Decomposition of plant material as an indicator of ecosystem disturbance in tropical land use systems

We report on an attempt to distinguish cropped from fallowed land and young from old fallow by the rate of decomposition of a standard material, to contribute towards the development of simple, yet reliable indicators of soil quality and agricultural sustainability on tropical soils. In three southern Cameroonian villages, Senna spectabilis leaves and wood were incubated in undisturbed bush fallow of about 4 years, young secondary regrowth of about 12 years and secondary forest of at least 25 years and in the same fallow age class after clearing and cropping. The decomposition of S. spectabilis leaves distinguished fallowed from cropped land throughout a period of 14 to 280 days after incubation, independent of the fallow type that was cleared and the location. Fallow types were distinguished over the same period, with higher leaf mass loss in secondary forest systems than young regrowth and bush fallow. In all cases mass loss followed significant logarithmic functions. Soil chemical properties were not correlated to leaf mass loss. Mass loss from S. spectabilis wood was not suitable to distinguish either undisturbed from cropped or one fallow age class from another. Significant differences between land uses occurred only at the end of the incubation period. Fallow types could not be distinguished from each other. S. spectabilis leaf decomposition may be developed into one component of a soil quality or soil function indicator if decomposition can be linked to crop yields in cleared sites and biomass accumulation in undisturbed sites and other soil properties

Atomistic two-, three- and four-body potentials. Spatial and material settings

In molecular dynamics or molecular statics (MD/MS) multi-body potentials empirically capture the energetic interactions in atomistic systems enabling the computation of the corresponding atomistic forces as energetic conjugates to the atomistic positions. We distinguish here between spatial and material atomistic positions and consequently between the corresponding spatial and material atomistic forces. In quasi-statics, i.e. MS, the former, also denoted as deformational atomistic forces, contribute to the classical deformational mechanics (i.e., equilibrium) problem that seeks to minimise the total potential energy of an atomistic system with respect to the atomistic positions relative to the ambient space. The latter, also denoted as configurational atomistic forces, contribute to the configurational mechanics (i.e., non-equilibrium) problem that determines the release of total potential energy of an atomistic system upon variation of the atomistic positions relative to the ambient material, i.e., due to perturbations of the material (initial) atomistic configuration. The importance of material atomistic forces is that they drive energetically favourable re-organisations of the material atomistic configuration, thereby characterising the tendency of generic atomistic defects to propagate. In this contribution we focus on two-, three-, and four-body potentials, whereby we distinguish between novel stretch- and classical angle-based potentials for the two latter cases. Taken together, as the main contribution, we derive expressions for the corresponding spatial and, for the first time, material atomistic forces and highlight their striking formal similarity. The derivations are detailed but the final expression compact and well-suited for numerical implementation