Abstract/Summary

Soil organic carbon (SOC), although a small component of the soil, is fundamental to delivering the physical, biological, and chemical functions that underpin soil ecosystem goods and services. The loss of SOC causes considerable negative impacts on the global environment. However, our knowledge of SOC is primarily drawn from studies that focus on the topsoil layer (0-30cm) while the majority of SOC stocks are stored in the subsoil layer. The mechanisms responsible for subsoil SOC preservation are far less well understood. This study generally aimed to quantify, compare, and mechanistically explain carbon storage in the top 1 metre of soil under different land uses. To examine the preservation mechanisms responsible for SOC storage under different land uses down the soil profile in the UK, soil sampling was carried out up to 1 m depth under the three UK dominant land uses (woodland, grassland and arable). The samples were analysed to quantify key soil properties (SOC, pH, C/N ratio, and texture) at 10 cm increments and SOC physical fractionation and ammonium oxalate extractable Al, Fe, and Mn was determined on selected soil layers, representing topsoil and subsoil, to investigate the mechanisms of SOC preservation down the soil profile. The results indicated that woodland soils contain greater SOC, N, and a higher C/N ratio than grassland and arable, while all these properties decreased down the soil profile. SOM fractionation revealed that the mineral-free particulate organic matter (fPOM) fraction was significantly greater in both the topsoil and subsoil under woodland than under grassland or arable. The mineral associated organic carbon (MinOC) fraction was proportionally higher in the subsoil than topsoil under all land uses, indicating that SOM protection in subsoils is primarily regulated by soil organo-mineral interactions, mainly amorphous Fe and Mn concentrations. Fourier-transform infrared (FTIR) spectroscopy and δ 13C analysis of the fPOM fraction of topsoils and subsoils sampled down the soil profile under woodland, grassland, and arable land uses were used to determine the chemical recalcitrance and degree of microbial decomposition of fPOM and attribute this to the isotopic composition and presence of functional groups. The results showed a clear influence of land use on fPOM characteristics where soils under more natural vegetation had higher fPOM, SOC, and N concentrations, more oxygenated functional groups (e.g. carbohydrates and carboxylic acids), and were iv depleted in 13C due to the absence of C4 crops. Subsoil fPOM was chemically different to the surface layers due to being more microbially decomposed. The effect of forest conversion to cinnamon plantation on an Indonesian Andosol was investigated by collecting and analysing samples collected down the top 1 m of the soil profile from three different prominent cinnamon producing locations (Lempur, Pungut, Renah Kayu Embun (RKE)) in Kerinci Regency, Sumatera, Indonesia. Soil samples were collected under natural forest vegetation and at 1, 5, and 10 years after clearance of the forest and establishment of a cinnamon plantation. The results revealed that, despite short term increases in SOC stocks due to pyrogenic inputs resulting from slash and burn, SOC stocks were ultimately lower 10 years after forest conversion, reflecting decreases in both topsoil and subsoil layers. FTIR spectroscopy revealed an increase in the degree of decomposition of fPOM down the soil profile 10 years after forest conversion to cinnamon plantation, probably due to less fresh litter input and greater microbial activity. The findings of this study are that SOC quality and quantity are altered when natural vegetation is converted to an agricultural land use, largely influenced by the lower contribution of plant litter input due as source OM. Natural vegetation have greater OM inputs, with a higher proportion of carbohydrate compounds, than converted land and there is tendency that the SOC quantity decrease as the converted land gets older. Moreover, this study confirmed that mechanisms of SOC protection differ with depth; topsoil is dominated by free particulate organic matter (fPOM) and subsoil dominated by SOM bound to mineral surfaces (amorphous Al, Fe and Mn). Topsoil is provided with direct fresh litter and experienced faster decomposition process while subsoil was dominated by less microbially processed OM. All these observations indicate that SOC, and related physical chemical properties, are clearly influenced by land use and soil depth which is important knowledge to advance SOC preservation.