SSS5.5

Fertilization experiments provide insights into elemental imbalances in soil microbial communities and their consequences for soil nutrient cycling. By addition of selected nutrients, other nutrients become deficient and limiting for soil microorganisms as well as for plants. In this study we focused on microbial nitrogen (N) cycling in a long-term nutrient manipulation experiment. In many soils, the rate-limiting step in N cycling is depolymerization of high-molecular-weight nitrogen compounds (e.g., proteins) to oligomers (e.g., peptides) and monomers (e.g., amino acids) rather than the subsequent steps of mineralization (ammonification) and nitrification. The aim of our study was to determine whether nutrient deficiency directly or indirectly – via changes in plant carbon (C) inputs - affects soil microbial N processing.

We collected soil samples from a fertilization experiment, established in 1946 on a hay meadow close to Admont (Styria, Austria). The field experiment consisted of a full factorial combination of inorganic N, P, and K fertilization and a control with no fertilizers. Furthermore, liming (Ca-addition) and organic fertilizer application treatments (solid manure and liquid slurry) were established. In the experiment, plant biomass is harvested three times per year, inducing strong nutrient limitation in plots that have not received nutrient additions (fully deficient or deficient in a single element). We determined gross rates of microbial protein depolymerization, N-mineralization and nitrification via isotope pool dilution assays with ¹⁵N-labeled amino acids, NH₄⁺, and NO₃^-. We hypothesized that N deficiency (lack of N fertilization) would stimulate microbial N mining (depolymerization), and reduce subsequent N mineralization and nitrification. In contrast, we expected that organic fertilization would alleviate microbial C and N limitations, reducing N depolymerization rates and increasing mineralization and nitrification.

Our results show that organically fertilized and limed soils have significantly lower gross protein depolymerization rates than plots receiving inorganic N. No significant differences were found comparing gross N-mineralization and gross nitrification rates across the different treatments. Given the higher rates of protein depolymerization in inorganically fertilized soils as compared to organically fertilized and limed soils, microbial N processes seem to be controlled by plant C input and/or soil pH rather than by direct soil nutrient availability. However, depolymerization of macromolecular N does not only supply N to the soil microbial community but also organic C. Thus, the reduced plant C input compared to fully fertilized soils may have caused microorganisms to increase their mining for a C-containing energy source, thereby increasing protein depolymerization rates. In summary, this study suggests that long term nutrient deficiency or nutrient imbalances may affect soil nutrient cycling indirectly by changing plant C inputs (via reduced primary production) and/or changing soil pH, rather than directly, by nutrient availability. This further indicates that soil microbial communities are rather C than nutrient limited.

How to cite: Spiegel, F., Fuchslueger, L., Canarini, A., Schnecker, J., Schmidt, H., Martin, V., Wiesenbauer, J., Jenab, K., Watzka, M., Pötsch, E. M., Wanek, W., Kaiser, C., and Richter, A.: Controls of microbial N cycling in agricultural grassland soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12730, https://doi.org/10.5194/egusphere-egu21-12730, 2021.

The coupled cycles and interactions of soil carbon (C), nitrogen (N), and phosphorus (P) are fundamental for soil quality and soil organic matter (SOM) formation. Low C:N ratios through nitrogenous fertilizer addition may accelerate SOM cycling and promote C mineralization in soil, whereas P limitations may decline C storage by reducing plant and microbial biomass production. Deeper soil layers’ C-N-P stoichiometry has an important role in regulating SOM formation in subsoils. However, there is little information on soil C:N:P stoichiometry in deep soil layers of farmland. In this study, soil columns up to one meter were collected from 32 farms distributing across Finland with different soil texture and agricultural management history. The one-meter soil columns were cut into 10 cm deep slices and analyzed for the total organic carbon (TOC), total nitrogen (TN) by dry combustion method and total phosphorus (TP) contents by aqua regia digestion and ICP-OES method. Overall, the TOC, TN and TP contents all dropped sharply in 30-40 cm soil layers, but TP contents rose again in deep soil. The role of agricultural management practice (including crop rotation, crop cover, crop diversity and fertilization) on soil C:N:P stoichiometry as well as organic matter accumulation in the deep soil layers were explored. The preliminary results will be presented in the poster. The data deepens our understanding of soil C, N and P coupling and interaction related to soil C sequestration.

How to cite: Wang, S., Uhlgren, O., Salonen, A.-R., and Heinonsalo, J.: Soil contents and stoichiometry of carbon, nitrogen, and phosphorus in Finnish farmland and feedbacks on management patterns, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12350, https://doi.org/10.5194/egusphere-egu21-12350, 2021.

Elevated carbon dioxide in the atmosphere (eCO₂) has been found to influence soil C by altering the belowground balance between the decomposition of existing soil organic matter (SOM) and the accumulation of plant-derived C inputs. Even small changes in this balance can have a potentially large effect on future climate. The relative availability of soil nutrients, particularly N and P, are crucial mediators of both decomposition and new C accumulation, but both these two processes are rarely assessed simultaneously. We asked if the effect of eCO₂ on soil C decomposition was mediated by soil N and P availability, and if the effect of CO₂ and soil N and P availability on soil C decomposition was dependent on C pools (existing SOM C, newly added C). We grew Eucalyptus grandis and a C3 grass (Microlaena stipoides) from seed in an experimentally manipulated atmosphere with altered δ¹³C signature of CO₂, which allowed the separation of plant derived C, from the existing SOM C. Then we manipulated N and P relative abundance via nutrient additions. We evaluated how the existing SOM and the new plant-derived C pool, and their respiration responded to eCO₂ conditions and nutrient treatments. SOM respiration significantly increased in the eucalypts when N was added but was not affected by CO₂. In the grass the SOM respiration increased with eCO₂ and added N and SOM respiration per unit of SOM-derived microbial was significantly higher in both the added P and added N+P nutrient treatments. The rhizosphere priming of SOM was suppressed in both the added P and added N+P nutrient treatments. The heterotrophic respiration of plant-derived C was contingent on nutrient availability rather than eCO₂ and differed by species. The grass-derived respiration was significantly higher than the eucalypt and was higher in both added P and added N+P nutrient treatments. Thus, nutrient stoichiometry had similar effects on SOM and plant derived C, but e CO₂ only affected SOM and only for the Eucalyptus.  This study shows how species differences have large effects on rhizosphere C cycling responses to eCO2 and stoichiometric conditions.      

How to cite: Pihlblad, J., Andresen, L. C., Macdonald, C., Ellsworth, D., and Carrillo, Y.: Stochiometric control of SOM and plant derived soil C pools dynamics under elevated CO2 , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14375, https://doi.org/10.5194/egusphere-egu21-14375, 2021.

Human activity has caused imbalances in nitrogen (+N) and phosphorus (+P) loadings of ecosystems around the world, causing widespread P limitation of many biological processes. Soil phosphatases catalyze the hydrolysis of P from a range of organic compounds, representing an important P acquisition pathway. Therefore, a better understanding of soil phosphatase activity as well as the underlying mechanisms to individual and combined N and P loadings could provide fresh insights for wise P management. Here we show, using a meta-analysis of 188 published studies and 1277 observations that +N significantly increased soil phosphatase activity by 14%, +P significantly repressed it by 30%, and +N+P led to non-significant responses of soil phosphatase activity. Responses of soil phosphatase activity to +N were positively correlated with soil C and N content, whereas the reverse relationships were observed for +P and +N+P. Similarly, effects of +N on soil phosphatase activity were positively related to microbial biomass C, microbial biomass C:P, and microbial biomass N:P, whereas reverse relationships were observed for +P. Although we found no clear relationship between soil pH and soil phosphatase activity, +N-induced reductions in soil pH were positively correlated with soil phosphatase activity. Our results underscore the integrated control of soil and microbial C, N and P stoichiometry on the responses of soil phosphatase activity to +N, +P, and +N+P, which can be used to optimize future P management.

How to cite: Chen, J., Luo, Y., Cao, J., Jørgensen, U., Moorhead, D., and Sinsabaugh, R. L.: Contrasting responses of soil phosphatase activity to nitrogen and phosphorus loadings: Implications for phosphorus management, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1655, https://doi.org/10.5194/egusphere-egu21-1655, 2021.

With climate change, much of the world will experience devastating shifts in weather patterns like increased flooding, intensifying periods of soil saturation. Soil carbon (C), nitrogen (N) and phosphorus (P) cycles are sensitive to changes in soil saturation, where exchange between the mineral-bound and the soluble bioavailable pools can occur with increases in moisture content. With soil saturation, C, N, and P may be mobilized either through greater diffusion or reduced conditions that cause desorption of mineral-bound C, N and P into their respective soluble pools. De-sorption, resorption and diffusion dynamics of C, N, and P may or may not reflect the stoichiometry of the mineral bound pool. Changes in bioavailable soluble C, N and P that could occur with soil saturation and drying may cause unknown consequences for microbial biomass C:N:P. With increases in soil moisture, simultaneous changes in both substrate stoichiometry and microbial growth may occur that impact microbial biomass stoichiometry.  Such changes in microbial stoichiometry and microbial retention of C, N, and P may affect the post-flood fate of soluble C, N, and P. Understanding how releases in mineral bound C, N and P alter the bioavailable C:N:P and how this in turn impacts microbial activity and accumulation of these substrates can inform predictions of retention or losses of C, N and P following soil saturation events.

To determine if mineral-bound, soluble and microbial biomass stoichiometry is maintained or altered during and after soil saturation events, we used a laboratory incubation approach with manipulated soil saturation and duration. Soil incubations were maintained at three water-holding capacity (WHC) levels: 20% (control), 50%, (moderate) and 100% (severe). We maintained the moderate and severe water-logging treatments for  0.5 h, 24 h, 1 week, followed by air-drying to 20% WHC to examine the influence of flood duration. To understand the exchanges of C, N and P between different pools during flooding, we compared changes in soluble and mineral bound soil C, N and P and impacts on microbial C, N, and P exo-cellular enzymes, and microbial biomass C:N:P. Preliminary results indicate that greater soil moisture content increases soluble P and that the 24 hour flood period captures shifts in the mineral bound P pool that do not remain for the longer flood period (1 week). Enzyme activity similarly reflects an increase in microbial activity in the soil held at 50% and 100% moisture content for 24 hours. We also discuss how soil moisture levels and flood duration impact soluble and mineral bound C relative to P, and how microbial biomass C:N:P tracks these fractions. By exploring the combined response of mineral-bound and soluble C, N, and P to variation in soil saturation, we can better understand how different flood scenarios will impact soil C, N and P retention.

How to cite: Lieberman, H., von Sperber, C., Rothman, M., and Kallenbach, C.: Understanding the interdependent cycles of soil carbon, nitrogen and phosphorus during soil saturation events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6703, https://doi.org/10.5194/egusphere-egu21-6703, 2021.

Tree species capable of forming a symbiosis with N-fixing bacteria may affect P availability in reclaimed technosols. The objective of this study was to compare the effect of N-fixing tree species and non-N-fixing species on phosphorus forms in technosols developing from various materials. Soil samples were taken under black locust (Robinia pseudoaccaccia), black alder (Alnus glutinosa), silver birch (Betula pendula) and Scots pine (Pinus sylvestris) from two depths (0-5 cm and 5 – 20 cm). The soil substrates were fly ashes, sands and clays. In the soil samples measured were concentrations of total P (P_t),  water soluble P (P_H2O),  dilute salt-extractable P (P_ex), microbial biomass P (P_mic) and total labile P (P_labil). Multifactor ANOVA revealed that tree species did not influence contents of P_t, P_ex and P_H20. However, there was a statistically significant effect of soil substrate and soil horizon on these forms of P. The factors tree species, soil substrate and soil horizon had statistically significant effect on P_mic content whereas content of P_labil was affected by tree species and soil horizon. Multiple Range Tests by tree species showed that soils under Scots pine contained significantly less P_mic than soils under other tree species studied. There were no significant differences in P_mic between the soils under silver birch, black alder and black locust. The soils under Scots pine contained also significantly less P_labil than the soils under black locust and silver birch. Our study included P forms that are considered labile (except P_t). The obtained results indicated that the effect of N-fixing trees on these forms of P was weak. Instead we noticed that Scots pine had negative effect on some forms of labile P. 

The study was financed by The National Science Centre, Poland, grant No. 2018/31/B/ST10/01626.

How to cite: Sroka, K., Chodak, M., and Pietrzykowski, M.: Phosphorus forms in technosols afforested with N-fixing and non-N-fixing tree species, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7375, https://doi.org/10.5194/egusphere-egu21-7375, 2021.

It is estimated that over 37 % of degraded soils in the European Union are polluted by heavy metals [1], which are non-biodegradable and persistent pollutants in soils. The application of organic amendments to soils for their remediation has been worldwide used [2]. Several studies have shown that biochar, the carbonaceous material produced by pyrolysis of organic residues, has a high potential to stabilize trace elements in soils [3]. Biochars usually have an alkaline pH and high water holding capacity (WHC), large specific surface area and cation exchange capacity, which are appropriate characteristics to reduce the availability of heavy metals in the environment [4]. Nevertheless, recent studies exhibited that biochar recalcitrance could be much lower than assumed [5].  Beside this, the effects of the addition of biochar as a soil amendment on the composition of soil organic matter (SOM) are largely unknown. Thus, the aim of this study is to investigate the effects of the application of biochars from rice husk (RHB) and olive pit (OPB) in a Typic Xerofluvent polluted with trace-elements after 24 months at field in 12 plots installed at the surroundings of the Guadiamar Green Corridor (37° 23' 7.152"N, 6° 13' 43.175"; Southwest Spain). Specifically, for this study the effects of biochar amendment on soil physical properties (pH, water holding capacity-WHC, moisture, etc), elemental composition, total SOM, the content of oxidizable SOM as well as the content and composition of humic acids (HAs) have been assessed.

Biochar application caused an increase in soil pH (around 0.4 units), soil moisture (from 6-7% to 10-18 %) and WHC. In addition, the total organic carbon and HAs content increased slightly. Preliminary results show that biochar could become part of the humified SOM in a shorter time than initially expected. Nevertheless, the spectroscopic analyses (FT-IR and ¹³C NMR spectroscopy) documented that the qualitative composition of soil HAs was not altered due to the biochar amendment.

References:

[1] EEA; 2007. CSI 015. Copenhagen, Denmark: European Environmental Agency.

[2] Madejón, E.; Pérez de Mora, A.; Burgos, P.; Cabrera, F.; 2006. Environ. Pollut. 139, 40-52.

[3] Campos, P., De la Rosa, J.M., 2020. Sustainability 12, 6025.Uchimiya, M.; Klasson, K.T.; Wartelle, L.H.; Lima, I.M.; 2011. Chemosphere 82, 1438-1447.

[4] Campos, P., Miller, A.Z., Knicker, H., Costa-Pereira, M.F., Merino, A., De la Rosa, J.M., 2020. Waste Manag. 105, 256-267.

[5] De la Rosa, J.M.; Rosado, M.; Paneque, M.; Miller, A.Z.; Knicker, H.; 2018. Sci. Tot Environ. 613-614, 969-976.

Acknowledgements: The Spanish Ministry of Economy, Industry and Competitiveness (MINEICO), CSIC and AEI/FEDER are thanked for funding the project CGL2016-76498-R. P. Campos thanks the “Fundación Tatiana Pérez de Guzmán el Bueno” for funding her PhD.

How to cite: Santa-Olalla, A., Fernandez-Boy, E., Campos, P., Knicker, H., Lopez, R., Gonzalez-Pérez, J. A., and De la Rosa, J. M.: Impact of biochar amendment on soil organic matter composition in a heavy-metals polluted soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-865, https://doi.org/10.5194/egusphere-egu21-865, 2021.

Tree species capable of forming symbiosis with N-fixing bacteria planted on reclaimed wastelands may increase not only their N content but also increase availability of P. The aim of our study was to compare the effect of rhizobial and actinorhizal N-fixing tree species and non-N-fixing species on the activity of phosphatases in various technosols. Soil samples were taken under black locust (Robinia pseudoaccaccia), black alder (Alnus glutinosa), silver birch (Betula pendula) and Scots pine (Pinus sylvestris) from two depths (0-5 cm and 5 – 20 cm) of technosols developing from different parent materials (Quaternary sands, fly ashes after lignite combustion,  acid and alkaline Tertiary clays). The samples were measured for the activities of acid and alkaline phosphatase, inorganic pyrophosphatase, microbial biomass (C_mic), texture, as well as contents of organic C (C_org), total N (N_t) and total P (P_t). Activities of acid (Pho_Aci), alkaline (Pho_Alk), total phosphatase (Pho_Sum) and inorganic pyrophosphatase (Pyro_Pho) were expressed per soil dry mass and per unit of C_mic (specific enzyme activities - Pho_Aci_SP, Pho_Alk_SP and Pho_Sum_SP for acid, alkaline and total phosphatase, respectively, Pyro_Pho_SP for pyrophosphatase). The soils under black locust exhibited higher Pho_Aci activity and higher specific activities of all enzymes (Pho_Aci_SP, Pho_Alk_SP,, Pho_Sum_SP and Pyro_Pho_SP) than the soils under both non-N-fixing trees. For alder  Pho_Aci activity was significantly higher only when compared to pine. However, the values of Pho_Aci_SP and Pho_Sum_SP were higher under alder than under both non-N-fixing trees. There were no differences in the activities or specific activities of measured enzymes between the soils under pine and birch. Our results indicated that rhizobial black locust stimulated activity of soil enzymes involved in P cycling much stronger than non-N-fixing tree species. This effect of black locust was consistent in technosols developing from various parent materials. The effect of actinorhizal black alder was less pronounced, but also evident.  The results of our study indicated that both N-fixing trees stimulated activity of enzymes involved in P cycling stronger than the non-fixing trees. Thus, the N-fixing trees may alleviate P deficiency in technosols as they stimulate development of phosphatase releasing microorganisms and increase P availability.

The study was financed by The National Science Centre, Poland, grant No. 2018/31/B/ST10/01626.

How to cite: Chodak, M., Sroka, K., and Pietrzykowski, M.: Activity of phosphatases in technosols afforested with N-fixing and non-N-fixing tree species, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4005, https://doi.org/10.5194/egusphere-egu21-4005, 2021.

Land cover, productivity and carbon stocks are among the widely acknowledged indicators of the land’s degradation and development status. The indicators’ assess-ability, however, differs across global ecosystems and location. Despite the complexity of carbon stocks, soil carbon in particular is receiving increasing attention for its potential in both climate change mitigation and economic growth in developing carbon markets. 
The degraded drylands of Jordan have been targeted by multiple investment programs to rehabilitate their arid agro-pastures, including through the application of mechanized micro-water harvesting structures combined with the plantation of native shrub seedlings. Whilst both local and remote land cover and biomass change monitoring indicate variable rehabilitation success, the related carbon stock changes remain largely under-investigated and unclear.
An international research consortium designed and implemented a study to investigate the actual and potential future carbon stocks per ecosystem status at an agro-pastoral research site located in central Jordan’s ‘Badia’, considering both conventionally managed (degraded) and rehabilitated lands. Field experiments conducted by scientists and  local and former tribal community collaborators were combined with carbon modeling using RothC. This enabled the development of multiple scenarios considering both natural and enhanced, or human induced, processes; for example, through landscape modification (mechanized micro-water harvesting), vegetation plantation as well as optional soil amendment through biosolids. Preliminary results suggest that the implementation of water harvesting structures leads to a pronounced increase in soil carbon sequestration when compared to baseline conditions of between 15% and 45% over a 5 year period , with work ongoing to quantify the uncertainties around these results. The selected rehabilitation scenarios match the criteria for vast potential upscaling across global drylands. The study outcomes will eventually support a comprehensive ecosystem services valuation approach with (soil) carbon as an integral factor and moving towards reversing degradation and crediting the dry ecosystem’s values beyond their marginal agricultural services.

How to cite: Hall, L., Haddad, M., Strohmeier, S., Rawashdeh, H., Bani-Hani, N., Al-Widyan, J., Hasan, H., and Sterk, G.: Carbon stock changes through dryland rehabilitation: a case study from central Jordan’s agro-pastures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15998, https://doi.org/10.5194/egusphere-egu21-15998, 2021.

Anaerobic digestion (AD) of organic wastes is a promising alternative to landfilling for reducing greenhouse gas emission and it is encouraged by current regulation in Europe. AD represents a source of green energy, while the by-product digestate still generates concerns for a safely disposal. The sustainability of AD plants partly depends on the management of digestion residues. Digestate could be used in organic amendment straightaway or after composting to limit possible phytotoxicity effects on crops. This study has been focused on the environmental benefits of digested olive mill wastewater (OMW), recalcitrant agricultural waste. OMW require a complex management due to high production volume in a limited time, fermentative processes occurring during the storage, and toxicity due to phenol compounds. These latter might compromise the AD process affecting microbial metabolism. As biochar is able to adsorb and retain organic and inorganic pollutants, we used biochar as additives during AD to remove phenols, stimulate microbial activity and therefore hydrogen and methane production. The resulted digestates including biochar could be used in order to increase the carbon stock in soil as a valid alternative to other organic amendments.

The aim of this work was to evaluate the effect of solid and liquid digestates, obtained from the AD process of OMW with biochar (30 and 45%), as additive, on soil chemical and biochemical properties in order to validate its use in organic amendment in lab-scale experiment. The liquid and solid digestates were added to soil according to the maximum dose allowed by the Italian nitrates directive concerning non-vulnerable areas (91/676/EEC, DGR 209/2007). Pots containing soil differently amended with liquid and solid digestates were prepared also for the growth of Lactuca sativa L. seedlings.

Thirty days after treatments, positive changes in chemical and biochemical properties in soil pots with biochar-treated digestates, in particular with liquid ones, occurred. Soil organic carbon, microbial biomass carbon and some soil enzymatic activities such as dehydrogenase, phosphomonoesterase, β-glucosidase and fluorescein diacetate hydrolysis significantly improved. Besides an enhancement of lettuce biomass, a significant decrease of nitrate content in plant tissue was registered when pots were amended with biochar-treated digestates.

The assessment of the agronomic quality of liquid and solid digestates, obtained by biochar assisted AD of OMW, as organic soil amendment, demonstrated that also critical biomass such as OMW, if opportunely treated, can entry in a re-use process where biogas and by-products can be part of virtuous circular economy.

This work was part of the project “Mitigation of the environmental impact of olive mill waste water through sustainable bioprocess with energy recovery” funded by the Università degli Studi di Napoli Federico II.

How to cite: Di Rauso Simeone, G., Cesarano, G., Micoli, L., Toscano, G., Turco, M., and Rao, M. A.: Chemical and biochemical changes in soil after biochar-treated OMW digestate amendment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10407, https://doi.org/10.5194/egusphere-egu21-10407, 2021.

In the context of global soil degradation, biochar is being promoted as a potential solution to improve soil quality, besides its carbon sequestration potential. Burying biochar in soils is known to effect soil physical quality in the short-term (<5 years), and the intensity of these effects depends on soil texture. However, the long-term effects of biochar remain largely unknown yet and are important to quantify given biochar’s persistency in soils. The objective of this study was therefore to assess the long-term effect of biochar on soil physical properties as a function of soil texture and biochar concentration.  For this purpose, soil physical properties (particle density, bulk density, porosity, water retention and hydraulic conductivity curves) were measured in the topsoil of three fields with former kiln sites containing charcoal more than 150 years old in Wallonia (southern Belgium).  The fields had a silt loam, loam and sandy loam texture.  Samples were collected along 3 transects in each field, from the center of the kiln sites outwards. 

Particle density and bulk density slightly decreased as a function of charcoal content. Because particle density and bulk density were affected to a similar extent by charcoal content, total porosity was not affected by the presence of century-old charcoal. Regarding the soil water retention curve, charcoal affected mostly water content in the mesopore range. This effect was strongest for the sandy loam. On the other hand, the presence of century-old charcoal increased significantly the hydraulic conductivity at pF between 1.5 and 2 for the silt loam, while no effect of charcoal was observed for the loamy soil.  The study highlights a limited effect of century-old charcoal on the pore size distribution (at constant porosity) and on the resulting soil physical properties for the range of soils and charcoal concentrations investigated here.  Further research may be needed to confirm the observed trends over a wider range of soil types. 

How to cite: Zanutel, M., Garré, S., and Bielders, C.: Long-term effect of biochar on soil physical properties of agricultural soils with different textures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8588, https://doi.org/10.5194/egusphere-egu21-8588, 2021.

Soil surveys are critical for maintaining sustainable use of natural resources while minimizing harmful impacts to the ecosystem. A key soil attribute for many environmental parameters, such as CO2 budget, soil fertility and sustainability, is soil organic matter (SOM), and its sequestration. Soil spectroscopy is a popular method to assess SOM content rapidly in both field and laboratory domains. However, the SOM source composition differs from soil to soil and the use of spectral-based models for quantifying SOM may present limited accuracy when applying a generic approach for SOM assessment. We therefore examined the extent to which the generic approach can assess SOM contents of different origin using spectral-based models. We created an artificial big dataset composed of pure dune sand as a SOM-free background which was artificially mixed with increasing amounts of different organic matter (OM) sources obtained from commercial compost of different origins. Dune sand has high albedo and yields optimal conditions for SOM detection. This study combined two methods: partial least squares regression for the prediction of SOM content from reflectance values across the 400–2500 nm region, and soil spectral detection limit (SSDL) to judge the prediction accuracy. Spectral-based models to assess SOM content were evaluated with each OM source as well as with a merged dataset that contained all of the generated samples (generic approach). The latter was concluded to have limitations for assessing low amounts of SOM (<0.6%), even under controlled conditions. Moreover, some of the OM sources were more difficult to monitor than others; accordingly, caution is advised when different SOM sources are present in the examined population.

How to cite: Francos, N., Ogen, Y., and Ben-Dor, E.: Spectral assessment of organic matter with different composition using reflectance spectroscopy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-190, https://doi.org/10.5194/egusphere-egu21-190, 2021.

The conversion of tropical forest for cassava cultivation is widely known to decrease the soil organic matter (OM) and nutrient contents of highly weathered soils in the tropics. Amazonian Dark Earth (ADE) might be more resistant to this process due to their historical anthropogenic amelioration with e.g. charcoal, ceramics and bones, leading to higher soil OM and nutrient concentrations. In this study, we analyzed the effect of land use change on the OM dynamics under tropical conditions and how this is related with P distribution at the microscale, using ADE and an adjacent Acrisol (ACR) as model systems. Soil samples were obtained south of Manaus (Brazil), from a secondary forest and an adjacently located 40-year-old cassava plantation. The land use change induced a severe decrease of organic carbon (OC) concentrations in ADE (from 35 to 15 g OC kg^‑1) while OC in the adjacent ACR was less affected (18 to 16 g OC kg^‑1). The analysis by ¹³C NMR spectroscopy showed that the conversion of secondary forest to cassava changed the chemical composition of OM to a more decomposed state (increase of alkyl:O/N-alkyl ratio) in the ADE whereas the OM in ACR changed to a less decomposed state (decrease of alkyl:O/N-alkyl ratio). According to neutral sugar and lipid extraction analyses, land use change led to a larger impact on the microbial-derived and plant-derived compounds in the ADE compared to the ACR. In order to analyze the interactions of OC and P at the microscale, we conducted an incubation experiment with ¹³C glucose for the analysis with Scanning X-ray Microscopy (SXM) and Nano scale Secondary Ion Mass Spectrometry (NanoSIMS). In both soil types ADE and ACR, land use change caused a reduction of the total ¹³C glucose respiration by approximately one third in a 7-days incubation, implying lower microbial activity. Microorganisms in both soil types appear to be more readily active in soils under forest, since we observed a distinct lag time between ¹³C glucose addition and respiration under cassava planation. This indicated differences in microbial community structure, which we will be assessed further by determining the ¹³C label uptake by the microbial biomass and the microbial community structure using ¹³C PLFA analysis. Preliminary results from synchrotron-based STXM demonstrate a distinct arrangement of OM at fine-sized charcoal-particle interfaces. From ongoing NanoSIMS analyses, we expect further insights on the co-localization of P and ¹³C-labelled spots at the microscale. Despite the high loss of OC in the ameliorated ADE through land use change, the remaining OM might foster nutrient dynamics at the microscale thanks to charcoal interactions compared to the ACR. Our results contribute to a better understanding of the C and P interactions and how these respond to land use change in highly weathered tropical soils.

How to cite: Jarosch, K. A., Colocho Hurtarte, L. C., Gavazov, K., Westphal Muniz, A., Müller, C., Angst, G., and Schweizer, S.: Consequences of land use change on soil organic matter composition and C-P relationships in Amazonian Dark Earth and Acrisol, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15580, https://doi.org/10.5194/egusphere-egu21-15580, 2021.