Enrichment Of Soil Organic Matter Research Articles

A dual mechanism drives the enrichment of pedogenic magnetic particles derived from red beds

Iron oxides and associated magnetic properties are considered good indicators of the pedogenic environment and regional climate. However, these indicators are under debate for soils developed from sediments. The accumulation of magnetic particles derived from sediments depends not only on the formation of iron oxides but also on the existence and transformation of detrital iron oxides. To explore the formation mechanism of magnetic particles accompanied by pedogenetic processes, a toposequence consisting of six profiles derived from red beds with contrasting drainage conditions under a subtropical monsoon climate is examined. We generally observed upward magnetic enhancement but inconsistent correlations between magnetism and iron oxide phases. The magnetic enhancement in the well drained profiles was dominated by the reverse transformation of detrital hematite to fine maghemite particles, whereas in the poorly drained profiles, coarse magnetite particles formed with soil organic matter enrichment in an anaerobic way. The lower magnetism in the transition profiles is attributed to the hydration of iron oxides into goethite before the pedogenic environment became anaerobic. The different magnetic enrichment mechanisms explain the nonlinear responses of magnetism and color to climate across temporal and spatial scales. These findings also suggest that soil magnetism is promising for tracing soil moisture and ecological evolution, especially in red soils and sediments across the surfaces of the Earth.

Pomegranate's (Punica granatum L.) fruit quality and nutrient content are influenced by soil fraction × root architecture interaction

AbstractAmong the most important factors influencing plant nutrient uptake and water access are soil fraction and root architecture. However, little is known about how soil fraction × root architecture interaction affects woody plants. This study considered the differentiation of soil fertility parameters on layers through the root zone of two different soils and the quality and nutrient responses of three newly bred pomegranate varieties to soil fractions based on root architecture in clay and clay loam orchards. According to the findings, increasing clay content in the deeper layers has a negative impact on soil fertility, organic matter, phosphorus (P), and potassium (K). In clayey conditions, pomegranate varieties yielded lower in a range of 2.74% to 6.10% and significantly lower macro and micronutrient insertion. Conversely, the shallow‐rooted variety accumulated significantly more boron (B) and P and set 50% more fruits in the clayey orchards. Soil fractions × root architecture significantly altered fruit quality characteristics, nutrient ingredients, and their relationships supported by a relatively low linear relationship in the Mantel test (r = 0.42). This study suggests that pomegranate tree responses to soil fractions are determined by root architecture. A proper approach to obtaining high‐quality and nutritionally fortified pomegranates will incorporate appropriate cultivation techniques, such as suitable fertigation regime regulation, harvest date optimization, and soil organic matter enrichment, to provide uniform nutrient uptake based on the soil type and variety's root architecture.

Some biological properties of the Kuidusun depression soils of Oymyakon

The aim of the study. To investigate some microbiological properties of the catenal series of soils in the Kuidusun depression of the Oymyakon Highlands. Location and time of the study. The study area is located in the vicinity of the Tomtor village (N63°15'41.94''; E143°10'26.91'') of the Oymyakonsky district of the Republic of Sakha (Yakutia, Russia). Soil samples were collected in the fall of 2011 at the main positions of the catena. The length of the catena is 38 km, the altitude a.s.l. is 727–821m. Soils are classified as Cambic Cryosols (fragic, мurshic), (forest); Skeletic Cryosols (aridic), (steppe meadow), Cambic Cryosols (fluvic), (typical meadow); Folic Cryosols (swampy meadow). Methods. The content of labile soil organic matter (LOC) was determined photocalorimetrically in soil extracts with phosphate buffer (1/15 M, pH 6,8). The number of colony-forming units (CFU) was determined using standard MPA, KAA, and Czapek media. The functional spectrum of the soil microbiota was assessed using the community level physiological profile (CLPP) method. All properties were studied in 10 cm thick layers to a depth of 40 cm. Soil urease activity was assessed by ammonium production from urea. Results. At the time of sampling moisture content of soil samples down the catena increased from 5 to 20%, whereas the LOC content increased from 2 to 6 g C /kg. In the 0–10 cm layer, the greatest accumulation of LOC was observed in the soil under a typical meadow. The highest number of CFU (150 million/g soil) in a dilute MPA medium was also found there. An increase in the proportion of CFU in the dormant state was revealed in the soil series from the lower position of the catena to the upper one. In the same series, an increase in the oligotrophic index of the community was noted. The highest metabolic activity was observed in the soil under a swampy meadow, whereas the lowest was shown by the soil under a forest. The enrichment of soil organic matter with microbiota cells increased in the series of soils f in the following order: a swampy meadow – a typical meadow – a steppe meadow, whereas the specific activity of the microbiota decreased. Urease activity of the soil was detected mainly in the 0–10 cm layer; in the underlying layers the activity was more than 500 times lower. Conclusions. The biological properties of the studied soils corresponded to the known patterns established for the catenal series of soils in other soil and climatic conditions. The peculiarity of the studied soils was related to the extremely sharp differentiation of the microbiological profile. The absolute predominance of actinomycetes CFU in the soil under the steppe meadow was the major characteristic feature of the culturable part of the soil microbial community.

Interpreting δ13C Values Obtained on SOM from Ancient Maya Reservoirs and Depressions

Elemental analyzer (EA) Isotope Ratio Mass Spectrometry was used to measure ∂13C values on soil organic matter from reservoirs and depressions at the ancient Maya urban centers of Tikal, Guatemala and Yaxnohcah, Mexico. Variation in δ13C values on soil organic matter were > −2.0‰, which suggests enrichment from C4 plants including maize, other tropical grasses (Poaceae), and tropical sedges (Cyperaceae), CAM plants (Clusia sp.), and cyanobacteria (blue-green algae). Cyanobacteria were likely a major contributor to the 13C enrichment of soil organic matter in Maya reservoirs and depressions, which has obfuscated our understanding of ancient Maya maize production. It is possible that the Maya used cyanobacteria as a fertilizer, which enriched agricultural field soil organic matter.

The Variation of Soil Phosphorus Fractions and Microbial Community Composition under Consecutive Cucumber Cropping in a Greenhouse

The distribution of phosphorus (P) fractions in soil plays a decisive role in soil P bioavailability; however, the characteristics of soil P fractions under consecutive cropping in a solar greenhouse remain unclear. To evaluate the effects of the long-term successive vegetable cropping on soil P fractions and the microbial community composition in greenhouse soil, a continuous long-term cropping experiment was conducted using cucumber (Cucumis sativus L.) in a solar greenhouse starting from 2006 to 2018. Soil P fractions and the microbial community composition were determined using the Hedley continuous extraction method and the phospholipid fatty acid (PLFA) method, respectively, in the 1st, 9th, 13th, and 21st rounds of cultivation. The soil total phosphorus (TP) content increased from 0.90 g·kg−1 in the 1st round to 3.07 g·kg−1 in the 21st round of cucumber cultivation. With an increase in continuous cropping rounds, soil available phosphorus (AP) increased and the phosphorus activation coefficient (PAC) decreased, with no significant difference between the 13th and 21st rounds. After 21 rounds of continuous cropping, the soil organic matter (SOM) content was 16.34 g·kg−1, 1.42 times that of the 1st round. The abundance of soil bacteria, actinomycetes, Gram-negative bacteria (G−), Gram-positive bacteria (G+), and total PLFAs initially increased with continuous cropping rounds, but then decreased significantly, and the ratios of fungi:bacteria (F/B) and G+/G− bacteria also increased significantly with continuous cropping rounds. The contents of soil labile P, moderately labile P, and non-labile P increased significantly over 21 continuous cropping rounds. Together, these results demonstrate that long-term continuous cropping can directly lead to the accumulation of P fractions, but it can also affect the abundance of actinomycetes through SOM enrichment, which indirectly leads to the accumulation of non-labile P. This study provides a theoretical basis for future soil P fertilizer management and vegetable production sustainability.

Modifying effect of ant colonization on soil heterogeneity along a chronosequence of tropical forest restoration on slash-burn lands

Ants act as ecosystem engineers in regulating soil heterogeneities. Little is known about the degree and direction of these modifications on soils across a restoration chronosequence of tropical forests. Our objectives aimed to explore the effect of belowground-nesting ants on soil characteristics across four forest restoration stages (i.e., 12-, 28-, 42- and 53-yr olds) on slash-burn lands in the tropical Xishuangbanna, southwestern China. We confirmed the hypotheses about a positive effect of ant colonization on soil physical characteristics, and on the enrichment of microbial carbon and mineral nutrients in nest soils across the four restoration stages. Ant nests had the highest enrichment of soil organic matter (103%), readily oxidizable carbon (78%), total nitrogen (114%), available nitrogen (126%), NH4+ (133%) and NO3– (140%) at the 12-yr old stage compared with the surrounding soils. In contrast, the highest enrichment of microbial carbon (110%) in nest soils was showed at the 53-yr old stage. The enrichment of microbial carbon in nest soils increased with restoration age, but that of soil mineral nutrients would not follow the forest restorations. A higher enrichment of mineral nutrients in nest soils at the early restoration stage can improve soil fertility, which might promote the spontaneous forest restorations. A higher abundance and area of ant nests at the older restoration stage may create a higher bare space for plant development. Therefore, our results suggest that ant colonization can regulate forest restorations, possibly through creating and maintaining higher soil nutrient heterogeneity at earlier stage, and greatly stirring microbial growth and opening up space for plant development at older stage on slash-burn tropical lands.

Soil aggregate-associated distribution of DDTs and HCHs in farmland and bareland soils in the Danjiangkou Reservoir Area of China

Soil organic matter (SOM) is the principal aggregating agent for soil aggregation and also the main adsorbent for organochlorine pesticides (OCPs) such as dichlorodiphenyltrichloroethane (DDT) and hexachlorocyclohexane (HCH), which may thereby affect OCP distribution in soils subjected to different land use types. However, the potential effects of land use on SOM and OCP distribution patterns in soil aggregates are not well understood. In this study, soils from farmlands and barelands in the Danjiangkou Reservoir area were analyzed to determine the influence of land use on OCP distribution and composition in different aggregate fractions (>3, 1–3, 0.25–1, and <0.25 mm). The results showed that the levels of ∑DDTs ranged from 9.01 to 27.48 with a mean of 14.40 ng g−1, and ∑HCHs ranged from 2.06 to 4.66 with a mean of 3.19 ng g−1 in farmland soils. In comparison, bareland soils were less contaminated, with total DDTs and HCHs fell in the range of 0.75–5.01 ng g−1 and not detected (n.d.)-1.40 ng g−1 respectively. In regard to the distribution patterns in soil aggregates, the residual levels of ∑DDTs and ∑HCHs tended to a certain degree to enrich in microaggregates (<0.25 mm) relative to bulk soils. A further analysis revealed that the enrichment of ∑DDTs and ∑HCHs in microaggregates were mainly attributed to the accumulation of p,p'-DDE and β-HCH. Moreover, SOM was found also enriched in microaggregates. The enrichment of SOM was significantly and positively correlated with these of ∑DDTs, ∑HCHs, and the dominant metabolites (i.e., DDE and β-HCH) in both land use types. Such results indicated that the variations in behavior of OCPs could be linked to the processes of soil aggregate turnover. These findings may help to enrich the theory of soil OCPs sequestration and establish targeted strategies to mitigate their health risks in the environment.

Emissions of carbon dioxide and methane from fields fertilized with digestate from an agricultural biogas plant

AbstractDigestate from biogas plants can play important role in agriculture by providing nutrients, improving soil structure and reducing the use of mineral fertilizers. Still, less is known about greenhouse gas emissions from soil during and after digestate application. The aim of the study was to estimate the emissions of carbon dioxide (CO2) and methane (CH4) from a field which was fertilized with digestate. The gas fluxes were measured with the eddy covariance system. Each day, the eddy covariance system was installed in various places of the field, depending on the dominant wind direction, so that each time the results were obtained from an area where the digestate was distributed. The results showed the relatively low impact of the studied gases emissions on total greenhouse gas emissions from agriculture. Maximum values of the CO2and CH4fluxes, 79.62 and 3.049 µmol s−1m−2, respectively, were observed during digestate spreading on the surface of the field. On the same day, the digestate was mixed with the topsoil layer using a disc harrow. This resulted in increased CO2emissions the following day. Intense mineralization of digestate, observed after fertilization may not give the expected effects in terms of protection and enrichment of soil organic matter.

Сорняку в агроценозах можно найти разумное применение

Цель исследований — выявление возможности сокращения количества обработок при подготовке почвы и накопления дополнительного количества энергии в пожнивной период в виде зеленой массы естественного фитоценоза в условиях Терско-Сулакской низменности Прикаспия. Методика предусматривала исследование эффективности двух систем содержания почвы в этот период: энергозатратной, основанной на многократных обработках почвы по существующим в зоне системам, и энергонакопительной, предусматривающей создание оптимальных условий увлажнения почвы для достижения высокой продуктивности естественного фитоценоза, считающимся в агрономии «сорняком». Установлено, что в орошаемых условиях низменности указанный период надо использовать не для многократных обработок почвы по разным системам, а для оптимизации условий функционированию естественного фитоценоза. Установлено, что термические и энергетические ресурсы региона позволяют получать порядка 20 т/га зеленой массы естественного фитоценоза в пожнивной период с содержанием 90,72 кг азота, 19,99 кг P2O5, 23,88 кг K2O. Запашка этой массы способствует обогащению почвы органическим веществом и минеральными элементами питания растений, улучшению водно-физических и агрохимических показателей плодородия почвы, повышению урожайности зерна последующей в севообороте озимой пшеницы на 17,2 % и получению 4,2 тыс. руб./га дополнительного чистого дохода. Систему содержания почвы в пожнивной период, основанный на многократных обработках, предлагается считать энергозатратной, а использование этого же периода для формирования естественного фитоценоза на зеленое удобрение при минимальных затратах труда и средств за счет одного полива после уборки предшественника — энергонакопительной.

Clipped Cowpea (Vigna unguiculata (L.) Walp) Fodder Management: A Potential for Soil Organic Matter Enrichment in Degraded Savannah Soils of Nigeria

Severe nutrient depletion of the soils of the savannah tropics of Africa over the years has made it difficult to improve the productivity of varieties of crops using cultural practices alone. But interestingly, it has been observed that when a cowpea (Vigna unguiculata (L.) Walp) crop is cut (clipped) before senescence; it can regenerate after defoliation (provided there is enough soil moisture). And when the clipped organic fodder is added and/or ploughed back into the soil, it enriches the soil organic matter (SOM) content that in turn enhances crop productivity. Thus, this study was carried out with the objective of determining the influence of intra-row spacing, clipping height and time on the productivity of cowpea and SOM; at the Institute for Agricultural Research, Ahmadu Bello University, Samaru, Zaria, Nigeria; during the 2002-2005 wet seasons. The experimental lay out was a Randomized Complete Block Design (RCBD), replicated three times. The collected data was analyzed statistically using the analysis of variance test (ANOVA); and the means separation was done using the Duncan Multiple Range Test (DMRT). Results showed that the textural class of the experimental site soil was loam silt; with a conducive pH of (6.6 in 2002 and 5.5 in 2005), for crop growth. The soil organic carbon content (SOC), nitrogen (N) and cation exchange capacity (CEC) were very low (0.30 g kg-1, 0.88 g kg-1 and 4.90 respectively) in 2002. These increased to 10.37 g kg-1, 2.2 g kg-1 and 11.10 respectively in 2005. Total rainfall in 2002 was 1007.9mm and 871.5mm in 2005. Mean air temperature, relative humidity and sunshine hours in 2002 ranged between 21.0-31.5 °C, 16.0-90.6 and 4.5-8.5 respectively; while mean air temperature and relative humidity ranged between 24.7-32.0 °C and 40.5-85.3% respectively in 2005. Total harvested clipped fodder yield was 15t ha-1; and this was added to the soil; and it effectively increased SOM content by about 42%. Consequently, it was concluded that the adoption of this innovative clipping management technology, holds great potential of improving soil pH, increasing soil CEC, SOM and crop productivity generally, for the low technology, resource poor, and subsistence farmers in the region. Without doubt, overcoming SOM decline is a major component in the development of more sustainable agro-systems.

Sulfur speciation in well‐aerated and wetland soils in a forested catchment assessed by sulfur K‐edge X‐ray absorption near‐edge spectroscopy (XANES)

AbstractIn forested catchments, retention and remobilization of S in soils and wetlands regulate soil and water acidification. The prediction of long‐term S budgets of forest ecosystems under changing environmental conditions requires a precise quantification of all relevant soil S pools, comprising S species with different remobilization potential. In this study, the S speciation in topsoil horizons of a soil toposequence with different groundwater influence and oxygen availability was assessed by synchrotron‐based X‐ray absorption near‐edge spectroscopy (XANES). Our investigation was conducted on organic (O, H) and mineral topsoil (A, AE) horizons of a Cambisol–Stagnosol–Histosol catena. We studied the influence of topography (i.e., degree of groundwater influence) and oxygen availability on the S speciation. Soil sampling and pretreatment were conducted under anoxic conditions. With increasing groundwater influence and decreasing oxygen availability in the sequence Cambisol–Stagnosol–Histosol, the C : S ratio in the humic topsoil decreased, indicating an enrichment of soil organic matter in S. Moreover, the contribution of reduced S species (inorganic and organic sulfides, thiols) increased systematically at the expense of intermediate S species (sulfoxide, sulfite, sulfone, sulfonate) and oxidized S species (ester sulfate, SO$ _4^{2-} $). These results support the concept of different S‐retention processes for soils with different oxygen availability. Sulfur contents and speciation in two water‐logged Histosols subject to permanently anoxic and temporarily oxic conditions, respectively, were very different. In the anoxic Histosol, reduced S accounted for 57% to 67% of total S; in the temporarily oxic Histosol, reduced S was only 43% to 54% of total S. Again, the extent of S accumulation and the contribution of reduced S forms to total S closely reflected the degree of O2 availability. Our study shows that XANES is a powerful tool to elucidate key patterns of the biogeochemical S cycling in oxic and anoxic soil environments. In contrast to traditional wet‐chemical methods, it particularly allows to distinguish organic S compounds in much more detail. It can be used to elucidate microbial S‐metabolism pathways in soils with different oxygen availability by combining soil inventories and repeated analyses of a sample in different stages of field or laboratory incubation experiments under controlled boundary conditions and also to study (sub)microspatial patterns of S speciation in aggregated soils.

Arbuscular mycorrhizal fungi contribute to 13C and 15N 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.

The fate of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O consumed in soils

Abstract. Soils are capable to consume N2O. It is generally assumed that consumption occurs exclusively via respiratory reduction to N2 by denitrifying organisms (i.e. complete denitrification). Yet, we are not aware of any verification of this assumption. Some N2O may be assimilatorily reduced to NH3. Reduction of N2O to NH3 is thermodynamically advantageous compared to the reduction of N2. Is this an ecologically relevant process? To find out, we treated four contrasting soil samples in a flow-through incubation experiment with a mixture of labelled (98%) 15N2O (0.5–4 ppm) and O2 (0.2–0.4%) in He. We measured N2O consumption by GC-ECD continuously and δ15N of soil organic matter before and after an 11 to 29 day incubation period. Any 15N2O assimilatorily reduced would have resulted in the enrichment of soil organic matter with 15N, whereas dissimilatorily reduced 15N2O would not have left a trace. None of the soils showed a change in δ15N that was statistically different from zero. A maximum of 0.27% (s.e. ±0.19%) of consumed 15N2O may have been retained as 15N in soil organic matter in one sample. On average, 15N enrichment of soil organic matter during the incubation may have corresponded to a retention of 0.019% (s.e. ±0.14%; n=4) of the 15N2O consumed by the soils. We conclude that assimilatory reduction of N2O plays, if at all, only a negligible role in the consumption of N2O in soils.

Evidence of plaggen soils in European North Russia (Arkhangelsk region)

AbstractTo clarify whether a particular group of soils of Archangelsk region (European N Russia) with humus‐rich topsoils exceeding the plowing zone supports an anthropogenic formation, four exemplary profiles were investigated. The investigation sites are characterized by distinct elevated surfaces, and the soils show thick toplayers of up to 60 cm with enrichment of soil organic matter and artifacts like brick, charcoal, and peat fragments, all indicating an anthropogenic origin. Increased phytolith amounts and high P contents of up to 800 mg kg–1 citric acid–soluble P and up to 1,400 mg kg–1 total P in the top horizons support an anthropogenic influence. These properties are very similar to the Plagganthrepts of NW Europe. The same is true regarding the main management aims: increasing soil fertility and overcoming the need of bedding materials. Having the required depths of the anthropogenic topsoil, the properties of the soils of the Archangelsk region allow a classification as Agrozems (Russian classification), Plaggenesche (German classification), and Plagganthrepts (US taxonomy). Since the high base saturation of the topsoil excludes a designation as plaggic horizon, the topsoil has to be considered as terric horizon, which leads to a classification as Terric Anthrosol according to WRB.

Composition, Dynamics, and Fate of Leached Dissolved Organic Matter in Terrestrial Ecosystems: Results from a Decomposition Experiment

Fluxes of dissolved organic matter (DOM) are an important vector for the movement of carbon (C) and nutrients both within and between ecosystems. However, although DOM fluxes from throughfall and through litterfall can be large, little is known about the fate of DOM leached from plant canopies, or from the litter layer into the soil horizon. In this study, our objectives were to determine the importance of plant-litter leachate as a vehicle for DOM movement, and to track DOM decomposition [including dissolve organic carbon (DOC) and dissolved organic nitrogen (DON) fractions], as well as DOM chemical and isotopic dynamics, during a long-term laboratory incubation experiment using fresh leaves and litter from several ecosystem types. The water-extractable fraction of organic C was high for all five plant species, as was the biodegradable fraction; in most cases, more than 70% of the initial DOM was decomposed in the first 10 days of the experiment. The chemical composition of the DOM changed as decomposition proceeded, with humic (hydrophobic) fractions becoming relatively more abundant than nonhumic (hydrophilic) fractions over time. However, in spite of proportional changes in humic and nonhumic fractions over time, our data suggest that both fractions are readily decomposed in the absence of physicochemical reactions with soil surfaces. Our data also showed no changes in the 13 C signature of DOM during decomposition, suggesting that isotopic fractionation during DOM uptake is not a significant process. These results suggest that soil microorganisms preferentially decompose more labile organic molecules in the DOM pool, which also tend to be isotopically heavier than more recalcitrant DOM fractions. We believe that the interaction between DOM decomposition dynamics and soil sorption processes contribute to the 13 C enrichment of soil organic matter commonly observed with depth in soil profiles.

MICROBIAL IMMOBILIZATION AND THE RETENTION OF ANTHROPOGENIC NITRATE IN A NORTHERN HARDWOOD FOREST

To determine the importance of microorganisms in regulating the retention of anthropogenic NO3−, we followed the belowground fate and flow of 15NO3− in a mature northern hardwood forest, dominated by Acer saccharum Marsh. Total recovery of added 15N (29.5 mg 15N/m2 as NaNO3) in inorganic N, microbial immobilization in forest floor and soil microbial biomass, soil organic matter, and root biomass pools (0–10 cm depth) was 93% two hours following application of the 15NO3− but rapidly dropped to ∼29% within one month, presumably due to movement of the isotope into other plant tissues or deeper into soil. Microbial immobilization was initially (i.e., at 2 h) the largest sink for 15NO3− (21% in forest floor; 16% in soil microbial biomass). After one month, total 15N recovery varied little (24–18%) throughout the remainder of the growing season, suggesting that the major N transfers among pools occurred relatively rapidly. At the end of the four-month experiment, the main fates of the 15N label were in soil organic matter (7%), root biomass (6%), and N immobilized in forest floor and soil microbial biomass (6%). Temporal changes in the 15N enrichment (atom % excess 15N) of plant and soil pools during the first month of the experiment indicated the dynamic nature of NO3− cycling in this forest. The 15N enrichment of soil microbial biomass and the forest floor significantly increased two hours after isotope additions, suggesting rapid microbial immobilization of NO3−. In contrast, the 15N enrichment of soil organic matter did not peak until day 1, presumably because much of the added 15N cycled through microorganisms before becoming stabilized in soil organic matter, or it directly entered soil organic matter via physical processes. Furthermore, the 15N enrichment of root biomass (<0.5-mm diameter and 0.5–2.0 mm diameter) was greatest between day 7 and day 28, following significant increases in the 15N enrichment of soil organic matter (day 1) and, more importantly, NH4+ (day 2). From these data we conclude that microorganisms are immediate, short-term sinks for anthropogenic NO3−. Although the long-term fate of NO3− additions to this forest is likely in soil organic matter and plants, the cycling of N through microorganisms appears to be the major short-term factor influencing patterns of NO3− retention in this ecosystem.

Influence of organic and mineral fertilisers on soil biological and physical properties

The aim of this research was to study in a field experiment the influence of different fertiliser applications on soil biological and physical properties. Vermicompost (VC) from biological sludge, stabilised dairy manure or mineral nitrogen fertiliser (NH 4NO 3) were applied to a corn crop ( Zea mays L.) at 200 kg N ha −1. Soil enzyme activity (acid phosphatase, dehydrogenase and protease BAA) and CO 2 production were measured as indices of soil biological activity. These measures of metabolic activity were correlated to soil physical properties such as soil porosity. The soluble fractions of C and N were taken as indicators of fertiliser effects on soil fertility. There were positive correlations between soil porosity, enzymatic activity and CO 2 production in organic and mineral treatments. The addition of organic fertilisers improved soil physical and biological properties. The increase in macropores, ranging from 50–500 μm, in soil treated with organic fertilisers was mainly due to an increase in elongated pores, which are considered very important both in soil–water–plant relationships and in maintaining a good soil structure. Organic treatments stimulated soil biological activity probably due to an enrichment of soil organic matter. Mineral fertiliser enhanced soil porosity by increasing regular and irregular pores and caused a priming effect of native soil organic matter.

Bacterial and Fungal Cell‐Wall Residues in Conventional and No‐Tillage Agroecosystems

Agricultural management practices have been shown to influence the decomposer community in soils, with no‐tillage (NT) systems favoring fungi as compared with conventional tillage (CT) systems. In this study, we examined six North American agroecosystems with respect to the effects of NT vs. CT management systems on the accrual of microbial cell‐wall residues in surface soil. We used total amino sugar contents to estimate living and decomposing microbial cell‐wall mass in soil and the contents of glucosamine and muramic acid to separate fungal and bacterial contributions to microbial‐derived soil organic matter (SOM). Compared with estimates of glucosamine and muramic acid present in living biomass of fungi and bacteria, total concentrations of these compounds (745–2076 mg glucosamine kg−1 soil and 37–79 mg muramic acid kg−1 soil) were larger by factors of 54 to 745 and 26 to 82, respectively. At three sites, the ratios of glucosamine to muramic acid in NT soils (32.0, 30.0, 42.2) significantly exceeded those in the respective CT soils (18.8, 22.1, 23.0) because of a higher enrichment of glucosamine. This coincided with higher values for fungal biomass, particulate organic matter carbon (POM‐C), mean weight diameter of water‐stable aggregates (MWD), and total organic carbon (TOC). Analysis of aggregate‐size classes showed that the additional glucosamine accumulated in &gt;53‐mm aggregates but not in smaller particles. The enrichment of SOM in fungal‐derived glucosamine suggests that the accrual of hyphal cell‐wall residues is an important process in the three NT agroecosystems which leads to higher SOM storage in surface soil concurrent with an increase in aggregate stability. The other soils, having a lower clay plus silt content, exhibited no significant differences in POM‐C, MWD, and total amino sugars between NT and CT management systems. We suggest that at lower clay plus silt contents the beneficial potential for NT to sequester microbial‐derived SOM is lower because of limited physical stabilization.

Differential thermal analysis, thermogravimetry and in-source pyrolysis-mass spectrometry studies on the formation of soil organic matter

Differential thermal analysis (DTA) thermogravimetry (TG) and pyrolysis-field ionization mass spectrometry (Py-FIMS) were compared in studies of soil organic matter (SOM) in particle-size fractions in different years of a long-term soil formation experiment with loamy marl. The DTA curves and TG weight losses indicated the presence of different amounts of SOM, quartz and carbonates in the various size fractions and experimental years. The soil development from the 2nd to the 34th experimental year was characterized by the enrichment of SOM in the size fractions investigated, which also resulted in an accelerated weathering of carbonates. The comparison between DTA curves, TG weight losses and Py-FIMS thermograms and weight losses, respectively, showed a shift of the maximum of thermal decomposition of SOM to higher temperatures in Py-FIMS. This was explained by the higher heating rate (0.5 K s −1) and the particularly large sample amounts (5 mg) used in the mass spectrometric studies of particle-size fractions of whole soils with organic carbon (C org) concentrations between 0.2 and 1.9%. However, the onset of thermal evolution was in good agreement with DTA and TG data. Similar results were also obtained in the thermal behaviour of particle-size fractions and changes during soil development. By Py-FIMS, complementary information was obtained about the molecular chemical composition of SOM in the fractions by the evaluation of summed and averaged mass spectra and by the assignment of signals to the principal classes of chemical compounds. The abundances of phenols and lignin monomers and N-compounds decreased with increasing particle size; the reverse was true for lignin dimers and fatty acids. The latter two compound classes were also enriched in the samples from the 34th experimental year owing to SOM maturation. A unique feature of Py-FIMS as a novel method of whole soil analysis was its high analytical resolution which allowed monitoring of the thermal evolution of selected classes of compounds on a molecular chemical basis. This was demonstrated for the first time for phenols and lignin monomers, lignin dimers and alkylaromatics which are important soil constituents for SOM formation as well as for agricultural and environmental problems.