Fertilization in pastoral and Pinus radiata D. Don silvopastoral systems developed in forest and agronomic soils of Northwest Spain

Abstract

The effects of fertilization, pasture sowing and tree plantation on soil fertility and tree and pasture production can vary depending on the soil type. Tree plantation is recognized as a way to reduce nutrient leaching and increase land profitability in agronomic and forest soils, meanwhile pasture fertilization and sowing is usually associated to better pasture productivity and quality. Fertilization can be performed with mineral fertilizers, which have become expensive in recent times, or with organic fertilizers like sludge, which is being promoted worldwide. This study aims to evaluate the effects of sludge fertilization, tree planting and pasture sowing on different variables of soil (KCl-pH, cation exchange capacity, total N, total and Mehlich P, nitrate and soil organic matter) and pasture (production, botanical composition, crude protein and P concentration) in treeless and agroforestry systems established in forest and agronomic soils. The experimental design was a randomized block following an incomplete factorial design with three replicates and nine treatments including two types of soils (forestry and agronomic), two types of vegetation (natural and sown), two types of fertilization (sludge fertilization and mineral fertilization, with a no fertilizer control) in afforested and treeless pastures. Pasture production and quality was better under agronomic soils, which also had higher levels of KCl-pH, cation exchange capacity, nitrate, total N and P than forest soils. Tree establishment did not modify nitrate or P leaching, probably due to the youth of the trees when most of nitrate was leached at the beginning of the experiment, but reduction of soil KCl pH and pasture crude protein was found in forest soils, when trees and pasture were together established, probably due to the high extractions of these systems compared with unsown forests. Moreover, the sludge inputs increased pasture production better than the mineral fertilizer in the forest soils, probably due to the greater amount of nutrients applied by the former. Sowing enhanced the presence of sown grasses in the forest understory, but their presence reduced pasture quality, and they disappeared within a short period of time. Therefore, the use of the sludge as fertilizer allows nutrient recycling of this residue in soils of low fertility and increases productivity and preserves fertility compared with mineral fertilizer at short (forest soils) and medium (agronomic soils) term.