Physical-hydric Attributes and Soil CO2 Efflux in Pastoral Systems in a Brazilian Semi-Arid Environment

The positive balance between inputs/outputs of carbon in wooded pastures may be associated with soil physical-hydric attributes. These relationships are not totally known. This study aims to examine the relation between soil physical-hydric parameters with balance between primary production (carbon input) and soil respiration (carbon output). The study was carried out in three grazing systems: A Urochloa mosambicensis grass pasture (LU1), a silvopastoral system with U. mosambicensis and tree legume (LU2), and an area with U. mosambicensis and a herbaceous legume (LU3). The soil CO2 efflux (ECO2) and ten indicators of soil physical-hydric quality were measured: macroporosity, aeration capacity, plant-available water capacity (PAWC), relative field capacity (RFC), characteristic pore radius, quantity of hydraulically active pores, sorptivity, saturated hydraulic conductivity, soil bulk density, and soil penetration resistance. The leaf area index was used as an indicator of forage production. In the dry period, LU2 had averages of 0.17 m3 m−3 (PAWC) and 0.63 (RFC), while LU1 and LU3 both had an PAWC of 0.14 m3 m−3 and RFC of 0.54 and 0.57, respectively. This study provides the first evidence that soil physical-hydric attributes, specifically the plant-available water capacity and the connectivity of pores with wooded pastures, in semi-arid regions, favors a positive carbon balance in these systems (higher input/lower output).