Soil organic matter dynamics in grassland soils under elevated CO2: Insights from long-term incubations and stable isotopes

Elevated atmospheric carbon dioxide (CO₂) levels generally stimulate carbon (C) uptake by plants, but the fate of this additional C largely remains unknown. This uncertainty is due in part to the difficulty in detecting small changes in soil carbon pools. We conducted a series of long-term (170-330 days) laboratory incubation experiments to examine changes in soil organic matter pool sizes and turnover rates in soil collected from an open-top chamber (OTC) elevated CO₂ study in Colorado shortgrass steppe. We measured concentration and isotopic composition of respired CO₂ and applied a two-pool exponential decay model to estimate pool sizes and turnover rates of active and slow C pools. The active and slow C pools of surface soils (5-10 cm depth) were increased by elevated CO₂, but turnover rates of these pools were not consistently altered. These findings indicate a potential for C accumulation in near-surface soil C pools under elevated CO₂. Stable isotopes provided evidence that elevated CO₂ did not alter the decomposition rate of new C inputs. Temporal variations in measured δ¹³C of respired CO₂ during incubation probably resulted mainly from the decomposition of changing mixtures of fresh residue and older organic matter. Lignin decomposition may have contributed to declining δ¹³C values late in the experiments. Isotopic dynamics during decomposition should be taken into account when interpreting δ¹³C measurements of soil respiration. Our study provides new understanding of soil C dynamics under elevated CO₂ through the use of stable C isotope measurements during microbial organic matter mineralization.     

The turnover of organic carbon in subsoils. Part 2. Modelling carbon turnover

A new model, RothPC‐1, is described for the turnover of organic C in the top metre of soil. RothPC‐1 is a version of RothC‐26.3, an earlier model for the turnover of C in topsoils. In RothPC‐1 two extra parameters are used to model turnover in the top metre of soil: one, p, which moves organic C down the profile by an advective process, and the other, s, which slows decomposition with depth. RothPC‐1 is parameterized and tested using measurements (described in Part 1, this issue) of total organic C and radiocarbon on soil profiles from the Rothamsted long‐term field experiments, collected over a period of more than 100 years. RothPC‐1 gives fits to measurements of organic C and radiocarbon in the 0–23, 23–46, 46–69 and 69–92 cm layers of soil that are almost all within (or close to) measurement error in two areas of regenerating woodland (Geescroft and Broadbalk Wildernesses) and an area of cultivated land from the Broadbalk Continuous Wheat Experiment. The fits to old grassland (the Park Grass Experiment) are less close. Two other sites that provide the requisite pre‐ and post‐bomb data are also fitted; a prairie Chernozem from Russia and an annual grassland from California. Roth‐PC‐1 gives a close fit to measurements of organic C and radiocarbon down the Chernozem profile, provided that allowance is made for soil age; with the annual grassland the fit is acceptable in the upper part of the profile, but not in the clay‐rich Bt horizon below. Calculations suggest that treating the top metre of soil as a homogeneous unit will greatly overestimate the effects of global warming in accelerating the decomposition of soil C and hence on the enhanced release of CO₂ from soil organic matter; more realistic estimates will be obtained from multi‐layer models such as RothPC‐1.     

The turnover of organic carbon in subsoils. Part 1. Natural and bomb radiocarbon in soil profiles from the Rothamsted long-term field experiments

The Rothamsted long‐term field experiments, started more than 150 years ago, provide unique material for the study of carbon turnover in subsoils. Total organic C, ¹⁴C and ¹³C were measured on soil profiles taken from these experiments, before and after the thermonuclear bomb tests of the mid‐20th century. Four contrasting systems of land management were sampled: land cultivated every year for winter wheat; regenerating woodland on acid soil; regenerating woodland on calcareous soil; and old grassland. The mean radiocarbon ages of all the pre‐bomb samples from cultivated land were 1210 years (0–23 cm), 2040 years (23–46 cm), 3610 years (46–69 cm) and 5520 years (69–92 cm). Bomb radiocarbon derived from thermonuclear tests was present throughout the profile in all the post‐bomb samples, although below 23 cm the amounts were small and the pre‐ and post‐bomb radiocarbon measurements were often not significantly different. Values of δ¹³C increased down the profile, from ?26.3‰ (0–23 cm layer, mean of all measurements) to ?25.2‰ for the 69–92 cm layer. The C/N ratios decreased with depth in virtually all of the profiles sampled. Excluding the surface (0–23 cm) soils from the old grassland, the hyperbola m = 152.1 ? 2341/(1 + 0.264n) gave a close fit to the radiocarbon data from all depths, all sampling times and all sites, where n is the organic C content of the soil, in t ha^?1, and m is the radiocarbon content of the soil, in Δ¹⁴C units, corrected for expansion or contraction of soil layers with time. The aberrant grassland soils almost certainly contained coal: one of them was shown by ¹³C‐NMR to contain 0.82% coal C. In Part 2 (this issue) of this pair of papers, these radiocarbon and total C measurements are used to develop and test a new model for the turnover of organic C in subsoils.     

Distribution of polycyclic aromatic hydrocarbons in particle‐size separates and density fractions of typical agricultural soils in the Yangtze River Delta,east China

Soil organic matter can be divided into different organic carbon (C) pools with different turnover rates. The organic pollutants in soils associated with these organic C pools may have different bioavailability and environmental risks during the decomposition of soil organic matter. We studied the distribution patterns of 15 USEPA priority polycyclic aromatic hydrocarbons (PAHs) in different particle‐size separates (clay, fine silt, coarse silt, fine sand and coarse sand) and density fractions (light and heavy fractions) of nine agricultural topsoils (0–20 cm depth) from a contaminated area in the Yangtze River Delta region of east China. There was a decreasing trend in PAH concentration in particle‐size separates with decreasing particle size. However, the different particle‐size separates had similar PAH composition. The concentration of PAHs in the light fraction ranged from 13 037 to 107 299 μg kg^?1, far higher than in the heavy fraction, which ranged from 222 to 298 μg kg^?1. Although the light fraction accounted for only 0.4–2.3% of the soils, it was associated with 31.5–69.5% of soil PAHs. The organic matter in coarse silt had the strongest capacity for enrichment with PAHs. Combining the distributions of PAHs and the turnover rates of organic matter in different soil fractions, the environmental risks of PAH‐polluted soils may be due mainly to the PAHs associated with sand and the light fraction.     

Soil organic carbon fluxes abd turnover rates estimated from radiocarbon enrichments

A simple method is described for analysing data on soil organic carbon and enriched ¹⁴C, to provide estimates of the carbon input rate, decomposition time and downward diffusivity in the soil column. Data on atmospheric ¹⁴C enrichment, needed for such calculations on soil cores sampled between 1960 and 1990, are provided. Five New Zealand pasture soils and a forest soil are analysed by this method. The results show the marked effect of earthworms on increasing the C input, reducing the decomposition time and increasing the downward diffusion rate of soil organic C. An undeveloped sandy soil had a small input rate and a rather low downward diffusion rate. The forest soil studied had an organic C input similar to that in good pasture soils but almost no downward diffusion, resulting in a very shallow layer of soil organic C.     

Decomposition of labelled roots and root-C and -N allocation between soil fractions in mountain grasslands

Given the high turnover of fine roots in mountain grasslands, knowledge of their decomposition rates and the capacity of mountain grassland soils to stabilize root-derived C are central to understand the role of these ecosystems as potential C sinks. Here we studied the decomposition of fine roots in mountain grasslands and estimated the rates at which root-C and -N incorporated into protected pools at two soil depths. For this purpose, we incubated standard ¹³C- and ¹⁵N-labelled wheat roots mixed with unlabelled soil at 5 and 20 cm depth in two mountain grassland sites. Particle size fractionation allowed the quantification of the labelled wheat root-C and -N allocated to each size fraction (coarse sand, fine sand and silt plus clay sized) as well as their incorporation rates into the finest fraction. Between 62% and 78% root-C remained in the soil after one year of field incubation, faster decomposition being registered at the warmest site. In the following two years, roots decomposed much more slowly. In contrast to reports in the literature, our results indicate that decay rates during the first year were highest in the deep layer. The incorporation of wheat root-derived organic matter into the silt plus clay size fraction was also much greater during the first year of decomposition than in the following two years and also slightly higher in the deep soil than in topsoil. The incorporation rates of root-¹³C and root-¹⁵N into this fraction also suggest that the wheat-derived organic matter associated with this fraction was N-enriched and less recalcitrant (i.e., less resistant to acid hydrolysis) than that recovered from the coarser fractions. Furthermore, recalcitrant organic matter incorporated much more slowly than labile organic matter did. We conclude that the conditions of the subalpine grassland subsoil are more favourable for root decomposition than the topsoil and that the organic matter that incorporates into the protected pool is characterised by a high N content and low biochemical recalcitrance.     

Bacterial and enchytraeid abundance accelerate soil carbon turnover along a lowland vegetation gradient in interior Alaska

Boreal wetlands are characterized by a mosaic of plant communities, including forests, shrublands, grasslands, and fens, which are structured largely by changes in topography and water table position. The soil associated with these plant communities contain quantitatively and qualitatively different forms of soil organic matter (SOM) and nutrient availability that drive changes in biogeochemical cycling rates. Therefore different boreal plant communities likely contain different soil biotic communities which in turn affect rates of organic matter decomposition. We examined relationships between plant communities, microbial communities, enchytraeids, and soil C turnover in near-surface soils along a shallow topographic soil moisture and vegetation gradient in interior Alaska. We tested the hypothesis that as soil moisture increases along the gradient, surface soils would become increasingly dominated by bacteria and mesofauna and have more rapid rates of C turnover. We utilized bomb radiocarbon techniques to infer rates of C turnover and the ¹³C isotopic composition of SOM and respired CO₂ to infer the degree of soil humification. Soil phenol oxidase and peroxidase enzyme activities were generally higher in the rich fen compared with the forest and bog birch sites. Results indicated greater C fluxes and more rapid C turnover in the surface soils of the fen sites compared to the wetland forest and shrub sites. Quantitative PCR analyses of soil bacteria and archaea, combined with enchytraeid counts, indicated that surface soils from the lowland fen ecosystems had higher abundances of these microbial and mesofaunal groups. Fungal abundance was highly variable and not significantly different among sites. Microbial data was utilized in a food web model that confirmed that rapidly cycling systems are dominated by bacterial activity and enchytraeid grazing. However, our results also suggest that oxidative enzymes play an important role in the C mineralization process in saturated systems, which has been often ignored.     

Long-term nitrogen additions increased surface soil carbon concentration in a forest plantation despite elevated decomposition

Forests cover one-third of the Earth’s land surface and account for 30-40% of soil carbon (C). Despite numerous studies, questions still remain about the factors controlling forest soil C turnover. Present understanding of global C cycle is limited by considerable uncertainty over the potential response of soil C dynamics to rapid nitrogen (N) enrichment of ecosystems, mainly from fuel combustion and fertilizer application. Here, we present a 15-year-long field study and show an average increase of 14.6% in soil C concentration in the 0-5 cm mineral soil layer in N fertilized (defined as N+ hereafter) sub-plots of a second-rotation Pinus radiata plantation in New Zealand compared to control sub-plots. The results of ¹⁴C and lignin analyses of soil C indicate that N additions significantly accelerate decomposition of labile and recalcitrant soil C. Using an annual-time step model, we estimated the soil C turnover time. In the N+ sub-plots, soil C in the light (a density < 1.70 g cm⁻³) and heavy fractions had the mean residence times of 23 and 67 yr, respectively, which are lower than those in the control sub-plots (36 and 133 yr in the light and heavy fractions, respectively). The commonly used lignin oxidation indices (vanillic acid to vanillin and syringic acid to syringaldehyde ratios) were significantly greater in the N+ sub-plots than in the control sub-plots, suggesting increased lignin decomposition due to fertilization. The estimation of C inputs to forest floor and δ¹³C analysis of soil C fractions indicate that the observed buildup of surface soil C concentrations in the N+ sub-plots can be attributed to increased inputs of C mass from forest debris. We conclude that long-term N additions in productive forests may increase C storage in both living tree biomass and soils despite elevated decomposition of soil organic matter.     

Soil respiration in a chihuahuan desert rangeland

Soil respiration of a desert soil was measured at the New Mexico State University Ranch in Southern New Mexico. Respiration rates were highest during late July and August after summer rains. Soil respiration data were used to estimate soil organic matter turnover which was 54 yr using summer data and 20 yr using both summer and winter data. The long turnover estimate for summer measurements resulted from temperatures above optimum in June and July. Diurnal soil respiration was also measured after a simulated 2.54 cm rain event. For both wetted and dry soils, temperature controlled the patterns of soil respiration with an optimum of near 41°C. Activation energy values decreased from 84.91 to 39.5 kJ mol^?1 when the soil was wetted. A light-dark container method was tested as a possible means of estimating algal uptake of CO₂, however, the method was not feasible for desert soils.     

Assessment, based on a climosequence of soils in tussock grasslands, of soil carbon storage and release in response to global warming

A soil climosequence in tussock grasslands in South Island, New Zealand, encompassing climates ranging from cold to warm temperate provided a spatial analogue of climate change for investigating the effects of global warming on soil C contents and turnover. Mean annual temperature (T) and annual precipitation (P) ranged from 2 to 10°C, and 350 to 5000 mm, respectively. Soil C contents were curvilinearly related to T/P across the sequence (r=−0.95, significant at P<0.0l), indicating that east of the Southern Alps, increased decomposition of organic matter with global warming would provide a positive feedback to further increase atmospheric CO₂. This decrease in New Zealand's soil C, estimated to be up to 10% of the current content for a global temperature rise of 0.03 K a⁻¹ to 2050, could contribute about 0.5 × 10¹⁵ g C to the atmosphere over the next 60 years. These conclusions were generally supported by changes in soil C turnover estimated from ‘bomb’¹⁴C enrichment. The unexpectedly slow turnover found for two soils was explained by a ‘memory’ effect from the former southern beech forest that grew on these soils in prehistoric times. Accumulation of Al-humus under the forest may be responsible for the slow C turnover observed.     

Location and chemical composition of stabilized organic carbon in topsoil and subsoil horizons of two acid forest soils

The ¹⁴C age of soil organic matter is known to increase with soil depth. Therefore, the aim of this study was to examine the stabilization of carbon compounds in the entire soil profile using particle size fractionation to distinguish SOM pools with different turnover rates. Samples were taken from a Dystric Cambisol and a Haplic Podzol under forest, which are representative soil types under humid climate conditions. The conceptual approach included the analyses of particle size fractions of all mineral soil horizons for elemental composition and chemical structure of the organic matter by ¹³C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy. The contribution of phenols and hydroxyalkanoic acids, which represent recalcitrant plant litter compounds, was analyzed after CuO oxidation.In the Dystric Cambisol, the highest carbon concentration as well as the highest percentage of total organic carbon are found in the <6.3 μm fractions of the B and C horizons. In the Haplic Podzol, carbon distribution among the particle size fractions of the Bh and Bvs horizons is influenced by the adsorption of dissolved organic matter. A relationship between the carbon enrichment in fractions <6.3 μm and the ¹⁴C activity of the bulk soil indicates that stabilization of SOM occurs in fine particle size fractions of both soils. ¹³C CPMAS NMR spectroscopy shows that a high concentration of alkyl carbon is present in the fine particle size fractions of the B horizons of the Dystric Cambisol. Decreasing contribution of O-alkyl and aromatic carbon with particle size as well as soil depth indicates that these compounds are not stabilized in the Dystric Cambisol. These results are in accordance with data obtained by wet chemical analyses showing that cutin/suberin-derived hydroxyalkanoic acids are preserved in the fine particle size fractions of the B horizons. The organic matter composition in particle size fractions of the top- and subsoil horizons of the Haplic Podzol shows that this soil is acting like a chromatographic system preserving insoluble alkyl carbon in the fine particle size fractions of the A horizon. Small molecules, most probably organic acids, dominate in the fine particle size fractions of the C horizons, where they are stabilized in clay-sized fractions most likely due to the interaction with the mineral phase. The characterization of lignin-derived phenols indicated, in accordance with the NMR measurements, that these compounds are not stabilized in the mineral soil horizons.     

The 14C age and residence time of organic matter and its lipid constituents in a stagnohumic gley soil

A combination of radiocarbon (¹⁴C) dating and biomarker analyses of the aliphatic hydrocarbons in soil lipids is proposed as a novel and improved method for studying the environmental history of peaty soils. The radiocarbon concentration of unfractionated bulk organic matter, hydrolysed soil residues and two lipid fractions (the aliphatic hydrocarbons and carboxylic acids) recovered from a stagnohumic gley soil, were compared using AMS (accelerator mass spectrometry) and radiometric ¹⁴C dating techniques. The radiocarbon ages recorded by the aliphatic hydrocarbon fractions were consistently older than those measured from the unfractionated soil, and were in most cases older than the residues remaining after acid hydrolysis. This pattern was observed at three different depths in the soil profile. The apparent age difference between the hydrocarbon fraction and its unfractionated substrate increased with depth. An abundance of long–chain n–alkanes, similar to those found in higher plant waxes, characterized the aliphatic hydrocarbon fraction from the deepest soil (at 21.5–24.5–cm depth). The radiocarbon age of this basal organic component (13470± 170 years bp ) indicated that it derived from the initial re–establishment of vegetation on the local deglaciated landscape with the onset of the Windermere Interstadial (c. 14000–13000 ¹⁴C years bp ). Bulk organic detritus within the basal horizon dated at some 3000 years younger, and presumably as a result of the downward penetration and retention of some mobile organic residues produced later in the development of the soil profile. The survival and apparent stratigraphical stability of these recoverable aliphatic hydrocarbons provides the opportunity, via the development of AMS dating, to measure an unambiguous radiocarbon age for the origin of organic residues retained in soils and sediments.     

Microbial biomass formed from 14C, 15N-labelled plant material decomposing in soils in the field

The decomposition of ¹⁴C, ¹⁴N-labelled medic (Medicago littoralis) material and the net formation and decay of isotope-labelled biomass have been measured in four South Australian soils in the field over 4 yr. The field sites were in similar climatic zones but two sites received about twice as much rainfall as the others. The soils were calcareous and of similar pH, but differed in texture and organic matter content. The decomposition of the organic-¹⁴C and organic-¹⁵N residues were, for a given site, similar. Initially, the concentrations of labelled residues decreased rapidly, then very slowly. Decomposition rates in a heavy clay soil were significantly less than in the other soils during the first 16 weeks after incorporation of plant material, but thereafter, rates of decomposition in all soils were similar, despite differences in soil texture and climate. More than 50% of the medic-¹⁴C had disappeared from all soils after 4 weeks of decomposition and only 15–20% of the medic-¹⁴C remained as organic residues after 4 yr. Of the medic-¹⁵N 60–65% remained as organic residues after 32 weeks decomposition; the percentage decreased to 45–50% after 4 yr.The amounts of ¹⁴C, ¹⁴N-labelled biomass, formed from decomposing plant material, were maximal 4–8 weeks after incorporation of plant material into the soils. In samples taken at 8 weeks from the sandy Roseworthy soil, biomass-¹⁴C and -¹⁵N accounted for 14 and 22% respectively of the total organic-¹⁴C and -¹⁵N residues present. Thereafter in this soil, the concentrations of biomass-¹⁴C and -¹⁵N decreased, rapidly at first then more slowly. Nevertheless, throughout most of the decomposition the rates of decrease in the concentrations of biomass-¹⁴C and -¹⁵N exceeded those of the non-biomass, labelled organic residues.The proportions of ¹⁴C, ¹⁵N-labelled materials accounted for in the labelled biomass varied between soils. Soils of higher clay content generally retained higher proportions of residual organic-¹⁴C and -¹⁴N in the biomass, even though the net rates of decomposition of total labelled residues did not differ significantly between soils during most of the decomposition.     

The role of amino acids and nucleic bases in turnover of nitrogen and carbon in soil humic fractions

Incorporation of labelled ¹⁵N and ¹⁴C amino acids and nucleic bases into soil humus fractions as well as humus turnover was investigated under field conditions. The dynamics of ¹⁵N and ¹⁴C incorporation into organic matter was characterized by the following main steps: rapid incorporation of the labelled substance prevailing for the first 1–3 weeks, and decomposition of included labelled fragments prevailing beyond one month after substance addition. The annual turnover rates of N and C in humus fractions due to incorporation of amino acids and nucleic bases were calculated. The turnover rate of N in humus is two to three times that of C. The contribution of amino acids to organic matter generation is about twice as great as that of nucleic bases and other N-containing organic substances. This indicates the important role of amino acids in the humification process and humus turnover. Turnovers of humic acids (0·002 year^?1 for C and 0·02 for N) are the most rapid of humic fractions investigated, and humin is characterized by the slowest turnover (0·0002 year^?1 for C and 0·007 for N). There are no significant differences in the turnover rates of fulvic acid fractions (0·0002 year^?1 for C) with different molecular weight.     

Chemical and mineral control of soil carbon turnover in abandoned tropical pastures

We investigated changes in soil carbon (C) cycling with reforestation across a long-term, replicated chronosequence of tropical secondary forests regrowing on abandoned pastures. We applied CP MAS ¹³C NMR spectroscopy and radiocarbon modeling to soil density fractions from the top 10 cm to track changes in C chemistry and turnover during secondary forest establishment on former pastures. Our results showed that inter-aggregate, unattached, particulate organic C (free light fraction) and particulate C located inside soil aggregates (occluded light fraction) represent distinct soil C pools with different chemical composition and turnover rates. The signal intensity of the O-alkyl region, primarily representing carbohydrates, decreased, and alkyl C, attributed to recalcitrant waxy compounds and microbially resynthesized lipids, increased from plant litter to soil organic matter and with incorporation into soil aggregates. The alkyl/O-alkyl ratio, a common index of humification, was higher in the occluded than in the free light fraction. Greater variability in the chemical makeup of the occluded light fraction suggests that it represents material in varied stages of decomposition. Mean residence times (¹⁴C-based) of the free light fraction were significantly shorter (4 ± 1 years) than for the heavy fraction. We report two scenarios for the occluded light fraction, one fast-cycling in which the occluded and free light fractions have similar turnover rates, and one slow-cycling, in which the occluded light fraction resembles the heavy fraction. Mean residence times of the occluded light fraction and heavy fraction in active pastures and 10-year old secondary forests in the earliest stage of succession were longer than in older secondary forests and primary forests. This is likely due to a preferential loss of physically unprotected C of more labile composition in the pastures and in the youngest successional forests, resulting in an increase in the dominance of slow-cycling C pools. Soil carbon turnover rates of the mineral-associated C in secondary forests recovering from abandoned pasture resembled those of primary forests in as little as 20 years of succession.     

Untersuchungen zur Kohlenstoffdynamik und Bioturbation von Mollisolen

Investigations regarding carbon dynamics and bioturbation in Mollisols Bioturbation in Mollisols (mollic horizons) is not really leading to a homogenisation of C-atoms of different ¹⁴C-ages. Age versus depth curves indicate rather flatly stretched or steeper shapes, dependent on either slow development via an extended pararendzina phase or faster growth of the mollic epipedon under more half bog like conditions. 291 ¹⁴C-dates of Mollisols versus depth of soil result in one mutual and several individualized regression lines for the countries of origin with variable coefficients of X. Highest levels of X indicate steepest increase of age towards depth, i.e. weakest bioturbation in the sense of homogenization. When eliminating 16 sand covered Mollisol samples from Tunisia 275 dates remain, which, plotted as age/profile maximum age versus depth, produce regressions of the date collectives of different countries in relation to organic matter decomposition. The steepest regression line indicates maximization of old carbon species and most intensive decomposition of the young organic matter; the flat ones stand for slowed down decomposition. Comparative ¹⁴C-dates of texture fractions of the aquic Hapludolls ?Asel”? and ?Söllingen”? show age maxima in the middle clay and fine clay fractions. ¹⁴C-dating of the body – C of layerwise collected earthworms as well as of fillings of fossil rainworm or Citellus tracks reveal for the worm – C exclusively recent bomb – C (> 100% NBS) with youngest C species in the top soil and in the greatest depth of the soil profile. The fossil rainworm and Citellus track-fillings are above the measured total soil age. Studies of the decomposition of organic matter from different soil depth indicate in case of the almost neutral agricultural soil ?Söllingen”? expectedly lowest CO₂-release from the organo-mineralically stabilized soil layer at maximum depth (in mgC/100 g soil as well as in mg C/g C_org). In case of the more acid forest soil ?Asel”? the same trend of diminishing CO₂ release with depth prevails at normal soil-pH as well as under condition of uniformly adjusted pH to 7.5–8.3. The two deepest layers (80–100 cm) fall of the trend by higher rates of decomposition. The unfavorable ratio of organic C (0.139%) versus carbonate-C (1.54%) under synchroneous pH-increase is considered responsible.     

Predicting soil organic matter stability in agricultural fields through carbon and nitrogen stable isotopes

In order to evaluate the sustainability and efficiency of soil carbon sequestration measures and the impact of different management and environmental factors, information on soil organic matter (SOM) stability and mean residence time (MRT) is required. However, this information on SOM stability and MRT is expensive to determine via radiocarbon dating, precluding a wide spread use of stability measurements in soil science. In this paper, we test an alternative method, first developed by Conen et al. (2008) for undisturbed Alpine grassland systems, using C and N stable isotope ratios in more frequently disturbed agricultural soils. Since only information on carbon and nitrogen concentrations and their stable isotope ratios is required, it is possible to estimate the SOM stability at greatly reduced costs compared to radiocarbon dating. Using four different experimental sites located in various climates and soil types, this research proved the effectiveness of using the C/N ratio and δ¹⁵N signature to determine the stability of mOM (mineral associated organic matter) relative to POM (particulate organic matter) in an intensively managed agro-ecological setting. Combining this approach with δ¹³C measurements allowed discriminating between different management (grassland vs cropland) and land use (till vs no till) systems. With increasing depth the stability of mOM relative to POM increases, but less so under tillage compared to no-till practises. Applying this approach to investigate SOM stability in different soil aggregate fractions, it corroborates the aggregate hierarchy theory as proposed by Six et al. (2004) and Segoli et al. (2013). The organic matter in the occluded micro-aggregate and silt & clay fractions is less degraded than the SOM in the free micro-aggregate and silt & clay fractions. The stable isotope approach can be particularly useful for soils with a history of burning and thus containing old charcoal particles, preventing the use of ¹⁴C to determine the SOM stability.     

Arbuscular mycorrhizal fungi contribute to C and N enrichment of soil organic matter in forest soils

Increasing evidence suggests that accretion of microbial turnover products is an important driver for isotopic carbon (C) and nitrogen (N) enrichment of soil organic matter (SOM). However, the exact contribution of arbuscular mycorrhizal fungi (AMF) to soil isotopic patterns remains unknown. In this study, we compared ¹³C and ¹⁵N patterns of glomalin-related soil protein (GRSP), which includes a main fraction derived from AMF, litter, and bulk soil in four temperate rainforests. GRSP was an abundant C and N pool in these forest soils, showing significant ¹³C and ¹⁵N enrichment relative to litter and bulk soil. Hence, cumulative accumulation of recalcitrant AMF turnover products in the soil profile likely contributes to ¹³C and ¹⁵N enrichment in forest soils. Further research on the relationship between GRSP and AMF should clarify the exact extent of this process.     

Comparison of forest soil carbon dynamics at five sites along a latitudinal gradient

The aim of this study was to compare the turnover time of labile soil carbon (C), in relation to temperature and soil texture, in several forest ecosystems that are representative of large areas of North America. Carbon and nitrogen (N) stocks, and C:N ratios, were measured in the forest floor, mineral soil, and two mineral soil fractions (particulate and mineral-associated organic matter, POM and MOM, respectively) at five AmeriFlux sites along a latitudinal gradient in the eastern United States. Sampling at four sites was replicated over two consecutive years. With one exception, forest floor and mineral soil C stocks increased from warm, southern sites (with fine-textured soils) to cool, northern sites (with more coarse-textured soils). The exception was a northern site, with less than 10% silt-clay content, that had a soil organic C stock similar to the southern sites. A two-compartment model was used to calculate the turnover time of labile soil organic C (MRT_U) and the annual transfer of labile C to stable C (k₂) at each site. Moving from south to north, MRT_U increased from approximately 5 to 14 years. Carbon-13 enrichment factors (ε), that described the rate of change in δ¹³C through the soil profile, were associated with soil C turnover times. Consistent with its role in stabilization of soil organic C, silt-clay content was positively correlated (r = 0.91; P ≤ 0.001) with parameter k₂. Latitudinal differences in the storage and turnover of soil C were related to mean annual temperature (MAT, °C), but soil texture superseded temperature when there was too little silt and clay to stabilize labile soil C and protect it from decomposition. Each site had a relatively high proportion of labile soil C (nearly 50% to a depth of 20 cm). Depending on unknown temperature sensitivities, large labile pools of forest soil C are at risk of decomposition in a warming climate, and losses could be disproportionately higher from coarse textured forest soils.     

Long‐term effects of tillage,nutrient application and crop rotation on soil organic matter quality assessed by NMR spectroscopy

Crop and land management practices affect both the quality and quantity of soil organic matter (SOM) and hence are driving forces for soil organic carbon (SOC) sequestration. The objective of this study was to assess the long‐term effects of tillage, fertilizer application and crop rotation on SOC in an agricultural area of southern Norway, where a soil fertility and crop rotation experiment was initiated in 1953 and a second experiment on tillage practices was initiated in 1983. The first experiment comprised 6‐yr crop rotations with cereals only and 2‐yr cereal and 4‐yr grass rotations with recommended (base) and more than the recommended (above base) fertilizer application rates; the second experiment dealt with autumn‐ploughed (conventional‐till) plots and direct‐drilled plots (no‐till). Soil samples at 0–10 and 10–30 cm depths were collected in autumn 2009 and analysed for their C and N contents. The quality of SOM in the top layer was determined by ¹³C solid‐state NMR spectroscopy. The SOC stock did not differ significantly because of rotation or fertilizer application types, even after 56 yr. However, the no‐till system showed a significantly higher SOC stock than the conventional‐till system at the 0–10 cm depth after the 26 yr of experiment, but it was not significantly different at the 10–30 cm depth. In terms of quality, SOM was found to differ by tillage type, rate of fertilizer application and crop rotation. The no‐till system showed an abundance of O‐alkyl C, while conventional‐till system indicated an apparently indirect enrichment in alkyl C, suggesting a more advanced stage of SOM decomposition. The long‐term quantitative and qualitative effects on SOM suggest that adopting a no‐tillage system and including grass in crop rotation and farmyard manure in fertilizer application may contribute to preserve soil fertility and mitigate climate change.