Ernesto Bosatta

Nitrogen saturation of terrestrial ecosystems

Nitrogen saturation, in the sense that nitrogen additions to an ecosystem lead to losses of the same order of magnitude, is analyzed as an interplay between a plant subsystem and a soil subsystem. The plant system is defined by its nitrogen productivity, which allows calculations of the maximum amount of nitrogen that can be held in, and the maximum nitrogen flux density that can be utilized by, the plant subsystem. The most important response of the soil subsystem is a change in the microbial nitrogen concentration, from which the nitrogen absorption capacity can be derived. It is shown that of the two subsystems the soil must always saturate first. The time to reach saturation depends strongly on site history in terms of the sources of litter forming the soil organic matter and on the ratio between the external nitrogen inflows and the litter nitrogen flow.

Soil organic matter quality interpreted thermodynamically

Abstract Soil organic matter quality in the sense of how easily carbon in the soil organic matter can be mineralised is a major determinant of soil carbon storage and rate of mineralisation of nutrients. Its origin has so far remained elusive and a number of indices, such as C-to-N-ratio, lignin concentration and other combinations of chemical constituents have been used as substitutes for quality. We suggest here that quality is the number of enzymatic steps required to release as carbon dioxide a carbon atom from an organic compound . The larger the number of steps the lower is the quality of the carbon atom. Such a measure connects quality to thermodynamics. It also explains the rapid decrease in decomposition rate with decreasing quality suggested in the q-theory of organic matter dynamics and shows that the decomposition rate of low quality substrates has a stronger temperature dependence than that of high quality substrates.

Theoretical analysis of decomposition of heterogeneous substrates

A theory is proposed for the decomposition of a heterogeneous substrate, in which the heterogeneity is described by a continuously varying quality variable, q. Two microbial properties, efficiency in substrate utilization, e(q), and rate of substrate utilization, u(q), depend on the quality variable and decrease with decreasing substrate quality. General results of the theory can be displayed either in terms of time or quality. It turns out that the quality representation is both more general and more lucid. Only very weak specifications of the functions u(q) and u(q) are necessary to determine whether the decomposition process will end after a finite time and whether all substrate eventually will become mineralized. The final nitrogen-to-carbon ratio is shown to be independent of these two functions but depends on the initial nitrogen concentration and quality of the substrate. Using specific functions for u(q) and u(q) it is possible to derive a number of models used to describe decomposition and the variation in the critical nitrogen-to-carbon ratio with specific decomposition rate of the substrate. The theoretical predictions are compared to a number of decomposition experiments.

Quality : a bridge between theory and experiment in soil organic matter studies

The abstract concept quality was introduced by us ten years ago to formalise the dynamics of C, N, P, and S in soil organic matter. We present here an interpretation of decomposition studies of 19 different litter types performed at 16 different localities and including a total of 978 observations in terms of the continuous quality theory showing how quality can be estimated from conventional chemical fractionation. This provides us with a powerful tool to unify our understanding of the physical-chemical-biological complex behind the processes controlling soil organic matter dynamics.

Reconciling differences in predictions of temperature response of soil organic matter

Abstract Global warming has long been assumed to lead to an increase in soil respiration and, hence, decreasing soil carbon stores. This assumption has been based on short-term studies of litter and soil organic matter incubations. However, some recent studies seem to indicate that soil organic matter is less temperature sensitive than previously thought. We will in this paper use the continuous-quality theory to show that the temperature dependence of decomposition of soil organic matter depends on whether one studies soils at their native temperatures or soils that have been perturbed from their native temperatures. Turnover times of soil organic matter are more sensitive to temperature changes when they are estimated from typical incubation experiments with different temperatures than when they are estimated from soils at their native temperatures because the variation in turnover rate with native soil temperature is not the same as the temperature response of turnover rate of a given soil. This reconciles some seemingly incompatible results in the literature.

Energy or nutrient regulation of decomposition: Implications for the mineralization-immobilization response to perturbations

A model developed previously to describe the turnover of forest soil nitrogen is modified here to explain the effects of carbon and nitrogen additions on their dynamics. The model, which is structurally very simple, seems to explain correctly, among other phenomena, the negative correlation between N mineralization and CO2 evolution observed in many experimental situations. An important variable used to explain this behaviour is the deficiency factor, which is related to the critical C-to-nutrient ratio and which gives a measure of the C or nutrient deficiency in the substrate with respect to the needs of the decomposers. Ways are discussed in which the model output can be used to explain the observed retention in the soil of fertilizer N added to mature forest soils.

Theoretical analysis of microbial biomass dynamics in soils

Abstract Microbial biomass can serve as a rapid indicator of state and change of soil properties. We have extended a published theory in order to incorporate the dynamics of microbial biomass in the soil. The extension adds a requirement of information about microbial mortality. Theoretical predictions are given for the ratio between carbon in living microbial biomass and carbon in the substrate in decomposing litter cohorts, in the soil at steady state and in chronosequences of forests and agricultural systems. These predictions compare well with empirical studies. The extended theory allows a new interpretation to the dispersion function, which relates to processes in the food chain of the microbial community. Effects of soil texture are also discussed.

Theoretical analyses of soil texture effects on organic matter dynamics

Abstract Soil organic C and N tend to increase and mineralisation rates decrease with clay content. We show how these observations can be explained within the framework of the continuous quality theory by letting the quality dependence of decomposer growth rate depend on clay concentration. Theoretical predictions are shown to agree with a number of empirical findings on both stores of C and N and N mineralisation rates from various ecosystems.

Theoretical analyses of carbon and nutrient dynamics in soil profiles

Abstract The study of C, N, P, and S profiles in soils can tell us about the rates and pathways within and interactions between the cycles of the four elements in the whole ecosystem. We now add a spatial dimension to the continuous quality theory such that it can describe the distribution of the four elements along the vertical direction in the soil. This extension requires the introduction of a new function describing the velocity with which the organic matter moves along the soil profile. The theory is used to calculate properties of storage and mineralisation rates of C, N, P and S in soil profiles; for example, how is the position of the maximum of the density profiles determined by the rate of decomposition relative to the rate of transport. Predictions of the theory agree well with empirical information from agricultural and forest soils. A refined analysis would require better knowledge of the velocity of transport and its explicit dependence on quality and abiotic factors.

The power and reactive continuum models as particular cases of the q-theory of organic matter dynamics

The reactive continuum model (Boudreau and Ruddick, 1991), the power model (Janssen, 1984; Middelburg, 1989), and the q-theory (Bosatta and Agren, 1991a) have been used to describe decomposition of organic matter; the reactive continuum model has also been used to model the kinetics of kerogen cracking, i.e., of oil and gas generation (Burnham et al., 1987). The reactive continuum model describes the organic matter as being composed of a continuous distribution of reactive types; the basic premise in the power model is that the reactivity of organic matter decreases with time. The q-theory is based on the idea that organic matter is composed of an infinite spectrum of interacting (through microbial and physicochemical processes) reactive types. We show here that the reactive continuum model and the power model can be deduced as particular cases of the q-theory. The q-theory reduces to the reactive continuum model if all interactions between the components of the spectrum are neglected, whereas the power model is obtained under specific assumptions about microbial properties. The q-theory is, therefore, not only mathematically more general but has also a larger explanatory power.