Plant Residue Inputs Research Articles

Application of RothC model to predict soil organic carbon stock on agricultural soils of Slovakia

Soil organic matter (SOM) takes part in many environmental functions and, depending on the conditions, it can be a source or a sink of the greenhouse gases. Presently, the changes in soil organic carbon (SOC) stock can arise because of the climatic changes or changes in the land use and land management. A promising method in the estimation of SOC changes is modelling, one of the most used models for the prediction of changes in soil organic carbon stock on agricultural land being the RothC model. Because of its simplicity and availability of the input data, RothC was used for testing the efficiency to predict the development of SOC stock during 35-year period on agricultural land of Slovakia. The received data show an increase of SOC stock during the first (20 years) phase and no significant changes in the course of the second part of modelling. The increase of SOC stock in the first phase can be explained by a high carbon input of plant residues and manure and a lower temperature in comparison with the second modelling part.

Effects of nitrogen mineralization on paddy rice yield under low nitrogen input conditions in irrigated rice-based multiple cropping with intensive cropping of vegetables in southwest China

The farm household responsibility system (FHRS) was adopted in Chinese rural areas during the economic reform in the early 1980s. Since then, many farm households have increased cropping intensity by using large quantities of nitrogen (N) fertilizers in their responsible fields to increase agricultural income. However, intensive cropping systems with low N input are still common in remote places of the southwestern region of China. Maintenance and improvement of soil quality in intensive cropping systems is critical for sustaining agricultural productivity and environmental quality for future generations. The effects of intensive cropping of vegetables on paddy rice (Oryza sativa L.) yield using small quantities of N fertilizers through N mineralization of paddy soil in irrigated rice-based multiple cropping systems were studied in 15 paddy fields in Sichuan Province, China for 3 years. Intensification of vegetable cropping with negative N balance and removal of vegetable crop residues has greatly decreased total N (TN) contents in paddy soil leading to low levels of effective cumulated soil temperature and thickness of plow layer. As a result, the N mineralization in paddy field during paddy rice growing period was decreased. In addition to the low levels of chemical fertilizer N input and residual mineral N input, the lower level of N mineralization in paddy fields and low N recovery efficiency decreased the amount of N accumulated in aboveground biomass of paddy rice at maturity, resulting in limited rice yields. The amount of mineralized N only correlated with TN (partial correlation analysis). Therefore, in paddy fields with very low N input, the N mineralization in paddy soil during the paddy rice-growing period was the major limiting factor affecting the total yield increases. In addition, a decline in soil fertility can be determined using TN as an indicator. To improve paddy rice yield and avoid soil deterioration, the development and adoption of rational soil management programs are needed. These include input of plant residues, conscientious soil tillage for the maintenance of soil temperature and thickness of the plow layer, and the split application of fertilizer for the improvement of N recovery efficiency.

Modelling soil organic carbon turnover in improved fallows in eastern Zambia using the RothC-26.3 model

Scarcity of simple and reliable methods of estimating soil organic carbon (SOC) turnover and lack of data from long-term experiments make it difficult to estimate attainable soil C sequestration in tropical improved fallows. Testing and validating existing and widely used SOC models would help to determine attainable C storage in fallows. The Rothamsted C (RothC) model, therefore, was tested using empirical data from improved fallows at Msekera in eastern Zambia. This study (i) determined the effects of nitrogen fixing tree (NFT) species on aboveground organic C inputs to the soil and SOC stocks, (ii) estimated annual net organic C inputs to the soil using the RothC, and (iii) tested the performance of RothC model using empirical data from improved fallows. Soil samples (0–20 cm) were collected from coppicing and non-coppicing fallow experiments in October 2002 for determination of SOC by LECO CHN-1000 analyser. Data on surface litter, maize and weed biomasses, and on weather, were supplied by the Zambia/ICRAF Agroforestry Project. Measured SOC stocks to 20 cm depth ranged from 32.2 to 37.8 t ha −1 in coppicing fallows and 29.5 to 30.1 t ha −1 in non-coppicing fallows compared to 22.2–26.2 t ha −1 in maize monoculture systems. Coppicing fallows accumulated more SOC (680–1150 g m −2 year −1) than non-coppicing fallows (410–789 g m −2 year −1). While treatments with NFTs accumulated more SOC than NFT-free systems, SOC stocks increased with increasing tree biomass production and tree rotation. For food security and C sequestration, coppicing fallows are a potentially viable option. The RothC-26.3 model calculates the effect of annual above- and below-ground plant residue inputs to the soil on total organic C, microbial biomass, and radiocarbon age of the soil over a period ranging from a few years to centuries. As plant residue inputs from roots during plant growth are rarely known, the model is most often run in ‘inverse’ mode to generate total annual plant residue inputs from known soil, site, and weather data. The model, run in reverse, estimated the annual net organic C inputs required to maintain SOC stocks. Estimates ranged from 2.8 to 6.1 t ha −1 in coppicing fallows, 2.2–5.7 t ha −1 in non-coppicing fallows, and from 1.4 to 2.7 t ha −1 in controls. Modelled inputs comprising above- and below-ground organic residues in fallows were 12–104% greater than measured above-ground inputs alone. The model provided a good fit to empirical SOC data in fertilized maize monoculture, and in coppicing and non-coppicing fallows. Modelled inputs for Leucaena, Gliricidia, Senna, Sesbania, and Cajanus closely matched plant C input values estimated in separate studies, suggesting that RothC is giving reasonable simulations of soil C changes under improved fallow conditions in Zambia. However, the DPM/RPM ratio for plant C inputs in fallows was increased from 0.25 to 1.10 to suit their biodegradability characteristics. The RothC model can be used to calculate annual organic C inputs and SOC stocks in improved fallows provided suitable DPM:RPM ratios are used.

Management factors affecting size and structure of soil Fusarium communities under irrigated maize in Australia

Fungi in the genus Fusarium are ubiquitous in agricultural soils and have a wide range of ecological roles. We conducted a series of field trials, glasshouse trials, and soil microcosm experiments to assess the effects of stubble and N management, crop type, incorporation of residue originating from different plants and seed-applied fungicides on soil Fusarium. A community level approach was used based on PCR-DGGE and a semi-quantitative nested qPCR approach. Soil Fusarium communities were found to be primarily affected by plant residue inputs. In a field trial, incorporation of maize residue into soil significantly increased the relative abundance of Fusarium Ef1α gene copies ( P < 0.001) and increased the diversity of Fusarium genotypes present in the field when compared to stubble burned treatments. In particular, the important mycotoxin-producing Fusarium verticillioides was present where stubble had been retained and absent where stubble was burnt. Fusarium oxysporum was common in soil regardless of stubble management. In the field trial, the application of mineral N fertiliser (329 kg N ha −1 year −1) had no effect on abundance or diversity of soil Fusarium ( P > 0.05). Glasshouse trials tested the effects of fungicides and crop rotation on the diversity and abundance of soil Fusarium spp. Using soil repeatedly cropped with maize (5 years), the subsequent growth of neither canola, wheat nor maize significantly altered the relative abundance of Fusarium in soil ( P < 0.05), but growth of adzuki bean resulted in a significant decrease ( P < 0.05). Furthermore, the growth of different plants had no effect on the species of Fusarium present in the soil. However, in a microcosm experiment, the return of residues of these plants to soil (at equivalent dry weights) resulted in increases to the size and change to the composition of the Fusarium community ( P = 0.001). Treatment of maize seed with the fungicides metalaxyl-M, tolclofos-methyl, or fludioxonil increased plant growth and the relative abundance of Fusarium Ef1α genes in DNA extracted from the bulk soil. This may have occurred by suppressing soil microorganisms competing with Fusarium for plant residues. Fungicide addition also effected a short-term change to the Fusarium community structure. This work clearly demonstrates the effects of crop residue quantity and quality (origin) on soil Fusarium communities, indicating its importance for integrated management of Fusarium in production systems.

Storage and stability of organic matter and fossil carbon in a Luvisol and Phaeozem with continuous maize cropping: A synthesis

AbstractQuantitative information about the amount and stability of organic carbon (OC) in different soil organic‐matter (OM) fractions and in specific organic compounds and compound‐classes is needed to improve our understanding of organic‐matter sequestration in soils. In the present paper, we summarize and integrate results performed on two different arable soils with continuous maize cropping (a) Stagnic Luvisol with maize cropping for 24 y, b) Luvic Phaeozem with maize cropping for 39 y) to identify (1) the storage of OC in different soil organic‐matter fractions, (2) the function of these fractions with respect to soil‐OC stabilization, (3) the importance and partitioning of fossil‐C deposits, and (4) the rates of soil‐OC stabilization as assessed by compound‐specific isotope analyses. The fractionation procedures included particle‐size fractionation, density fractionation, aggregate fractionation, acid hydrolysis, different oxidation procedures, isolation of extractable lipids and phospholipid fatty acids, pyrolysis, and the determination of black C. Stability of OC was determined by 13C and 14C analyses. The main inputs of OC were plant litter (both sites) and deposition of fossil C likely from coal combustion and lignite dust (only Phaeozem).

Soil organic matter mineralization and residue decomposition of spring wheat grown under elevated CO 2 atmosphere

The influence of elevated atmospheric CO 2 concentrations ([CO 2]) on the decomposition of spring wheat ( Triticum aestivum L. cv. Triso) residues remaining in the soil after harvest was simulated in a microcosm incubation experiment in the lab. Undisturbed soil cores with and without visible wheat residues were taken in the third year after establishment from a Mini-free-air carbon dioxide enrichment (FACE) system, in which we used 13C-depleted CO 2 to determine the contribution of plant-derived carbon to overall carbon mineralization. The Mini-FACE system is located on a Gleyic Cambisol near Hohenheim (Baden-Württemberg, Germany). Carbon dioxide production and leaching of nitrogen and inorganic and organic carbon were measured during 191 days of incubation. Rates of CO 2 production were generally highest in all treatments during the first two weeks of the incubation and this was followed by a steady decrease until day 58. After this day mineralization rates declined only weakly until the end of the incubation. Cumulative carbon mineralization was similar in the two treatments without visible wheat residues, but significantly lower in the elevated (−19.0%) versus ambient [CO 2] treatment with visible plant residues (significant [CO 2] × residue interaction; F 1,13 = 7.17; P = 0.019). This result demonstrated reduced decomposition of wheat residues grown under elevated [CO 2]. The contribution of plant-derived carbon to soil respiration was highest in the beginning, followed by a steady decrease until the end of the incubation. Irrespective of incubation time, the amount of mineralized, plant-derived carbon was higher in the treatment with visible wheat residues. Leaching of inorganic carbon (DIC) tended to be affected by [CO 2] ( F 6,8 = 4.50; P = 0.057), with more DIC leached in the elevated [CO 2] treatment without (+47.2%) and with visible plant residues (+29.5%) than in the respective ambient CO 2 treatments. The amount of carbon potentially sequestered as DIC in the wheat cropping system was small compared to the effects of elevated [CO 2] on the amounts and decomposition of plant residues. Increased input of plant residues and reduced decomposition of plant-derived carbon are discussed as possible mechanisms for enhanced carbon sequestration under elevated atmospheric CO 2 concentration.

Chemical and Biological Characteristics of Physically Uncomplexed Organic Matter

Physical fractionation methods are based on the premise that soil organic matter (SOM) can be divided into pools of functional relevance. Physically uncomplexed organic matter (OM) is isolated on the basis of particle size and/or density. Our objective here is to review research on the biological and chemical characteristics of physically uncomplexed OM that demonstrates its value (or otherwise) as a meaningful pool of SOM. Chemical characterization indicates that fractions isolated by size are not identical to those separated by density; even materials separated using variations of a particular fractionation method (i.e., different sizes or different densities) have different chemical and biological properties. Physically uncomplexed OM often contains a substantial portion of whole soil carbon (C) and nitrogen (N) and, compared with the whole soil or heavy fraction, has a wide C/N ratio and high O‐alkyl (i.e., carbohydrates) and low carbonyl (i.e., proteins) C contents. The response of physically uncomplexed OM to changes in land use and management practices is greater than that of other labile OM fractions or the whole soil C and N. Studies to elucidate the nutrient availability of physically uncomplexed OM suggest that it is not an immediate source of nutrients. That the quantity of physically uncomplexed OM is not always related to the amount of plant residue inputs suggests that other factors may control its accumulation in soil. Thus the quantity and the biological and chemical properties of physically uncomplexed OM are affected by the amount, composition, and accessibility of plant residues entering the soil; environmental conditions that may enhance or constrain decomposition; and the fractionation technique used.

How are soil use and management reflected by soil organic matter characteristics: a spectroscopic approach

SummaryWe studied the quantitative and qualitative changes of soil organic matter (SOM) due to different land uses (arable versus grassland) and treatments (organic manure and mineral fertilizer) within an agricultural crop rotation in a long‐term field experiment, conducted since 1956 at Ultuna, Sweden, on a Eutric Cambisol. The organic carbon (OC) content of the grassland plot was 1.8 times greater than that of the similarly fertilized Ca(NO3)2 treated cropped plots. The comparison of two dispersion techniques (a low‐energy sonication and a chemical dispersion which yield inherent soil aggregates) showed that increasing OC contents of the silt‐sized fractions were not matched by a linear increase of silt‐sized aggregates. This indicated saturation of the aggregates with OC and a limited capacity of particles to protect OC physically. Thermogravimetric analyses suggested an increase of free organic matter with increasing OC contents. Transmission FT‐IR spectroscopy showed relative enrichment of carboxylic, aromatic, CH and NH groups in plots with increasing OC contents. The silt‐sized fractions contained the largest SOM pool and, as revealed by 13C NMR spectroscopy, were qualitatively more influenced by the plant residue versus manure input than the clay fractions. Alkyl and O‐alkyl C in the silt‐sized fractions amounted to 57.4% of organic carbon in the animal manure treated plots and 50–53% in the other treatments.

Effects of plant residues on crop performance, N mineralisation and microbial activity including field CO2and N2O fluxes in unfertilised crop rotations

The impacts of crop rotation and input of organic matter in the form of green manure crops, straw residues and incorporation of catch crops on crop yield, nitrogen uptake, microbial biomass and activity were studied in unfertilised crop rotations differing in input of plant residues, i.e., high-input rotations with a grass-clover crop and catch crops included and low-input cereal rotations without catch crops. The parameters studied included substrate induced respiration (SIR), hydrolysis of fluorescein diacetate (FDA), arylsulfatase activity (ASA), N mineralisation, N2O emission, and soil respiration. These parameters were measured in bare soil plots, to estimate the effects of previous years' crops and input of plant residues. In neighbouring plots crop performances were registered by measuring yields, above-ground biomass and nitrogen uptake during the growing season. Generally, all measured parameters were significantly higher in the high-input than in low-input rotations. Estimates of metabolic quotients indicated that the microbial communities in the low-input rotations were less efficient in utilising the C sources than those in the high-input rotations. Calculations of N2O emission factors indicate that the current IPCC methodology for estimating N2O emission from plant residues needs to be improved.

Utility of 13C for ecosystem carbon turnover estimation in grazed mixed grass prairie

Differences in 13C inputs between two adjacent Canadian mixed grass prairie communities were used to estimate turnover of two soil organic matter fractions with increasing depth below the surface. Partial cultivation and abandonment of a C 4-dominated grassland ca. 1930, and natural recovery to a native C 3-dominated grassland changed the 13C/ 12C ratio (reported in units of δ 13C‰) of plant residue inputs to soil organic matter. After 70 years, macro-organic matter carbon (MOMC) differed up to 5.6‰, and fine soil organic carbon (FSOC) by 2.9‰ near the surface; differences declined to a depth of 1 m. Mean residence time estimates differed among fractions from a short 35 years for MOMC 0–5 cm, to a longer 413 years for FSOC 5–10 cm. The background carbon pool on this site was substantial, and the minimum change in soil carbon of 10.29 Mg C ha −1 over 70 years represented less than 7% of the carbon pool to a 1-m depth. Considerable vertical and horizontal variation in soil organic carbon mass and 13C content were observed. Variation due to small sample size was a more substantial limitation than cumulative analytical variation for calculating turnover estimates >500 years or at depths >20 cm. Corrections for bias were also necessary since imperfect physical separation of soil organic matter fractions, or CO 3 2− contamination, were issues.

Modeling carbon dynamics in vegetation and soil under the impact of soil erosion and deposition

Soil erosion and deposition may play important roles in balancing the global atmospheric carbon budget through their impacts on the net exchange of carbon between terrestrial ecosystems and the atmosphere. Few models and studies have been designed to assess these impacts. In this study, we developed a general ecosystem model, Erosion‐Deposition‐Carbon‐Model (EDCM), to dynamically simulate the influences of rainfall‐induced soil erosion and deposition on soil organic carbon (SOC) dynamics in soil profiles. EDCM was applied to several landscape positions in the Nelson Farm watershed in Mississippi, including ridge top (without erosion or deposition), eroding hillslopes, and depositional sites that had been converted from native forests to croplands in 1870. Erosion reduced the SOC storage at the eroding sites and deposition increased the SOC storage at the depositional areas compared with the site without erosion or deposition. Results indicated that soils were consistently carbon sources to the atmosphere at all landscape positions from 1870 to 1950, with lowest source strength at the eroding sites (13 to 24 gC m−2 yr−1), intermediate at the ridge top (34 gC m−2 yr−1), and highest at the depositional sites (42 to 49 gC m−2 yr−1). During this period, erosion reduced carbon emissions via dynamically replacing surface soil with subsurface soil that had lower SOC contents (quantity change) and higher passive SOC fractions (quality change). Soils at all landscape positions became carbon sinks from 1950 to 1997 due to changes in management practices (e.g., intensification of fertilization and crop genetic improvement). The sink strengths were highest at the eroding sites (42 to 44 gC m−2 yr−1), intermediate at the ridge top (35 gC m−2 yr−1), and lowest at the depositional sites (26 to 29 gC m−2 yr−1). During this period, erosion enhanced carbon uptake at the eroding sites by continuously taking away a fraction of SOC that can be replenished with enhanced plant residue input. Overall, soil erosion and deposition reduced CO2 emissions from the soil into the atmosphere by exposing low carbon‐bearing soil at eroding sites and by burying SOC at depositional sites. The results suggest that failing to account for the impact of soil erosion and deposition may potentially contribute to an overestimation of both the total historical carbon released from soils owing to land use change and the contemporary carbon sequestration rates at the eroding sites.

Soil organic matter transformations induced by Hieracium pilosella L. in tussock grassland of New Zealand

To study the effect of Hieracium pilosella L. invasion on the transformations of soil organic matter of New Zealand tussock grassland soils (Ustochrepts), plant material and soils underneath Hieracium, the surrounding halo, and the adjacent herbfield (depleted tussock grassland) were examined for their chemical composition. An attempt was made to reveal possible changes in chemical composition of the soil organic matter induced by H. pilosella invasion. Small differences were detected by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy in the composition of the plant and soil materials from these zones. Most of the differences in soil organic matter occurred due to differences in the amount and quality of plant-residue inputs. Comparable amounts of phenolic C were detected in the solid-state 13C NMR spectra of H. pilosella and herbfield vegetation, while alkaline CuO oxidation yielded considerable lower lignin oxidation products for H. pilosella. A slightly higher proportion of these compounds in H. pilosella soil revealed an accumulation and a low degradation rate of lignin compounds under H. pilosella. The HCl hydrolysis and solid-state 15N NMR spectroscopy showed similar chemical compositions of the N fractions of the three different soils. The absence of 15N NMR signal intensity assignable to aniline derivatives or aromatic heterocyclic N indicates that the condensation of phenolic compounds with N groups plays a minor role in N sequestration in these soils.

Soil-released carbon dioxide from microbial biomass carbon in the cultivated soils of karst areas of southwest China

Soil microbial biomass and the emission of CO2 from the soil surface were measured in yellow soils (Ultisols) of the karst areas of southwest China. The soils are relatively weathered, leached and impoverished, and have a low input of plant residues. The measurements were made for a 1-year period and show a reciprocal relationship between microbial biomass and surface CO2 efflux. The highest (42.6±2.8 mg CO2-C m–2 h–1) and lowest (15.6±0.6 mg CO2-C m–2 h–1) CO2 effluxes are found in the summer and winter, respectively. The cumulative CO2 efflux is 0.24 kg CO2-C m–2 year–1. There is also a marked seasonal variation in the amount of soil microbial biomass carbon, but with the highest (644±71 μg C g–1 soil) and lowest (270±24 μg C g–1 soil) values occurring in the winter and summer, respectively. The cumulative loss of soil microbial biomass carbon in the top 10 cm of the soil was 608 μg C g–1 year–1 soil over 17 sampling times. The mean residence time of microbial biomass is estimated at 105 days, suggesting that the carbon in soil microbial biomass may act as a source of the CO2 released from soils.

Seasonal changes of microbial biomass carbon related to climatic factors in soils from karst areas of southwest China

The seasonal responses of soil microbial biomass C to changes in atmospheric temperature, soil moisture and soluble organic C were studied in soils from the karst areas of southwest China. These soils are relatively weathered, leached and impoverished, and have a low input of plant residues. Over 1 year, an inverse relationship between soil microbial biomass C and atmospheric temperature was found. The highest microbial biomass C occurred in winter and the lowest in summer, and ranged from 231–723 μg g–1 dry soil. Although there was no obvious relationship between microbial biomass C and soil moisture, a negative correlation existed between microbial biomass C and soluble organic C. In the ecosystem studied, the marked changes in soil microbial biomass C at above 20 °C were ascribed to fluctuations of soil moisture, which were controlled by climatic factors and geomorphic conditions. The patterns of soluble organic C turnover were similar to those of soluble carbohydrate C, both of which were controlled by soil drying-rewetting cycles. It was concluded that the lowest amounts of soil microbial biomass C, measured in the summer, resulted in increases in soluble organic C due to higher turnover rates of the former at warmer air temperatures. Thus, there was a marked seasonal change in soil microbial biomass C.

Composition and Decomposition of Soybean and Sorghum Tissues Grown under Elevated Atmospheric Carbon Dioxide

AbstractIt has been hypothesized that changes in both quantity and quality of plant residue inputs to soils as atmospheric carbon dioxide (CO2) concentration increases may alter carbon (C) and nitrogen (N) turnover rates and pool sizes. We determined the effect of elevated atmospheric CO2 on plant tissue quality, and how modifications in tissue quality affect C and N mineralization. Soybean [C3; Glycine max (L.) Merr. cv. Stonewall] and sorghum [C4; Sorghum bicolor (L.) Moen. cv. Savanna 5] were grown under elevated (704.96 ± 0.33 µmol CO2 mol−1) and ambient (357.44 ± 0.12 µmol CO2 mol−1) atmospheric CO2 in open‐top chambers. Leaf and stem tissues were separated from harvested plants and analyzed for C, N, lignin, and cellulose. Tissues were applied to Norfolk loamy sand (fine‐loamy, siliceous, thermic Typic Kandiudult) and aerobically incubated for 70‐d to determine C and N mineralization, C turnover, relative N mineralization, and C/N mineralized. Elevated CO2 had no effect on plant residue C concentration, but N concentration of soybean leaves and stems and sorghum stems was reduced; however, CO2 enrichment increased C/N ratio and lignin concentration for only sorghum stems and soybean leaves, respectively. Source of plant residue (i.e., produced under either elevated or ambient CO2) had no impact on soil C turnover, relative N mineralization, cumulative C and N mineralization, and C/N mineralized. These data suggest that increasing atmospheric CO2 will have little effect on composition or decomposition of field crop residues. Thus, since CO2 enrichment results in increased photosynthetic C fixation, the possibility exists for increased soil C storage under field crops in an elevated CO2 world.

Dynamics of Soil Organic Matter and Corn Residues Affected by Tillage Practices

AbstractThis study was conducted to determine the influence of tillage practices on the decomposition of corn (Zea mays L.) residues and turnover of soil organic matter (SOM). Measurements of δ13C were made of the whole soil, the microbial biomass, and two particle‐size fractions (&lt;50 and &gt;50 µm) in soils that had been under corn production for 11 yr and from an adjacent meadow. Meadow‐derived C in total SOM (0–24 cm) decreased under corn cropping at the same rate under all tillage treatments. Corn‐derived C was evenly distributed with depth in the moldboard plow treatment and accumulated at the surface in the shallow, reduced‐tillage treatments. The incorporation of corn residue C into SOM in the 0‐ to 24‐cm layer was not significantly affected by tillage and was estimated to be ≈41 g C m−2 yr−1, which represents ≈30% of the corn residue inputs. Both the macroorganic matter (&gt;50 µm) and microbial biomass had a greater enrichment (up to 35%) in corn‐derived C than either the whole soil or the microorganic matter (&lt;50 µm), indicating that these pools are important recipients of plant residue inputs. Nevertheless, the microorganic matter was also a significant sink for C input, acounting for ≈50% of the total corn‐derived C remaining in the surface (0–8 cm) soil. Under the conditions of this study, tillage practices influenced the vertical distribution of SOM and corn residues but had no detectable effect on SOM turnover and on the fate of corn residues when the whole Ap horizon was considered.

Application of the Rothamsted carbon turnover model to soils in degraded semi-arid land in New Zealand

The Rothamsted soil-carbon turnover model was used to determine rates of change of organic carbon in soils of a degraded semi-arid land system of New Zealand. Estimates of annual inputs, net primary production (NPP), and recovery time to raise carbon from a degraded state to a sustainable level of production were obtained. The annual inputs and hence the NPP were estimated by running the Rothamsted soil-carbon turnover model in reverse. Inputs included the organic C content of the soil, clay content, monthly temperature and rainfall and the rate of decay of annual inputs. An estimate of the biologically inert organic matter fraction was obtained from the radiocarbon content of soil organic matter. The soil recovery time was estimated by determining NPP for a relatively undegraded soil and and applying carbon at this rate to a degraded soil. The predicted total annual plant residue inputs were 1.81 t ha −1 y −1 and the predicted inert organic matter content from radiocarbon data is 5.12 t ha −1. The estimated time needed to raise carbon from a degraded state to the level of an undegraded site is 48 years. This is most likely an underestimate, because in initial years the vegetation will not be capable of adding the same annual carbon input as the undegraded site. The refinement of this estimate of recovery time will require data on the rate of increase in net primary productivity with revegetation.

Free-air CO2 enrichment effects on soil carbon and nitrogen

Since the onset of the industrial revolution, atmospheric CO2 concentration has increased exponentially to the current 370 μmol mol−1 level, and continued increases are expected. Previous research has demonstrated that elevated atmospheric CO2 results in larger plants returning greater amounts of C to the soil. However, the effects of elevated CO2 on C and N cycling and long-term storage of C in soil have not been examined. Soil samples (in 0–50, 50–100, and 100–200 mm depth increments) were collected after 3 years of cotton (Gossypium hirsutum L.) production under free-air CO2 enrichment (FACE, at 550 μmol CO2 mol−1), in combination with 2 years of different soil moisture regimes (wet, 100% of evapotranspiration replaced, or dry, 75% and 67% of evapotranspiration replaced in 1990 and 1991, respectively) on a Trix clay loam (fine, loamy, mixed (calcareous), hyperthermic Typic Torrifluvent) at Maricopa, Arizona. Ambient plots (370 μmol CO2 mol−1 (control)), in combination with the wet and dry soil moisture regimes, were also included in the study. Soil organic C and N concentrations, potential C and N mineralization, and C turnover were measured. Increased input of cotton plant residues under FACE resulted in treatment differences and trends toward increased organic C in all three soil depths. During the first 30 days of laboratory incubation, available N apparently limited potential C mineralization and C turnover in all treatments. Between 30 and 60 days of incubation, soils from FACE plots had greater potential C mineralization with both water regimes, but C turnover increased in soils from the dry treatment and decreased in soils where cotton was not water stressed. These data indicate that in high-CO2 environments without water stress, increased C storage in soil is likely, but it is less likely where water stress is a factor. More research is needed before the ability of soil to store additional C in a high-CO2 world can be determined.

Abundance and distribution of earthworms in relation to landscape factors on the Georgia Piedmont, U.S.A.

Abundance and distribution of earthworms were studied on the Georgia Piedmont of the Southeastern U.S.A., at sites representing various ecosystem types, management practices, landscape positions, soil textures and soil erosion status. Earthworm abundance showed distinct seasonal patterns, with winter/spring maxima and summer minima. Numbers and biomass ranged from zero in plowed. mono-cropped soil at an upland site to over 1000 m -2 ( > 25g ash-free dry wt m -2) in no-tillage, double-cropped soil on bottomlands. Numbers and biomass in plowed, double-cropped soil, in a bottomland forest, and in grass meadows at both upland and bottomland sites were intermediate. Soil texture, as influenced by water erosion, strongly affected earthworm abundance. Moderately and severely eroded sandy clay loam supported significantly higher earthworm numbers and biomass than did slightly eroded soil with higher sand content. This effect may have resulted from low organic content and water holding capacity of the sandy soils. Of the soil texture variables, silt content was most highly correlated with earthworm biomass. Earthworm abundance was also related to quantity and quality of plant residue inputs in the agroecosystems, and to standing stocks of soil organic carbon across all sites studied. At most agroecosystem and forest sites, the predominant earthworm species were European lumbricids; native Diplocardia spp were most prominent in a meadow soil with high organic content.

Response of microbial populations in Arctic tundra soils to crude oil

Experimental crude oil spillages of 5 and 12 litre/m2 were established on the four major topographically distinguished soils of Arctic coastal polygonized tundra. The response of microbial populations to contaminating oil was found to depend on soil type and depth. Increases in numbers of heterotrophs were initially restricted to the top 2 cm of the soils. Increase in oil-degrading populations were found in oil-treated soils. Increases in microbial populations in subsurface soils paralleled downward migration of the oil. Some of the observed population increases probably resulted from input of plant residues and products from oil biodegradation.