Enzyme kinetics inform about mechanistic changes in tea litter decomposition across gradients in land-use intensity in Central German grasslands.

Grassland ecosystems provide important ecosystem services such as nutrient cycling and primary production that are affected by land-use intensity. To assess the effects of land-use intensity, operational and sensitive ecological indicators that integrate effects of grassland management on ecosystem processes such as organic matter turnover are needed. Here, we investigated the suitability of measuring the mass loss of standardized tea litter together with extracellular enzyme kinetics as a proxy of litter decomposition in the topsoil of grasslands along a well-defined land-use intensity gradient (fertilization, mowing, grazing) in Central Germany. Tea bags containing either green tea (high-quality litter) or rooibos tea (low-quality litter) were buried in 5 cm soil depth. Litter mass loss was measured after three (early-stage decomposition) and 12 months (mid-stage decomposition). Based on the fluorescence measurement of the reaction product 4-methylumbelliferone, Michaelis-Menten enzyme kinetics (Vmax: potential maximum rate of activity; Km: substrate affinity) of five hydrolases involved in the carbon (C)-, nitrogen (N)- and phosphorus (P)-cycle (ß-glucosidase (BG), cellobiohydrolase (CBH), cellotriohydrolase (CTH), 1,4-ß-N-acetylglucosaminidase (NAG), and phosphatase (PH)) were determined in tea litter bags and in the surrounding soil. The land-use intensity index (LUI), summarizing fertilization, mowing, grazing, and in particular the frequency of mowing were identified as important drivers of early-stage tea litter decomposition. Mid-stage decomposition was influenced by grazing intensity. The higher the potential activity of all measured C-, N- and P-targeting enzymes, the higher was the decomposition of both tea litters in the early-phase. During mid-stage decomposition, individual enzyme parameters (Vmax of CTH and PH, Km of CBH) became more important. The tea bag method proved to be a suitable indicator which allows an easy and cost-effective assessment of land-use intensity effects on decay processes in manged grasslands. In combination with enzyme kinetics it is an appealing approach to identify mechanisms driving litter break down.

Soil phosphorus status and P nutrition strategies of European beech forests on carbonate compared to silicate parent material.

Sustainable forest management requires understanding of ecosystem phosphorus (P) cycling. Lang et al. (2017) [Biogeochemistry, https://doi.org/10.1007/s10533-017-0375-0] introduced the concept of P-acquiring vs. P-recycling nutrition strategies for European beech (Fagus sylvatica L.) forests on silicate parent material, and demonstrated a change from P-acquiring to P-recycling nutrition from P-rich to P-poor sites. The present study extends this silicate rock-based assessment to forest sites with soils formed from carbonate bedrock. For all sites, it presents a large set of general soil and bedrock chemistry data. It thoroughly describes the soil P status and generates a comprehensive concept on forest ecosystem P nutrition covering the majority of Central European forest soils. For this purpose, an Ecosystem P Nutrition Index (ENI P ) was developed, which enabled the comparison of forest P nutrition strategies at the carbonate sites in our study among each other and also with those of the silicate sites investigated by Lang et al. (2017). The P status of forest soils on carbonate substrates was characterized by low soil P stocks and a large fraction of organic Ca-bound P (probably largely Ca phytate) during early stages of pedogenesis. Soil P stocks, particularly those in the mineral soil and of inorganic P forms, including Al- and Fe-bound P, became more abundant with progressing pedogenesis and accumulation of carbonate rock dissolution residue. Phosphorus-rich impure, silicate-enriched carbonate bedrock promoted the accumulation of dissolution residue and supported larger soil P stocks, mainly bound to Fe and Al minerals. In carbonate-derived soils, only low P amounts were bioavailable during early stages of pedogenesis, and, similar to P-poor silicate sites, P nutrition of beech forests depended on tight (re)cycling of P bound in forest floor soil organic matter (SOM). In contrast to P-poor silicate sites, where the ecosystem P nutrition strategy is direct biotic recycling of SOM-bound organic P, recycling during early stages of pedogenesis on carbonate substrates also involves the dissolution of stable Ca-Porg precipitates formed from phosphate released during SOM decomposition. In contrast to silicate sites, progressing pedogenesis and accumulation of P-enriched carbonate bedrock dissolution residue at the carbonate sites promote again P-acquiring mechanisms for ecosystem P nutrition. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10533-021-00884-7.

Above- and belowground biodiversity jointly tighten the P cycle in agricultural grasslands.

Experiments showed that biodiversity increases grassland productivity and nutrient exploitation, potentially reducing fertiliser needs. Enhancing biodiversity could improve P-use efficiency of grasslands, which is beneficial given that rock-derived P fertilisers are expected to become scarce in the future. Here, we show in a biodiversity experiment that more diverse plant communities were able to exploit P resources more completely than less diverse ones. In the agricultural grasslands that we studied, management effects either overruled or modified the driving role of plant diversity observed in the biodiversity experiment. Nevertheless, we show that greater above- (plants) and belowground (mycorrhizal fungi) biodiversity contributed to tightening the P cycle in agricultural grasslands, as reduced management intensity and the associated increased biodiversity fostered the exploitation of P resources. Our results demonstrate that promoting a high above- and belowground biodiversity has ecological (biodiversity protection) and economical (fertiliser savings) benefits. Such win-win situations for farmers and biodiversity are crucial to convince farmers of the benefits of biodiversity and thus counteract global biodiversity loss.

Carbohydrate depletion in roots impedes phosphorus nutrition in young forest trees.

Nutrient imbalances cause the deterioration of tree health in European forests, but the underlying physiological mechanisms are unknown. Here, we investigated the consequences of decreasing root carbohydrate reserves for phosphorus (P) mobilisation and uptake by forest trees. In P-rich and P-poor beech (Fagus sylvatica) forests, naturally grown, young trees were girdled and used to determine root, ectomycorrhizal and microbial activities related to P mobilisation in the organic layer and mineral topsoil in comparison with those in nongirdled trees. After girdling, root carbohydrate reserves decreased. Root phosphoenolpyruvate carboxylase activities linking carbon and P metabolism increased. Root and ectomycorrhizal phosphatase activities and the abundances of bacterial genes catalysing major steps in P turnover increased, but soil enzymes involved in P mobilisation were unaffected. The physiological responses to girdling were stronger in P-poor than in P-rich forests. P uptake was decreased after girdling. The soluble and total P concentrations in roots were stable, but fine root biomass declined after girdling. Our results support that carbohydrate depletion results in reduced P uptake, enhanced internal P remobilisation and root biomass trade-off to compensate for the P shortage. As reductions in root biomass render trees more susceptible to drought, our results link tree deterioration with disturbances in the P supply as a consequence of decreased belowground carbohydrate allocation.

Enhanced tomato plant growth in soil under reduced P supply through microbial inoculants and microbiome shifts.

Soil microbial communities interact with roots, affecting plant growth and nutrient acquisition. In the present study, we aimed to decipher the effects of the inoculants Trichoderma harzianum T-22, Pseudomonas sp. DSMZ 13134, Bacillus amyloliquefaciens FZB42 or Pseudomonas sp. RU47 on the rhizosphere microbial community and their beneficial effects on tomato plants grown in moderately low phosphorous soil under greenhouse conditions. We analyzed the plant mass, inoculant colony forming units and rhizosphere communities on 15, 22, 29 and 43 days after sowing. Selective plating showed that the bacterial inoculants had a good rhizocompetence and accelerated shoot and root growth and nutrient accumulation. 16S rRNA gene fingerprints indicated changes in the rhizosphere bacterial community composition. Amplicon sequencing revealed that rhizosphere bacterial communities from plants treated with bacterial inoculants were more similar to each other and distinct from those of the control and the Trichoderma inoculated plants at harvest time, and numerous dynamic taxa were identified. In conclusion, likely both, inoculants and the rhizosphere microbiome shifts, stimulated early plant growth mainly by improved spatial acquisition of available nutrients via root growth promotion. At harvest, all tomato plants were P-deficient, suggesting a limited contribution of inoculants and the microbiome shifts to the solubilization of sparingly soluble soil P.

An inter-laboratory comparison of gaseous and liquid fumigation based methods for measuring microbial phosphorus (Pmic) in forest soils with differing P stocks.

In an inter-laboratory trial, gaseous ("CFE") and liquid fumigation ("Resin") based methods for measuring microbial phosphorus (Pmic) were compared, based on the analysis of soil samples from five forests, which differ in their P stocks. Both methods reliably detected the same Pmic gradient in the different soils. However, when the individual recovery rates of spiked P were taken into account, the "CFE" based methods consistently generated higher Pmic values (factor 2) compared to the "Resin" based approaches.

Impacts of temperature increase and change in precipitation pattern on crop yield and yield quality of barley.

Spring barley was grown in a field experiment under moderately elevated soil temperature and changed summer precipitation (amount and frequency). Elevated temperature affected the performance and grain quality characteristics more significant than changes in rainfall. Except for the decrease in thousand grain weight, warming had no impacts on aboveground biomass and grain yield traits. In grains, several proteinogenic amino acids concentrations were increased, whereas their composition was only slightly altered. Concentration and yield of total protein remained unaffected under warming. The concentrations of total non-structural carbohydrates, starch, fructose and raffinose were lower in plants grown at high temperatures, whereas maltose was higher. Crude fibre remained unaffected by warming, whereas concentrations of lipids and aluminium were reduced. Manipulation of precipitation only marginally affected barley grains: amount reduction increased the concentrations of several minerals (sodium, copper) and amino acids (leucine). The projected climate changes may most likely affect grain quality traits of interest for different markets and utilisation requirements.

Rhizosphere bacteria affected by transgenic potatoes with antibacterial activities compared with the effects of soil, wild-type potatoes, vegetation stage and pathogen exposure.

A greenhouse experiment was performed to analyze a potential effect of genetically modified potatoes expressing antibacterial compounds (attacin/cecropin, T4 lysozyme) and their nearly isogenic, nontransformed parental wild types on rhizosphere bacterial communities. To compare plant transformation-related variations with commonly accepted impacts caused by altered environmental conditions, potatoes were cultivated under different environmental conditions, for example using contrasting soil types. Further, plants were challenged with the blackleg pathogen Erwinia carotovora ssp. atroseptica. Rhizosphere soil samples were obtained at the stem elongation and early flowering stages. The activities of various extracellular rhizosphere enzymes involved in the C-, P- and N-nutrient cycles were determined as the rates of fluorescence of enzymatically hydrolyzed substrates containing the highly fluorescent compounds 4-methylumbelliferone or 7-amino-4-methyl coumarin. The structural diversity of the bacterial communities was assessed by 16S rRNA-based terminal restriction fragment length polymorphism analysis, and 16S rRNA gene clone libraries were established for the flowering conventional and T4 lysozyme-expressing Desirée lines grown on the chernozem soil, each line treated with and without E. carotovora ssp. atroseptica. Both genetic transformation events induced a differentiation in the activity rates and structures of associated bacterial communities. In general, T4 lysozyme had a stronger effect than attacin/cecropin. In comparison with the other factors, the impact of the genetic modification was only transient and minor, or comparable to the dominant variations caused by soil type, plant genotype, vegetation stage and pathogen exposure.

Root exudation, phosphorus acquisition, and microbial diversity in the rhizosphere of white lupine as affected by phosphorus supply and atmospheric carbon dioxide concentration.

White lupine (Lupinus albus L.) was used as a phosphorus (P)-efficient model plant to study the effects of elevated atmospheric CO(2) concentrations on (i) P acquisition, (ii) the related alterations in root development and rhizosphere chemistry, and (iii) the functional and structural diversity of rhizosphere microbial communities, on a P-deficient calcareous subsoil with and without soluble P fertilization. In both +P (80 mg P kg(-1)) and -P treatments (no added P), elevated CO(2) (800 micromol mol(-1)) increased shoot biomass production by 20 to 35% and accelerated the development of cluster roots, which exhibit important functions in chemical mobilization of sparingly soluble soil P sources. Accordingly, cluster root formation was stimulated in plants without P application by 140 and 60% for ambient and elevated CO(2) treatments, respectively. Intense accumulation of citrate and increased activities of acid and alkaline phosphatases, but also of chitinase, in the rhizosphere were mainly confined to later stages of cluster root development in -P treatments. Regardless of atmospheric CO(2) concentrations, there was no significant effect on accumulation of citrate or on selected enzyme activities of C, N, and P cycles in the rhizosphere of individual root clusters. Discriminant analysis of selected enzyme activities revealed that mainly phosphatase and chitinase contributed to the experimental variance (81.3%) of the data. Phosphatase and chitinase activities in the rhizosphere might be dominated by the secretion from cluster roots rather than by microbial activity. Alterations in rhizosphere bacterial communities analyzed by denaturing gradient gel electrophoresis (DGGE) were related with the intense changes in root secretory activity observed during cluster root development but not with elevated CO(2) concentrations.

Effects of heavy metal contamination of soils on micronucleus induction in Tradescantia and on microbial enzyme activities: a comparative investigation.

The aim of this study was to investigate correlation between genotoxic effects and changes of microbial parameters caused by metal contamination in soils. In total, 20 soils from nine locations were examined; metal contents and physicochemical soil parameters were measured with standard methods. In general, a pronounced induction of the frequency of micronuclei (MN) in the Tradescantia micronucleus (Trad-MN) assay was seen with increasing metal concentration in soils from identical locations. However, no correlations were found between metal contents and genotoxicity of soils from different locations. These discrepancies are probably due to differences of the physicochemical characteristics of the samples. Also, the microbial parameters depended on the metal content in soils from identical sampling locations. Inconsistent responses of the individual enzymes were seen in soils from different locations, indicating that it is not possible to define a specific marker enzyme for metal contamination. The most sensitive microbial parameters were dehydrogenase and arylsulfatase activity, biomass C, and biomass N. Statistical analyses showed an overall correlation between genotoxicity in Tradescantia on the one hand and dehydrogenase activity, biomass C, and the metabolic quotient on the other hand. In conclusion, the results of the present study show that the Trad-MN assay is suitable for the detection of genotoxic effects of metal contamination in soils and furthermore, that the DNA-damaging potential of soils from different origin cannot be predicted on the basis of chemical analyses of their metal concentrations.