Brachiaria brizantha in Silvopastoral and Monoculture Systems: Soil, Trees, and Microclimate in an Altitudinal Gradient of the Amazon

The aim was to determine the edaphic and microclimatic influence of an altitudinal gradient on the production of Brachiaria brizantha in monoculture (MCS) and silvopastoral systems (SPS). Twenty-four farms were located along an altitudinal gradient from 150 to 1260 masl. Soil properties were found to be similar between MCS and SPS, but with significant differences (95% CI) along the altitudinal gradient, with higher values of pH, electrical conductivity, phosphorus, potassium, cation exchange capacity and clay at 503 and 661 m in alkaline soils associated with CaCO₃, while soils at 170 and 1110 m were acidic. In the SPS, over 20 tree families and 40 species were identified, with 58% of the families consisting of Simaroubaceae, Asteraceae, Fabaceae, and Malvaceae, and the highest number of families and species found at the mid-altitude of 170 m. SPS regulate the microclimate by decreasing temperature and solar radiation while increasing relative humidity, contrary to MCS. These microclimatic conditions in SPS resulted in higher plant height, leaf length, and leaf width compared to MCS. The growth and development of B. brizantha did not respond to soil properties in either MCS or SPS, showing similar behavior in slightly alkaline soils as in acidic soils, nor did it respond to higher availability of phosphorus, potassium, and higher cation exchange capacity. However, the altitudinal gradient influenced both SPS and MCS, where the highest values for plant height, leaf length, and leaf width were found at mid-altitudes.