Dissolved Organic C Research Articles

Timing patterns of nitrogen application alter plant production and CO2 efflux in an alpine meadow on the Tibetan Plateau, China

Abstract Nitrogen (N) availability is an important factor that determines ecosystem productivity and respiration, especially in N-limited alpine ecosystems. However, the magnitude of this response depends on the timing and amounts of N input. Moreover, we have only a limited understanding of the potential effects of the timing of N fertilization on ecosystem carbon (C) and N processes, and activities of the soil microbes. A nitrogen fertilization experiment was conducted in an alpine meadow on the Tibetan Plateau to determine how plant productivity and ecosystem respiration ( RE ) respond to the timing and amount of N application. In this study, half of the N was added either in the early spring (ES), before the growing season, or in the late fall (LF), after the growing season. All treatments received the other half of the N in mid-July. Three N levels (10, 20, 40 kg N hm −2 yr −1 ) were used for each of two N treatments, with no N addition used as a control. Plant aboveground biomass, ecosystem respiration ( RE ) and soil respiration ( RS ) were measured for the 2011 and 2012 growing seasons. The LF treatment enhanced ecosystem CO 2 efflux compared with the ES treatment at high N addition levels, resulting from an increase of soil dissolved organic C (DOC) and soil microbial activity. The ES treatment resulted in increased plant aboveground biomass when compared with LF during both growing seasons, although this increase accounted for little variation in ecosystem and soil respiration. Overall, the ES treatment is likely to increase the ecosystem C pool, while the LF treatment could accelerate ecosystem C cycling, especially for the high N treatment. Our results suggest that supplying N during the early stage of the growing season benefits both forage production and soil C sequestration in this alpine ecosystem.

Changes in soil heterotrophic respiration, carbon availability, and microbial function in seven forests along a climate gradient

AbstractSoil microbial communities play an essential role in soil carbon (C) emission and C sequestration in forest ecosystems. However, little information is available regarding the relationship between soil C dynamics and microbial substrate utilization at large scales. Along the North–South Transect of Eastern China (NSTEC), seven forests representative of boreal, temperate and tropical biomes were examined. Soil heterotrophic respiration (Rh), soil dissolved organic C (DOC), microbial biomass C (MBC), and microbial community‐level physiological profiles (CLPPs) were investigated using biochemical measurements, static chamber‐gas chromatography analysis, and Biolog‐Eco microplates, respectively. We found that soil Rh rates were significantly higher in subtropical and boreal forests than in temperate forests. Conversely, the concentrations of soil DOC and MBC, as well as microbial metabolic activity and functional diversity, were consistently higher in temperate forests than in subtropical forests. There were considerably different substrate utilization profiles among the boreal, temperate, and subtropical forests. Soil microorganisms from the temperate and boreal forests mainly metabolized high‐energy substrates, while those from the subtropical forests used all substrates equally. In addition, soil Rh rates were significantly and negatively related to soil labile C concentrations, total metabolic activity, and the intensity of individual substrate utilization, indicating that soil microbes assimilated more soil substrates, thereby reducing CO2 emissions. Overall, our study suggests that climate factors, as well as substrate availability, dominate the activities and functions of soil microbes at large scales.

Effects of short-term conservation management practices on soil organic carbon fractions and microbial community composition under a rice-wheat rotation system

The objective of this study was to investigate the effects of short-term (less than 2 years) conservation managements [no-tillage (NT) and crop residue returning] on top soil (0–5 cm) microbial community composition and soil organic C (SOC) fractions under a rice-wheat rotation at Junchuan town of Hubei Province, China. Treatments were established following a split-plot design of a randomized complete block with tillage practices [conventional tillage (CT) and NT] as the main plot and residue returning level [no residue returning (0) and all residues returned to fields from the preceding crop (S, 2,146 kg C ha−1)] as the subplots. The four treatments were CT with or without residue returning (CT0 and CTS) and NT with or without residue returning (NT0 and NTS). The abundances of microbial groups [total FLFAs, fungal biomass, bacterial biomass, fungal biomass/bacterial biomass (F/B), monounsaturated fatty acids/saturated fatty acids (MUFA/STFA), and microbial stress] were determined by phospholipid fatty acid (PLFA) analysis of soil. The ratio of MUFA/STFA reflects aeration of soil and greater MUFA/STFA means better aeration condition of soil. Moreover, the microbial stress, the ratio of cy19:0 to 18:1ω7, was regarded as an indicator of physiological or nutritional stress of microbial community. PLFA profiles were dominated by the fatty acids iC15:0 (9.8 %), C16:0 (16.5 %), 10Me17:0 (9.9 %), and Cyc19:0 (8.3 %), together accounting for 44.6 % of the total PLFAs. Compared with CT, NT significantly increased microbial biomass C (MBC) by 20.0 % but did not affect concentrations of total organic C (TOC), dissolved organic C (DOC), easily oxidizable C (EOC), and SOC of aggregates. Residue returning significantly increased MBC by 18.3 % and SOC content of 2–1-mm aggregate by 9.4 %. NT significantly increased total PLFAs by 9.8 % and fungal biomass by 40.8 % but decreased MUFA/STFA by 15.5 %. Residue returning significantly enhanced total PLFAs, bacterial biomass, fungal biomass, F/B, and MUFA/STFA by 31.1, 36.0, 95.9, 42.5, and 58.8 %, respectively, but decreased microbial stress by 45.9 %. Multivariate analysis (redundancy analysis and partial correlation analysis) indicated that SOC of 2–1-mm aggregate was related to changes in the composition of soil microbial groups, suggesting that SOC of 2–1-mm aggregate was sensitive to changes in soil microbial community composition affected by short-term conservation management practices in our study.

Soil microbial substrate properties and microbial community responses under irrigated organic and reduced-tillage crop and forage production systems.

Changes in soil microbiotic properties such as microbial biomass and community structure in response to alternative management systems are driven by microbial substrate quality and substrate utilization. We evaluated irrigated crop and forage production in two separate four-year experiments for differences in microbial substrate quality, microbial biomass and community structure, and microbial substrate utilization under conventional, organic, and reduced-tillage management systems. The six different management systems were imposed on fields previously under long-term, intensively tilled maize production. Soils under crop and forage production responded to conversion from monocropping to crop rotation, as well as to the three different management systems, but in different ways. Under crop production, four years of organic management resulted in the highest soil organic C (SOC) and microbial biomass concentrations, while under forage production, reduced-tillage management most effectively increased SOC and microbial biomass. There were significant increases in relative abundance of bacteria, fungi, and protozoa, with two- to 36-fold increases in biomarker phospholipid fatty acids (PLFAs). Under crop production, dissolved organic C (DOC) content was higher under organic management than under reduced-tillage and conventional management. Perennial legume crops and organic soil amendments in the organic crop rotation system apparently favored greater soil microbial substrate availability, as well as more microbial biomass compared with other management systems that had fewer legume crops in rotation and synthetic fertilizer applications. Among the forage production management systems with equivalent crop rotations, reduced-tillage management had higher microbial substrate availability and greater microbial biomass than other management systems. Combined crop rotation, tillage management, soil amendments, and legume crops in rotations considerably influenced soil microbiotic properties. More research will expand our understanding of combined effects of these alternatives on feedbacks between soil microbiotic properties and SOC accrual.

Properties of Dissolved Organic Matter in Peatland: Implications for Water Quality after Harvest

We studied the properties of dissolved organic matter (DOM) in drained boreal peatland in stem‐only‐harvested (SOH), whole‐tree‐harvested (WTH), and unharvested sites. The DOM derived from both aerobic and anaerobic peat layers was divided into four different molecular‐weight fractions with ultrafiltration, and the fractions were measured for the concentrations of dissolved organic C (DOC), N, and carbohydrates, aromaticity (specific ultraviolet [UV] absorbance at 254 nm), pH, and bioavailability to bacteria. The percentage of DOC in the low‐molecular‐weight (LMW) fraction was higher in the deeper than in the upper layer. We suggest that easily degradable LMW compounds with low aromatic character and high bioavailability had not degraded in the deeper layer under anaerobic conditions. This had produced a relative enrichment of LMW DOC in the anaerobic layer. A different relation to aerobicity was observed in alternating aerobic conditions in the upper layer alone, where the high‐molecular‐weight compounds seemed to increase under more anaerobic conditions. Thus the properties and processes of DOM in peatlands seem to be controlled by the aerobicity in a complicated manner. We suggest that operations that affect the water table level in peatland affect also the properties of peat DOM. The properties of DOM that could be connected to clear‐cut harvest‐induced changes in the recent inputs of C and N were small. Peat DOM was more N rich in harvested than in control sites. Thus it seems possible that N is susceptible to leaching after harvest in naturally relatively N‐rich sites and especially in mineral form.

Particle-facilitated transport of lindane in water-saturated tropical lateritic porous media.

The persistent insecticide lindane [(1α,2α,3β,4α,5α,6β)-1,2,3,4,5,6-hexachlorocyclohexane] is still in use in many tropical countries and remains a threat to soil and water quality. We studied the sorption and transport of lindane onto and through lateritic soils in both the absence and presence of lignite particles, onto which lindane may preferably sorb. We determined a linear distribution coefficient of lindane onto the soil matrix of 3.38 ± 0.16 L kg. Soil particles were not released from the porous medium on changing ionic strength, and also transport of lindane was not affected by changes in ionic strength. We fitted coupled transport models for lindane and the particles to the data, revealing that: (i) sorption kinetics of lindane onto the matrix is described best by intraparticle diffusion; (ii) 20% of the total porosity of the lateritic sample is intraparticle porosity; and (iii) only lignite particles with a median diameter <0.45 μm were not retained in the porous medium and thus facilitated the transport of lindane. We conclude that although lindane and similar pollutants may sorb on tropical lateritic porous media, their transport may be facilitated by particles with high organic-C content or dissolved organic C (DOC). This may be of relevance in farmlands and swamp groundwater systems where DOC, produced by leaching or slow biodegradation of surface organic matter, could cause rapid groundwater contamination by sorbing pollutants. Moreover, the results of this study can help to understand nanoparticle behavior in lateritic soils as the size of particles that facilitate lindane transport approaches the nanoparticle size range.

Mineralization of Eroded Organic Carbon Transported from a Loess Soil into Water

The fate of soil-derived organic C (SOC) transported during erosion is a large uncertainty in assessing the impact of soil erosion on aquatic environments and in balancing C budgets. In our study, we determined C mineralization from solid soil organic C and dissolved organic C (DOC) translocated from a loess soil into surface water. We used runoff generated during rainfall simulation experiments. Both total runoff C and DOC were incubated to measure CO2 evolution during 28-d experiments. Cumulative CO2 emissions from runoff accounted for 3.9 to 4.8% of initial runoff C. It was estimated that 3.3 to 3.7% of initial solid SOC was mineralized contributing to 69 to 80% of total C mineralization from runoff. Mineralization of DOC was larger (7.3–30.2% of initial DOC) and showed a much larger variability than mineralization from solid SOC. However, DOC mineralization contributed to 20 to 31% of total C mineralization from runoff only because of the much smaller amounts of DOC than solid SOC. We could confirm a preferential removal of labile C from soils by water erosion. Nevertheless, the majority of this C will contribute to an aquatic C sink with less than 5% being potentially mineralizable. Our results indicated that the base level of C mineralization from translocated C was derived from the solid phase whereas the variability depends largely on DOC.

Water-Soluble Carbon and the Carbon Dioxide Pulse are Regulated by the Extent of Soil Drying and Rewetting

It is well established that rewetting dry agricultural soils causes a pulse of CO2 to be emitted due to rapid mineralization of organic C (“Birch effect”). The factors controlling the size of the CO2 pulse are not well known or understood. The objective of this laboratory study was consequently to determine the main factors affecting CO2 pulse size resulting from drying repacked cores of a clay loam soil (fine, loamy, mixed, mesic Typic Argiaquoll) to 45, 30, 20, or 10% water-filled pore space (WFPS) and subsequent rewetting to 75 or 90% WFPS. A distinct CO2 pulse was observed after rewetting soil dried to 10 and 20% WFPS, but no pulse was emitted after rewetting soil dried to 30 or 45% WFPS. Significant positive linear correlations occurred between CO2 pulse size and water-soluble soil C content in the form of dissolved organic C (DOC) and hot water extractable C (HWEC); the greatest increase in both DOC and HWEC occurred with the most extensive soil drying (i.e., drying to 10% WFPS). There were no significant correlations between CO2 pulse size and peak intensity of functional groups in HWEC, as determined using Fourier transform infrared (FT-IR) spectroscopy. However, the magnitude of the C-O stretching peak associated with soil polysaccharides was significantly related to the amount of CO2 emitted in the first few hours after rewetting dry soil. The total amount of C mineralized over the entire drying–rewetting process was found to be lower than that when soils were maintained at 75 or 90% constantly. Hence, the CO2 pulse following rewetting did not compensate for the reduced CO2 emission during the drying phase. It was also concluded that the amount of CO2 emitted on rewetting this clay loam soil depended primarily on the amount of water-soluble C (WSC; DOC, HWEC) released from the soil drying–rewetting process.

Identifying the Transport Pathways of Dissolved Organic Carbon in Contrasting Catchments

Dissolved organic C (DOC) plays an important role in the cycling and distribution of energy and nutrients. However, factors controlling the transport of DOC both within and between ecosystems are not clear. The aim of this work was to identify the contributing pathways for transport of DOC to surface water in catchments contrasting in land use and hydrogeology and during different flow regimes. Stream water was sampled to observe temporal variation of DOC concentrations and quality both seasonally and at the time scale of a rain event. Major cation and silica concentrations in stream water, groundwater, soil pore water, precipitation/throughfall, and riparian zone water samples were combined in an end‐member mixing analysis to determine the contributing end‐members for DOC delivery at the catchment outlet. Results show that the change in DOC concentrations and quality observed in the stream water during a rain event can be explained by a change in contribution of the different end‐members. In the forested catchments with deep groundwater tables, the main pathway for DOC transport from the soil to the surface water during base flow was via the groundwater. Rising stream DOC concentrations during rainfall events were attributed to additional throughfall and riparian zone transport pathways. In the grassland catchments with shallow groundwater tables, DOC in the stream mainly originated from seeps. During rain events, contributions from a surficial transport pathway and riparian zone water gained importance. The importance of contributing pathways changed seasonally and highly depended on the degree of saturation of the vadose zone.

Response of Dissolved Organic Matter in the Forest Floor of a Temperate Spruce Stand to Increasing Throughfall

In temperate spruce forests, dissolved organic matter (DOM) from forest floors is the major source of organic matter entering the mineral soil and thus it determines important soil properties and element cycling through the ecosystem. We examined effects of doubling locally collected throughfall for 6 yr on the concentrations of dissolved organic C (DOC) and properties of DOM (aromaticity, degree of molecule complexity) in the forest floor. Forest floor solutions below the Oi, Oe, and Oa horizons were sampled every 2 to 4 wk using tension lysimeters. For the controls, the average DOC concentrations in 2002 to 2007 were 43.8 ± 2.6 mg L−1 below the Oi, 49.6 ± 2.7 mg L−1 below the Oe, and 61.0 ± 2.0 mg L−1 below the Oa horizon. Doubling throughfall resulted in average DOC concentrations of 37.4 ± 1.8 mg L−1 (Oi), 49.3 ± 1.6 mg L−1 (Oe), and 50.1 ± 8.0 mg L−1 (Oa). The decreases in concentrations due to throughfall addition as well as the effects on DOM properties were, however, not statistically significant. It is commonly assumed that throughfall inputs are linearly related to water fluxes within and from the forest floor. Under that assumption, the results suggest that DOM fluxes are controlled by water fluxes rather than by the quantity of C that can be mobilized from the soil organic matter. Hence, increasing precipitation due to future climate changes presumably will result in enhanced DOM fluxes into the mineral horizons.

Variability and relationships between Pb, Cu, and Zn concentrations in soil solutions and forest floor leachates at heavily polluted sites

AbstractSeasonal variability of Cu, Pb, and Zn concentrations in litter leachates and soil solutions was examined in an afforested zone surrounding a copper smelter in SW Poland. Litter leachates (with zero‐tension lysimeters) and soil solutions (with MacroRhizon suction‐cup samplers, installed at a depth of 25–30 cm) were collected monthly at three sites differing in contamination levels in the years 2009 and 2010 (total Cu: 2380, 439, and 200 mg kg–1, respectively). Concentrations of Cu in the litter leachate were correlated with dissolved organic C (DOC), whereas Zn and Pb were mainly related to leachate pH. Metal concentrations in the soil solution were weakly influenced by their total content in soils and the monthly fluctuations reached 300, 600, and 700% for Cu, Pb, and Zn, respectively. Metal concentrations in soil solutions (Cu 110–460 μg L–1; Zn 20–1190 μg L–1; Pb 0.5–36 μg L–1) were correlated with their contents in the litter leachates. Chemical speciation, using Visual Minteq 3.0, proved organically‐complexed forms even though the correlations between metal concentrations and soil solution pH and DOC were statistically insignificant. The flux of organically‐complexed metals from contaminated forest floors is believed to be a direct and crucial factor affecting the actual heavy metal concentrations and their forms in the soil solutions of the upper mineral soil horizons.

Effects of nutrient limitation on the release and use of dissolved organic carbon from benthic algae in Lake Michigan

Compared to phytoplankton, relatively little information exists about the importance of benthic algae as a source of dissolved organic C (DOC) in lakes. We enriched outdoor tanks with N and P in a full factorial design (unenriched, +N, +P, N+P) to evaluate effects of nutrients on the release, composition, and decomposition of DOC from Cladophora glomerata (L.) Kütz. in Lake Michigan. After 10 d of enrichment, biomass-specific rates of gross primary productivity (GPP) were significantly greater in the N+P treatment than in the +N or +P treatments. The fraction of fixed C released as DOC was ∼37% of GPP in the N+P treatment, which was similar to material collected from the lake (initial samples). Biomass-specific rates of DOC release did not vary significantly with treatment, whereas GPP declined in the nutrient-limited treatments (+N, +P, and unenriched). Consequently, DOC release represented a higher percentage of GPP (>100%) under severe nutrient limitation. Specific ultraviolet (UV) absorbance, an indicator of aromatic compounds, indicated that DOC from material grown in the N+P treatment (DOCN+P) was more labile than DOC from material grown in the +N treatment (DOC+N). Both DOCN+P and DOC+N supported an exponential increase in bacterial density during a 19-d incubation, but DOCN+P was used more rapidly and supported higher bacterial density than DOC+N. Cladophora has become a major contributor to primary production in the shallow nearshore areas of the lower Laurentian Great Lakes, and our data suggest that ≥⅓ of this productivity is available for the benthic microbial loop as DOC. The amount of DOC released remains relatively constant as nutrient limitation reduces productivity, but the nutrient status of the alga affects the quality of DOC as a substrate for heterotrophic bacteria.

Effect of preceding crops and their residues on availability of zinc in a calcareous Zn-deficient soil

Limited information is available on the effect of preceding crop on phytoavailability of zinc (Zn) in soil. This pot experiment examined the effect of four preceding crops including clover, sunflower, safflower, and sorghum residues on shoot and grain Zn uptake by two wheat genotypes differing in Zn-deficiency tolerance Back Cross and Kavir in a calcareous Zn-deficient soil. Incorporation of all preceding crop residues into the soil significantly increased organic matter (OM) content, dissolved organic C (DOC), and diethylene triamine pentaacetic acid (DTPA) extractable Zn concentration in the soil. Residues of safflower and clover had the greatest effect on increasing DOC. Shoot and grain Zn concentrations were increased by incorporating all pre-crop residues into the soil although this increase was greater at safflower and clover treatments. Incorporation of sorghum residues into the soil had a negative effect on shoot and grain dry matter yield of wheat. Incorporation of safflower and clover residues into the soil increased Zn uptake by wheat shoot and grain. There was a positive significant correlation between shoot and grain Zn concentration with DOC in soil solution. It shows that DOC, produced from decomposition of crop residues, has facilitated Zn uptake by roots of wheat plants and particularly its transfer to grains.

Biochar suppressed the decomposition of organic carbon in a cultivated sandy loam soil: A negative priming effect

Conversion of plant residues to biochar is an attractive strategy for mitigation of atmospheric carbon dioxide (CO2) emission and enhancement of carbon (C) storage in soil. However, the effect of biochar application on the decomposition of soil organic C (SOC) as well as its mechanisms is not well understood in the sandy loam soil of North China Plain. We investigated how biochar affected the decomposition of native SOC, using stable δ13C isotope analyses by applying biochar produced from corn straw (a C4 plant, δ13C = −11.9‰) to a sandy loam soil (δ13C of SOC = −24.5‰) under a long-term C3 crop rotation. The incubation experiment included four treatments: no amendment (Control), biochar amendment (BC, 0.5% of soil mass), inorganic nitrogen (N) amendment (IN, 100 mg N kg−1) and combined biochar and N amendments (BN). Compared with Control, N amendment significantly (P < 0.05) increased total soil CO2 emission, even when combined with biochar amendment. In contrast, biochar alone amendment did not affect total soil CO2 emission significantly. However biochar, even when combined with N amendment, significantly (P < 0.05) reduced CO2 emission from native SOC by 64.9–68.8%, indicating that biochar inhibited the decomposition of native SOC and the stimulation effect of inorganic N on native SOC degradation, a negative priming effect. N addition immediately stimulated the growth of microorganisms and altered microbial community structure by increasing Gram-positive bacteria compared to Control as measured by phospholipid fatty acid. Biochar amendment did not alter microbial biomass during the 720-h incubation period except at 168 and 720 h, but significantly (P < 0.05) lowered dissolved organic C (DOC) content in soil, primarily due to sorption of DOC by the biochar. Our study suggested that biochar application could effectively reduce the decomposition of native organic C and a potential effective measure for C sequestration in the test soil of the North China Plain.

Soil organic carbon redistribution by water erosion--the role of CO2 emissions for the carbon budget.

A better process understanding of how water erosion influences the redistribution of soil organic carbon (SOC) is sorely needed to unravel the role of soil erosion for the carbon (C) budget from local to global scales. The main objective of this study was to determine SOC redistribution and the complete C budget of a loess soil affected by water erosion. We measured fluxes of SOC, dissolved organic C (DOC) and CO2 in a pseudo-replicated rainfall-simulation experiment. We characterized different C fractions in soils and redistributed sediments using density fractionation and determined C enrichment ratios (CER) in the transported sediments. Erosion, transport and subsequent deposition resulted in significantly higher CER of the sediments exported ranging between 1.3 and 4.0. In the exported sediments, C contents (mg per g soil) of particulate organic C (POC, C not bound to soil minerals) and mineral-associated organic C (MOC) were both significantly higher than those of non-eroded soils indicating that water erosion resulted in losses of C-enriched material both in forms of POC and MOC. The averaged SOC fluxes as particles (4.7 g C m−2 yr−1) were 18 times larger than DOC fluxes. Cumulative emission of soil CO2 slightly decreased at the erosion zone while increased by 56% and 27% at the transport and depositional zone, respectively, in comparison to non-eroded soil. Overall, CO2 emission is the predominant form of C loss contributing to about 90.5% of total erosion-induced C losses in our 4-month experiment, which were equal to 18 g C m−2. Nevertheless, only 1.5% of the total redistributed C was mineralized to CO2 indicating a large stabilization after deposition. Our study also underlines the importance of C losses by particles and as DOC for understanding the effects of water erosion on the C balance at the interface of terrestrial and aquatic ecosystems.

Land use and topographic position control soil organic C and N accumulation in eroded hilly watershed of the Loess Plateau

Land use and topography strongly influence soil organic C (SOC) and N accumulation in eroded hilly regions. However, their combined effects and the underlying mechanism remain unclear. In this study, five land uses and three topographic positions across an eroded hilly watershed of the Loess Plateau were selected to investigate their effects on SOC and N accumulation. The restored grassland, shrubland and woodland (25 to 30years) increased SOC and total N by 32% to 119% in the slope and 17% to 81% in the gully, respectively compared with the cropland. These restored vegetation increased soil dissolved organic C (DOC) and microbial biomass C and N (MBC and MBN) by 1.1- to 3.0-fold in the slope and 30% to 108% in the gully, respectively. Similar increases were observed in soil aggregates, MBN/total N, soil C:N and microbial C:N ratios. These improvements were higher in the shrubland than in the other land uses. The SOC significantly decreased from the gully to the slope, and the magnitudes decreased in a sequence of cropland, grassland, shrubland and woodland. Soil DOC, total N, and MBN also decreased from the gully to the slope in the cropland, but remained unchanged in the grassland, shrubland, and woodland. Land use (P<0.05), topographic position (P<0.05), and their interaction (P<0.1) influenced SOC, total N, DOC, MBC, MBN, soil C:N and microbial C:N ratios. There were close relationships among soil aggregates, SOC and total N, and DOC, MBC and MBN. Therefore, land use, topographic position (erosion and deposition), and their interactions regulate SOC and N accumulation and their labile fractions in the eroded hilly region at a watershed scale. Our results suggested that converting cropland to shrubland is an initial strategy to restore degraded ecosystems and increase soil C sequestration in eroded hilly region of the Loess Plateau.

Terrestrial Salmon Carcass Decomposition: Nutrient and Isotopic Dynamics in Central Idaho

Terrestrial salmon carcass decomposition and nutrient dynamics have been reported for coastal marine ecosystems of the Pacific Northwest but are lacking for semi-arid inland ecosystems of this region. Understanding these processes is a critical step in accurately understanding the biochemical responses to natural salmon deposition and appropriately mitigating for its loss. Additionally, the movement of carcasses for nutrient enhancement poses the risk of spreading pathogens and disease. Freezing and pasteurization of the carcasses reduces this risk. However, the effect of these treatments on decomposition processes is unknown. Decomposition rate and nutrient dynamics were investigated in semi-arid central Idaho by decomposing salmon carcasses in soil-filled microcosms. Rapid thermal accumulation in this semi-arid climate resulted in completion of soft tissue decomposition (skeletonization) in 16 days. Soil dissolved organic C (DOC) and dissolved total N (DTN) increased dramatically with respective increases from pretreatment concentrations of 7 and 48 fold by the time skeletonization occurred. Isotopic analysis of fluids beneath the carcasses revealed up to a 6‰ change in fluid δ13C and δ15N during decomposition as well as an overall 4‰ enrichment in mean fluid δ515N, relative to whole reference carcasses. Freezing and pasteurization of carcasses only yielded differences during the first few days of decomposition relative to the fresh carcasses. These results suggest that decomposition of carcasses in inland riparian forests proceeds very rapidly and that treated carcasses are suitable surrogates for fresh carcasses in semi-arid regions of central Idaho.

Land use and hydrologic flowpaths interact to affect dissolved organic matter and nitrate dynamics

The transport and transformation of dissolved organic matter (DOM) and dissolved inorganic nitrogen (DIN) through the soil profile impact down-gradient ecosystems and are increasingly recognized as important factors affecting the balance between accumulation and mineralization of subsoil organic matter. Using zero tension and tension lysimeters at three soil depths (20, 40, 60 cm) in paired forest and maize/soybean land uses, we compared dissolved organic C (DOC), dissolved organic N (DON) and DIN concentrations as well as DOM properties including hydrophilic-C (HPI-C), UV absorption (SUVA254), humification index and C/N ratio. Soil moisture data collected at lysimeter locations suggest zero tension lysimeters sampled relatively rapid hydrologic flowpaths that included downward saturated flow through the soil matrix and/or rapid macropore flow that is not in equilibrium with bulk soil solution whereas tension lysimeters sampled relatively immobile soil matrix solution during unsaturated conditions. The effect of land use on DOC and DON concentrations was largely limited to the most shallow (20 cm) sampling depth where DOC concentrations were greater in the forest (only zero tension lysimeters) and DON concentrations were greater in the cropland (both lysimeter types). In contrast to DOC and DON concentrations, the effect of land use on DOM properties persisted to the deepest sampling depth (60 cm), suggesting that DOM in the cropland was more decomposed regardless of lysimeter type. DOC concentrations and DOM properties differed between lysimeter types only in the forest at 20 cm where soil solutions collected with zero tension lysimeters had greater DOC concentrations, greater SUVA254, greater humification index and lower HPI-C. Our data highlight the importance of considering DOM quality in addition to DOC quantity, and indicate long-term cultivation reduced the delivery of relatively less decomposed DOM to all soil depths.

Soil labile organic carbon and microbial activity changes with age in citrus (Citrus sinensis Osb.) plantations in China

SummarySoil labile organic carbon fractions are important in ensuring soil productivity and improving soil quality. Plantations offer an opportunity to sequester carbon (C), but fundamental information is needed on labile fraction dynamics in soils over long periods. Two replicate sites of three stand ages (10, 20 or 30 y) of citrus in the Three Gorges Reservoir area of China were studied. The results showed that soil organic C (SOC) concentration and storage, and readily oxidisable C (ROC) content increased in the surface soil with stand age, but dissolved organic C (DOC) and the proportion of the SOC present as ROC and DOC showed no such significant difference. The microbial biomass C (MBC) and water-soluble carbohydrate (WSCh) increased gradually, and tended to be highest in the 20-y-old stand; thereafter MBC decreased again and WSCh became constant. Basal soil respiration (BSR) measured over a 24-h period at 25°C was lower under mature stands (after 20 y) than under younger (10 y) stands. BSR and qCO2 (expressed as BSR per unit microbial biomass) were negatively correlated with SOC fractions except DOC. The rate increase with age was highest in WSCh among the labile fractions measured and it was closely correlated with stand age, SOC and other labile organic C fractions; this suggested that WSCh could be the most sensitive fraction for detecting changes in organic C due to the long-term growth of citrus.

Microbial interactions affect sources of priming induced by cellulose

The recently developed 3-source-partitioning approach: addition of 14C labeled organics to soil after C3–C4 vegetation changes, was used to distinguish C sources in three compartments, namely CO2, microbial biomass and dissolved organic C (DOC) during decomposition of labeled cellulose. Microbial community structure (based on PLFA composition) and functions (based on enzyme activities and on microbial growth parameters) revealed mechanisms and drivers of priming effects (PE) induced by cellulose addition.14C-cellulose input caused negative PE within the first week and was accompanied by fast consumption of unlabelled DOC and its incorporation into microbial biomass. Microbial activation however, was not confirmed by substrate-induced respiration, nor by hydrolytic enzymes activity or by PLFA changes. A remarkable exception was a 2-fold increase in protozoan PLFA. Such an increase indicates that microorganisms feeding on cellulose and on DOC were quickly grazed by protozoans acting as a driver of microbial succession. This experimentally demonstrates the functioning of the microbial interactions: protozoan grazers provided for rapid recycling of nutrients and facilitated the succession of cellulose-degrading microorganisms during the second week of cellulose decomposition. An increase in the activity of cellulolytic enzymes caused short-term real PE accompanied by increase in abundance of slow-growing fungi and G(−) bacteria. Long-term real PE observed between 14 and 60 days after cellulose input was due to decomposition of SOM-originated hemicelluloses by fungi and G(+) bacteria. The CO2 released by primed soil organic matter (SOM) decomposition was originated mainly from C younger than 12 years (63%) and only 37% were older than 12 years despite the recent and old C contributed almost equally (51 and 49%, respectively) to SOM under Miscanthus giganteus. This indicates that the SOM pools are involved in PE according to their availability. Despite 71% of the applied cellulose-C was sequestered in the soil, the net soil C-gain amounted only for 28% of the applied cellulose-C after factoring in the C losses by the PE. Our study emphasizes the role of food webs in the PE dynamics: cellulose input served as a driver activating the food chain through the microbial loop.