Higher Soil Organic Matter Content Research Articles

Spatial distribution characteristics of organic matter and total nitrogen of marsh soils in river marginal wetlands

Contents of soil organic matter (SOM) and total nitrogen (TN) in the surface soils and subsurface soils were measured in five types of floodplains classified with different flood frequencies in river marginal wetlands of Erbaifangzi, China, in 1999. Contour maps and profile maps were constructed to describe the spatial distribution of SOM and TN in order to identify the influences of flood frequency on them. Results showed that spatial distributions of both SOM and TN were very similar in soil profiles (0–120 cm) of the five areas, decreasing gradually with depth except an accumulation peak in the flooded floodplain (B area). Also, the accumulation peak in the soil profile of B area was relevant to water table, nitrogen leaching, denitrification and mineralization. However, their horizontal distributions in surface soils (0–10 cm) were different in the five areas. Although the flood could bring the deposit of nutrients and sands, the highest content of SOM or TN did not appear in B area but in the floodplain with certain flood frequency. For example, SOM content (6.76%) in 5-year floodplain wetland was highest, and the highest content of TN (3666.4 mg/kg) appeared in 1-year floodplain wetland. However, SOM and TN contents in soils of B area were 4.08% and 2605.4 mg/kg, respectively. Soil clay content, wetland plant ( Phragmites australis) litter inputs, soil moisture and water table greatly affected the spatial distribution of SOM and TN in floodplain wetlands. The ratios of carbon to nitrogen of wetland soils in this region were relatively lower than those in paddy soils. SOM and TN contents were significantly correlated with total phosphorus (TP) contents in floodplain wetlands except the 100-year floodplain wetland, but they were significantly influenced by soil pH values only in B area. Denitrification and ammonia volatilization were the main mechanisms resulting in nitrogen loss of surface soils in B area. Flood frequency significantly influenced the ecological functions such as nutrient retention and water quality maintenance of floodplains.

Building materials as a source of PCB pollution in Bergen, Norway

Although PCB is a globally recognised pollutant, an understanding of its transport from man-made building materials to the environment is poorly constrained. This paper presents data from a study that was conducted in order to determine the extent of PCB usage in plaster on building façades in the Bergen area, Norway. The study was to determine whether PCB concentrations vary according to building usage type and age. One aim was also to determine the nature and extent of displacement of PCB from the façade into the surrounding soil. Buildings built between 1952 and 1979 were chosen for the study. Three different media were sampled during the study; surface soil, plaster and paint. Samples were then analysed for PCB7 content. The results show that there is a difference in PCB usage in buildings of different age and usage type. Residential buildings and schools demonstrated higher PCB concentrations in both soil and plaster than buildings designated for office use, storage, or for industrial purposes. Buildings erected in the 1950s and 1960s also show a higher PCB concentration than buildings from a later date. It appears that the usage of PCB for these purposes decreased in the 1970s. Thirty percent of the soil samples showed a higher PCB concentration than the Norwegian action level. The soil samples tend to have a higher concentration than the corresponding plaster from the adjacent wall, which probably has its cause in the high soil organic matter contents that retains PCB. Plaster has not been considered a pollution source in previous studies; therefore this study demonstrates a new source that needs to be considered in emission calculations.

Soil organic matter, microbial properties, and aggregate stability under annual and perennial pastures

The use of annually sown pastures to provide winter forage is common in dairy farming in many regions of the world. Loss of organic matter and soil structural stability due to annual tillage under this management may be contributing to soil degradation. The comparative effects of annual ryegrass pastures (conventionally tilled and resown each year), permanent kikuyu pastures and undisturbed native vegetation on soil organic matter content, microbial size and activity, and aggregate stability were investigated on commercial dairy farms in the Tsitsikamma region of the Eastern Cape, South Africa. In comparison with soils under sparse, native grassy vegetation, those under both annual ryegrass and permanent kikuyu pasture had higher soil organic matter content on the very sandy soils of the eastern end of the region. By contrast, in the higher rainfall, western side, where the native vegetation was coastal forest, there was a loss of organic matter under both types of pasture. Nonetheless, soil organic C, K2SO4-extractable C, microbial biomass C, basal respiration, arginine ammonification and fluorescein diacetate hydrolysis rates and aggregate stability were less under annual than permanent pastures at all the sites. These results reflect the degrading effect of annual tillage on soil organic matter and the positive effect of grazed permanent pasture on soil microbial activity and aggregation. Soil organic C, microbial biomass C, K2SO4-extractable C, basal respiration and aggregate stability were significantly correlated with each other. The metabolic quotient and percentage of organic C present as microbial biomass C were generally poorly correlated with other measured properties but negatively correlated with one another. It was concluded that annual pasture involving conventional tillage results in a substantial loss of soil organic matter, soil microbial activity and soil physical condition under dairy pastures and that a system that avoids tillage needs to be developed.

Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoal - a review

Rapid turnover of organic matter leads to a low efficiency of organic fertilizers applied to increase and sequester C in soils of the humid tropics. Charcoal was reported to be responsible for high soil organic matter contents and soil fertility of anthropogenic soils (Terra Preta) found in central Amazonia. Therefore, we reviewed the available information about the physical and chemical properties of charcoal as affected by different combustion procedures, and the effects of its application in agricultural fields on nutrient retention and crop production. Higher nutrient retention and nutrient availability were found after charcoal additions to soil, related to higher exchange capacity, surface area and direct nutrient additions. Higher charring temperatures generally improved exchange properties and surface area of the charcoal. Additionally, charcoal is relatively recalcitrant and can therefore be used as a long-term sink for atmospheric CO2. Several aspects of a charcoal management system remain unclear, such as the role of microorganisms in oxidizing charcoal surfaces and releasing nutrients and the possibilities to improve charcoal properties during production under field conditions. Several research needs were identified, such as field testing of charcoal production in tropical agroecosystems, the investigation of surface properties of the carbonized materials in the soil environment, and the evaluation of the agronomic and economic effectiveness of soil management with charcoal.

Response of cotton (Gossypium hirsutum) to organic amendments and weed- management practices

A field experiment was conducted during 201516 and 201617 at Department of Agronomy, TNAU, Coimbatore, Tamil Nadu to determine the effects of organic amendments and weed-control methods on weed con- trol, yield and fibre quality of cotton (Gossypium hirsutum L.). Different organic amendments included (farmyard manure, vermicompost, composted poultry manure and goat manure), while weed-control methods comprised manual weeding, twin wheel hoe weeder and power-operated weeder for improvement in growth and yield of cot- ton. The results of pooled data indicated that, composted poultry manure along with hand-weeding twice at 20 and 45 days after sowing (DAS) recorded significantly lower weed density (27.29 No./m2), higher weed-control effi- ciency (84.80%) and higher crop dry-matter production (4,637 kg/ha), sympodial branches/plant (19.74), fruiting points/plant (54.01), bolls/plant (20.87), seed-cotton yield (1,602 kg/ha) compared with the other treatments. Appli- cation of organic materials along with manual weeding significantly improved available nutrient content, soil micro- bial biomass carbon, decreased soil electrical conductivity in short-term, and resulted in higher cotton yield and higher soil organic matter content. These results indicated that, application of composted poultry manure in addi- tion with hand-weeding or weeding with power-operated weeder would do more to improve soil organic matter content, increase soil organism abundance, reduce weed population and improve seed-cotton yield.

Long-term nutrient cycling patterns in Douglas-fir and red alder stands: a simulation study

Long-term patterns in nutrient cycling in regrowing Douglas-fir ( Pseudosuga menziesii Mirb. Franco) and red alder ( Alnus rubra Bong.) on native soils plus soils previously occupied by other species were simulated using the nutrient cycling model. Simulations of regrowing stands were also compared with observations of nutrient cycling in mature Douglas-fir and red alder. We hypothesized that (1) prolonged presence of red alder will cause a depletion in soil base cations due to increased nitrification and NO 3 − leaching; (2) lower base cation availability under red alder will ultimately cause biomass production to decline; (3) high N availability in red alder soils will favor regrowth of Douglas-fir; (4) higher base cation and P status of the Douglas-fir soils will favor growth of red alder both in the short- and long-term, since N is not limiting to red alder; and (5) in regrowing red alder, NO 3 − leaching will increase with time as a result of increased N fixation. All hypotheses were confirmed, but the effect of soil type on biomass production was minimal both for red alder and Douglas-fir. The higher soil organic matter content in the mature red alder stand most likely reflected previous occupation by old-growth Douglas-fir and also a large litter input from the understory vegetation. In general, the nutrient cycling model simulated differences in nutrient cycling patterns at least qualitatively between Douglas-fir and red alder and was helpful in identifying potential gaps in the understanding of biogeochemical cycling as well as uncertainties in the data. The nutrient cycling model did not fully elucidate differences in P cycling between Douglas-fir and red alder and overestimated weathering rates under Douglas-fir. Uncertainties in the data included: (1) temporal patterns in N fixation in the regrowing stands; (2) understory litterfall; and (3) site history and, consequently, presence of pre-existing differences in site conditions.

Process-based modelling of nitrous oxide emissions from different nitrogen sources in mown grassland

The process-based Pasture Simulation Model (PaSim 2.5) has been extended to simulate N2O production and emission from grassland caused by nitrogen inputs from different sources. The model was used to assess the influence of management on N2O emissions, such as the effect of shifts in the amount and timing of fertilizer application. Model performance has been tested against season-long field measurements at two different field sites. Simulation results agreed favourably with measured N2O emission and soil air concentrations, except during an extremely wet period at one site when grass growth was very poor. The results of short-term and long-term simulation runs demonstrated the potential of the model to estimate N2O emission factors under various conditions. During the first growing season, simulated emissions from organic fertilizers were lower than from synthetic fertilizers because more of the nitrogen was used to build up soil organic matter. The relative difference between the fertilizer types became larger with increasing application rate. The difference between fertilizer types was smaller at steady-state when higher soil organic matter content from repeated application of organic fertilizer over time led to enhanced mineralization and N2O emissions. The dependence of simulated N2O emissions on N input was close to linear at low, but non-linear at high fertilization rates. Emission factors calculated from the linear part of the curve suggested that the factors used in the current IPCC method underestimate the long-term effects of changes in fertilizer management. Furthermore the simulations show that N2O emissions caused by nitrogen inputs from the decomposition of harvest losses and from biological fixation in grassland can be considerable and should not be neglected in national emission inventories.

Spatial ecology of the Japanese beetle, Popillia japonica

AbstractA cornerstone of spatial ecology is the quantification of the patchy nature of animal and plant populations in space. By using spatial covariance, total covariance, and quantile variance, we found that patchiness of Japanese beetle grub populations varied more between years than between fairways at a central New York golf course. We also observed that populations tended to shrink and swell around patches with perennially low density, that locations with perennially low grub density were associated with high soil organic matter content, and that locations with frequently high grub density were associated with intense adult beetle activity in the vicinity.

PATTERNS IN NUTRIENT AVAILABILITY AND PLANT DIVERSITY OF TEMPERATE NORTH AMERICAN WETLANDS

Few wetland studies from temperate North America have related either species richness or plant community composition to any direct measure of nutrient availability, or examined changes in species composition following experimental nutrient additions. Studies of wetlands in western Europe and of other terrestrial ecosystems in North America frequently show that nutrient enrichment leads to changes in species composition, declines in overall plant species diversity, and loss of rare and uncommon species. We therefore used an extensive literature survey and analysis of data on plant species composition, species richness, productivity or standing crop, and C:N:P stoichiometry in plant tissues and surface soils to draw conclusions about the nature of nutrient limitation in temperate North American bogs, fens, marshes, and swamps, and to infer their potential response to nutrient enrichment. We searched all major bibliographic data bases for studies containing such data (through March 1998) and added relevant data from our own ongoing research. We analyzed plant and soil data sets by wetland type and by wetland soil type, and the plant data set also by growth form. Existing studies appear to confirm common generalizations: (1) plant community type changes across broad nutrient gradients; (2) species richness declines as various indicators of nutrient availability increase beyond some threshold; and (3) rare and uncommon species are almost always associated with species-rich communities. However, (1) these generalizations do not always hold within community types; (2) for many community types, the threshold beyond which richness declines has not been established, and high or low diversity may occur below that threshold; and (3) the failure of many studies to include bryophytes precludes drawing strong conclusions about nutrient availability and diversity in peatlands. Marshes had significantly lower mean nitrogen:phosphorus (N:P) ratios in live tissue than other wetland types (bogs, fens, and swamps), which did not differ significantly from each other. Mean N:P ratios in live tissues were significantly higher in peatlands than in mineral-soil wetlands. Nitrogen:phosporus ratios in litter did not differ significantly between peatlands and mineral-soil wetlands but were higher than in live tissues. Among growth forms, the highest mean N:P ratios in live tissues occurred in bryophytes, and the lowest in vascular herbaceous species. Only bryophyte live tissues differed significantly from other growth forms; litter N:P ratios were not significantly different among growth forms. Average N:P ratios in surface soils were lower in marshes and swamps than in bogs and fens. Wetlands on mineral soils had lower average N:P ratios than wetlands on peat soils. Average surface soil N:P ratios rose sharply at high soil organic-matter contents (≥90%) and were generally greater than 16 at organic-matter concentrations above 20%. In combination, plant tissue and surface soil N:P ratios suggest that a large proportion of North American wetlands are either P limited or co-limited by N and P, especially those occurring on organic soils. Only marshes have N:P ratios in both live tissues and surface soils that consistently indicate N limitation, although soils data suggest that the majority of swamps are also N limited. Vascular herbaceous species appear to be N limited, but no pattern is apparent among other growth forms. Inferences drawn from N:P stoichiometry need to be verified by examining ecosystem response to experimental fertilization. High variances in plant and soil N:P ratios suggest that understanding nutrient limitation at both the community and individual-species level may be necessary to predict changes in species composition and richness with nutrient enrichment.

Patterns in Nutrient Availability and Plant Diversity of Temperate North American Wetlands

Few wetland studies from temperate North America have related either species richness or plant community composition to any direct measure of nutrient availability, or examined changes in species composition following experimental nutrient additions. Studies of wetlands in western Europe and of other terrestrial ecosystems in North America frequently show that nutrient enrichment leads to changes in species composition, declines in overall plant species diversity, and loss of rare and uncommon species. We therefore used an extensive literature survey and analysis of data on plant species composition, species richness, productivity or standing crop, and C:N:P stoichiometry in plant tissues and surface soils to draw conclusions about the nature of nutrient limitation in temperate North American bogs, fens, marshes, and swamps, and to infer their potential response to nutrient enrichment. We searched all major bibliographic data bases for studies containing such data (through March 1998) and added relevant data from our own ongoing research. We analyzed plant and soil data sets by wetland type and by wetland soil type, and the plant data set also by growth form. Existing studies appear to confirm common generalizations: (1) plant community type changes across broad nutrient gradients; (2) species richness declines as various indicators of nutrient availability increase beyond some threshold; and (3) rare and uncommon species are almost always associated with species-rich communities. However, (1) these generalizations do not always hold within community types; (2) for many community types, the threshold beyond which richness declines has not been established, and high or low diversity may occur below that threshold; and (3) the failure of many studies to include bryophytes precludes drawing strong conclusions about nutrient availability and diversity in peatlands. Marshes had significantly lower mean nitrogen:phosphorus (N:P) ratios in live tissue than other wetland types (bogs, fens, and swamps), which did not differ significantly from each other. Mean N:P ratios in live tissues were significantly higher in peatlands than in mineral-soil wetlands. Nitrogen:phosporus ratios in litter did not differ significantly between peatlands and mineral-soil wetlands but were higher than in live tissues. Among growth forms, the highest mean N:P ratios in live tissues occurred in bryophytes, and the lowest in vascular herbaceous species. Only bryophyte live tissues differed significantly from other growth forms; litter N:P ratios were not significantly different among growth forms. Average N:P ratios in surface soils were lower in marshes and swamps than in bogs and fens. Wetlands on mineral soils had lower average N:P ratios than wetlands on peat soils. Average surface soil N:P ratios rose sharply at high soil organic-matter contents (≥90%) and were generally greater than 16 at organic-matter concentrations above 20%. In combination, plant tissue and surface soil N:P ratios suggest that a large proportion of North American wetlands are either P limited or co-limited by N and P, especially those occurring on organic soils. Only marshes have N:P ratios in both live tissues and surface soils that consistently indicate N limitation, although soils data suggest that the majority of swamps are also N limited. Vascular herbaceous species appear to be N limited, but no pattern is apparent among other growth forms. Inferences drawn from N:P stoichiometry need to be verified by examining ecosystem response to experimental fertilization. High variances in plant and soil N:P ratios suggest that understanding nutrient limitation at both the community and individual-species level may be necessary to predict changes in species composition and richness with nutrient enrichment.

Distinguishing characteristics of temporary pond habitat of endangered northeastern bulrush,Scirpus ancistrochaetus

Habitat characteristics of wetlands that contain the federally endangered sedge,Scirpus ancistrochaetus, were investigated in a one-point-in-time field survey. Sixteen adjacent seasonal ponds, four of which supported populations ofS. ancistrochaetus. were sampled for 26 habitat variables. Using only five of these variables, we were able to separate ponds with and withoutS. ancistrochaetus in a linear discriminant analysis. These five variables are wetland area, percent cover of overlying forest canopy, soil percent organic matter, soil exchangeable [Na], and water [H−]. Ponds containingS. ancistrochaetus had higher soil organic matter content, highet soil [Na], greater area, lower water [H+], and lower percent forest canopy cover compared to ponds that did not support this species. This information can be used toward understanding factors that control the distribution of this species and also toward evaluating possible habitat for reintro-duction of this endangered species.

Effects of soil moisture, organic matter and grass mulching on the carpogenic germination of sclerotia and infection of bean by Sclerotinia sclerotiorum

Microplot experiments were carried out during the autumn and winter of 1995 to determine the effects of soil moisture and levels of organic matter on the carpogenic germination of Sclerotinia sclerotiorum and subsequent infection of Phaseolus vulgaris. Soil moisture treatments were established by irrigation to field capacity: (a) when soil had dried to − 25 KPa, (b) when soil had dried to − 54 KPa, and (c) eight days after the soil had dried to − 54 KPa. Three levels of organic matter were established by mixing compost and yellow‐red latosol in the following proportions (by volume): (a) 0% compost and 100% soil, (b) 50% compost and 50% soil, and (c) 100% compost and 0% soil. More apothecia and diseased plants were observed in the plots with higher moisture levels. Drier treatments resulted in a reduction in number (in the winter experiment) or elimination (in the autumn experiment) of apothecia. Other experiments were designed to examine the effect of grass mulching and organic matter levels on the carpogenic germination of S. sclerotiorum. In one experiment, grass mulching was compared with no mulching, and in another, the effects of different depths of mulch on the soil surface (0, 1.5, 3, 6 and 9 cm) were examined. Fewer apothecia were formed in mulched soil and the effect was most pronounced with 6 and 9 cm of mulch. The highest level of mulch (9 cm) impaired the vigour and yield of bean plants. In these experiments, carpogenic germination was increased by high soil organic matter content. Control of soil moisture and grass mulching may reduce the production of apothecia of S. sclerotiorum even in soils rich in organic matter.

Efecto de la transformación de la selva a pradera sobre la dinámica de los nutrientes en un ecosistema tropical estancional en México

El presente trabajo analiza los efectos de la transformación de la selva a pradera sobre la dinámica de los nutrientes en el suelo, en un ecosistema tropical estacional de la costa central del Pacífico en México. De acuerdo a las características de la dinámica de los nutrientes en los suelos de la selva, se establecieron dos condiciones: 1] el suelo en cresta, donde los procesos biológicos edáficos son los determinantes y 2] el suelo en ladera, donde el principal factor son las características del relieve. Cuando la selva es talada y quemada, los pastos son cultivados para uso de ganadería extensiva, lo cual genera cambios en las propiedades físicas y químicas del suelo. Cada elemento del relieve responde de una manera diferente a la transformación. Por ejemplo, el suelo en la cresta tuvo mayor capacidad de retención de nutrientes, lo cual está relacionado con el mayor contenido de la materia orgánica. Por el contrario, el suelo de la ladera tuvo una menor capacidad de protección de nutrientes. Sin embargo, las pérdidas totales de nutrientes después de 11 años de uso continuo fueron iguales para ambos elementos del relieve: 700 kg N ha- 1, 2,500 kg Ca ha-1 y 500 kg P ha-1.

Enset farming in Ethiopia. I. Soil nutrient status in Shoa and Sidamo regions

Soil samples were collected from farms in two enset (Ensete ventricosum) growing regions of Ethiopia and analyzed for plant nutrients. For comparative reasons, soil samples were taken both from fields where enset had been continuously cultivated for several decades and from other fields: annual cropping, grazing, or uncultivated. The results reveal that the enset fields studied had higher soil organic matter contents and better nutritional status than fields used otherwise, not only in the topsoil but also in the subsoil. This indicates more sustainable cultivation with maintained fertility in the enset fields. Possible explanations of this are discussed.

Kinetics and Equilibrium of Simazine Sorption on Soil Colloids.

Adsorption and desorption of simazine on a Hiroshima loam soil and its colloidal size fractions were studied in batch experiments. The colloidal size fractions were isolated by repeated gravity sedimentation. Two-step sorption kinetics (step I-with initially non-polluted soil and step II-with soil initially polluted during step I) were well described by a modified Freundlich type kinetics sorption model. Both for the whole soil and soil size fractions the sorption kinetics were characterized by a rapid initial adsorption where the major part of the pesticide was adsorbed within the first 10 minutes followed by a slow, diffusion-controlled sorption. The sorption kinetics parameter N was well correlated with soil organic matter content, and N decreased from step I to step II, dependent on the total amount of simazine adsorbed during step I. This suggests that adsorption kinetics were controlled by both the actual organic matter content and the previous sorption history of each soil size fraction. Adsorption and desorption isotherms for whole soil and loam soil size fractions were well described by the classical Freundlich isotherm equation. A fine silt size fraction with a relatively higher soil organic matter content adsorbed the most simazine and, also, released the least simazine after 10 steps desorption. A large difference between the adsorption and desorption isotherms and therefore a hysteretic sorption behavior for all soil size fractions was observed. Significant correlation was found between adsorption and desorption coefficients and the soil organic matter content.

Influence of lime, fertilizer and manure applications on soil organic matter content and soil physical conditions: A review

The effects of lime, fertilizer and manure applications on soil organic matter status and soil physical properties are of importance to agricultural sustainability. Their effects are complex and many interactions can occur. In the short-term, liming can result in dispersion of clay colloids and formation of surface crusts. As pH is increased the surface negative charge on clay colloids increases and repulsive forces between particles dominate. However, at higher lime rates, Ca2+ concentrations and ionic strength in soil solution increase causing compression of the electrical double layer and renewed flocculation. When present in sufficient quantities, both lime and hydroxy-Al polymers formed by precipitation of exchangeable Al, can act as cementing agents bonding soil particles together and improving soil structure. Liming often causes a temporary flush of soil microbial activity but the effect of this on soil aggregation is unclear. It is suggested that, in the long-term, liming will increase crop yields, organic matter returns, soil organic matter content and thus soil aggregation. There is a need to study these relationships on existing long-term liming trials. Fertilizers are applied to soils in order to maintain or improve crop yields. In the long-term, increased crop yields and organic matter returns with regular fertilizer applications result in a higher soil organic matter content and biological activity being attained than where no fertilizers are applied. As a result, long-term fertilizer applications have been reported, in a number of cases, to cause increases in water stable aggregation, porosity, infiltration capacity and hydraulic conductivity and decreases in bulk density. Fertilizer additions can also have physico-chemical effects which influence soil aggregation. Phosphatic fertilizers and phosphoric acid can favour aggregation by the formation of Al or Ca phosphate binding agents whilst where fertilizer NH4 + accumulates in the soil at high concentrations, dispersion of clay colloids can be favoured. Additions of organic manures result in increased soil organic matter content. Many reports have shown that this results in increased water holding capacity, porosity, infiltration capacity, hydraulic conductivity and water stable aggregation and decreased bulk density and surface crusting. Problems associated with large applications of manure include dispersion caused by accumulated K+, Na+ and NH4 + in the soil and production of water-repellant substances by decomposer fungi.

The dynamics of carbon in particle-size fractions of soil in a forest-cultivation sequence

Cultivation of forest and grassland soils induces heavy changes in soil organic matter (SOM) dynamics. To better predict the effect of cultivation, there is a need to describe which organic pools are affected and to which extent. We used a chronosequence of thick humic forest soils converted to maize cultivation for 40 yr in southwest France. The dynamics of soil carbon was investigated through particle-size fractionation and the use of 13C allowed to distinguish forest-derived organic matter and new crop-derived organic matter. This partitioning of soil carbon by size on one hand and by age on the other provided a precise description of carbon turnover. The level towards which tend the organic pools under cultivation showed that the decay rates of soil carbon were one order of magnitude higher under cultivation than under forest. SOM can thus be considered as deprotected under cultivation. All size fractions appeared to be deprotected to the same extent. A progressive transfer of silt-sized C to clay-sized C was nevertheless suspected and attributed to the decreasing stability of fine silt-sized microaggregates with cultivation. SOM furthermore contained some very stable C present as silt-sized and possibly clay-sized particles. The turnover times of maize-derived organic matter was the same as that observed in similar soils cultivated for centuries. This indicated that the new conditions induced by cultivation were reached in the very first years after forest clearing and that the high initial SOM content and high mineralization rate of initial organic matter did not affect the dynamics of newly incorporated carbon.

Clomazone for Weed Control in Transplanted Cole Crops (Brassica oleracea)

Experiments were conducted at Lexington and Quicksand, KY, in 1989 and 1990 to determine the effect of preplant incorporated clomazone on weed control, crop injury, and yield of broccoli, cauliflower, green cabbage, red cabbage, and pak choi. Oxyfluorfen and trifluralin were included as standard treatments. Greater than 80% weed control was observed at both locations with 0.8 kg ai ha−1clomazone, with the exception of smooth pigweed at Lexington. All clomazone treatments caused crop injury 2 WAT. Injury was still evident 8 WAT at clomazone rates of 1.7 and 3.4 kg ha−1, but was minor with rates of 0.8 kg ha−1or less on broccoli, cauliflower, and red cabbage. Injury was less at Quicksand in both years and seasons over all clomazone rates, most likely due to higher soil organic matter content Yields of broccoli and cauliflower treated with 0.8 kg ha−1clomazone were similar to yields with oxyfluorfen on a 3% organic matter soil at Lexington in 1989. However, clomazone at 0.8 kg ha−1at Lexington reduced 1989 spring yields of green cabbage, red cabbage, and pak choi and 1990 spring yields of all cole crops as compared to oxyfluorfen. Clomazone at 0.8 kg ha−1at Quicksand reduced yield of green cabbage and pak choi in spring 1990 only on a 5.2% organic matter soil. Fall yields of broccoli and cauliflower in both years were not reduced by clomazone at 0.8 kg ha−1or less at either location. Our studies indicated potential for utilization of clomazone on cole crops in higher organic matter soils, especially if some early season crop injury and occasional yield loss can be tolerated.

Organic matter and nitrogen dynamics in conventional versus integrated arable farming

Carbon and nitrogen cycling and crop yields in integrated (INT) arable farming, with 35% of N fertilisation by organic manures, were compared with those in conventional (CONV) management, using only mineral-N fertiliser, in a long-term field trial on a calcareous silt loam soil. In fields with a relatively high initial soil organic matter (SOM) content, crops, especially potatoes, seemed to benefit from the increased soil N supply. Under INT management, original SOM levels were maintained in fields with 2.2% and 2.8% SOM in the 0–25 cm layer; under CONV management these levels decreased to 2.15% and 2.6% SOM in 6 years. In situ, periods of N immobilisation as well as of N mineralisation were found, while in vitro only net N mineralisation occurred. Model calculations of C and N turnover in soil were used to estimate the N mineralisation pattern and the pool sizes of microbial biomass and young humus. Mineral-N balance calculations for potatoes suggested N losses of 0–100 kg ha−1 over the 1989 growing season. For the rotation cycle (1988–1991), calculated N losses to the environment were less than 300 kg ha−1 and 170 kg ha−1, at the low and the high SOM level, respectively. High losses may be partly explained by high levels of rainfall in 1988 and 1990, with denitrification as a likely cause of N loss. The highest (risk of) N loss in spring is for potatoes and sugar beet, where high nitrate levels are present in soil and N uptake starts relatively late. Calculated average efficiencies of N inputs were 88% and 72% for CONV management started on soil with 2.8% and 2.2% SOM, respectively; for INT management these values were 82% and 65%. Values of N efficiencies and N losses are still preliminary because steady state had not yet been attained.

Nitrogen mineralization and nitrification in successional ecosystems on the Tanana River floodplain, interior Alaska

Nitrogen (N) mineralization and nitrification were compared among ecosystems representing a primary successional sequence on the Tanana River floodplain of interior Alaska. These processes displayed marked seasonality, were closely related to substrate chemistry, and reflected the impact of vegetation clearing. The highest rates of N mineralization were encountered in the June to July incubation periods, and rates generally declined during the remainder of the summer. The early season period (June to July) was the interval of most favorable litter and mineral soil temperature and most available energy supply for microbial mineralization of detrital materials. Minimal rates were encountered during the winter. Litter layer N mineralization rates were highest in the early-successional poplar–alder (Populusbalsamifera–Alnustenuifolia (Nutt.) stage and declined with advancing succession in poplar (Populusbalsamifera) and mature white spruce (Piceaglauca) (Moench) Voss) stands. The poplar–alder stage displayed the highest rate of nitrification. Nitrate constituted 98% of the mineralized N in early-successional poplar–alder forest floors but fell to 4 and 0% in poplar and white spruce forest floors, respectively. Nitrogen mineralization was closely related to significant increases in the lignin/N ratio across the sequence of vegetation types. The rate of surface mineral soil net N mineralization increased with succession in response to higher soil organic matter content. The range of average total seasonal net N mineralization (260–1600 mg N•m−2) for litter layer plus mineral soil among successional stages in this study was generally lower than the 1200–8400 mg N•m−2 reported by investigators for other studies in temperate latitudes. Vegetation clearing increased the magnitude of temporal fluxes as well as total annual mineral N production. The most consistent increases were encountered in the poplar–alder vegetation type. The average seasonal total net N mineralization for forest floor plus mineral soil in this vegetation type increased from 1500 to 3264 mg N•m−2 as a result of clearing. Soil temperature declined with advancing succession and generally increased as a consequence of clearing. However, these changes were not as closely correlated with N mineralization as were the changes in substrate chemistry encountered across this successional sequence.