Soil Organic Matter Pools Research Articles

Soil organic carbon and nitrogen dynamics induced by continuous maize cropping compared to maize–soya bean rotation

SummaryAgricultural measures that alter soil carbon (C) and nitrogen (N) dynamics might affect biogeochemical cycling. We combined soil chemical fractionation with isotope analyses to investigate C and N dynamics of soil organic matter pools (SOMP) after 25 years of cropping with the application of manure. Soil samples (0–20‐ and 20–40‐cm depths) were taken from the control (CK, i.e. the initial soil condition) and two cropping treatments: continuous maize (Zea mays L.) (MM) and maize–soya bean (Glycine max (L.) Merr.) rotation (MS). Samples were separated into recalcitrant and labile fractions and analysed for C and N content, δ13C and δ15N values, and soil C and N recalcitrance indices (RIC and RIN). Long‐term MM and MS cropping with manure application increased the soil organic carbon (SOC) storage considerably (1852.69 g C m−2 on average) because of enhanced recalcitrant C (RC) and labile C (LC). Soil organic nitrogen (SON) storage (166.79 g C m−2 on average) was also increased in MM and MS treatments from the accumulation of recalcitrant N (RN). In contrast, labile N (LN) was reduced in soil under MM and MS compared with CK. Storage of SOC was greater for MS than MM, but RIC was larger for MM than MS in the top layer, indicating that MM cropping might be more effective for future soil C sequestration because of the greater proportion of stable C stored. The RIN values were larger in cropped soil than in CK at 0–20‐cm depth. Moreover, the δ13C values indicated that the turnover rates of soil C were faster in MS than in MM. The more enriched δ15N values and decreased C:N ratios of SOMP occurred in soil under MM than under MS, indicating a greater loss of soil N with MM cropping.Highlights We investigated soil C and N dynamics under long‐term cropping combined with manure. We combined soil chemical fractionation with isotope analyses to determine C sequestration and turnover. Long‐term cropping increased recalcitrant C and N and labile C but decreased labile N. Continuous maize cropping would be better for stable C accumulation than maize–soya bean rotation.

Pyrogenic Carbon Erosion: Implications for Stock and Persistence of Pyrogenic Carbon in Soil

Pyrogenic carbon (PyC) constitutes an important pool of soil organic matter, particularly for its reactivity and because of its assumed long residence times in soil. In the past, research on the dynamics of PyC in the soil system has focused on quantifying stock and mean residence time of PyC in soil, as well as determining both PyC stabilization mechanisms and loss pathways. Much of this research has focused on decomposition as the most important loss pathway for PyC from soil. However, the low density of PyC and its high concentration on the soil surface after fire indicates that a significant proportion of PyC formed or deposited on the soil surface is likely laterally transported away from the site of production by wind and water erosion. Here, we present a synthesis of available data and literature to compare the magnitude of the water-driven erosional PyC flux with other important loss pathways, including leaching and decomposition, of PyC from soil. Furthermore, we use a simple first-order kinetic model of soil PyC dynamics to assess the effect of erosion and deposition on residence time of PyC in eroding landscapes. Current reports of PyC mean residence time (MRT) range from 250 to 660 years. Using a specific example-based model system, we find that ignoring the role of erosion may lead to the under- or over-estimation of PyC MRT on the centennial time scale. Furthermore, we find that, depending on the specific landform positions, timescales considered, and initial concentrations of PyC in soil, ignoring the role of erosion in distributing PyC across a landscape can lead to discrepancies in PyC concentrations on the order of several hundred g PyC m-2. Erosion is an important PyC flux that can act as a significant control on the stock and residence time of PyC in the soil system.

Contributions of residue-C and -N to plant growth and soil organic matter pools under planted and unplanted conditions

Soil microorganisms are considered the most effective decomposers of applied crop residues, but it is poorly understood which communities are primarily responsible for decomposition under different conditions. A pot experiment was conducted in a greenhouse to follow the cycling of C and N derived from maize (Zea mays L.) residues labeled with both 13C and 15N to a subsequent winter wheat (Triticum aestivum L.) crop and to soil pools under planting with winter wheat (+P) or an unplanted control (–P), both in soil maintained at field moisture capacities of 40% and 80%. Soil microbes involved in residue decomposition were investigated by 13C phospholipid fatty acid (13C-PLFA) analysis technique. At wheat maturity, a total of 68% of residue N was recovered in the +P treatments, in which 26% was recovered from wheat plants and another 42% from soil total N (TN), independent of the water regimes, while only 50% was recovered from TN in the –P treatments. More residue C was recovered as soil organic carbon in +P than –P treatments (33% vs. 27%), and the trend became more significant with soil moisture. In addition, the +P soil had 35–48% larger microbial biomass carbon (MBC) than the –P soil, and more residue C was recovered as MBC in +P than –P treatments (7% vs. 4%), suggesting the induced microbial utilization of the applied residues. The distribution of the residue-derived PLFA-C showed that only 16:1ω7c and 18:1ω7c had larger relative abundances in the +P than the –P soils, suggesting that they were mainly stimulated by the presence of wheat and that they may be the most important fatty acids to define the different recoveries of residue N and C between the planted and unplanted conditions. Our results demonstrate that the enhanced recovery of residue-C and -N by the presence of wheat plants was mainly from the induced microbial utilization of applied residues by altering the activities of specific microorganisms.

Nitrogen Alters Initial Growth, Fine-Root Biomass and Soil Organic Matter Properties of a Eucalyptus dunnii Maiden Plantation in a Recently Afforested Grassland in Southern Brazil

Nitrogen (N) fertilization effects on Eucalyptus growth and soil carbon (C) stocks are still controversial. We set up an N fertilization experiment in southern Brazil to evaluate initial tree growth and changes in soil organic matter (SOM). Four N levels (24–Reference, 36, 48 and 108 kg ha−1 of N) were tested and tree growth was assessed during the first two years. Afterwards, representative trees were chosen to evaluate fine-root biomass (FRB) and its spatial distribution. Soil was sampled to a 40-cm depth and SOM was fractionated in Particulate (POM) and Mineral-Associated Organic Matter (MAOM) for C and N content, and δ13C determination. Positive N effect on tree growth was seen only for tree height. N addition resulted in higher FRB. Changes in SOM were more expressive in top-soil layers. Overall, afforestation had positive effects on soil C. Increasing reference N dose resulted in higher C and N content in both SOM fractions. C and N dynamics were tightly correlated, especially in MAOM. Eucalypt-derived C was on average three-fold higher in POM. In summary, we showed that N fertilization may have positive but limited effects on tree growth, nevertheless it enhances fine-root biomass and C and N accumulation in SOM pools.

Dynamics of Rice Photosynthesized Carbon Input and Its Response to Nitrogen Fertilization at the Jointing Stage:13 C-CO2 Pulse-labeling

Photosynthesized carbon (C) is an important source of soil organic C in paddy fields, and its input and distribution are affected by rice growth and soil fertility. Fertilizer application plays an important role in rice growth. The 13C pulse-labeling method was used to quantify the dynamics and distribution of input photosynthesized C in the rice-(rhizosphere-and bulk-) soil system and its response to nitrogen fertilizer (N) application. The results suggested that N fertilization significantly increased the rice aboveground and the root biomass and decreased the rice biomass root/shoot ratio. The amount of assimilated 13C gradually decreased in the rice plants but gradually decreased over 0-6 days and increased over 6-26 days in the rhizosphere and bulk soil during rice growth. N fertilization significantly increased the amount of assimilated 13C in the rhizosphere soil by 9.5%-32.6% compared with the control. In comparison to the unfertilized treatment, the application of N fertilization resulted in higher photosynthetic13C in rice aboveground and in the root by 24.5%-134.7% and 9.1%-106%, respectively. With the N fertilized and unfertilized treatments, 85.5%-93.2% and 91.3%-95.7%, respectively, of input photosynthetic 13C was distributed in the rice plants. The results suggested that N fertilization significantly affected the distribution of photosynthesized C in the rice-soil system (P<0.01). After 26 days of pulse labeling, the distribution of photosynthetic 13C into rice aboveground was increased by 13.4%, while the distribution into the rhizosphere and bulk soil were decreased by 21.9% and 52.2%, respectively, in the N fertilized treatments compared with the unfertilized treatments. Therefore, the N application increased the distribution of photosynthesized carbon in the soil-rice system but decreased the accumulation in the rhizosphere and bulk soil. The findings of this study provided a theoretical basis for our understanding of the dynamic of photosynthetic C in the plant-soil system and the assimilation of the soil organic matter pool in the paddy soil ecosystem.

Effects of Long-Term Fertilization on Organic Carbon and Nitrogen Dynamics in a Vertisol in Eastern China

Mineral fertilizers and organic amendment can affect the various soil organic matter (SOM) pools and the distribution of organic carbon (OC) and nitrogen (N) in these pools. It is unknown how OC and N are distributed in different SOM pools under different long-term fertilization regimes. Therefore, this study aimed to examine the effects on OC and N concentrations in various SOM pools after 33 years of application of chemical fertilizer and organic amendment in Anhui Province in the Huang-Huai-Hai Plain, eastern China. This long-term experiment consisted of five fertilization treatments measuring changes in the OC and N concentrations in the soils and different SOM fractions of each experiment plot. Organic amendment increased the OC and N concentrations in the mineral-associated fraction, the coarse mineral-associated fraction and the aggregates compared with the values obtained without fertilizer application. Mineral fertilizer application alone increased the abovementioned indexes, but this increase was small. There was a small but significant increase in the OC and N concentrations in the free particulate fraction, and the change in magnitude had no obvious effect on the total OC (TOC) and total N (TN) concentrations in soils. More than 80% of the water-stable aggregate-associated C was stored in macroaggregates >2 mm in size. More than 60% of the TOC and TN accumulated within mineral associations in the soil, and organic amendment increased this proportion to 80%. These results suggest that the OC in Vertisols is dominated by mineral-associated OC and that the effect of organic amendment on mineral-associated OC is obvious.

Excitation‐Emission‐Matrix Fluorescence Spectroscopy of Soil Water Extracts to Predict Nitrogen Mineralization Rates

Core Ideas EEM spectroscopy was used to characterize soluble OM pools in agricultural soils. PARAFAC was used to quantify OM meaningful EEM spectral components. PARAFAC components were correlated to N mineralization rates in specific sites. NPLS modeling of EEM data can estimate N mineralization rates in diverse soils. Rapid, easy, and frequent prediction of gross or potential nitrogen mineralization rates (GNMR and PNMR, respectively) is desirable for improved understanding and quantification of soil N dynamics and for enabling advanced sustainable N management. Our goal was to extend the use of excitation–emission matrix (EEM) fluorescence spectroscopy to characterize constituents of soluble organic matter (OM) pools in agricultural soils and to couple these characterizations with advanced chemometric techniques to improve prediction of PNMR and GNMR. To achieve this, EEM‐based predictions must be valid across diverse soils, climates, and management systems. Accordingly, we analyzed soil water extracts spanning a broad range of OM contents from midwest United States (MUS) and Israeli (ISL) agroecosystems under organic‐ and mineral‐based N management strategies. Parallel factors analysis, a multiway data analysis method, was used to quantify meaningful EEM spectral components, which were used to detect changes in labile soil OM pools and to predict N mineralization rates. N‐way partial least squares regression (NPLS) was also applied to EEM data to obtain spectral factors correlated to total organic carbon concentration potential and gross N mineralization rates. This NPLS analysis led to reliable estimation of all three tested properties for ISL and MUS soils.

Agricultural practices for crop residue transformation into soil organic matter in cold humid temperate agroecosystems.

Abstract Understanding how agricultural practices affect crop residue retention in soil organic matter (SOM) fractions is necessary to apply strategies that maintain or increase the SOM level in cold humid temperate agroecosystems. This review describes how SOM fractions, i.e. physically uncomplexed, aggregate-occluded and mineral-associated SOM, are influenced by crop residue inputs, considering that the amount of residue and its residence time within each fraction is impacted by climate, edaphic properties and residue characteristics. We also investigate how agricultural practices may improve residue retention. In cold humid temperate regions, crop residues can account for 17-62% of the light fraction (as physically uncomplexed SOM), 20-60% of aggregate-occluded SOM and a considerable amount of mineral-associated SOM, depending on the time of year when samples are collected. Crop residue retention within each fraction is strongly influenced by clay content and mineralogy. Physico-chemical characteristics of crop residues affect their transformation into SOM fractions. Agricultural practices within a cropping system, such as tillage, fertilization and residue management, determine the amount and timing of residue introduction to the agroecosystem, as well as the quantity of residues that are retained in the soil. Practices leading to residue retention within mineral-associated SOM, which has the longest residence time, are important to sustain the SOM pool. We recommend agricultural practices that build and sustain the aggregate-occluded and mineral-associated SOM reserves, including tillage practices that promote residue retention, judicious fertilization and continuous cropping.

A rapid method for estimating labile carbon and nitrogen pools in Mollisols under no-tillage

ABSTRACTThe objective of this study was to adapt the partial chemical digestion method for estimation of labile soil organic matter pools by evaluating the effect of different digestion times in Mollisols of the Argentine Pampas. The soils were sampled from nine agricultural fields under no-tillage at the 0–20 cm depth. A chemical method was performed through partial soil digestion with dilute sulphuric acid at 100°C on the basis of four digestion times: 1 (Nd1), 2 (Nd2), 4 (Nd4) and 6 (Nd6) hours. Soil organic carbon (C) and nitrogen (N) fractions were determined. The extracted organic N (Nd) ranged from 0.076 g kg−1 to 0.273 g kg−1, with a mean of 0.154 g kg−1. Statistically, the means for each digestion time indicated highly significant differences (P = 0.008). High correlations were found between Nd for different times and labile C and N fractions. However, the best fit prediction was observed between Nd2 and soil total carbohydrates (CHt), with a high coefficient of determination (R2 = 0.94). Partial chemical digestion for 2 h can be used as a rapid indicator to accurately predict CHt. Because of its speed and simplicity, this method may also be useful for rapid soil quality assessments.

Linking soil organic matter thermal stability with contents of clay, bound water, organic carbon and nitrogen

Thermogravimetry is a technique measuring mass change during programmed heating. In soil analysis, it is used for determination of content of volatile fractions, thermally labile and stable fractions of soil organic matter and minerals. One method of data analysis uses the determination of mass losses in 10°C temperature areas. In the past, their mutual correlation revealed several larger temperature areas of mass losses, which appeared to be universal for all types of soils equilibrated at the same relative humidity. However, it is unclear if mass losses in these temperature areas are connected with biogeochemical functions or processes in soil. In this work, using data from >300 soils of different types, geographical origin, and land uses we demonstrate their linear correlation with content of organic C, total N and clay content and biological activity or their combinations. In particular, the results showed that mass losses between 200 and 300°C, which is related to thermally labile fraction, correlates best with total organic C and less with total organic N. From 300 to 450°C represents a more stabilized soil organic matter pool and can be best described by either by C or N contents. Mass loss 450–550°C correlates strongly with clay content, which suggests a connection to organo-clay complexes. The low temperature interval of 30–200°C, which corresponded to weakly and strongly bound water showed a strong connection with clay content, but a weaker connection to microbial activity. The developed equations were corrected and verified using additional soil sample sets. The correlations along with their universal applicability lead to conclusion about the possible connection of mass losses in these temperature areas to, still unknown, biogeochemical soil functions. The obtained equations may represent a new approach of rapid and universally applicable “mathematical” fractionation requiring only contents of soil organic carbon, total nitrogen, clay and water in soil equilibrated at 76% relative humidity.

Diverse Soil Carbon Dynamics Expressed at the Molecular Level

AbstractThe stability and potential vulnerability of soil organic matter (SOM) to global change remain incompletely understood due to the complex processes involved in its formation and turnover. Here we combine compound‐specific radiocarbon analysis with fraction‐specific and bulk‐level radiocarbon measurements in order to further elucidate controls on SOM dynamics in a temperate and subalpine forested ecosystem. Radiocarbon contents of individual organic compounds isolated from the same soil interval generally exhibit greater variation than those among corresponding operationally defined fractions. Notably, markedly older ages of long‐chain plant leaf wax lipids (n‐alkanoic acids) imply that they reflect a highly stable carbon pool. Furthermore, marked 14C variations among shorter‐ and longer‐chain n‐alkanoic acid homologues suggest that they track different SOM pools. Extremes in SOM dynamics thus manifest themselves within a single compound class. This exploratory study highlights the potential of compound‐specific radiocarbon analysis for understanding SOM dynamics in ecosystems potentially vulnerable to global change.

Persistent carbon loss from the humus layer of tilled boreal forest soil

SummaryEffects of forest floor tillage on the soil organic matter (SOM) content of a podzol were studied across the boreal forest zone in Finland. At each of the 93 study sites, where an old natural forest stand is bordered by a tilled or non‐tilled clear‐cut‐regenerated stand, 20 topsoil cores were collected from both the old and regenerated stand for SOM determination. An analysis of covariance (ancova) revealed a significant decrease in the SOM pool related to tilled soil on the clear‐cut stands compared with the adjacent old‐forest stands. A relative increase in SOM on several of the most recent ‘clear‐cut’ sites appears to be a transient feature attributable to logging residue. The average decrease in SOM for all the &gt; 10‐year tilled sites was 1260 g OM m−2, corresponding to a decline of about 15% from the old‐forest average. For clear‐cut sites without tillage, there was an average loss of only 300 g OM m−2, or 3.9%. Based on the current rates of forest floor tillage in Finland (1200 km2 annually), CO2 emissions were estimated to be in the order of 2.8 Tg year−1, which suggests a total of about 130 Tg CO2 has been emitted since the 1960s (about 58 000 km2 of forests have been tilled so far). The persistent reduction in carbon stocks observed in tilled forest soil contradicts the carbon balance neutrality claimed for the intensive forestry currently practised in Finland.Highlights Effects of clear‐cut forestry and soil tillage on SOM stores in Finland. Soil at clear‐cut borders of old‐forest stands reveals effect of forestry on soil C balance. Tilling of soil on clear‐cuts typically reduces SOM by 10–15% on average. The results suggest that present Finnish UNFCCC reporting for boreal forest soil is erroneous.

Cropping System Effects on Soil Monosaccharides in Western Burkina Faso

Labile pools of soil organic matter (SOM), including soil sugars, are important to the formation and stabilization of soil aggregates and to microbial activity and nutrient cycling. The effects of cropping systems at farm level in tropical areas on SOM labile pool dynamics have not been adequately studied and the results are sparse and inconsistent. The objective of this study was to determine the effects of soil management intensity on soil sugar monomers derived from plant debris or microbial activity in cotton (Gossypium herbaceum)-based cropping systems of western Burkina Faso. Thirty-three (33) plots were sampled at 0-15 cm soil depth considering field-fallow successions and tillage intensity. Two pentose (arabinose, xylose) and four hexose (glucose, galactose, mannose, glucosamine) monomers accounted for 2 to 18% of soil organic carbon (SOC) content. Total sugar content was significantly less with tillage, especially for the hexose monomeric sugars glucose and mannose, the latter of microbial origin. Soil mannose was 63 and 80% less after 10 years of cultivation, without and with annual ploughing respectively, compared with fallow conditions. Soil monosaccharide content was rapidly restored with fallow and soon approached the equilibrium level observed under old fallow lands. Therefore, the soil monosaccharides, in particular galactose and mannose from microbial synthesis are early indicators of changes in SOC.

The temperature sensitivity of soil organic matter decomposition is constrained by microbial access to substrates

Soils can be sources or sinks of carbon depending on the balance between carbon inputs from plants and losses from the decomposition of soil organic matter (SOM). A good understanding of the temperature sensitivity of SOM decomposition is critical for forecasting whether soils in a warming world will lose or gain carbon, and therefore accelerate or mitigate the rate of increasing atmospheric carbon dioxide (CO2) concentration.We provide new evidence to show that the response of SOM decomposition to temperature may be constrained by substrate availability to microbial decomposers. We used laboratory incubations of a grassland soil to compare the temperature sensitivity of SOM decomposition with unmodified substrate availability with that of the same soil in which substrate availability was reduced by adding allophane, a clay-size mineral with a high capacity for binding SOM. In the soil with no added allophane, the decomposition rate increased about 7-fold over the temperature range from 1 to 40 °C. With added allophane, decomposition rate increased only about 3-fold over the same temperature range.We then used a non-disruptive, natural abundance isotopic technique at our field site to partition total soil respiration into CO2 efflux from newly released, 13C-depleted SOM (root respiration and rhizosphere decomposition) from CO2 efflux from older 13C-enriched SOM from the decomposition of more stable SOM. We found no increase in the decomposition rate of the 13C-enriched pool of SOM between 11 and 28 °C. That finding contrasts with most previous studies that have generally reported strong increases in SOM decomposition with temperature. We hypothesised that the large temperature sensitivity observed in laboratory incubations was due to substrate becoming readily available as a result of the disturbance involved in collecting soil samples. In undisturbed field conditions, the limiting step for the decomposition of the more stable SOM pool may be the rate at which decomposable substrate becomes available for decomposition.Our findings will have important implications for the feedbacks between soil carbon storage and the rate of increase in atmospheric CO2 concentration mediated by global warming.

Relationships between soil organic matter pools and nitrous oxide emissions of agroecosystems in the Brazilian Cerrado

In the Brazilian Cerrado, despite the increasing adoption of no-till systems, there are still extended areas under conventional soil management systems that reduce soil carbon (C) and nitrogen (N) stocks and increase the emissions of greenhouse gases, such as nitrous oxide (N2O). Conservation agroecosystems, such as no-till, have been proposed as a strategy to mitigate agriculture-induced climatic changes through reductions in N2O emissions. However, the relationship between organic matter and N2O emissions from soils under different agroecosystems is not yet clear. This study hypothesized that agroecosystems under no-till promote an accumulation of labile and stable SOM fractions along with a reduction of N2O emissions. This study evaluated the effects of crop-rotation agroecosystems: i) on C and N pools and labile and stable SOM fractions; ii) on cumulative N2O emissions; and iii) on the relationships between SOM fractions and N2O emissions. The agricultural systems consisted of: (I) soybean followed by sorghum under no-tillage (NT1); (II) maize followed by pigeon pea under no-tillage (NT2); (III) soybean under conventional tillage followed by fallow soil (CT); (IV) and native Cerrado (CER). After CT for 18years, following the replacement of CER, the soil C stock in the 0–20cm layer was reduced by 0.64tha−1year−1. The no-till systems were more efficient in accumulating labile and stable C fractions with values close to those observed under CER, and were directly related to lower soil N2O emissions. The cumulative pattern of N2O emissions was inverse to that of the following SOM fractions: microbial biomass carbon, permanganate-oxidizable carbon, particulate organic carbon, inert carbon, and humic substances. Based on principal component analysis, the CT was generally separated from the other land use systems. This separation was strongly influenced by the low C contents in the different SOM fractions and higher N2O emissions promoted by the CT.

Soil microbial diversity drives the priming effect along climate gradients: a case study in Madagascar.

The priming effect in soil is proposed to be generated by two distinct mechanisms: 'stoichiometric decomposition' and/or 'nutrient mining' theories. Each mechanism has its own dynamics, involves its own microbial actors, and targets different soil organic matter (SOM) pools. The present study aims to evaluate how climatic parameters drive the intensity of each priming effect generation mechanism via the modification of soil microbial and physicochemical properties. Soils were sampled in the center of Madagascar, along climatic gradients designed to distinguish temperature from rainfall effects. Abiotic and biotic soil descriptors were characterized including bacterial and fungal phylogenetic composition. Potential organic matter mineralization and PE were assessed 7 and 42 days after the beginning of incubation with 13C-enriched wheat straw. Both priming mechanisms were mainly driven by the mean annual temperature but in opposite directions. The priming effect generated by stoichiometric decomposition was fostered under colder climates, because of soil enrichment in less developed organic matter, as well as in fast-growing populations. Conversely, the priming effect generated by nutrient mining was enhanced under warmer climates, probably because of the lack of competition between slow-growing populations mining SOM and fast-growing populations for the energy-rich residue entering the soil. Our study leads to hypotheses about the consequences of climate change on both PE generation mechanisms and associated consequences on soil carbon sequestration.

Effects of soil macro- and mesofauna on litter decomposition and soil organic matter stabilization

Soil fauna consumes substantial amounts of litter and can even consume the entire annual litterfall in some ecosystems. The assimilation efficiency of fauna may reach 50% but is usually much smaller. Soil fauna may affect soil organic matter (SOM) dynamics not only by assimilating litter but also by modifying the soil environment at many spatiotemporal scales. Litter processing by fauna usually results in a short-term increase in microbial activity in feces; this activity than decreases such that feces over the long term may decompose more slowly than the original litter. During passage through the guts of litter-feeding fauna, litter modifications include fragmentation, consumption of associated microorganisms, pH and redox changes, removal of easily decomposed polysaccharides, increase in the proportion of lignin, and decrease in soluble polyphenols and carbon:nitrogen (C:N) ratios. The coating of litter with clay during passage through earthworms reduces microbial access to the litter as well as conditions for microbial activity by reducing the diffusion of nutrients and oxygen. At a larger scale, soil fauna affects leaching and the release of particulate organic matter (POM), which in turn affect microbial activity in soil. Fauna also affects the distribution of organic matter in the soil profile and determine whether litter decomposes on the soil surface or as POM bound to soil particles, which substantially affects the microbial community and the rate of decomposition. Fauna affects the amount of organic matter entering different SOM pools, and this effect depends on litter quality and the degree of soil C saturation. At an even larger scale, fauna can change the soil profile, soil properties, and the plant community, which may in turn affect microbial activity and the decomposition rate. The effect of soil fauna on litter decomposition and soil C storage can be positive or negative. Faunal effects tend to be greatest in ecosystems under transition, e.g. ecosystem developing after some disturbance during primary or secondary succession.

Adaptation of microbial resource allocation affects modelled long term soil organic matter and nutrient cycling

In order to understand the coupling of carbon (C) and nitrogen (N) cycles, it is necessary to understand C and N-use efficiencies of microbial soil organic matter (SOM) decomposition. While important controls of those efficiencies by microbial community adaptations have been shown at the scale of a soil pore, an abstract simplified representation of community adaptations is needed at ecosystem scale.Therefore we developed the soil enzyme allocation model (SEAM), which takes a holistic, partly optimality based approach to describe C and N dynamics at the spatial scale of an ecosystem and time-scales of years and longer. We explicitly modelled community adaptation strategies of resource allocation to extracellular enzymes and enzyme limitations on SOM decomposition. Using SEAM, we explored whether alternative strategy-hypotheses can have strong effects on SOM and inorganic N cycling.Results from prototypical simulations and a calibration to observations of an intensive pasture site showed that the so-called revenue enzyme allocation strategy was most viable. This strategy accounts for microbial adaptations to both, stoichiometry and amount of different SOM resources, and supported the largest microbial biomass under a wide range of conditions. Predictions of the holistic SEAM model were qualitatively similar to precitions of the SYMPHONY model, which explicitly represents competing microbial guilds. With adaptive enzyme allocation under conditions of high C/N ratio of litter inputs, N that was formerly locked in slowly degrading SOM pools was made accessible, whereas with high N inputs, N was sequestered in SOM and protected from leaching.The findings imply that it is important for ecosystem scale models to account for adaptation of C and N use efficiencies in order to represent C-N couplings. The combination of stoichiometry and optimality principles is a promising route to yield simple formulations of such adaptations at community level suitable for incorporation into land surface models.

Soil Organic Carbon Pools as Early Indicators for Soil Organic Matter Stock Changes under Different Tillage Practices in Inland Pacific Northwest

Soil organic matter (SOM) is essential for sustaining soil health and crop productivity. However, changes in SOM stocks in response to agronomic practices are slow and show years later when it is too late for adjustments in management. Identifying early indicators of SOM dynamics will allow early management decisions and quick remedial action. The objectives of this study were to evaluate long-term effects of tillage intensity and timing on SOM pools and determine the most responsive SOM pools to tillage practice. Soil from a long-term (53 yrs) winter wheat (Triticum aestivum L.) - spring pea (Pisum sativum L.) rotation and undisturbed grass pasture (GP) in inland Pacific Northwest (iPNW) was sampled to evaluate the effect of four tillage systems [no-till (NT), disk/chisel (DT/CT), spring plow (SP), and fall plow (FP)] on soil organic carbon (SOC, proxy for SOM), total nitrogen (TN), particulate organic matter carbon (POM-C) and nitrogen (POM-N), permanganate oxidizable carbon (POXC), water extractable organic carbon (WEOC), total dissolved nitrogen (TDN), KCl-extractable nitrogen (KEN), microbial biomass carbon (MBC) and nitrogen (MBN), basal respiration (BR), carbon mineralization (Cmin), and metabolic quotient (qCO2). GP had higher levels of SOC pools than cultivated treatments. On average, tillage significantly decreased SOC and TN by 28% and 26%, respectively, compared to GP. Among the cultivated soils, tillage had no significant effect on SOC and TN, except for DT/CT that had slightly higher SOC than FP (P=0.08). On the contrary, NT and DT/CT significantly (P<0.05) increased levels of POM-C, POM-N, POXC, WEOC, MBC, BR, Cmin, and qCO2 over FP or SP. However, tillage did not affect TDN, MBN, and KEN. The C-pools (POM-C, POXC, MBC, WEOC, BR, and Cmin) were more strongly correlated with SOM than the N-pools (TDN, MBN, and KEN), with an exception to POM-N. Under wheat-pea rotation in the iPNW, reduced tillage systems (NT and DT/CT) have a potential to maintain or increase SOM, which can be assessed early through its physical (POM), chemical (POXC, WEOC), and microbiological (MBC, BR, Cmin) indicators. POXC and WEOC were the most sensitive indicators of tillage-induced changes in SOM dynamics.

Linear and non-linear responses of vegetation and soils to glacial-interglacial climate change in a Mediterranean refuge

The impact of past global climate change on local terrestrial ecosystems and their vegetation and soil organic matter (OM) pools is often non-linear and poorly constrained. To address this, we investigated the response of a temperate habitat influenced by global climate change in a key glacial refuge, Lake Ohrid (Albania, Macedonia). We applied independent geochemical and palynological proxies to a sedimentary archive from the lake over the penultimate glacial-interglacial transition (MIS 6–5) and the following interglacial (MIS 5e-c), targeting lake surface temperature as an indicator of regional climatic development and the supply of pollen and biomarkers from the vegetation and soil OM pools to determine local habitat response. Climate fluctuations strongly influenced the ecosystem, however, lake level controls the extent of terrace surfaces between the shoreline and mountain slopes and hence local vegetation, soil development and OM export to the lake sediments. There were two phases of transgressional soil erosion from terrace surfaces during lake-level rise in the MIS 6–5 transition that led to habitat loss for the locally dominant pine vegetation as the terraces drowned. Our observations confirm that catchment morphology plays a key role in providing refuges with low groundwater depth and stable soils during variable climate.