A number of approaches have been used to model and thus describe N mineralization kinetics in soils. In this paper, we review and present a comparative evaluation of some approaches to modelling adopted for this purpose. These may broadly be categorized as: (1) simple functional approaches to predict net N mineralization and, (2) mechanistic approaches that include a description of microbial biomass processes to predict long-term C and N turnover in soils. The following conclusions emerged. First, the simple functional models try to quantify one or more active fractions of organic matter with associated rate constants to predict net N mineralization. A minimum of two organic N fractions are considered essential to significantly contribute towards N mineralization. Second, the mechanistic models have the advantage that model algorithms represent the basic mechanisms believed to influence mineralization. The input parameters of the models could be obtained independently of the test data. However, it is difficult to validate the models as some of the presumed functional pools cannot be quantified by physical, chemical and biological techniques. Consequently, the models have to be calibrated by adjusting the rate coefficients and pool sizes (site-specific) to fit the measured data. Generally, the performance of a model depends on site-specific calibration. In future, a major challenge will be to free simulations from site-specific calibration and to devise experimental methods for providing initial values to run a model. The amount of N mineralized during laboratory incubation experiments and the parameter estimates for different models depend on methods used to pretreat the soil prior to incubation and their exposure to experimental conditions (such as temperature, soil moisture content) during the incubation. To obtain comparable values amongst studies it is, therefore, necessary to standardize methods of sample pretreatment as well as incubation conditions.

A critical review of some approaches to modelling nitrogen mineralization

A critical review of nitrogen mineralization in biosolids-amended soil, the associated fertilizer value for crop production and potential for emissions to the environment.

Evaluation of sewage sludge, septic waste and sludge compost applications to corn and forage: yields and N, P and K content of crops and soils.

Currently, sludge is not considered as a waste any more, since it is capable of producing valuable products. Besides land disposal and thermochemical processes (i.e. pyrolysis, combustion, gasification), biological processes appear as promising valorisation routes to treat wastewater sludge efficiently. Anaerobic digestion (AD) processes are already being applied at industrial scales for the effective disposal and valorisation of sludge. However, methane yields from sludge anaerobic digestion remain low compared to other types of organic waste. Thus, pretreatment and co-digestion contribute to improve the degradability of organic matter and methane potential of sludge, respectively. This paper reviews the recent achievements in sludge pretreatment and co-digestion with other substrates such as the organic fraction of municipal solid waste, fatty waste, lignocellulosic and algal biomass. Furthermore, recent studies combining co-digestion and pretreatment are examined. The paper also provides recommendations to better manage sludge recovery by taking into account multiple aspects such as techno-economic feasibility, the effect of pretreatment on both the physico-chemical properties of sludge and the quality of digestate. The socio-environmental and legislative aspects are also essential in order to ensure the sustainability of the process.

F. Pomares

Papers

Nitrogen mineralization of sludge-amended soil