Water-extractable Organic C Research Articles

Non‐Legume Cover Crops Can Increase Non‐Growing Season Nitrous Oxide Emissions

Core Ideas Nitrous oxide emissions were greater in winter than spring or fall. Tillage radish increased over‐winter N2O fluxes. Non‐legume cover crops increased N2O fluxes under apparent NO3 limiting conditions. Cover crops retain post‐harvest nutrients but how they impact non‐growing season nitrous oxide (N2O) emissions is unclear. Therefore, we quantified how cover crop type (fall rye [Secale cereale L.] or oilseed radish [Raphanus sativus L.]) and fertilizer source (compost or inorganic fertilizer) affected N2O emissions, soil water‐extractable organic C (WEOC) and nitrate (NO3) dynamics over two non‐growing seasons. A treatment with no fertilizer or cover crop was also included. Weekly, N2O fluxes were determined using vented static chambers; soil WEOC and NO3–N concentrations were measured monthly. Each non‐growing season, mean N2O fluxes were 74 to 450% greater in the winter (21 December–20 March) than spring (21 March–20 June) or fall (22 September–20 December). In winter 2014–2015, oilseed radish increased the mean N2O flux by 39 and 323% compared with fall rye and no cover crop, respectively, while the mean N2O fluxes were strongly correlated to the pre‐winter (16 Dec. 2014) NO3 concentrations (r = 0.96; P < 0.001), indicating NO3 levels < 6 mg NO3–N kg–1 limited N2O fluxes. In 2014–2015, fall rye and oilseed radish had 76 and 154% greater cumulative N2O emissions than amended soils with no cover crop, respectively. Across both winters, an exponential model explained 67% of variability between the pre‐winter WEOC to NO3 ratio and N2O fluxes, indicating that organic C and NO3 controlled over‐winter N2O fluxes. Non‐legume cover crops increased non‐growing season N2O emissions, suggesting that cover crops concentrate denitrification substrates in root‐associated soil to enhance N2O fluxes.

Effect of rate and source of phosphorus application on soil organic carbon pools under rice (Oryza sativa)-wheat (Triticum aestivum) cropping system

A field experiment was conducted during 2006-2013 on rice (Oryza sativa L.) - wheat (Triticum aestivum L.) cropping system at research farm of Punjab Agricultural University, Ludhiana, India. The study aimed to evaluate the effect of two rates of phosphorus (P) application, viz. 0 and 30 kg P2O5/ha to rice in rice-wheat sequence on soil organic C pools. Phosphorus was applied through either single super phosphate (SSP) or rock phosphate (RP) with and without farmyard manure (FYM). After 7 years of cropping total organic carbon (TOC) and labile C fractions were higher under FYM and RP treated plots and the lowest in unfertilized control. Plots treated with FYM and fertilizer-P either individually or in combination significantly (P<0.05) increased TOC and water extractable organic C (WEOC) in surface (0-7.5 cm) and subsurface (7.5-15 cm) soil. Beneficial effects of fertilizer-P application through RP were lower than SSP. Potassium permanganate oxidizable C (KMnO4-C) comprised the largest labile pool of soil organic C and represented 13.6% of TOC. Carbon management index (CMI) improved with FYM and fertilizer-P applications indicating the favorable impact of these treatments on C stabilization in soil. Conjoint application of FYM and RP improved soil organic C pools to a greater extent than their individual application, suggesting the need for integrated use of FYM and RP in these alluvial soils.

Sensitivity of Labile Soil Organic Carbon Pools to Long-Term Fertilizer, Straw and Manure Management in Rice-Wheat System

Labile soil organic carbon (SOC) pools, estimated through chemical fractionation techniques, are considered sensitive indicators of management-induced changes in quality and composition of soil organic matter. Although the impacts of organic manure and crop residue applications on C sequestration in rice-wheat system are fairly well documented, their influence on labile SOC pools is relatively less known. Impacts of organic manure, rice straw, and inorganic fertilizer nitrogen (N) applications on soil total organic carbon (TOC) and SOC pools including water-extractable organic C (WEOC), hot water-soluble organic C (HWOC), potassium permanganate-oxidizable organic C (KMnO4-C), microbial biomass C (MBC), mineralizable organic C (Cmin), and the oxidizable fractions of decreasing oxidizability (easily-oxidizable, oxidizable, and weakly-oxidizable) were investigated in an 11-year field experiment under rice-wheat system. The field experiment included treatments of different combinations of farmyard manure, rice straw, and fertilizer N application rates, with C inputs estimated to be in the range from 23 to 127 Mg ha−1. After 11 years of experiment, WEOC, HWOC, and KMnO4-C were 0.32%–0.50%, 2.2%–3.3%, and 15.0%–20.6% of TOC, respectively. The easily-oxidizable, oxidizable, and weakly-oxidizable fractions were 43%–57%, 22%–27%, and 10%–19% of TOC, respectively. The applications of farmyard manure and rice straw improved WEOC, HWOC, KMnO4-C, easily-oxidizable fraction, Cmin, and MBC, though the rates of change varied considerably from –14% to 145% and –11% to 83% of TOC, respectively. At the C input levels between 29 and 78 Mg C ha−1 during the 11-year period, the greatest increase was observed in WEOC and the minimum in KMnO4-C. Water-extractable organic C exhibited a relatively greater sensitivity to management than TOC, suggesting that it may be used as a sensitive indicator of management-induced changes in soil organic matter under rice-wheat system. All the other labile SOC pools exhibited almost the same sensitivity to management as TOC. Most of the SOC pools investigated were positively correlated to each other though their amounts differed considerably. Long-term applications of farmyard manure and rice straw resulted in build-up of not only the labile but also the recalcitrant pool of SOC, emphasizing the need for continued application of organic amendments for permanence of the accrued C under the experimental conditions.

Translocation of 13C-labeled leaf or root litter carbon of beech (Fagus sylvatica L.) and ash (Fraxinus excelsior L.) during decomposition – A laboratory incubation experiment

The aim was to quantify medium term litter type and litter mixture effects on the translocation and transformation dynamics of root and leaf litter C during decomposition. Partitioning of 13C-labeled root or leaf litter C (beech – Fagus sylvatica L., ash – Fraxinus excelsior L.) to CO2, water-extractable organic C (WEOC), microbial biomass C (CMB) and light (LF) and heavy soil fraction (HF) was determined in a laboratory decomposition experiment of 206 days. The proportions of C mineralized from ash leaf (34%) and root litter (29%) were higher than those from beech leaf (24%) and root litter (23%). In mixture with beech, the mineralization of ash leaf litter was enhanced. Mineralization was positively correlated with litter-derived WEOC until day 29. Water-extractable organic C declined with time, until <0.1% of litter C remained in this fraction. Litter-C recovery in CMB was higher for ash (0.7–1.0%) than for beech (0.2–0.4%). The litter C recovery in HF (4–12%) was positively correlated with that in WEOC (days 9 and 29) and CMB, but did not differ between treatments. Ash leaf litter mineralization showed different behavior in mixed treatments from pure treatments. Thus, the ability to transfer results from pure to mixed treatments is limited. The litter differed in chemical composition and in mineralization dynamics, but differences in partitioning to HF, WEOC and MB were finally of minor importance.

Effects of Different Rates of Ca2+ Addition on Respiration and Sorption of Water-Extractable Organic C to a Vertisol Subsoil

It is well known that calcium (Ca2+) plays an important role in binding organic matter to clay. However, most previous studies were conducted with either topsoil or pure aluminosilicates. Less is known about the effect of Ca2+ on binding of organic matter to clay-rich subsoils, which have lower organic-matter contents than topsoils, and their clays are more strongly weathered than pure aluminosilicates. Two experiments were conducted with a Vertisol subsoil (69% clay): a laboratory incubation and a batch sorption. The mineral substrate in the incubation experiment was pure sand alone or sand amended with 300 g clay kg−1. Powdered calcium sulfate (CaSO4) at rates of 0, 5, 10, and 15 g Ca kg−1 and mature wheat residue at a rate of 20 g kg−1 were added to this mineral substrate and the water content was adjusted to 70% of water-holding capacity. Carbon dioxide release was measured for 28 days. Cumulative respiration per g soil organic carbon (C) (SOC from clay and residues) was increased by clay addition. Increasing Ca2+ addition rate decreased cumulative respiration in the sand with clay but had no effect on respiration in the pure sand. Clay and Ca2+ addition had no significant effect on microbial biomass carbon (MBC) per g SOC but clay addition reduced the concentration of potassium sulfate (K2SO4)–extractable C per g SOC. For the batch sorption experiment, the subsoil was mixed with 0 to 15 g Ca kg−1 and water-extractable organic C (WEOC) derived from mature wheat straw was added at 0, 1485, 3267, and 5099 mg WEOC kg−1. Increasing Ca2+ addition rate increased sorption of WEOC, particularly at the greatest concentration of WEOC added, and decreased desorption. This study confirmed the importance of Ca2+ in binding organic matter to clay and suggests that Ca2+ addition to clay-rich subsoils could be used to increase their organic C sequestration.

Nitrogen fertilization and tillage reversal affected water-extractable organic carbon and nitrogen differentially in a Black Chernozem and a Gray Luvisol

Reversing land management from no tillage to conventional tillage (tillage reversal) to deal with weed infestation and accumulation of crop residue in long-term no tillage systems may dramatically alter soil carbon (C) dynamics. We studied the impact of nitrogen (N) fertilization and tillage reversal on the quantity and quality of water-extractable organic C (WEOC) and N (WEON) in the 0–10cm soil layer in two contrasting soil types located at Ellerslie (high organic matter content) and Breton (low organic matter content) in central Alberta, Canada. We used a split-plot design with N assigned to the main plot and tillage to the subplot. Each treatment had two levels which included addition of 0 (N0) vs. 100kgNha−1yr−1 (N100) N fertilizer and long-term no tillage (NT) vs. tillage reversal (TR); straw was retained on site in all treatments as part of the management regime. Our results showed that soil organic C and N storage were not affected by long-term N fertilization or tillage reversal at Ellerslie but were increased at Breton. Soil WEOC was significantly higher under N100 than under N0 at both sites. Soil WEOC was TR<NT at Breton but was not affected by tillage at Ellerslie. Soil WEON was influenced by the interaction effects of N fertilization and tillage reversal at both sites. The highest WEON concentration was in the N100–TR treatment combination (17.8±1.5 and 10.5±0.7μgg−1 at Ellerslie and Breton, respectively). Nitrogen fertilization decreased the aromaticity of WEOC at both sites but had different effects on WEOC condensation between Ellerslie and Breton. Nitrogen fertilization increased non-aromatic compounds in WEOC and the stability of WEOC at Breton but not at Ellerslie. Neither tillage nor tillage×fertilizer interaction affected the quality of WEOC in either soil. Therefore, N fertilization was the main factor controlling the quality and quantity of WEOC in the studied soils.

Temperature and duration of extraction affect the biochemical composition of soil water-extractable organic matter

Water-extractable organic matter is regarded as readily available substrate for soil microbes. However, little is known about the influence of extraction temperature and duration on its biochemical composition. The effect of temperature (20 vs. 80 °C) and extraction duration (1–24 h) on the amounts of water-extractable organic C (WEOC), water-extractable organic N (WEON), carbohydrates, phenolics, ninhydrin-reactive organic N (NRON), and mineral N were evaluated using four agricultural soils from New Zealand and eastern Canada. More WEOC and WEON were extracted in hot than in cold water, and the same was found for all biochemical compounds tested. Biochemical components increased rapidly during the first 4 h of extraction at 80 °C, and at a slower rate thereafter. In contrast, at 20 °C, concentrations of all measured components were at or close to their maximum after 1 h and showed little change thereafter. Glucose and nitrate in the 20 °C extracts both declined substantially between 1 and 24 h, likely due to microbial activity. Moreover, the disappearance of glucose induced a decline in the ratio of carbohydrate-C to phenolic-C. The carbohydrates measured in water extracts at both temperatures were predominantly of microbial origin after 1 h. However, the proportion of microbial carbohydrates gradually declined and the proportion of plant carbohydrates increased from 1 to 24 h at 80 °C. Based on these findings, we recommend limiting extractions to 1 h or less at 20 °C and to 4 h or less at 80 °C to minimize compositional changes that may occur during longer extraction periods.

Respiration and Sorption of Water-Extractable Organic Carbon as Affected by Addition of Ca2+, Isolated Clay or Clay-Rich Subsoil to Sand

Clay addition to light-textured soils is used to ameliorate water repellency and to increase nutrient retention. However, clay addition may also increase the potential to bind organic matter and thus C sequestration. Divalent calcium ions (Ca2+) play an important role in binding of organic matter to clay because they provide the bridge between the clay particles and organic matter which are both negatively charged. In the first experiment, quartz sand was mixed with clay isolated from a Vertosol at rates of 0, 50 and 300 g kg−1, finely ground mature wheat residues (20 g kg−1) and powdered CaSO4 at 0, 5 and 10 g kg−1. Soil respiration was measured over 28 d. Compared to the sand alone, addition of isolated clay at 300 g kg−1 increased cumulative respiration with a stronger increase than that at 50 g kg−1. Addition of CaSO4 increased electrical conductivity, decreased sodium adsorption ratio and reduced cumulative respiration. The latter can be explained by enhanced sorption of organic matter to clay via Ca2+ bridges. In a second experiment, isolated clay or subsoil of the Vertosol without or with powdered CaSO4 at 10 g kg−1 were used for a batch sorption with water-extractable organic C (WEOC) from wheat straw followed by desorption with water. Addition of 10 g kg−1 CaSO4 increased sorption and decreased desorption of WEOC in both subsoil and isolated clay. In the third experiment, subsoil of the Vertosol was used for a batch sorption in which WEOC was added repeatedly. Repeated addition of WEOC increased the concentration of sorbed C but decreased the sorbed proportion of the added WEOC. This indicates that sorption of WEOC may be underestimated if it is added only once in batch sorption experiments.

Erosion-induced changes in soil biogeochemical and microbiological properties in Swiss Alpine grasslands

Soil erosion can alter the storage of carbon (C) and other biogeochemical properties in both eroding and depositional soils. Little is yet known about soil microbial responses to erosion-induced changes in the quantity and quality of organic matter in mountain grasslands. To examine biogeochemical and microbiological responses to soil erosion, we compared the concentrations and stable isotope ratios of C and N, and microbial properties in eroding upslope (oxic), and depositional downslope (oxic) and wetland soils among three grasslands in the Swiss Alps. Compared to the reference site (Moos), the eroding upslope soils (Laui and Bielen) tended to have lower N concentrations and δ15N. The depositional wetland soils had higher δ13C and lower δ15N and C and N concentrations compared to the reference wetland, reflecting the influence of dry, oxic soils from eroding slopes. The depositional wetland soils had lower water-extractable organic C (WEOC) concentrations and optical intensities (UV absorbance and humic- and protein-like fluorescence) compared to the reference wetland. The activity of soil enzymes was positively related to most of the measured parameters indicative of organic matter quantity (e.g., %C and %N) and quality (e.g., WEOC and protein-like fluorescence), exhibiting significantly lower values in the sheet erosion-affected wetland (Bielen) than at the other sites. 16S rRNA gene copy numbers in the wetland were smaller than in the upland soil at all sites and greatest at Laui among three sites, indicating a potential alteration of the microbial community by the deposited oxic soils and attached microbial cells. The results suggest that soils deposited from the eroding slopes can slow down organic matter decomposition in the depositional wetland soils through decreases in the availability of labile organic matter and enzyme activity. Further research is required to elucidate erosion-induced changes in the activity and abundance of wetland microbial communities.

Effect of mono- and divalent cations on sorption of water-extractable organic carbon and microbial activity

Sorption is an important process for retention of organic carbon (C) in soils. The effect of Na+ and Ca2+ on sorption of organic C has been studied in salt-affected soils, but little is known about the effect of K+ and Mg2+ ions on sorption of water-extractable organic C (WEOC). The effect of Na+, K+, Ca2+ and Mg2+ ions on sorption of WEOC and its decomposition were investigated in a loamy sand (7.5 % clay) and a sandy clay loam (34.4 % clay). Salinity was developed with NaCl, KCl, MgCl2 or CaCl2 to obtain different concentrations of exchangeable Na+, K+, Ca2+ and Mg2+ at an electrical conductivity in a 1:5 soil/water extract (EC1:5) of 1 dS m−1. Water-extractable organic C was derived from wheat straw, and microbial activity after sorption was quantified by measuring CO2 emission from the soils for 27 days. The concentration of sorbed C was higher in the sandy clay loam than in the loamy sand and decreased in the treatment order Ca2+ > Mg2+ > K+ > Na+, but cumulative CO2-C emission after sorption was highest from the Na+ and lowest from the Ca2+ treatments. The strong binding in the Ca2+ and Mg2+ treatments can be explained by the low zeta potential and the high covalency index of cation binding with C, whereas zeta potential was high and the covalency index was low in the Na+ treatments. Although K+ is also monovalent, WEOC was more strongly bound in the K+ than in the Na+ treatment. The weak binding with Na increased the accessibility of the sorbed C to soil microbes and, thus, microbial activity. Our results suggest that monovalent cations may enhance decomposition and leaching of WEOC in saline soils with Na+ having a greater effect than K+. Divalent cations, particularly Ca2+, enhance the binding of organic matter and thus organic C stabilization, whereas Mg2+ ions have a smaller effect.

Retention and loss of water extractable carbon in soils: Effect of clay properties

Clay sorption is important for organic carbon (C) sequestration in soils, but little is known about the effect of different clay properties on organic C sorption and release. To investigate the effect of clay content and properties on sorption, desorption and loss of water extractable organic C (WEOC), two experiments were conducted. In experiment 1, a loamy sand alone (native) or mixed with clay isolated from a surface or subsoil (78 and 96% clay) resulting in 90, 158 and 175gclaykg−1 soil. These soil treatments were leached with different WEOC concentrations, and then CO2 release was measured for 28days followed by leaching with reverse osmosis water at the end of experiment. The second experiment was conducted to determine WEOC sorption and desorption of clays isolated from the loamy sand (native), surface soil and subsoil. Addition of clays isolated from surface and subsoil to sandy loam increased WEOC sorption and reduced C leaching and cumulative respiration in percentage of total organic C and WEOC added when expressed per g soil and per g clay. Compared to clays isolated from the surface and subsoil, the native clay had higher concentrations of illite and exchangeable Ca2+, total organic C and a higher CEC but a lower extractable Fe/Al concentration. This indicates that compared to the clay isolated from the surface and the subsoil, the native clay had fewer potential WEOC binding sites because it had lower Fe/Al content thus lower number of binding sites and the existing binding sites are already occupied native organic matter. The results of this study suggest that in the soils used here, the impact of clay on WEOC sorption and loss is dependent on its indigenous organic carbon and Fe and/or Al concentrations whereas clay mineralogy, CEC, exchangeable Ca2+ and surface area are less important.

Chemical and microbiological soil quality indicators and their potential to differentiate fertilization regimes in temperate agroecosystems

The study examined the interrelationships between chemical and microbiological quality indicators of soil and their ability to differentiate plots under contrasting fertilization regimes. The study was based on a long-term field experiment established on an Udic Ustocrepts in 1966. The soil was cropped with maize (Zea mays L.) and winter wheat (Triticum aestivum L.) and received no organic fertilization (control), wheat straw and maize stalk (crop residue) or cattle manure (manure) in combination with increasing levels of mineral N (N0 and N200). To asses whether seasonal fluctuations of measured properties might mask the effects of fertilization, soil samples were collected four times within a growing season. Manure amendment increased soil TOC and TN, while crop residue amendment had no significant effects. Mineral N increased TN only in April, while in September it decreased water extractable organic C (WEOC). Data of diffuse reflectance Fourier transform mid-infrared spectroscopy (DRIFTS) gave evidence for a higher relative contribution of the aliphatic peak at 2930cm−1 and a lower relative contribution of the aromatic peaks at 1620cm−1 and 1520cm−1 under manure. Manure amendment stimulated enzymatic activities, increased microbial biomass carbon (Cmic) and total phospholipids (PLFAs), and reduced the metabolic quotient (qCO2). Patterns of PLFAs indicated that manure amendment increased the ratio of Gram-positive to Gram-negative bacteria. Crop residue amendment had no significant effects, while in September mineral N inhibited protease activity and reduced the Gram-positive to Gram-negative ratio. Microbial-related parameters fluctuated over time but their seasonality did not hamper the identification of fertilization-induced effects. The selected properties proved to be valuable indicators of long-term changes of soil quality and were strongly interrelated: changes in soil organic matter content and composition induced by manure amendment were accompanied by changes in abundance and function of the soil microbial community. Partial least square analysis obtained relating DRIFTS spectra to measured soil properties produced accurate predictive models for TOC and PLFAs, and moderately accurate models for Cmic, showing the potential of DRIFTS to be used as a rapid soil testing technique for soil quality monitoring.

Soil Organic C:N vs. Water-Extractable Organic C:N

Traditionally, soil-testing laboratories have used a variety of methods to determine soil organic matter, yet they lack a practical method to predict potential N mineralization/immobilization from soil organic matter. Soils with high micro-bial activity may experience N immobilization (or reduced net N mineralization), and this issue remains unresolved in how to predict these conditions of net mineralization or net immobilization. Prediction may become possible with the use of a more sensitive method to determine soil C:N ratios stemming from the water-extractable C and N pools that can be readily adapted by both commercial and university soil testing labs. Soil microbial activity is highly related to soil organic C and N, as well as to water-extractable organic C (WEOC) and water-extractable organic N (WEON). The relationship between soil respiration and WEOC and WEON is stronger than between respiration and soil organic C (SOC) and total organic N (TON). We explored the relationship between soil organic C:N and water-extractable organic C:N, as well as their relationship to soil microbial activity as measured by the flush of CO2 following rewetting of dried soil. In 50 different soils, the relationship between soil microbial activity and water-extractable organic C:N was much stronger than for soil organic C: N. We concluded that the water-extractable organic C:N was a more sensitive measurement of the soil substrate which drives soil microbial activity. We also suggest that a water-extractable organic C:N level > 20 be used as a practical threshold to separate those soils that may have immobilized N with high microbial activity.

Effects of soil aggregate size, moisture content and fertilizer management on nitrous oxide production in a volcanic ash soil

A laboratory incubation study was conducted to determine the effects of soil aggregate size, soil moisture content and manure application on nitrous oxide (N2O) production through nitrification and denitrification. In Southern Hokkaido, soil samples were taken from a mineral soil layer (2.5–10 cm) of a grassland to which chemical fertilizer and manure had been applied. The soil aggregates were air-dried and sieved with 4.5 mm and 2 mm sieves, and the soil moisture was adjusted to 60% and 80% of field water capacity (FWC). Immediately after moistening, incubation was initiated and lasted for 9 days at 20°C. Following the start of incubation, a flush of N2O, carbon dioxide (CO2) and nitric oxide (NO) was observed. Production of all gases was higher in larger aggregates from the manure-applied soil. Productions of CO2 and NO were not significantly influenced by soil moisture content, but N2O production was considerably higher in 80% FWC as compared with 60% FWC. Based on the results of the N2O–nitrogen (N)/NO–N ratio, the process of N2O production was mainly due to nitrification in 60% FWC and denitrification in 80% FWC. Soil chemical properties, especially ammonium–N –N), nitrate–N (NO3–N) and water extractable organic C (WEOC) and microbial biomass C (MBC) also changed immediately after moistening. These changes were higher in larger aggregates from the manure-applied soil. Potential denitrification enzyme activity (DEA) was significantly higher in larger aggregates from manure-applied soil with higher moisture content. The N2O production in both 60% and 80% FWC correlated significantly with MBC and DEA. Regardless of soil moisture conditions, MBC correlated significantly with DEA, WEOC consumption and apparent N mineralization. These facts suggest that larger soil aggregates could have quickly developed suitable internal conditions for microbial activity inside the aggregates and consequently stimulated N2O production through nitrification and denitrification.

Soil extracellular enzyme activities from a forest harvest and climate manipulation experiment in central Pennsylvania

We have previously shown that warming and wetting a post-harvest forest soil increased C and N mineralization rates. A series of interacting processes may lead to these observed patterns including shifts in microbial allocation among extracellular enzymes. We monitored soil extracellular enzyme activity (EEA) for 2.5 years in a post-harvest, climate manipulated field experiment. A 2 ha forest in Central Pennsylvania was whole-tree harvested in 2007 and climate manipulations of +2 °C (warmed), and +20% mean monthly historical precipitation (wetted) were added to plots and in combination in a factorial design. The following enzymes were quantified: β-1,4-glucosidase (BG), cellobiohydrolase (CBH), leucine aminopeptidase (LA), N-acetyglucosaminidase (NAG), peroxidase (PER), and polyphenol oxidase (PPO).Soil hydrolase enzymes (BG, CBH, LA, and NAG) all showed greater mean activities than those reported from other studies, including study sites with similar climatic and edaphic characteristics. This is indicative of high overall soil microbial activity after forest harvest. Only BG and NAG showed significant treatment effects with a two-factor interaction (warming+wetting) on BG (p = 0.029) and warmed effect on NAG (p = 0.007). Warming decreased BG relative to ambient and NAG relative to wetted plots. There was substantial variability in the season and time since harvest, but water-extractable organic C (WEOC), C:N, and total N best explained the variability of EEAs over the experiment (p’s < 0.05). Two exoenzyme ratios previously used in the literature, BG:(LA+NAG) and BG:PPO, did not vary among treatments. However, the ratio of nitrogen acquiring enzymes (LA+NAG) to the sum of all other enzymes was higher in single-factor treatments compared to the ambient treatment (p = 0.007). Extracellular enzyme activity in a post-harvest forest soil was affected more by increased temperature than by increased precipitation, warming decreasing BG and NAG. Warming has been shown to decrease EEA in other ecosystem warming studies, but this usually is accompanied by a decrease in soil C and N fluxes (also called “attenuation to heating”). Surprisingly the warming treatment had the opposite effect on soil C and N fluxes, increasing fluxes compared to ambient. This inconsistency indicates a physiological change, including a possible shift towards alternate C and N acquiring enzymes, and that disturbed soil EEAs may not respond to climate change in the same way as undisturbed systems.

Soil microbial activity under different grass species: Underground impacts of biofuel cropping

Microbial and plant communities interact to determine local nutrient cycling rates. As lands are converted to bioenergy crops, including corn and cellulosic grasses, focus has been on changes in soil carbon sequestration. Little attention has been paid to impacts of such land conversion on the activity of belowground communities. We hypothesized that in addition to affecting soil organic carbon (SOC), monoculture species establishments have appreciable effects on microbial community activity, as evidenced by N and C mineralization rates. We compared soil microbial response in soils under long-term corn (Zea mays L.) production to soils under 10-year old monocultures of four warm-season perennial grasses (switchgrass (Panicum virgatum L.), coastal bermudagrass [Cynodon dactylon (L) Pers.], sideoats grama [Bouteloua curtipendula (Michaux) Torrey] and buffalo grass [Bouteloua dactyloides (Nutt.) Columbus]). All assayed perennial systems had higher SOC and water extractable organic C (WEOC) than the annual corn system. However, of all the perennial grasses, switchgrass soils had the lowest SOC and WEOC values, and the lowest 28-day C and N mineralization rates. This study indicates that soil microbial activities under buffalograss stands are more active than those under sideoats grama, switchgrass, coastal bermudagrass, or corn.

Effect of lithological substrate on microbial biomass and enzyme activity in brown soil profiles in the northern Apennines (Italy)

Abstract The aim of the present study was to investigate the microbial activity along forest brown soil profiles sequence developed on different lithological substrates (carbonate or non-carbonated cement in sandstone formations) at different altitudes. The main question posed was: does carbonate affect the biochemical activity of brown soil profiles at different altitudes? For the purpose of this study, four soil profiles with different amounts and compositions of SOM developed on different lithological substrates were selected: two with carbonate (MB and MZ) and the other two with non-carbonated cement in the sandstone formations (MF1 and MF2). Chemical and biochemical properties of soil were analysed along soil profiles in order to assess the SOM quantity and quality, namely total organic C (Corg), water extractable organic C (WEOC) and humification indices (HI, DH, HR). Microbial biomass (Cmic and Nmic) content, as well as the specific activities of acid phosphatase, β-glucosidase and chitinase enzymes were chosen as indicators of biochemical activity. The soil biochemical properties provided evidence of better conditions for microorganisms in MB than in MF1, MF2 and MZ soil profiles, since patterns of microbial biomass content and activity might be expected in response to the amount and quality of organic substances. The different lithological substrates did not show any clear effect on soil microbial biomass content, since similar values were obtained in MF1, MF2 (with non-carbonated cement) and MZ (with carbonate). However, the specific activities of acid phosphatase (per unit of Corg and per unit of Cmic) were higher in soils with no carbonate (MF1 and MF2) than in soils with carbonate (MB and MZ). In conclusion, the biochemical activity along brown soil profiles was mainly regulated by different soil organic matter content and quality, while the two different lithological substrates (with carbonate or non-carbonated cement in the sandstone formations) did not show any direct effect on microbial biomass and its activity. However, the activity of acid phosphatase per unit of C was particularly enhanced in soil with non-carbonate cement in the sandstone formations.

Influence of different litter quality on the abundance of genes involved in nitrification and denitrification after freezing and thawing of an arable soil

Due to disruption of soil aggregates and cell lysis and the subsequent release of organic C and N, increased microbial N transformation processes can be observed after freeze–thaw cycles. In a microcosm study, we investigated the influence of plant residues with different C/N ratios (lucerne-clover-grass-mix and wheat straw) on N transformations and the abundance pattern of the corresponding functional genes in an arable soil after freezing and thawing. Unfrozen soil samples, continuously incubated at 10°C, served as control. Concentration of soil NH4+, NO3−, and water-extractable organic C (WEOC) as well as genes involved in nitrification and denitrification, quantified by real-time PCR, were determined before freezing and 1, 3, and 7 days after thawing. The amounts of inorganic N and WEOC as well as the investigated gene abundance pattern did hardly differ between control samples and samples subjected to freezing and thawing that have been amended with straw. In contrast, clear alterations of the measured parameters and abundances were observed after freezing and thawing in samples being amended with the lucerne-clover-grass-mix compared to the control samples.

Influence of Temperature on Water‐Extractable Organic Matter and Ammonium Production in Mineral Soils

Cold (room temperature) and hot water (70–80°C) extraction methods have been used to assess the availability of organic matter and N in soil; however, the influence of temperature on their extractability is largely unknown. Twenty‐one mineral soils from New Zealand and eastern Canada were incubated for 16 h in water (1:6 soil/water ratio) at temperatures ranging from 20 to 80°C. After centrifugation and removal of the supernatant solution, the soil residue was extracted with 2 mol L−1 KCl to recover NH4–N released during incubation. In all cases, water‐extractable organic C (WEOC) and N (WEON) increased exponentially with temperature. The temperature dependence of organic matter solubility differed considerably among the soils (estimated increase in WEOC and WEON: 1.1–5.3% °C−1). These differences were not explained by management history or soil physicochemical properties, although there was evidence that organic matter extractability in some clay soils was less sensitive to temperature. Ammonium N was released in significant amounts at all temperatures. Peak NH4–N release typically occurred at 50°C, where NH4–N comprised an average of 49% of the total recovered N (WEON plus NH4–N). Factors that may account for NH4–N production during the 16‐h incubation include mineralization at the lower temperatures and abiotic processes (e.g., release of clay‐fixed NH4 and thermal degradation of organic N) at higher temperatures. The possibility that mineralization made a significant contribution to NH4–N production at temperatures as high as 50°C warrants further investigation.

Determination of the fate of regularly applied13C‐labeled‐artificial‐exudates C in two agricultural soils

AbstractExudates are part of the total rhizodeposition released by plant roots to soil and are considered as a substantial input of soil organic matter. Exact quantitative data concerning the contribution of exudates to soil C pools are still missing. This study was conducted to reveal effects of13C‐labeled exudate (artificial mixture) which was regularly applied to upper soil material from two agricultural soils. The contribution of exudate C to water‐extractable organic C (WEOC), microbial biomass C (MBC), and CO2‐C evolution was investigated during a 74 d incubation. The WEOC, MBC, and CO2‐C concentrations and the respective δ13C values were determined regularly. In both soils, significant incorporation of artificial‐exudate‐derived C was observed in the WEOC and MBC pool and in CO2‐C. Up to approx. 50% of the exudate‐C amounts added were recovered in the order WEOC &lt;&lt; MBC &lt; CO2‐C in both soils at the end of the incubation. Newly built microbial biomass consisted mainly of exudates, which substituted soil‐derived C. Correspondingly, the CO2‐C evolved from exudate‐treated soils relative to the controls was dominated by exudate C, showing a preferential mineralization of this substrate. Our results suggest that the remaining 50% of the exudate C added became stabilized in non‐water‐extractable organic fractions. This assumption was supported by the determination of the total organic C in the soils on the second‐last sampling towards the end of the incubation. In the exudate‐treated soils, significantly more soil‐derived C compared to the controls was found in the WEOC on almost all samplings and in the MBC on the first sampling. This material might have derived from exchange processes between the added exudate and the soil matrix. This study showed that easily available substrates can be stabilized in soil at least in the short term.