N and P dynamics in the litter layer and soil of Mexican semi-arid forests, state of Morelos

In central Mexico, it is common for farmers to retain useful trees in abandoned lands after maize cultivation, creating a park-like landscape of scattered trees for extensive livestock grazing, among other land uses (mature forests, secondary forests, and livestock grazing in secondary forests). Among these trees Acacia cochliancantha and Ipomoea arborescens are the most common species associated with this land use in the region. The objective of this study was to assess the effects of both tree species on soil N and P recycling. To this end, we measured N and P concentrations in leaves of both species; and the seasonal N and P (total and dissolved) content in the litter, and total N and P, inorganic N, and bicarbonate-extractable-P concentrations, and the N transformations in the soil, in samples collected under crown of Acacia and Ipomoea and in open areas. Trees of different species varied in their capacity to cycle N. The leaves of Acacia were richer in N than those of Ipomoea (29.7 and 25.0 mg N g⁻¹, respectively), and nutrient resorption was higher in leguminous trees than in Ipomoea (by 20% in the case of N, and 35% in the case of P). Acacia trees had higher effects on soils than Ipomoea trees, like consistent increases of N concentrations (by 30% in the case of total N, and by 50% in the case of inorganic N) and transformations (N mineralization and nitrification in rainy season increases by a factor of 20 and 36, respectively). On the other hand, Ipomoea produced senescent leaves and accumulated forest litter with less P concentration (0.8 and 0.7 mg P g⁻¹, respectively) in relation to Acacia (senescent leaves: 1.3 mg P g⁻¹; litter: 1.1 mg P g⁻¹), reflecting the lower availability of the nutrient in the soil. The total litter N and P pools decreased in the rainy season under crown of both species, as the dissolved P pool did. The total soil N and P concentrations did not change with sampling season. However, potential N transformations and bicarbonate P under both species were higher in dry season than in rainy season samples. Comparison with other land uses in the region suggests that the Acacia–Ipomoea system cycles low quantity of nutrients, but there are no notable differences in the availability of P in the soils. These results demonstrate that scattered trees improve the N and P cycling following the discontinuation of agricultural practices, and the effects will vary depending on the tree species.