Soil fertility, physical and chemical organic matter fractions, natural 13C and 15N abundance in biogenic and physicogenic aggregates in areas under different land use systems

The dynamics of soil aggregation, including aggregate formation by biogenic and physicogenic processes, is affected by the soil management system. The present study aimed to divide soil aggregates into biogenic and physicogenic, and to characterise the aggregates in terms of soil exchange complex, distribution of total organic carbon (TOC), nitrogen (N) levels, and natural 13C and 15N abundance, and determine C levels in the soil organic matter (SOM) fractions of the aggregates. The areas studied were under a no-till system (NTS) established 15 years before, conventional tillage system (CTS) managed with ploughing and harrowing for 56 years, and a secondary forest (Forest) and pasture grass (Axonopus compressus) (Pasture) (both ≥30 years old). Undisturbed soil samples were collected at 0–5 and 5–10cm. CTS was unsuitable for biogenic aggregate formation, and NTS and Forest areas showed similar biogenic and physicogenic aggregate content. The Pasture area promoted the formation of biogenic aggregates. In contrast to CTS, NTS increased TOC, calcium, magnesium, nitrogen and phosphorus levels. Considering the parameters evaluated, the Forest area showed the most variation (differences) between biogenic and physicogenic aggregates. Biogenic aggregates exhibited higher TOC and nutrient levels than their physicogenic counterparts. In the CTS area, the absence of legumes and the long-term and constant use of N-fertilisers resulted in 15N soil enrichment (at 0–5cm). In the Pasture area, the absence of legumes and the constant deposition of cattle manure also increased 15N content relative to NTS and Forest areas. The Forest area had the lowest 15N values in the two layers and aggregate types studied. The levels of 13C reflect the type of material incorporated in the soil (mostly C3 plants in Forest and NTS areas and C4 plants in Pasture and CTS areas). The Pasture area favoured the formation of stable SOM, with higher levels of C content in humin fraction (C-HUM) and mineral-associated OC (MOC). CTS compromised SOM stabilisation, producing lower levels of C-HUM, MOC, and moderately oxidisable and resistant fractions. The biogenic aggregates enhanced soil fertility and contained higher C content in SOM fractions than the physicogenic aggregates. We conclude that biogenic rather than physicogenic aggregates can be considered reliable indicators of soil quality.