Dynamic soil depth differences in soil nutrient cycling in boreal podzol destined for land use conversion and land use intensification

Tingskou, Richard (2023) Dynamic soil depth differences in soil nutrient cycling in boreal podzol destined for land use conversion and land use intensification. Masters thesis, Memorial University of Newfoundland.

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Abstract

Chapter 2. - Boreal lands have long been considered unsuitable for agriculture. Climate change projections support a shift towards increased agricultural production in boreal ecosystems via land use conversion. Depending on the conversion protocol the new plough layer may variably reflect the properties of different soil depths. This study assessed how adding mineral or organic fertiliser sources to distinct soil layers within ploughing depth (0-15cm [topsoil] or 15-30cm [subsoil]) affects nutrient uptake and crop productivity. Locally relevant organic waste fertilisers (dairy manure [DMN], recirculating aquaculture system supernatant [SUP] and slurry [RAS]), were compared against mineral fertiliser. Tall fescue was used to verify agronomic responses. Starting available nutrient concentrations were balanced as necessary with mineral N and P fertilisers. Except for NH4-nitrogen, both soils were of apparent similar fertility (total C <1%); unfertilized, neither could support significant plant growth. Subsoil-based growth was fast but limited in time reflecting immediately available nutrients. Topsoil led to delayed, extended growth. This might be due to dichotomous priming effects: during the duration of the experiment up to 36% of subsoil carbon was lost, while the topsoil gained up to 49% carbon, most apparent for DMN. While RAS led to the greatest N and P acquisition efficiencies for topsoil, it had no such effect for subsoil, suggesting distinct fixation and mineralization processes. Dissimilar C cycling and nutrient acquisition for the two soil depths indicates that site-specific considerations that include soil health parameters to the commonly tested available nutrients are needed where Podzols are farmed after land-use conversion. Chapter 3. - Climate change projections are changing perspectives on boreal ecosystems with a focus shifting to the potential necessity for agricultural productivity. One of the dominate soil types in the boreal region are Podzols that are known for their low organic matter (OM) content. OM a common and well defined indicator of soil quality. Stoichiometric ratios have been applied in ecology to measure the dynamics of ecosystem functions and may be employed to enhance the understanding and monitoring of SOM based soil quality assessments. This work set out to identity the potential application of stoichiometric ratios in boreal podzol land use conversion events where organic matter content may not be the ideal indicator of soil quality. Two soil depths (0-15cm [topsoil] and 15-30cm [subsoil]) were collected and amended with locally relevant organic waste fertilisers (dairy manure [DMN], recirculating aquaculture system supernatant [SUP] or slurry [RAS]), with mineral fertilisation as control. Tall fescue served as a model for biomass accumulation of C:N:P and was used to measure the transfer of N and P to plants as a measure of functional capacity of the soil to sustain continued production. Starting available nutrient concentrations were equalized as necessary with mineral N and P fertilisers. Noted differential C:N:P ratios in test crop biomass were due to interactive effects of soil depth and nutrient source; while the test crop was provided with the same apparent quantity of nutrients, the plant biomass C:N:P ratios were found to differ significantly. P uptake and availability proved to subscribe to the law of the minimum with respect to subsoil interactive effects with predominantly organic sourced P. The use of plant tissue nutrient ratios was able to help identify the limiting components of the system providing evidence for further exploration in to field based settings. Boreal podzol soil layers led to differential nutrient uptake and utilisation of the same fertilisers, an observation of interest for the sustainable nutrient management in podzol when soil layers are variably mixed at land use conversion.

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