Do residue quality and tillage systems modify nitrogen partitioning into different soil organic matter fractions?

Abstract

The crop residue transformation into soil organic matter (SOM) is a complex process in which the allocation of carbon (C) and nitrogen (N) into soil fractions does not follow a direct or predictable pattern. The main objective of our study was to determine the partitioning of N from plant residues with different quality into SOM fractions under contrasting tillage systems. We evaluated the percentage of N derived from plant residues (%NDR) allocated into SOM fractions [particulate (POM) and associated with minerals (MAOM)]. Contrasting tillage systems [conventional tillage (CT) or no-till (NT)] and ¹ ⁵N enriched plant residues (corn and soybean) were compared. Over one year, the %NDR in SOM ranged from 35 % (NT-Corn) to 79 % (CT-Soybean), with tillage and residue quality significantly affecting N partitioning. The N from the corn residue was recovered in greater proportion in the POM than soybean. However, in CT the %NDR differences between crop residues were smaller than in NT. The soybean residue had a proportionally greater contribution to MAOM than corn (+7.7 % on average). The C-POM increased immediately after residue addition with a maximum in CT-Corn (+0.35 g kg−1 compared to the initial value), twice as much as the other treatments. The N-POM also increased after residue application, reaching a maximum in NT-Soybean (26 % more than other treatments). A temporary depletion of C-MAOM and N-MAOM was observed, which was greater in CT compared to NT, and with corn residues compared to soybean. The novel findings of our study are that: 1. Nitrogen incorporation from crop residues into POM occurs simultaneously with POM mineralization; 2. Nitrogen direct stabilization from residues into MAOM occurs at the same time as MAOM mineralization, which is stimulated by residues input; 3. Low C:N crop residues are retained more efficiently than high C:N crop residues in SOM and its fractions.