Hot Water-extractable C Research Articles

Effects of land use and site on organic carbon fractions in some humic soil profiles of KwaZulu-Natal, South Africa

Soil organic matter (SOM) and its different carbon (C) fractions play a vital role in storing C and are a source of plant nutrients in natural and managed ecosystems. This study was conducted at six sites in KwaZulu-Natal Province to determine the effect of land use on the distribution of C fractions in selected humic soil profiles from different sites, using chemical and physical fractionation. Land use at some sites had been converted from natural grassland to maize and cultivated pasture and at others from native forest/wattle plantation to sugarcane farming. Humic acids (HA), fulvic acids (FA), hot water extractable C (HWEC), particulate organic matter C (POM-C) and mineral-associated organic C (MAC) were analysed to 1 m depth. Cropping lowered POM-C and HWEC, in the top 100 mm, irrespective of site differences, under pasture, grassland, forest and wattle plantation. Amongst the most labile fractions, FA predominated, resulting in a HA:FA ratio < 1 for pastures. The MAC was not affected by land use, irrespective of site differences. These findings reveal that organic carbon in humic soils can be decreased by cultivation but does not inevitably indicate the loss of C storage in the stable pool.

Recovering of soil organic matter and associated C and N pools on regenerated forest ecosystems at different tree species influence on post-fire and reclaimed mine sites

Understanding the processes in soils after disturbances could be helpful in better designing restoration practices. The labile fraction of soil organic carbon (LSOC) plays a vital role in recovering soil properties and quality. This study compared labile C and N fractions, including microbial biomass C (MBC), cold-water-soluble C (WSC), water-soluble N (WSN), hot-water-extractable C (HWC), hot-water-extractable N (HWN), particulate organic carbon (POC), and particulate organic nitrogen (PON) in post-mining, post-fire, and undisturbed soils under different tree cover (Scots pine, common birch, and black alder). Nearly 30 years after the disturbance, post-mining soils were characterized by lower soil organic carbon (SOC) and total nitrogen (TN) as well as their labile fractions compared to those of post-fire and undisturbed soils. Disturbed soils had less stable SOC than undisturbed soils. Compared to the post-fire site, the higher proportion of WSC in SOC in post-mining soils suggests higher SOC turnover and leaching potential during primary succession compared to secondary succession. The post-fire soils had a similar SOC stock to undisturbed soils but differed in the C and N labile fractions stock. Post-fire soils were characterized by higher POC stocks due to charcoal admixture and lower WSN, HWC, and HWN fractions than undisturbed soils. The studied tree species differently affected the recovery of SOM properties after disturbances. Post-mining soils under alder had higher SOC, TN, and labile fractions (except MBC) than soils under birch and pine cover. However, this was not true for the post-fire site, where only WSN was higher under the alder. The soils under birch stands had the highest MBC stocks across all sites. The soils under pine had less stable SOM than those under birch and alder, which may accelerate podzolization processes. Results indicate a longer time for the recovery of C and N pools during primary succession than in secondary succession on sandy soils. Alder, as phytomelioration species that increases the C and N pools in post-fire sites, has limited usefulness.

Soil Labile Organic Carbon Fractions and Carbon Management Index in Response to Different Fertilization under Organic Maize Farming System

Organic farming has been known to improve soil quality by enhancing soil organic carbon (SOC) contents. The labile organic carbon (LOC) pools and carbon management index (CMI) are commonly used as very sensitive indicators of changes in SOC and assessment of soil quality. This study was conducted to investigate the effect of organic farming practices on soil quality by LOC fractions and CMI analysis in a 6-year field experiment. Four treatments were included: compost (COM), green manure (GM), inorganic fertilizer (NPK), and no fertilization (NF). This study was designed to explore changes in SOC concentrations, soil labile organic C fractions (microbial biomass C (MBC), water and hot water-extractable C (WEC, HWEC), particulate organic C (POC), light fraction organic C (LFOC) and permanganate oxidizable C (POXC)) and CMI within the bulk soil under organic corn cultivation condition. Organic fertilization significantly increased SOC concentrations and stocks by 10 - 55% compared to NPK and NF, especially, compost treatment. All labile carbon fractions were higher in COM and GM compared to NPK and NP, except MBC content. Among the LOC fractions, POC showed the highest proportion (32 - 87%) on total SOC. The CMI varied from 0.87 to 2.77, organic fertilized treatments increased by 1.7 - 3.2 times over NPK. These results showed that POC and POXC could be used as a rapid and informative indicator to assess soil quality and SOC changes. Hence, organic farming management could therefore contribute to improved nutrient cycling services and higher soil quality.Effect of different fertilization on labile organic C fractions; water extractable C (WEC), hot-water extractable C (HWEC), microbial biomass C (MBC), permanganate oxidizable C (POXC), particulate organic C (POC) and light fraction organic C (LFOC) in soil.

Content of soil organic carbon and labile fractions depend on local combinations of mineral-phase characteristics

Abstract. Soil organic matter (SOM) is an indispensable component of terrestrial ecosystems. Soil organic carbon (SOC) dynamics are influenced by a number of well-known abiotic factors such as clay content, soil pH, or pedogenic oxides. These parameters interact with each other and vary in their influence on SOC depending on local conditions. To investigate the latter, the dependence of SOC accumulation on parameters and parameter combinations was statistically assessed that vary on a local scale depending on parent material, soil texture class, and land use. To this end, topsoils were sampled from arable and grassland sites in south-western Germany in four regions with different soil parent material. Principal component analysis (PCA) revealed a distinct clustering of data according to parent material and soil texture that varied largely between the local sampling regions, while land use explained PCA results only to a small extent. The PCA clusters were differentiated into total clusters that contain the entire dataset or major proportions of it and local clusters representing only a smaller part of the dataset. All clusters were analysed for the relationships between SOC concentrations (SOC %) and mineral-phase parameters in order to assess specific parameter combinations explaining SOC and its labile fractions hot water-extractable C (HWEC) and microbial biomass C (MBC). Analyses were focused on soil parameters that are known as possible predictors for the occurrence and stabilization of SOC (e.g. fine silt plus clay and pedogenic oxides). Regarding the total clusters, we found significant relationships, by bivariate models, between SOC, its labile fractions HWEC and MBC, and the applied predictors. However, partly low explained variances indicated the limited suitability of bivariate models. Hence, mixed-effect models were used to identify specific parameter combinations that significantly explain SOC and its labile fractions of the different clusters. Comparing measured and mixed-effect-model-predicted SOC values revealed acceptable to very good regression coefficients (R2=0.41–0.91) and low to acceptable root mean square error (RMSE = 0.20 %–0.42 %). Thereby, the predictors and predictor combinations clearly differed between models obtained for the whole dataset and the different cluster groups. At a local scale, site-specific combinations of parameters explained the variability of organic carbon notably better, while the application of total models to local clusters resulted in less explained variance and a higher RMSE. Independently of that, the explained variance by marginal fixed effects decreased in the order SOC &gt; HWEC &gt; MBC, showing that labile fractions depend less on soil properties but presumably more on processes such as organic carbon input and turnover in soil.

The effect of long-term soil surface mulching on SOC fractions and the carbon management index in a semiarid agroecosystem

Soil organic carbon (SOC) and its fractions are widely considered to be valuable indicators for monitoring soil carbon (C) pools and dynamics induced by mulching practices. Thus, a 10-year experiment with three treatments, including no mulching (CK), gravel mulching (GM) and film mulching (FM), was established on the Loess Plateau. The results showed that SOC under GM treatment (7.88 g kg−1) in surface soil (0–20 cm) decreased significantly compared with that of CK (8.59 g kg−1), and that of FM (8.09 g kg−1) did not change significantly. Compared with CK in surface soil, water soluble C (WSC) under FM treatment decreased by 9.36% on average, and hot-water extractable C (HWC) under GM and FM treatments increased by 12.36% and 24.56% on average, respectively. Labile organic C (LOC) determined by the potassium permanganate (P-P) method did not decrease significantly under FM treatment in surface soil, but recalcitrant organic C (ROC), determined by the P–P method, significantly decreased by 9.88% on average. Compared with CK, less labile C (CLL), determined by the modified Walkley-Black (M–W–B) method under GM and FM treatments, significantly decreased by 11.21% and 17.54% on average, respectively, which suggested that CLL was converted to very labile C (CVL) and labile C (CL). This result indicated that the M-W-B method was superior in explaining C turnover. Combined with the C management index (CMI) results of previous studies, CMI was consistently lower under GM treatment, and it was unstable under FM treatment. The sensitivity index (SI) results indicated that HWC was the most sensitive SOC fraction, and LOC and ROC determined by the M–W–B method were more sensitive than those determined by the P–P method. Moreover, CMI calculated by the M–W–B method (10–35%) was more sensitive than that calculated by the P–P method (9–12%). In conclusion, the decrease in SOC under long-term mulching is mainly from ROC, and the M–W–B method is superior to the P–P method.

Effect of conservation agriculture on soil organic carbon dynamics and mineral nitrogen under different fertilizer management practices in maize (Zea mays)-wheat (Triticum aestivum) cropping system

Soil organic carbon (SOC) is the center of all physical, chemical and biological properties of soils, its maintenance and buildup in soils is necessary to sustain the intensive cereal-based cropping system of Indo Gangetic plains. Intensive tillage, residue removal and indiscriminate use of fertilizers led to a continuous deterioration of soil health in Indo Gangetic parts of the country. Under this context, a field experiment on conservation agriculture (CA) was conducted at Taroari, Karnal, comprising four combinations of tillage and residue management [i.e. conventional tillage (CT) with complete residue removal (CT-RR), CT with 20% residue incorporation of wheat, 50% residue incorporation of maize and incorporation of green gram residue (CT+RI+GI), permanent beds (PB) with 20% residue retention of wheat and 50% residue retention of maize (PB+RR), and PB with 20% residue retention of wheat, 50% residue retention of maize and retention of green gram residue (PB+RR+GR)] and three nutrient management options [viz. farmers fertilization practice (FFP), recommended fertilizer dose (RDF) and site-specific nutrient management (SSNM)]. The results showed that, there was an increase in SOC mainly the active SOC pools, permanganate oxidizable-C (1.70 g/kg), hot water extractable-C (0.32 g/kg) and soil microbial biomass C (310 mg/kg) under CA-based treatment (PB+RR+GR) compared to CT (0.58, 0.23 g/kg, 183 mg/kg, respectively). Also, the mineral N was invariably greater under residue treatments. Therefore, crops residue retention as well as balanced fertilization (RDF and SSNM) under CA helped in improving SOC; mineral N and soil aggregation stability which can lead to increased sustainability under cereal-based intensive cropping systems.

Soil quality indicators as influenced by 5-year diversified and monoculture cropping systems

AbstractIncreasing crop diversity has been highly recommended because of its environmental and economic benefits. However, the impacts of crop diversity on soil properties are not well documented. Thus, the present study was conducted to assess the impacts of crop diversity on selected soil quality indicators. The cropping systems investigated here included wheat (Triticum aestivumL.) grown continuously for 5 years as mono-cropping (MC), and a 5-year cropping sequence [(wheat–cover crop (CC)–corn (Zea maysL.)–pea (Pisum sativumL.) and barley (Hordeum vulgareL.)–sunflower (Helianthus annuus L.)]. Each crop was present every year. This study was conducted in the northern Great Plains of North America, and soil quality data were collected for 2016 and 2017. Selected soil quality indicators that include: soil pH, organic carbon (SOC), cold water-extractable C (CWC) and N (CWN), hot water-extractable C (HWC) and N (HWN), microbial biomass carbon (MBC), bulk density (BD), water retention (SWR), wet soil aggregate stability (WAS), and urease andβ-glucoside enzyme activity were measured after the completion of 5-year rotation cycle and the following year. Crop diversity did not affect soil pH, CWC, CWN, HWC, HWN and SWR. Cropping systems that contained CC increased SOC at shallow depths compared to the systems that did not have CC. Crop diversity increased WAS, MBC, and urease andβ-glucoside enzyme activity compared with the MC. Comparison of electrical conductivity (EC) measured in this study to the baseline values at the research site prior to the establishment of treatments revealed that crop rotation decreased EC over time. Results indicate that crop diversity can improve soil quality, thus promoting sustainable agriculture.

Tradeoff between the Conservation of Soil C Stocks and Vegetation Productivity in Temperate Grasslands

Grassland management affects ecosystem services such as the conservation of C stocks. The aim of this study was to analyze the relation between vegetation production and soil C stocks for a set of seven temperate grasslands of various productivity levels. We estimated vegetation production directly through measurements of aboveground biomass (&gt;5 cm), stubble and root biomass, and indirectly via plant community functioning. Soil C stocks were measured for bulk soil (organic C, SOC) and hot-water-extractable C (HWC) of topsoil. Plant community functioning was characterized by community-weighted mean (CWM) traits and functional diversity index. Results show a negative relation between biomass production and SOCstock. The tradeoff between productivity and SOCstock could be linked to plant community functioning and particularly Leaf Dry Matter content (LDMCCWM) which appeared to be the most relevant descriptor of plant community functioning. High SOCstock could be associated to low productivity, conservative strategy (high LDMCCWM), low soil labile C content and grassland age. Our results show a strong direct effect of management and grassland age on plant community, which in turn affects plant tissue quality and subsequent organic matter mineralization. Old permanent grasslands appeared less productive but represent an occasion for C storage and thus global change mitigation.

Soil degradation and recovery – Changes in organic matter fractions and structural stability

The combination of concurrent soil degradation and restoration scenarios in a long-term experiment with contrasting treatments under steady-state conditions, similar soil texture and climate make the Highfield land-use change experiment at Rothamsted Research unique. We used soil from this experiment to quantify rates of change in organic matter (OM) fractions and soil structural stability (SSS) six years after the management changed. Soil degradation included the conversion of grassland to arable and bare fallow management, while soil restoration comprised introduction of grassland in arable and bare fallow soil. Soils were tested for clay dispersibility measured on two macro-aggregate sizes (DispClay 1–2 mm and DispClay 8–16 mm) and clay-SOM disintegration (DI, the ratio between clay particles retrieved without and with SOM removal). The SSS tests were related to soil organic carbon (SOC), permanganate oxidizable C (POXC) and hot water-extractable C (HWC). The decrease in SOC after termination of grassland was greater than the increase in SOC when introducing grassland. In contrast, it was faster to restore degraded soil than to degrade grassland soil with respect to SSS at macro-aggregate scale. The effect of management changes was more pronounced for 8–16 mm than 1–2 mm aggregates indicating a larger sensitivity towards tillage-induced breakdown of binding agents in larger aggregates. At microscale, SSS depended on SOC content regardless of management. Soil management affected macroscale structural stability beyond what is revealed from measuring changes in OM fractions, underlining the need to include both bonding and binding mechanisms in the interpretation of changes in SSS induced by management.

Exchangeable cation effects on hot water extractable carbon and nitrogen in agricultural soils

Hot water is believed to extract bio-available soil organic matter (SOM), including organic compounds from the biomass of soil microbes. The role of soil physico-chemical factors in relation to extractability of SOM in hot water is not well understood. We evaluated the influence of exchangeable sodium (Na) on the quantity and quality of organic matter extracted in hot water from soils with a range of total and extractable C (total C 19–60 g kg–1; hot water extractable C (HWC) 659–3292 mg kg–1). The soils were pre-treated with different rates of Na (0–156 cmolc kg–1, as NaCl) to establish a range of exchangeable Na percentages (ESP), and then extracted with hot water (80°C) for 16 h. Hot water extractable C increased linearly as ESP increased, but the rate of increase differed between soils (the increase in HWC per unit increase in ESP ranged within 19–71 mg kg–1). At ESP 15, a threshold used to separate sodic and non-sodic soils, HWC was 30–60% greater than that measured without added Na. Ultraviolet absorbance (260 nm) data indicated that aromatic organic matter was preferentially released following Na pre-treatment. The proportion of HWC in phenolic form was generally little affected by Na treatment but there was a consistent increase in protein in response to Na (hot water extractable organic N in protein form increased from an average of 5.5 ± 2.2% without added Na to 11.0 ± 3.6% at the highest Na rate). The Na-induced increases in UV absorbance may be largely attributable to release of proteins containing aromatic amino acids (tryptophan and tyrosine). Our results suggest that organic matter desorbed from mineral surfaces may be an important contributor to hot water extractable C and N, and factors that affect the adsorption–desorption process may significantly influence organic matter extractability in hot water.

Decadal Changes in Soil Organic Matter Due to Microaggregate and Hot Water Extractable Pools

Core Ideas Hot water extractable and microaggregate C explained 60% of soil C change over time. Hot water extractable and microaggregate N explained 47% of soil N change over time. No significant soil order effects on changes in C and N in fractions. Storage of C and N within aggregates is important for long‐term stabilization of soil organic matter (SOM). We investigated whether changes in C and N associated with physical soil fractions and the hot water extractable pool were correlated to changes in topsoil C and N over three decades. Archived soil samples from three soil orders collected from 46 sites across New Zealand were physically fractionated and the aggregate abundance (and C and N contents) of fractions determined; hot water extractable C (HWC) and hot water extractable N (HWN) were also measured. Together the change of C and N in hot water extractable SOM, microaggregate within macroaggregate, and free microaggregate fractions explained 60 and 47% of the change in whole soil C and N, respectively. Soil order was not a significant factor in the model suggesting that similar processes were operating in all three soil types. In summary, the development of strategies that enhance the storage of labile SOM and microaggregates could reverse the trend of loss of SOM and its associated ecosystems services.

Quantification of Soil Properties with Hyperspectral Data: Selecting Spectral Variables with Different Methods to Improve Accuracies and Analyze Prediction Mechanisms

We explored the potentials of both non-imaging laboratory and airborne imaging spectroscopy to assess arable soil quality indicators. We focused on microbial biomass-C (MBC) and hot water-extractable C (HWEC), complemented by organic carbon (OC) and nitrogen (N) as well-studied spectrally active parameters. The aggregation of different spectral variable selection strategies was used to analyze benefits for reachable estimation accuracies and to explore spectral predictive mechanisms for MBC and HWEC. With selected variables, quantification accuracies improved markedly for MBC (laboratory: RPD = 2.32 instead of 1.33 with full spectra; airborne: 2.35 instead of 1.80) and OC (laboratory: RPD = 3.08 instead of 2.36; airborne: 2.20 instead of 1.94). Patterns of selected variables indicated similarities between HWEC and OC, but significant differences between all other soil variables. This agreed to our results of indirect approaches in which both (i) wet-chemical data of OC and N and (ii) spectra fitted to measured OC and N values were used to estimate MBC and HWEC. Compared to these approaches, we found marked benefits of laboratory and airborne data for a direct spectral quantification of MBC (but not for HWEC). This suggests specificity of spectra for MBC, usable for the determination of this important soil parameter.

Wetting and drying cycles improving aggregation and associated C stabilization differently after straw or biochar incorporated into a Vertisol

Wetting and drying (WD) cycle drives soil structure dynamics more intensively in swelling soils than in non-swelling soils, and its feedback on soil organic C stabilization. Such relationship may be further dependent on the quality of organic amendment. Our objectives were to evaluate the role of WD cycle on soil aggregation after organic incorporation and to determine its physical protection on C stabilization in aggregates. Two organic inputs (maize straw and straw-derived biochar) were incorporated into a Vertisol subjected to two WD intensities (constant soil moisture and WD cycles). After 56day incubation, CO2, microbial biomass C (MBC), aggregate stability (MWD), hot water extractable C (HWC), and δ13C were measured. Our results showed that straw amendment significantly enhanced soil respiration, MBC, and hence improved aggregation (P<0.05), as compared to the control treatment but biochar did not. The WD cycle improved soil aggregation process (MWD) but decreased the MBC relative to the constant moisture condition in organic amended soils or not (P<0.05). Natural δ13C indicates that straw derived C joined soil aggregation process as a binding agent greater than biochar derived C did (P<0.05). The WD cycle further enhanced the new C stabilization in large macroaggregates in straw amended soils. However, this improvement was observed in microaggregates and silt+clay fraction in biochar amended soils. Thus, our results demonstrate that the WD cycle can improve aggregation and associated C stabilization after organic amendment in the Vertisol, but this process is organic type dependent.

The influence of the type of crop residue on soil organic carbon fractions: An 11-year field study of rice-based cropping systems in southeast China

Retaining crop residue is an important agronomic practice in sustainable agriculture, particularly to maintain soil organic carbon (SOC). To evaluate the effect of the retention of different types of crop residues on SOC and labile fractions, a long-term rice-based crop rotation experiment was established with five different winter cropping practices: (1) rice-fallow (RF), (2) rice–wheat (RW), (3) rice–potato with rice straw mulch (RP), (4) rice–green manure (Chinese milk vetch; RG), and (5) rice–oilseed rape (RO). The results showed that the annual rice yields in the RP, RO, RG, and RW treatments were not significantly different from each other, but 13.3%, 10.2%, 10.3%, and 8.4% higher, respectively, than in the RF treatment. Furthermore, the soil properties in the RP system improved significantly compared with those at the initiation of the study, with increases of 14.7% in total N, 17.5% in total P, 77.2% in available K, and 13.9% in SOC content. Considering the slight rice yield increase and superiority in the annual net income, the RP treatment seems to be a promising rotation system to meet the sustainability requirements for the agro-ecosystem. The responses of the labile SOC fractions (i.e., microbial biomass C (MBC), dissolved organic C (DOC), hot-water extractable C (HWC), permanganate-oxidizable C (KMnO4-C), and particulate organic C in the 0–20cm layer) to the crop rotation systems with the retention of different crop residues over the period of 11 years were specific to the types of the rotation system and closely related to the soil fertility properties. The RP treatment had comparatively higher content of DOC, HWC, MBC, and KMnO4-C than the other treatments in both 0–10cm and 10–20cm depths. The RW treatment had higher KMnO4-C content but lower MBC and POC content than the other treatments. The RO treatment had lower MBC content compared to the other treatments, while the opposite results were found in the RG treatment. Furthermore, the DOC content was improved by the winter crop growth. The changes in labile SOC might be attributable to the types of residues retained.

Responses of the soil microbial catabolic profile and diversity to vegetation rehabilitation in degraded semiarid grassland

Changes in the soil labile C pools induced by vegetation rehabilitation on semiarid lands are assumed to correlate with changes in the microbial catabolic profiles. To verify this, soil was sampled under the canopies of 12-year-old (R12Y) and 30-year-old (R30Y) planted Caragana microphylla shrubs, as well as native C. microphylla shrubs, all located in the Horqin Sandy Land, China. Community Level Physiological Profiles (CLPPs) of soils were determined to reveal any qualitative and quantitative shifts in soil microbial catabolism linked with the revegetation process. water-soluble C (WSC) and hot-water extractable C (HWEC) were measured to indicate the availability of labile C pools. As the restoration proceeds, soil organic C (Corg) increased 2–3 times at the R30Y habitat compared to the R12Y habitat. The total utilization rate of the 15C substrates also increased from 0.68μg CO2-Cg−1 soilh−1 at the R12Y habitat to 4.90μg CO2-Cg−1 soilh−1 at the R30Y habitat, but a slight decline in catabolic diversity, from 2.53 to 2.29, was recorded. Principal component analysis and redundancy analysis revealed that soil microbial communities colonizing the R12Y habitat exhibited an affinity to carbohydrates, which positively correlated to the relative concentration of WSC. Those colonizing the R30Y and native habitats mainly preferred carboxylic acids and 3,4-dihydroxybenzoic acid due to higher Corg content and relative concentration of HWEC.

Unexpected stimulation of CH4 emissions under continuous no-tillage system in mono-rice paddy soils during cultivation

No-tillage (NT) is known as an effective method of soil management that has the potential to increase soil organic carbon (SOC) accumulation in arable lands. Unlike upland soils, the increase of SOC accumulation in paddy soils under continuous NT is likely to increase methane (CH4) emissions during rice cultivation. However, the interaction between the SOC accumulation and CH4 emission characteristics associated with continuous NT in rice paddies has not been well elucidated. To investigate the effects of continuous NT on SOC accumulation and CH4 emissions, conventional tillage (CT) and NT plots were installed in a typical mono-rice paddy soil classified as a fine-silty, mixed, nonacid mesic Typic endoaquept in the southern part of the Korean peninsula in 2007. In the 1st, 2nd and 5th years after installation, the CH4 emission patterns were characterised during rice cultivation and rice grain yield and soil properties were investigated at the harvesting stage. Compared with CT (381 – 363 kg CH4 ha-1), NT effectively decreased total CH4 fluxes by approx. 20-27% in the 1st and 2nd years (279 – 291 kg CH4 ha-1) after installation. However, a much higher CH4 flux (approx. 36%) was observed in the NT (385 kg CH4 ha-1) than the CT (287 kg CH4 ha-1) plots in the 5th year. The SOC content in the NT plots clearly increased over the study years (14.5 – 15.6 g kg-1) compared with that under CT (14.4 - 14.3 g kg-1) which did not change significantly during the study period. Similar to the increase of the SOC content observed under NT, the concentrations of labile C forms such as water-extractable C (WEC) and hot water-extractable C (HWEC) and labile C availability in the surface soil dramatically increased over the study years, which may have increased mcrA gene copies as a methanogen population abundance (5.7 × 106mcrA gene copy number g-1 soil) and CH4 production potentials in the 5th year compared with CT (4.2 × 106mcrA gene copy number g-1 soil). Rice productivity was slightly lower in the NT than the CT treatment, though this difference was not statistically significant across the study years. These findings led to the conclusion that because continuous NT can increase CH4 emissions during rice cultivation under flooded paddy soil conditions due to the increased availability of labile forms of SOC, therefore, other soil management regimes that can decrease CH4 emissions, such an intermittent drainage, should be introduced along with continuous no-tillage.

Use of A Portable Camera for Proximal Soil Sensing with Hyperspectral Image Data

In soil proximal sensing with visible and near-infrared spectroscopy, the currently available hyperspectral snapshot camera technique allows a rapid image data acquisition in a portable mode. This study describes how readings of a hyperspectral camera in the 450–950 nm region could be utilised for estimating soil parameters, which were soil organic carbon (OC), hot-water extractable-C, total nitrogen and clay content; readings were performed in the lab for raw samples without any crushing. As multivariate methods, we used PLSR with full spectra (FS) and also combined with two conceptually different methods of spectral variable selection (CARS, “competitive adaptive reweighted sampling” and IRIV, “iteratively retaining informative variables”). For the accuracy of obtained estimates, it was beneficial to use segmented images instead of image mean spectra, for which we applied a regular decomposing in sub-images all of the same size and k-means clustering. Based on FS-PLSR with image mean spectra, obtained estimates were not useful with RPD values less than 1.50 and R2 values being 0.51 in the best case. With segmented images, improvements were marked for all soil properties; RPD reached values ≥ 1.68 and R2 ≥ 0.66. For all image data and variables, IRIV-PLSR slightly outperformed CARS-PLSR.

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.

Effect of biochar on carbon fractions and enzyme activity of red soil

To evaluate the changes in the pool of organic carbon fractions, aggregate stability and activity of enzymes, degraded red soil was amended with three different rates (0.5, 1.0 and 2%) of oak wood biochar (W0.5, W1.0, W2.0) and bamboo biochar (B0.5, B1.0, B2.0), with control as 0%. After 372days, the incubated soils were analyzed for total organic C (TOC), potassium permanganate oxidizable C (POXC), light fraction organic C (LFOC), water soluble organic C (WSC), hot-water extractable C (HWC) and microbial biomass C (MBC), macroaggregates (>0.25mm), dehydrogenase, β-glucosidase and urease. The highest macroaggregates, POXC, LFOC, HWC, MBC and enzyme activities were measured in the lowest rates (W0.5 and B0.5). MBC positively correlated with all labile organic C and macroaggregates, indicating that microbial activities result in mineralization of organic matter (OM) and contribute on bonding agent for macroaggregation. The C/N of the experimental soil negatively correlated with most of labile organic carbons and macroaggregates, which could be the effect of limited N availability on labile organic carbon fraction and aggregation. As compared to the control, lability index (LI) (changes in the lability of soil carbon) increased in W0.5 and B0.5 by 4 and 6%, respectively, whereas the carbon management index (CMI) (changes of the total carbon in the soil and its lability) increased by ranges of 50 to 286% in the treatments, and implies sequestration of organic C in soil. The high CMI is largely caused by high C sequestration and low lability differences between the treatments. Our results suggest that biochar application increases total organic carbon, stimulates microbial activities, in turn increases macroaggregation, and thus soil quality.

Interannual variations of soil organic carbon fractions in unmanaged volcanic soils (Canary Islands, Spain)

The stability over time of the organic C stocked in soils under undisturbed ecosystems is poorly studied, despite being suitable for detecting changes related to climate fluctuations and global warming. Volcanic soils often show high organic C contents due to the stabilization of organic matter by short-range ordered minerals or Al-humus complexes. We investigated the dynamics of different organic C fractions in volcanic soils of protected natural ecosystems of the Canary Islands (Spain) to evaluate the stability of their C pools. The study was carried out in 10 plots, including both undisturbed and formerly disturbed ecosystems, over two annual periods. C inputs to (litterfall) and outputs from (respiration) the soil, root C stocks (0–30 cm), soil organic C (SOC) fractions belonging to C pools with different degrees of biogeochemical stability –total oxidisable C (TOC), microbial biomass C (MBC), water soluble C (WSC), hot-water extractable C (HWC), humic C (HSC), – and total soil N (TN) (at 0–15 and 15–30 cm) were measured seasonally.A statistically significant interannual increase in CO2 emissions and a decrease in the SOC, mainly at the expense of the most labile organic forms, were observed, while the root C stocks and litterfall inputs remained relatively constant over the study period. The observed changes may reflect an initial increase in SOC resulting from low soil respiration rates due to drought during the first year of study. The soils of nearly mature ecosystems were more apparently affected by C losses, while those undergoing the process of active natural regeneration exhibited disguised C loss because of the C sequestration trend that is characteristic of progressive ecological succession.