Soil Water-extractable Organic Carbon Research Articles

Residual Effect of Compost and Biochar Amendment on Soil Chemical, Biological, and Physical Properties and Durum Wheat Response

The residual effect of compost and biochar amendment on soil properties and durum wheat response was evaluated under field conditions in a Mediterranean environment. The treatments compared in a randomized complete block experimental design with three replications were: mineral fertilizer (100 kg N ha−1), compost applied at the rate of 25 Mg ha−1, biochar applied at the rates of 10 and 30 Mg ha−1, unfertilized control. Wheat was the second crop included in a sorghum–wheat cropping system and did not receive fertilizer supply. A hierarchical statistical analysis was carried out to investigate how different treatments could impact the cropping system performance. The findings highlight the significant influence of soil properties, particularly total N, WEOC, and TOC, on wheat and protein yield. One year after the amendment and fertilizer application, compost and biochar significantly increased soil total organic carbon content. The highest soil water extractable organic carbon was found with the compost application (76.9 mg kg−1), whereas the lowest value (50 mg kg−1) was with the highest rate of biochar. Soil respiration rates and hydraulic properties were not affected by the investigated treatments. This behavior is probably related to the short experimental duration and to the silty clayey soil texture. Significant correlations were observed between bulk density and water content at pressure heads in the −20 and −100 cm range; this range accounts for the effect of soil macro and mesopores. Multiple linear regression analysis revealed strong predictive power for grain (R2_adj = 0.78; p < 0.001) and protein yield (R2_adj = 0.77; p < 0.001). The highest grain yield (3.36 Mg ha−1) was observed with compost, and the lowest (2.18 Mg ha−1) with biochar at a rate of 30 Mg ha−1. These findings lay the basis for understanding how different soil amendment management may impact soil quality and wheat performance, even in consideration of climate change.

Potential carbon mineralization assays are confounded by different soil drying temperatures

Measuring carbon dioxide (CO2) produced after re-wetting previously dried soil is an increasingly popular soil health assay, but there is disagreement on the optimal soil drying temperature. We tested whether soil drying temperature impacts water-extractable organic carbon (WEOC) and soil CO2 emissions following rewetting. Soils were collected from corn/soybean croplands and adjacent perennial vegetation at four sites in Iowa, USA. Soil replicates were dried at 22 °C, 35 °C, 55 °C, 85 °C, and rewetted for incubation at 22 °C. Soil WEOC and CO2 emissions after re-wetting increased nonlinearly with drying temperature. Effects on CO2 were largest after four days of incubation, but cumulative differences persisted even after 42 days. Responses of CO2 and its stable isotope ratio (δ13C) to increased drying temperature varied among sites and vegetation types, indicating shifts in C sources. Soil health assays performed with different soil drying temperatures may not be directly comparable, effects of drying temperature may vary idiosyncratically among samples, and drying at 22 °C or 35 °C as opposed to higher temperatures may be preferable to avoid increasing C availability.

Reclaimed Water Use in Agriculture: Effects on Soil Chemical and Biological Properties in a Long-Term Irrigated Citrus Farm

In Mediterranean regions, the scarcity of freshwater for agricultural purposes is leading to the use of alternative water sources. This study aimed to evaluate the effects of long-term irrigation with reclaimed water on chemical and biological soil properties. On a mandarin tree orchard (Citrus clementina, cv. Orogrande), freshwater (FW) and tertiary reclaimed water (RW) were supplied for irrigation. In spring 2017, a soil sampling was carried out, collecting from each experimental plot four samples at 0–0.20 m depth. Chemical and biochemical soil properties were determined on air dried and sieved soil and on fresh and field-moist soil, respectively. The irrigation with reclaimed water significantly increased the soil water extractable organic carbon (WEOC), available P, Mg, and Na content, and the electrical conductivity (EC). Although not significant, the respiration rates and enzymatic activities were higher in RW treatment. The results of this research highlighted that the irrigation with reclaimed water, providing organic carbon and other nutrients, could have, in the long-term, beneficial effects on soil microorganism and their activities. In any case, especially in arid and semi-arid environments, a proper management of wastewater should be recommended to avoid soil degradation due to salt accumulation in the rootzone.

The undetected loss of aged carbon from boreal mineral soils

The boreal forest is among the largest terrestrial biomes on earth, storing more carbon (C) than the atmosphere. Due to rapid climatic warming and enhanced human development, the boreal region may have begun transitioning from a net C sink to a net source. This raises serious concern that old biogenic soil C can be re-introduced into the modern C cycle in near future. Combining bio-decay experiments, mixing models and the Keeling plot method, we discovered a distinct old pre-bomb organic carbon fraction with high biodegradation rate. In total, 34 ± 12% of water-extractable organic carbon (WEOC) in podzols, one of the dominating boreal soil types, consisted of aged (~ 1000 year) labile C. The omission of this aged (i.e., Δ14C depleted) WEOC fraction in earlier studies is due to the co-occurrence with Δ14C enriched modern C formed following 1950s nuclear bomb testing masking its existence. High lability of aged soil WEOC and masking effects of modern Δ14C enriched C suggests that the risk for mobilization and re-introduction of this ancient C pool into the modern C cycle has gone undetected. Our findings have important implications for earth systems models in terms of climate-carbon feedbacks and the future C balance of the boreal forest.

Catchment topography and the distribution of electron donors for denitrification control the nitrate concentration in headwater streams of the Lake Hachiro watershed

ABSTRACT We examined the linkages between topography and electron donors for denitrification on in-stream NO3 − concentration in headwater catchments in the Lake Hachiro watershed having marine sedimentary rock, Japan. In 35 headwater catchments (0.07–16.9 km2), we sampled stream water every season in 2 years. The water samples were analyzed for NO3 –, dissolved nitrous oxide (dN2O), and SO4 2 – concentrations. Stream sediment was sampled once for the measurement of denitrification potential (DP). Water-extractable soil organic carbon (WESOC) and easily oxidizable sulfide (EOS) in the sediment, which can be considered the principal potential electron donors for denitrification, were measured. The topographical features of each catchment were calculated using a digital elevation model with 10-m grid cells. Stream NO3 – concentrations displayed large spatial variation among catchments, ranging from 0.06 to 0.52 mg N L–1, and were negatively correlated with topographic wetness index (TWI) (P < 0.01) and were positively correlated with catchment slope (P < 0.01), indicating that NO3 – concentrations decreased in wetter and gentle slope catchments. Sediment DP and the WESOC content in sediments were positively correlated with TWI, significantly. These results suggested denitrification was likely to occur in higher TWI catchments. Generalized linear model showed that TWI, slope aspect, and sediment DP significantly affected in-stream NO3 – concentration and WESOC was a significant explanatory variable for sediment DP. EOS content in riverbed sediments was not selected as a significant explanatory variable for either in-stream NO3 − concentrations or sediment DP. But higher soil DP with higher EOS was detected in the stream bank subsoil at the catchment where the higher EOS content in the riverbed sediment was observed, which suggested EOS in riverbed sediments can contain site-specific information about denitrification hotspot driven by sulfides. We conclude that catchment topography and the distribution of electron donors in riverbed sediment can be important factors to explain the spatial variation in in-stream NO3 – concentration and sediment DP.

Investigation on labile organic carbons in soils developed from limestone and sandstone in karst areas of southwest China

Although labile soil organic carbons (LSOC) represent only a small part of soil organic carbon (SOC), they are an important source of energy for the belowground portion of the ecosystem and play a crucial role regarding changes in the quality of soil organic matter (SOM). This study measured and compared the content of LSOC and SOC in limestone soil (developed from limestone) and sandstone soil (developed from sandstone) in the karstic area of southwest China to better understand the impacts of different soil types on LSOC (water-extractable and readily oxidizable organic carbon) and evaluate the existence of a possible relationship between LSOC and SOC in the soil. The results demonstrated that the amounts of SOC, soil readily oxidizable carbon (SROC), and soil water-extractable organic carbon (WEOC) (hot-water extractable organic carbon (hot-WEOC) and cold-water extractable organic carbon (cold-WEOC)) were the highest in the surface soil and decreased with increasing depth in both types of soil profiles. In addition, the content of SOC and LSOC were significantly higher in the limestone soil profiles than those in the sandstone soil profiles. This may be as a result of the occurrence of higher pH values and higher content of calcium, magnesium, and clay minerals in the limestone soil than in the sandstone soil. The proportions of WEOC in total SOC extracted at 25°C and 70°C in the sandstone soil profiles displayed higher average values of 0.040% and 0.527%, respectively, whereas the limestone soil profiles displayed lower average values of 0.015% and 0.255% for the WEOC extracted at 25°C and 70°C, respectively. Regarding the SROC content, both soil profiles showed higher values than WEOC content, ranging from 39.4%–55.2%. The results also showed that for the two types of soils significant correlations were found between samples of SROC, WEOC, and SOC (P < 0.01). Among the LSOC, the hot-WEOC showed clear differences between different soil types, and significant relationships were observed for not only SOC but also SROC. The correlation coefficients between hot-WEOC and SOC or SROC were higher than those between cold-WEOC and SOC or SROC. This study, therefore, suggests that hot-WEOC could be a useful indicator of changes in the quality of SOM in different soil types.

Dynamics of soil water extractable organic carbon and inorganic nitrogen and their environmental controls in mountain forest and meadow ecosystems in China

We investigated the differences among two types of forest (pine and oak forests) and a meadow in soil water extractable organic carbon (WEOC), ammonium (NH4+) and nitrate (NO3−) concentrations in the Qinling Mountains, China. Our results showed that soil WEOC, NH4+ and NO3− concentrations were higher in the meadow than in the forests, and they were higher in the pine forest than in the oak forest. The soil surface temperature and soil water content were positively correlated with WEOC and inorganic nitrogen concentrations (P < 0.01). In addition, there was a significantly negative correlation between soil WEOC concentration and pH (P < 0.01). Moreover, soil WEOC concentration was positively correlated with the inorganic nitrogen concentration (P < 0.01). Generally, soil WEOC concentration decreased in the forests, and increased in the meadow under warming condition. The effects of warming on soil WEOC and dissolved inorganic nitrogen (DIN) concentrations varied with time. We elucidated that warming had a significant influence on soil WEOC concentration in the pine forest, whereas soil DIN concentration was affected significantly by warming only in the meadow. Our findings indicate that soil WEOC and DIN show different responses to warming in the mountain forest and meadow ecosystems. This study could provide useful insights for the prediction of soil carbon and nitrogen cycling and their responses to climate change in the complex mountain ecosystem.

Short-term legacy effects of feedlot manure amendments on water-extractable organic carbon in a clay loam soil profile

Limited research exists on legacy effects of land application of feedlot manure on accumulation, redistribution, and leaching potential of water-extractable organic carbon (WEOC) in soil profiles. We sampled a clay loam soil at six depths (0–1.50 m) 2 yr after the last application (2014) of 17 continuous annual manure applications (since 1998). The amendment treatments were stockpiled (SM) or composted (CM) feedlot manure containing straw (ST) or wood-chip (WD) bedding at three application rates (13, 39, and 77 Mg ha−1dry basis). There was also an unamended control (CON) and inorganic fertilizer (IN) treatment. The soil samples were analyzed for concentrations of WEOC. The total mass or accumulation of WEOC in the soil profile was greater (P ≤ 0.05) by 1.2–3.3 times for the CM-ST-77 treatment than 12 of 14 other treatments, and it was significantly greater for amended than CON or IN treatments. The total WEOC mass was 14%–20% greater for CM-ST than CM-WD, SM-ST, and SM-WD treatments, and it was 16%–22% greater for CM than SM at the 39 and 77 Mg ha−1rates. The 77 Mg ha−1rate of the four manure type-bedding treatments had the significantly greatest (by 37%–527%) concentrations of WEOC at the six depths compared with other treatments, suggesting greater redistribution and leaching potential. Significant manure effects occurred on soil WEOC 2 yr after the manure was last applied following 17 continuous applications, and it indicated an increased risk of leaching potential at the higher application rate.

Drought-Induced Accumulation of Root Exudates Supports Post-drought Recovery of Microbes in Mountain Grassland.

Droughts strongly affect carbon and nitrogen cycling in grasslands, with consequences for ecosystem productivity. Therefore, we investigated how experimental grassland communities interact with groups of soil microorganisms. In particular, we explored the mechanisms of the drought-induced decoupling of plant photosynthesis and microbial carbon cycling and its recovery after rewetting. Our aim was to better understand how root exudation during drought is linked to pulses of soil microbial activity and changes in plant nitrogen uptake after rewetting. We set up a mesocosm experiment on a meadow site and used shelters to simulate drought. We performed two 13C-CO2 pulse labelings, the first at peak drought and the second in the recovery phase, and traced the flow of assimilates into the carbohydrates of plants and the water extractable organic carbon and microorganisms from the soil. Total microbial tracer uptake in the main metabolism was estimated by chloroform fumigation extraction, whereas the lipid biomarkers were used to assess differences between the microbial groups. Drought led to a reduction of aboveground versus belowground plant growth and to an increase of 13C tracer contents in the carbohydrates, particularly in the roots. Newly assimilated 13C tracer unexpectedly accumulated in the water-extractable soil organic carbon, indicating that root exudation continued during the drought. In contrast, drought strongly reduced the amount of 13C tracer assimilated into the soil microorganisms. This reduction was more severe in the growth-related lipid biomarkers than in the metabolic compounds, suggesting a slowdown of microbial processes at peak drought. Shortly after rewetting, the tracer accumulation in the belowground plant carbohydrates and in the water-extractable soil organic carbon disappeared. Interestingly, this disappearance was paralleled by a quick recovery of the carbon uptake into metabolic and growth-related compounds from the rhizospheric microorganisms, which was probably related to the higher nitrogen supply to the plant shoots. We conclude that the decoupling of plant photosynthesis and soil microbial carbon cycling during drought is due to reduced carbon uptake and metabolic turnover of rhizospheric soil microorganisms. Moreover, our study suggests that the maintenance of root exudation during drought is connected to a fast reinitiation of soil microbial activity after rewetting, supporting plant recovery through increased nitrogen availability.

A case study on the method-induced difference in the chemical properties and biodegradability of soil water extractable organic carbon of a granitic forest soil

Water extractable organic carbon (WEOC) plays important roles in soil dissolved organic matter (DOM) research. In the present study, we have detected the chemical properties and biodegradability of WEOC obtained from one granitic forest soil with four commonly used or suggested extraction methods, to study the potential methodological influence in soil DOM research. Results showed great difference in both chemical properties and biodegradation of WEOC from various methods. For the chosen soil, compared to that from fresh soil, WEOC from dried soil contained large proportion of HIN, Base fractions and labile O-alkyl components which might be derived from microbial cell lysis, and showed low fluorescence characteristics, exhibiting great biodegradability. Similarly, WEOC extracted under low temperature and short time conditions showed low fluorescence characteristics and exhibited considerable biodegradability. Conversely, WEOC, which might be potentially subjected to decomposition and loss during extraction, contained higher percentages of HOA fractions and aromatic alkyl and aryl components, and showed high fluorescence characteristics, exhibiting low biodegradability. WEOC extracted in moderate time and temperature showed moderate biodegradability.These method-induced differences implied the direct comparison of the results from similar works is difficult, as we considered here a specific forest soil and other authors other soil types and uses. However, the complexity in comparison reminds that the methodological influence be paid more attention in future soil WEOC researches.

The sensitivity of water extractable soil organic carbon fractions to land use in three soil types

ABSTRACTSoil organic carbon (SOC) has a high impact on the sustainability of ecosystems, global environmental processes, soil quality and agriculture. Long-term tillage usually leads to SOC depletion. The purpose of this study was to determine the impact of different land uses on water extractable organic carbon (WEOC) fractions and to evaluate the interaction between the WEOC fractions and other soil properties. Using an extraction procedure at 20°C and 80°C, two fractions were obtained: a cold water extractable organic carbon (CWEOC) and a hot water extractable organic carbon (HWEOC). The results suggest that there is a significant impact from different land uses on WEOC. A lower relative contribution of WEOC in SOC and a lower concentration of labile WEOC fractions are contained in arable soil compared to forestlands. Chernozem soil was characterized by a lower relative contribution of WEOC to the SOC and thus higher SOC stability in contrast to Solonetz and Vertisol soils. Both CWEOC and HWEOC are highly associated with SOC in the silt and clay fraction (<53 µm) and were slightly associated with SOC in the macroaggregate classes. The WEOC fractions were highly and positively correlated with the SOC and mean weight diameter.

Changes in Water-Extractable Organic Carbon with Cover Crop Planting under Continuous Corn Silage Production

Long-term row crop agricultural production has dramatically reduced the pool of soil organic carbon. The implementation of cover crops in Midwestern agroecosystems is primarily to reduce losses of nitrogenous fertilizers, but has also been shown to restore soil carbon stocks over time. If labile carbon within agricultural soils could be increased, it could improve soil health, and if mobilized into subsurface drainage, it may positively impact watershed biogeochemistry. We tested for potential differences in water-extractable organic carbon (WEOC) at two different soil profiles (0-5 cm and 5-20 cm) between plots planted with cereal rye/daikon radish (cover crop), corn, and zero control (no vegetation) within the Illinois State University Research and Teaching Farm. We also tested for potential differences in denitrification within the upper soil profile throughout the growing year. We modeled excitation–emission matrices from soil cores through parallel factor analysis. We found no difference in WEOC concentrations between each crop treatment ( P = 0.2850), but concentrations of WEOC were significantly lower in the 5-20 cm profile than that in the upper (0-5 cm) profile ( P = 0.0033). There was a significant increase in WEOC after each treatment in samples after cover crop termination. The parallel factor analysis model found humic and fulvic acids to be the dominant fractions of WEOC in all soils tested. Humic and fulvic acids accounted for ~70% and 30% of model variation. Denitrification rates did not differ across treatments ( P = 0.3520), which is likely attributed to soil WEOC being in limiting quantities and in primarily recalcitrant fractions. After three years, cover crops do not appear to alter soil WEOC quantity and type. Restoring the availability of carbon within agricultural soils will not be a short-term fix, and fields will likely be a net carbon sink, contributing minimal labile carbon to receiving waterways.

중적외선분광분석법을 이용한 토양 유기 탄소 분획 분석

기후변화 문제와 관련하여 탄소 격리와 토양 유기물의 중요성에 대한 관심이 증가하고 있다. 토양 탄소 격리를 산정하기 위해서는 물 추출 토양 유기탄소(WESOC)와 토양 호흡에 의해 이산화탄소로 배출되는 탄소량과 같은 토양 유기탄소를 분석하는 것이 중요하다. 이러한 성분의 분석에는 시간이 많이 소요된다. 따라서 본 연구에서는 분석시간이 짧게 소요되는 중적외선분광분석법으로 물 추출 유기탄소와 토양 호흡에 의한 이산화탄소량을 분석할 수 있는 가능성을 알아보았다. 토양을 <TEX>$100^{\circ}C$</TEX>와 <TEX>$350^{\circ}C$</TEX> 건조오븐에서 처리하고 중적외선분광계로 분석하여 WESOC와 30일, 60일, 90일, 120일 간 토양호흡에 의해 발생하는 이산화탄소량과의 상관을 분석하였다. 물 추출 토양 유기탄소에 대한 예측 모델에서는 표준 일반 변수화(SNV) 전처리를 통해 0.6937의 결정 계수를 보였고 30일간의 토양 호흡 발생 이산화탄소 예측 모델에서는 <TEX>$350^{\circ}C$</TEX> 건조 토양에 대해 1차 도함수 전처리를 통해 0.8933의 결정 계수를 보여 중적외선분광분석법을 사용하여 토양 중 유기탄소의 분획별 정량에 사용할 수 있는 가능성을 보였다. For concerning the climate change issues, the carbon sequestration and importance of soil organic matter are receiving high attention. To evaluate carbon sequestration in soil is important to determine the soil organic carbon (SOC) fractions such as WESOC (Water extractable soil organic carbon), and <TEX>$CO_2$</TEX> emission by soil microbial respiration. However, the analyses for those contents are time-consuming procedure. There were studied the feasibility of MIRS (Mid-Infrared Spectroscopy), which has short analysis time for determining the WESOC and an incubated carbon in this study. Oven-dried soils at <TEX>$100^{\circ}C$</TEX> and <TEX>$350^{\circ}C$</TEX> were scanned with MIRS and compared with the chemically analyzed WESOC and cumulative carbon dioxide generated during 30, 60, 90, and 120 days of incubation periods, respectively. It was observed that an optimized determination coefficient was 0.6937 between WESOC and untreated soil processed by spectrum vector normalization (SNV) and 0.8933 between cumulative <TEX>$CO_2$</TEX> from 30 days incubation and soil dried at <TEX>$350^{\circ}C$</TEX> after subtracting air-dried soil processed by 1st derivatives. Therefore, it was shown that Quantification of soil organic carbon fractions was possibility to be analyzed by using MIRS.

Solubility and Leaching Risks of Organic Carbon in Paddy Soils as Affected by Irrigation Managements

Influence of nonflooding controlled irrigation (NFI) on solubility and leaching risk of soil organic carbon (SOC) were investigated. Compared with flooding irrigation (FI) paddies, soil water extractable organic carbon (WEOC) and dissolved organic carbon (DOC) in NFI paddies increased in surface soil but decreased in deep soil. The DOC leaching loss in NFI field was 63.3 kg C ha−1, reduced by 46.4% than in the FI fields. It indicated that multi-wet-dry cycles in NFI paddies enhanced the decomposition of SOC in surface soils, and less carbon moved downward to deep soils due to less percolation. That also led to lower SOC in surface soils in NFI paddies than in FI paddies, which implied that more carbon was released into the atmosphere from the surface soil in NFI paddies. Change of solubility of SOC in NFI paddies might lead to potential change in soil fertility and sustainability, greenhouse gas emission, and bioavailability of trace metals or organic pollutants.

Spatial variation of denitrification potential of grassland, windbreak forest, and riparian forest soils in an agricultural catchment in eastern Hokkaido, Japan

Riparian forest soils have a high denitrification potential (DP), but the effect of their spatial distribution on DP within a catchment has rarely been assessed. To provide insights into this effect, we evaluated the spatial distribution of DP and its control factors in the Shibetsu River Catchment (SRC), eastern Hokkaido, Japan. Soil samples from August 2008 were used in the lab to determine the DP (using the acetylene inhibition method) of grassland, windbreak forest, and riparian forest soils in the SRC. Soil DP ranged from 0.001 to 64mgNkgsoil−1d−1 and followed a log-normal distribution. Soil DP was highest in the surface peat soil of a low-elevation riparian forest. The land-use type and soil depth both significantly affected soil DP, with higher DP in riparian forests and surface soils. Soil DP increased with increasing soil moisture content and water-extractable soil organic carbon (WESOC). The clay, moisture, and WESOC contents in the surface soils decreased significantly with increasing elevation, resulting in increased soil DP in low-elevation riparian forests. The stream NO3− concentration was significantly correlated with the upland (grassland and cropland) proportion (%) of the catchment's area, and the ratio of stream NO3− concentration to this proportion tended to be lower in low-elevation areas, indicating higher NO3− denitrification. We conclude that low-elevation riparian forest soils have a higher DP due to the higher moisture, clay, and WESOC contents of their soils, and that these factors control riverine NO3− export in the SRC.

Water-extractable soil organic matter inhibits phosphatase activity

We show that water-extractable soil organic matter may inhibit activity of added phosphatase. The inhibition was greater for the topsoils than the corresponding subsoils. The enzyme kinetics in buffer and in soil solutions followed Michaelis–Menten kinetics. Vmax decreased and Km increased with increasing soil water-extractable organic carbon. Both kinetic parameters followed similar trends with the intensities of both fluorescence peaks of the solutions. The effect was more marked at pH below the optimum pH. Inhibition was complex, particularly for topsoil solutions where Vmax was decreased and Km increased. This effect could lead to underestimation of catalytic activity of enzymes extracted from soil and could result in the overestimation of decreased specific activity in the adsorbed phase of enzymes added to soils.

Water-Extractable Soil Organic Carbon and Nitrogen Affected by Tillage and Manure Application

Water-extractable organic matter (WEOM) contains labile organic carbon (C) and nitrogen (N) and is sensitive to soil management. The objective of this study was to determine the quantity and spectral properties of soil WEOM responding to types of nutrient addition and tillage managements. Soil samples were taken from treatments (conventional tillage, no-tillage, poultry litter, and NH4NO3 application) of a 10-year experiment in Watkinsville, Georgia, at different times and soil depths. Air-dried soil samples were extracted by deionized water followed by filtration (<0.45 μm), and soluble C and N, absorptivity at 254 nm, and fluorescence excitation/emission matrix in the extract were determined. Results showed that poultry litter application accumulated higher amounts of water-extractable organic C and N than NH4NO3 application. But no significant difference was found between tillage and no-tillage for both water-extractable C and N. Absorptivity at 254 nm decreased during cropping years for conventional tillage and NH4NO3 application, indicating accumulation of aliphatic organic compounds in WEOM, but the fluorescence-measured humification index showed an opposite trend. For no-tillage and poultry litter application treatments, there was no difference in absorptivity over the sampling time. Parallel factor analysis of excitation/emission matrix data showed that WEOM consisted of three fluorophore components: humic-, fulvic-, and tyrosine-like "compounds," and relative distribution of these components differed among treatments and changed over soil depths. The tyrosine-like component tended to accumulate in the lower sampling depth (>15 cm). Overall, the quantity and spectral properties of WEOM responded to the types of nutrient application and tillage practices.