5–13% and 2.9–5.7%, respectively, in three different soils throughout the entire incubation period. Compared with traditional commonly used methods, the newly developed Approach C (VCM model) provides a more accurate and rapid estimation of the temperature response of SOM decomposition and can be used for large-scale estimation of Q 10.