Incorporation of S into soil organic matter in the field as determined by the natural abundance of stable S isotopes

In agricultural systems with low S inputs, soil organic matter is a major source of S and the transformations between organic and inorganic S pools are important for the supply of S to plants. This study was conducted to determine the effect of S fertilizer on the size and activity of organic S pools. For 5 years S fertilizer with a known composition of stable S isotopes was applied to a rotation on a loamy soil and a coarse sandy soil at rates higher than the plant demand. Total organic S in soil organic matter was not affected by sulphur application, but a small increase occurred in the sulphate ester fractions (P<0.05). Inorganic sulphate concentrations in the soil reflected the S application in the year of sampling, whereas S applied in earlier years was not recognized. Organic matter below the plough layer in both soils was enriched with S, possibly as a result or organic matter leaching or an increased clay content in the subsoils. At 0–20 cm, the C:S ratio in organic matter was ca. 100 for both soils, decreasing to 73 and 46 at 60–80 cm for the coarse sandy soil and the loamy soils, respectively. In both soils, isotope data showed that ca. 30% of organic-bonded S at 0–20 cm originated from fertilizer S applied during the last 5 years, irrespective of the S application rate. At 20–40 cm the rate of incorporations was lower and at 40–60 cm no incorporation of fertilizer S into organic matter was recognized. The fertilizer application did not induce net changes in the total organic S fraction, but isotope data indicated that a considerable part of the organic S pool was involved in S cycling in the field.     

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.     

Soil organic matter dynamics and land use change at a grassland/forest ecotone

In the grassland/forest ecotone of North America, many areas are experiencing afforestation and subsequent shifts in ecosystem carbon (C) stocks. Ecosystem scientists commonly employ a suite of techniques to examine how such land use changes can impact soil organic matter (SOM) forms and dynamics. This study employs four such techniques to compare SOM in grassland (Bromus inermis) and recently forested (∼35 year, Ulmus spp. and Quercus spp.) sites with similar soil types and long-term histories in Kansas, USA. The work examines C and nitrogen (N) parameters in labile and recalcitrant SOM fractions isolated via size and density fractionation, acid hydrolysis, and long-term incubations. Size fractionation highlighted differences between grassland and forested areas. N concentration of forested soils’ 63-212 μm fraction was higher than corresponding grassland soils’ values (3.0±0.3 vs. 2.3±0.3 mg g_fraction⁻¹, P<0.05), and N concentration of grassland soils’ 212-2000 μm fraction was higher than forested soils (3.0±0.4 vs. 2.3±0.2 mg g_fraction⁻¹, P<0.05). Similar trends were observed for these same fractions for C concentration; forested soils exhibited 1.3 times the C concentration in the 63-212 μm fraction compared to this fraction in grassland soils. Fractions separated via density separation and acid hydrolysis exhibited no differences in [C], [N], δ¹⁵N, or δ¹³C when compared across land use types. Plant litterfall from forested sites possessed significantly greater N concentrations than that from grassland sites (12.41±0.10 vs. 11.62±0.19 mg g_litter⁻¹). Long-term incubations revealed no differences in C or N dynamics between grassland and forested soils. δ¹³C and δ¹⁵N values of the smallest size and the heavier density fractions, likely representing older and more recalcitrant SOM, were enriched compared to younger and more labile SOM fractions; δ¹⁵N of forested soils’ 212-2000 μm fraction were higher than corresponding grassland soils (1.7±0.3‰ vs. 0.5±0.4‰). δ¹³C values of acid hydrolysis fractions likely reflect preferential losses of ¹³C-depleted compounds during hydrolysis. Though C and N data from size fractions were most effective at exhibiting differences between grassland and forested soils, no technique conclusively indicates consistent changes in SOM dynamics with forest growth on these soils. The study also highlights some of the challenges associated with describing SOM parameters, particularly δ¹³C, in SOM fractions isolated by acid hydrolysis.     

Changes in δC composition of soil carbonates driven by organic matter decomposition in a Mediterranean climate: A field incubation experiment

The current view on the relationship between the δ¹³C of pedogenic carbonates and soil organic matter is based on static studies, in which soil profiles are analysed at a given moment of their development. A dynamic approach to this question should also be possible by studying under field conditions how the δ¹³C of carbonates changes as organic matter decomposes. No such study has been undertaken owing to the slowness of the changes in the δ¹³C of carbonates, since it has been calculated that a detectable change will occur only after millenia. Nevertheless, this may not be true where soil CO₂ efflux is intense, as expected in soil zones with high microbial activity. In this paper we test the latter assumption by incubating mixtures of plant material and carbonate-rich red earth in the field at depths of 5, 20 and 40 cm. Four types of plant material were tested: Medicago sativa, Eucalyptus globulus, Quercus ilex and Pinus halepensis. Because the isotopic composition of these plant materials is known, we can determine the isotopic composition of the respired C and study how it relates to the (expected) changes in the δ¹³C. After two years of field incubation, the changes in δ¹³C of carbonates were high enough to be reliably detected and quantified, thus showing that the isotopic composition of soil carbonates can change quite rapidly in biologically active soil horizons. The observed changes are possible only if we assume that the increase in δ¹³C in the overall path respired C → pedogenic carbonate is much higher than the usually applied standard factors (about 15‰). These enrichments can be explained by assuming, as does the currently accepted paradigm, that the precipitation of new carbonates occurs in an open system in which the penetration of free-air CO₂ plays a major role. On the other hand, these enrichments can also be explained by an alternative interpretation, which assumes that the dissolution–precipitation carbonate cycles occur in systems that can be at least temporarily closed. Thus, we suggest that both possibilities (carbonate dissolution and precipitation in either an open or closed system) can coexist in a given soil, even though one or the other will dominate in any given time period.     

Spatial variation of soil organic carbon concentrations and stable isotopic composition in 1-ha plots of forest and pasture in Costa Rica: implications for the natural abundance technique

The goal of this study was to examine spatial variation of soil organic C and its stable isotopic composition (δ¹³C) in 1-ha plots of mature rain forest and a cattle pasture dominated by C4 grasses in Costa Rica. Soil samples were taken from 80 mapped locations per plot and analyzed for organic C and δ¹³C. The range of values for soil C concentrations was similar between forest and pasture, although the mean values were higher in the forest. δ¹³C was narrowly constrained in the forest (from −27.96 to −26.09‰) but varied from −15.09 to −28.59‰ in the pasture. Variograms revealed spatial autocorrelation of soil C and δ¹³C in the pasture and organic C concentration in the forest soil. The large range and spatial variability of δ¹³C in the pasture site may be due to varying contributions of C3 and C4 vegetation to the soil C pool, which may limit the usefulness of the natural abundance technique as a precise tracer of soil C dynamics in this pasture.     

Long‐term changes of aggregate‐associated and labile soil organic carbon and nitrogen after conversion from forest to grassland and cropland in northern Turkey

Soil management systems can have great effect on soil chemical, physical and biological properties. Conversion of forest to grassland and cropland can alter C and N dynamics. The objective of this study was to evaluate the changes in aggregate‐associated and labile soil organic C and N fractions after conversion of a natural forest to grassland and cropland in northern Turkey. This experiment was conducted on plots subject to three different adjacent land uses (forest, grassland and cropland). Soil samples were taken from 0–5, 5–15 and 15–30 cm depths from each land use. Some soil physical (soil texture, bulk density), chemical (soil pH, soil organic matter, lime content, total organic C and N, inorganic N, free and protected organic C) and biological (microbial biomass C and N, mineralizable C and N) properties were measured. The highest and lowest bulk densities were observed in grassland (1.41 g cm⁻³) and cropland (1.14 g cm⁻³), respectively. Microbial biomass C and total organic C in forest were almost twice greater than grassland and four‐times greater than cropland. Cultivation of forest reduced total organic N, mineralizable N and microbial biomass N by half. The great portion of organic C was stored in macroaggregates (>250 µm) in all the three land uses. Free organic C comprised smaller portion of soil organic C in all the three land uses. Thus, this study indicated that long‐term conversion of forest to grassland and cropland significantly decreased microbial biomass C, mineralizable C and physically protected organic C and the decreases were the greatest in cropland. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of repeated prescribed burning on incorporation of C from cellulose by forest soil fungi as determined by RNA stable isotope probing

Repeated prescribed burning is frequently used as a forest management tool and can influence soil microbial diversity and activity. Soil fungi play key roles in carbon and nutrient cycling processes and soil fungal community structure has been shown to alter with increasing burning frequency. Such changes are accompanied by changes to soil carbon and nitrogen pools, however, we know little regarding how repeated prescribed burning alters functional diversity in soil fungal communities. We amended soil with ¹³C-cellulose and used RNA stable isotope probing to investigate the effect of biennial repeated prescribed burning over a 34-year period on cellulolytic soil fungi. Results indicated that repeated burning altered fungal community structure. Moreover, fungal community structure and diversity in ¹²C and ¹³C fractions from the unburned soil were not significantly different from each other, while those from the biennial burned soils differed from each other. The data indicate that fewer active fungi in the biennially burned soil incorporated ¹³C from the labelled cellulose and that repeated prescribed burning had a significant impact on the diversity of an important functional group of soil fungi (cellulolytic fungi) that are key drivers of forest soil decomposition and carbon cycling processes.     

Carbon dynamics determined by natural C abundance in microcosm experiments with soils from long-term maize and rye monocultures

Understanding carbon dynamics in soil is the key to managing soil organic matter. Our objective was to quantify the carbon dynamics in microcosm experiments with soils from long-term rye and maize monocultures using natural ¹³C abundance. Microcosms with undisturbed soil columns from the surface soil (0-25 cm) and subsoil (25-50 cm) of plots cultivated with rye (C₃-plant) since 1878 and maize (C₄-plant) since 1961 with and without NPK fertilization from the long-term experiment ‘Ewiger Roggen’ in Halle, Germany, were incubated for 230 days at 8 °C and irrigated with 2 mm 10⁻² M CaCl₂ per day. Younger, C₄-derived and older, C₃-derived percentages of soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass (C_mic) and CO₂ from heterothropic respiration were determined by natural ¹³C abundance. The percentage of maize-derived carbon was highest in CO₂ (42-79%), followed by C_mic (23-46%), DOC (5-30%) and SOC (5-14%) in the surface soils and subsoils of the maize plots. The percentage of maize-derived C was higher for the NPK plot than for the unfertilized plot and higher for the surface soils than for the subsoils. Specific production rates of DOC, CO₂-C and C_mic from the maize-derived SOC were 0.06-0.08% for DOC, 1.6-2.6% for CO₂-C and 1.9-2.7% for C_mic, respectively, and specific production rates from rye-derived SOC of the continuous maize plot were 0.03-0.05% for DOC, 0.1-0.2% for CO₂-C and 0.3-0.5% for C_mic. NPK fertilization did not affect the specific production rates. Strong correlations were found between C₄-derived C_mic and C₄-derived SOC, DOC and CO₂-C (r≥0.90), whereas the relationship between C₃-derived C_mic and C₃-derived SOC, DOC and CO₂-C was not as pronounced (r≤0.67). The results stress the different importance of former (older than 40 years) and recent (younger than 40 years) litter C inputs for the formation of different C pools in the soil.     

The proportional mineralisation of microbial biomass and organic matter caused by air-drying and rewetting of a grassland soil

During the first few days after rewetting of an air-dried soil (AD-RW), microbial activity increases compared to that in the original moist soil, causing increased mineralisation (a flush) of soil organic carbon (C) and other nutrients. The AD-RW flush is believed to be derived from the enhanced mineralisation of both non-biomass soil organic matter (due to its physical release and enhanced availability) and microbial biomass killed during drying and rewetting. Our aim was to determine the effects of AD-RW on the mineralisation of soil organic matter and microbial biomass during and after repeated AD-RW cycles and to quantify their proportions in the CO₂-C flushes that resulted. To do this, a UK grassland soil was amended with ¹⁴C-labelled glucose to label the biomass and then given five AD-RW cycles, each followed by 7 d incubation at 25 °C and 50% water holding capacity. Each AD-RW cycle increased the amount of CO₂-C evolved (varying from 83 to 240 μg g⁻¹ soil), compared to the control with, overall, less CO₂-C being evolved as the number of AD-RW cycles increased. In the first cycle, the amount of biomass C decreased by 44% and microbial ATP by 70% while concentrations of extractable C nearly doubled. However, all rapidly recovered and within 1.3 d after rewetting, biomass C was 87% and ATP was 78% of the initial concentrations measured prior to air-drying. Similarly, by 2 d, extractable organic C had decreased to a similar concentration to the original. After the five AD-RW cycles, the amounts of total and ¹⁴C-labelled biomass C remaining in the soil accounted for 60 and 40% of those in the similarly incubated control soil, respectively. Soil biomass ATP concentrations following the first AD-RW cycle remained remarkably constant (ranging from about 10 to 14 μmol ATP g⁻¹ biomass C) and very similar to the concentration in the fresh soil prior to air-drying. We developed a simple mathematical procedure to estimate the proportion of CO₂-C derived from biomass C and non-biomass C during AD-RW. From it, we estimate that, over the five AD-RW cycles, about 60% of the CO₂-C evolved came from mineralisation of non-biomass organic C and the remainder from the biomass C itself.     

Effects of past, present and future atmospheric CO2 concentrations on soil organic matter dynamics in a chaparral ecosystem

Owing to the continuously increasing concentration of atmospheric CO₂, it has become a priority to understand if soil organic matter (SOM) will behave as a sink or a source of CO₂ under future environmental changes. Although many studies have addressed this question, a clear understanding is still missing, particularly with respect to long-term responses. In this study, we quantified soil C stores and dynamics in relationship to soil aggregation and pool composition in a Californian chaparral ecosystem exposed for 6 years to a gradient of atmospheric CO₂ concentrations, ranging from pre-industrial levels 250 to 750 μl l⁻¹ CO₂. Fossil fuel-derived CO₂ depleted in ¹³C was used for the fumigation, thus providing a tracer of C input from the vegetation to the soil.Long-term CO₂ exposure invariably affected soil aggregation, with a significant decrease in the macroaggregate fraction at highest CO₂ levels relative to the other two size fractions (i.e. microaggregates and silt and clay). This soil structural change most likely reduced the stability and protection of SOM, and C content generally decreased in most fractions over the CO₂ treatments, and induced faster turnover of recently fixed C at high CO₂ levels. The strongest response was found in the C content of the microaggregates, which decreased significantly (P<0.05) with rising levels of CO₂. We conclude that increasing atmospheric CO₂ concentrations will decrease soil C in chaparral ecosystems, and that the microaggregate fraction is the most responsive to increasing concentrations of atmospheric CO₂.     

Carbon and nitrogen degradation on molecular scale of grass-derived pyrogenic organic material during 28 months of incubation in soil

The present study focuses on the microbial recalcitrance of pyrogenic organic material (PyOM) on a molecular scale. We performed microcosm incubation experiments using ¹³C- and ¹⁵N-enriched grass-derived PyOM mixed with a sub soil material taken from a Haplic Cambisol. Solid-state ¹³C and ¹⁵N NMR studies were conducted to elucidate the humification processes at different stages of PyOM degradation. The chemical structure of the remaining PyOM after incubation was clearly different from the initial pyrogenic material. The proportion of O-containing functional groups was increased, whereas that of aryl C and of N-containing heterocyclic structures had decreased, probably due to mineralisation and conversion to other C and N groups. After 20 months of incubation the aryl C loss reached up to 40% of the initial amount and up to 29% of the remaining PyOM C was assigned to carboxyl/carbonyl C and O-aryl C. These reactions alter the chemical and physical properties of the char residue and make it more available for further microbial attack but also for adsorption processes. Our study presents direct evidence for the degradation of N-heterocyclic domains in charred plant remains adding new aspects to the understanding of the N cycling in fire-affected ecosystems.     

Perspectives of the Fritz-Scheffer Awardee 2021: Profile- to ecosystem-scale perspectives on soil organic matter formation as demonstrated by woody debris in forest dynamics

Soil organic matter (SOM) forms along a continuum from individual particles, pores, and aggregates to litter–soil profiles and larger ecosystems such as forests. However, forest management of SOM stocks and the carbon therein requires knowledge on which processes and factors at which scales determine SOM formation from forest biomass. As evident from woody debris at the profile scale, SOM forms through additions, transformations, translocations, and removals of litter by soil organisms and environmental components. Yet SOM stocks only increase if litter additions-to-removals are out of steady state or enter a new steady state that ignores older litter. Both happen through disturbance and self-selecting feedback processes in ecosystems consisting of autotrophs, heterotrophs, and their physical environment. One such positive feedback process is litter-SOM transformation by heterotrophs that releases nutrients that promote plant productivity and thus litter input. Stocks of litter-SOM, heterotrophs, nutrients, and plants thus exhibit Lotka–Volterra dynamics (i.e., predator–prey interactions) and only increase when attractor states (i.e., steady series or sets of states) change due to disturbance. Evidence of evolving feedback processes and disturbance in SOM would help identify limits, potentials, and precariousness of ecosystems in light of global change, but remains to be found.     

Chemistry of soil organic matter as related to C : N in Norway spruce forest (Picea abies(L.) Karst.) floors and mineral soils

Due to high nitrogen deposition in central Europe, the C : N ratio of litter and the forest floor has narrowed in the past. This may cause changes in the chemical composition of the soil organic matter. Here we investigate the composition of organic matter in Oh and A horizons of 15 Norway spruce soils with a wide range of C : N ratios. Samples are analyzed with solid‐state ¹³C nuclear magnetic resonance (NMR) spectroscopy, along with chemolytic analyses of lignin, polysaccharides, and amino acid‐N. The data are investigated for functional relationships between C, N contents and C : N ratios by structural analysis. With increasing N content, the concentration of lignin decreases in the Oh horizons, but increases in the A horizons. A negative effect of N on lignin degradation is observed in the mineral soil, but not in the humus layer. In the A horizons non‐phenolic aromatic C compounds accumulate, especially at low N values. At high N levels, N is preferentially incorporated into the amino acid fraction and only to a smaller extent into the non‐hydrolyzable N fraction. High total N concentrations are associated with a higher relative contribution of organic matter of microbial origin.     

Effect of mineral and organic fertilization on crop yield,nitrogen uptake,carbon and nitrogen balances,as well as soil organic carbon content and dynamics: results from 20 European long-term field experiments of the twenty-first century

Assembled results from 20 European long-term experiments (LTE), mainly from the first decade of the twenty-first century, are presented. The included LTEs from 17 sites are the responsibility of institutional members of the International Working Group of Long-term Experiments in the IUSS. Between the sites, average annual temperatures differ between 8.1 and 15.3°C, annual precipitation between 450 and 1400 mm, and soil clay contents between 3 and 31%. On average of 350 yield comparisons, combined mineral and organic fertilization resulted in a 6% yield benefit compared with mineral fertilization alone; in the case of winter wheat, the smallest effect was 3%, the largest effect, seen with potatoes, was 9%. All unfertilized treatments are depleted in soil organic carbon (SOC), varying between 0.36 and 2.06% SOC. The differences in SOC in unfertilized plots compared with the respective plots with combined mineral (NPK) and organic (10 t ha^?1 farmyard manure) fertilization range between 0.11 and 0.72%, with an average of 0.3% (corresponding to ～15 t ha^?1). Consequently, the use of arable soils for carbon sequestration is limited and of low relevance and merely depleted soils can temporarily accumulate carbon up to their optimum C content.