Model-Based Assessment of Grazing Impact on Soil Organic Carbon Stocks and Dynamics of a Kenyan Rangeland

As the largest terrestrial ecosystem carbon (C) sink, soils store about 2500 Pg C in the top 1 m depth. Rangelands make up around 40 % of global land surface and contain about 30 % of global terrestrial soil organic carbon (SOC). Grazing is the most important use of rangelands worldwide which when managed properly, can make rangelands potential sequesters of significant amounts of previously lost C. This will not only offset anthropogenic C emissions and contribute to climate change (CC) mitigation but also improve soil quality and productivity to ensure food security in the worlds’ poorest regions. Rangelands in East Africa cover 75 % of the total land area, but despite the importance outlined, little scientific studies have been carried out to quantify the effects of grazing management on SOC sequestration potentials and there exists a knowledge gap. The heterogeneous nature of rangelands worldwide makes it impossible to generalize grazing management recommendations across different regions as same studies have produced different results in comparable places. We aimed to investigate the long-term effect (30 years) of four grazing regimes on three selected sites under mid-future CC scenarios of the Representative Concentration Pathways (RCPs) 4.5 and 8.5 on SOC stocks of a rangeland in Southern Kenya. Heavy (HG) lead to SOC losses in all sites with the most loss of 206 kg C ha-1 observed on one sandy site “sand_light”. Moderate grazing (MG) increased SOC stocks in all three sites, the highest of 141 kg C ha-1 occurred at our clay site “clay_heavy”. RG (yearly rotation) and ALT (monthly rotation) caused SOC to increase only for the clay_heavy site. We observed small reductions in SOC stocks from the baseline scenario with a more negative impact of RCP4.5 than RCP8.5 but the observed differences were not dramatically different. The possible reasons for higher SOC stocks under RCP8.5 could be attributed to higher primary productivity of the C4 grassland under elevated CO2 and also more production of lignified plant materials that are less decomposable. We conclude that grazing management and soil texture will be the major factors controlling SOC stocks dynamics in the future as major differences was observed under those than under the CC scenarios

N₂O emission factors for cattle urine: effect of patch characteristics and environmental drivers

Urine patches from grazing cattle are hotspots of nitrous oxide (N2O) emissions. The default IPCC emission factor for urine patches (EFᵤᵣᵢₙₑ) is 0.77% for wet climates and 0.32% for dry climates. However, literature reports a considerable range of cattle urine EF values and urine characteristics used in experimental studies, revealing contrary results on the effects of urine patch characteristics and seasonal pattern. Therefore, we examined N2O emissions and corresponding EFᵤᵣᵢₙₑ values in relation to urine patch characteristics (urine N concentration, urine volume, patch area, urine composition) and environmental drivers (precipitation, water filled pore space, soil temperature). Ten artificial urine application experiments were performed from July 2020 to June 2022 on a pasture located in Eastern Switzerland. Urine N concentration, patch area, volume and urine N composition showed no significant effects on the EFᵤᵣᵢₙₑ value (p > 0.05). EFᵤᵣᵢₙₑ varied, however, strongly over time (0.17–2.05%). A large part of the variation could be predicted either by cumulative precipitation 20 days after urine application using a second order polynomial model (Adj. R² = 0.60) or average WFPS 30 days after urine application using a linear model (Adj. R² = 0.45). The derived precipitation model was used to simulate EFᵤᵣᵢₙₑ weekly over the last 20 years showing no significant differences between the seasons of a year. The resulting overall average EFᵤᵣᵢₙₑ was 0.67%. More field studies are needed across sites/regions differing in climate and soil properties to implement a country-specific EF₃ for Switzerland and to improve the quantification of N₂O emissions at the national scales.ISSN:1385-1314ISSN:1573-086