Impact of biological nitrogen fixation and livestock management on the manure transfer from grazing land in mixed farming systems.

Soil fertility in mixed farming systems relies on the manure produced by livestock and its recycling in the entire system. In the particular case of crop-livestock system with grazing area, the proper functioning of the system also depends on the presence of nitrogen-fixing plants in the area where livestock grazes (the grazing land). In this paper, we study the impact of biological nitrogen fixation (BNF) and livestock management on the flux of manure exported outside the grazing land. We address this issue using a modeling approach. We consider a plant-soil model composed of a set of nonlinear ordinary differential equations that represents the grazing land. We assume that the manure produced by the grazing livestock can be partially exported as a fertilizer outside of this area. Through the mathematical analysis of the model, and analytical and numerical optimization, we then determine the optimal livestock management in terms of grazing rate and manure recycling percentage that lead to the maximal flux of exported manure. We focus more precisely on the role of nitrogen-fixing plants and their impact on the optimal livestock management. When grazing rate is high and the capacity of plants to fix nitrogen is important, we showed that it is necessary to recycle some of the manure produced by the livestock in the grazing land to maximize the flux of exported manure. On the contrary, if we can optimize both the grazing rate and the manure recycling percentage, then it is better to transfer all the produced manure and to adapt the grazing rate accordingly to minimize nitrogen losses from the soil. Finally, to maximize the flux of exported manure, it is also necessary to bring the system to a state in which the plants fix nitrogen. In this way, we can benefit from the nitrogen fixation which provides an additional input of nitrogen in the system.

Maximization of fertility transfers from rangeland to cropland: The contribution of control theory.

In traditional mixed farming systems, soil fertility in cropland relies on the transfer of fertility from rangeland through the transfer of manure produced by livestock that grazes in rangeland. In this work, we introduce a simple meta-ecosystem model in which the mixed farming system is represented by a cropland sub-system connected to a rangeland sub-system by nutrient fluxes. The livestock plays the role of nutrient-pump from the rangeland sub-system to the cropland sub-system. We use this model to study how spatial organization and practices of livestock management such as the control of grazing pressure and night corralling can help optimize both nutrient transfers and crop production. We argue that addressing the optimization of crop production requires different methods, depending on whether the agricultural practice in focus is constant or variable over time. We first used classical optimization methods at equilibrium to address optimization when the grazing pressure was assumed to be constant over time. Second, we address optimization for a more realistic configuration of our model, where grazing pressure was assumed to vary over the course of a year. In this case, we used methods developed in the field of the control theory. Classical methods showed the existence of an optimal level of constant grazing pressure that maximizes the transfers from rangeland to cropland, leading to the maximization of crop production. Control methods showed that by varying the grazing pressure adequately an additional gain of production is possible, with higher crop production and lower nutrient transfer from rangeland to cropland. This additional gain arises from the fact that the requirement of nutrient by crops is variable along the year. Consequently, a constant adjustment of the grazing pressure allows a better match between nutrient transfer and nutrient requirement over time, leading to a substantial gain of crop biomass. Our results provide new insights for a "smarter" management of fertility transfers leading to higher crop production with less rangeland surface.

Facilitation- vs. competition-driven succession: the key role of resource-ratio.

Symbiotic nitrogen (N)-fixing plants are abundant during primary succession, as typical bedrocks lack available N. In turn, fixed N accumulates in soils through biomass turnover and recycling, favouring more nitrophilous organisms. Yet, it is unclear how this facilitation mechanism interacts with competition for other limiting nutrients such as phosphorus (P) and how this affects succession. Here, we introduce a resource-explicit, community assembly model of N-fixing species and analyze successional trajectories along resource availability gradients using contemporary niche theory. We show that facilitation-driven succession occurs under low N and high enough P availabilities, and is characterised by autogenic ecosystem development and relatively ordered trajectories. We show that late facilitation-driven succession is sensitive to catastrophic shifts, highlighting the need to invoke other mechanisms to explain ecosystem stability near the climax. Put together with competition-driven succession, these results lead to an enriched version of Tilman's resource-ratio theory of succession.

Analysis of arsenic and antimony distribution within plants growing at an old mine site in Ouche (Cantal, France) and identification of species suitable for site revegetation.

One of the objectives of this study was to assess the contamination levels in the tailings of an old antimony mine site located in Ouche (Cantal, France). Throughout the 1.3 ha site, homogenous concentrations of antimony and arsenic, a by-product of the operation, were found along 0-0.5 m-deep profiles. Maximum concentrations for antimony and arsenic were 5780 mg kg(-1) dry tailings and 852 mg kg(-1) dry tailings, respectively. Despite the presence of the contaminants and the low pH and organic matter contents of the tailings, several patches of vegetation were found. Botanical identification determined 12 different genera/species. The largest and most abundant plants were adult pines (Pinus sylvestris), birches (Betula pendula) and the bulrush (Juncus effusus). The distribution of the metalloids within specimens of each genera/species was analysed in order to deduce their concentration and translocation capacities. This was the second goal of this work. All plant specimens were highly contaminated with both metalloids. Most were root accumulators with root to shoot translocation factors <1. Whereas contamination levels were high overall, species with both a low translocation factor and a low root accumulation coefficient were identified as suitable candidates for the complete revegetation of the site. Species combining those characteristics were the perennials P. sylvestris, B. pendula, Cytisus scoparius and the herbaceous Plantago major, and Deschampsia flexuosa.