Effects of green manure mulching on soil aggregates and organic carbon fractions in orchards

Green manure mulching is a sustainable soil management model for orchards. We conducted an experiment with four different green manure treatments, Vicia villosa (T₁), Arachis pintoi (T₂), Raphanus sativus (T₃), and a mixed sowing of V. villosa, R. sativus and P. wetsfeteinii (T₄), with clear tillage as control, to investigate the effects of green manure mulching on soil aggregates and organic carbon fractions in a citrus orchard. The results showed that: 1) Green manure mulching significantly enhanced the distribution and stability of soil aggregates. The proportion of large aggregates (>2 mm) increased by 10.7%, 6.6%, 11.1%, and 9.3% for the T₁-T₄ treatments, respectively, while the mean weight diameter of the aggregates increased by 1.1 to 1.3 times. 2) Mulching significantly improved total organic carbon (TOC) content and its fractions. TOC was increased by 2.5% to 31.7%, while dissolved organic carbon, particulate organic carbon, mineral-bound organic carbon, and easily oxidizable organic carbon was increased by 23.7%-97.9%, 38.2%-118.3%, 2.8%-35.5%, and 8.6%-49.5%, respectively. 3) The proportion of aggregates of each particle size correlated with its contribution to the organic carbon fractions. Aggregates greater than 2 mm were the primary carriers of TOC and its fractions. Additionally, the stability of these aggregates was significantly correlated with TOC. In summary, green manure mulching markedly increased the content of large aggregates, enhanced the stability of soil aggregates, improved granular soil structure, and increased organic carbon content in citrus orchard soil. Notably, the effects varied among treatments, with V. villosa and A. pintoi having the most effective amelioration.