Influence of crop rotation, tillage and fertilization on chemical and spectroscopic characteristics of humic acids

Abstract

The changes in soil organic matter composition induced by anthropogenic factors is a topic of great interest for the soil scientists. The objective of this work was to identify possible structural changes in humic molecules caused by a 2-year rotation of durum wheat with faba bean, lasted for a decade, and conducted with different agricultural practices in a Mediterranean soil. Humic acids (HA) were extracted at three depths (0-30, 30-60 and 60-90 cm) from a Mediterranean soil subjected to different tillage (no tillage, minimum tillage and conventional tillage), crops (faba bean and wheat), and fertilization. The changes in HA quality were assessed by several chemical (ash, yield and elemental analysis) and spectroscopic techniques (solid-state 13C nuclear magnetic resonance, Fourier transform infrared and fluorescence). The results suggest that the different agronomic practices strongly affected the quality of HA. Smaller but more aromatic molecules were observed with depth, while the fertilization induced the formation of simpler and less aromatic molecules due to the enhanced decomposition processes. Under no tillage, more stable humic molecules were observed due to the less soil aeration, while under conventional tillage larger and more aromatic molecules were obtained. Compared to wheat, more aromatic and more oxidized but less complex molecules were observed after faba bean crop. The inorganic fertilization accelerates the decomposition of organic substances rather than their stabilization. At the end of each crop cycle, humic matter of different quality was isolated and this confirms the importance of the rotation practice to guarantee a diversification of the soil organic matter with time. Finally, no tillage induces the formation of more stable humic matter.

Fig 1. FT-IR spectra of HA samples isolated from faba bean soils, year 2015.

a: MT unfertilized; b: MT fertilized; c: CT unfertilized; d: CT fertilized; e: NT unfertilized; f: NT fertilized. Sampling depth: Blu spectra, 0–30 cm; Red spectra, 30–60 cm; Green spectra, 60–90 cm. X-axis: wavenumbers (cm^-1); Y-axis: transmittance (%).

Fig 2. FT-IR spectra of HA samples isolated from wheat soils, year 2016.

a: MT unfertilized; b: MT fertilized; c: CT unfertilized; d: CT fertilized; e: NT unfertilized; f: NT fertilized. Sampling depth: Blu spectra, 0–30 cm; Red spectra, 30–60 cm; Green spectra, 60–90 cm. X-axis: wavenumbers (cm^-1); Y-axis: transmittance (%).

Fig 3. Excitation/Emission matrices of HA samples isolated from faba bean soils, year 2015.

Fig 4. Excitation/Emission matrices of HA samples isolated from wheat soils, year 2016.

Fig 5. ¹³C CP MAS NMR spectra of humic acids isolated from various soil samples.

A. NT 2015 0–30 cm; B. NT 2015 30–60 cm; C. NT 2015 60–90 cm; D. CT fertilized 2015 0–30 cm; E. CT 2015 0–30 cm; F. MT 2015 30–60 cm; G. MT 2016 30–60 cm; H. CT 2016 30–60 cm; I. NT 2016 30–60 cm.