Appraisal of management types on agroecological quality in different vineyard soils developed on volcanic ash (Tenerife, Canary Islands)

Abstract

The influence of agricultural management in soil carbon cycle is not completely understood especially in soils developed on volcanic ash materials which are traditionally postulated to exert a significant enhancement in organo-mineral complexes. This interaction is reflected in the performance both in soil carbon sequestration and in soil agroecological quality. A total of 30 humic acids collected from undisturbed and cultured soils from Tenerife (Canary Islands) were studied. The sampling sites were representative for different types of vineyards on volcanic materials either consisting of pumites or soils with more or less marked allophanic character. The vineyard management types consisted of traditional, intensive and minimum tillage, including organic agriculture. In the former case, a series of anthrosols consisting of allochthonous volcanic materials (pumites or andic horizons) were also included. The sampling points were also arranged in an altitudinal catena from 100 to 1100 m.a.s.l. The humic acids were characterized by derivative visible, infrared and nuclear magnetic resonance spectroscopies followed by analytical pyrolysis. This technique was carried out at 500 ºC in a Pyrojector (SGE instruments) connected to a GC/MS system Finnigan Trace GC Ultra with a Trace DSQ mass spectrometer. The chromatograph was provided with an HP-1 capillary column (30m × 250 ¿m), and the helium was adjusted to 1ml min-1. The oven temperature was 50 ºC for 1 min, then increased up to 100 ºC at 30 ºC min-1, from 100 to 300 ºC at 10 ºC min-1 and isothermal at 320 ºC. The results suggested significant interaction between mineralogical substrate (compositional gradient between allophonic materials and coarse pumite fractions) and management types (minimum tillage vs. conventional tillage, intensive or ecological management, plant vs. mineral cover) that was reflected in a series of spectroscopic features and pyrolytic patterns that could be interpreted in terms of soil complexity of the soil trophic system and the biogeochemical performance. A set of soils (mainly developed on pumites) showed humic acids with typical patterns of resistant plant biomacromolecules as well as a weakly condensed macromolecular structure (low optical density and comparatively small amounts of perylenequinone chromophors) also reflected by the substantial yields of lignin-derived methoxyphenolic units. Other set of soils, where the allophanic substrate is frequently associated with traditional management, in general show humic acids with large condensation and aromaticity (high optical density), and pyrograms with comparatively infrequent methoxyl-substituted aromatic compounds, and higher yields of not especially diagnostic compounds (mainly styrene, benzene) and methylfurfural. This suggests complex organic matter forms with stable aromatic domain not amenable to analytical pyrolysis, but reflected in the 13C-NMR spectra. The overall interpretation of the biogeochemical information suggest the possibility to control the humification processes by the farmer action, which¿in the study zone¿may exert a significant role on soil organic matter characteristics, with a stronger effect than that expected from the geological and topographical gradients.