Nitrogen cycling in shaded and unshaded coffee plantations in the Central Valley of Costa Rica.

Abstract

Coffee (Coffee arabica L.) management in Costa Rica is changing from plantations associated with shade trees to monocultures; both of which receive large additions of N through fertilization. Understanding patterns and processes of N cycling within coffee agro-ecosystems could lead to the development of management practices that maintain productivity while minimizing environmental damage. My study investigated the N dynamics in paired shaded and unshaded coffee plantations in the Central Valley, all fertilized with about 300 kg N ha$\sp{-1}$ y$\sp{-1}$. I quantified net N mineralization and nitrification, and microbial biomass N. Mean daily mineralization rates did not significantly differ between treatments (shaded = 38.9 mg N m$\sp{-2}$ d$\sp{-1}$; unshaded = 29.4 mg N m$\sp{-2}$ d$\sp{-1}$). However, net mineralization was significantly higher in shaded plantations (148 kg N ha$\sp{-1}$ y$\sp{-1}$) than in the unshaded (111 kg N ha$\sp{-1}$ y$\sp{-1}$). Small pools of microbial biomass N (shaded = 0.48 g m$\sp{-2}$; unshaded = 0.46 g m$\sp{-2}$) suggested that N retention primarily occurs through plant uptake. I also investigated NO$\sb3\sp-$ leaching and factors controlling denitrification, such as NO$\sb3\sp-$, C, and O$\sb2$ availability. Annual NO$\sb3\sp-$ leaching losses were 3 times greater in unshaded plantations (238 kg NO$\sb3\sp-$-N ha$\sp{-1}$ y$\sp{-1}$) than those in the shaded (87 kg ha$\sp{-1}$ y$\sp{-1}$). In contrast, mean denitrification rates in shaded plantations were 60% higher (6774 ug N$\sb2$O-N g$\sp{-1}$ d$\sp{-1}$) than those in unshaded plantations (4340 ug N$\sb2$O-N g$\sp{-1}$ d$\sp{-1}$). Carbon additions (C$\sb6$ H$\sb{12}$ O$\sb6$) elicited nearly a three-fold increase ($-$C = 2985 ug N$\sb2$O-N g$\sp{-1}$ d$\sp{-1}$ to +C = 8396 ug N$\sb2$O-N g$\sp{-1}$ d$\sp{-1}$) in both agroecosystems. Anaerobic conditions increased denitrification to a lesser extent (+O$\sb2$ = 4331 ug N$\sb2$O-N g$\sp{-1}$ d$\sp{-1}$; $-$O$\sb2$ = 6656 ug N$\sb2$O-N g$\sp{-1}$ d$\sp{-1}$). In both plantation types, potential N loss via denitrification was small compared to NO$\sb3\sp-$ leaching. My results indicate that large N-fertilizer additions are unwarranted measures to sustain high coffee yields. In light of the large N loss through leaching, the presence of shade trees is an important factor reducing N loss.