Impact of crop aerial and root biomass inputs on soil nitrifiers and cellulolytic microorganisms

Abstract

In agricultural no-till systems, crop residue management has an important impact on the activity and abundance of soil microorganisms, which play a crucial role in soil health and quality. In these systems, when cover crops are included in order to increase the amount of crop residues, aerial biomass remains on the soil surface, whereas root biomass is directly incorporated into the soil. Although both aerial and root biomass may have a differential effect on the abundance and activity of nitrifiers and cellulolytic microorganisms, to our knowledge, such effects have not been individually studied in the literature. The aims of this research were to study the abundance and activity of nitrifiers and cellulolytic microorganisms, and their efficiency indexes (EIs), as affected by: i) aerial cover crop residues and roots of cover crops at two soil depths (0-0.5 and 0.05-0.2 m), and ii) soil conditions at 30, 90, and 150 days after the cover crop killing date. We conducted an experiment during two consecutive years in Paraná, Argentina (-31°50.9′S; -60°32.3′W). Treatments included two cover crop species that were added to the soil either as aerial input or as root input. Treatments were aerial input of wheat (Triticum aestivum L.) (AW) on fallowed soil, aerial input of white sweet clover (Melilotus albus Medik.) (ASC) on fallowed soil, in situ grown roots of wheat (RW), in situ grown roots of white sweet clover (RSC) and a control (C) on fallowed soil without aerial or root inputs. In each of the growing seasons, at 30, 75 and 150 days after the killing date, soil samples were collected at depths of 0-0.05 m and 0.05-0.20 m. From the soil samples, soil water content, activity and abundance of nitrifiers and cellulolytic microorganisms were determined. The average activity of nitrifiers was higher (p < 0.0001) in the topsoil layer than in the subsurface layer (0.06 vs. 0.04 mg NO3− g soil-1). However, the average cellulolytic activity did not differ among depths and treatments. The average abundance of nitrifiers was similar between soil layers (ca. 5-log g soil-1), and the abundance of cellulolytic activity ranged from 6.7 to 8.4 log g-1 soil. Our results show that in the topsoil, the microbiological variables under study had a more important effect via the source of the inputs, i.e. whether the source belongs to the aerial or roots biomass rather than the cover crop species. In addition, we detected that in the topsoil, regardless of the crop species, aerial residues promoted cellulolytic activity, whereas root inputs promoted nitrifier activity. The study allowed us to identify how the microbiological variables were affected by the source of the input at the field level.