Carbon Sequestration in Response to Grassland-Shrubland-Turfgrass Conversions and a Test for Carbonate Biomineralization in Desert Soils, New Mexico, USA

This study uses an experimental pedology approach to examine (i) how the conversion of native C-4 grassland to C-3 woody shrubs then to irrigated C-4 turfgrass affects both soil organic C (SOC) and soil inorganic C (SIC) and (ii) whether SIC can be enhanced by microbial biomineralization. Three sites were studied in the Chihuahuan Desert of New Mexico. At each site, SOC, SIC, and their delta C-13 values were measured on control soil samples and compared to samples treated with liquid growth medium and Ca(OH)(2) at 10 and 40 cm. The treated samples were left to react for 1 mo in February, May, and August of 2014. Using a space-for-time substitution, soil organic C decreased when native grasslands converted to desert shrubs, then sharply increased after desert shrubs were converted to irrigated turfgrass. Most surprising, however, was the increase of SIC in the turfgrass site, which doubled in 6 yr. The delta C-13 values of both SOC and SIC reflected the change from C-4 to C-3 then back to C-4 vegetation and showed how rapidly SOC and SIC can change their isotopic signatures. Soil inorganic C formation was slightly higher for the liquid growth medium, but no statistically significant differences were observed between the treatments and control samples. In addition, no biomineralization was observed with microscopy, perhaps because the 1-mo reaction time was too short and the amount applied was too small. Although SIC is typically viewed as a soil mineral that requires centuries to accumulate, our study indicate that SIC can be generated in months to years if the soil environment is suitable.