Soil N dynamics associated with deforestation, biomass burning, and pasture conversion in a Mexican tropical dry forest

Abstract

Deforestation and land conversion in deciduous dry forests are occurring throughout the Neotropics, yet few studies have described soil N dynamics associated with this land-use/land-cover change. Soil nitrogen dynamics were measured before, and after, slash burning in two different fire-severity treatments (a high-severity fire with 80% of the aboveground biomass consumed, and a low-severity fire where 63% was consumed) in a neotropical dry forest ecosystem near Chamela, Jalisco, Mexico. We repeated these measurements three months after burning (corn field), the first year after slash-and-burn (pasture), and before, and after, a fire ≈2 years after the initial slash fires. Soil pH, total and inorganic soil nitrogen pools, and potential net mineralization and nitrification rates were measured. Slash burning resulted in dramatic but short-term increases in inorganic nitrogen and soil pH. Mineral N pools in surface soils (0–10 cm) increased from 9 to 44 kg ha −1 in the low-severity treatment and from 18 to 57 kg ha −1 in the high-fire severity treatment. This was due to an increase in soil NH 4-N; NO 3-N concentrations were lowered by fire. Surface soil pH increased 1.1 units following the low-severity fires and 2.3 units following the high-severity fires. Soil pH remained elevated in the perturbed sites for the duration of the study. Immediately after the initial slash fires, N mineralization was higher in the burned plots compared to the adjacent undisturbed forest. Potential nitrification was >9.5 μg NO 3-N g −1 soil day −1 in the slash-and-burn plots, and 3.9 μg NO 3-N g −1 soil day −1 in an adjacent reference forest. Potential nitrification was significantly higher in the high-severity treatment compared to the low-severity treatment. The increases in mineralization were likely a response to the large pulse of NH 4-N created via pyromineralization during the slash fires. The pattern of increased mineral N pools and potential rates of N mineralization in perturbed sites was short-lived. Compared to the reference forest, N mineralization rates in burned plots were similar 3 months after the fire, and lower 2 years following conversion. In contrast to the initial slash fire, we did not find a significant increase in available mineral N (NH 4-N) pools, or potential mineralization immediately following the pasture fires in 1995. Soil surface total N concentrations decreased from 4.3 and 5.4 mg g −1 at the start of the study, to 2.5 and 2.4 mg g −1 at the end of the study in the low- and high-fire severity treatments, respectively.