Comparing NaOH‐extractable organic matter of acid forest soils that differ in their pedogenic trends: a pyrolysis‐GC/MS study

Soil organic matter (SOM) in Alu‐andic Andosols and Alu‐humic Umbrisols is believed to accumulate because of the protection caused by binding to aluminium (Al). We investigated soils that differed in the abundance of organo‐Al complexes to determine the effect of such binding on SOM chemistry. For this, the surface horizons of three types of acid soils in the Basque Country (northern Spain) under forest stands were studied: (i) Alu‐andic Andosols (AND soils) on basalts and trachytes, (ii) Umbrisols or so‐called ‘aluminic’(ALU) soils also on basalts and trachytes and (iii) soils with a podzolizing trend (POD), on quartzites. Values of Al extractable with sodium pyrophosphate (Al_p) in the surface horizons of these soils ranged between 8.5 and 13.1, 1.9 and 9.3, and 0.8 and 3.7 g kg^?1 dry weight, for the AND, ALU and POD soils respectively. For POD and ALU soils, surface horizons were sampled at two depths, 0–5 and 5–20 cm, whereas the AND soils were sampled at different depths down to the B horizon. NaOH‐extractable SOM from three AND soils, 12 ALU soils and 12 POD soils was studied by pyrolysis‐gas chromatography/mass spectrometry. The POD soils had the largest loads of plant‐derived markers (lignin, long‐chain alkanes and alkenes, methyl ketones, fatty acids); SOM of the AND soils had the smallest amounts of plant‐derived SOM and the largest amounts of microbial products (microbial sugars and N‐compounds) of the soils studied. ALU soils had an intermediate pattern, as expected. The results indicate that the SOM of Alu‐andic Andosols, developed from basalt and trachyte rocks, is essentially dissimilar to that of soils derived from quartz‐rich parent material, under the same climate conditions and similar forest stands. The dominance of secondary (microbial‐derived) SOM in Alu‐andic Andosols, also observed in previous research on Sil‐andic Andosols (these are dominated by short‐range ordered Si compounds in contrast to the dominance of organo‐Al complexes in Alu‐andic Andosols), reveals the small contribution of primary (plant‐derived) material to SOM in soils with andic properties.