PS 33-153 - CANCELLED - Plant decomposition modifies active carbon availability and nitrogen dynamics in an organic vegetable agroecosystem in the Upper Midwest

Tuesday, August 16, 2022

5:00 PM – 6:30 PM EDT

Located in: ESA Exhibit Hall

Presenting Author(s)

GH

Gabriela C. Hidrobo

University of Minnesota, United States

Coauthor(s)

MS

Marie Schaedel

PhD Candidate
Department of Horticultural Science/ University of Minnesota
St Paul, Minnesota, United States
AF

Adria Fernandez

University of Minnesota, United States
EM

Ezra Moses

University of Minnesota, United States
BM

Bonsa Mohamed

University of Minnesota, United States
JG

Julie Grossman, PhD

Associate Professor
Department of Horticultural Science/ University of Minnesota
St Paul, Minnesota, United States

Background/Question/Methods

Soil fertility is often enhanced by cover crop (CC) biomass incorporation in organic vegetable rotations. However, the effects of organic carbon (C) and nitrogen (N) inputs on ecosystem function, specifically decomposition, may vary depending on CC species selection and plant growth stage at termination date. Further, the temporal dynamics of microbial mineralization in vegetable agroecosystems are not well defined, even though they are key to understanding management and conservation strategies. Our objectives were to identify temporal patterns and determine effects of cover crop species on 1) labile soil organic C content and 2) N-cycling enzyme activity during cover crop biomass decomposition. Spring-planted CC were established using a RCBD with four replications in early May and terminated in mid-July 2021 at two locations in Minnesota, after which a vegetable crop was transplanted. Treatments included a legume, a forb, a legume/grass mixture, and two controls: a weedy fallow and weed-free fallow. A total of 240 soil samples were collected at six time points across ninety days of CC decomposition and vegetable crop growth. Permanganate oxidizable carbon (POX-C) was measured from dry soil samples to estimate active C; nitrification potential (NP) and denitrifying enzyme activity (DEA) were quantified from fresh soil samples.

Results/Conclusions

Differences in POX-C among cover crop treatments were not significant (p > 0.05) at any time point. Therefore, we combined data from all treatments to determine changes in POX-C over time. POX-C is considered a fraction of labile organic matter which is readily used by microorganisms. POX-C values were highest forty-five days into the CC decomposition study period, and differed between the two experimental sites by up to 180%. DEA and NP were positively correlated with POX-C and also showed the greatest values after 45 days into CC decomposition, which may be associated with favorable soil moisture content and warm temperatures in mid- summer. DEA is a potential route of microbially mediated N loss, and we expected a greater response from the legume-only treatment due to the lower C:N of the decomposing biomass; however, no differences were observed across sampling time points or locations. Therefore, results suggest that legume CC could help achieving N fertility and retention goals through biological nitrogen fixation but without stimulating denitrifying enzyme activity. Ongoing analysis will measure microbial biomass and hydrolytic enzyme activity during the CC decomposition process to further elucidate nutrient cycling dynamics in Upper Midwest cover cropping systems.