Bulk optical characterization of dissolved organic matter from semiarid wheat-based cropping systems Authors: Carlos M. Romero, Richard E. Engel, Juliana D'Andrilli, Chengci Chen, Catherine Zabinski, Perry R. Miller, & Roseann Wallander. NOTICE: this is the author’s version of a work that was accepted for publication in Geoderma. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Geoderma, [vol. 306, November 2017]. DOI#10.1016/ j.geoderma.2017.06.029 Romero C.M., R.E. Engel, J. D'Andrilli, C. Chen, C. Zabinski, P.R. Miller, R. Wallander, "Bulk optical characterization of dissolved organic matter from semiarid wheat-based cropping systems," Geoderma 306, (November 2017): 40-49. Made available through Montana State University’s ScholarWorks scholarworks.montana.edu http://scholarworks.montana.edu/ http://scholarworks.montana.edu/ https://doi.org/10.1016/j.geoderma.2017.06.029 https://doi.org/10.1016/j.geoderma.2017.06.029 Bulk optical characterization of dissolved organic matter from semiarid wheat-based cropping systems Carlos M. Romero, Richard E. Engel Juliana D'Andrilli, Chengci Chen, Catherine Zabinski, Perry R. Miller, Roseann Wallander Keywords: Semiarid soil Winter wheat Conservation agriculture Excitation emission matrix Parallel factor analysis Dissolved organic matter (DOM) plays a critical role in the cycling of nutrients and long-term agricultural sustainability. The composition of DOM in soil is likely altered due to management, yet there is limited knowledge on the effect of long-term cropping on DOM chemical character. Here, we characterized water ex-tractable DOM composition along a gradient of soil organic carbon (SOC) affected by differing cropping and tillage intensity in a semiarid climate of the northern Great Plains, USA. Soil samples (0–10, 10–20, 20–30 cm) were collected from conventional till-fallow winter wheat (Triticum aestivum L.; Ftill-W), no-till spring pea/oil-seed-wheat (Pisum sativum L.; Pg/O-W), and no-till continuous wheat (W-W) fields, and analyzed using UV/Vis absorbance and excitation-emission matrix fluorescence spectroscopy. The concentration of DOM decreased with depth and was significantly greater (P < 0.05) under W-W or Pg/O-W than Ftill-W. The absorbance at 254 nm (Abs254), a proxy for DOM aromatic nature, indicated that aromaticity decreased with depth and lower biomass-C inputs (i.e. W-W≥ Pg/O-W ≥Ftill-W). Multidimensional parallel factor (PARAFAC) analysis revealed humic-like (C1, C2), monolignol-like (C3), and protein/ tannin-like (C4) components with varying fluorescence in-tensities as a function of cropping system and soil depth. DOM humification, indicated by the humification index (HIX), increased significantly with depth (P < 0.05) and was higher for Ftill-W (2.95) than W-W (2.61) or Pg/O-W (2.28). Overall, DOM became depleted of plant-derived constituents and was enriched by more decomposed, condensed substances in Ftill-W, as compared to W-W or Pg/O-W soils. DOM composition is strongly affected by cropping intensity and such changes are important drivers controlling SOC accretion in arable soils. Dissolved organic matter (DOM) is the most dynamic and reactive component of soil organic carbon (SOC) in terrestrial ecosystems (Bolan et al., 2011). Although this pool accounts for a minimal portion of SOC (i.e. < 0.5–1%), DOM components exert a critical control over the biogeochemistry of soils by serving as a substrate for microbial activity and influencing the availability of plant nutrients and metal ions (Kalbitz et al., 2000; Bolan et al., 2011). The occurrence of DOM in soil is also critical to regulate both CH4 and N2O production (Bolan et al., 2011). Furthermore, terrestrial DOM is an important source of organic matter in freshwater ecosystems, affecting drinking water supplies through eutrophication and nutrient runoff (Wilson and Xenopoulos, 2009). DOM is a heterogeneous mixture of plant and microbial derived constituents with varying degrees of reactivity (Bolan et al., 2011). Chemically, DOM can be conceptualized in terms of biologically active and stable humified fractions. Biologically active compounds are both aliphatic (i.e. amino acids, carbohydrates) and aromatic constituents (i.e. fulvic acids) of low molecular weight rapidly metabolized by soil microorganisms (Vázquez et al., 2016; Pan et al., 2017). Contrarily, humified pools are complex aromatic substances derived from decom-posing lignocellulosic polymers (Stevenson, 1994; Vázquez et al., 2013) that can exhibit strong resistance against biodegradation (Marinari et al., 2010). Several factors affect the quantity and quality of SOC and the water extractable DOM fraction in arable soils (Kalbitz et al., 2000). Besides soil temperature and water content (Burke et al., 2008), management practices (i.e. tillage, crop rotation) can influence SOC occurrence by (i) modifying soil physical structure and aggregation (Bongiovanni and Lobartini, 2006; Dieckow et al., 2009), (ii) changing the amount and quality of biomass-C inputs returned to the system (Chantigny, 2003; MARK Vázquez et al., 2016), and (iii) altering mineralization and humification pathways of plant residues in soil (Kalbitz et al., 2000; Vázquez et al., 2016). No-tillage (NT) management is often claimed to promote SOC accretion in surface soil (i.e. 0–30 cm) (Dieckow et al., 2009; Norton et al., 2012). However, the effect of long-term cropping intensity on SOC quality is not clear and still under debate; management practices may differentially affect SOC composition depending on site-specific environmental factors, such as climate and soil type (Embacher et al., 2007; Borisover et al., 2012). In the northern Great Plains of North America, conversion from native semiarid shortgrass steppe to dryland wheat-fallow (W-F) crop- ping has resulted in severe soil erosion and a net loss of nutrients (DeLuca and Zabinski, 2011). A century of cultivation has decreased surface SOC and associated C fractions by ~50–60% (Norton et al., 2012; Hurisso et al., 2013). Recently, NT management has become increasingly popular, currently comprising 60% of dryland agriculture in this region (Hansen et al., 2012). With NT management, precipita- tion storage has improved and led to greater cropping system in- tensification with legume and/or oilseed crops being grown in rotation with wheat (Miller et al., 2015; Chen et al., 2015). More diverse and intensified NT systems have prompted SOC recovery through increased biomass-C inputs and minimal disturbance of soils. Several reports have documented higher SOC contents under NT annual cropping than conventional tillage (CT) with W-F systems (reviewed by Watts et al., 2011 and Collins et al., 2012). A recent study by Engel et al. (2017) quantified SOC levels among eight management systems in southern Montana, including CT fallow-wheat (Ftill-W), NT spring pea/oilseed- wheat (Pg/O-W) and NT continuous wheat (W-W). After 10 yr, cumu- lative SOC stocks (0–30 cm) for NT WeW, NT Pg/O-W and Ftill-W sys- tems, average of two N levels, were equivalent to 38.4, 37.9, and 33.7 Mg C ha−1, respectively. The degree to which SOC accumulated in soil was directly related to biomass-C inputs, particularly root and/or root-derived C. Engel et al. (2017) report SOC maintenance levels at 2.6 Mg ha−1 yr−1 of shoot + root + rhizodeposit C, or 7.0 Mg ha−1 yr−1 of net primary productivity (NPP), with accretion or loss occurring above or below these thresholds, respectively. Accumulation of SOC after 10 yr of NT annual cropping (Engel et al., 2017) suggests recovery trends towards improved soil quality. Within this context, some differences in SOC composition were ex- pected. The objective of this study was to characterize the chemistry of SOC following a continuum from minimally disturbed soils with high biomass-C inputs (i.e. NT annual cropping) to more strongly degraded fields (CT W-F). We extracted and further analyzed DOM from soil samples (0–30 cm) collected in 2012 along Ftill-W, Pg/O-W and W-W fields (Engel et al., 2017) by combined use of non-destructive spec- troscopic techniques such as UV/Vis absorbance and excitation emission matrix fluorescence spectroscopy (EEMs). We hypothesized that (i) additional biomass-C inputs under NT annual cropping (Pg/O- W, W-W) would increase plant-derived DOM constituents when com- pared to Ftill-W, (ii) such changes would be associated with increasing SOC sequestration rates, and (iii) the influence of cropping systems on DOM properties is attenuated from surface soil to subsoil layers. 2. Materials and methods 2.1. Experimental site and study design The soil analyzed in this study came from a long-term cropping system study established at Montana State University's Arthur H. Post Agronomy Research Farm, located 10 km west of Bozeman, Montana, USA (45° 40′ N, 111° 09′ W, elevation 1450 m). The location is char- acterized by a semiarid cold climate with annual precipitation of 411 mm and a mean annual temperature of 6.7 °C (Western Regional Climate Center, 2014). The soil is a Mollisol-type, classified as an Amsterdam silt loam (fine-silty, mixed, superactive, frigid Typic Hap- lustolls), generally deep and well-drained, and derived from loess-like deposits mixed with volcanic ash (Soil Survey Staff, 2013). Selected soil properties to a depth of 30 cm are presented in Table 1. The field study consisted of eight management systems replicated four times in a ran- domized complete block design, previously described by Engel et al. (2017). The water extractable DOM was characterized in three of the eight cropping systems, including Ftill-W, Pg/O-W and W-W managed at a high N fertility level, and where large differences in SOC mass ac- cretion were observed. 2.2. Soil sampling and processing Soil sampling occurred in September 2012, or 10 yr after initiation of the field trial, using a truck mounted hydraulic probe. Triplicate soil cores (5 cm dia.) were collected to a depth of 30 cm from each plot, separated into three depth increments (0–10, 10–20, and 20–30 cm) and composited per depth. The composite core samples were placed in plastic-lined bags, dried (50 °C), and then crushed to pass through a 2- mm sieve. A subsample of the sieved material was then separated, and all identifiable crop residue material including roots were removed by hand-picking with tweezers prior to its analysis. 2.3. Dissolved organic matter Soil (50 g) was added to an acid-washed (10% HCl), pre-combusted (425 °C, 4 h) 250-mL Erlenmeyer flask with 100 mL of ultrapure Milli- Q® water (≤18.2 MΩ cm−1). The flasks were shaken (15 min, Soil depth Texture BDa CECb,g ECc,h pHi ICd SOCe,j TNf,j C:N Sand Silt Clay cm g kg−1 g kg−1 g kg−1 g cm−3 cmolc kg−1 dS m−1 g kg−1 g kg−1 g kg−1 0–10 173 613 214 1.16 18.41 0.11 6.28 0.58 13.0 1.3 9.7 10–20 163 606 231 1.34 17.97 0.07 6.59 0.87 9.6 1.1 8.7 20–30 158 630 212 1.43 19.42 0.05 7.22 1.64 7.7 0.9 8.5 a BD = bulk density. b CEC = cation exchange capacity. c EC = electrical conductivity. d IC = inorganic carbon. e SOC = soil organic carbon. f TN = total nitrogen. g Unbuffered 0.2 M NH4Cl. h 1:2 soil:water extract. i 1:1 soil:water extract. j LECO dry combustion (LECO Corp., St. Joseph, MI). Data taken from Engel et al. (2017). Table 1 Selected physical and chemical properties of soil at the experimental site. 100 rpm) and then allowed to settle for 15 min at room temperature (25 °C). The supernatant was subsequently passed through a pre-com- busted Fisherbrand Glass Fiber Circle (Grade 6, 1.6 μm) and then va- cuum filtered through a pre-combusted 0.7 μm Millipore glass fiber filter (EMD Millipore, Merk KGaA, Germany). Extracts were transferred to opaque, acid-washed, and pre-combusted glass bottles and stored in the dark at 4 °C prior to analysis within 24 h of sample extraction. Filtered Milli-Q® water blanks were collected in the same way as for the soil extracts. Water-extractable DOM concentration, expressed as dis- solved organic carbon ([DOC]; mg C L−1) and total nitrogen ([WEN]; mg N L−1), was quantified in 15 mL aliquots using a total C and N Shimadzu combustion analyzer (TOC-VCSH, Shimadzu Corp., Kyoto, Japan). 2.4. UV–Vis absorbance spectroscopy Ultraviolet and visible light absorbance was measured in a 1 cm path-length quartz cuvette using a Genesys 10 Series spectrophotometer from 190 to 1100 nm (Thermo-Scientific, Rochester, NY) and VISIONlite software. Specific UV absorbance at 254 nm (SUVA254, L mg−1 C m−1), a proxy for DOM aromatic nature, was calculated as follows: SUVA254 = 100 × [Abs254 / (DOC)], where L is the optical path length (cm) and DOC is in mg C L−1 (Weishaar et al., 2003). Samples with absorbance values at 254 nm (Abs254) > 0.3 a.u. (absor- bance units) were diluted to volumes with Abs254 < 0.3 a.u. prior to collecting the entire absorbance and fluorescence spectra (Miller and McKnight, 2010). Dilution factors (2 and 4; diluted by half to a quarter the original concentrations) were subsequently used when post-pro- cessing the EEMs fluorescent data; UV/Vis absorbance values from 190 to 1100 nm were used to calculate necessary spectral corrections for primary and secondary inner filter effects (Tucker et al., 1992). Meth- odological and system blanks were run at the beginning of the experi- ment and between five sample sets. 2.5. Fluorescence spectroscopy EEMs were obtained with a Horiba Jobin Yvon Fluoromax-4 spec- trofluorometer (Horiba) equipped with a Xe lamp light source and 1 cm path-length quartz cuvette at 25 °C with the following specifications: excitation wavelengths of 240–450 nm scanned over 5 nm intervals, emission wavelengths of 300–560 nm recorded in 2 nm increments, data integration period 0.25 s, and 5 nm band pass for both excitation and emission monochromators. All data were generated in signal/re- ference mode to normalize the emission signal relative to the excitation light intensity. EEMs were normalized by the area under the Raman peak of a Milli-Q® water sample each day at Ex = 350 nm and Em = 365–450 nm. Post-collection data manipulation was performed to correct for inner filter effects, Raman scattering, and our method blank subtraction following the protocol outlined in D'Andrilli et al. (2017). System blanks of Milli-Q® water were scanned daily to monitor instrument performance, and also were used to post-process the EEMs of our methodological blanks to check for contamination. DOM humification was measured from the EEMs data by calculating the humification index (HIX) (Zsolnay et al., 1999) for each sample. Briefly, the area in the upper quarter (434–480 nm; Em peak fluores- cence intensities) was divided by the area in the lower quarter (300–344 nm) at Ex = 255 nm (Zsolnay et al., 1999). HIX is propor- tional to DOM condensation or humified state, and has been used to differentiate sources of DOM (i.e. fulvic acids vs. microbial biomass) upon soil drying (Zsolnay et al., 1999). The Fluorescence Index (FI) was calculated as the ratio of fluorescence emission intensity 470 nm and 520 nm (in Raman Units; R.U.), obtained at an excitation wavelength of 370 nm (Cory and McKnight, 2005). This index allows for discrimina- tion between microbial (high FI ~1.8, bacteria leachate, algae) or terrestrially derived (low FI ~1.2, plant and soil) DOM (Cory and McKnight, 2005). 2.6. PARAFAC modelling – Multivariate analysis Parallel factor (PARAFAC) analysis was applied to the EEMs data using drEEM (Decomposition Routines for Excitation Emission Matrices; v. 0.3.0) and the N-way toolbox in MATLAB R2016b (The MathWorks, Inc., Natick, MA) (Murphy et al., 2013; Stedmon and Bro, 2008). A non-negativity constraint was applied to both excitation and emission loadings, considering that negative fluorescent intensities and concentrations are chemically impossible (Ohno and Bro, 2006). No samples were identified as outliers, and PARAFAC analysis resolved four components comprising the 36 EEMs normalized data set, ex- plaining 99.8% of the total variance. The PARAFAC model was vali- dated by split-half analysis with all the components of the split model test finding a match with a Tucker correlation coefficient > 0.95 (Murphy et al., 2013). The Fluorescence Efficiency (FE, R.U. cm−1), or quantum yield, was calculated by taking the ratio of PARAFAC com- ponent fluorescence maxima divided by the Abs254, with higher FE indicating lower DOM size and/or molecular weight (Wu et al., 2003). 2.7. Linear statistical analyses All statistical analyses were carried out using PROC GLM of SAS 9.4 (SAS Institute, Cary, NC). The effects of cropping system and soil depth on DOM size and spectroscopic features were analyzed with a two-way ANOVA. The assumptions of normal distribution and equal variance were tested using Shapiro–Wilk and Levene's tests, respectively. When significant effects were observed, treatment means were compared using Fisher's least significant difference (LSD) procedure at α = 0.05. Single degree of freedom orthogonal contrasts were performed for the cropping system x soil depth interaction. Pearson correlation coeffi- cients (r-value) were used to examine relationships between DOM properties. All figures were developed with SigmaPlot 13.0 software (Systat Software, San Jose, CA). 3. Results and discussion 3.1. Dissolved organic carbon concentration and C:N ratio The distribution of DOC concentration, among each of the three soil depths and cropping systems, was summarized by box and whisker plots (Fig. 1). DOC concentration decreased with soil depth (P < 0.0001; Table 2). The 0–10 cm layer contained more DOC than the 10–20 and 20–30 cm layers, averaging 27.90, 14.23, and 12.85 mg C L−1, respectively (Fig. 1). The contrast cropping system vs. soil depth showed larger DOC decreases under Pg/O-W and W-W than Ftill-W (−50.05, −58.42% and −41.19%, respectively) (P = 0.0178; Table 2). DOC concentration was lowest in Ftill-W, and highest in Pg/O- W or WeW, averaging 14.25, 19.31, and 21.42 mg C L−1, respectively (Fig. 1). Furthermore, DOC concentration was strongly correlated to bulk SOC (r = 0.81, P < 0.0001) and TN levels (g kg−1) (r = 0.75, P < 0.0001) reported by Engel et al. (2017) (Table 1). The effect of cropping system on DOC concentration was consistent with the 10 yr change in C supply associated with NT annual cropping (Engel et al., 2017). Higher cumulative above- and below-ground C inputs (i.e. shoots, roots, and rhizodeposits) under Pg/O-W and W-W (29.1 and 30.7 Mg C ha−1) when compared to Ftill-W (21.1 Mg C ha−1) increased SOC accretion and associated soil DOM fractions. This result was expected given the fact DOM is mostly replenished in solution by bulk SOC (Kalbitz et al., 2000; Akagi and Zsolnay, 2008). Soils at this field site (O'Dea et al., 2015) managed under F-W have poorer ag- gregate stability than NT annual cropping (101 and 192–220 g kg−1), suggesting that physical protection likely prompted the accumulation of DOM in W-W or Pg/O-W fields. The C:N ratio of DOM decreased with soil depth from 11.04 to 5.87 (Fig. 1). However, no significant differences were noted in DOM C:N ratio values among cropping systems (P > 0.05) (Table 2). The observed higher C:N in the 0–10 cm layer clearly indicated an enrich- ment of N upon increasing soil depth (Toosi et al., 2012) regardless of the cropping system. 3.2. Optical characterization of DOM 3.2.1. Spectroscopic indices Abs254, SUVA254, HIX, FI, and FE Estimates of Abs254 were significantly affected by soil depth and cropping system (Table 2). The Abs254 decreased from 0.55 to 0.10 with increasing soil depth (Fig. 2). This is consistent with results reported by Hassouna et al. (2012) and Toosi et al. (2012) and is mainly ascribed to adsorptive interactions of aromatic moieties with reactive mineral surfaces (Kaiser and Guggenberger, 2000), e.g. Ca2+-bridging in neu- tral/alkaline soils (Oades, 1988). The Abs254 properties of DOM also changed with cropping system, and averaged 0.22, 0.31, and 0.35 for Ftill-W, Pg/O-W, and W-W, respectively (Fig. 2). Higher inputs of plant biomass-C under NT annual cropping likely increased the concentration of lignin-like materials and/or other hydrophobic structures in soil (Capriel, 1997; Vázquez et al., 2016), further increasing DOM aromatic nature (Hassouna et al., 2012; Gao et al., 2017). The latter was sub- stantiated by the modest (r = 0.49) but significant positive correlation (P < 0.01) between the Abs254 and the C:N ratio (Table 3). The overall depth pattern for SUVA254 was similar to Abs254; SUVA254 decreased from 1.96 to 0.76 with increasing soil depth (Fig. 2). However, such trend was stronger under Ftill-W than Pg/O-W and W-W (−55.77, −32.37% and −28.95%, respectively) (P = 0.0121; Table 2). Crop- ping system did not affect DOM SUVA254 estimates (P > 0.05) (Table 2). Similarly, Abs254 yielded more pronounced relationships with other DOM properties than SUVA254 (Table 3). HIX increased significantly with soil depth from 2.22 to 2.93 (Fig. 2). This implied that DOM potentially contained old, highly pro- cessed structures within subsoil layers (Rumpel and Kogel-Knabner, 2011). Such a possibility is consistent with the modest, but significant negative correlation of the HIX and the C:N ratio (r = −0.44; P < 0.01) (Table 3). The content of N within soil organic matter is typically enriched by humification (Ewing et al., 2006). The Fig. 1. Box and whisker plots of (a) dissolved organic carbon (DOC) concentration (mg C L−1) and (b) C:N ratio from different cropping systems (Ftill-W, conventional till fallow winter wheat; Pg/O-W no-till spring pea/oilseed- wheat; W-W no-till continuous wheat) and soil depths (0–10, 10–20 and 20–30 cm). Box plots show the 25 and 75 percentile ranges; solid and dashed inbox lines represent the median and mean, respectively; whiskers delimit the non-outlier ranges; and the symbol (x) represent outlier values. Different letters indicate significant differences (LSD Fisher test, P < 0.05). Table 2 Two-way ANOVA of the effect of cropping system and soil depth on dissolved organic carbon (DOC) concentration, C:N ratio, absorbance at 254 nm (Abs254), specific UV absorbance at 254 nm (SUVA254), humification index (HIX) and fluorescence index (FI) of DOM soil extracts. Factors df DOC C:N Abs254 SUVA254 HIX FI F P F P F P F P F P F P Cropping system (CS) 2 6.70 0.0049 0.17 0.8439 4.14 0.0286 2.16 0.1369 3.91 0.0338 2.07 0.1484 Soil depth (D) 2 34.19 < 0.0001 11.87 0.0003 46.57 < 0.0001 57.63 < 0.0001 4.64 0.0197 1.82 0.1839 CS × D 4 2.37 0.0806 1.18 0.3476 0.70 0.6017 2.00 0.1274 0.19 0.9411 0.23 0.9217 0–10 vs. 10–30 × Ftill-W vs. NT CSa 1 6.47 0.0178 0.52 0.4785 2.13 0.1579 7.38 0.0121 0.37 0.5482 0.44 0.5114 a Ftill-W, conventional till fallow winter wheat; NT CS (Pg/O-W no-till spring pea/oilseed-wheat and W-W no-till continuous wheat). Fi g. 2. Bo x an d w hi sk er pl ot s of (a ) ab so rb an ce at 25 4 nm (A bs 2 5 4 ), (b ) sp ec ifi c U V ab so rb an ce at 25 4 nm (S U V A 2 5 4 ), (c ) hu m ifi ca ti on in de x an d (d ) fl uo re sc en ce in de x fr om di ff er en tc ro pp in g sy st em s (F ti ll -W ,c on ve nt io na lt ill fa llo w w in te r w he at ; P g /O -W no -t ill sp ri ng pe a/ oi ls ee d- w he at ;W -W no -t ill co nt in uo us w he at ) an d so il de pt hs (0 –1 0, 10 –2 0 an d 20 –3 0 cm ). Bo x pl ot s sh ow th e 25 an d 75 pe rc en ti le ra ng es ;s ol id an d da sh ed in bo x lin es re pr es en t th e m ed ia n an d m ea n, re sp ec ti ve ly ; w hi sk er s de lim it th e no n- ou tl ie r ra ng es ;a nd th e sy m bo l (x ) re pr es en t ou tl ie r va lu es .D iff er en t le tt er s in di ca te si gn ifi ca nt di ff er en ce s (L SD Fi sh er te st ,P < 0. 05 ). complexation of microbial biomass within mineral surfaces buffers N into more condensed, recalcitrant forms; narrowing soil C:N ratios (Ewing et al., 2006; Normand et al., 2017), yet increasing HIX (Martins et al., 2011). Across all depths, HIX varied from 2.95 in Ftill-W to 2.28 in Pg/O-W soil (Fig. 2). The higher HIX under Ftill-W suggested that DOM was depleted of more labile-C components. Highly disturbed arable soils have more humified SOC than NT (Milori et al., 2006; Martins et al., 2011) or grassland soils (Kalbitz et al., 1999). The physical breakdown of aggregates under CT fosters the microbial oxidation of native SOC and precludes the stabilization of fresh plant- and/or mi- crobial-degradation byproducts (Ewing et al., 2006; Dieckow et al., 2009). The distribution of FI was not affected by cropping system or soil depth (Table 2), and oscillated between 1.23 and 1.44 (Fig. 2); lower than the estimated average for arable soils (i.e. 1.50–1.90) (Hassouna et al., 2012; Sun et al., 2017). Terrestrial, plant-derived material dominated DOM fluorescence (Cory and McKnight, 2005); our sampling scheme was perhaps not deep enough to catch microbial, auto- chthonous signatures of DOM occurring in subsoil layers (Hassouna et al., 2012). FE increased sharply with soil depth and was generally higher under Ftill-W than Pg/O-W or W-W (Table S1). This indicated DOM was comprised by low molecular size constituents within subsoil layers, regardless of its intrinsic chemical character (Wu et al., 2003). Reduced biomass-C inputs under Ftill-W relative to Pg/O-W or W-W likely prompted soil microorganisms to further process the already existing pool of SOC (González-Pérez et al., 2007); particularly when considering a microbial community shift towards simple, energy-poor DOM (Nunan et al., 2015). Humification is typically associated with aromaticity and the poly- condensed status of organic structures in SOC (Stevenson, 1994); sev- eral reports show high correlations between humification and aromatic humic substances in soil (Fuentes et al., 2006; Marinari et al., 2010). However, an opposite trend was observed in our study among Abs254, SUVA254 and HIX (Table 3). DOM composition was more humified and less aromatic as a function of soil depth. Our results are in agreement with findings by Sanderman et al. (2008) and Hassouna et al. (2012) in grassland and arable soils, and indicate a shift from fresh plant-derived to older, highly decomposed DOM upon increasing soil depth. 3.2.2. EEMs and PARAFAC components of DOM Average EEMs, representative of cropping system by soil depth fluorescent profiles, are shown in Fig. S1. All samples contained DOM species fluorescing at low (< 240–280/300–330 nm) and high (< 240–275/390–540 nm) excitation/emission (Ex/Em) wavelengths, including fluorescence maxima overlapping EEM regions commonly associated with fluorophore peaks B (tyrosine-like), T (tryptophan- like), A (humic-like), and C (humic-like) (Coble, 1996; Coble et al., 1998). Fluorescence intensities decreased upon increasing soil depth and NT annual cropping (i.e. Pg/O-W and W-W) contained higher fluorescence intensities than Ftill-W soils (Fig. S1). Since DOM is a complex mixture of biomolecules that can fluoresce in overlapping Ex/ Em wavelength regions, the application of PARAFAC analysis was used to resolve the EEMs into individual DOM fluorescing components characterized by their Ex/Em maxima (Coble et al., 2014; D'Andrilli et al., 2017). Four different fluorescent components, hereafter referred to as components one through four (C1-C4), were identified by PAFAFAC analysis (contour plots Fig. 3a; loading scores Fig. S2). The spectral Ex/ Em maxima of these components and descriptions of similar DOM markers reported in the literature are summarized in Table 4. PARAFAC C1 and C2 resembled mid- and large-size humic-like fluorophores characteristic of soil, sediment, and freshwater environments (Ishii and Boyer, 2012). Fluorescent markers in this region are linked with re- calcitrant plant or soil-derived constituents highly resistant to microbial decomposition (Cory and McKnight, 2005), but susceptible to photo- chemical and abiotic degradation (Coble et al., 2014). PARAFAC C3 showed maximum fluorescence in a region analogous to the secondary fluorescence of B-peak (Ex/Em: 230/305 nm); a region commonly ascribed to microbial-derived DOM, but also more recently attributed to simple phenolic compounds (i.e. monolignols) of plant origin (Coble et al., 2014). PARAFAC C3 also contained fluorescence indicative of more complex lignin-like precursors, e.g. Ex 240 nm spanning Em 308–556 nm. Thus, we attribute C3 fluorescence to in- clude simple and somewhat more complex lignin-like chemical species. PARAFAC C4 exhibited maximum fluorescence in a region analogous to the primary fluorescence of B-peak and secondary fluorescence of T- peak (Ex/Em 275/305–340 nm) (Coble, 1996; Coble et al., 1998); overlapping fluorescence between protein- and tannin-like DOM mar- kers (Cuss and Gueguen, 2013; Coble et al., 2014). In terrestrial ecosystems, DOM exists as a mixture of organic frag- ments at various stages of oxidation and decay (Masoom et al., 2016). Microbial processing of vegetative inputs alters DOM reactivity by in- creasing soil aggregation and adsorptive processes within mineral sur- faces (Lehmann and Kleber, 2015). Unfortunately, we cannot determine the absolute soil versus plant origin of DOM by fluorescence spectro- scopy, thus can only consider C1, C2, C3 and C4 to be a mixture of terrestrially-derived signatures with minimal biological contribution (i.e. FI: 1.23–1.44). Nevertheless, our PARAFAC model identified che- mical species at different stages of decomposition. Average FE (n = 36) was higher for C1 (1.92 ± 0.25) followed by C3 (1.77 ± 0.26), C2 (1.19 ± 0.16), and C4 (0.67 ± 0.08), indicating molecular weights decreased in the order C4 > C2 > C3 > C1. Taken together, we infer fluorescent DOM chemical character is best described when con- sidering the progressive transformation of plant and microbial-derived biopolymers in solution. Future research should incorporate a DOM DOCa Abs254 SUVA254 HIX FI C:N C1b C2 C3 C4 Abs254 0.94⁎⁎⁎ 1.00 SUVA254 0.56⁎⁎⁎ 0.77⁎⁎⁎ 1.00 HIX −0.52⁎⁎ −0.50⁎⁎ −0.42⁎ 1.00 FI 0.24 0.14 −0.12 −0.15 1.00 C:N 0.42⁎ 0.49⁎⁎ 0.55⁎⁎⁎ −0.44⁎⁎ 0.05 1.00 C1 0.92⁎⁎⁎ 0.98⁎⁎⁎ 0.74⁎⁎⁎ −0.42⁎⁎ 0.24 0.44⁎⁎ 1.00 C2 0.88⁎⁎⁎ 0.97⁎⁎⁎ 0.80⁎⁎⁎ −0.41⁎ 0.05 0.48⁎⁎ 0.97⁎⁎⁎ 1.00 C3 0.72⁎⁎⁎ 0.75⁎⁎⁎ 0.61⁎⁎⁎ −0.81⁎⁎⁎ 0.01 0.51⁎⁎ 0.70⁎⁎⁎ 0.72⁎⁎⁎ 1.00 C4 0.90⁎⁎⁎ 0.92⁎⁎⁎ 0.63⁎⁎⁎ −0.64⁎⁎⁎ 0.21 0.50⁎⁎ 0.89⁎⁎⁎ 0.85⁎⁎⁎ 0.79⁎⁎⁎ 1.00 a Dissolved organic carbon (DOC) concentration, absorbance at 254 nm (Abs254), specific UV absorbance at 254 nm (SUVA254), humification index (HIX), fluorescence index (FI), and C:N ratio of soil extracts. b C1, C2, C3 and C4 refer to PARAFAC fluorescent components. ⁎ Significant at P < 0.05. ⁎⁎ Significant at P < 0.01. ⁎⁎⁎ Significant at P < 0.001. Table 3 Pearson correlation coefficients (r) between various DOM parameters for all soil depths and cropping systems. continuum model (Lehmann and Kleber, 2015), rather than classifying fluorescent components as only humic- and/or protein-like chemical species. 3.2.3. Variations of PARAFAC components The distributions of PARAFAC component Ex/Em fluorescent maxima (Fmax; R.U.) among soil depths and cropping systems are presented in Fig. 3b. The Fmax was assumed to be a proxy for the re- lative concentration of each PARAFAC component in each sample (Murphy et al., 2013). In general, Fmax decreased with soil depth (Fig. 3b; Fig. 4), with a further significant decrease in Fmax below the 10–20 cm depth only detected for C2 (Table 5). As already discussed, adsorption processes within the soil matrix can gradually affect the vertical occurrence of DOM fractions (Kaiser and Guggenberger, 2000). Alternatively, microbial degradation of labile-C, occurring in surface soil, can also limit the downward movement of DOM (Marinari et al., 2010). Across all depths, mean Fmax of C1 and C3 were similar among Ftill- W, Pg/O-W and W-W (Table 5). However, significant increases in average Fmax were reported for NT annual cropping (Pg/O-W and WeW) for both C2 (P = 0.0422) and C4 (P= 0.0171) over Ftill-W (Table 5). No significant variation in Fmax was determined for each component when comparing Pg/O-W and W-W cropping systems. Ac- cordingly, NT annual cropping increased Fmax of PARAFAC compo- nents, whereas Ftill-W led to a more depleted fluorescent DOM pool. This suggested DOM composition to be mainly driven by the amount of biomass-C inputs rather than the quality of crop residues returned to the system. Moreover, cumulative changes in DOM chemical character influenced bulk SOC dynamics. The occurrence of PARAFAC compo- nents at high Ex/Em wavelengths (C1 and C2) reflected reasonably well with the ΔSOC gradient imposed by Ftill-W, Pg/O-W, and W-W (Fig. 5), which averaged −3.80, 1.51 and 2.70 Mg C ha−1, respectively (Engel Fig. 3. PARAFAC analysis results of soil DOM showing a) components one through four (C1, C2, C3, and C4) and b) fluorescence maxima (Fmax) in Raman units (R.U.) of each component (n = 36) from different cropping systems (Ftill- W, conventional till fallow winter wheat; Pg/O-W no-till spring pea/oilseed-wheat; W-W no-till continuous wheat) and soil depths (0–10, 10–20 and 20–30 cm). et al., 2017). The higher SOC accretion rate by W-W vs. Pg/O-W likely resulted from slightly greater biomass-C inputs under W-W than Pg/O- W and lower quality (high C:N ratio) of WeW residues (O'Dea et al., 2015; Duval et al., 2016). This response was consistent with reports by Kaiser and Guggenberger (2000) and Kramer et al. (2012) that re- calcitrant, aromatic DOM is the greatest contributor for long-term SOC storage. Strong positive correlations were found between all four PARAFAC components, Abs254, and DOC concentrations (Table 3). Both UV-ab- sorbing compounds and fluorescent molecules contributed to the bulk pool of DOM concentration (Borisover et al., 2012; Sharma et al., 2017). Fmax values increased strongly with higher SOC concentrations (Fig. 4). The depth pattern was similar among PARAFAC components; Fmax was higher in surface soil, particularly under NT annual cropping (Pg-O/W, W-W). It is important to note, however, the trend was more pronounced within C1 and C3, than C2 or C4 (Fig. 4), describing highly condensed DOM with lower molecular weights and aromaticity at depth. 4. Conclusions The results of this study supported our hypothesis that changes in SOC accretion patterns altered the chemical structure of SOC. Characterization of soil water extractable DOM revealed contrasting structural features after 10 yr of cropping intensity and tillage (Engel et al., 2017). Annual cropping with NT management increased the DOM pool concentration and compositional diversity; higher inputs of bio- mass-C promoted the accumulation of fresh, plant-derived DOM con- stituents in solution relative to the conventional Ftill-W management. Contrarily, DOM extracted from Ftill-W was more humified than Pg/O-W or W-W, which may reflect the more decomposed, recalcitrant native SOC pool. DOM composition surveyed by EEMs can be used to track SOC quality changes and provide further insights on the mechanisms driving SOC microbial processing, decomposition, and sequestration. Accretion of SOC was accompanied by the presence of a larger and more enriched DOM pool, likely favoring the uptake of nutrients by winter wheat. Component no. Current study (λex/λem, nm) Previous models (λex/λem, nm) Literature component description Previous PARAFAC assignment Current study component description C1 < 240–250/390–440 < 240–260/374–450a Terrestrial and marine humic- like C3a,C2b,C2c,C2h Terrestrial humic-like (plant/soil derived)240/405–408b < 250/410c < 250/397h C2 < 240–275/455–540 < 240–275/434–520a Terrestrial humic-like C2a,C1b,C4c,C3d, C2f,C1h Terrestrial humic-like (plant/soil derived)240/474–480b 260/500c 260/490d > 240/465f < 250/460h C3 < 240/303–312 240/312–315b Tyrosine-like C3b,C5f, C5i,C3j Monolignol-like (lignin precursor) 273/309f < 230–300i 220/320j C4 280/300–330 280/328d Tryptophan-like; phenol-like C6d,C2e,C4f,C3g,C4h Protein-like; tannin-like 260/340e 270/354f 285/354g 260/320h a Ishii and Boyer (2012). b Zhao et al. (2012). c Sharma et al. (2017). d Murphy et al. (2008). e Pan et al. (2017). f Ohno and Bro (2006). g Borisover et al. (2012). h Cao et al. (2016). i Zhou et al. (2017). j Gao et al. (2017). Fig. 4. Relationship between average fluorescence maxima (Fmax) in Raman units (R.U.) of a) high- (C1, C2) and b) low- (C3, C4) Ex/Em PARAFAC components and soil or- ganic carbon (SOC) concentration (g kg−1) from different soil depths (0–10, 10–20 and 20–30 cm). Filled and semi- filled squares represent annual cropping (Pg/O-W and W- W) and Ftill-W, respectively. Table 4 Approximate location of peaks [excitation and emission (λex/λem, nm)] of PARAFAC identified components for DOM soil extracts. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.geoderma.2017.06.029. Acknowledgements This work was supported by USDA-NIFA project #MONB00346 “Nutrient cycling and management in Montana's agricultural soils” granted to Dr. Richard E. Engel. We thank Dr. Christine M. Foreman for use of laboratory instrumentation at Montana State University to con- duct the bulk optical spectroscopic DOM analyses. 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Cropping system (CS) Ftill-W 0.32 ± 0.04 0.19 ± 0.02 0.28 ± 0.05 0.11 ± 0.02 Pg/O-W 0.35 ± 0.04 0.21 ± 0.02 0.39 ± 0.05 0.15 ± 0.03 W-W 0.39 ± 0.04 0.23 ± 0.02 0.33 ± 0.04 0.15 ± 0.02 Soil depth (D) 0–10 cm 0.50 ± 0.03a 0.27 ± 0.02a 0.45 ± 0.05a 0.22 ± 0.02a 10–20 cm 0.32 ± 0.02b 0.20 ± 0.01b 0.33 ± 0.03b 0.10 ± 0.01b 20–30 cm 0.25 ± 0.01b 0.16 ± 0.01c 0.23 ± 0.03b 0.09 ± 0.01b ANOVA Factor df C1 C2 C3 C4 F P F P F P F P CS 2 1.77 0.1927 2.56 0.0980 2.27 0.1251 3.38 0.0510 Ftill-W vs. NT CSd 1 2.45 0.1305 4.60 0.0422 3.15 0.0886 6.56 0.0171 Pg/O-W vs. W-W 1 1.08 0.3092 0.52 0.4769 1.39 0.2504 0.19 0.6647 D 2 30.06 < 0.0001 25.67 < 0.0001 8.30 0.0018 37.39 < 0.0001 CS × D 4 0.43 0.7850 0.19 0.9431 0.16 0.9580 0.29 0.8805 Data are mean ± SE (n = 12). Means followed by a common letter within a column are not significantly different (LSD Fisher P < 0.05). d NT CS, no-till Pg/O-W and W-W. Fig. 5. 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IRScholarworksPDFCoverPage 17-042_Bulk_optical_characterization_of_dissolved Bulk optical characterization of dissolved organic matter from semiarid wheat-based cropping systems Introduction Materials and methods Experimental site and study design Soil sampling and processing Dissolved organic matter UV–Vis absorbance spectroscopy Fluorescence spectroscopy PARAFAC modelling – Multivariate analysis Linear statistical analyses Results and discussion Dissolved organic carbon concentration and C:N ratio Optical characterization of DOM Spectroscopic indices Abs254, SUVA254, HIX, FI, and FE EEMs and PARAFAC components of DOM Variations of PARAFAC components Conclusions Acknowledgements References