Impact of different fallow durations on soil aggregate structure and humus status parameters

Abstract

Soil aggregate structure and soil organic matter are closely interrelated and commonly considered as key indicators of soil quality. The aim of this study was to evaluate the effects of different fallow durations on indices of soil structure and humus status indicators. Studies were conducted on abandoned agricultural fields (15, 20 and, 35 years after abandonment). As a reference site, we used a cultivated field in the area. The experimental soil fields are classified as Gleyic Cambisols. Soil macroaggregates were separated with the sieve (dry sieve) to seven aggregate size fractions, i.e.> 10, 10–5, 5–2, 2–1, 1–0.5, 0.5–0.25 and < 0.25 mm. The humus status parameters of soils included the following indicators: soil organic carbon (C<sub>org</sub>), humus reserves (Q<sub>H</sub>), the degree of humification of organic matter (SOM<sub>dh</sub>), fractions of humic acids (HA) (free and bound with monovalent cations and Al<sub>2</sub>O<sub>3</sub>, Fe<sub>2</sub>O<sub>3</sub>, bound with Са<sup>2+</sup> which forms humates, bound with clay minerals), fulvic acids (FA) (free aggressive) and ratio of HA to FA (C<sub>HA</sub><sub> </sub>: C<sub>FA</sub>). After a fallow period of more than 20 years on the surface formation of a sod layer. A long-term fallow period had an impact on the mean weight diameter of the aggregates (MWD) and agronomically valuable aggregates (AVA). Fallow soils have a significantly better structure than soils under a cultivated field. Long-term cultivation leads to the deterioration of soil structure and the formation of large aggregates (>10 mm). The C<sub>org</sub> content remains at the level of the background content when the soils are left fallow for less than 15 years and increases over time. The C<sub>org</sub> in the upper 0–20 cm soil layer has been shown to increase from 3.55 to 8.74% on arable land that has been fallow for 35 years and has been largely associated with significant accumulation of organic matter within the plant root mass. Mature sites are characterized by an increase of fulvic acids in the humus composition in comparison with their arable analogues. The abandonment of soil agricultural use and the cessation of mechanical tillage results in the restoration of the natural structure of soils and the improvement of their agrophysical properties. Such studies have not been previously conducted in the Primorsky region of the Russian Far East.