Microscale spatial distribution and soil organic matter persistence in top and subsoil

The spatial distribution of organic substrates and microscale soil heterogeneity significantly influence organic matter (OM) persistence as constraints on OM accessibility to microorganisms. However, it is unclear how changes in OM spatial heterogeneity driven by factors such as soil depth affect the relative importance of substrate spatial distribution on OM persistence. This work evaluated the decomposition and persistence of 13C and 15N labeled water-extractable OM inputs over 50 days as either hotspot (i.e., pelleted in 1 – 2 mm-size pieces) or distributed (i.e., added as OM < 0.07 µm suspended in water) forms in topsoil (0-0.2 m) and subsoil (0.8-0.9 m) samples of an Andisol. We observed greater persistence of added C in the subsoil with distributed OM inputs relative to hotspot OM, indicated by a 17% reduction in cumulative mineralization of the added C and a 10% higher conversion to mineral-associated OM. A lower substrate availability potentially reduced mineralization due to OM dispersion throughout the soil. NanoSIMS (nanoscale secondary ion mass spectrometry) analysis identified organo-mineral associations on cross-sectioned aggregate interiors in the subsoil. On the other hand, in the topsoil, we did not observe significant differences in the persistence of OM, suggesting that the large amounts of particulate OM already present in the soil outweighed the influence of added OM spatial distribution. Here, we demonstrated under laboratory conditions that the spatial distribution of fresh OM input alone significantly affected the decomposition and persistence of OM inputs in the subsoil. On the other hand, spatial distribution seems to play a lower role in topsoils rich in particulate OM. The divergence in the influence of OM spatial distribution between the top and subsoil is likely driven by differences in soil mineralogy and OM composition.Microscale spatial distribution and soil organic matter persistence in top and subsoilpublishedVersio

Fine scale spatial variability of microbial pesticide degradation in soil: scales, controlling factors, and implications

Pesticide biodegradation is a soil microbial function of critical importance for modern agriculture and its environmental impact. While it was once assumed that this activity was homogeneously distributed at the field scale, mounting evidence indicates that this is rarely the case. Here, we critically examine the literature on spatial variability of pesticide biodegradation in agricultural soil. We discuss the motivations, methods, and main findings of the primary literature. We found significant diversity in the approaches used to describe and quantify spatial heterogeneity, which complicates inter-studies comparisons. However, it is clear that the presence and activity of pesticide degraders is often highly spatially variable with coefficients of variation often exceeding 50% and frequently displays nonrandom spatial patterns. A few controlling factors have tentatively been identified across pesticide classes: they include some soil characteristics (pH) and some agricultural management practices (pesticide application, tillage), while other potential controlling factors have more conflicting effects depending on the site or the pesticide. Evidence demonstrating the importance of spatial heterogeneity on the fate of pesticides in soil has been difficult to obtain but modelling and experimental systems that do not include soil’s full complexity reveal that this heterogeneity must be considered to improve prediction of pesticide biodegradation rates or of leaching risks. Overall, studying the spatial heterogeneity of pesticide biodegradation is a relatively new field at the interface of agronomy, microbial ecology, and geosciences and a wealth of novel data is being collected from these different disciplinary perspectives. We make suggestions on possible avenues to take full advantage of these investigations for a better understanding and prediction of the fate of pesticides in soil

Succession of N cycling processes in biological soil crusts on a Central European inland dune

Biological soil crusts (BSCs) are microbial assemblages that occur worldwide and facilitate ecosystem development by nitrogen (N) and carbon accumulation. N turnover within BSC ecosystems has been intensively studied in the past; however, shifts in the N cycle during BSC development have not been previously investigated. Our aim was to characterise N cycle development first by the abundance of the corresponding functional genes (in brackets) and second by potential enzyme activities; we focussed on the four processes: N fixation (nifH), mineralisation as proteolysis and chitinolysis (chiA), nitrification (amoA) and denitrification (nosZ). We sampled from four phases of BSC development and from a reference located in the rooting zone of Corynephorus canescens, on an inland dune in Germany. BSC development was associated with increasing amounts of chlorophyll, organic carbon and N. Potential activities increased and were highest in developed BSCs. Similarly, the abundance of functional genes increased. We propose and discuss three stages of N process succession. First, the heterotrophic stage (mobile sand without BSCs) is dominated by mineralisation activity. Second, during the transition stage (initial BSCs), N accumulates, and potential nitrification and denitrification activity increases. Third, the developed stage (established BSCs and reference) is characterised by the dominance of nitrificatio

Exploring the limits of mineral- associated organic carbon formation in temperate soils

Soils store almost three times more organic carbon (OC) than the atmosphere. This makes the sequestration of soil organic carbon (SOC) a promising strategy for mitigating climate change. SOC can be divided into particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). POC is more susceptible to decomposition by microorganisms because it is not bound to mineral surfaces and therefore breaks down more rapidly. In contrast, MAOC is more stable, persisting for decades to centuries due to its strong associations with mineral particles. This longevity makes MAOC a key target for SOC sequestration. According to the concept of SOC saturation concept, the ability of a soil to store additional MAOC is limited by its silt and clay content, beyond which additional C is less protected. Soils with a lower SOC content have a higher sequestration potential due to a larger SOC saturation deficit, i.e., the difference between the current and the maximum MAOC storage capacity. In this dissertation, the potential limitations of the SOC saturation concept are critically analyzed using three complementary studies. The first study found no evidence of an upper limit for MAOC storage in different soils. The second study confirmed that soils stabilize OC even beyond theoretical saturation limits. The third study showed that the efficiency of MAOC formation remained constant despite increasing carbon inputs in both the topsoil and subsoil. These results challenge the existing concept of SOC saturation as they show that SOC stabilization is determined by sustained carbon inputs and not by mineral limitations. These results emphasize the untapped potential for long-term SOC sequestration and the need to prioritize C input from agricultural management in the context of climate change adaptation

An investigation on the effects of lime and/or phosphorus fertilizer applications on soil organic matter preservation : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Manawatu, New Zealand

The poor understanding of the mechanisms through which soil organic matter (OM) is lost with ongoing land-use intensification hampers the development of food security and climate-smart agricultural management practices. The overall objective of this thesis was to investigate the effect of lime and/or phosphorus (P) amendment on OM preservation in a volcanic soil classified as an Andosol – the mineral soil group with the largest organic carbon (OC) content worldwide and characterised by its abundance in aluminium (Al)-OM complexes (e.g., Al³⁺-OM and allophane-OM complexes). Special attention was paid to the response of OM stabilisation and mineralisation with depth to these amendments. Firstly, we hypothesised that (i) lime and P application has an impact on OM stabilisation through different mechanisms, and (ii) their effect is synergic. To have a direct understanding of the effect of lime and/or P application on OM preservation in the Andosol under study, we conducted a batch of water extractions. We extracted the bulk soil and its heavy fraction (>1.6 g/cm3, indicative of the presence of OM-mineral associations) with added lime and/or P to reveal the individual and combined influence of lime and P amendments on water-extractable OM (WEOM), which has been deemed to be an indicator of OM destabilisation. The results obtained from quantitative analyses of WEOM showed that adding lime and/or P significantly increased the WEOM, along with a decrease in its carbon (C)/nitrogen (N) ratio (C/N) and an increase in its aromaticity. The chemical composition of WEOM measured by pyrolysis-gas chromatography/mass spectrometry suggested that lime and P addition (at high application rate) caused an enrichment in WEOM in the poly- and monophenolic, and nitrogenised fraction, as well as in plant-derived polysaccharides. If we consider the effect on the heavy fraction, the increase in WEOM was still consistent with that observed in the bulk soil when lime was applied, but the response to P addition alone was smaller. These findings indicate that lime and P amendment to soils rich in Al-OM complexes cause destabilisation of OM, but through different mechanisms. Phosphate was found to mainly impact Al³⁺-OM complexes (partly present in the removed free particulate OM) by outcompeting organic ligands for Al³⁺, whereas alkalisation was able to disrupt both the Al³⁺-OM and allophane-OM complexes, and the stability of aggregates. These could be hastened by combined lime and P addition, as made evident by the larger impact of combined lime and P amendments than that of either P or lime addition alone (Chapter 3). After confirming the occurrence of OM destabilisation in the Andosol upon lime and/or P application, we hypothesised that the response of OM preservation (OM stabilisation and mineralisation) to these amendments varies with soil depth. We conducted a 6-month incubation experiment to have an in-depth understanding of the influence of these amendments on OM preservation in soil at different depths. A topsoil (rich in Al³⁺-OM complexes) and a subsoil (with a greater abundance of allophane) of an alu-andic Andosol was incubated with/without inorganic amendments (either lime, phosphate or lime+phosphate) in the presence or absence of an organic amendment (¹³C- and ¹⁵N- labelled barley, Hordeum vulgare L.). By conventional chemical analyses of the bulk soil, we showed an increase in WEOM in both topsoil and subsoil samples that received amendments, particularly of lime (with/without P). However, through a nano-scale secondary ion mass spectrometry analysis of OM-mineral associations in soil microaggregates, we noted that lime amendments decreased OM coverage (particularly plant-derived OM) on the mineral surface in topsoil, but increased it in subsoil (with enhanced coverage of plant-derived OM). These suggested that at these two soil depths with different biogeochemistry, lime addition resulted in OM destabilisation through different mechanisms associated with (i) the displacement of OM from inorganic surfaces in microaggregates in the topsoil, and (ii) the release of OM previously protected within macroaggregates in the subsoil. The total cumulative carbon dioxide (CO₂) emissions and stable C isotopic signature (δ¹³C) of CO₂ showed that lime amendments caused an increase in OM decomposition in the subsoil from both inherited OM (priming) and OM newly formed from barley litter decomposition, but not in topsoil. The increase in OM mineralisation observed in the subsoil (a harsher environment for microbes, with limited bioavailable OM) is consistent with the fact that more favourable conditions were generated by the lime and P addition, which caused an increase in WEOM (Chapter 4). To further understand the distinct responses in OM mineralisation with depth to lime and/or P amendments, we investigated soil bacterial and fungal community composition and their functional profile through high-throughput sequencing analysis. A shift in bacterial and fungal community composition and their functional composition was found in the limed topsoil but not in the limed subsoil. Through structural equation modelling analysis, it was found that in the topsoil, microbial properties, particularly the fungal community composition and functional profile, had a significant relationship with OM mineralisation (with a relatively greater positive or negative coefficient value than other factors). However, in the subsoil, OM mineralisation was only significantly correlated with labile OM in the subsoil. These findings suggested that in the Andosol, the key regulator controlling the response of OM mineralisation to lime and/or phosphate addition shifted with depth from microbial composition and functionality to bioavailable C substrate (Chapter 5). All the results obtained in this thesis contribute to providing a mechanistic understanding of the effect of lime and/or P amendments on OM stabilisation and mineralisation, and have implications for designing climate-smart agricultural management practices of soils with abundant Al-OM complexes

PEARL model for pesticide behaviour and emissions in soil-plant systems : description of the processes in FOCUS PEARL v 1.1.1

The use of pesticides in agriculture presents risks to the environment, which are increasingly evaluated by using computation models. The new PEARL model simulates the behaviour of pesticides in soil-plant systems and their emissions to the environment. The pesticide model is used in combination with the hydrological model SWAP. Various agricultural situations and ways of applying the pesticides can be simulated. The model accounts for different sorption mechanisms, in equilibrium and non-equilibriumdomains of the soil. Pesticide transport in the liquid and gas phases is described by the convection-dispersion-diffusion type equation, which is supplemented with sink terms. Comprehensive reaction schemes are processed in matrix form. The rate in first-order transformation kinetics is dependent on temperature, soil moisture content and depth in the soil. Besides computing persistence and distribution of the pesticidal compounds in soil, the model computes volatilization into the air, lateral drainage to water courses and leaching to groundwater

Fate and stability of dissolved organic carbon in topsoils and subsoils under beech forests

Dissolved organic carbon (DOC) from Oa horizons has been proposed to be an important contributor for subsoil organic carbon stocks. We investigated the fate of DOC by directly injecting a DOC solution from 13C labelled litter into three soil depths at beech forest sites. Fate of injected DOC was quantified with deep drilling soil cores down to 2 m depth, 3 and 17 months after the injection. 27 ± 26% of the injected DOC was retained after 3 months and 17 ± 22% after 17 months. Retained DOC was to 70% found in the first 10 cm below the injection depth and on average higher in the topsoil than in the subsoil. After 17 months DOC in the topsoil was largely lost (− 19%) while DOC in the subsoil did not change much (− 4.4%). Data indicated a high stabilisation of injected DOC in the subsoils with no differences between the sites. Potential mineralisation as revealed by incubation experiments however, was not different between DOC injected in topsoil or subsoils underlining the importance of environmental factors in the subsoil for DOC stabilisation compared to topsoil. We conclude that stability of DOC in subsoil is primary driven by its spatial inaccessibility for microorganisms after matrix flow while site specific properties did not significantly affect stabilisation. Instead, a more fine-textured site promotes the vertical transport of DOC due to a higher abundance of preferential flow paths. © 2020, The Author(s)

Fate and stability of dissolved organic carbon in topsoils and subsoils under beech forests

Dissolved organic carbon (DOC) from Oahorizons has been proposed to be an important contributor for subsoil organic carbon stocks. We investigated the fate of DOC by directly injecting a DOC solution from 13C labelled litter into three soil depths at beech forest sites. Fate of injected DOC was quantified with deep drilling soil cores down to 2 m depth, 3 and 17 months after the injection. 27 ± 26% of the injected DOC was retained after 3 months and17 ± 22% after 17 months. Retained DOC was to 70% found in the first 10 cm below the injection depth and on average higher in the topsoil than in the subsoil. After 17 months DOC in the topsoil was largely lost (-19%) while DOC in the subsoil did not change much (-4.4%). Data indicated a high stabilisation of injected DOC in the subsoils with no differences between the sites. Potential mineralisation as revealed by incubation experiments however, was not different between DOC injected in topsoil or subsoils underlining the importance of environmental factors in the subsoil for DOC stabilisation compared to topsoil. We conclude that stability of DOC in subsoil is primary driven by its spatial inaccessibility for microorganisms after matrix flow while site specific properties did not significantly affect stabilisation. Instead, a morefine-textured site promotes the vertical transport of DOC due to a higher abundance of preferential flow paths