Assessing the impacts of biochar and fertilizer management strategies on N and P balances in subtropical pastures

Abstract

Improper nutrient management strategies often results in soil N and P imbalances, which can lead to unintended environmental consequences. Nutrient budgets can be useful tools to identify nutrient imports and exports and assess the agronomic and environmental performance of different pasture nutrient management options. In this 3-yr study, we used a farm-gate nutrient budgeting approach to determine N and P use efficiency in subtropical perennial pasture systems subjected to different nutrient management strategies. Treatments consisted of repeated application of biosolids and inorganic fertilizer (at an equivalent rate of 160 kg plant available N ha−1 yr−1) either alone or in combination with (20 Mg ha−1 yr−1). Major N and P inputs and outputs were: N and P added via fertilizer and biochar, plant uptake, leaching, and denitrification (N budget only). Nitrogen recovery in forage above-ground biomass accounted for a significant proportion of applied N (~64%) and P (22%). Greater N leaching occurred in inorganic fertilizer (29% of applied N) vs. biosolids treatments (average of 7% of applied N). Negligible P leaching (0.1–0.2% of total P applied) occurred, regardless of the treatment. Biochar did not affect either crop N and P use efficiency or P leaching, but reduced N leaching for treatments receiving inorganic fertilizer. Treatments showed no impact on soil N, but vertical distribution of soil total P changed over 3 yr, suggesting downward movement of P. Despite the high initial soil test P levels, biosolids application at P loads exceeding agronomic recommendations improved agronomic N and P efficiency relative to control treatments with no impact on N or P leaching.