Long-term Soil Organic Matter Research Articles

Trade-offs between the short- and long-term effects of residue quality on soil C and N dynamics

The decline of soil organic matter (SOM) and its associated fertility is one of the most important constraints to enhanced crop productivity in sub-Saharan Africa. Integrated soil fertility management recognizes the potential benefits of the combined use of organic residue and mineral fertilizer inputs for improved crop yield and SOM build up. However, these benefits may be controlled by residue quality. We examined the short- to long-term C and N dynamics following application of different quality residues with and without N fertilizer in a series of experiments comprising different timescales of measurement in a Kenyan Humic Nitisol. The combined results of these studies indicate that residue quality and fertilizer additions alter short-term C and N mineralization. Combining low quality residue and fertilizer inputs immobilized a greater amount of fertilizer-N than high quality residue. Under field conditions, this reduction in available N induced by the combination of low quality residue and fertilizer reduced environmental N losses and created a positive interactive effect on crop N uptake. While input management manipulated short-term nutrient dynamics, it did not influence long-term SOM stabilization. The input of residue, regardless of quality, contributed to long-term soil fertility improvement. In conclusion, organic residue quality can be manipulated to optimize short-term nutrient dynamics while still conferring the same benefits to long-term SOM contents.

Assessing the sustainability of wheat-based cropping systems using APSIM: model parameterisation and evaluation

Assessing the sustainability of crop and soil management practices in wheat-based rotations requires a well-tested model with the demonstrated ability to sensibly predict crop productivity and changes in the soil resource. The Agricultural Production Systems Simulator (APSIM) suite of models was parameterised and subsequently used to predict biomass production, yield, crop water and nitrogen (N) use, as well as long-term soil water and organic matter dynamics in wheat/chickpea systems at Tel Hadya, north-western Syria. The model satisfactorily simulated the productivity and water and N use of wheat and chickpea crops grown under different N and/or water supply levels in the 1998–99 and 1999–2000 experimental seasons. Analysis of soil-water dynamics showed that the 2-stage soil evaporation model in APSIM’s cascading water-balance module did not sufficiently explain the actual soil drying following crop harvest under conditions where unused water remained in the soil profile. This might have been related to evaporation from soil cracks in the montmorillonitic clay soil, a process not explicitly simulated by APSIM. Soil-water dynamics in wheat–fallow and wheat–chickpea rotations (1987–98) were nevertheless well simulated when the soil water content in 0–0.45 m soil depth was set to ‘air dry’ at the end of the growing season each year. The model satisfactorily simulated the amounts of NO3-N in the soil, whereas it underestimated the amounts of NH4-N. Ammonium fixation might be part of the soil mineral-N dynamics at the study site because montmorillonite is the major clay mineral. This process is not simulated by APSIM’s nitrogen module. APSIM was capable of predicting long-term trends (1985–98) in soil organic matter in wheat–fallow and wheat–chickpea rotations at Tel Hadya as reported in literature. Overall, results showed that the model is generic and mature enough to be extended to this set of environmental conditions and can therefore be applied to assess the sustainability of wheat–chickpea rotations at Tel Hadya.

Effect of natural and agricultural factors on long-term soil organic matter dynamics in arable soddy-podzolic soils—modeling and observation

Changes in topsoil C content were measured in 60 experiments on soddy-podzolic soils in the Russian Federation, Belarus, Ukraine, Lithuania and Latvia with different management characteristics. The data were compiled with a range of actual climatic parameters for the years of the experiments. The combined impact of different management practices (inorganic fertilizers, organic manure and different rotations), climate gradients and soil properties (initial soil C, clay content) was analysed using a static model. The importance of initial C level interactions with the soil and climatic variables are demonstrated as one of the major factors of soil organic matter (SOM) dynamics. The effect of different treatments for seven long-term experiments at the same site (Barybino, the Moscow Region) was explored using both static and dynamic modeling approaches. The SUNDIAL dynamic model was used to estimate the dynamics of different C pools for the whole 35-year experimentation period. Some adjustments to parameters were necessary to obtain satisfactory results. The management practices tested lead to changes not only in total SOM but also in SOM quality parameters. This was established using a complex of chemical and biological methods. For cropping systems with 50% of row crops manure application increased steady state C values by about 0.2%. High inorganic N fertilization decreased the steady state C values. The negative effect of high inorganic N fertilization was seen to be more pronounced on soil with a higher C content. This influence might be a result of increased organic C mineralization after autumn application of mineral N where there may be no crop present to utilize the fertilizer, resulting in adverse changes in SOM quality. The mineral fertilization contributes significantly to the formation of labile humic substances at the expense of Ca-forms of SOM which are more resistant to microbial decomposition and act as the binding agents of aggregate stability.

EuroSOMNET – a European database of long-term experiments on soil organic matter: the WWW metadatabase

Since 1997, the EuroSOMNET project, funded by the EU-ENRICH programme, has assembled a metadatabase, and separate experimental databases, of European long-term experiments that investigate changes in soil organic matter. In this paper, we describe the WWW-based metadatabase, which is a product of this project. The database holds detailed records of 110 long-term soil organic matter experiments, giving a wide geographical coverage of Europe, and includes experiments from the European part of the former Soviet Union, many of which have not been available previously. For speed of access, records are stored as hyper-text mark-up language (HTML) files. In this paper, we describe the metadatabase, the experiments for which records are held, the information stored about each experiment, and summarize the main characteristics of these experiments. Details from the metadatabase have already been used to examine regional trends in soil organic matter in Germany and eastern Europe, to construct and calibrate a regional statistical model of humus balance in Russia, to examine the effects of climatic conditions on soil organic matter dynamics, to estimate the potential for carbon sequestration in agricultural soils in Europe, and to test and improve soil organic matter models. The EuroSOMNET metadatabase provides information applicable to a wide range of agricultural and environmental questions and can be accessed freely via the EuroSOMNET home page at URL: http://www.iacr.bbsrc.ac.uk/aen/eusomnet/index.htm.

Initialisation of the soil organic matter pools of the Daisy model

Like most other contemporary models of long-term soil organic matter (SOM) dynamics, Daisy partitions the refractory component of the SOM into two or more pools. In long-term simulations, the initial distribution of SOM between these two pools influences the simulations. However, as these pools do not correspond to any measurable entities the distribution of SOM cannot be initialised by a simple measurement. Daisy has usually been initialised by using a standard distribution, which almost corresponds to equilibrium. In applications of other models, the initial distribution between the different pools of organic matter has been chosen by assuming equilibrium at the beginning of the simulation, by using standard values or by calibrating the initial distribution to optimise simulation performance. However, as we show here the initial distribution of SOM between the different pools influences simulations notably, and the appropriate distribution is dependent on changes in management and climate at the site before the onset of a simulated experiment. This is done by simulating a scenario and initialising the Daisy model in to different ways: (1) by assuming equilibrium at the beginning of the simulation; and (2) by simulating the preexperimental management history of the site. This shows that it can be important to use plausible assumptions to initialise SOM models, and that often the only way to initialise the model is to simulate the preexperimental period of the site.

Temporal variations in plant δ 13C values and implications for using the 13C technique in long-term soil organic matter studies

The 13C natural labeling technique in combination with modeling could potentially be used to analyze SOM changes in long-term experiments involving complex cropping systems such as agroforestry systems. It requires information on quantity and 13C abundance of all plant materials entering the soil throughout the experiment. The occurrence of significant year-to-year changes of δ 13C values of the plant inputs may complicate the technique, as it would require a systematic isotopic analysis of all plant inputs over the entire life span of the experiment. By analyzing crop and tree samples from different sampling times in a long-term agroforestry experiment under a sub-humid tropical climate we established that year-to-year variations in plant δ 13C values were of the order of only 0.4–0.6‰, i.e. only 3–4% of the maximum tracer signal difference between C 3 and C 4 plants (∼15‰). Hence plant sampling and isotopic analysis can be limited to 2–3 years or even a single representative year in terms of weather. Important and consistent within-year variations were, however, observed for legume tree prunings (up to 2.4‰) and weeds (up to 7‰), pointing to the need for frequent sampling within a single year if such seasonal variation is suspected. Observed δ 13C values for the weed vegetation revealed a clear shift in weed composition in terms of C 3 and C 4 weeds within the season and across treatments, as was confirmed by a visual weed species identification in the field.

Soil organic matter in rainfed cropping systems of the Australian cereal belt

The Australian cereal belt stretches as an arc from north-eastern Australia to south-western Australia (24˚S–40˚S and 125˚E–147˚E), with mean annual temperatures from 14˚C (temperate) to 26˚C (subtropical), and with annual rainfall ranging from 250 mm to 1500 mm. The predominant soil types of the cereal belt include Chromosols, Kandosols, Sodosols, and Vertosols, with significant areas of Ferrosols, Kurosols, Podosols, and Dermosols, covering approximately 20 Mha of arable cropping and 21 Mha of ley pastures. Cultivation and cropping has led to a substantial loss of soil organic matter (SOM) from the Australian cereal belt; the long-term SOM loss often exceeds 60% from the top 0–0.1 m depth after 50 years of cereal cropping. Loss of labile components of SOM such as sand-size or particulate SOM, microbial biomass, and mineralisable nitrogen has been even higher, thus resulting in greater loss in soil productivity than that assessed from the loss of total SOM alone. Since SOM is heterogeneous in nature, the significance and functions of its various components are ambiguous. It is essential that the relationship between levels of total SOM or its identif iable components and the most affected soil properties be established and then quantif ied before the concentrations or amounts of SOM and/or its components can be used as a performance indicator. There is also a need for experimentally verifiable soil organic C pools in modelling the dynamics and management of SOM. Furthermore, the interaction of environmental pollutants added to soil, soil microbial biodiversity, and SOM is poorly understood and therefore requires further study. Biophysically appropriate and cost-effective management practices for cereal cropping lands are required for restoring and maintaining organic matter for sustainable agriculture and restoration of degraded lands. The additional benefit of SOM restoration will be an increase in the long-term greenhouse C sink, which has the potentialto reduce greenhouse emissions by about 50 Mt CO2 equivalents/year over a 20-year period, although current improved agricultural practices can only sequester an estimated 23% of the potential soil C sink.

Soil stripping and replacement for the rehabilitation of bauxite-mined land at Weipa. III. Simulated long-term soil organic matter development

Long-term trends in soil organic carbon (C) and nitrogen (N) under current and alternative rehabilitation practices at Weipa were simulated using the CENTURY model. After 100 years, predicted organic C in the surface soils (0–20 cm) of each treatment had risen to new dynamic equiliPbria. Since the ‘passive’ pool of recalcitrant organic C, which occupied 47% of organic C, changed little over the simulation period, the new equilibria differed according to initial organic C content. Most organic matter recovery occurred in the ‘slow’ fraction, although the greatest rate of change occurred in the ‘active’ C pool, which stabilised within 50 years at levels similar to the native forest. Similarly, ‘slow’ C accumulated in all treatments to new equilibria which were similar to that in undisturbed forest soil. The main difference between treatments was in the predicted time until a stable equilibrium in the ‘slow’ pool was reached: between 90 and 160 years depending on the soil stripping and replacement operation used. Successful development of new equilibria was highly sensitive to the amount of legume N2 fixation in the system and also to the severity of C and N losses during fire events. Reasonable agreement was found between simulated organic C accumulation and that observed in surveyed rehabilitation of up to 15 years of age (r2 = 0.67 for freshly replaced soils, r2 = 0.72 for soils stockpiled before respreading).

Effect of straw management, tillage timing and timing of fertilizer nitrogen application on the crop utilization of fertilizer and soil nitrogen in an irrigated cereal rotation

In irrigated grain-growing soils on Canada's prairies, straw management can affect nitrogen (N) fertility and long-term soil organic matter reserves. We conducted a 2-year field experiment in southern Alberta, on a Dark Brown Chernozemic Lethbridge loam (Typic Boroll), to determine the effects of straw removal, tillage, and fertilizer timing on crop uptake of soil and fertilizer N. During the study (1991 and 1992), the crop was oat ( Avena sativa L.) and wheat ( Triticum aestivum L.), respectively, in an experiment that had been in a wheat-wheat-oat-wheat rotation since 1986. Five straw-tillage treatments were: straw-fall plow, straw-pring plow, no straw-fall plow, no straw-spring plow and no straw-direct seeding. Fertilizer N was applied in fall or spring. Ammonium nitrate (5 at.% 15N) was added at 100 kg N ha −1 in fall 1990 or spring 1991. For oat (1991), plant N derived from soil was higher under fall plow than under spring plow, higher with tillage than direct seeding, and unaffected by straw removal. The plant N derived from fertilizer was not affected by straw removal in fall plow treatments, but under spring plow, it was higher with straw removal. The plant N derived from fertilizer showed a significant straw-tillage × fertilizer timing interaction; with fall incorporated straw, plant N derived from fertilizer was 44.0 kg N ha −1 for spring-applied, and 30.6 kg N ha −1 for fall-applied N, but in other straw-tillage treatments there was no effect of fertilizer timing. Cumulative fertilizer N recovery (plant + soil) over the 2 years averaged 64.2%, and was unaffected by straw-tillage treatment. Fertilizer N recovery, however, was less with fall-applied N (61.3%) than spring applied N (66.8%). At mid-season, fall plow treatments had higher soil inorganic N and inorganic N derived from fertilizer than spring plow treatments, apparently because of less immobilization. The fall plow treatments also retained higher inorganic N after harvest. Straw removal and fertilizer timing did not influence soil inorganic N and soil inorganic N derived from fertilizer. N removal in straw (16 kg N ha −1 yr −1) could deplete soil N in the long-term. Long-term effects of tillage timing on soil N will depend on the relative amount of N lost by leaching with fall plowing and that lost by denitrification under spring plowing. With direct seeding, crop yield and uptake of soil N was less, and N losses by denitrification could be greater. Application of N in spring, rather than fall, should enhance crop N uptake, reducing N losses and enhancing long-term soil organic N.

Analysis of impact of farming practices on dynamics of soil organic matter in northern China

The objectives of this study are to predict the long-term soil organic matter (SOM) dynamics in arable land in northern China, and to suggest measures for the maintenance and improvement of SOM content. With a newly developed model calibrated to relevant data collected from this region, the course of SOM content and SOM quality was calculated over a time span of 50 years. Two levels of annual grain yields were considered: `mean yields' of 7500 kg/ha and `high yields' of 15 000 kg/ha, and three scenarios for organic inputs into the soil: (1) roots and stubble; (2), as (1) plus one-third of straw produced in situ; (3) as (2) plus farmyard manure (FYM) made with another one-third of the straw produced in situ. It was predicted that with mean yields SOM contents can be maintained at 10, 13 and 15 g/kg under the three scenarios, and with high yields at 15, 19 and 21 g/kg. The present SOM contents are often below 10 g/kg. Depending on whether present SOM contents are below or above the steady-state levels, they will increase or decrease, respectively. The fraction of annual mineralization of SOM will increase with the magnitudes of annual organic inputs, indicating improvement in SOM quality. Hence, in most arable fields in northern China, both quantity and quality of SOM will rise upon increases in annual crop production, or when a large portion of straw is returned to the soil, directly or indirectly via FYM.

The Ultuna long-term soil organic matter experiment. II. Posphorus status and distribution in soils

In 1991, soil samples were taken from the long-term (40 years old) field trial at Ultuna in order to investigate soil P status and the distribution of its various forms. Among the treatments investigated, two were inorganic PK additions only – one to continuous fallow (PK-fallow) and the other to cropped fields (PK). There were also treatments amended with PK in combination with applications of straw, green manure composed of grass (GM), farmyard manure (FYM) or sewage sludge (SS). A total of 720, 720, 883, 1154, 1941 and 6617 kg P h-1 had been supplied in the PK-fallow, PK, Straw, GM, FYM and SS treatments, respectively up to 1991. The soil P distribution was determined by step-wise fractionation using anion exchange resin (resin-P), sodium bicarbonate (bicarb-P), sodium hydroxide (hyd-P), and HCl (HCl-P). Finally, the soil was digested to obtain residual P (resid-P). The amendments resulted in a significant (p=0.05) enrichment of total P in soils relative to the initial value. A breakdown of the bicarb-P and hyd-P into inorganic P (Pi) and organic P (Po) was manifested as considerable transformations within these P compartments compared with the initial values. Thus, total Pi (Σresin-P, bicarb-Pi, hyd-Pi, HC1-P, resid-P)/total Po (Σbicarb-Po, hyd-Po) ratios markedly decreased in all treatments relative to control. The two P compartments were significantly and negatively (p =0.05) correlated. On average, the total Po increase was about 380 mg kg-1 (range 270–715). The results suggested that an equilibrium between Pi immobilization and Po mineralization was difficult to attain under any of the experimental management regimes used, which exclude inorganic N application. The balance sheet calculations revealed P deficits ranging from about 10 to 60 kg ha-1, indicating that some P had migrated to the subsoil.

Litter quality of native herbaceous legumes in a burned pine forest of the Georgia Piedmont

The objectives of this study were to compare the nitrogen retranslocation efficiency and the litter substrate quality of eight native and one naturalized herbaceous legume species growing in a periodically burned pine forest of the Georgia Piedmont. A survey of the nine species revealed a wide range of nitrogen retranslocation (8.3–51.9%) from senescent leaf tissue, as well as differences in nitrogen concentrations (0.9–2.5%), C:N ratios (17.7–51.2), and initial lignin contents (8.6–20.1%) of senescent leaves. Intraspecific differences in Desmodiumviridiflorum L., Lespedezahirta L., and Lespedezaprocumbens Michx. were found in the leaf, stem, coarse- and fine-root tissue C:N ratio and in the lignin concentrations. Overall, the litter of these nitrogen-fixing herbaceous legumes is predicted to decompose at moderate rates and may have a significant effect upon the long-term soil organic matter and soil nitrogen accumulations of frequently burned forest sites.

Modelling soil organic matter levels after long-term applications of crop residues, and farmyard and green manures

A computer simulation model for long-term soil organic matter dynamics was developed and evaluated with data from long-term field trials in Belgium, Germany and The Netherlands. The model distinguishes four pools of soil organic components (including a microbial biomass pool) with different chemical properties. Transformation rates are described by (pseudo) first order kinetics. Effects of temperature and soil moisture tension were included. Simulation results were in agreement with experimental data from arable farming practices where common input rates were applied. Model calculations overestimated soil organic matter levels when green manures or exceptionally high input rates were applied. Inadequate experimental estimations of organic matter input rates and insufficient modelling of the soil preservation capacity for organic matter and biomass are likely reasons. After changes in the soil organic matter-input management it may take more than a century to reach new equilibrium levels.