Relative degradability of exogenous organic carbon in function of its concentration and the interaction with native soil organic matter content

The soil environment impacts carbon turnover in a multitude of ways. It has been shown that C emissions are higher when exogenous carbon is locally concentrated than when it is spread in the soil matrix. Hence, the spatial separation between decomposers and substrate may cause ineffective dispersion of the produced enzymes within the soil matrix. It is not clear, however, if and how lower doses of exogenous C impact its degradability and if an interaction exists with native soil organic matter (SOM) level. Therefore, a seventy-day incubation experiment with six varying exogenous OM doses (0-6 g C kg-1) in soil mesocosms of loamy sand subsoil (WFPS 50%) with three different native SOM contents (i.e. 0.1, 0.5, and 1.0 %) was carried out. C emission and isotopic signature measurements of 13CO2 by means of a CRDS-analyser were used to track the degradation of added maize shoots with a δ13C=-12.7‰ that contrasts to native SOM (δ13C=-25‰). Mineral N was added, and the redox potential (Eh) monitored. Eh suggested that O2 may have not become limiting for C mineralisation, even at higher doses, but it did not clearly relate to the added C dose. Cumulative CO2 emissions increased proportionally with C dose and so our hypothesis that a lower dose dispersion limits microbial activity was disproved. However, a higher native SOM level led to higher relative degradability of added C for the highest doses (i.e. 3 and 6 g kg-1 soil). Furthermore, a negative impact of OM doses above 1 g kg-1 soil on native SOM degradation was found. We conclude that positive priming of native SOM mineralisation did not occur. A disproportional response of microbial biomass carbon and soil C mineralisation at lower doses suggests that dose does impact the microbial metabolic quotient. Ongoing PLFA analyses will help us interpret these observations

Soil texture can predominantly control organic matter mineralization in temperate climates by regulating soil moisture rather than through direct stabilization

Soil organic carbon (OC) levels generally increase with increasing clay and silt content under a similar climatic zone because of increased association of OC to clay minerals and stronger occlusion inside aggregates. Surprisingly though, in Western Europe many silt loam soils actually bear low topsoil OC levels compared to lighter textured soils. Soil texture obviously also strongly controls moisture availability with consequent indirect impact on heterotrophic activity. We hypothesized that with increasingly frequent summer drought: 1) soil microbial activity in sandy soils is more likely impeded due to their limited water holding capacity retention during droughts, while soil OC mineralization in silty soils remain be less drought-limited; 2) capillary rise from sufficiently shallow groundwater would, on the other hand, alleviate the water stress in lighter textures. To test these hypotheses, we established a one-year field trial with manipulation of soil texture, monitoring of soil moisture and maize-C decomposition via 13/12C-CO2 emissions. The upper 0.5 m soil layer was replaced by sand, sandy loam and silt loam soil with low soil OC. Another sandy soil treatment with a gravel layer was also included beneath the sand layer to exclude capillary rise. Soil texture did not affect maize-C mineralization (Cmaize-min) until April 2019 and thereafter Cmaize-min rates were higher in the silt loam than in the sandy soils (P=0.01). θv correlated positively with the Cmaize-min rate for the sand-textured soils only but not for the finer textures. These results clearly highlight that soil texture controlled Cmaize-min indirectly through regulating moisture under the field conditions starting from about May, when soils faced a period of drought. By the end of the experiment, more added Cmaize was mineralized in the silt loam soil (81%) (P<0.05) than in the sandy soil (56%). Capillary rise did not result in a significant increase in cumulative Cmaize-min in the sandy soil, seemingly because the capillary fringe did not reach the sandy topsoil layer. These results imply that, under future climate scenarios the frequency of drought is expected to increase, the largely unimpeded microbial activity in silty soils might lead to a further stronger difference in soil OC with coarser textured soils under similar management

Soil texture controls activity of heterotrophs in soil indirectly via soil moisture

Soil organic carbon (SOC) plays a vital role in soil fertility and quality, resilience against degradation, and consequently food security. Therefore, it is crucially important to understand how current SOC stocks will be affected under future climate change scenarios. Silt soils in Western-Europe have superior soil physical quality but have lower SOC contents than sandy soils. We hypothesize that both moisture availability and aeration in silt soils strongly favour soil microbial activity, especially in warmer periods. To test our hypothesis, we have currently started a field trail of 1 year-round monitoring of CO2 emissions in Bottelare, Belgium. We replaced the upper 50 cm from the soil profile (sandy loam texture) by subsoil with sandy, sandy loam and silty textures, obtained from 50 to 100 cm depth to eliminate SOC content difference. Ground maize stalk residues were added to the plots in order to partition the total soil CO2 efflux by isotopic mass balance in situ via high-precision isotopic CO2 cavity ring-down spectroscopy (CRDS) measurements. Nitrogen was also added to avoid differences in nitrogen availability from different textures. Soil moisture at 10 and 25 cm depth is measured by TDR200 hourly. O2 and soil redox potential (Eh) is also measured to study O2 diffusions in different textures. The microbial community composition will be estimated via PLFA to evaluate the changes of microorganisms during degradation of organic carbon. The detailed and unique measurements of soil moisture levels and CO2 emission, complemented with O2, and Eh at small depth increments will allow better understanding of effects of soil texture on microbial activity in field conditions and provide valuable information for evaluating global SOC stocks

Mineralisation of ryegrass and soil organic matter as affected by ryegrass application doses and changes in soil structure

The addition of exogenous organic matter (EOM) generally stimulates microbial activity and shapes soil structure, likely with stronger effects at a higher application dose. It is not known, however, if and how the application dose of EOM would impact its own mineralisation and that of native soil organic carbon (SOC) by such mediation of soil pore network structure. In 90-day soil incubation experiments, we compared mineralisation of ± 2 cm pieces of 13C-labelled ryegrass as EOM reference at two doses (0.5 g kg−1 vs. 5 g kg−1) and mineralisation of SOC in sandy loam and silt loam soils that were initially subjected to different degrees of disturbance, namely soil sieving at  300 μm class by 130% vs. unamended control) and macroporosity (i.e. EOM doses 0.5 g kg−1 and 5 g kg−1 increased the vol% of the 60–100 μm class by 80 and 120% vs. unamended control) formation in the sandy loam soil, whereas the porosity in the silt loam soil remained unaffected. The % of EOM mineralised was independent of EOM dose and therefore did not relate to the changes in soil pore network structure. However, the observed development of meso- and macropores in the sandy loam soil appeared to be associated with stronger stimulation of SOC mineralisation at increasing EOM dose (30% and 71% more SOC mineralisation at low and high dose vs. unamended control). In the silt loam soil, only the high EOM dose-stimulated SOC mineralisation vs. unamended control (28%) but soil pore network structure remained unresponsive to EOM dose. This suggests that stimulation of SOC mineralisation following EOM addition might also be in part linked to the formation of larger pores in the sandy loam soil, and that this stimulation is not always proportional to EOM dose

Relative degradability of exogenous organic carbon in function of its concentration and the interaction with native soil organic matter content

The soil environment impacts carbon turnover in a multitude of ways. It has been shown that C emissions are higher when exogenous carbon (C) is locally concentrated than when it is spread in the soil matrix. Hence, the spatial separation between decomposers and their substrate may cause ineffective dispersion of the produced enzymes within the soil matrix. It is not clear, however, if and how lower doses of exogenous C also impact its degradability and if an interaction exists with native soil organic matter (SOM) level. Therefore, a seventy-day incubation experiment with six varying exogenous OM doses (0-6 g C kg-1) in soil mesocosms of loamy sand subsoil (WFPS 50%) with three different native SOM contents (i.e. 0.1, 0.5, and 1.0 %) was carried out. C emission and isotopic signature measurements of 13CO2 by means of a CRDS-analyser were used to track the degradation of added maize shoots with a δ13C=-12.7‰ that contrasts to native SOM (δ13C=-25‰). Mineral N was added, and the redox potential (Eh) monitored. Eh suggested that O2 may have not become limiting for C mineralisation, even at higher doses, but it did not clearly relate to the added C dose. Cumulative CO2 emissions increased proportionally with C dose and so our hypothesis that a lower dose dispersion limits microbial activity was disproved. However, a higher native SOM level led to higher relative degradability of added C for the highest doses (i.e. 3 and 6 g kg-1 soil). Furthermore, a negative impact of OM doses above 1 g kg-1 soil on native SOM degradation was found. Growth of the microbial biomass was only stimulated proportionally above 1 g C kg-1, and this extra biomass was unexpectedly not involved in degradation of native SOM but instead exclusively in decomposition of the added maize residues. Hence, we can conclude that positive priming of native SOM mineralisation did not occur. The resulting disproportional response of microbial biomass and soil C mineralisation at lower doses suggests that dose does impact the apparent microbial metabolic quotient. Ongoing PLFA analyses will further reveal the soil microbial community structure along the different treatments and help us interpret these observations

Control of paddy soil redox condition on gross and net ammonium fixation and defixation

Abiotic fixation and defixation of ammonium (NH4+) in silicate interlayers are common processes in paddy soils, owing to their often high levels of 2:1 type clay minerals. Fixed NH4+ hence forms a buffer during the supply and loss of plant-available N. The soil redox potential (E-h) is suspected to influence NH4+ (de)fixation by its impact on the negative charge of Fe3+-containing clay minerals, yet experimental confirmations are scarce. We examined the effect of a fluctuating or generally increased E-h on dynamics of NH4+ (de)fixation in a Bangladeshi paddy soil (total N: 1.9 g kg(-1) vertical bar CEC: 43 cmol kg(-1)) during two incubation experiments. In those, we either N-15-labelled the fixed NH4+ pool (with initially 87 mg kg(-1) exchangeable versus 393 mg kg(-1) fixed NH4+-N) or the exchangeable pool (with initially 141 mg kg(-1) exchangeable versus 249 mg kg(-1) fixed NH4+-N), and this under three E-h-controlling treatments: continuous flooding (CF), CF with extra MnO2 added (3.5 g kg(-1) Mn-4(+)) (CF-MnO2), and alternate wetting and drying (AWD). The CF treatment overall led to somewhat higher net NH4+ fixation than AWD and, less clearly, also compared to the CF-MnO2 treatment, resulting from higher gross NH4+ fixation as well as lower gross defixation. These fluxes always led to a dynamic equilibrium between the exchangeable and fixed NH4+ pool within a few days of incubation, and this equilibrium was the same for both experiments but depended on the E-h treatment. However, by altering other NH4+ transformation processes, E-h treatments also indirectly impacted NH4+ (de)fixation fluxes, for which the equilibrium ratio between exchangeable versus fixed NH4+ turns out to be the determining factor

Stable isotopes to understand paddy soil biogeochemical processes under water-saving irrigation

To cope with climate change, the use of water-saving irrigation practices in rice production is increasingly promoted, instead of the traditional practice of continuous flooding. Yet, it is unclear how the periodic introduction of oxygen during drying events impacts some fundamental biogeochemical processes in paddy soils because of the difficulty to quantify them. This knowledge gap actually complicates the development of mechanistic biogeochemical models, which are key for the prediction of paddy soil nutrient dynamics over a range of edaphic, agronomic and climatic settings. However, stable isotopes may help to shed some light on dynamics of important soil nutrients, i.e., by exploiting their natural or artificial (by labelling) variations over different biogeochemical transformations. In an effort to improve their implementation in soil models, we experimentally investigated the effect of water-saving irrigation management on three prominent but understudied carbon and nitrogen transformations in paddy soils from Italy and Bangladesh by means of stable isotopes. In particular, we assessed the oxidation of methane (CH4) in situ, the decomposition of soil organic carbon (SOC) after the incorporation of organic residues, and the fixation and defixation of ammonium (NH4+) in clay interlayers, always as a function of irrigation management. As it turns out, the adoption of water-saving irrigation practices had the potential to relatively stimulate in situ CH4 oxidation, in spite of some methodological difficulties, which implies that the commonly observed lower CH4 emissions in paddy fields under water-saving irrigation probably do not only stem from lower CH4 production rates under less anaerobic conditions, but also from this mitigation mechanism. Secondly, the stimulation of native SOC dissolution after the incorporation of organic residues was restricted under water-saving irrigation, and redox-active iron oxides with sorbed SOC potentially play a role in this. However, this effect on SOC dissolution was not proportionally reflected in SOC-derived emissions of CH4 and carbon dioxide (CO2), so that irrigation management probably does not influence these emissions and the long-term SOC balance through the proposed mechanism. Thirdly, the effect of irrigation management on abiotic fixation and defixation dynamics of NH4+ into or from interlayers of clay minerals was rather limited, but these processes – which determine the bioavailability of NH4+ in paddy soil – were instead predominantly dictated by an equilibrium between bioavailable and clay-fixed NH4+ phases. Overall, with a well-considered experimental setup in which untargeted isotope contrasts can be quantified and the appropriate pools sampled, stable isotopes are apparently a powerful tool to quantify gross nutrient transformations in paddy soil

Do interactions between application rate and native soil organic matter content determine the degradation of exogenous organic carbon?

Although the amendment of various forms of exogenous organic matter (EOM) is a common practice in cropland production, it is to date not clear if its mineralisation in soil depends on application rate. Previous research suggested that spatial concentration of EOM in soil positively impacts its degradability. Here, we seek to test these reports and furthermore to investigate if an interactive effect exists with native soil organic carbon (SOC) concentration. We hypothesised that at a low EOM dose, more EOM remains undecomposed in soil and that this effect becomes stronger at lower SOC level. Moreover, as a secondary goal, we explored if priming of native SOC mineralisation depends on EOM dose. Therefore, we set up seventy-day soil incubation experiments with six varying C4-maize residue (813C = - 12.7 parts per thousand) doses (0-6 g kg- 1) in soil mesocosms of loamy sand subsoils (813C = - 25.7 parts per thousand) with three different native SOC levels (i.e. 0.1, 0.5, and 1.0%). Soil CO2 emissions and isotopic signature measurements of CO2 allowed to discern EOM and SOC mineralisation. We found that EOM-derived C mineralisation increased proportionally to EOM dose, refuting the hypothesised relative undecomposed EOM at low concentration. Volumes of larger pore neck size diameter classes (60-100 and >300 mu m) almost doubled at high EOM dose, demonstrating formation of macropores. Yet, this apparently did not impact EOM mineralisation, probably because O2 supply was always sufficient to allow unlimited activity of EOM degraders, even at higher EOM doses, as indicated by generally higher measured redox potential. With EOM dose, fungal marker PLFA abundances increased in the 1% SOC soil and protozoan abundances increased in all three soils, but apparently these shifts did not result in an enhanced relative degradation of the EOM. Increasing EOM doses induced negative priming, e.g. EOM >= 1 g kg- 1 reduced SOC mineralisation by >43% and >24% compared to the control in the 0.1 and 0.5% SOC soils, respectively; whereas no priming occurred in soil with 1% SOC. These results were largely explained by the amount of added C relative to microbial biomass carbon, and the theorised switch of slow decomposers (so called K-strategists and involved in recalcitrant compound decomposition) from SOM to preferentially decompose EOM at higher doses. We also postulate that at low SOC %, the obvious increased O2 consumption with higher EOM dose more readily results in local anaerobic conditions in finer pores, i.e. where SOC is located and mineralised. We conclude that on the short term, agricultural management for SOM thus does not need to consider EOM doses but only the total amount of EOM

Relative degradability of exogenous organic carbon in function of its concentration and the interaction with native soil organic matter content

The soil environment impacts carbon (C) turnover in a multitude of ways. It has been shown that C emissions are higher when exogenous carbon is locally concentrated than when it is spread in the soil matrix. Hence, the spatial separation between decomposers and their substrate may cause ineffective dispersion of the produced enzymes within the soil matrix. It is not clear, however, if and how lower doses of exogenous C also impact its degradability and if an interaction exists with native soil organic matter (SOM) level. Therefore, a seventy-day incubation experiment with six varying exogenous OM doses (0-6 g C kg-1) in soil mesocosms of loamy sand subsoil (WFPS 50%) with three different native SOM contents (i.e. 0.1, 0.5, and 1.0 %) was carried out. CO2 emissions and isotopic signature measurements of 13CO2 by means of a CRDS-analyser were used to track the degradation of added maize shoots with a δ13C=-12.7 ‰ that contrasts to native SOM (δ13C=-25 ‰). Mineral N was added, and the redox potential (Eh) monitored. Eh suggested that O2 may have not become limiting for C mineralization, even at higher doses, but it did not clearly relate to the added C dose. Cumulative CO2 emissions increased proportionally with C dose and so our hypothesis that a lower dose dispersion limits microbial activity was disproved. However, a higher native SOM level led to higher relative degradability of added C for the highest doses (i.e. 3 and 6 g kg-1 soil). Furthermore, a negative impact of OM doses above 1 g kg-1 soil on native SOM degradation was found. Growth of the microbial biomass was only stimulated proportionally above 1 g C kg-1, and this extra biomass was unexpectedly not involved in degradation of native SOM but instead exclusively in the decomposition of the added maize residues. Hence, we can conclude that positive priming of native SOM mineralisation did not occur, but rather negative priming effect was found for higher doses (> 1 g C kg-1). The resulting disproportional response of microbial biomass carbon (MBC) and soil C mineralisation at lower doses suggests that dose does impact the apparent microbial metabolic quotient (qCO2). Ongoing PLFA analyses will further reveal the soil microbial community structure along the different treatments and help us interpret these observations