Olof Andrén

Temperature dependence of organic matter decomposition: a critical review using literature data analyzed with different models

Abstract The literature was reviewed regarding laboratory incubation studies where C mineralization was measured. Experiments were selected in which the same substrate was incubated at least at two different temperatures and where time-series were available with at least four measurements for each substrate and temperature. A first-order one-component model and a parallel first-order two-component model were fitted to the CO2–C evolution data in each experiment using a least-squares procedure. After normalising for a reference temperature, the temperature coefficient (Q10) function and three other temperature response functions were fitted to the estimated rate constants. The two-component model could describe the dynamics of the 25 experiments much more adequately than the one-component model (higher R2, adjusted for the number of parameters), even when the rate constants for both were assumed to be equally affected by temperature. The goodness-of-fit did not differ between the temperature response models, but was affected by the choice of the reference temperature. For the whole data set, a Q10 of 2 was found to be adequate for describing the temperature dependence of decomposition in the intermediate temperature range (about 5–35  °C). However, for individual experiments, Q10 values deviated greatly from 2. At least at temperatures below 5  °C, functions not based on Q10 are probably more adequate. However, due to the paucity of data from low-temperature incubations, this conclusion is only tentative, and more experimental work is called for.

ICBM: THE INTRODUCTORY CARBON BALANCE MODEL FOR EXPLORATION OF SOIL CARBON BALANCES

A two-component model was devised, comprising young and old soil C, two decay constants, and parameters for litter input, “humification,” and external influences. Due to the model’s simplicity, the differential equations were solved analytically, and parameter optimizations can be made using generally available nonlinear regression programs. The calibration parameter values were derived from a 35-yr experiment with arable crops on a clay soil in central Sweden. We show how the model can be used for medium-term (30 yr) predictions of the effects of changed inputs, climate, initial pools, litter quality, etc., on soil carbon pools. Equations are provided for calculating steady-state pool sizes as well as model parameters from litter bag or 14C-labeled litter decomposition data. Strategies for model parameterization to different inputs, climatic regions, and soils, as well as the model’s relations to other model families, are briefly discussed.

Biodiversity and species redundancy among litter decomposers

We discuss biodiversity in relation to ecosystem processes, particularly litter decomposition. Three hypotheses concerning the relations between organism groups, diversity and decomposition rates are proposed and tested against data from a two-year straw decomposition experiment. Barley straw mass loss and chemical composition, soil temperature and moisture, and the abundance of bacteria, fungi (total and FDA-active), protozoa, nematodes, microarthropods and enchytraeids were monitored.

ICBM regional model for estimations of dynamics of agricultural soil carbon pools

Swedish arable land covers 3 Mha and its topsoil contains about 300 Mton C. The mineral soils seem to be close to steady-state, but the organic soils (about 10% of total arable land) have been estimated to lose ca. 1 Mton/year. We have devised a conceptual model (ICBMregion), using national agricultural crop yield/manuring statistics and allometric functions to calculate annual C input to the soil together with a five-parameter soil carbon model (ICBMr), calibrated using long-term field data. In Sweden, annual yield statistics are reported for different crops, for each of eight agricultural regions. Present topsoil carbon content and regional distribution of soil types have recently been measured. We use daily weather station data for each region together with crop type (bulked from individual crop data) and soil type to calculate an annual soil climate parameter for each crop/soil type permutation in each region. We use 14 soil types and 9 crop types, which gives 126 parameter sets for each year and region, each representing a fraction of the regions area. For each year, region, crop and soil type, ICBMregion calculates the change in young and old soil carbon per hectare, and sums up the changes to, e.g., national changes. With eight regions, we will have 1008 parameter sets per year, which easily can be handled, and what-if scenarios as well as comparisons between benchmark years are readily made. We will use the model to compare the soil C pools between the IPCC benchmark year 1990 and the present. In principle, we use inverse modelling from the sampled, recent soil C pools to estimate those in 1990. In the calculations, soil climate and yield for each year from 1990 onwards are taken into account. Then we can project soil C balances into the future under different scenarios, e.g., business as usual, land use change or changes in agricultural crops or cultivation practices. Projections of regional climate change are also available, so we can quite easily make projections of soil C dynamics under, e.g., different climate scenarios. We can follow the dynamic effects of carbon sequestration efforts – and estimate their efficiency. The approach is conceptually simple, fairly complete, and can easily be adapted to different needs and availability of data. However, perhaps the greatest advantage is that the results from this comprehensive approach used for, e.g., a 10-year period, can be condensed into a very simple spreadsheet model for calculating effects of management/land use changes on C stocks in agricultural soils.

Wheat root biomass and nitrogen dynamics—effects of daily irrigation and fertilization

Root biomass, root nitrogen content, and root distribution down to 50 cm depth in winter wheat were determined by soil coring on five dates in four different treatments: control (C), drought (D), daily irrigation (I), and daily irrigation and fertilization (IF). The first three treatments received the N fertilizer application as a single dose in spring, whereas in IF daily doses of N were supplied in the irrigation water using a drip-tube system, according to the estimated nutrient demand of the crop. All treatments received 20 g N m−2 year−1.The maximum root biomass (104 g m−2) was reached earliest in IF. On 6 June, root samples were taken down to a depth of 100 cm, and the proportion of deep roots (50–100 cm) was least in I, indicating that it had the shaklowest root system. The root biomass as a fraction of the total plant mass decreased during crop development in all treatments down to about 4% at harvest. The decrease was more rapid in I and C than in D and IF. The higher proportion of roots during spring in D and IF coincided with a low nitrogen concentration in the roots, which was attributed to the restricted water supply and to the relative shortage of nitrogen during early crop development in D and IF, respectively. The dynamics of mass and nitrogen in macroscopic organic debris in the soil suggested that root turnover rates were high. ei]{gnB E}{fnClothier}

Pedotransfer functions for estimating plant available water and bulk density in Swedish agricultural soils

Abstract Pedotransfer functions (PTFs) to estimate plant available water were developed from a database of arable soils in Sweden. The PTFs were developed to fulfil the minimum requirements of any agro-hydrological application, i.e., soil water content at wilting point (θ wp ) and field capacity (θ fc ), from information that frequently is available from soil surveys such as texture and soil organic carbon content (SOC). From the same variables we also estimated bulk density (ρ) and porosity (ϵ), which seldom are included in surveys, but are needed for calculating element mass balances. The seven particle-size classes given in the data set were aggregated in different ways to match information commonly gained from surveys. Analysis of covariance and stepwise multiple linear regression were used for quantifying the influence of depth, particle size class, textural class and soil organic carbon on the characteristic variables. PTFs developed from other data sets were also tested and their goodness-of-fit and bias was evaluated. These functions and those developed for the Swedish database were also tested on an independent data set and finally ranked according to their goodness of fit. Among single independent variables, clay was the best predictor for θ wp , sand (or the sum of clay and silt) for θ fc and SOC for ρ and ϵ. A large fraction of the variation in θ wp and θ fc is explained by soil texture and SOC (up to 90%) and root mean square errors (RMSEs) were as small as 0.03 m3 water m−3 soil in the best models. For the prediction of ρ and ϵ in the test data set, the best PTF could only explain 40–43% of the total variance with corresponding RMSEs of 0.14 g cm−3 and 5.3% by volume, respectively. Recently presented PTFs derived from a North American database performed very well for estimating θ wp (low error and bias) and could be recommended for Swedish soils if measurements of clay, sand and SOC were available. Although somewhat less accurately, also θ fc could be estimated satisfactorily. This indicates that the determination of plant available water by texture and SOC is rather independent of soil genesis and that certain PTFs are transferable between continents.

Root dynamics in barley, lucerne and meadow fescue investigated with a mini-rhizotron technique

Root development, including depth distribution, was followed in pure barley stands (Hordeum distichum, L.) with or without nitrogen fertilization and in barley undersown with lucerne (Medicago sativa L.) or meadow fescue (Festuca pratensis, Huds.). The number of roots per 5 cm depth level down to 1 m was counted frequently during the growing season using mini-rhizotrons, i.e., transparent tubes inserted into the soil. Root biomass at different depths down to 1 m was estimated from soil cores taken one month before harvest. The results from the two methods were compared and root counts in the different treatments were compared with the above-ground growth and production.Nitrogen-fertilized barley in pure stand had the highest biomass both above and below ground. According to the mini-rhizotron observations this treatment also had a deeper and denser root system, until barley harvest, than the other treatments. After barley harvest, roots from the undersown lucerne continued to increase, whereas the number of roots in the undersown meadow fescue remained the same. The root system in barley/meadow fescue did not penetrate into the subsoil, where more than 60% of the number of roots in barley undersown with lucerne were found. In general, the mini-rhizotron results indicated a higher relative abundance of roots in the deeper layers than the root biomass estimated with the soil coring method.

Growth dynamics of reed canarygrass ( Phalaris arundinacea L.) and its allocation of biomass and nitrogen below ground in a field receiving daily irrigation and fertilisation

Biomass and nitrogen in the roots, rhizomes, stem bases and litter of reed canarygrass (Phalaris arundinacea L.) were repeatedly estimated by soil coring, and root growth dynamics of this potential energy crop was studied for two years using minirhizotrons. Results are discussed in relation to above-ground biomass and nitrogen fertilisation. Five treatments were used: C0, unfertilised control; C1, fertilised with solid N fertiliser in spring; I1, irrigated daily, fertilised as in C1; IF1 , irrigated as I1 and fertilised daily through a drip-tube system; IF2, as in IF1 but with higher N fertiliser rates. Biomass of below-ground plant parts of reed canarygrass increased between the first and second years. Up to 50% of total plant biomass and nitrogen were recovered below-ground. The highest proportions were found in C0. The calculated annual input via root turnover ranged between 80 and 235 g m-2. In absolute terms, up to 1 kg and 10 g m-2 of biomass and nitrogen, respectively, were found in below-ground plant fractions. High inputs of stubble and accumulated below-ground biomass will occur when the ley is ploughed, which will result in a highly positive soil carbon balance for this crop in comparison with that of conventional crops such as cereals.

Modelling the effects of moisture on barley straw and root decomposition in the field

Abstract In a field experiment with barley on a clay soil, four treatments were compared: the control (C) was not nitrogen fertilized or irrigated; drought treatment (D) was identical to C, except that screens had been installed to divert rainwater; irrigation (I) and irrigation-fertilization (IF) treatments were irrigated using a drip-tube system, with liquid fertilizer (260kg N ha−1yr−1) applied daily in IF according to predicted plant uptake. Litter-bags with barley straw were buried in all treatments at 10 cm depth on 19 May 1988 and sampled five times during the growing season. Decomposition rates were calculated assuming exponential decay. The rates were highly dependent on soil moisture, and the constants ranged from 0.54% day−1 in D to 0.92% day−1 in IF. A simulation model with driving variables based on Q10 temperature dependence and a log-linear relationship between soil water tension and activity was fitted to the same data set. The rate constants became quite similar between the four treatments, ranging from 1.1 to 1.3% optimum day−1. Thus it was concluded that the assumptions and parameters used for temperature and moisture dependence were valid. Roots collected from each of the four treatments on four occasions were put in litter-bags, buried in the soil and taken up after about 20 days. Observed root decomposition rates were lowest in D. The rates for all treatments were highest early in the growing season and decreased thereafter. When temperature and moisture effects were compensated for using the climatic correction factors for barley straw, the decreasing trend remained, which was attributed to reduced decomposability of the roots during the growing season. The results indicate that using a constant value for root decomposability when calculating root turnover can give misleading results.

The impact of altered management on long-term agricultural soil carbon stocks – a Swedish case study

Land use in general and particularly agricultural practices can significantly influence soil carbon storage. In this paper, we investigate the long-term effects of management changes on soil carbon stock dynamics on a Swedish farm where C concentrations were measured in 1956 at 124 points in a regular grid. The soil was re-sampled at 65 points in 1984 and at all grid points in 2001. Before 1956 most of the fodder for dairy cattle was produced on the farm and crop rotations were dominated by perennial grass leys and spring cereals with manure addition. In 1956 all animals were sold; crop rotations were thereafter dominated by wheat, barley and rapeseed. Spatial variation in topsoil C concentration decreased significantly between 1956 and 2001. C stocks declined in fields with initially large C stocks but did not change significantly in fields with moderate C stocks. In the latter fields, soil C concentrations declined from 1956 to 1984, but increased slightly thereafter according to both measurements and simulations. Thus, the decline in C input due to the altered management in 1956 was partly compensated for by increasing crop yields and management changes, resulting in increased C input during the last 20 years. A soil carbon balance model (ICBM) was used to describe carbon dynamics during 45 years. Yield records were transformed to soil carbon input using allometric functions. Topsoil C concentrations ranging between 1.8 and 2.4% (depending on individual field properties) seemed to be in dynamic equilibrium with C input under recent farming and climatic conditions. Subsoil C concentrations seemed to be unaffected by the management changes.