Nitrogen transformations and loss over winter in manure-amended soils with cover crops

Many farmers apply manure in the fall (autumn season), but without an actively growing crop in the ground, the nitrogen (N) in the manure is susceptible to over-winter losses. Periods of freeze-thaw cycling can exacerbate N losses by stimulating soil microbes to transform reactive substrates like soil mineral N into nitrous oxide (N2O), a potent greenhouse gas. The uptake of reactive N from fall-applied manure by a fall-sown cover crop may reduce over-winter N losses. The objective of my research was to investigate the effect of combining fall manure application with cover cropping on soil N dynamics over winter and during periods of freeze-thaw cycling under field and laboratory conditions. I also examined the relationship between N2O production and reactive soil substrate concentrations. The field experiment was a full factorial in a randomized complete block design with three manure treatments (none, liquid, solid) and four cover crop types (no cover crop, 100% ryegrass [Lolium multiflorum Lam.], a 75% ryegrass/25% hairy vetch [Vicia villosa Roth] mixture and a 50% ryegrass/50% vetch mixture). The experiment was established at two field sites in Québec, Canada. A partial N mass balance (g N m-2) was calculated in fall (sum of the fall soil N stock to 0.15 m depth, N in fall-applied manure, and N in cover crop biomass) and in spring (sum of the spring soil N stock to 0.15 m depth and N in the winter-killed cover crop) for each treatment combination. After terminating the cover crop, spring wheat (Triticum aestivum L.) was planted, and each main plot was split into two subplots that received either 100 kg N ha-1 urea fertilizer or no fertilizer. Wheat samples were taken at tillering, flowering, and maturity to determine N content. Final yield was also measured. Cover crops were not effective at retaining manure N (≤7% uptake) and there was no difference in the fall and spring N balance among the manure and non-manure plots. Residual N was not supplied from fall-applied manure to the spring wheat in the next growing season, and average wheat yields were 11–14% less in the subplots that received no spring N fertilizer than those that received 100 kg N ha-1. In the laboratory, pots with 280–285 g soil received four N fertilizer treatments (none, liquid manure, solid manure, urea), with or without an annual ryegrass cover crop. The pots were exposed to 0, 1, 2, or 3 simulated freeze-thaw cycles (FTCs) at -4 to +4°C. The N2O production was measured at 0, 3, 6 and 9 h for each FTC, then pots were destructively sampled to determine the soil mineral N concentration. There was no difference in N2O production among the treatment combinations across all FTCs, but the pots that received urea or liquid manure had the highest soil mineral N concentration. The cover crop had minimal effect on the soil mineral N concentration. Soil mineral N explained approximately 14% of the variation in N2O production. Pots that underwent FTCs had a remarkable 937–1000% increase in N2O production compared to unfrozen pots. This suggests that N2O-producing microbial activity occurred in the frozen soils at -4oC, causing N2O to accumulate under ice and be released when the soils thawed at 4oC, mostly within the first 3 h. The results of both the field and laboratory studies suggests that microbial N transformations do not stop during the winter months, leading to substantial losses of N in fertilized soils during the non-growing season in cold humid temperate regions

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