More trees less loss: Nitrogen leaching losses decrease with increasing biomass in coffee agroforests

The mitigation of nutrient leaching losses is an important ecosystem service, and is easily affected by small-scale management decisions. This study was conducted on eight coffee agroforests in Costa Rica, in order to identify particular mechanisms preventing nutrient leaching losses. First, we examined how nitrogen (N) and phosphorus (P) leaching losses differed between coffee agroforests amended with mineral or organic fertilizers. Monthly N and P concentrations were measured in soil water using tension lysimeters (at 15 and 100 cm) between October 2008 and September 2009, and a water balance model was developed to estimate annual nutrient leaching losses from these agroforests. Second, we examined the effects of human interventions (fertilizer type and quantity), biology (shade trees), and chemistry (soil properties) on nutrient leaching losses. Despite differences in the quantity and form of fertilizer inputs, N and P losses at 100 cm did not differ significantly between organically and conventionally managed agroforests (119 kg N ha−1 yr−1 and 1.5 kg P ha−1 yr−1, respectively). Rather, N losses declined linearly with increasing shade tree biomass, which is determined by farmers. Phosphorus losses, on the other hand, declined with increasing soil iron pools, which are independent of management decisions.

► We measured N and P leaching losses in organic and conventional agroforests.
► We determined the effects of human interventions, biology, and chemistry on N and P leaching losses.
► N leaching losses declined linearly with increasing shade tree biomass.
► P leaching losses declined linearly with increasing soil Fe pools.