DYNAMICS OF SOIL ORGANIC MATTER QUALITY IN ALTERNATIVE SYSTEMS TO SHIFTING AGRICULTURE IN THE PERUVIAN AMAZON

Shifting cultivation based on slash-and-burn practices significantly contributes to soil degradation in the Peruvian Amazon, causing qualitative and quantitative changes in soil organic matter (SOM) pools. Agroforestry systems (AFS) represent sustainable alternatives with the potential to reduce SOM losses in the humid tropics.

During five years, SOM pool dynamics were evaluated during dry and wet seasons across 15 soil use systems (SUS) in the Amazon jungle (2200 mm. annual rainfall). Low jungle SUS included primary and secondary forest; multistrata AFS with Cedrelinga cateniformis and Centrosema macrocarpum (10% and 4% clay content); Bertholletia excelsa intercropped with coffee (Coffea arabica); crop systems, including Elaeis guineensis Pueraria phaseoloides), Bactris gasipaes as fruit and hearth of palm with kudzu; improved Brachiaria brizantha with Centrosema; and degraded native grasses. High jungle and SUS comprised cacao (Theobroma cacao L.) under secondary forest shade; cacao with planted timber shade trees; cacao without shade and with Centrosema cover crop; and improved and traditional shaded coffee.

SOM exhibited a cyclical trend, with higher contents in topsoil. SOM levels were lower in the low jungle (0.5–3.4%) than in the high jungle (1.0–5.0%), showing accumulation during dry periods and enhanced mineralization during wet seasons. After five years, SOM decreased in agroforestry and crop systems, with a more pronounced decline in the high jungle (up to 1.9%). This decrease was reflected in a significant reduction of the humin fraction (up to 1.9%) and an increase in the fulvic acid fraction (up to 0.72%), while humic acid pools decreased in cacao-based AFS.