## 2 Materials and Methods
### Modeling Details  
#### 2.2.1 CenW 4.2 Overview  
CenW is an open-source process-based model, combining the major carbon, energy, nutrient, and water fluxes in an ecosystem [52]. For the present work, we used CenW version 4.2, which is available for download, together with its source code and a list of relevant equations available in cenW documentation, version 4.1.1 (_A growth and C balance simulation model, (c) 2017_). A number of additional routines were added to run the model for managed pastures [56]. A list of relevant parameters is given in Appendix B. The CenW model runs on a daily time step and encompasses major ecosystem processes (canopy photosynthesis, allocation and growth, litterfall, decomposition, autotrophic and heterotrophic respiration), and their relationships to climatic drivers to simulate the behavior of the ecosystem over time, which are further modified through management practices (i.e., moving, grazing, N fertilizer applications, plowing, and sowing). The main CO2 fluxes are photosynthetic carbon gain by plants which is integrated over the whole canopy and the whole daytime period [72] and CO2 losses through autotrophic respiration by plants and heterotrophic respiration by soil organisms and grazing animals, when they are present on the modeled paddock. CenW simulates soil heterotrophic respiration individually for growth and maintenance. These fluxes are modified by temperature and nutrient and water balances. Plant growth is determined by the dynamic allocation of fixed carbon to the different plant organs, which depends on the plant root/shoot ratio, vegetation type and development stages, and water and nutrient stresses. The model contains a fully integrated nitrogen cycle as well as a coupled multilayer bucket water model. Water is gained by rainfall and lost through evapotranspiration. Any amount of water exceeding the soil's water-holding capacity is lost by deep drainage beyond the root zone, with important controls by soil depth and water-holding capacity. CenW simulates total evapotranspiration by modeling separately canopy and soil evaporation rates, and plant transpiration. These individual fluxes are calculated using the Penman-Monteith equation, with canopy resistance for calculating transpiration explicitly linked to photosynthetic carbon gain. This module of CenW is particularly important, as it is likely that soil water availability constituted an important constraint on plant productivity over the summer months at the experimental site, which is prone to summer droughts. The soil organic matter component of CenW is based on the CENTURY model [55], which was originally developed for grasslands. The model includes three soil organic matter pools (active, slow, and resistant) with different potential decomposition rates. Leaves and roots senescence and litter production are controlled by plant type and phenology, and by water, temperature, and specific senescence parameters which depend on plant species. Dead foliage can either fall onto the soil surface and become part of the decomposing litter pool, remain standing for some time where it either decomposes during wet periods, or eventually falls onto the soil surface after some time. It was important to model these processes as the estimates of foliage biomass included a component of dead standing biomass that was not separated out in the data. These processes were modeled by assuming that all senescence, or drought-induced leaf death, initially transferred foliage from a live to a dead foliage pool [56]. The soil is divided into multiple layers and the same calculations driving the behavior of organic matter are applied to all of them, with each layer having its own complement of all organic matter pools. Layers only differ by the amounts and qualities of litter entering each layer. In addition, a small fraction of each pool is transferred to the corresponding pool in the layer below [53; 73]. This allows changes in organic matter and C:N ratios in the surface litter layer and with depth in the soil to be simulated.