Dissolved Organic C Research Articles

Effects of Different Tillage and Straw Return on Soil Organic Carbon in a Rice-Wheat Rotation System

Soil management practices, such as tillage method or straw return, could alter soil organic carbon (C) contents. However, the effects of tillage method or straw return on soil organic C (SOC) have showed inconsistent results in different soil/climate/cropping systems. The Yangtze River Delta of China is the main production region of rice and wheat, and rice-wheat rotation is the most important cropping system in this region. However, few studies in this region have been conducted to assess the effects of different tillage methods combined with straw return on soil labile C fractions in the rice-wheat rotation system. In this study, a field experiment was used to evaluate the effects of different tillage methods, straw return and their interaction on soil total organic C (TOC) and labile organic C fractions at three soil depths (0–7, 7–14 and 14–21 cm) for a rice-wheat rotation in Yangzhong of the Yangtze River Delta of China. Soil TOC, easily oxidizable C (EOC), dissolved organic C (DOC) and microbial biomass C (MBC) contents were measured in this study. Soil TOC and labile organic C fractions contents were significantly affected by straw returns, and were higher under straw return treatments than non-straw return at three depths. At 0–7 cm depth, soil MBC was significantly higher under plowing tillage than rotary tillage, but EOC was just opposite. Rotary tillage had significantly higher soil TOC than plowing tillage at 7–14 cm depth. However, at 14–21 cm depth, TOC, DOC and MBC were significantly higher under plowing tillage than rotary tillage except for EOC. Consequently, under short-term condition, rice and wheat straw both return in rice-wheat rotation system could increase SOC content and improve soil quality in the Yangtze River Delta.

Short-term responses of soil enzyme activities and carbon mineralization to added nitrogen and litter in a freshwater marsh of Northeast China

Soil organic matter decomposition is regulated by nutrient availability. Adding nitrogen (N) and litter could affect the stability of soil organic carbon (SOC). An incubation experiment was conducted to examine the effects of ammonium nitrate (AN), urea (U), and litter amendment on activities of soil microbial extracellular enzyme, soil microbial biomass C (MBC), dissolved organic C (DOC), and C mineralization in freshwater marsh. The results showed that adding N including AN and U decreased soil urease activity, MBC, DOC, and soil pH. However, litter amendment and combined litter and N amendment increased the enzymes activities of urease and invertase, MBC and DOC concentrations. The response of soil C mineralization to N and litter additions was different, being inhibited with AN addition while stimulated with littler addition and the combined litter and N addition, initially stimulated but inhibited thereafter with urea addition. Our results suggest that adding N is helpful for improving marshland soil C stocks via decreasing soil cumulative CO2–C emissions, labile organic carbon concentration of MBC and DOC. However, adding litter could reduce C storage stability by stimulating soil C mineralization and increasing soil labile C fractions and enzyme activities.

Carbon Dioxide Fixation by Metallosphaera yellowstonensis and Acidothermophilic Iron-Oxidizing Microbial Communities from Yellowstone National Park

The fixation of inorganic carbon has been documented in all three domains of life and results in the biosynthesis of diverse organic compounds that support heterotrophic organisms. The primary aim of this study was to assess carbon dioxide fixation in high-temperature Fe(III)-oxide mat communities and in pure cultures of a dominant Fe(II)-oxidizing organism (Metallosphaera yellowstonensis strain MK1) originally isolated from these environments. Protein-encoding genes of the complete 3-hydroxypropionate/4-hydroxybutyrate (3-HP/4-HB) carbon dioxide fixation pathway were identified in M. yellowstonensis strain MK1. Highly similar M. yellowstonensis genes for this pathway were identified in metagenomes of replicate Fe(III)-oxide mats, as were genes for the reductive tricarboxylic acid cycle from Hydrogenobaculum spp. (Aquificales). Stable-isotope ((13)CO2) labeling demonstrated CO2 fixation by M. yellowstonensis strain MK1 and in ex situ assays containing live Fe(III)-oxide microbial mats. The results showed that strain MK1 fixes CO2 with a fractionation factor of ∼2.5‰. Analysis of the (13)C composition of dissolved inorganic C (DIC), dissolved organic C (DOC), landscape C, and microbial mat C showed that mat C is from both DIC and non-DIC sources. An isotopic mixing model showed that biomass C contains a minimum of 42% C of DIC origin, depending on the fraction of landscape C that is present. The significance of DIC as a major carbon source for Fe(III)-oxide mat communities provides a foundation for examining microbial interactions that are dependent on the activity of autotrophic organisms (i.e., Hydrogenobaculum and Metallosphaera spp.) in simplified natural communities.

Eutrophication reverses whole-lake carbon budgets

Lakes play a large role in global atmospheric and landscape carbon (C) processes, but their role may change as they become polluted with nutrients. Geographic regions rich in surface waters are also prone to agricultural and urban development and so may become increasingly eutrophic as the population rises. Here we develop C budgets of highly eutrophic lakes. These analyses show that lakes undergoing eutrophication can become atmospheric carbon dioxide (CO2) sinks because of the CO2 disequilibrium caused by extreme primary production. C budgets of such lakes show they absorb both landscape and atmospheric C, converting it into lake sediments and passing additional dissolved organic C (DOC) downstream. Eutrophication may cause a reversal in the role played by oligotrophic lakes by promoting atmospheric C sequestration as sediment and DOC. This means that as eutrophication increases from agriculture and urbanization, the expected large CO2 evasion to the atmosphere by natural lakes will decline substantially and inland C sequestration and enrichment of DOC in waters flowing to the sea will be augmented. Thus, we suggest that the global C role of eutrophication is worthy of future consideration because it represents an interface between 2 large, converging environmental problems, whose interaction may reverse the role of lakes in the global C cycle.

Response of soil respiration to grazing in an alpine meadow at three elevations in tibet.

Alpine meadows are one major type of pastureland on the Tibetan Plateau. However, few studies have evaluated the response of soil respiration (R s) to grazing along an elevation gradient in an alpine meadow on the Tibetan Plateau. Here three fenced enclosures were established in an alpine meadow at three elevations (i.e., 4313 m, 4513 m, and 4693 m) in July 2008. We measured R s inside and outside the three fenced enclosures in July–September, 2010-2011. Topsoil (0–20 cm) samples were gathered in July, August, and September, 2011. There were no significant differences for R s, dissolved organic C (DOC), and belowground root biomass (BGB) between the grazed and ungrazed soils. Soil respiration was positively correlated with soil organic C (SOC), microbial biomass (MBC), DOC, and BGB. In addition, both R s and BGB increased with total N (TN), the ratio of SOC to TN, ammonium N (NH4 +-N), and the ratio of NH4 +-N to nitrate N. Our findings suggested that the negligible response of R s to grazing could be directly attributed to that of respiration substrate and that soil N may indirectly affect R s by its effect on BGB.

Response of Soil C and N, Dissolved Organic C and N, and Inorganic N to Short-Term Experimental Warming in an Alpine Meadow on the Tibetan Plateau

Although alpine meadows of Tibet are expected to be strongly affected by climatic warming, it remains unclear how soil organic C (SOC), total N (TN), ammonium N (NH4 +-N) , nitrate N (NO3 +-N), and dissolved organic C (DOC) and N (DON) respond to warming. This study aims to investigate the responses of these C and N pools to short-term experimental warming in an alpine meadow of Tibet. A warming experiment using open top chambers was conducted in an alpine meadow at three elevations (i.e., a low (4313 m), mid-(4513 m), and high (4693 m) elevation) in May 2010. Topsoil (0–20 cm depth) samples were collected in July–September 2011. Experimental warming increased soil temperature by ~1–1.4°C but decreased soil moisture by ~0.04 m3 m−3. Experimental warming had little effects on SOC, TN, DOC, and DON, which may be related to lower warming magnitude, the short period of warming treatment, and experimental warming-induced soil drying by decreasing soil microbial activity. Experimental warming decreased significantly inorganic N at the two lower elevations,but had negligible effect at the high elevation. Our findings suggested that the effects of short-term experimental warming on SOC, TN and dissolved organic matter were insignificant, only affecting inorganic forms.

Biological lability of dissolved organic carbon in stream water and contributing terrestrial sources

Terrestrial sources of dissolved organic C (DOC) provide energy for stream microbial heterotrophs. Hydrologic conditions alter flow paths to the stream and influence terrestrial source quantity and quality. We used bioreactors to measure biodegradable DOC (BDOC) in water from soils, shallow wells, and spring seeps under baseflow conditions, overland flow during storms, and a stream during base flow and storm flow. Stream DOC concentrations ranged from 0.7 to 15.5 mg C/L and total BDOC ranged from 0.1 to 8.7 mg C/L. Under baseflow conditions, DOC was generally <2 mg C/L and BDOC was <0.7 mg C/L. BDOC made up 37.8% of DOC (8.2% labile and 29.6% semilabile constituents) and most DOC (62.2%) was recalcitrant. Storms increased DOC concentrations 6- to 12-fold and selectively mobilized BDOC increasing concentrations 8- to 27-fold. Labile and semilabile constituents increased 2-fold to 17.3% and 55.4%, respectively, so that storm flow DOC was largely biodegradable (73%). Terrestrial DOC concentrations declined from 2.15 mg C/L in soil water to 1.78 mg C/L in shallow wells to 1.26 mg C/L in spring seeps, and labile BDOC concentrations declined from 0.1 mg C/L in soil water to 0.04 mg C/L in shallow wells but increased to 0.13 mg C/L in spring seeps. Overland flow waters had elevated DOC (11.0 mg C/L) and labile BDOC (1.7 mg C/L) concentrations. Peak storm concentrations for 2 biological reactivity classes were coincident during 2 storms but divergent during another storm, suggesting temporal variation in hydrologic pathways. Our results demonstrate that storms deliver increased BDOC loads to a headwater stream and reveal the temporal variability of labile and semilabile BDOC constituents. Storms increase stream depth and velocity, resulting in increased uptake lengths for BDOC constituents, exporting them from the basin and probably providing a downstream subsidy.

Stream water carbon controls in seasonally snow-covered mountain catchments: impact of inter-annual variability of water fluxes, catchment aspect and seasonal processes

Stream water carbon (C) export is one important pathway for C loss from seasonally snow-covered mountain ecosystems and an assessment of overarching controls is necessary. However, such assessment is challenging because changes in water fluxes or flow paths, seasonal processes, as well as catchment specific characteristics play a role. For this study we elucidate the impact of: (i) changes in water flux (by comparing years of variable wetness), (ii) catchment aspect [north-facing (NF) vs. south-facing (SF)] and (iii) season (snowmelt vs. summer) on all forms of dissolved stream water C [dissolved organic C (DOC), chromophoric dissolved organic matter (CDOM) and dissolved inorganic C (DIC)] in forested catchments within the Valles Caldera National Preserve, New Mexico. The significant correlation between annual water and C fluxes (e.g. DOC r2 = 0.83, p < 0.02) confirms annual stream water discharge as the overarching control on C efflux, likely from a well-mixed ground water reservoir as indicated by previous research. However, CDOM exhibited a dominantly terrestrial fluorescence signature (59–71 %) year round, signaling a strong riparian and near stream soil control on CDOM composition. During snowmelt, the role of water as C transporter was superimposed on its control as C reservoir, when the NF stream transported significantly more soil C (40 % DOC, 56 % DIC) than the SF stream as a result of hillslope flushing. Inter-annual variations in winter precipitation were paramount in regulating annual stream C effluxes, e.g., reducing C effluxes three-fold after a dry (relative to wet) winter season. During the warmer summer months % dissolved oxygen saturation decreased, δ13CDIC increased and CDOM assumed a more microbial signature, consistent with heterotrophic respiration in the stream and riparian soils. As a result of stream C incubation and soil respiration, \( P_{{{\text{CO}}_{2} }} \) increased up to 12 times atmospheric values leading to substantial degassing.

Soil profile carbon and nitrogen in prairie, perennial grass–legume mixture and wheat-fallow production in the central High Plains, USA

Conversion of native prairie land for agricultural production has resulted in significant loss and redistribution of soil organic matter (SOM) in the soil profile ultimately leading to declining soil fertility in a low-productivity semiarid agroecosystem. Improved understanding of such losses can lead to development of sustainable land management practices that maintain soil fertility and enhance soil quality. This study was conducted to determine whether conservation practices impact soil profile carbon (C) and nitrogen (N) accumulation in central High Plains. Soil samples were taken at four-depth increments to 1.2m in July of 2011 from five unfertilized fields under long-term management with varying degrees of soil disturbance: (1) historic wheat (Triticum aestivum)-fallow (HT) – managed with tillage alone, (2) conventional wheat-fallow (CT) – input of herbicides for weed control and fewer tillage operation than historic wheat-fallow, (3) no-till wheat-fallow (NT) – not plowed since 2000 and herbicides used for weed control, (4) grass–legume mixture – established in 2005 as in the Conservation Reserve Program (CRP), and (5) native mixed grass prairie (NP) – representing a relatively undisturbed reference location. Cumulative soil organic C (SOC) was not significantly different among the three wheat-fallow systems when the whole profile (0–120cm) was analyzed. However, SOC, dissolved organic C (DOC), and total soil N contents decreased in the direction NP>CRP≥NT>HT≥CT in the surface 0–30cm depth. In the surface 0–30cm depth, estimated annual SOC storage rate averaged 0.28Mg Cha−1year−1 since the cessation of tillage in 2000 and 0.58Mg Cha−1year−1 since the establishment of CRP grass–legume mixture in 2005. Cumulative soil inorganic C (SIC) accumulation ranged between 8.1 and 24.9Mgha−1and was greatest under wheat-fallow systems, particularly at deeper soil layers, relative to the perennial systems (NP and CRP). Results from this study suggest that repeated soil disturbance induced by cropping and fallow favored large accumulation of SIC which presence may result in decline in soil fertility and productivity; whereas conversion from tilled wheat-fallow to CRP grass–legume mixture offers great SOC storage potential relative to NT wheat-fallow practices.

Soil carbon and nitrogen storage in alluvial wet meadows of the Southern Sierra Nevada Mountains, USA

Purpose Wet meadows formed on alluvial deposits potentially store large amounts of soil carbon (C) but its stability is subject to the impacts of management practices. The objective of this study was to quantify and characterize soil organic carbon (SOC) and nitrogen (N) in mountain wet meadows across ranges of meadow hydrology and livestock utilization. Materials and methods Eighteen wetlands in the southern Sierra Nevada Mountains representing a range of wetness andlivestockutilizationlevelswereselectedfor soilsampling. In each wetland meadow, whole-solum soil cores delineated by horizon were analyzed for total and dissolved organic C (DOC) total (TN) and mineral nitrogen and soil water content (SWC). Multiple regression and GIS analysis was used to estimatethe roleofwet meadowsinC storageacrossthestudy area landscape. Results and discussion Average solum SOC contents by wetland ranged from 130 to 805 Mg ha �1 .A ll SOC and TN components were highly correlated with SWC. Regression analyses indicated subtle impacts of forage utilization level on SOC and TN concentrations, but not on whole-solum, mass-per-area stocks of SOC and TN. Proportions of DOC and TN under seasonally wet meadows increased with increasing utilization. GIS analysis indicated that the montane landscape contains about 54.3 Mg SOC ha �1 , with wet meadows covering about 1.7% of the area and containing about 12.3% of the SOC. Conclusions Results indicate that soil organic C and N content of meadows we sampled are resilient to current light to moderate levels of grazing. In seasonally wet meadows, higher proportions of DOC and N with increasing utilization indicate vulnerability to loss. Partial drying of the wettest and seasonally wet meadows could result in losses of over five % of landscape SOC.

Shifts in the abundance and community structure of soil ammonia oxidizers in a wet sclerophyll forest under long-term prescribed burning

Fire shapes global biome distribution and promotes the terrestrial biogeochemical cycles. Ammonia-oxidizing bacteria (AOB) and archaea (AOA) play a vital role in the biogeochemical cycling of nitrogen (N). However, behaviors of AOB and AOA under long-term prescribed burning remain unclear. This study was to examine how fire affected the abundances and communities of soil AOB and AOA. A long-term repeated forest fire experiment with three burning treatments (never burnt, B0; biennially burnt, B2; and quadrennially burnt, B4) was used in this study. The abundances and community structure of soil AOB and AOA were determined using quantitative PCR, restriction fragment length polymorphism and clone library. More frequent fires (B2) increased the abundance of bacterium amoA gene, but tended to decrease archaeal amoA genes. Fire also modified the composition of AOA and AOB communities. Canonical correspondence analysis showed soil pH and dissolved organic C (DOC) strongly affected AOB genotypes, while nitrate-N and DOC shaped the AOA distribution. The increased abundance of bacterium amoA gene by fires may imply an important role of AOB in nitrification in fire-affected soils. The fire-induced shift in the community composition of AOB and AOA demonstrates that fire can disturb nutrient cycles.

Increased CO2 fluxes under warming tests and soil solution chemistry in Histic and Turbic Cryosols, Salluit, Nunavik, Canada

Cryosols in tundra ecosystems contain large stocks of organic carbon as peat and as organic cryoturbated layers. Increased organic mater decomposition rate in those Arctic soils due to increasing soil temperatures and to permafrost thawing can lead to the release of greenhouse gases, thus potentially creating a positive feedback on global warming. Instrumentation was installed on permafrost terrain in Salluit (Nunavik, Canada; 62°14′N, 75°38′W) to monitor respiration of two Cryosols under both natural and experimental warmed conditions and to simultaneously monitor the soil solution composition in the active layer throughout a thawing season. Two experimental sites under tussock tundra vegetation were set up: one is on a Histic Cryosol (H site) in a polygonal peatland; the other one is on a Turbic Cryosol reductaquic (T site) on post-glacial marine clays. At each site an open top chamber was installed from mid-July to the end of August 2010 to warm the soil surface. Thermistors and soil moisture probes were installed both in natural (N), or non-modified, surface thermal conditions and in warmed (W) stations, i.e. under an open top chamber. At each station, ecosystem respiration (ER) was measured three times per day every second day with an opaque closed chamber linked to a portable IRGA. Soil solutions were also sampled every alternate day at 10, 20 and 30 cm depths and analysed for dissolved organic C (DOC), total dissolved nitrogen (TDN) and major elements. The experimental warming thickened the active layer in the Histic soil while it did not in the Turbic soil. In natural conditions, average ER at the HN station (1.27 ± 0.32 μmol CO2 m−2 s−1) was lower than at the TN station (1.96 ± 0.41 μmol CO2 m−2 s−1). A soil surface warming of 2.4 °C lead to a ∼64% increase in ER at the HW station. At the TW station a ∼2.1 °C increase induced an average ER increase of ∼48%. Temperature sensitivity of ER, expressed by a Q10 of 2.7 in the Histic soil and 3.9 in the Turbic Cryosol in natural conditions, decreased with increasing temperatures. There was no difference in soil solution composition between the N and W conditions for a given site. Mean DOC and TDN contents were higher at the H site. The H site soil solutions were more acidic and poorer in major solutes than the T ones, except for NO3-. The induced warming increased CO2 fluxes in both soils; this impact was however more striking in the Histic Cryosol even if ER was lower than in the Turbic Cryosol. In the Histic Cryosol, the thickening of the active layer would made available for decomposition new organic matter that was previously frozen into permafrost; due to acidic conditions, CO2 would be directly emitted to atmosphere. In contrast, the smaller increase in ER in the Turbic Cryosol may indicate the lack of organic matter input and carbon stabilization because of cold, non-acidic and more concentrated soil solutions; at this site warming mainly stimulates plant-derived respiration without decomposing a newly available carbon pool.

Effects of Plant Growth-Promoting Rhizobacteria and NPK Fertilizers on Biochemical and Microbial Properties of Soils Under Ginger (Zingiber officinale) Cultivation

For this study, plant growth-promoting rhizobacteria (PGPR) were isolated from soils under ginger and shortlisted based on their nutrient mobilization traits. The promising PGPR (Burkholderia cepacia, Klebsiella sp., Serratia marcescens, and Enterobacter sp.) were either applied alone or in combination with varying rates of NPK fertilizers to determine their effect on sensitive biochemical and microbial properties of soils under ginger (Zingiber officinale Rosc.). The properties studied were soil organic carbon, dissolved organic-C (DOC), and -N, microbial biomass-C (C MIC), -N (N MIC) and -P (P MIC), net N mineralized (N MIN), soil respiration (SR), metabolic quotient ( $$ q_{{CO_{2} }} $$ ) and activities of dehydrogenase (DHA), acid phosphatase (AcP), β-glucosidase (βG), and urease (UR). Results revealed a 24 % increase in mean DOC level in treatments with PGPR + NPK compared to control. Similarly, mean C MIC and N MIC levels were greater by 27 and 71 %, respectively, in treatments involving PGPR + NPK compared to treatments with only fertilizers. Also, combined application of PGPR and fertilizers positively influenced P MIC and N MIN rates compared to sole application of PGPR or NPK. While SR did not vary considerably among the treatments, $$ q_{{CO_{2} }} $$ levels across PGPR + NPK treatments were lower by 15–20 % relative to treatments with only NPK or PGPR. Results also revealed that DHA activity was on an average greater by 49.0 %, UR by 15 %, AcP by 40 %, and βG by 35 % in PGPR + NPK treatments compared to only NPK.

Long-term effects of no-tillage management practice on soil organic carbon and its fractions in the northern China

The influence of different tillage practices on soil organic carbon levels is more significant under long-term tillage compared to short-term tillage. Despite the great interest in the effect of no-tillage (NT) management practice on carbon sequestration, the long-term effect of NT practice on soil organic carbon and its fractions in northern China remain unclear. We evaluated the long-term effects (after 17years) of NT and conventional tillage (CT) practices on soil organic carbon and its fractions at different depths ranging from 0 to 60cm using a cinnamon soil in Shanxi, China. A randomised block design with three replications was used to evaluate both the tillage and its effects on the yield performance of winter wheat (Triticum aestivum L.). After 17years, the soil organic carbon (SOC) concentration in the NT soil was greater than that of the CT soil, but only in the layer that was located between 0 and 10cm. There was a significant accumulation of SOC (0–60cm) in the NT soil (50.2MgCha−1) compared to that observed in the CT soil (46.3MgCha−1). The particulate organic matter C (POM-C), dissolved organic C (DOC), and microbial biomass C (MBC) levels in the 0–5cm layer under NT treatment were 155%, 232%, and 63% greater, respectively, compared to the CT treatment. The POM-C, DOC, and MBC in the 5–10cm layer under NT treatment were 67%, 123%, and 63% greater, respectively, compared to the CT treatment. Below 10cm, the labile carbon observed in the NT treatment did not differ from that of the CT treatment. Significantly positive correlations were observed between the SOC and the labile organic C fractions. Moreover, the winter wheat (T. aestivum L.) yield increased 28.9% in the NT treatment compared to the CT treatment. The data show that NT is an effective and sustainable management practice that improves carbon sequestration and increases soil fertility, resulting in higher winter wheat yields in the rainfed dryland farming areas of northern China.

Evaluating the potential contribution of vegetation as a nutrient source in snowmelt runoff

Elliott, J. 2013. Evaluating the potential contribution of vegetation as a nutrient source in snowmelt runoff. Can. J. Soil Sci. 93: 435-443. On the Canadian prairies, most nutrient transport to surface waters takes place during snowmelt. The potential for a range of 11 residue types to release nitrogen (N), phosphorus (P) and carbon (C) was assessed by snowmelt simulation. Interactions between soils and residues were measured for two contrasting residues. Samples (taken in late fall) were frozen prior to snowmelt simulations that consisted of three diurnal temperature cycles from -5°C to +9°C followed by a final melt at +5°C. Releases of total and total dissolved P (TP and TDP), total dissolved N (TDN), and dissolved organic C (DOC) during simulated snowmelt were greater from actively growing residues than from crop stubble and were significantly related to plant moisture and nutrient contents. Nutrient release from wheat stubble (WS) was statistically similar to that from the underlying surface soil but...

Evaluating the potential contribution of vegetation as a nutrient source in snowmelt runoff

Elliott, J. 2013. Evaluating the potential contribution of vegetation as a nutrient source in snowmelt runoff. Can. J. Soil Sci. 93: 435–443. On the Canadian prairies, most nutrient transport to surface waters takes place during snowmelt. The potential for a range of 11 residue types to release nitrogen (N), phosphorus (P) and carbon (C) was assessed by snowmelt simulation. Interactions between soils and residues were measured for two contrasting residues. Samples (taken in late fall) were frozen prior to snowmelt simulations that consisted of three diurnal temperature cycles from −5°C to +9°C followed by a final melt at +5°C. Releases of total and total dissolved P (TP and TDP), total dissolved N (TDN), and dissolved organic C (DOC) during simulated snowmelt were greater from actively growing residues than from crop stubble and were significantly related to plant moisture and nutrient contents. Nutrient release from wheat stubble (WS) was statistically similar to that from the underlying surface soil but releases of P and ammonia (NH3) from winter wheat (WW) were at least four times greater than for the corresponding soil. When combined samples of residue and soil were tested, releases of most nutrients were less than when the residue and soil were tested separately. Potential release of nutrients from vegetation is a factor for consideration in the design of practices to reduce nutrient transport.

The Effect of Different Flooding Duration on Soil Organic Carbon and its Labile Fraction in the Riparian Zone of Three Gorges Reservoir Area

The labile fractions of organic carbon (OC) in soil can respond rapidly to changes in C supply and have been suggested as a sensitive indicator of soil quality. The object is to determine soil organic C and its labile fractions at the duration of different flooding. Three elevation gradient sites were chose: below 145m (permanently flooding site), 145-175m (seasonal flooding site), and above 175m (never flooding site) in the riparian zone of Three Gorges reservoir Area. The results showed that soil OC, dissolved organic C (DOC) and water-soluble carbohydrate (WSCh) were lower under seasonal flooding sites than those of never flooding upland and permanently flooding sites, whereas microbial biomass C (MBC) decreased with the flooding duration. Correlation analysis showed that soil OC and MBC had significantly positive relationship with DOC. The results suggested that seasonal flooding significantly affected soil OC and its labile fractions in the riparian zone of Three Gorges reservoir Area.

Labile soil organic carbon and microbial activity in three subtropical plantations

The effects of tree species on soil properties have been attracted much attention, but the specific responses of labile soil organic carbon (SOC) and microbial activity to changes in tree species of subtropical forest ecosystems remain unknown. In this study, we investigated the labile fractions of SOC from three different single species plantations, namely Pinus massoniana (PM), Cinnamomum camphora (CC) and Schima superba (SS) in subtropical China. Specifically, we analysed the soil microbial biomass C (MBC), dissolved organic C (DOC) and permanganate-oxidizable C (POC), soil respiration, and activities of six enzymes in surface mineral soil (0–20 cm). The MBC, POC and soil respiration, as well as the activities of urease, acid phosphatase and polyphenol oxidase significantly differed among the three plantations in the study. In contrast, changes in the DOC as well as the activities of invertase, catalase and cellulase were not significant. The CC soil exhibited the highest POC, DOC and urease activity. PM soil had the highest MBC, soil respiration and polyphenol oxidase activity, but also had the lowest POC, qCO2, urease activity and acid phosphatase activity. DOC and MBC were significantly correlated with the soil respiration. Urease activity was significantly related to the SOC fractions, except for MBC. Invertase and polyphenol oxidase activities were correlated with MBC. The results suggested that the tree species had different effects on the labile SOC and microbial activity and the observed differences seemed not to be explained by the differences in the litter quality.

Nitrogen, organic carbon and sulphur cycling in terrestrial ecosystems: linking nitrogen saturation to carbon limitation of soil microbial processes

Elevated and chronic nitrogen (N) deposition to N-limited terrestrial ecosystems can lead to ‘N saturation’, with resultant ecosystem damage and leaching of nitrate (NO3 −) to surface waters. Present-day N deposition, however, is often a poor predictor of NO3 − leaching, and the pathway of the ecosystem transition from N-limited to N-saturated remains incompletely understood. The dynamics of N cycling are intimately linked to the associated carbon (C) and sulphur (S) cycles. We hypothesize that N saturation is associated with shifts in the microbial community, manifest by a decrease in the fungi-to-bacteria ratio and a transition from N to C limitation. Three mechanisms could lead to lower amount of bioavailable dissolved organic C (DOC) for the microbial community and to C limitation of N-rich systems: (1) Increased abundance of N for plant uptake, causing lower C allocation to plant roots; (2) chemical suppression of DOC solubility by soil acidification; and (3) enhanced mineralisation of DOC due to increased abundance of electron acceptors in the form of $${{\text{SO}}_{ 4}}^{ 2-}$$ and NO3 − in anoxic soil micro-sites. Here we consider each of these mechanisms, the extent to which their hypothesised impacts are consistent with observations from intensively-monitored sites, and the potential to improve biogeochemical models by incorporating mechanistic links to the C and S cycles.

Soil organic matter in restored rangelands following cessation of rainfed cropping in a mountainous semi-arid landscape

Agricultural abandonment is known to influence plant cover composition and C inputs into the soil with a consequence for changes in soil organic matter (SOM) storage and dynamics in rangeland ecosystems. This study was conducted on a chronosequence of high altitude rangelands (1) cultivated with rainfed wheat (CR0), (2) abandoned for 4 (AR4), 12 (AR12) and 45 (AR45) years and (3) uncultivated (reference) rangelands (UR) with three replicates in Zagros Mountains, Central Iran. We studied the changes in the concentrations and stocks of bulk soil organic carbon (OC), total N, particulate organic C (POC) and N (PON), dissolved organic C (DOC), microbial biomass C (MBC), and potentially mineralizable C (Min-C) at 0–0.15 and 0.15–0.3 m soil depths. Results showed that the concentrations and stocks of OC, N, and labile fractions increased with the abandonment of agriculture at both soil sampling depths. After 4–45 years of agricultural abandonment, soil OC and N stocks increased logarithmically by 3.8–46 % and 2.8–32 % in the whole 0–0.3 m, respectively. Although, the stocks of labile fractions decreased slightly 4 years after agricultural abandonment, there were considerable increases (logarithmic) in these fractions after 12–45 years of abandonment (POC, 65–148 %; PON, 68–147 %; DOC, 76–139 %; MBC, 24–62 %). The study shows that rangelands abandoned for 45 years contained lower soil OC and N concentrations and stocks compared to uncultivated rangelands, reflecting 45 years of abandonment would not be sufficient for SOM to attain the level of uncultivated rangelands. The present study provided evidence that the rate of increases in POC and DOC stocks was greater than that of OC and MBC stocks, demonstrating POC and DOC fractions of total SOM pool may be suitable and sensitive indicators for detecting the effects of agricultural abandonment on soil OC changes and storage in these restored semi-arid rangelands. Soil bulk density decreased, while the mean weight diameter (MWD) and aggregate ratio as measures of aggregate stability increased considerably within the abandoned rangelands with increasing time of abandonment. Results from a multivariate analysis suggested that soil variables such as bulk density, OC, TN, DOC, POC, PON, MBC, MWD and metabolic quotient (qCO2) were successful in separating land uses. In brief, the abandonment of agricultural activities in previously cultivated high altitude rangelands can potentially lead to an increase of total and labile SOM and also sequestration of C in these semi-arid rangelands.