LAUSR

Abstract Land management and global warming affect the dynamics of soil organic matter (SOM) and nutrients such as nitrogen (N) and phosphorus (P). The effects depend on the type of SOM and on the temperature sensitivity of the SOM mineralization. There is insufficient data on tropical soils, which generally have low nutrient availability, to be able to predict the changes in SOM dynamics and, particularly, N and P availability with global warming. We studied at field scale, the influence of two land management systems, agroforestry (AGF) and traditional slash and burn agriculture (SB) on total carbon (C), available P and total N stocks in the 0–30 cm horizon of a Ferralsol in Madagascar. The soil under AGF stored more C and available P than the soil under SB. There was no clear difference for total N. A soil incubation laboratory experiment tested the effect on SOM mineralization of three factors: (i) land management (AGF or SB), (ii) increasing the temperature from 25 °C to 35 °C, and (iii) adding farmyard manure (FYM). The mineralization was assessed by measurements of microbial soil respiration (CO2), available P (resin P) and total mineral N over a 59 day soil incubation experiment at 25 °C and 35 °C with or without FYM. During incubation, the soil respiration was higher for AGF than for SB but it was slightly less temperature sensitive for AGF (Q10 = 1.2) than for SB (Q10 = 1.4). The incubation temperature did not have a clear effect on the available P for either AGF or SB, suggesting that temperature did not affect P-fixation processes in the Ferralsol studied which had a high P sorption capacity. The temperature sensitivity of N mineralization was quite high in both systems (Q10 = 3.4 after 42 days of incubation). The nutrient cycling processes (immobilization, mineralization) were faster at the higher temperature. Adding FYM increased slightly SOM mineralization and available P and mineral N, although exchange reactions in the Ferralsol limited P availability. Adding FYM modified the temperature sensitivity of SOM mineralization. It increased the temperature sensitivity of soil respiration, especially for AGF, but seemed to decrease the temperature sensitivity of N mineralization. Land use and management of Ferralsols affected the temperature sensitivity of SOM and nutrient dynamics and, in AGF soils, SOM seemed to be less temperature sensitive and have higher levels of nutrients.