An investigation of organic matter quality and quantity in acid soils as influenced by soil type and land use

Knowledge of the molecular composition of soil organic matter (OM) and the interaction of OM with soil minerals is needed to fundamentally understand how the persistence of OM is affected by land use. We investigated organic carbon (C) fractions, content of short-range order constituents (SRO) (i.e., Al and Fe oxy-hydroxides) and OM chemistry of 45 top soils across a range of soil orders and land uses in New Zealand. The objective of the study was to assess the influence of different land uses on the OM quality and quantity of soils that differed in their content of SRO constituents. The C fractions considered were cold + hot water-soluble C (CH2O), C recovered in the residuum after HF treatment (CHF-residuum), and C not so recovered (CHF-mobile). Carbon in particulate OM (CPOM) was determined in non-Allophanic soils, and C extractable with sodium pyrophosphate (Cp) in Allophanic soils. The chemistry of the HF-residual OM was investigated using pyrolysis-GC/MS. The highest C content was found under grazed grasslands and, among soil orders, in Allophanic soils, which had the largest CHF-mobile and CHF-residuum contents. Yet compared to non-Allophanic, Allophanic soils were more vulnerable to loss of C (CHF-mobile and CHF-residuum) when used for cropping. The relative contribution of microbial- vs. plant-derived OM was influenced by soil order and land use: microbial-derived OM increased as the presence of SRO constituents increased, these being more abundant in Allophanic soils; soils under ungrazed grasslands had the largest contribution of fresh plant-derived molecules to OM (and of CHF-residuum to total C) while cropping had a negative impact on the contribution of plant-derived OM, consistent with a decrease in CPOM. Overall, the results showed that not only is the ability of New Zealand soils to store C soil-specific, but so too is their vulnerability to losing it when under specific land use.