Ingrid K. Thomsen

Turnover of organic matter in differently textured soils: I. Physical characteristics of structurally disturbed and intact soils

Abstract Soil type effects on organic matter turnover are most often ascribed directly to differences in soil clay content. Since soil texture determines the physical characteristics of soil, aggregation and water holding capacity may be more relevant to address in the search for controls of organic matter turnover. Most studies of microbial processes in soils are based on structurally disturbed soil, where the abiotic conditions for the microbial activity may be quite different from those in intact soils. In this study, basic physical characteristics were determined for structurally disturbed and intact soil samples from differently textured soils. Bulk soil was retrieved from 0–20 cm depth at six locations along a textural gradient in an arable field on Weichselian morainic deposits in Denmark. The samples (NA1 to NA6) ranged in clay from 11 to 45% and in silt from 7 to 15%. Clay and silt-sized organomineral complexes were isolated from NA2 soil by ultrasonic dispersion and sedimentation in water. The clay and silt fractions were added individually and in varying proportions to NA1 soil, providing three clay-amended (CL2, CL4 and CL6) and three silt-amended (SI2, SI4 and SI6) soils. All 12 soils were crushed in air dry state to 100 μm). Air diffusivity and permeability measurements showed disturbed soils to have a less continuous and more tortuous pore system than undisturbed reference samples. Water-filled pore space at a critical level of air diffusion potential was significantly higher for undisturbed than for disturbed samples, especially in soils high in clay. Drop cone measurements showed disturbed soils to be structurally weaker than undisturbed ones. Intact and structurally disturbed soils were found to differ significantly in physical properties even after 17 months of soil structure regeneration. Water-filled pore space seems to reflect the potential of available water and aeration status to regulate aerobic microbial activity of structurally disturbed soil, but not of intact field soil.

Turnover of organic matter in differently textured soils. II. Microbial activity as influenced by soil water regimes

To evaluate the effect of soil texture and soil water content on decomposition of organic carbon (OC), turnover of partially stabilized 14C-labelled ryegrass residues was studied at four matric potentials in twelve differently textured soils of similar origin and cropping history. Six soils were from a naturally occurring clay gradient and had 11, 16, 21, 31, 37 and 45% clay (termed NA1 to NA6). Three clay-amended soils (CL2, CL4, CL6) and three silt-amended soils (SI2, SI4, SI6) were prepared by adding clay or silt sized organomineral complexes extracted from the NA2 soil to a portion of the NA1 soil. After 14C-labelled ryegrass had decomposed for eight months under field-like conditions, soil cores were sampled, adjusted to four matric potentials (-30, -100, -500 and -1500 hPa) and incubated at 20°C for 15 weeks. The content of native soil organic carbon (SOC) in the NA soils was not related to texture. The SOC content increased with clay and silt in the CL and SI soils because of OC contained in the applied size separates. The relative CO2-evolution from CL and SI soils was lower than from the texturally corresponding NA soils, indicating a slower turnover of C supplied with the clay separate than of bulk OC. Differences in the decomposability of native SOC and residues of 14C-ryegrass were better explained by soil moisture parameters than by soil textural composition. Within each set of soils, evolution of CO2 from native SOC was highly correlated with the volumetric water content. The same was true for 14CO2-evolution, but correlations were significantly improved when 14CO2 was related to water retained in soil pores with diameters >0.2 m. This indicated that the water available for the turnover of residues from ryegrass and of native SOC was retained in different fractions of the pore volume. Our study suggested that water was the main factor in controlling turnover of SOC. Texture effects were indirect and expressed through soil structure which in turn defined the soil pore system and thus the ability of the soils to retain water of different availability to the decomposer organisms.

C and N mineralization of composted and anaerobically stored ruminant manure in differently textured soils.

Three animal manures cross-labelled with 15 N in either the urine, faeces or straw fractions were prepared. After a storage period of 86 days when the manures were exposed to either composting or to anaerobic storage, portions of the manures were incubated in six differently textured soils with clay contents ranging from 11 to 45%. Evolved CO 2 -C was determined during a 266 day incubation and inorganic N and 15 N in soil were measured at the termination of the incubation. The mineralization of C was analysed using first-order kinetics, and two C pools with fast (P 1 ) and slow (P 2 ) turnover rates were estimated. The total conversion of added C (P s ) was estimated as P s =P 1 +P 2 . The cumulated CO 2 production was considerably higher from soils incubated with anaerobically stored manure compared with soils amended with composted manure. CO 2 production levelled off after c . 60 days in the three sandier soils whereas CO 2 continued to be produced throughout the incubation from the three soils with the highest clay content. More C was assigned to the easily decomposable P 1 pool in the sandiest soils whereas the more recalcitrant P 2 pool was larger in the soils with higher clay content. Because of the different relationships between soil texture and C pools, P s ended up being similar for five of the six soils. When taking C losses during the preceding storage into account, the accumulated C losses during storage and after incubation in soil accounted for 60 and 54% of C initially present in the composted and anaerobically stored manure, respectively. Net N mineralization which averaged 16% of applied organic N took place in all soils amended with composted manure. Soils with anaerobically stored manure showed net immobilization after the 266 days of incubation. The amount of N immobilized accounted for up to 30% of the inorganic N applied with the manure. As anaerobically stored manure generally loses less inorganic N during storage, it may contain more inorganic N than composted manure at the time of field application. Because of the immobilization that takes place after application of anaerobically stored manure to soil, the immediate levels of plant available N in soil may not be as different from soil supplied with composted manure as could be expected from the inorganic N content in the two types of manure. However, when considering the manure as a N resource, anaerobic storage is superior to composting.

C and N turnover in structurally intact soils of different texture

The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at –10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg-1 were horizontially sliced and 15N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20°C and the evolution of CO2-C determined continuously for 177 d. Inorganic and microbial biomass N and 15N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO3-N present in soil before faeces was applied, net nitrification became negative indicating that NO3-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and 15N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces 15N was recovered in the microbial biomass in the amended soils. CO2-C evolution increased with clay content in amended and unamended soils. CO2-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the water content and in the long term result in similar total C content irrespective of texture.

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.

Net mineralization of soil N and 15N-ryegrass residues in differently textured soils of similar mineralogical composition

The net mineralization of soil nitrogen (N) and of 15N-labelled ryegrass residues was studied in twelve differently textured soils of similar mineralogical composition and cropping history. Six soils with 11, 16, 21, 31, 37 and 45% clay (termed NA1 to NA6) were from an arable field with a naturally occurring texture gradient. Three clay amended soils (CL2, CL4, CL6) and three silt amended soils (SI2, SI4, SI6) were prepared by spiking portions of the NA1 soil with clay and silt sized organomineral complexes isolated from the NA2 soil. 15N-labelled ryegrass and soil (<8 mm) were mixed and adjusted to −10 kPa before being incubated at 20°C. Sufficient replicates were prepared to allow for ten destructive samplings during the 31 weeks of incubation. Following incubation, samples were analysed for 15N-labelled and unlabelled 1 M KCl extractable mineral N (NH4++NO3−), total N and microbial biomass N using chloroform fumigation–extraction methodology. When described by first order kinetics, the pool of potentially mineralizable native soil N (N0) was similar in the three SI soils. N0 was not significantly affected by clay content in the NA soils. N0 decreased with increasing clay content in the CL soils and the proportion of N held in the N0 pool was smaller than in the NA soils. At the end of the incubation, 28–36% of the 15N applied with the ryegrass had become mineralized. The influence of clay differed for the three sets of soils. The NA soils showed a decreasing and the CL soils an increasing 15N-mineralization with increasing clay content. However, the effect of clay in the NA and CL soils was relatively small. 15N-turnover in the SI soils showed no response to soil texture. Between 12 and 16% of the 15N initially added with ryegrass was rapidly incorporated into the microbial biomass in the NA and CL soils. Less 15N (8–10%) was found in the biomass of the SI soils. 15N in the microbial biomass declined during incubation. At the end of the incubation, about 5% of the added 15N resided in microbial biomass regardless of soil characteristics. The differently textured NA soils were comparable in mineralogical composition, cropping history and moisture status during incubation. Under such conditions the textural composition of the soil appears to play a minor role for N turnover, suggesting that the capacity of soils to stabilize organically bound N in clay organomineral complexes is less important to short-term N dynamics.

Recovery of nitrogen from composted and anaerobically stored manure labelled with 15N

Abstract Three solid ruminant manures labelled with 15 N in either the faeces, urine or straw component were prepared. Half of each manure was composted, the other half stored anaerobically by preventing any oxygen supply. The paper describes how the previous storage conditions affect the N availability of the manure components when incorporated in August before the sowing of winter wheat ( Triticum aestivum L.). Wheat supplied with composted manure received a total of 22.6 g N m −2 , and wheat with anaerobically stored manure received 16.9 g N m −2 . Unmanured wheat plots were also included. Dry matter yield, total N and 15 N offtake were determined in the wheat crop in December and March and again at wheat maturity in August. The residual value of the animal manures was measured in spring barley ( Hordeum vulgare L.) grown in the second year after manure application. In December, wheat dry matter was similar on manured and unmanured plots. In March and at maturity, wheat dry matter and N offtake were higher on manured plots but were not influenced by manure type. At maturity, the wheat crop had recovered similar amounts of applied faeces 15 N (7.2 and 7.6%) from composted and anaerobically stored manure. From the anaerobically stored manure 10.1% urine 15 N was taken up compared with 8.5% from composted manure. Recovery of straw 15 N was 5.6% in the anaerobically stored manure and 8.8% in the composted manure. The recovery of total manure 15 N in the mature winter wheat was 8.1% for composted manure and 9.6% for anaerobically stored manure. Total N recovery in soils and crop after the first year was 65% for composted manure and 76% for anaerobically stored manure. No residual effect of the manures could be detected in the yield of spring barley grown in the second growth season and only 2.6% of the labelled manure N was taken up by the barley crop. I conclude that solid manure applied before the sowing of winter wheat has a relatively low utilization. The storage conditions did not seem to have a large influence on the availability of N remaining after storage. However, when adjusting for N losses during storage, differences between recoveries increased. Thus, the adjusted first year 15 N recovery in winter wheat was equivalent to 8% of N initially present before the anaerobic storage and to only 4% of initial N before composting.

Recovery of nitrogen by spring barley following incorporation of 15N-labelled straw and catch crop material

Abstract The recovery by spring barley ( Hordeum vulgare L.) of nitrogen mineralized from 15 N-labelled straw and ryegrass material was followed for 3 years in the field. The effects of separate and combined applications of straw and ryegrass were studied using cross-labelling with 15 N. Reference plots receiving 15 NH 4 15 NO 3 were included. Plant samples were taken every second week until maturity during the first growing season and at maturity in the two following years. Incorporation of plant material had no significant influence on the above-ground dry matter yield of the barley. The barley recovery of N derived from straw was not significantly different whether straw was incorporated alone or in combination with ryegrass material. The mean recovery of straw N was 4.5% in the first barley crop and 2.7% and 1.1% in the second and third crop. During the first growing season, recovery of ryegrass N in the barley was higher when the catch crop material was incorporated without straw, but the differences were only significant at one sampling date. At maturity 7.8% and 10.2% of the ryegrass N was recovered in the barley crop, when ryegrass was incorporated with or without straw, respectively. Mean recoveries of ryegrass N were 2.3% in the second year and less than 1% in the third year after incorporation. recovery of mineral fertilizer in the year of application was relatively low (29–40%), probably due to long periods of spring drought in all 3 years. The recovery of N from residual mineral fertilizer was in the second and third barley crop similar to the recovery of N from incorporated plant residues.

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.

What are the effects of agricultural management on soil organic carbon (SOC) stocks

BackgroundChanges in soil organic carbon (SOC) stocks significantly influence the atmospheric C concentration. Agricultural management practices that increase SOC stocks thus may have profound effects on climate mitigation. Additional benefits include higher soil fertility since increased SOC stocks improve the physical and biological properties of the soil. Intensification of agriculture and land-use change from grasslands to croplands are generally known to deplete SOC stocks. The depletion is exacerbated through agricultural practices with low return of organic material and various mechanisms, such as oxidation/mineralization, leaching and erosion. However, a systematic review comparing the efficacy of different agricultural management practices to increase SOC stocks has not yet been produced. Since there are diverging views on this matter, a systematic review would be timely for framing policies not only nationally in Sweden, but also internationally, for promoting long-term sustainable management of soils and mitigating climate change.MethodsThe systematic review will examine how changes in SOC are affected by a range of soil-management practices relating to tillage management, addition of crop residues, manure or other organic “wastes”, and different crop rotation schemes. Within the warm temperate and the snow climate zones, agricultural management systems in which wheat, barley, rye, oats, silage maize or oilseed rape can grow in the crop rotation will be selected. The review will exclusively focus on studies conducted over at least 10 years. Searches will be made in 15 publication databases as well as in specialist databases. Articles found will be screened using inclusion/exclusion criteria at title, abstract and full-text levels, and screening consistency will be evaluated using Kappa tests. Data from articles that remain after critical appraisal will be extracted using a predefined spreadsheet. Subgroup analyses will be undertaken to elucidate statistical relationships that are specific to particular type of management interventions. Meta-regression within subgroups will be performed as well as sensitivity analysis to investigate the impact of removing groups of studies with low or unclear quality.