Soil carbon storage and greenhouse gas fluxes in forested and grassland ecosystems of Gujarat undergoing anthropogenic and climatic disturbances

Carbon storage in soils is a significant nature-based solution for adaptation and mitigation of climate change. However, soil carbon storage varies with the change in land cover and the association of organic matter with differently sized soil aggregates. To compare the effects of these parameters on the carbon cycling of soils, we have analyzed the soil carbon content and greenhouse gas emissions from soils under different land cover in Gujarat, India. We sampled forest soils from regions covered with dry deciduous trees, moist trees, forest fires, and forest conversion to agriculture. We further sampled grassland soils in the areas showing matured grasses, harvested grasses, salinity-affected grasses, grazing-affected grasses, and the grassland region impacted by the invasion by Prosopis juliflora species. We classified some of these land features as a result of climatic disturbances (increase in salinity, invasion by Prosopis, forest fire) and others as anthropogenic disturbances (cattle grazing, harvesting, agriculture). Physico-chemical properties and greenhouse gases emitted from soils were measured using handheld probes and chamber methods, respectively. Laboratory analysis of soil carbon and its isotopes was performed for bulk soils and the soil density-size fractions (particulate fractions, sand-sized fractions, clay and silt size fractions, and recalcitrant organic matter fractions). Our analysis reveals the change in carbon emissions and carbon storage in soils of arid-moist stretch in Gujarat is caused by different land cover and management practices. The presence of aboveground biomass has a significant control on the carbon storage capacity of soils highlighting the importance of afforestation and ecosystem restoration in building up the soil carbon stock. Carbon emissions were also higher in soils with large aboveground vegetation; however, this mainly represents soil microbial respiration and thus indicates healthy soil and rich pedo-biodiversity. Most of the carbon in grassland soils was associated with silt and clay-sized particles whereas the carbon content in forested soils is higher within sand-sized particles. Soils from both grasslands and forests acted as a sink for atmospheric methane except for soils from grazed grasslands, indicating the importance of grazing management in grassland ecosystems. Climatic stressors like an increase in salinity and Prosopis invasion showed no significant impact on soil carbon stock and greenhouse gas emissions and behaved similarly to harvested grasslands or dry-deciduous forests. Forest fire, on the other hand, can change the texture and carbon and nitrogen association in the soils. Conversion of forests to croplands has the most detrimental impact on soil carbon storage and can lead to loss of stored carbon in the form of high greenhouse gas emissions. Forest land conversions should, therefore, also consider this aspect of forest conversion in its management plan.