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Carbon accumulates in organo-mineral complexes after long-term liquid dairy manure application

Quantifying and understanding the impact of animal manure on soil organic carbon (OC) is important for agronomic and environmental purposes. The influence of liquid manure on soil OC stocks has been less studied and was found to be more variable than that of solid manure. In addition, only a few studies have analyzed the effects of animal manure on stable fractions of soil OC. Our objective was to quantify OC in the whole soil and in specific physical fractions of organic matter (free and intra-aggregate light fractions, and sand and silt+clay-size heavy fractions) within a 0–50cm soil profile after 17 years of applications of liquid dairy manure (LDM) and mineral fertilizer on a perennial grass sward. The mineral fertilizer and LDM were applied at nominally 200 and 400kg mineral-Nha−1year−1 (low and high rates, respectively) to tall fescue (Festuca arundinacea Schreb. var. Festorina), grown on a Monroe silt in a maritime climate near Agassiz, British Columbia, Canada. The LDM applications provided an average of 3.8 and 7.3MgCha−1year−1 for the low and high rates, respectively. The whole-soil OC stock was significantly higher in the top 20cm of soils amended with LDM than either soils with mineral fertilizer or unamended soils. There were no differences in OC below 20cm, which may be linked to the absence of C transfer at depth, or to an insufficient C input to offset a possible LDM-induced priming effect on soil OC mineralization. Compared to the unamended soil, mineral fertilization led to higher OC stocks in only the top 5cm of soil. The application of LDM favored the incorporation of C into organo-mineral complexes rather than into the free light fraction of soil organic matter. The size separation of the heavy (density>1.8gmL−1) soil fraction revealed the presence of sand-size organo-mineral complexes that were responsive to treatments, and particularly to LDM application. This sand-size heavy fraction could be given greater scrutiny to detect more finely the effect of management changes on soil OC stocks.

Contribution of legume tree residues and macrofauna to the improvement of abiotic soil properties in the eastern Amazon

The potential advantages of positive interactions between the functional group components of the soil macrofauna and soil residue cover may increase the sustainability and efficiency of tropical agrosystems. The objective of this study was to link the ecological requirements of soil macrofauna with their impacts on the principal chemical and physical indicators of a hardsetting soil from a humid tropical region. The soil of a no-tillage system covered with legumes tree residues was subjected to the following treatments: Leucaena+Clitoria (L+C); Leucaena+Gliricidia (L+G); Leucaena+Acacia (L+A); Gliricidia+Clitoria (G+C); Gliricidia+Acacia (G+A); and a control with bare soil (BS) and no legumes. The application of leguminous tree residue with a high C/N ratio (G+A) extended essential ecosystem functions in the no-tillage agrosystem due to the favoring abundant functional groups, including soil engineers, predators and litter transformers. Litter transformers are associated with mulching effects that enhance multiple attributes, such as water infiltration, soil porosity, soil density, the litter carbon stock, the free light fraction (FLF) and total organic C. The use of fast-growing leguminous trees can increase soil acidity and decrease soil macrofauna diversity, but the harmful effects of leguminous tree cover are minor relative to the environmental benefits.

Large differences in estimates of soil organic carbon turnover in density fractions by using single and repeated radiocarbon inventories

Radiocarbon measurements are frequently used to model the turnover of soil organic carbon (OC) fractions. The assumption of homogeneous turnover in these fractions is typically stated, but consequences of its violation have not been tested. We used samples of a repeated soil inventory of a German beech forest from the litter layer to 50cm depth in 2004 and 2009 to determine the suitability of short-term repeated radiocarbon inventories for estimating the turnover times of OC in soil fractions. Samples from 0–5 and 10–20cm soil depth were density separated into a free light fraction (fLF), an occluded light fraction (oLF), and a heavy, mineral-associated fraction (HF). Samples were analysed for radiocarbon (∆14C), OC, and total nitrogen (TN) contents.Similar portions of OC were stored in the HF, but contributions of fLF and oLF varied between study years, probably due to interannual variations or methodological constraints. Following declining atmospheric CO2–14C, also ∆14C values at 0–5cm depth declined significantly between 2004 and 2009. Exchange of old for new OC was largest in the fLF and smallest in the HF, which confirms slow turnover of OC associated with minerals. Model results revealed that turnover time estimates based on single-pool models were not in agreement with observed changes in any of the fractions, suggesting all of them to be mixtures, to varying degrees, of fast and slow cycling pools. While single-pool models suggest average turnover times of 115years for HF-OC at 0–5cm depth, thus being a stable fraction, fitting a two-pool model to the two-point measurements of radiocarbon suggested the presence of a fast cycling pool of 15–25years of turnover time. It was however only possible to constrain the portion of this fast pool as being between 50 and 85% of total HF-OC. Increasing ∆14C in bulk soil and density fractions between study years at 10–20cm depth suggest that OC enters deeper soil layers with a longer time lag than topsoil layers, e.g., by slow transport, and 5years was not enough to induce significant changes.Even 40years after the bomb peak, radiocarbon time series are still suitable to detect OC fractions of decadal turnover and hint at time lags and translocation processes. Nevertheless, they do not allow for fixing the portions of fast and slow cycling OC in two-pool models and their turnover times.

The soil matrix increases microbial C stabilization in temperate and tropical forest soils

Microbial biomass represents a substantial source of labile C contributing to soil organic matter (SOM) maintenance. Microbial residues may associate with the soil matrix through a variety of mechanisms, reducing its bioavailability and increasing its persistence in soil. Our objective was to examine soil matrix effects on the stability of non-living microbial C inputs in two contrasting forest ecosystems by following microbial residues (Fungi, Actinobacteria, Gram-positive bacteria (Gm +), Gram-negative bacteria (Gm −)) into SOM fractions in a temperate forest in California (CA) and a tropical forest in Puerto Rico (PR) for 3 and 2 years, respectively. We isolated 3 SOM fractions: (i) free light fraction (FLF), (ii) occluded light fraction (OLF), and (iii) dense fraction (DF). Additionally, we characterized SOM fraction chemistry to infer quality and source of native fraction SOM. Our results showed greater stabilization as mineral-associated microbial C (i.e., as DF and OLF), compared with loose detrital C (i.e., FLF). There was no microbial group effect (i.e., differences in fraction C recovery among different microbial cell types). Our findings suggest that mineral association is more important for stabilizing non-living microbial C in soil than the cellular structure of the initial source of microbial inputs, with site specific edaphic factors as the major controllers of the amount of microbial residues stabilized.

Dynamics of Soil Organic Carbon Fractions and Aggregates in Vegetable Cropping Systems

Fertilisers significantly affect crop production and crop biomass inputs to soil organic carbon (SOC). However, the long-term effects of fertilisers on C associated with aggregates are not yet fully understood. Based on soil aggregate and SOC fractionation analysis, this study investigated the long-term effects of organic manure and inorganic fertilisers on the accumulation and change in SOC and its fractions, including the C concentrations of free light fraction, intra-aggregate particulate organic matter (POM) and intra-aggregate mineral-associated organic matter (MOM). Long-term manure applications improved SOC and increased the concentrations of some C fractions. Manure also accelerated the decomposition of coarse POM (cPOM) into fine POM (fPOM) and facilitated the transformation of fPOM encrustation into intra-microaggregate POM within macroaggregates. However, the application of inorganic fertilisers was detrimental to the formation of fPOM and to the subsequent encrustation of fPOM with clay particles, thus inhibiting the formation of stable microaggregates within macroaggregates. No significant differences were observed among the inorganic fertiliser treatments in terms of C concentrations of MOM, intra-microaggregate MOM within macroaggregate (imMMOM) and intra-microaggregate MOM (imMOM). However, the long-term application of manure resulted in large increases in C concentrations of MOM (36.35%), imMMOM (456.31%) and imMOM (19.33%) compared with control treatment.

Controls on soil carbon storage and turnover in German landscapes

Soil organic carbon (OC) storage across regions is influenced by climate and parent materials, which determine soil properties like clay content and mineralogy. Within homogeneous soil regions, land use and management practices are further important controls for soil OC contents and turnover. Here, we studied the impact of study region, land use (forest, grassland), forest management (spruce and beech forest under age-class management, unmanaged beech forest), and grassland management (meadow, mown pasture, unmown pasture) on stocks and turnover (based on Δ14C values) of soil OC in density frations of topsoil horizons. Samples were taken from 36 plots in the regions Hainich–Dun (HAI) and the Schwabische Alb (ALB) in Germany. They were separated into two light fractions (free light fraction (LF1), occluded light fraction (LF2)) and the mineral-associated organic matter (MOM) fraction using sodium polytungstate with a density of 1.6 g cm−3. Overall most soil OC was stored in the MOM fraction (73%). Soil OC concentrations and stocks in the MOM fraction differed between study regions, probably due to larger amounts of pedogenic Al- and Fe-oxides in the ALB than in the HAI region. Within each region, forest soils stored significantly higher proportions of total OC in the two LF (33±1.9 %) than grassland soils (20±2.3 %). Different management of forests and grasslands affected the C:N ratio of density fractions, but not OC storage. While modelled soil OC turnover in the MOM was longest of all fractions, all fractions had average Δ14C values above atmospheric values, suggesting a significant fast-cycling component in all of them. Different from stocks, turnover of OC in the MOM fraction were not affected by study region or contents of pedogenic oxides. Radiocarbon contents in the LF were higher for forest than for grassland sites, indicating faster turnover of OC at grassland sites. However, some of the observed difference could originate from different average lifetimes of roots in forests and grasslands. Applying different lag-times for OC input for forests and grasslands significantly reduced the differences in modelled turnover times. Lower Δ14C values of mown pastures than pasture soils in both regions suggest a management effect on soil C turnover in grasslands.We conclude that OC storage in the MOM of topsoil layers is more affected by regional differences in soil texture and mineralogy than by land use and management, while its turnover could not be explained with the studied soil properties. Soil OC storage and turnover in the two LFs is influenced by land use (forest or grassland) and management, but ecosystem specific lag-times have to be considered for modelling OC turnover in these fractions.

Transformation and stabilization of pyrogenic organic matter in a temperate forest field experiment.

Pyrogenic organic matter (PyOM) decomposes on centennial timescale in soils, but the processes regulating its decay are poorly understood. We conducted one of the first studies of PyOM and wood decomposition in a temperate forest using isotopically labeled organic substrate, and quantified microbial incorporation and physico-chemical transformations of PyOM in situ. Stable-isotope (¹³C and ¹⁵N) enriched PyOM and its precursor wood were added to the soil at 2 cm depth at ambient (N0) and increased (N+) levels of nitrogen fertilization. The carbon (C) and nitrogen (N) of added PyOM or wood were tracked through soil to 15 cm depth, in physically separated soil density fractions and in benzene polycarboxylic acids (BPCA) molecular markers. After 10 months in situ, more PyOM-derived C (>99% of initial 13C-PyOM) and N (90% of initial ¹⁵N-PyOM) was recovered than wood derived C (48% of 13C-wood) and N(89% under N0 and 48% under N+). PyOM-C and wood-C migrated at the rate of 126 mm yr ⁻¹ with 3-4% of PyOMC and 4-8% of wood-C recovered below the application depth. Most PyOM C was recovered in the free light fraction(fLF) (74%), with 20% in aggregate-occluded and 6% in mineral associated fractions – fractions that typically have much slower turnover times. In contrast, wood C was recovered mainly in occluded (33%) or dense fraction (27%).PyOM addition induced loss of native C from soil (priming effect), particularly in fLF (13%). The total BPCA-C content did not change but after 10 months the degree of aromatic condensation of PyOM decreased, as determined by relative contribution of benzene hexa-carboxylic acid (B6CA) to the total BPCA C. Soil microbial biomass assimilated 6-10% of C from the wood, while PyOM contributions was negligible (0.14–0.18%). The addition of N had no effect on the dynamics of PyOM while limited effect on wood.

Changes in Soil Organic Carbon and Carbon Fractions Under Different Land Use and Management Practices After Development From Parent Material of Mollisols

Soil organic carbon (SOC) is important to soil nutrient status in agroecosystems. Some of the soils of the Northeast of China, noted for their high SOC content, suffer from serious soil erosion to the point of having the parent material exposed or near the surface, which has raised concerns for food security. The Chinese Mollisols were derived from loamy Quaternary loess that developed from parent material. To effectively restore parent material to productive soils, information on the effects of land use/management practices on SOC concentration and C fractions in loess parent material of Chinese Mollisols is needed. The main objective of this study was to investigate the changes in C sequestration and C density fractions by physical and chemical fractionation (humic substances) occurring in the process of soil development from parent material under different management practices and land use. Six treatments were imposed in plots of loess parent material in a 5-year experiment: (1) natural fallow without weed control; (2) alfalfa; (3) soybean-maize rotation (S-M), straw of unfertilized maize removed; (4) S-M, straw of chemically fertilized maize removed; (5) S-M, straw of chemically fertilized maize and dried soybean powder incorporated; (6) S-M, biomass, including grain, of chemically fertilized maize incorporated. The SOC content increased by 15% to 77% depending on treatments. In the process of soil development, the C fractions of the parent material changed rapidly. The heavy fraction C pool accounted for a larger proportion of total SOC (78%–89%) than both the free light fraction (2.1%–10.2%) and the occluded light fraction (1.3%–12.9%) pools. The occluded light fraction was more sensitive than the free light fraction as indicator of soil C changes because of different land use and management practices. Humin accounted for a larger proportion (29.9%–54.7%) of SOC than fulvic acid (18.0%–34.4%), which was larger than the humic acid fraction (11.8%–14.8%). Our results indicate that SOC increase in loess parent material depends on types and amounts of organic matter inputs. The treatments, in which aboveground crop biomass and grain were incorporated, contributed more to C sequestration, distributions of density fraction, and humic substances than the treatments without organic matter. Management practices maximizing biomass inputs are recommended to restore SOC in degraded Chinese Mollisols in order to restore their fertility.

Changes to particulate versus mineral-associated soil carbon after 50 years of litter manipulation in forest and prairie experimental ecosystems

Models of ecosystem carbon (C) balance generally assume a strong relationship between NPP, litter inputs, and soil C accumulation, but there is little direct evidence for such a coupled relationship. Using a unique 50-year detrital manipulation experiment in a mixed deciduous forest and in restored prairie grasslands in Wisconsin, combined with sequential density fractionation, isotopic analysis, and short-term incubation, we examined the effects of detrital inputs and removals on soil C stabilization, destabilization, and quality. Both forested sites showed greater decline in bulk soil C content in litter removal plots (55 and 66 %) compared to increases in litter addition plots (27 and 38 % increase in surface soils compared to controls). No accumulation in the mineral fraction C was observed after 50 years of litter addition of the two forested plots, thus increases in the light density fraction pool drove patterns in total C content. Litter removal across both ecosystem types resulted in a decline in both free light fraction and mineral C content, with an overall 51 % decline in mineral-associated carbon in the intermediate (1.85–2.4 g cm−3) density pool; isotopic data suggest that it was preferentially younger C that was lost. In contrast to results from other, but younger litter manipulation sites, there was with no evidence of priming even in soils collected after 28 years of treatment. In prairie soils, aboveground litter exclusion had an effect on C levels similar to that of root exclusion, thus we did not see evidence that root-derived C is more critical to soil C sequestration. There was no clear evidence that soil C quality changed in litter addition plots in the forested sites; δ13C and Δ14C values, and incubation estimates of labile C were similar between control and litter addition soils. C quality appeared to change in litter removal plots; soils with litter excluded had Δ14C values indicative of longer mean residence times, δ13C values indicative of loss of fresh plant-derived C, and decreases in all light fraction C pools, although incubation estimates of labile C did not change. In prairie soils, δ13C values suggest a loss of recent C4-derived soil C in litter removal plots along with significant increases in mean residence time, especially in plots with removal of roots. Our results suggest surface mineral soils may be vulnerable to significant C loss in association with disturbance, land use change, or perhaps even climate change over century–decadal timescales, and also highlight the need for longer-term experimental manipulations to study soil organic matter dynamics.

Can Soil Organic Carbon Pools Indicate the Degradation Levels of Pastures in the Atlantic Forest Biome?

In Brazil, a large part of the Atlantic Forest was deforested in pastures, which were developed on the basis of natural fertility and organic matter content of the new deforested soil. However, as time went by the organic matter content in these areas have been decreasing causing soil degradation. The objective of this study was to evaluate the organic matter as a quality indicator of Ultisols in different levels of pasture degradation in the region of Governador Valadares-MG. Four sites of pastures were chosen at different levels of degradation, observed visually, two capoeiras in different stages of natural recovery and forest site (reference). Soil samples were collected at three depths (0.00-0.05, 0.05-0.10 and 10-0.20 m) in two seasons of the year (rainy and dry seasons) in the middle third of the landform with three replicates. The following attributes were determined: soil organic C, free light fraction, particulate organic matter, particulate organic C, mineral-associated organic C and dissolved organic matter. The experimental results showed that the soil organic matter appeared efficient in discriminating soil quality between degraded forest/pasture, as well as between degraded capoeira/pasture. But, it was not sensitive to discriminate within levels of degraded pastures. Among the organic matter pools studied, the most sensitive soil quality indicators were: particulate organic matter and dissolved organic matter, in the following degraded order verified from these indicators: forest < capoeira 1 = capoeira 2 < pastures (1, 2, 3 and 4).

Soil carbon and nitrogen sequestration over an age sequence of Pinus patula plantations in Zimbabwean Eastern Highlands

Forests play a major role in regulating the rate of increase of global atmospheric carbon dioxide (CO2) concentrations creating a need to investigate the ability of exotic plantations to sequester atmospheric CO2. This study examined pine plantations located in the Eastern Highlands of Zimbabwe relative to carbon (C) and nitrogen (N) storage along an age series. Samples of stand characteristics, forest floor (L, F and H) and 0–10, 10–30 and 30–60cm soil depth were randomly taken from replicated stands in Pinus patula Schiede & Deppe of 1, 10, 20, 25, and 30years plus two natural forests. Sodium polytungstate (density 1.6gcm−3) was used to isolate organic matter into free light fraction (fLF), occluded light fraction (oLF) and mineral associated heavy fraction (MaHF). In both natural and planted forests, above ground tree biomass was the major ecosystem C pool followed by forest floor’s humus (H) layer in addition to the 45%, 31% and 24% of SOC contributed by the 0–10, 10–30 and 30–60cm soil depths respectively. Stand age caused significant differences in total organic C and N stocks. Carbon and N declined initially soon after establishment but recovered rapidly at 10years, after which it declined following silvicultural operations (thinning and pruning) and recovered again by 25years. Soil C and N stocks were highest in moist forest (18.3kgCm−2 and 0.66kg of Nm−2) and lowest in the miombo (8.5kgm−2 of C and 0.22kg of Nm−2). Average soil C among Pinus stands was 11.4kg of Cm−2, being highest at 10years (13.7 of Ckgm−2) and lowest at 1year (9.9kg of Cm−2). Some inputs of charcoal through bioturbation over the 25year period contributed to stabilisation of soil organic carbon (SOC) and its depth distribution compared to the one year old stands. Nitrogen was highest at 10years (0.85kg of Nm−2) and least at 30years (0.22kg of Nm−2). Carbon and N in density fractions showed the 20year old stand having similar proportions of fLF and oLF while the rest had significantly higher fLF than oLF. The contribution of fLF C, oLF C and MaHF C to SOC was 8–13%, 1–7% and 90–91% respectively. Carbon and N in all fractions decreased with depth. The mineral associated C was significantly affected by stand age whilst the fLF and oLF were not. Conversion of depleted miombo woodlands to pine plantations yield better C gains in the short and long run whilst moist forest provide both carbon and biodiversity. Our results highlight the importance of considering forestry age based C pools in estimating C sink potential over a rotation and the possibility of considering conservation of existing natural forests as part of future REDD+projects.

Organic matter composition in density fractions of Cerrado Ferralsols as revealed by CPMAS 13C NMR: Influence of pastureland, cropland and integrated crop-livestock

Integrated crop-livestock (ICL) is a promising land use system for the Brazilian Cerrado, but little is known about what this system might change in chemical composition of soil organic matter. In three long-term experiments (9–11 years old), located on Cerrado Ferralsols in Dourados, Maracaju and Campo Grande (Mato Grosso do Sul State, Brazil), we assessed the impact of continuous cropland (CC), ICL, and permanent pasture of Brachiaria decumbens (PP) on the C concentration and composition of the free light fraction (FLF), occluded light fraction (OLF) and heavy fraction (HF) of soil in the 0–5cm layer. CPMAS13C NMR spectroscopy was used to determine the percentage of alkyl, O-alkyl, aromatic and carboxyl C types. In Dourados and Maracaju, PP had the highest concentrations of organic C in whole soil and physical fractions, while ICL was intermediate and CC lowest. In Campo Grande, soil organic C concentration was similar among management systems. Distribution of organic C across physical fractions was not affected by management nor by experimental site, and on average the FLF, OLF and HF contained 7%, 26% and 67% of the total storage, respectively. Signal peaks of the four main C types appeared in all CPMAS 13C NMR spectra, but at different intensities. O-alkyl was the major C type (about 50%), carboxyl was the minor representative (generally less than 10%) and alkyl and aromatic C were intermediates. From FLF to OLF, the alkyl and aromatic C concentrations increased, possibly due to selective preservation of waxes, resins, cutin, suberin and lignin. The HF had greater O-alkyl and lower aromatic C concentrations than OLF, which might have been related to the accumulation of microbial carbohydrates on mineral surfaces of the HF. Along the sequence CC–ICL–PP, the most evident changes were greater of O-alkyl and lower alkyl C types, practically in all fractions and sites. In FLF and OLF, these changes were attributed to greater biomass input and less seed drill-induced disturbance of soil surface (lower decomposition of residues) in the PP and ICL. Additionally, in OLF, greater O-alkyl concentration in PP and ICL was attributed to physical protection of particulate organic matter derived from grass roots occluded inside soil aggregates. Our results suggest that PP and ICL systems increased or maintained soil organic C concentrations compared to CC, associated with a qualitative increase of the chemically labile O-alkyl C type which was possibly related to greater biomass addition and less soil disturbance.

Impacts of organic matter amendments on carbon and nitrogen dynamics in grassland soils

Organic matter amendments have been proposed as a means to enhance soil carbon (C) stocks on degraded soils. However, only few data exist on rates of soil C sequestration or the fate of added C in grassland soils, which are generally thought to have high C storage potential. We measured changes in the amount of C and nitrogen (N) in soils and in the composition of soil organic matter (SOM) following a single application of composted organic matter in two annual grasslands from different bioclimatic zones (coastal and inland valley). There was a significant increase in bulk soil organic C content at the valley grassland, and a similar but non-significant trend at the coastal grassland. Physical fractionation of soil three years after organic matter amendment revealed increases in C and N in the free- and occluded light fractions in both the valley and coastal grasslands. Amendments resulted in a greater relative increase in the N stored in light soil fractions compared to C, leading to lower C:N ratios. Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy showed an increase in the ratio of carboxyl and carbonyl functional groups to aliphatic methyl and methylene groups in the free- and occluded light fractions. These data show that the organic matter amendment was incorporated in the free light and occluded light fractions over three years. Our results indicate that a single application of compost to grassland soils can increase soil C and N storage in labile and physically protected pools over relatively short time periods and contribute to climate change mitigation.

Free and intra‐aggregate organic matter as indicators of soil quality change in volcanic soils under contrasting crop rotations

AbstractSoil physical fractionation techniques may provide indicators of changing soil organic carbon (SOC) content; however, they have not been widely tested on volcanic soils (Andisols). In this study, we assessed two fractions as potential indicators in volcanic soils, using two sites in Chile converted from natural grassland to arable and mixed crop rotations, 8 and 16 yr previously. In the 8‐yr experiment, SOC had declined under all rotations, with smaller changes where the rotation included 3 or 5 yr of perennial pasture. Whereas the average SOC was only 76% of the level in the preceding natural grassland, the corresponding value after 16 yr for the second site was 98% (and 93% under continuous arable), probably reflecting its high allophane clay content. The fractionation procedure tested proved applicable to both Andisols, but the intra‐aggregate light fraction (IA‐SOM, isolated in sodium iodide solution at 1.80 g/cm3 after ultrasonic dispersion) accounted for a very small proportion of total SOC (&lt;1%). We suggest that in Andisols, the free light fraction (FR‐SOM, isolated in sodium iodide at solution of the same density, but prior to ultrasonic dispersion) is stabilised to a greater extent than in nonvolcanic soils, and the intra‐aggregate fraction plays a more minor role as a pool of intermediate turnover. The relative value of each fraction needs to be confirmed through dynamic experiments, using more sites, and including situations where SOC content is initially low.

Land use and management effects on soil organic matter fractions in Rhodic Ferralsols and Haplic Arenosols in Bindura and Shamva districts of Zimbabwe

Soil organic carbon (SOC) is a major attribute of soil quality that responds to land management activities which is also important in the regulation of global carbon (C) cycling. This study evaluated bulk soil C and nitrogen (N) contents and C and N dynamics in three soil organic matter (SOM) fractions separated by density. The study was based on three tillage systems on farmer managed experiments (conventional tillage (CT), ripping (RP), direct seeding (DS)) and adjacent natural forest (NF) in Haplic Arenosols (sandy) and Rhodic Ferralsols (clayey) of Zimbabwe. Carbon stocks were significantly larger in forests than tillage systems, being significantly lower in sandy soils (15 and 14MgCha−1) than clayey soils (23 and 21MgCha−1) at 0–10 and 10–30cm respectively. Nitrogen content followed the same trend. At the 0–10cm depth, SOC stocks increased under CT, RP and DS by 0.10, 0.24, 0.36Mgha−1yr−1 and 0.76, 0.54, 0.10Mgha−1yr−1 on sandy and clayey soils respectively over a four year period while N stocks decreased by 0.55, 0.40, 0.56Mgha−1 and 0.63, 0.65, 0.55Mgha−1 respectively. SOM fractions were dominated by mineral associated heavy fraction (MaHF) which accounted for 86–93% and 94–98% on sandy and clayey soils respectively. Tillage systems on sandy soils had the smallest average free light fraction (fLF) and occluded light fraction (oLF) C stocks (25.3±1.3 g m−2 and 7.3±1.2gm−2) at 0–30cm when compared with corresponding NF (58.4±4 gm2 and 18.5±1.0gm−2). Clayey soils, had the opposite, having all fLF C and N in tillage systems being higher (80.9±12gC m−2 and 2.7±0.4gNm−2) than NF (57.4±2.0gCm−2 and 2.4±0.3gNm−2). Results suggest that oLF and MaHF C and N are better protected under DS and RP where they are less vulnerable to mineralisation while fLF contributes more in CT. Thus, DS and RP can be important in maintaining and improving soil quality although their practicability can be hampered by unsupportive institutional frameworks. Under prevailing climatic and management conditions, improvement of residue retention could be a major factor that can distinguish the potential of different management practices for C sequestration. The exploitation of the benefits of RP or DS and the corresponding sustainability of systems need support for surface cover retention which should also be extended to conventional tillage.

Dynamics of soil organic carbon fractions one year after the re-conversion of poplar and willow plantations to arable use and perennial grassland

Due to the intensive soil tillage during the re-conversion of fast growing tree plantations back to agricultural use, high losses of accumulated soil organic matter are expected. A field trial was conducted at two former fast growing poplar plantations and one willow plantation in northern Germany to study the effects of different tillage depths and land use systems during such a re-conversion of the plantations on soil organic carbon (SOC) dynamics. Re-conversion was performed at tillage depths of 5, 15 and 30cm and rye-grass and maize cropping, were established. Directly after re-conversion and again one year after re-conversion, bulk soil C, distribution of C within various soil fractions (microbial biomass, water-stable aggregates, free and occluded light fraction organic matter) and C amounts added with coarse harvest residues were determined at 0–30cm soil depths. After re-conversion, the amount of C stored in the harvest residues was 17–39tCha−1. One year after re-conversion, it had declined distinctly but rarely significantly, due to the high spatial variability of the harvest residues in the field. Nevertheless, C of the bulk soil did not change, but a decrease of microbial biomass C, macroaggregate (250–2000μm)C and free light fraction organic matter indicate a loss of important fractions of soil organic C. More C was found in macroaggregates under rye-grass than under maize one year after re-conversion in loamy soils. Overall, one year after re-conversion yet no clear effects of tillage depth or land-use on SOC dynamics could be detected.

Organic layer and clay content control soil organic carbon stocks in density fractions of differently managed German beech forests

Forest management and associated litter inputs and decomposition rates are thought to affect the carbon storage in mineral soils. Here, we studied the effects of forest management on soil organic carbon (OC) stocks in density fractions of Ah-horizons in soils that developed on loess. We used 82 beech (Fagus sylvatica L.) dominated forest plots in Thuringia, Germany that differed in their management (unmanaged forest, forests under age-class management and forests under selection cutting forest). After density fractionation of the mineral soil with a 1.6gcm−3 polytungstate solution we determined OC concentrations and stocks as well as CN-ratios in the free (f-LF) light fraction, the occluded (o-LF) light fraction and in the mineral associated organic matter (MOM) fraction. In our study, Ah-horizons of beech forests stored on average 2.6±0.2kgm−2 (38.7±1.3kgm−3) OC. The results showed that 37% of the bulk soil OC was stored in the light fractions. We could show that OC stocks in the light fraction were significantly affected by the amount of C stored in organic layers (p=0.011). The OC stocks in the organic layers, in turn, were higher in unmanaged forests and in forests under selection cutting. This suggests a sensitivity of unprotected OC in the f-LF of beech forests against forest management. In contrast to the f-LF, the OC stocks in the MOM fraction are mainly controlled by pedogenic properties such as clay and iron oxide content. Even after several decades of forest management and with large sample size, an effect of forest management on the stable MOM fraction could not be detected.

Fracionamento químico e físico da matéria orgânica de um argissolo vermelho sob diferentes sistemas de uso

Objetivou-se com este estudo avaliar o carbono orgânico total e as frações físicas e químicas da matéria orgânica (MO) do solo em um Argissolo Vermelho Eutrófico arênico, submetido a diferentes sistemas de uso. Os sistemas de uso do solo avaliados foram: florestamento homogêneo de Eucalyptus grandis (EUC), sistema agrossilvipastoril na faixa (ASP) e campo nativo (CN), nas camadas de 0,000-0,025m e de 0,025-0,075m. A fração grosseira (CFG) e o carbono associado aos minerais (CAM) foram obtidos por meio de fracionamento físico granulométrico. As frações leve livre (FLL), leve oclusa (FLO) e pesada (FP) foram obtidas por meio de fracionamento físico densimétrico. As frações não húmicas (NH), ácido fúlvico (AF), ácido húmico (AH) e humina (HU) foram obtidas por meio de fracionamento químico. O sistema EUC promoveu maiores estoques de carbono orgânico total, CFG, FLL e FLO e na fração AF nas camadas avaliadas. Na camada superficial, os mecanismos de proteção da MO por recalcitrância molecular e estabilização química estão sobrepondo a estabilidade decorrente da oclusão em agregados. Com a dificuldade de formar agregados, devido a matrizes arenosas desse solo, o carbono jovem que entra no sistema é decomposto pelos microrganismos, entrando em um estágio mais avançado de decomposição e, nesse caso, formando associações com as partículas silte e argila, mesmo em solos em que o percentual de argila é baixo. Através do fracionamento químico, observou-se que a maior parte da MO do solo encontra-se armazenada na forma de HU.

Cover crops and no-till effects on physical fractions of soil organic matter

Physical fractions (free light fraction, intra-aggregate light fraction and heavy fraction) of soil organic matter (SOM) are good indicators of soil quality for sustainable land use. The objective of this study was to evaluate the effect of cover crops on total organic carbon (TOC) and physical fractions of soil organic matter in soil under a no-tillage system (NTS) and a conventional tillage system (CTS, one plowing and two disking). A three-year field experiment was carried out as a cover crop-rice (Oryza sativa)-cover crop-rice rotation. Treatments included cover crops (Panicum maximum, Brachiaria ruziziensis, Brachiaria brizantha, and pearl millet (Pennisetum glaucum), fallow, till or no till. The SOM was physically fractionated in free light fraction (FLF), intra-aggregates light fraction (IALF) and heavy fraction (HF). The levels of C in whole soil were also evaluated, as well as C in the light fractions (FLF+IALF) and in the HF. Results indicated that concentrations of C in the FLF and IALF in surface soils (0–0.05m) were much higher (10.8 and 1.95gkg−1, respectively) than that in the 0.05–0.1m soil depth (7.68 and 1.54gkg−1, respectively) and in the 0.1–0.2m soil depth (4.98 and 1.24gkg−1, respectively). The NTS resulted in higher levels of FLF (12.2gkg−1) and IALF (2.19gkg−1) than with CTS (1.37–7.30gkg−1). Millet had the highest C (19.5gkg−1) and N (1.1gkg−1) concentrations in soil. There was an accumulation of TOC and total N in the surface soil with cover crops, and concentrations of TOC were higher in the HF (79.0%) than in the light fractions (21.0%). Although SOM changed little during the two years of this experiment, the various C fractions were significantly affected by the tillage treatments. We conclude that SOM physical fractionation allowed seeing significant differences caused by the soil management in the organic matter dynamics in a short period of time.

Storage and stability of organic carbon in soils as related to depth, occlusion within aggregates, and attachment to minerals

Abstract. Conceptual models suggest that stability of organic carbon (OC) in soil depends on the source of plant litter, occlusion within aggregates, incorporation in organo-mineral complexes, and location within the soil profile. Density fractionation is a useful tool to study the relevance of OC stabilization in aggregates and in association with minerals, but it has rarely been applied to full soil profiles. We aim to determine factors shaping the depth profiles of physically unprotected and mineral associated OC and test their relevance for OC stability across a range of European soils that vary in vegetation, soil types, parent material, and land use. At each of the 12 study sites, 10 soil cores were sampled to 60 cm depth and subjected to density separation. Bulk soil samples and density fractions (free light fractions – fLF, occluded light fractions – oLF, heavy fractions – HF) were analysed for OC, total nitrogen (TN), δ14C, and Δ14C. Bulk samples were also incubated to determine CO2 evolution per g OC in the samples (specific mineralization rates) as an indicator for OC stability. Depth profiles of OC in the light fraction (LF-OC) matched those of roots for undisturbed grassland and forest sites, suggesting that roots are shaping the depth distribution of LF-OC. Organic C in the HF declined less with soil depth than LF-OC and roots, especially at grassland sites. The decrease in Δ14C (increase in age) of HF-OC with soil depth was related to soil pH as well as to dissolved OC fluxes. This indicates that dissolved OC translocation contributes to the formation of subsoil HF-OC and shapes the Δ14C profiles. The LF at three sites were rather depleted in 14C, indicating the presence of fossil material such as coal and lignite, probably inherited from the parent material. At the other sites, modern Δ14C signatures and positive correlations between specific mineralization rates and fLF-OC indicate the fLF is a potentially available energy and nutrient source for subsurface microorganisms throughout the profile. Declining specific mineralization rates with soil depth confirm greater stability of OC in subsoils across sites. The overall importance of OC stabilization by binding to minerals was demonstrated by declining specific mineralization rates with increasing contributions of HF-OC to bulk soil OC, and the low Δ14C values of HF-OC. The stability of HF-OC was greater in subsoils than in topsoils; nevertheless, a portion of HF-OC was active throughout the profile. While quantitatively less important than OC in the HF, consistent older ages of oLF-OC than fLF-OC suggest that occlusion of LF-OC in aggregates also contributes to OC stability in subsoils. Overall, our results indicate that association with minerals is the most important factor in stabilization of OC in soils, irrespective of vegetation, soil type, and land use.