Microtopography controls organic and inorganic carbon stocks in Vertisols: Insights from a climosequence on the Texas coastal plain

Abstract

Complex interactions between ecological and environmental factors govern soil organic carbon (SOC) and soil inorganic carbon (SIC) stocks across landscapes. In gilgai-bearing Vertisols, submeter differences in water availability result in micro-spatial variability in soil properties. This study quantifies SOC and SIC stocks along a climosequence of subtropical grassland Vertisols (n = 45) on the Texas coastal plain, considering regional mean annual precipitation (MAP) (804–1475 mm year⁻¹) and gilgai microrelief. Expected regional increases in SOC stocks with increasing MAP are obscured by field-scale variability in soil moisture between gilgai microlows (ML) and microhighs (MH). Expected regional decreases in SIC stocks with increasing MAP occur in MLs. In contrast, the greatest SIC stocks in MHs occur where there was the greatest relief between gilgai elements (udic–ustic intergrades). Importantly, mean SOC stocks did not differ between the udic and ustic soil moisture regimes at a given microtopographic position. Regardless of soil moisture regime, MLs are enriched in SOC and depleted in SIC relative to MHs; however, differences were more pronounced in udic soil moisture regimes than ustic soil moisture regimes. Because the distribution, morphology, and degree of development of gilgai vary at field and regional scales, the resultant meter-scale variations in soil carbon stocks are unlikely to be captured in most digital soil mapping or carbon modeling studies.