Studying the physical protection of soil carbon with quantitative infrared spectroscopy

Near infrared (NIR) and mid-infrared (mid-IR) reflectance spectroscopy are time- and cost-effective tools for characterising soil organic carbon (SOC). Here they were used for quantifying (i) carbon (C) dioxide (CO2) emission from soil samples crushed to 2 mm and 0.2 mm, at 18 degrees C and 28 degrees C; (ii) physical C protection, calculated as the difference between CO2 emissions from 0.2 mm and 2 mm crushed soil at a given temperature; and (iii) the temperature vulnerability of this protection, calculated as the difference between C protection at 18 degrees C and 28 degrees C. This was done for 97 topsoil samples from Tunisia, mostly calcareous, which were incubated for 21 days. Soil CO2 emission increased with temperature and fine crushing. However, C protection in 0.2-2 mm aggregates had little effect on the temperature vulnerability of CO2 emission, possibly due to preferential SOC protection in smaller aggregates. In general, NIR spectroscopy, and to a lesser extent mid-IR spectroscopy, yielded accurate predictions of soil CO2 emission (0.60 <= R-2 <= 0.91), and acceptable predictions of C protection at the beginning of incubation (0.52 <= R-2 <= 0.81) but not over the whole 21 day period (R-2 <= 0.59). For CO2 emission, prediction error was the same order of magnitude as, and sometimes similar to, the uncertainty of conventional determination, indicating that a noticeable proportion of the former could be attributed to the latter. The temperature vulnerability of C protection could not be modelled correctly (R-2 <= 0.11), due to error propagation. The prediction of SOC was better with NIR spectroscopy and that of soil inorganic C (SIC) was very accurate (R-2 >= 0.94), especially with mid-IR spectroscopy. Soil CO2 emission, C protection and its vulnerability were best predicted with NIR spectra, those of 0.2 mm samples especially. Mid-IR spectroscopy of 2 mm samples yielded the worst predictions in general. NIR spectroscopy prediction models suggested that CO2 emission and C protection depended (i) on aliphatic compounds (i.e. labile substrates), dominantly at 18 degrees C; (ii) on amides or proteins (i.e. microbial biomass), markedly at 28 degrees C; and (iii) negatively, on organohalogens and aromatic amines (i.e. pesticides). Models using mid-IR spectra showed a negative influence of carbonates on CO2 emission, suggesting they did not contribute to soil CO2 emission and might form during incubation. They also suggested that CO2 emission and C protection related to carboxylic acids, saturated aliphatic ones especially.