Soil nitrogen cycling under contrasting management systems in Amazon Coffea canephora agroecosystems

Abstract

Robusta coffee (Coffea canephora Pierre ex Froehner) is well suited to the humid tropical climate of the Amazon basin. It is often produced under contrasting conditions of low-input agroforestry systems and intensively managed monocultures that differ in N constraints on productivity. We evaluated indicators of soil N cycling and coffee plant N sufficiency using a full factorial of two input managements (organic vs. conventional) and the absence or presence of the interplanted leguminous tree (LT) Erythrina spp. in a replicated robusta coffee field trial in the Ecuadorian Amazon. Activities of soil protease, three aminopeptidases, N-acetyl-β-D-glucosaminidase, cellobiohydrolase, and β-glucosidase were evaluated in tandem with soil NH 4 -N and NO 3 -N, potentially mineralizable N (PMN), and permanganate oxidizable C (POXC), as well as coffee leaf N, leaf N/P ratio (N/P), and yield. The LT decreased soil enzyme activities, PMN, and extractable NH 4 -N, as well as leaf N and leaf N/P. Enzyme activities and NH 4 -N were greater under organic input, but conventional input resulted in greater PMN, leaf N, leaf N/P, and yield. Permanganate oxidizable C and NO 3 -N were similar across input and interplanting but were more variable in the presence of LT relative to its absence. Lower soil enzymatic activities, labile N pools, and leaf N in coffee systems with Erythrina spp. suggest that, although a common smallholder practice in this region, interplanting this leguminous perennial has marginal impacts on soil N cycling and may not necessarily improve N supply for robusta coffee. INTRODUCTIONCoffee is an important cash crop in many developing tropical countries as the economic basis of an estimated 20 mil-

Apparent nitrogen limitation of Robusta coffee yields in young agroforestry systems

“…Though the full sun had slightly lowered SOM-the source and thus a predictor of mineralizable N (Ros et al, 2011;Clark et al, 2019)-this decrease was not reflected in soil available NH 4 + -N. Unexpectedly, the Erythrina treatment had lower soil ammonium levels, potentially reflecting competition by this leguminous tree for soil N. We did not quantify N contributions by leguminous shade trees, and wide variation in N 2 fixation due to soil, management, and weather interactions (Nair, 1999) challenges even broad estimates. However, similar N contained in litter loads suggests minimal contribution of N 2 fixation by leguminous shade trees at our study site, consistent with soil N cycling being invariant with Erythrina vs. full sun shading treatments at our site (Ibarra et al, 2021). Though repeated pruning and application of Erythrina residues to soil can contribute up to 227 kg N ha -1 yr -1 -presumably mostly from N 2 fixation (Meylan et al, 2017) but potentially recycling soil N (Nygren & Ramírez, 1995) this can vary substantially with planting density and pruning frequency (Russo & Budowski, 1986).…”

Overlooked tools for studying soil nitrogen depolymerization: Aminopeptidase assays using nitroanilide substrates

“…The majority of studies identified (61%) did not report conducting controls for nonenzymatic sources of absorbance, and nearly one‐third of studies conducted controls for dissolved organic matter and abiotic hydrolysis of substrate. Correction for dissolved organic matter contribution to absorbance of p NA product is particularly important given the lower activities generally observed for aminopeptidases assayed by p NA compared with many other p NP‐assay enzymes (Daughtridge et al., 2021; López et al., 2021). However, no studies corrected for incomplete recovery of p NA, which, as for p NP‐based assays, will entail mis‐estimation of enzyme activity (Daughtridge et al., 2021; Margenot et al., 2018).…”

“…matrix type, assay duration and temperature, termination and controls for nonenzymatic absorbance. Finally, the selectivity of these (exo)aminopeptidases to the N-terminus amino acid (Bradshaw, 2013) and the carbon (C)/N of amino acids means that different aminopeptidases may offer distinct information related to C vs. N limitation (López et al, 2021), or even sulfur (S)-cycling (e.g., methionine). The objective of this work was to provide a systematic review of soil aminopeptidase assays using the chromogenic pNA substrates.…”

“…Key assay conditions that merit standardization include scale (slurry vs. direct incubation), substrate concentration, matrix type, assay duration and temperature, termination and controls for nonenzymatic absorbance. Finally, the selectivity of these (exo)aminopeptidases to the N‐terminus amino acid (Bradshaw, 2013) and the carbon (C)/N of amino acids means that different aminopeptidases may offer distinct information related to C vs. N limitation (López et al., 2021), or even sulfur (S)‐cycling (e.g., methionine).…”