Bent T. Christensen

Measurement of soil microbial biomass provides an early indication of changes in total soil organic matter due to straw incorporation

The straw and stubble of spring barley (Hordeum vulgare; 5t dry matter ha−1) were either burned or incorporated into soil annually for 18 yr in two field experiments in Denmark. Both experiments were on light soils situated at Studsgaard (loamy sand) and Ronhave (sandy loam). At both sites 18 yr of annual straw incorporation increased total soil organic C by only 5% and total N by about 10% but produced large increases in microbial biomass measured by the CHCl3-fumigation method. The increases in biomass C were 45 and 37% at Studsgaard and Ronhave, respectively: the corresponding increases in biomass N were 50 and 46%. Biomass measurements thus gave an early indication of slow changes in organic matter content long before these could be measured accurately against the background of organic matter already present in the soils. Increases in biomass P due to straw incorporation appeared to be even greater. However, the amounts of P released by CHC13 were small so the measurements of biomass P were less accurate than those of biomass C or N. During a 60-day laboratory incubation at 25°C, evolution of CO2-C was 55–79% greater in soil from straw incorporated plots than in soil from burned plots. Mineralization of N was 40–50% greater where straw had been incorporated, indicating thaf the long-term incorporation of straw had increased the quantity of mineralizable N in soil.

Effects of animal manure and mineral fertilizer on the total carbon and nitrogen contents of soil size fractions

SummarySoil was sampled in autumn 1984 in the 132 field (sandy loam soil) of the Askov long-term experiments (started in 1894) and fractionated according to particle size using ultrasonic dispersion and sedimentation in water. The unmanured plot and plots given equivalent amounts of N (1923–1984 annual average, 121 kg N/ha) in either animal manure or mineral fertilizer were sampled to a depth of 15 cm, fractionated and analysed for C and N. Mineral fertilizer and animal manure increased the C and N content of whole soil, clay (<2 μm) and silt (2–20 μm) size fractions relative to unmanured samples, while the C content of the sand size fractions (fine sand 1, 20–63 μm; fine sand 2, 63–200 μm; coarse sand, 200–2000 μm) was less affected. Clay contained 58% and 65°70 of the soil C and N, respectively. Corresponding values for silt were 30% and 26%, while sand accounted for 10% of the soil C. Fertilization did not influence this distribution pattern. The C : N ratio of the silt organic matter (14.3) was higher and that of clay (10.6) lower than whole-soil C:N ratios (12.0). Fertilization did not influence clay and silt C : N ratios. Animal manure caused similar relative increases in the organic matter content of clay and silt size fractions (36%). In contrast, mineral fertilizer only increased the organic matter content of silt by 21% and that of clay by 14%.

Cropping system and residue management effects on nitrate leaching and crop yields

Abstract The effect of cropping system and of crop residue management on crop yield and N uptake and on NO3 leaching was tested in a 2-year lysimeter experiment with sandy loam soil. One cropping system included a 4-year rotation of spring barley, ryegrass, winter wheat and sugarbeet. In the other system, two test crops of spring barley followed cereal rye, ryegrass or spring barley, grown continuously for 9 years. In the first autumn, straw was incorporated or removed in lysimeters previously in cereals, and sugarbeet top was returned or removed in lysimeters previously in sugarbeet. Lysimeters with and without straw incorporation received 1 g 15N m−2 of 99 at.% 15NH4NO3 in order to estimate N immobilization caused by straw and its potential for remineralization. In the second year, above ground residues (except for stubbles) were removed in all lysimeters. Straw incorporation caused more 15N to be retained in the soil but the leaching of total N in the first winter was not reduced significantly. The pooled NO3 loss over the two winters was not affected by the straw management. Between 12 and 48% of 15N immobilized by the straw was remineralized during the 2 years following straw incorporation. Sugarbeet top increased the leaching of N in the two winter periods. Losses were small from lysimeters with ryegrass but turnover of ryegrass residues appeared to enhance losses of NO3 in the second winter after its incorporation. Straw incorporation reduced the yield and N uptake of the first test crop of barley, indicating a prolonged N immobilization phase. Sugarbeet yields were unaffected by straw probably because of its longer growth period. Return of sugarbeet tops increased the yield and N uptake of the two succeeding crops. Spring barley grown after termination of permanent ryegrass yielded considerably better and had larger N uptakes than barley succeeding cereal crops. Despite the differential effects of residue management, the type of cropping system was much more decisive for total N turnover. Amounts of N exported in the crop rotation by NO3 leaching and plant uptake generally balanced the amounts of N applied in mineral fertilizer. More N was exported than applied, however, when spring barley was grown continuously. Incorporation of straw could only partly prevent the negative N balance.

Appearance of β-lactam Resistance Genes in Agricultural Soils and Clinical Isolates over the 20th Century

Debate exists about whether agricultural versus medical antibiotic use drives increasing antibiotic resistance (AR) across nature. Both sectors have been inconsistent at antibiotic stewardship, but it is unclear which sector has most influenced acquired AR on broad scales. Using qPCR and soils archived since 1923 at Askov Experimental Station in Denmark, we quantified four broad-spectrum β-lactam AR genes (ARG; blaTEM, blaSHV, blaOXA and blaCTX-M) and class-1 integron genes (int1) in soils from manured (M) versus inorganic fertilised (IF) fields. “Total” β-lactam ARG levels were significantly higher in M versus IF in soils post-1940 (paired-t test; p < 0.001). However, dominant individual ARGs varied over time; blaTEM and blaSHV between 1963 and 1974, blaOXA slightly later, and blaCTX-M since 1988. These dates roughly parallel first reporting of these genes in clinical isolates, suggesting ARGs in animal manure and humans are historically interconnected. Archive data further show when non-therapeutic antibiotic use was banned in Denmark, blaCTX-M levels declined in M soils, suggesting accumulated soil ARGs can be reduced by prudent antibiotic stewardship. Conversely, int1 levels have continued to increase in M soils since 1990, implying direct manure application to soils should be scrutinized as part of future stewardship programs.

Isolation and characterization of labile organic phosphorus pools in soils from the Askov long-term field experiments

Labile soil organic phosphorus (Po) plays a crucial role in plant P nutrition and in environmental eutrophication. This paper discusses recent studies on the nature of labile soil Po and its response to different fertilization practices. Soil material was obtained from the Askov long-term experiment on animal manure and mineral fertilizers. Our analytical approach combined a macroporous anion exchange resin to isolate labile Po and 31P NMR spectroscopy to assess the chemical composition and origin of NaOH-extractable Po. The analyses were carried out on fine earth (<2 mm) and particle-size fractions. The results suggest that the resin extraction isolates an active pool of soil Po that consists primarily of microbially-derived compounds. The size of the active Po pool reflects seasonal variations and P fertilization. However, the source of added P (NPK vs. animal manure) appears to have only little influence on the labile Po. Our results further suggest that most of the active soil Po is associated with clay sized separates (<2 μm), indicating this fraction being important in the short-term turnover of Po.

Abundance of 13C and 15N in emmer, spelt and naked barley grown on differently manured soils: towards a method for identifying past manuring practice.

The shortage of plant-available nutrients probably constrained prehistoric cereal cropping but there is very little direct evidence relating to the history of ancient manuring. It has been shown that the long-term addition of animal manure elevates the δ(15)N value of soil and of modern crops grown on the soil. We have examined the δ(15)N and δ(13)C values of soil and of the grain and straw fractions of three ancient cereal types grown in unmanured, PK amended and cattle manured plots of the Askov long-term field experiment. Manure increased biomass yields and the δ(15)N values of soil and of grain and straw fractions of the ancient cereal types; differences in δ(15)N between unmanured and PK treatments were insignificant. The offset in straw and grain δ(15)N due to manure averaged 7.9 and 8.8 ‰, respectively, while the soil offset was 1.9 ‰. The soil and biomass δ(13)C values were not affected by nutrient amendments. Grain weights differed among cereal types but increased in the order: unmanured, PK, and animal manure. The grain and straw total-N concentration was generally not affected by manure addition. Our study suggests that long-term application of manure to permanently cultivated sites would have provided a substantial positive effect on cereals grown in early agriculture and will have left a significant N isotopic imprint on soil, grains and straw. We suggest that the use of animal manure can be identified by the (15)N abundance in remains of ancient cereals (e.g. charred grains) from archaeological sites and by growing test plants on freshly exposed palaeosols.

Interactions between elevated CO2 and added N: effects on water use, biomass, and soil 15N uptake in wheat

The interaction between atmospheric CO2 (ambient=365 ppmv, elevated=500 ppmv) and fertilizer N (6, 12 and 18 g N m−2) was examined in spring wheat (Triticum aestivum, L.) grown on a 15N-labelled sandy loam soil. Wheat shoots were pulse-labelled once with 14CO2 to trace the photosynthetically fixed C within the soil/plant system. Fertilizer N increased root length and canopy water use efficiency (WUE). At each N level, root length and WUE were higher under elevated than under ambient CO2, but no CO2-N interaction was observed. The N concentration in roots, but not in shoots, was affected by a CO2-N interaction. Fertilizer N increased and elevated CO2 decreased shoot N concentrations. The root-to-shoot (R/S) ratio decreased with N addition and was affected by interactions between N and CO2. Plant biomass C and N increased significantly with increased N addition. The effect of elevated CO2 on shoots was relatively small regardless of N level, whereas roots experienced a clear CO2-N interaction. The 14C recovered one week after labelling was mainly in the shoot biomass (80%). The activity increased with N addition, reflecting a larger shoot biomass, but the relative distribution of 14C between shoots, roots, soil and rhizosphere respiration was little affected by CO2 level and N addition. Soil 15N contributed 7.1 to 9.0 g N m−2 to shoot N, the total increase (9.3 to 24.3 g N m−2) being dominated by contributions from fertilizer N. We conclude that effects of elevated CO2 on wheat tissue N cannot be compensated for by N additions, and that plant uptake of native soil N is marginally affected by added N and elevated CO2.

Consolidating soil carbon turnover models by improved estimates of belowground carbon input

World soil carbon (C) stocks are third only to those in the ocean and earth crust, and represent twice the amount currently present in the atmosphere. Therefore, any small change in the amount of soil organic C (SOC) may affect carbon dioxide (CO2) concentrations in the atmosphere. Dynamic models of SOC help reveal the interaction among soil carbon systems, climate and land management, and they are also frequently used to help assess SOC dynamics. Those models often use allometric functions to calculate soil C inputs in which the amount of C in both above and below ground crop residues are assumed to be proportional to crop harvest yield. Here we argue that simulating changes in SOC stocks based on C input that are proportional to crop yield is not supported by data from long-term experiments with measured SOC changes. Rather, there is evidence that root C inputs are largely independent of crop yield, but crop specific. We discuss implications of applying fixed belowground C input regardless of crop yield on agricultural greenhouse gas mitigation and accounting.

Suboptimal fertilisation compromises soil physical properties of a hard-setting sandy loam

Nutrient management affects not only crop productivity and environmental quality, but also soil physical properties related to soil tilth. Previous studies on soil physical properties have focussed on effects of fertiliser type, whereas the effect of fertiliser rate has been neglected. We examined the impact of no fertilisation (UNF) and different rates of mineral fertiliser (½NPK and 1NPK) and animal manure (1½AM) on an ensemble of soil physical characteristics, with the amount of fertiliser added at level 1 corresponding to the standard rate of plant nutrients for a given crop. Soil was from the Askov long-term field experiment, initiated in 1894 on a hard-setting sandy loam. We assessed clay dispersibility, wet-stability of aggregates, aggregate strength, bulk soil strength and soil pore characteristics. The soils receiving 1NPK and 1½AM had similar soil physical properties, the only differences being a wider range in the optimum water content for tillage and more plant-available water in the soil amended with 1½AM. Suboptimal fertiliser rates (UNF and ½NPK) increased clay dispersibility, soil cohesion and bulk density, and reduced aggregate stability. The physical properties of soils exposed to suboptimal fertilisation indicate that the level of soil organic matter, including active organic binding and bonding materials, has become critically low due to reduced inputs of crop residues. While long-term suboptimal fertilisation compromises soil physical properties, crop-yield-optimised rates of mineral fertilisers and animal manure appear to sustain several soil physical properties equally well.

Soil texture analysis revisited: Removal of organic matter matters more than ever

Exact estimates of soil clay (<2 μm) and silt (2–20 μm) contents are crucial as these size fractions impact key soil functions, and as pedotransfer concepts based on clay and silt contents are becoming increasingly abundant. We examined the effect of removing soil organic matter (SOM) by H2O2 before soil dispersion and determination of clay and silt. Soil samples with gradients in SOM were retrieved from three long-term field experiments each with uniform soil mineralogy and texture. For soils with less than 2 g C 100 g-1 minerals, clay estimates were little affected by SOM. Above this threshold, underestimation of clay increased dramatically with increasing SOM content. Silt contents were systematically overestimated when SOM was not removed; no lower SOM threshold was found for silt, but the overestimation was more pronounced for finer textured soils. When exact estimates of soil particles <20 μm are needed, SOM should always be removed before soil dispersion.