Effect of organic amendments and inorganic fertiliser application on nitrogen use efficiency and denitrification in controlled and field conditions

The amount of bioavailable nitrogen is directly linked to anthropogenic activity (Kuypers, Marchant, & Kartal, 2018), particularly with the intensive application of synthetic nitrogen fertilisers. Although high nitrogen inputs are required to support the ever-increasing need for food production, nitrogen use efficiency is in many cases low, to the extent that even with extra nitrogen inputs over time, increases of food production are small and slow (Battye, Aneja, & Schlesinger, 2017).

It has been suggested that roughly 40% of reactive nitrogen is denitrified in the soil (Seitzinger, et al., 2006), and most of the reactive nitrogen that results from human activities is removed by denitrification, with consequent production of N₂ and N₂O. However, even if most reactive N forms are removed by denitrification, this is an indicator that N use efficiency is not at optimum levels.

A study is being conducted in field and controlled conditions, that aims to understand denitrification and nitrogen use efficiency in a long-term experiment (running continuously since 2013) at Rothamsted Research. The experiment was designed to provide a clearer look at the effect of applications of organic amendments and/or inorganic fertilisers on nitrogen dynamics and crop yields in a conventional cereal-based cropping system.

Simultaneously, using yield data from the same trial, we aim to understand a) if the application of organic amendments leads to a reduction of the nitrogen threshold for optimum yields and, by using a modelling approach, b) if the eventual higher yields obtained with organic amendment application are due to the effect of the extra nutrients contained in the amendment or to some other effect caused by the amendments.

Soil and gas samples are being collected from a) different treatments of the field experiment (four different organic amendments: anaerobic digestate, compost, farmyard manure, straw and unamended control; and different nitrogen application rates; area of each plot: 54 m²) to assess nitrogen dynamics, and b) from soil columns (height 35 cm; width 25.5 cm)  placed in a controlled environment using soil collected from the same trial. Different measurements are being taken including leachate (measurements of mineralised nitrogen), microbiology and gas emissions (using a Picarro device that measures NH₃, N₂O, CO₂, CH₄, O₂, H₂O). Simultaneously, underground sensors are being used to understand moisture and temperature evolution in the soil column, while electrochemical nitrate sensors are being used to understand nitrate dynamics before and after application of organic amendments and inorganic fertilisers.

With this, we aim at having a better understanding on denitrification processes and nitrogen use efficiency issues that may occur when using a joint regime of organic amendments and inorganic fertilisers. The main objectives of the project are the validation of the effect of organic amendments in the Fosters long-term experiment and the quantification of nitrogen gas emissions with the application of organic amendments and nitrogen fertilisers.