Long-term effects of grazing intensities on soil aggregation and organic matter in a no-tilled integrated soybean-cattle system

Abstract

Grazing intensity in integrated crop-livestock systems (ICLS) can affect soil aggregation and C stabilization and, consequently, the soil condition and agricultural sustainability. This study evaluate the influence of 13 years of different grazing intensities on (i) soil aggregation, (ii) C content in different aggregate size, and (iii) C and N content in different fractions of soil organic matter (SOM), in a no-tilled with integrated soybean-beef production system on a subtropical Oxisol. Treatments consisted of four steers grazing intensities defined by sward height of 10, 20, 30 and 40 cm on mixed black oat and Italian ryegrass pasture in the winter under continuous stocking, plus an ungrazed treatment, in a randomized complete block design with three replicates. Soil aggregation was not affected by grazing intensities. There was no effect of the grazing intensities on organic C content in all aggregate-size fractions and in all soil layers evaluated, being lower in microaggregates than in large and small macroaggregates. The macroaggregates were predominant, representing 97% of the soil mass, of which 74% were large macroaggregates (>2 mm). The high macroaggregates stability contributed to the lack of difference in the C and N content on free light fraction and the light occlusal fraction of SOM. Most of C and N were observed heavy fraction of SOM (53–68%), with greater stock in the no grazing compared to the grazed treatments. Even with the animal trampling in the grazed systems, the soil maintained high stability of aggregates and consequently the labile forms of SOM.