Quality Of Water Extractable Organic Matter Research Articles

High content and distinct spectroscopic characteristics of water-extractable organic matter in rhizosheath soils in a semiarid grassland

Rhizosheaths are sheath-like soils widely observed on plant roots and often have higher bioavailability compared with non-rhizosheath rhizosphere soils. As the most labile soil fraction, the quantity and quality of water-extractable organic matter (WEOM) and its temporal dynamics in rhizosheath soils, however, remain unclear. Here, we used ultraviolet–visible and excitation-emission matrix fluorescence spectroscopy to elucidate the WEOM characteristics of rhizosheath and non-rhizosheath soils in an Inner Mongolian grassland and investigate the temporal dynamics of rhizosheath WEOM from early (June) to late (October) growth stages. The results showed that rhizosheath soils had 88% more bulk WEOM (as indicated by water-extractable organic carbon), 44% more chromophoric WEOM, 55% more humus-like (i.e., humic- and fulvic-like) fluorescent WEOM, and 152% more protein-like fluorescent WEOM compared with non-rhizosheath soils. The absorbance and fluorescence analyses revealed that rhizosheath WEOM exhibited lower aromaticity and molecular size and higher fresh microbial contributions. Further, rhizosheath WEOM was more depleted and showed much higher aromaticity and microbial contributions in the middle (August) growth stage that had the highest temperature and precipitation levels. These findings suggest an enrichment of labile organic compounds, which can serve as the primary contributor to the commonly reported high microbial biomass and activity in rhizosheath soils. Overall, the results supported the exceptionally high labile substrate availability for accelerated microbial carbon processing in the rhizosphere microenvironment of rhizosheath-forming plants.

Conversion of Brazilian savannah to agricultural land affects quantity and quality of labile soil organic matter

Most studies assessing land-use change (LUC) and management impacts on the soil organic carbon (SOC) budget mainly focused on total soil organic matter (SOM), neglecting its functional fractions such as water-extractable organic matter (WEOM). This may limit a better understanding of SOC dynamics along the soil profile as WEOM plays a key role in several soil and microbial processes. Here, we aimed to obtain a quantitative and qualitative assessment of WEOM dynamics to 1 m depth in areas after native vegetation conversion to long-term agriculture in Brazil. For this purpose, cropland areas under no-tillage (NT; ∼ 23 years old) and native vegetation (NV; i.e., Cerrado vegetation) were evaluated across a transect of 1000 km within Brazil’s new agricultural frontier – the so-called MATOPIBA region. We combined spectroscopic analyses (UV–Vis and DRIFT spectroscopies), C and nitrogen (N) determination in different SOM pools (i.e., bulk SOM, WEOM, and microbial biomass), and microbial measurements [e.g., microbial biomass C (MBC) and N (MBN), and β-glucosidase activity] to better understand changes in WEOM dynamics induced by LUC. We observed that although SOC decreased along the soil profile after NV (4.2–20.7 g kg−1) conversion to NT (3.8–14.2 g kg−1), water-extractable organic C levels (3.6–79.3 mg L−1) were similar between land uses. On the other hand, an increase in water-extractable total N was observed after NV (0.4–4.6 mg L−1) conversion to NT (0.5–19.3 mg L−1). Although MBC and MBN levels decreased for most study sites, β-glucosidase activity increased after LUC indicating intensive microbial processing of SOM. As a result, WEOM had lower aromaticity and molecular weight (i.e., low values of SUVA254 and A2/A3) in NT than NV; and a relative decomposition of aliphatic and polysaccharides over aromatic and amine/amide was observed after LUC through DRIFT spectroscopy. Our study indicates that LUC had stronger impacts on WEOM quality than its quantity, with agricultural areas having more biochemical changes in WEOM than NV.

Quantitative and Qualitative Responses of Soil Water-Extractable Organic Matter to Carbon and Nitrogen Management Practices in Loess Soil

Soil-dissolved organic matter (DOM) drives the carbon (C) and nitrogen (N) cycles in agroecosystems. Despite many studies on DOM dynamics, hardly any attention has been directed toward DOM quality, particularly DOM composition. The aim of this study was to elucidate how C and N management practices alter soil water-extractable organic matter (WEOM) in a loess soil agroecosystem. Field experiments were conducted with a winter wheat monoculture. Three N fertilization rates (0, 120, and 240 kg ha−1 year−1) were applied for 17 years (2002–2019), combined with five C practices (zero, low, and high rates of sheep manure or wheat straw) for three years (2016–2019). The results reveal that soil organic carbon (SOC) and water-extractable organic carbon (WEOC) concentrations in the topsoil (0–20 cm) were increased by organic amendments considerably but were not affected by N fertilization. The fluorescence excitation–emission matrix spectra (EEM) of WEOM were resolved to two humic-like components (C1 and C2) and two soluble microbial byproduct-like components (C3 and C4). The proportions of C1 and C2 were increased, while the proportion of C3 was decreased by both C and N management practices. In conclusion, organic amendments increased both WEOM quality and its proportion of humic-like components, whereas N fertilization increased the proportion of humic-like components without variations of WEOM quality in the topsoil of loess soil.

Responses of soil WEOM quantity and quality to freeze–thaw and litter manipulation with contrasting soil water content: A laboratory experiment

Future climatic change is likely to increase the occurrence of soil freeze–thaw (FT) events in high latitude and/or high altitude zones, which can substantially influence the dynamics of dissolved organic matter (DOM) released into the soils. However, it is not clear how the quantity and quality of soil DOM respond to changing FT patterns under different soil moisture and litter manipulation conditions in northern temperate forest stands. In this study, litter-amended and non-amended forest soils were incubated for 360 days at three soil moisture levels (30%, 60%, and 90% water-filled pore space) and three intensities of FT disturbance (low, high, and none). We quantified heterotrophic respiration, enzyme activity, microbial biomass, and water-extractable organic matter (WEOM) as a proxy for DOM in soils during the incubation experiment. The quality of WEOM was characterized by biodegradability, UV absorbance and parallel factor analysis modelling of fluorescence excitation emission matrices. Concentrations of water-extractable organic carbon (WEOC) and biodegradable WEOC declined continuously in all treatments over the 360-day incubation period. The dominant component of fluorescent WEOM shifted from humic- and fulvic- like components during the first 108 days of incubation, to protein-like components of microbial origin, characterized by high aromaticity, at the end of the incubation period. Litter amendment, FT disturbance, and their interaction increased WEOC concentrations in soils during the early 108-day incubation period, particularly in soils with low moisture and high FT intensity, but these increases disappeared after 252 days incubation at 15 °C. Litter-derived fluorescent WEOM in soils without FT disturbance was dominated by protein-like components after 14 days of incubation, but these were replaced by humic- and fulvic-like components after 108 days. This replacement effect was weaker in soils with FT disturbance, which we attribute to changes in microbial properties, including enzyme activity, microbial biomass and activity.

Impact of conventional agriculture on the concentration and quality of water-extractable organic matter (WEOM) in the surface horizons of Retisols—A case study from the Carpathian Foothills in Poland

In spite of many studies focused on the impact of land use changes on soil organic carbon (SOC) and the total nitrogen (TN) and soil organic matter (SOM) budget, less is known about the fate of water-extractable organic matter (WEOM) after the conversion of forestland to conventional farmland. The main aims of this study were: 1) to determine the concentration of water-extractable organic carbon (WEOC) and water-extractable total nitrogen (WETN) in topsoil horizons of forestland soils and cropland soils in the Carpathian Foothills in southern Poland and 2) to determine the quality of WEOM in topsoil horizons of forestland soils and cropland soils. Soil samples from topsoil horizons of forestland soils (Oi and Ah horizons) and cropland soils (Ap horizons) were collected and the concentration of WEOC and WETN was determined. In addition, the quality of WEOM was determined using ultraviolet-visible (UV–vis) spectroscopy and Fourier-transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy. The results of this comparative study clearly indicate that conventional agriculture significantly affects the concentration and quality of WEOM in surface soil horizons of Retisols in the Carpathian Foothills in southern Poland. Surface horizons of arable soils (Ap horizons) are characterized by a significantly lower mean concentration of WEOC and WETN in comparison with surface horizons of forestland soils (Oi and Ah horizons), and reflect changes in the content of bulk SOC and TN due to soil tillage. Mean concentrations of WEOC and WETN in the studied Ap horizons of arable soils are 1.98 mg l−1 and 0.15 mg l−1, respectively while the mean concentrations of WEOC and WETN in the Ah horizons of forestland soils are 15.53 mg l−1 and 0.84 mg l−1, respectively. The studied Oi horizons of forestland soils are characterized by the highest mean concentration of WEOC (44.53 mg l−1) and WETN (6.22 mg l−1). Mean specific UV absorbance values at wavelength of 254 nm (SUVA254) was the highest for WEOM from Ap horizons indicating the highest transformation of WEOM in these horizons. Mean FTIR-ATR absorption bands of WEOM obtained from Oi and Ah horizons are much more intensive than the bands of WEOM from Ap horizons, which is related to significantly lower concentration of WEOM in the latter horizons in comparison with Oi and Ah horizons of forestland soils.

Chemical characterization and source identification of organic matter in eroded sediments: Role of land use and erosion intensity

Soil erosion is a key variable in the biogeochemical cycle of carbon (C) on the Earth's surface. However, questions remain about the roles of land use and erosion intensity on the composition, source, and fate of soil C eroded from terrestrial to fluvial systems. In this study, chemical characteristics of eroded water-extractable organic matter (WEOM) in soils and sediments, as well as subsequent source identification, were inferred from UV–Visible absorption and fluorescence excitation emission matrix (EEM)-parallel factor analysis (PARAFAC) in study sites that include land uses and gully banks, experiencing three levels of erosion intensity in a semi-arid region of China. 13C and 15N isotopic signatures along with elemental ratios were also successfully employed to explore the source of bulk soil organic matter (SOM) in sediments. We found a much greater contribution of condensed aromatic structures and hydrophobic fraction of soluble organic compounds in forest soils compared to croplands at eroding sites, where these variables were greater than those of depositional sites. The results from fluorescence analysis in soil materials showed that erosion intensity has a negligible influence on WEOM quality. The EEM-PARAFAC with fluorescence indices indicated that biological production of soil substrates can also play a key role in the dynamics of WEOM induced by soil erosion. Our results from an isotopic mixing model analysis showed that gully bank soil was the primary sources of sedimentary SOM in all regions with different erosion intensities (mean probability estimate (MPE) 100% for the region with light erosion intensity, 36.18% for the region with high erosion intensity, and 99.25% for the region with extremely high erosion intensity). However, orchard and grassland were also the main contributors for the SOM in sediments in regions with high erosion intensity, accounting for MPE 29.93% and 33.89%, respectively. Our findings demonstrate that land use and erosion intensity have significant effect on nature of eroded OM.

Effect of crop rotation, fertilisation and tillage on main soil properties and its water extractable organic matter

The excessive use of fertilisers and frequent and deep tillage are not considered good agricultural practices because they increase production costs and reduce soil fertility. Water extractable organic matter (WEOM) is the fraction of soil organic matter responsible for nutrient transport and bioavailability. The aim of this work was to investigate the effect of a 2-year rotation of faba bean–wheat, cultivated for a decade, and the agricultural practices (conventional vs no tillage, and fertilisation vs no fertilisation) on selected soil parameters and WEOM quality. Results showed that the soil organic carbon (SOC) and the total nitrogen (TN) content increased after the wheat and faba bean, respectively. Plots managed with conventional tillage showed WEOM with higher OC content with respect to no-tillage plots. A higher WEOM OC content was observed also in the fertilised plots. The WEOM of fertilised and faba bean plots was derived from decomposition of native SOC, but the microbial community decomposition was the main origin of WEOM after wheat.

Water-extractable organic matter and its fluorescence fractions in response to minimum tillage and organic farming in a Cambisol

BackgroundMinimum tillage (MT) and organic farming (OF) are increasingly conducted in agricultural managements from the interest of optimizing soil conditions and developing sustainable agriculture. Our understanding of their effects on water-extractable organic matter (WEOM) is still insufficient.MethodsTo study the effects of MT and OF on WEOM, we analyzed soil materials sampled at two depths (0–8-cm-upper soil and 12–25-cm-deeper soil) from long-term field experiments using different farming and tillage methods. The content, composition, and quality of WEOM were examined.ResultsThe results showed organic farming significantly decreased water-extractable organic carbon and nitrogen, but had positive effect on WEOM humic-like components revealed by parallel factor analysis with excitation–emission matrix, soil organic carbon (SOC), total nitrogen (TN), as well as SOC/TN. In addition, organic farming increased the aromaticity and condensation of WEOM as indicated by specific UV absorption and humification index. MT had no effect on WEOM both quantitatively and qualitatively but significantly decreased SOC and TN of the whole investigated soil profile. The depth effect was significant with strong stratification of WEOM, WEOM components as well as SOC and total N in upper soil. Moreover, the WEOM spectroscopic quality showed sharp differences between the upper and deeper soils.ConclusionsThe results indicated that in the combined presence both tillage management and farming management, farming management imposed more influence on WEOM than tillage, and organic farming may facilitate the transformation of WEOM and lead to formation of WEOM with high stability. MT significantly changed the distribution of SOC and WEOM in soil, profile but did not increase the contents of SOC and WEOM in the site of the present study. However, the presence of larger pool of WEOM in MT + OF treatment at upper soil is likely to fuel possibly greater microbial activity and more rapid nutrient cycling in soil which can be favorable practice with potential in improving soil conditions in view of developing a sustainable ecosystem in the studied siteGraphical abstractThe impacts of agricultural practices on soil water extractable organic matter

Quantity and chemistry of water-extractable organic matter in surface horizons of Arctic soils under different types of tundra vegetation – A case study from the Fuglebergsletta coastal plain (SW Spitsbergen)

Water-extractable organic matter (WEOM) is the most labile, dynamic, and bioavailable fraction of soil organic matter (SOM). WEOM is a very important component of soil because it is responsible for the mobilization and translocation of many elements and pollutants in the soil environment and plays a crucial role in the biogeochemistry of carbon (C) and nitrogen (N), mineral weathering as well as pedogenesis. The main objectives of this study were: 1) to determine the quantity and chemistry of WEOM in the surface horizons of High Arctic soils under different types of tundra vegetation in the eastern part of the Fuglebergsletta coastal plain in the Hornsund area of southwestern Spitsbergen; 2) to determine relationships between quantity of WEOM and physical and chemical soil properties; and 3) to verify the hypothesis that soil surface horizons covered with different types of tundra vegetation and showing different physical and chemical properties exhibit different quantity and quality of WEOM. Soil samples (uppermost 10cm) in six replicates from surface horizons were randomly collected at sites differing in terms of tundra vegetation type (wet moss, ornithocoprophilous, lichen-heath, polygonal). The obtained results indicate that the highest concentration of water-extractable organic carbon (WEOC) and water-extractable total nitrogen (WETN) occurs in surface horizons of soils covered with wet moss and ornithocoprophilous tundra vegetation. Soils covered with lichen-heath and polygonal tundra vegetation exhibit a markedly and significantly lower concentration of WEOC and WETN. On average, WEOC constitutes maximally up to 8.0% of total soil organic carbon (SOC) and WETN constitutes maximally up to 1.5% of total nitrogen (Ntot) content in the studied soil surface horizons. The lowest proportion of WEOC to total SOC and WETN to Ntot occurs in soils containing the highest content of SOM. On the other hand, the highest proportion of WEOC to total SOC and WETN to Ntot is present in soils characterized by the lowest content of SOM. Concentration of WEOC and WETN is highly positively and significantly correlated to the content of SOC, Ntot, total phosphorus (Ptot), and the C/N ratio as well as negatively and significantly correlated to pH and ash content. Fourier-transform infrared attenuated total reflectance (FTIR-ATR) spectra indicate clear differences in the chemistry of WEOM between the studied soils. The WEOM in the surface horizons of soils under wet moss and ornithocoprophilous tundra vegetation is characterized by a prevalence of aromatic compounds over aliphatic compounds. On the other hand, WEOM in surface horizons of soils covered with polygonal tundra vegetation exhibits a prevalence of aliphatic compounds over aromatic compounds. WEOM in surface horizons of soils under lichen-heath tundra vegetation contains an almost equal concentration of aromatic compounds and aliphatic compounds.

Response of water extractable organic matter and its fluorescence fractions to organic farming and tree species in poplar and robinia-based alley cropping agroforestry systems

Organic farming and agroforestry both have the potential to develop sustainable and environmental-friendly agroecosystems and to sequester more soil organic C (SOC). In a long-term field trial, we evaluated the effect of 21-year organic farming and 4-year agroforestry (Robinia and Poplar-based alley cropping system) on water extractable organic matter (WEOM). The technique combining excitation emission matrix (EEM) spectra with parallel factor analysis (PARAFAC) was used to reveal the components of WEOM. In addition, WEOM was characterized by UV absorbance and fluorescence spectra. Organic farming generally increased SOC and total N contents but decreased the WEOM content as well as the WEOM components indicated by the maximum fluorescence intensity (Fmax). Specific UV absorbance (SUVA) and humification index (HIX) of WEOM in organic farming implied WEOM in the organic farming had more components with aromatic structure but less humified. Higher fluorescence (FI) and freshness indices (BIX) of WEOM in organic farming system indicated that a higher percentage of WEOM was microbial-derived in the organic than in the integrated farming system. Robinia showed positive effect on SOC and total N contents in comparison with poplar and had stronger effects on the WEOM components, although the WEOM content did not differ between the two tree species. The significant farming×trees interactions on SOC and water extractable organic carbon (WEOC) indicated that the robinia effects were more pronounced in the organic farming system. Thus, the change of SOC was the result of interactive effect of farming and hedgerow trees in an agroforestry system. The low-input organic farming and robinia tended to result in change of quality of WEOM and led to enrichment of substances of high stability in WEOM. From above, the combination of organic farming and robinia trees is an important means for developing sustainable agricultural systems and soil carbon sequestration.

Effect of long-term application of corn stalks and inorganic fertilizers on fluorescence characteristics of water extractable organic matter in a meadow black soil of northeast China

ABSTRACTReturning crop residues into fields, either alone or in combination with inorganic fertilizer, is considered as a practical way to enhance soil fertility. However, information concerning the effects of crop residues and inorganic fertilizer application on water extractable organic matter (WEOM) in soil is scarce. The aim of this study was to examine the 10-year effect of corn residue (CR) return with or without the application of nitrogen, phosphorus, and potassium (NPK) fertilizer on the quantity and quality of WEOM in a black soil of northeast China by means of ultraviolet absorbance, fluorescence excitation--emission matrix, and derived spectroscopic indices. The application of NPK fertilizers and CR, alone or together, increased the content of total soil organic carbon (SOC), water extractable organic carbon (WEOC), and ratio of WEOC/SOC, with the sequence being NPK + CR > CR > NPK > CK. Compared with control treatment, the individual application of NPK fertilizer decreased the aromaticity of WEOM. In contrast, elevated proportion of tryptophan-like fluorophore and microbially derived fulvic acid-like components with low molecular weight was detected in the WEOM. The amendment with CR alone resulted in increase in aromaticity of WEOM and proportion of plant-derived humic acid-like component with large molecular weight, accompanied by reduced proportion of tyrosine-like compounds. For the soil with CR restoration, the application of NPK fertilizer increased aromaticity of WEOM, and large molecular weight fulvic acid-like and humic acid-like compounds were found. However, the proportions of tyrosine-like and tryptophan-like compounds were diminished. The quantitative and qualitative analysis of WEOM using fluorescence spectroscopy describes responses to be observed in long-term different fertilization strategies.

Water-extractable organic matter and enzyme activity in three forest soils of the Mediterranean area

Soil enzyme activities mediate key ecosystem functions of degradation, transformation and carbon mineralization. The study of microbial activity and its relations with water-extractable organic matter (WEOM) can be crucial to understand the dynamics of soil organic carbon pool. We investigated FDA-hydrolytic, β-glucosidase, cellulase, o-diphenol oxidase activities in soils under Fagus sylvatica (beech), Quercus ilex (holm-oak) and Quercus cerris (turkey-oak) stands. We investigated WEOM by liquid-state nuclear magnetic resonance (NMR) spectroscopy, useful to highlight the major functionalities in this fraction of soil organic matter. The highest enzyme activities, on mass basis, were recorded in soil under beech, with the highest organic carbon content. Reporting enzyme activities on organic carbon basis, it was possible to reveal enzyme enrichment for β-glucosidase and diphenol oxidase activities in soil under turkey-oak, with low organic matter. The 1H NMR spectra of WEOM highlighted a great richness of soluble organic compounds in soils with high organic carbon content, such as beech and holm-oak soils. All spectra are dominated by carbohydrate resonances. Spectra of WEOM from each stand exhibited specific signals. In WEOM from holm-oak, signals from substituted aliphatics account for up 28% of the total spectrum; in this sample signals from acetic and formic acids predominate, likely relating to the lower microbial utilization, according to the low heterotrophic (FDA-hydrolytic) activity. Only in WEOM from beech, signals from aromatics were detected probably related to the lower lignin degradation in soil, as expressed by the low phenol oxidase activity. However, the relationships among WEOM quality, tree species and microbial activity need further investigations.

A comparison of extraction procedures for water‐extractable organic matter in soils

SummaryThe characteristics of dissolved organic matter (DOM) in soils are often determined through laboratory experiments. Many different protocols can be used to extract organic matter from soil. In this study, we used five air‐dried soils to compare three extraction methods for water‐extractable organic matter (WEOM) as follows: (i) pressurised hot‐water‐extractable organic carbon (PH‐WEOC), a percolation at high pressure and temperature; (ii) water‐extractable organic carbon (WEOC), a 1‐hour end‐over shaking; and (iii) leaching‐extractable organic carbon (LEOC), a leaching of soil columns at ambient conditions. We quantified the extraction yield of organic carbon; the quality of WEOM was characterized by UV absorbance, potential biodegradability (48‐day incubation) and parallel factor analysis (PARAFAC) modelling of fluorescence excitation emission matrices (FEEMs). Biodegradation of dissolved organic carbon (DOC) was described by two pools of organic C. The proportions of labile and stable DOC pools differed only slightly between the WEOC and LEOC methods, while PH‐WEOC contains more stable DOC. The mineralization rate constants of both labile and stable DOC pools were similar for the three methods. The FEEMs were decomposed into three components: two humic‐like fluorophores and a tryptophan‐like fluorophore. The effect of extraction method was poorly discriminant and the most similar procedures were PH‐WEOC and LEOC while WEOC extracts were depleted in humic‐like fluorophores. This study demonstrates that WEOM quality is primarily determined by soil characteristics and that the extraction method has a smaller, but still significant, impact on WEOM quality. Furthermore, we observed considerable interaction between extraction procedure and soil type, showing that method‐induced differences in WEOM quality vary with soil characteristics.

Effects of forest fires on water extractable organic matter and humic substances from Mediterranean soils: UV–vis and fluorescence spectroscopy approaches

Humic substances (HS) and water extractable organic matter (WEOM) represent an important part of soil organic matter (SOM) which plays many roles in this ecosystem. Forest fires are known to have potential impacts on soil organic matter. We studied these impacts, first on global HS and the three relevant fractions (humic acids (HA), fulvic acids (FA) and non-humified fraction (NHF)) and secondly on WEOM and the three corresponding fractions (hydrophilic (Hpi), transphilic (Tpi) and hydrophobic (Hpo)) fractionated by the use of XAD-8 and XAD-4 resins. Measurements of total organic carbon (TOC) and the use of both UV–vis and fluorescence spectroscopy gave us quantitative and qualitative results to evaluate the impacts of forest fires abundance and recurrence on the restoration process of the burned soils. As for the TOC-SOM, the quantitative impact of fires on TOC-WEOM is indirect and depends on the fire number. The fires have a direct impact on WEOM quality where, depending on the fraction properties, fires lead to the homogenization of the different compounds of the WEOM. Fires have a direct impact on HS quantity leading to a significant decrease of the amounts. The quality of HS is also affected by fires showing an increase in aromaticity of humified fractions for long time spans (more than 16 years). On a short term the non-humified fractions appear less structured and complex. In general, the more structured the compounds in the fraction, the greater recovery time.

The dynamics of water extractable organic matter (WEOM) in common arable topsoils: II. Influence of mineral and combined mineral and manure fertilization in a Haplic Chernozem

Data on the dynamics of dissolved or water extractable organic matter (DOM, WEOM) in soils are often contradicting, which is especially true for arable soils. Since a complex set of soil inherent and environmental factors affects these dynamics, there is still a great need for additional data. Especially DOM results from (arable) field studies and long-term trials are scarce. We sampled the WEOM of the soils under three fertilization treatments in a Haplic Chernozem differing in fertilization intensity for over 90 years: (i) no fertilization (Control), (ii) mineral fertilization (NPK), and (iii) mineral plus additional farmyard manure fertilization (NPK + FYM). We sampled the WEOM from 0–40 cm at 10 cm intervals over a three year period during three seasons (spring, summer, and fall). We measured WEOM quantity (WEOC and WEON concentrations) and investigated the quality of WEOM with UV (absorptivity) and fluorescence (humification index, HIX) as well as biodegradability (BWEOC). The total soil organic carbon (SOC) and nitrogen (TN) were also quantified. The overall results indicated that NPK did not affect SOC and TN but did increase WEOC and WEON even though NPK does not contain organic matter, implying that fertilization affected WEOM via the biomass. The more aromatic and condensed compound of WEOM were especially increased. The NPK + FYM treatment also increased SOC and TN and had a stronger effect on WEOM than NPK alone. However, BWEOC was not significantly affected by fertilization practices. The three sampled years varied strongly in total precipitation and in crop type. Nevertheless, with the exception of WEON and HIX, no significant overall annual fluctuations could be detected. A seasonal pattern was found in WEOM concentration and quality but, except for WEON and HIX, fertilization treatments did not influence this seasonal pattern. The effects of fertilization did not vary as a function of depth for the parameters WEOC, WEON, and BWEOC, presumably because of their mobile nature. For the immobile SOC and TN depth had an effect. The values in the plough layer (0–30 cm) were significantly higher than in the region below it (30–40 cm). Absorptivity and HIX also showed such a pattern, indicating that more aromatic and condensed compounds are either preferentially retained or not as well metabolized in the plough layer.

The dynamics of water extractable organic matter (WEOM) in common arable topsoils: I. Quantity, quality and function over a three year period

Compared to total soil organic matter (SOM) the concentrations of water extractable organic matter (WEOM) are very small. Nevertheless, it is linked to many important soil functions. Numerous factors affect WEOM dynamics, most of them interacting in the same or in different directions. Therefore, there are still marked gaps in knowledge about WEOM. The majority of field studies have been done in forest ecosystems, and results on arable soils are scarce, contradictory and often do not fit to laboratory studies. Our purpose was to elucidate WEOM parameters in arable soils. We followed the dynamics of WEOM in arable top soils located on three different research sites in Germany. We sampled nine soils differing in soil type, texture, and soil management. Samplings over three years were done in three depths (0–10, 10–20, 20–30 cm) three times a year (spring, summer, and fall). WEOM concentrations were quantified as WEOC after batch extraction. Quality was derived by optical methods (UV absorption, fluorescence spectroscopy). Additionally, WEON (water extractable organic nitrogen) and the biodegradable fraction (BWEOC) were quantified. Results are presented in three parts. For this first part we compared the differences in WEOM dynamics between three selected soils (Eutric Cambisol, Eutric Luvisol, and Haplic Chernozem) and the temporal variability of WEOM in each soil (seasonality). Concentrations averaged over three years and for 0–30 cm showed no significant differences. WEOC and WEON ranged between 34–42 mg WEOC kg − 1 d.m. and 1.9–2.9 mg WEON kg − 1 d.m. The lack of significant differences was explained by the marked seasonality on all sites. The Eutric Cambisol was characterized by a sharp decrease of WEOC/WEON concentrations with increasing depth (60.6 to 20.6 mg WEON kg − 1 d.m. (dry mass), 1.7 to 0.48 mg WEON kg − 1 d.m.) after more than 10 years under no-till management. SOM and total N also decreased from 12.9 g kg − 1 C (0–10 cm) to 2.8 g kg − 1 C (20–30 cm) and from 1.4 g kg − 1 N to 0.43 g kg − 1 . Highest SOM and total nitrogen contents of the Chernozem did not lead to highest WEOC and WEON concentrations. In contrast to quantity, the quality of WEOM differed temporally significantly between all soils, with the highest absorptivity in the Luvisol and highest humification in the Chernozem. Differences in BWEOC were small, ranging between 42 and 46%. All soils were characterized by a marked seasonality. Variability of concentrations was more pronounced than that in quality. ANOVA gave indications that the quality was mainly influenced by the factor “site and/or soil” and the concentrations by the factor “seasonality”. WEOM dynamics did not show uniform patterns when the same field crop was planted on different sites. A test for the suitability of spectral absorptivity measurements as predictor for biodegradability, as it is often mentioned in literature, gave weak results. While in the Cambisol ( r = − 0.61, r 2 = 37%) and Chernozem ( r = − 0.70, r 2 = 49%) negative correlations were found, no relation between BWEOC and absorptivity was detected in the Luvisol. Due to the marked dynamics of WEOM in arable soils general statements can hardly be concluded, even when results are based on iterated samplings.