Geochemical Properties of Variable Charge Soil Explain the Low Nitrogen Bioavailability

ABSTRACT

In the Vakinankaratra region of the Malagasy Highlands, rainfed rice cultivation has developed rapidly, which has led to rapid degradation of soil fertility and reduced productivity. This study involved a comparison of rainfed rice in rotation with maize/Brachiaria (MZ-BR) or maize/soybean (MZ-SB) intercrops, managed by direct seeding (DS) or plowed with residue burial (PB) with conventional mineral fertilization (FM) or without fertilizer (F0). The umbric Acrisol (hyperdystric) presented an anion exchange capacity (AEC) and variable charge according to the pH. Mineralizable nitrogen (N_minz.) values decreased sharply from the surface to the deep horizons and became almost nil in the 90–120 cm horizon, while the N_minz. contents of the DS treatments were systematically 5–7% higher than the PB treatment values for a given depth. The AEC of the 0–10 cm horizon was practically zero but increased significantly with depth and reached 2.58 cmol_(c) kg⁻¹. AEC values were inversely correlated with the organic carbon (C_org.) content: AEC = 1.81–0.375 C_org. The nitrate adsorption isotherms were linear and the nitrate partition coefficients (coef_nit) were correlated with the AEC values: coef_nit = 0.3363 AEC+0.1855 (R² = 0.954). The soil nitrate stock determinants were highlighted by the matrix of correlations of the principal component analysis: excheangeable acidity and aluminum, cation exchange capacity, ΔpH, C_org. and clay and total silt contents were particularly important for the AEC and coef_nit. The different cropping systems showed no significant impact after 12 years on soil characteristics that could improve nitrogen bioavailability. We thus recommend using organic fertilization to raise the soil pH, C_org. content and nitrogen bioavailability.

KEYWORDS:

• Anion exchange capacity
• nitrate adsorption
• nitrogen bioavailabity
• point of zero salt effect
• variable charge soil

Acknowledgments

The authors thank CIRAD for supporting for this study through different projects, and the Water, Soil and Plant Analysis service unit (CIRAD, US 49, Montpellier) for the laboratory analyses. We also thank David Manley for improving the English in this paper.

Additional information

Funding

The authors thank CIRAD for financially supporting for this study;Centre de Coopération Internationale en Recherche Agronomique pour le Développement;