Does phosphorus supply enhance soil-N mineralization in Brazilian pastures?

Abstract

Two Brachiaria decumbens paddocks on a sandy Entisol soil were compared to investigate whether degradation of tropical pastures may be due to phosphorus deficiency limiting plant growth and/or soil N mineralization processes. Grass dry matter (168 vs. 130 kg ha−1 week−1, wet season) and litter production (130 vs. 86 kg ha−1 week−1) as well as standing root biomass (3744 vs. 1683 kg ha−1, 0-60 cm) were considerably greater in the P fertilized (100 kg P2O5 ha−1) plot. In situ soil N mineralization measurements indicated substantially greater rates of N release under the P fertilized pasture only at the beginning of the dry season. Soil mineral N amounts present during the wet season were greater in the unfertilized plot, presumably due to the smaller N demand of P deficient plants.In a laboratory incubation experiment testing three acidic soils of varying texture, P supply increased the amount of soil-N mineralized with time especially in the more clayey, P-fixing soils. The effect was associated with slight increases in soil pH, probably due to a decrease in charge of (Al- and Fe-) hydroxide complexes. Biomass N was not significantly affected by P supply but biomass efficiency for N mineralization (mineralized N per unit biomass N) increased with P supply in the clayey soils.Our results suggest that in poor tropical sandy soils under grass pastures, the addition of P stimulated plant N uptake primarily by alleviation of plant P deficiency and more efficient N cycling rather than by a direct effect on soil N mineralization.