Disentangling Drought and Grazing Effects on Soil Carbon Stocks and CO2 Fluxes in a Semi-Arid African Savanna

Lawrence Munjonji,Tieho Paulus Mafeo,Malesela Vincent Mokoka,Kai Behn,Kingsley Kwabena Ayisi,Edwin I Mudongo,Anja Linstädter

doi:10.3389/fenvs.2020.590665

Abstract

Grasslands cover ca. 30% of the global land surface and provide critical ecosystem services. Among them, carbon storage is one of the most important. However, grasslands are increasingly threatened by drought and overgrazing which might negatively affect soil carbon stocks. Despite this threat, there is a dearth of information on how drought and grazing jointly impact soil carbon stocks and CO2 fluxes in dryland grasslands. With the aid of a large field experiment, we studied the combined effects of a five-year extreme drought and moderate grazing on soil carbon stocks, CO2 fluxes and soil chemical properties. Extreme drought was induced by reducing ambient rainfall by 66% using large rainout shelters. We found CO2 fluxes to strongly respond to the five-year experimental drought. Extreme drought reduced CO2 emission rates by 32% compared to ambient conditions. CO2 fluxes averaged 5.7 mg m-2min-1 under drought compared to 8.3 mg m-2 min-1 under ambient conditions. CO2 fluxes were however not influenced by grazing. At the end of the growth period, grazed plots under ambient rainfall had released 16.3 tons of CO2 ha-1 which was 58% higher than observed on grazed plots subjected to severe drought. Soil carbon stocks were higher under drought conditions due to slower decomposition rates. Drought resulted in increased concentrations of primary macronutrients (N, P, and K), micronutrients (Zn and Mn) and pH in the top 30 cm of the soil relative to ambient conditions. The results also showed that grazing reduced the concentration of N and P in the topsoil compared to the ungrazed plots. This study provided insights on the soil carbon storage, CO2 emission rates and nutrient dynamics in a semi-arid dryland grassland as influenced by both drought and grazing. Our study also revealed that long-term extreme drought may be favourable in terms of preserving the existing soil carbon stocks through reduced CO2 release. This finding is critical for understanding future soil carbon dynamics in dryland grasslands in the face of climate change.

Highlights

Grasslands are the largest terrestrial biome, covering up to 30% of the global land surface (Suttie et al, 2005)
CO2 fluxes ranged from 1.35 mg m−2 min−1 at the time of peak standing biomass (May 2019) to 14.03 mg m−2 min−1 recorded in February 2019, i.e., at the time of maximum plant growth (Figure 3)
The results showed that the average CO2 flux rate under ambient conditions was 8.3 mg m−2 min−1 equating to 43.6 tons CO2 ha−1 year−1, while under drought, the CO2 flux rate was 5.7 mg m−2 min−1, resulting in 30 tons CO2 ha−1 year−1

Summary

Introduction

Grasslands are the largest terrestrial biome, covering up to 30% of the global land surface (Suttie et al, 2005). They are a large reservoir of organic carbon, storing 10–30% of the global soil organic carbon (Zhou et al, 2019a). The frequency and intensity of drought have increased considerably in global drylands, while overgrazing continues to be a problem (IPCC, 2019). This is the case in drylands situated in Sub-Saharan Africa, SSA. Combined effects of grazing and drought can have strongly impacts on the ecosystem functions and services delivered by these ecosystems (Ruppert et al, 2015; Pfeiffer et al, 2019)

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