Characterizing Livestock Production in Portuguese Sown Rainfed Grasslands: Applying the Inverse Approach to a Process-Based Model

Grasslands are a crucial resource that supports animal grazing and provides other ecosystem services. We estimated the main properties of Portuguese sown biodiverse permanent pastures rich in legumes (SBP) starting from measured data for soil organic carbon (SOC) and using the Rothamsted Carbon Model. Starting from a dataset of SOC, aboveground production (AGP) and stocking rates (SR) in SBP, we used an inverse approach to estimate root to shoot (RS) ratios, livestock dung (LD), livestock intake (LI) and the ratio between easily decomposable and resistant plant material. Results for the best fit show that AGP and belowground productivity is approximately the same (RS is equal to 0.96). Animals consume 61% of the AGP, which is within the acceptable range of protein and energy intake. Carbon inputs from dung are also within the range found in the literature (1.53 t C/livestock unit). Inputs from litter are equally distributed between decomposable and resistant material. We applied these parameters in RothC for a dataset from different sites that only comprises SOC to calculate AGP and SR. AGP and SR were consistently lower in this case, because these pastures did not receive adequate technical support. These results highlight the mechanisms for carbon sequestration in SBP.

Carbon sequestration under subtropical perennial pastures II: Carbon dynamics

Here we take advantage of the stable carbon isotope shift that occurs when a C4 plant is sown into a soil previously dominated by C3 vegetation, to explore the movement and fate of newly sequestered soil organic carbon (SOC) following establishment of subtropical perennial pastures in temperate regions of Australia. In kikuyu-based pastures up to 33 years of age, SOC accumulated exclusively in the coarse size fraction (>50 μm) in the sandy soils of southern Western Australia. In South Australian loams, regardless of pasture age, new SOC was found to accumulate in both the coarse and fine (<50 μm) size fractions. These differential results suggest that in soils with low clay content, new SOC remains in an unprotected form that is highly vulnerable to loss through decomposition and erosion. The Rothamsted Carbon Model, modified to track changes in stable isotopes, was able to represent the changes in total SOC stocks in both regions; however, the model over-predicted the incorporation of the new C4-SOC into the soil. This difference between data and model output could be reconciled if a greater proportion of new SOC is rapidly mineralised without being incorporated into any sort of stabilised pool.